WO1989010829A1

WO1989010829A1 - Method of molding plastic and mold therefor

Info

Publication number: WO1989010829A1
Application number: PCT/US1989/001907
Authority: WO
Inventors: Frank J. Baresich
Original assignee: Baresich Frank J
Priority date: 1988-05-06
Filing date: 1989-05-05
Publication date: 1989-11-16
Also published as: JPH03505185A

Abstract

A method of molding plastic, especially injection molding or compression molding and a mold (6) therefor involves causing the surface of the mold cavity (13) to have a temperature above the solidification temperature of the molten plastic during molding of a part. The temperature of the surface of the mold cavity (13) is preferably mantained near or above the solidification temperature of the molten plastic as the center of the plastic in the mold cavity cools from a temperature above the solidification temperature toward the solidification temperature, thereby reducing flow-induced and temperature gradient-induced orientation stresses in the solidified plastic. A layer of insulation (14, 15) about the mold cavity and gate, if any, allows heat from the molten plastic to raise the temperature of the mold cavity surface to the required level.

Description

Description Method of Molding Plastic and Mold Therefor Technical Field:

The present invention is directed to an improved method of molding plastic and a mold therefor. The invention is particularly adapted for injection molding plastic for molding plastic optical parts such as optical discs and lenses, or for compression molding plastic to produce phonograph records, for example. Background Art:

In almost all molding operations pressure and heat are applied to cause a plastic to flow into the desired shape. The shape is then fixed by cooling. Thermoplastic materials must be cooled below the melting temperature or glass transition temperature before removal from the mold. Heating, flow under pressure and cooling under pressure are required in this sequence.

It is known to use an injection molding process to produce parts from amorphous or crystalline thermoplastics, for example. In this process, a hot plastic melt is forced through a small opening called a gate into the mold cavity that defines the finished part geometry. The mold is much colder than the plastic melt. Where the plastic melt initially contacts the mold surface a thin layer freezes.

In the case of amorphous plastic, regardless of the molecular orientation of the solidifying melt caused by the molten plastic flowing through the gate, the outer frozen region is relatively stress free. An intermediate layer is formed when the molecules in the melt contact the outer frozen region and freeze to it. The high flow shear stress causes these molecules to become oriented in the direction of flow. This intermediate layer is highly oriented and highly shear stressed.

The outer and intermediate layers together form a region referred to as the skin. This skin region thermally insulates the central core of flowing plastic melt. The core remains hot for a longer period than the skin providing time for reorientation of the molecules in the core region. The core region consequently has little flow-induced orientation and shear stress. The orientations and layers can be seen in Figs. 1-3 of the drawings. In Fig. 1 a rectangular mold cavity 1 is shown being filled with molten plastic 2 through a gate 3. The hydrodynamic skin-core structure in the mold cavity 1 during filling is shown in Fig. 2 wherein the skin 4 of the plastic melt within the mold cavity at the top and bottom surfaces thereof thermally insulates the central core 5 of flowing plastic melt during filling of the mold. As shown in Fig. 3, the skin is formed of an outer layer, identified as zone I, and an intermediate layer which is highly oriented and highly shear stressed, identified as zone II. The core zone is identified as zone III in Fig. 3. Figures 1, 2 and 3 are from "The Interrelationship Of Flow, Structure, And Properties In Injection Molding" by L.R. Schmidt, Corp. R&D, G.E., Schenectady, New York.

Crystalline polymers also show the effects of flow-induced orientation. The plastic contacts the cold mold surface and a thin oriented layer freezes forming some small crystalline regions. The crystals thus formed act as nucleating agents and because they are oriented, the crystals grown from them are also oriented. The intermediate and central core regions develop in much the same way as described for amorphous plastics. Under typical injection molding conditions, flow rates vary significantly throughout the cavity resulting in variable skin thickness. Mechanical and physical properties of the part vary considerably depending on the distribution of skin and core thicknesses as well as the degree of orientation and crystallinity at different locations in the part.

During flow, higher pressures in the core region cause higher shear stresses and molecular orientation in the intermediate layer as the core material is pushed through. When the cavity is completely full, pressure increases rapidly. High pressure is then maintained as the part continues to cool and the thickness of the frozen layer increases. The volume of the part decreases as it solidifies and the injection pressure packs in more plastic until the gate closes or pressure is removed. After the gate closes, the part continues to cool until the frozen layer is thick and rigid enough for part ejection. The pressure decreases as the part cools and the specific volume decreases. The outer frozen layer maintains part dimensions while the core region continues to cool and shrink thus creating additional residual stresses in this known injection molding process.

Several other problems are also inherent in this known injection molding process. Parts with varying wall thicknesses will pack differently in different locations in the parts. Over packing occurs in areas of a part that fill too early and in a cavity of a family or multicavity mold that fills before the other cavities. Over packing also occurs in the area near the gate where hot plastic is forced in with the relatively cool plastic. The over packed areas are highly stressed and exhibit lower strength, warpage and part hang-up in the mold. Common methods of reducing these problems include altering part wall thicknesses. relocating the gate, and profiling the injection pressure to reduce pressure during packing.

Differential densities, molecular orientations, thermal gradients, and crystallinity in plastic parts cause warping. In addition, weld lines are formed where two melt paths meet. If there are large enough temperature differences at the converging melt fronts, weak weld lines result. Conventional methods for minimizing this problem include adjusting gate locations, gate sizes or runner sizes. The part may even be redesigned to relocate the weld lines.

Birefringence is the characteristic of having two indices of refraction with different values causing the separation of a light beam passing through the material into two diverging beams. Orientation of the molecules in the plastic and residual stress cause birefringence in a plastic part. Orientation stress has been plotted against birefringence and a linear relationship was found. The greater the degree of orientation, the greater the birefringence. Both flow induced orientation stress and stress caused by rapid cooling through the glass transition temperature with large temperature gradients as in the known process of injection molding, cause birefringence. Birefringence is a major problem in plastic optical parts such as a lenses and optical discs .

Shoji Ohsawa, Morio Tsuge, Akimitsu Takatsu, Toyoji Okuneshi, Junji Tanaka and Shin-ichi Mikami in their article "Thermosetting Resin Substrate For Computer Optical Memory Disk" 15 Nov. 1986/Vol. 25, No. 22 of Applied Optics make the following statements:

"Birefringence is one of the most important properties of optical disk substrates when used for computer memory units, as this property is directly related to the carrier- to-noise ration (CNR) and bit error rate. In particular, a magnetooptical disk using a delicate Kerr rotation angle as a signal requires a birefringence of <5 nm (double pass) , and nearly 0 must be targeted.

Birefringence is calculated using the following equation (Brewster formula) :

<5λ = c t (~1 — ~l) ' 2-

where (δ->) /2π) = birefringence (single pass) (nm) ; δ = retardation (single pass) ; λ = wavelength of light (nm) ; c = photoelastic modulus (cm²/dyn) ; t = thickness of a substrate (nm) ; and {σ~ σ- ) = principal stress difference (dyn/cm²) .

The principal stress difference defined here is caused by (1) a heterogeneous residue of mold pressure and (2) and heterogeneity of mold shrinkage. In general, the higher the mold pressure and the larger the mold shrinkage, the more heterogeneous becomes the residual stress. This results in a larger principal stress difference. It is, therefore, necessary to minimize the residual stress for a minimum birefringence."

Close tolerances are difficult to achieve in crystalline plastic parts formed by the known injection molding processes because crystallization causes additional shrinkage and typical molding conditions cause large variations in crystallinity throughout the part. Crystal growth requires time 5 and high mobility of molecules so that molecular segments can get close together and align themselves to one another. The slower the cooling, particularly as the polymer passes from its melting temperature to its glass transition temperature, the higher the

TCύ degree of crystallization. Crystallization will also occur more readily under lower pressure because of increased molecular mobility. As discussed above, in known injection molding processes, the skin region and the core region cool at much different rates as

15 they pass from the melting temperature to the glass transition temperature, which causes variable crystallization.

It has been generally recognized in the literature, see for example, page 34 of the second

20 edition of "The Handbook of Plastic Optics", U.S. Precision Lens, Inc., Cincinnati, Ohio that all parts of the molded optic must cool at the same rate during the cooling cycle to minimize part irregularity. To this end, the text states generally that molds have

25^" been built with heating and cooling passages for controlling cooling rate. Such sophisticated localized cooling and heating is costly, may be operator dependent and may not be uniformly effective.

30 Heavy walled parts are very difficult to mold without voids, under typical molding conditions with known injection molding processes. For example, Amoco Chemicals Corp. Bulletin TAT-35 for Torion, a poly(amide-imide) , offers the design guide that

35 generally part thickness range is 0.03 to 0.5 inch and that 0.625 inch is possible with special resin grades. Walls and gates freeze too quickly in heavy walled parts to allow enough plastic to enter the cavity to compensate for skrinkage. One known 5 solution to this problem is to introduce the polymer slowly to hot molds that keep the plastic from solidifying until enough plastic is injected to compensate for shrinkage. However, such a solution is disadvantageous because it requires an additional IDG operation before molding can begin which adds to the cost and reduces the efficiency of the molding operation. Disclosure of the Invention:

An object of the present invention is to provide 15 an improved method of molding plastic material to form molded parts and a mold therefor which avoid the aforementioned problems of the known plastic molding processes. More particularly, an object of the invention is to provide an improved method of 20 injection molding and an injection mold therefor which minimize, eliminate, or otherwise control the formation of the skin layer previously described, and which reduce the temperature gradients throughout the part while it is solidifying in the mold. 5 A further object of the invention is to provide an improved method of injection molding and an injection mold therefor which result in minimum flow induced molecular orientation and stresses, minimum cooling induced stresses, lower injection pressure, 0 higher quality and more consistent parts from single, family, and multicavity molds, greater control of crystallinity, minimum birefringence, stronger weld lines, more homogeneous plastic parts, better tolerance control of crystalline plastic parts, less 5 warping of plastic parts, and void free heavy wall parts, as compared with the known injection molding processes.

These and other objects of the invention are attained according to the method of molding plastic material to form molded parts of the invention, the method comprising the steps of providing a mold having at least one mold cavity in the form of a part to be molded, introducing molten plastic into the mold cavity, allowing at least a portion of the molten plastic in the mold cavity to cool until it freezes and removing the part from the mold cavity, the improvement comprising raising the temperature of at least a substantial portion of the surface of the mold cavity during molding to a temperature above the solidification temperature of the molten plastic of the part. Further, according to the method, the temperature of at least a substantial portion of the surface of the mold cavity is maintained near or above the solidification temperature of the molten plastic for an extended period of time during molding while the mold cavity, in the case of injection molding, is filled with molten plastic and the center of the molten plastic in the mold cavity cools from a temperature above the solidification temperature toward the solidification temperature thereby reducing flow-induced and temperature gradient induced orientation stresses in the plastic material. Preferably, the temperature difference between the outer surface and the center of the plastic in the mold cavity does not exceed 0.5°F for every 0.001 inch of part thickness (62.5°F for 0.125 inch thick part) when the outer surface starts to solidify after at least a substantial portion of the surface of the mold cavity has been caused to have a temperature above the solidification temperature of the molten plastic.

A significant feature of the invention is that the heat to raise at least a substantial portion of the surface of the mold cavity to a temperature above 5 the solidification temperature of the molten plastic during molding comes from the heat of the molten plastic forced into the mold cavity. This is accomplished in the injection mold of the invention by insulating the mold cavity by providing a layer of D_ mater ial that has a low thermal diffusivity at or near the mold cavity. In one form of the invention, the layer of insulating material is located beneath a layer of another material which forms the surface of the mold cavity that contacts the molten plastic 5 during molding. In another form of the invention, the layer of insulating material itself forms at least a substantial portion of the surface of the mold cavity which contacts molten plastic during molding. The cavity surface may be coated or plated 0 as necessary to prevent welding or other interactions between the molten plastic and the insulator. Preferably, the entire surface of the mold cavity which contacts molten plastic during molding is insulated so that its temperature is raised to a temperature above the solidification temperature of the molten plastic during molding of the plastic.

During the method of injection molding plastic material according to the invention, the temperature of the surface of the mold cavity is much colder than the solidification temperature of the molten plastic when the plastic first contacts the mold surf ce. This causes a layer of the plastic contacting the surface of the mold cavity to freeze. The frozen layer is remelted during the molding process when at a least substantial portion of the surface of the mold is caused by the invention to have a temperature above the solidification temperature of the molten plastic.

The injection mold of the invention according to a disclosed embodiment comprises a pair of mold halves each being formed with a metal insert with means being provided for cooling the metal insert such that the metal inserts act as heat sinks. A layer of insulation material is provided over the metal insert of each mold half adjacent the mold cavity. The layer of insulation material itself can form the surface of the mold cavity that is contacted by the molten plastic during molding or the layer of insulation material can be located beneath an insert, which may take the form of a layer of another material, which forms the surface of the mold cavity.

The thermal diffusivity and thermal resistance of the intermediate material in the mold between the metal inserts and the mold cavity are selected so that sufficient heat is stored from the molten plastic injected into the mold cavity during molding to raise the temperature of the surface of the mold cavity above the solidification temperature of the plastic being molded. This intermediate material can be formed solely by the insulating layer or by the combination of an insulating layer and an insert which forms the surface of the mold cavity. The insulation layer can also be provided about the gate of the mold. In the disclosed form of the invention, the outer periphery of the metal insert or heat sink is also insulated to control the flow of heat from the mold cavity and insure that the surface of the mold cavity adjacent the outer edges of the mold cavity can be raised to a temperature above the solidification temperature of the plastic material being molded. The injection mold may also be formed with a plurality of mold cavities, each insulated for molding according to the injection molding method of the invention. 5 The method of injection molding of the invention is particularly applicable to molding plastic material to form optical parts such as an optical discs or lenses whereby the optical parts have minimum birefringence, residual stresses, and warping

ID. so that consistently high quality optical components can be made.

The method and mold of the invention also have applicability in compression molding to produce parts from plastic. In this process, the molding material

15 is heated and placed between two heated mold halves. The mold halves are then brought together under pressure to cause the material to flow into the desired shape According to the invention, the heat from the material being molded raises the mold

20 surface temperature above the plastic solidification temperature for an extended period. This eliminates the need to first heat and then cool the mold as in the prior art compression molding techniques. Such a compression molding technique is useful for producing 5 high-quality phonograph records from a thermoplastic resin such as polyvinyl chloride .

These and other objects, features and advantages of the present invention will become more apparent by the following description when taken in connection 0 with the accompanying drawings, which show, for purposes of illustration only, three embodiments in accordance with the present invention. Brief Description of the Drawings:

Fig. 1 is a schematic of the filling dynamics 5 for flow of molten plastic into a rectangular cavity according to a known injection molding process;

Fig. 2 is a schematic of the cross-section of the molding of Fig. 1 taken along the line II-II and illustrating hydrodynamic skin-core structure; 5 Fig. 3 is a schematic of the cross-section of a molding like that of Fig. 2 illustrating morphologic zones and zone formation;

Fig. 4 is a cross-sectional view through an injection mold according to a first embodiment of the IDα invention for making an optical disc;

Fig. 5 is a cross-sectional view through an injection mold according to a second embodiment of the invention;

Fig. 6 is a cross-sectional view through an 15 injection mold according to a third embodiment of the invention for making a double convex lens;

Fig. 7 illustrates the results of a computer thermal analysis of the temperature of crystalline polyethylene plastic which has been injection molded 20 in a conventional metal mold, the temperature profile being shown from the outer surface of the plastic to the center line thereof at the instant of introduction of the plastic melt into the mold cavity 0.0 seconds, and at intervals of 0.05, 2.32, 8.82 and 25 12.82 seconds thereafter;

Fig. 8 is a diagram like that of Fig. 7 showing the thermal analysis using an injection mold according to the invention to raise the temperature of the mold cavity surface above the solidification

30 temperature of the polyethylene;

Fig. 9 shows temperature changes as a function of time for center and surface of polyethylene and for surface of mold using mold of invention;

Fig. 10 shows the temperature difference between 35 the outer surface and the center of the plastic when the outer surface starts to solidify with varying thicknesses of an aluminum layer about the surface of the mold cavity in mold of the invention;

Fig. 11 illustrates the mold surface temperature 5 as a function of the time from the introduction of the molten plastic in the mold cavity for two different combinations of mold cavity lining and insulating materials in a mold according to the invention; Iff Fig. 12 illustrates the temperature of plastic within the mold cavity of a conventional metal mold when molding a polycarbonate plastic to form an optical disc according to a known method;

Fig. 13 shows temperature change as a function 15 of time for center and surface of polycarbonate and for surface of metal mold of Fig. 12;

Fig. 14 is a diagram like that of Fig. 13 where the optical disk is injection molded according to the present invention. 0 Fig. 15 again illustrates the mold shown in fig. 6, but with the dimensions for components thereof being shown for making a double convex lens according to the invention;

Figs 16A and 16B show constant temperature lines 5 (isotherms) in a double convex lens about the time the lens starts to solidify in an all metal mold

(Fig. 16A) and in a mold according to the invention

(Fig. 16B) ; and

Figs. 17A and 17B are like Figs. 16A and 16B 0 except that they relate to a time when the lens is almost completely solid. Best Mode for Carrying Out the Invention:

Referring now to the drawings, an injection mold 6 according to a first embodiment of the invention is shown in Fig. 4. The mold 6 comprises a pair of mold halves 7 and 8. Each mold half includes a mold heat sink in the form of metal inserts 9 and 10 which are provided with cooling channels 11 and 12, respectively. Coolant is flowed through the channels 11-and 12. for removing heat from the mold when molten pllasi-ricr is injected into the mold. When the mold halves 7 and 8 are placed adjacent to one another as shown in Fig. 4 they define a mold cavity 13 for injected molten plastic. Insulator layers or inserts 14 and 15 according to the invention are located on respective surfaces of the metal inserts 9 and 10. Surface inserts or layers 16 and 17 of metal, glass or some other material that provides a desired hardness, wear, chemical and temperature -resistance to the molten plastic are located against the respective insulator inserts 14 and 15 and define surfaces of the mold cavity with their inner surfaces as shown in Fig. 4. These surface layers may also be used to draw excess heat from the plastic to reduce total cooling time. In the mold 18 of the invention illustrated in Fig. 5, inserts 16 and 17 are not employed and the insulator inserts 14 and 15 define the surface of the mold cavity. The outer edges of the mold cavity in each of the molds 6 and 18 are also insulated about their outer peripheries by insulation members 19 and 20, respectively. A central gate 21 extends through the metal insert 10, . insulator insert 15, and with respect to the mold 6 in Fig. 4 and mold 18 in Fig. 5, the surface layer 17, for introduction of molten plastic under pressure during injection molding. Clamping plate 22, "A" plate 23, "B" plate 24 and supporting plate 25 are typical of standard mold bases. Item 4 is a sprue bushing.

The surface layers 16 and 17 and insulator inserts 14 and 15 in the mold 6 of Fig. 4 constitute an intermediate medium between the molten plastic injected into the mold cavity 13 by way of the central gate 21, and the mold heat sink constituted by the metal inserts 9 and 10 with cooling channels 11 and 12, respectively. In the mold 18 of Fig. 5, the intermediate medium between the molten plastic and the mold heat sink is formed solely by the insulator inserts 14 and 15. Thin coatings or platings may be applied to the cavity surfaces of the insulator inserts to provide the desired hardness, wear, chemical, temperature and welding resistance to the plastic melt if necessary.

Mold 28 in Fig. 6 differs from the other two molds in that 26 and 27 are nickel metal inserts which are added to act as heat sinks in the insulator inserts 14 and 15 to increase the cooling rate at the thicker locations of the plastic part to encourage more uniform temperature throughout the part and to draw off excess heat to reduce cooling time. In all molds, the intermediate medium are selected according to the invention to have a low thermal diffusivity so that the surface of the mold cavity rapidly increases in temperature to a temperature above the solidification temperature of the molten plastic upon injection of molten plastic into the mold cavity 13. The thermal resistance of the intermediate medium is selected, so that it is high enough to allow a controlled release of heat from the molten plastic in the mold cavity to the mold heat sink so as to retard melt cool-down and to minimize melt internal temperature gradients. Thus, the intermediate medium according to the invention acts as a barrier between the plastic melt and the mold heat sink to induce a nearly uniform melt cool-down and homogeneous parts. The thermal capacitance of the intermediate medium is a measure of the heat storage capability thereof and is equal to the product of density, specific heat and volume as shown in equation (1) below.

C = (p) (Cp) (V) , where

(1)

C = Thermal Capacitance (BTU/°F) , p = Density (lb/in³) ,

V = Volume (in3) , Cp = Specific Heat

The thermal resistance of the intermediate medium relates to the insulating quality thereof and is equal to the thickness divided by the product of thermal conductivity and cross-sectional area for heat flow as expressed in equation (2) below.

R = L(43200)/κA, where

(2)

R = Thermal Resistance (sec °F/BTU) , L = Thickness (in) , K = Thermal Conductivity (BTU/hrft°F)

A = Cross-section Area (in²)

The thermal diffusivity is the property that defines the rate that a material responds to temperature change. It is the thermal conductivity divided by the product of density and specific heat.

Note that it is also equal to the thickness squared divided by the product of thermal resistance and thermal capacitance.

a = ^L2 in²/sec (4)

CR

The values for thermal capacitance and resistance are determined by the thermal physical properties and geometry of the material for the intermediate medium, that is, the insulator layers or inserts 14 and 15 and the insulation members 19 and 20 and also the surface inserts or layers 16 and 17, when used, and also by designing the configuration of such material, particularly the thickness.

The effects produced by the molds 6, 18 and 28 of the invention are to cause the temperature of the surface of at least a substantial portion of the mold cavity, e.g., most and preferably the entire surface, during molding to be raised to a temperature above the solidification temperature of the molten plastic injected therein by means of the heat of the molten plastic itself. If the mold cavity is not filled too quickly, the skin on the injected plastic will be minimal to nonexistent during polymer flow and strong orientation and high shear stresses will not occur. During the earliest period of plastic contact with the surface of the mold cavity a very thin layer of plastic can solidify while the mold surface temperature is increasing. However, with the mold and according to the method of the present invention, this thin frozen skin remelts when the mold surface temperature increases to a temperature above the solidification temperature and stays there long enough to allow molecular disorientation.

The mold of the invention produces the beneficial thermal insulating effects to the whole part that the skin provides to the core in a conventional injection mold. With the mold and according to the method of the invention, the surface of the mold cavity and a region of the plastic melt near the cavity surface, i.e., adjacent the surface of the mold cavity, is kept near or above the solidification temperature of the plastic for a period of time while the temperature at the center of the plastic melt continues to decrease toward the solidification temperature. Thus, there are greatly reduced temperature gradients in the plastic melt during melt solidification according to the present invention as compared to molding with conventional molds. The plastic melt remains hot for a long enough period for reorientation of the molecules to happen and this occurs even if a thin skin forms and remelts. Because the invention does this for the entire part thickness and not just a central core, the part takes longer to cool than it does using conventional methods.

In the mold 6 in Fig. 4, the metal inserts 9 and 10 can be formed of copper or steel such as stainless steel with cooling channels 11 and 12 being formed therein. Cooling channels are used only if necessary. By way of example, the insulator layers or inserts 14 and 15 and insulating members 19 and 20 can be formed of a liquid crystal polymer such as Xydar SRT-300 produced by Dart Industries. The thickness of the insulator inserts 14 and 15 can, for example, be 0.062 inch thick and the surface layers 16 and 17, 0.062 inch thick aluminum for molding crystalline plastics such as 0.953 density 60% crystalline polyethylene plastic, Dow Chemical EP 245, with a mold cavity height or thickness 5 of 0.125 inch with the plastic being introduced at 440°F to a 70°F mold. The solidification temperature of this plastic is 255 - 260°F.

When the molten crystalline polyethylene plastic is injected by way of the central gate 21 into the mold

ID: cavity 13, the mold surface temperature rises above the solidification temperature of the polyethylene in a fraction of a second and remains at or near the solidification temperature for about a period of 30 seconds as the temperature of the melt at the center of

15 the mold cavity 13 cools toward the solidification temperature thereby producing a temperature gradient within the mold cavity which is less than 63°F (0.5°F x 125 mils) during solidification after the surface of the mold cavity has been raised to a temperature above 0 the solidification temperature of the plastic.

Another insulating material which can be used to form the insulating layers or inserts 14 and 15 is a polyimide resin such as Vespel produced by Dow Chemical. The surface inserts or layers 16 and 17 can 5 also be formed of glass instead of metal, or from some other material that provides the desired hardness, thermal capacitance and wear, chemical, temperature, and welding resistance. The surface layers 16 and 17 in the mold 6 can also be omitted as shown in mold 18 0 of Fig. 5. The cavities 13 in the molds 6 and 18 in

Figs. 4 and 5 are thin, flat annular cavities which may, for example, be useful in forming an optical element such as an optical disc from amorphous plastic. The cavities could, of course, have other 5 configurations such as convex for forming lenses as shown in Figure 6. An insulator material which can be used without a surface layer 16 or 17 is quartz glass, for example. The method of the invention is also useful for molding plastics other than the crystalline polyethylene plastic material referred to above. Other types of materials such as amorphous plastics and other thermoplastic materials can be molded with the mold and according to the method of the invention. The mold according to this invention can also be used for compression molding eliminating the need to heat the mold.

Thermal analyses of molds according to the invention and of a conventional metal mold common to conventional practice were performed for both crystalline and amorphous plastics using a personal computer and commercially available finite element analysis software, particularly the NISA II Program, a finite element program for personal computer with a heat transfer module. The computer models were one dimensional for all simulations except the convex lens which was two dimensional with thickness variation appropriate to the radius (Quasi-3 dimensional) .

For crystalline plastics, a 0.943 density 60% crystalline polyethylene plastic (Dow Chemical EP 245) with a total wall thickness of 0.125 inch was modeled. The plastic was introduced at 440^βF to the 97° surface of a 70" mold. Temperature variable thermal conductivity and specific heat values found in the literature were used for the analyses. The solidification temperature of the crystalline plastic is 255 to 260°F. The results of the analysis for the metal mold are shown in Fig. 7 wherein it is seen that the frozen skin thickness is 0.0025 inch at 0.05 second, 0.014 inch at 2.32 seconds, 0.031 inch (1/2 of the part thickness) at 8.82 seconds, and the full thickness of the part at 12.82 seconds. Temperature gradients of 300°F exist during solidification. These results are in good agreement with the 13.9 seconds time for a complete solidification published in 5 "Injection Molding Theory And Practice" by Ervin Rubin. Rubin's time was for 0.945 density polyethylene at 45°F injected into a 70° mold and solidifying at 266°F.

Results of the thermal analyses for molds

ID according- to the present invention using Xydar SRT-300 as the insulating (low thermal diffusivity) material and from 0 to 0.125 inch aluminum at the mold surface are shown in Fig. 8. Runs were made for 0.062 inch SRT-300 with surface aluminum thicknesses of zero,

15 0.032, 0.062, 0.070, 0.078, 0.100 and 0.0125 inch. As seen from Fig. 8, in all cases, the mold surface temperature rises above the solidification temperature of the polyethylene and remains near or above the solidification temperature for an extended period. The

20 polymer can be injected at lower pressure and at a slow rate without frozen skin formation during flow. During this period, the center of the part continues to cool as shown in Fig. 9. This produces the desired low thermal gradients throughout the part as it passes

25 through the solidification temperature at a rate that encourages crystallization. As seen in Fig. 8 and 9, the temperature gradients are less than 50°F when the surface starts to solidify after the surface of the mold and the adjacent outer surface of the plastic

30 adjacent the mold surface have been raised to a temperature above the solidification temperature of the plastic. The zero aluminum determination places the Xydar at the surface of the polyethylene melt which may weld to it if the cavity surface is not coated or

35. plated, the analysis being shown to determine sensitivity to surface aluminum thickness only. However, the cavity surface of the Xydar can be coated or plated as needed to prevent welding. Electroless nickel plating is an example of a suitable protective 5 coating. Other insulator materials such as quartz glass, resistant to the plastic melt, could be used for the insulator inserts, where these also form the mold cavity surface.

Fig. 10 is a curve of temperature difference

ID.; between the outer surface and the center of the plastic part for various aluminum thicknesses as the surface layers 16 and 17 in the mold 6 of Fig. 4 as derived by the aforementioned thermal analysis. As seen from Fig. 10, the minimum temperature gradient results when

15 about 0.085 inch thick aluminum is used between the plastic and the 0.062 inch Xydar SRT-300 insulating material. The center of the polyethylene is at the 260°F solidification temperature at 43.82 seconds for zero aluminum and 38.32 seconds for 0.125 inch aluminum

20 as the surface layer over the insulator insert. The time decreases for increasing aluminum thicknesses.

In the case of 0.062 inch thick aluminum at the surface of the mold cavity as surface layers 16 and 17, the temperature at the center of the plastic drops over

25 150°F while the surface is above solidification temperature going from 255° to 270° and back to 255°. Similarly, for 0.125 inch thick aluminum the temperature at the center of the plastic drops 100° while the surface goes from 255° to 260° and back to

30 255°. This behavior produces the desired small thermal gradients in the plastic as it cools below the solidification temperature. These temperature gradients are less than 0.5°F x part thickness in mils. With 0.125 inch aluminum, the frozen surface

3S skin of 0.002 inch that forms during the first 0.1 seconds is remelted 3.3 seconds later so that little flow-induced orientation remains. For the 0.062 inch thick aluminum surface layer, it takes only 1.3 seconds for the mold surface to exceed the solidification temperature of the plastic. For no aluminum between the Xydar insulator layers or inserts and the plastic melts, the surface of the insulator inserts forming the mold cavity are raised above the solidification temperature in less than 0.001 second. While the above-described thermal analyses have been with respect to the use of a Xydar insulation material for the insulator inserts 14 and 15, if another insulation material is used, the thickness of the new material required to cause the same temperature and the same time period as the Xydar insulation material can be estimated from the Schmidt finite difference method equation (5) below.

DELTA TIME = L SQUARED /2 TIMES THERMAL DIFFUSIVITY f⁵

where L is insulator thickness.

The relationship for Vespel insulation material thickness as compared with Xydar thickness is :

Ly squared /2 times thermal diffusivity = L_x squared /2 times thermal diffusivity

which can be expressed as Ly-

for L_x = 0.062 inch: Ly = 0.098 inch. If aluminum is used as a surface layer between the plastic in the mold cavity and the insulator material, the same heat storage occurs if:

A x L_a x _a C_a + A x Lχ X W_x C_x = A x L_aa x W_a 5 x C_a + A x Lv x W_v x C_v where: L is thickness W is density A is area

C is specific heat. 10. Rearranging:

Laa » (( x x W_x x C_x - Lv x W_v x C_v) / W_a x

^C _a ⁾ + La for L_a = 0.085 inch, L _a = 0.060 inch.

Thus, for example, the overall heat transfer

15 coefficient can be calculated for a specific combination or combinations of insulation materials and surface layers. For example, the heat flow is faster through a 0.098 inch thick Vespel insulation material with 0.060 inch aluminum surface layer than it is

20 through an intermediate medium having 0.062 inch thick Xydar with 0.085 inch thick aluminum as the surface layer. A thermal analysis of the type referred to^" above was conducted and the results thereof are shown in Fig. 11. As seen from Fig. 11, the thermal analysis

25 results confirm the calculated results that the mold surface cools faster for the 0.098 inch thick Vespel/0.060 inch thick aluminum surface layer. The mold surface temperature reaches 270° for the Xydar mold and 267.4°F for the vespel design, both

30 temperatures being above the solidification temperature. Another thermal analysis of a mold according to the present invention was compared with that for a conventional metal mold for molding optical discs with polycarbonate as the amorphous plastic being molded to 5 form an optical disc 0.047 inch thick, which is typical of optical discs. The polycarbonate has a solidification temperature of about 305°F. The results of the computer simulation for polycarbonate injected at 660°F into a metal mold having a mold cavity surface

ID at: 250°F and with a mold temperature at 200°F are shown in Fig. 12. As shown therein, a frozen skin starts to form immediately upon introduction of the molten plastic into the mold cavity. By 0.2 second after introduction, the skin is about 0.001 inch thick.

15 Typical fill time for optical disc mold cavities is 0.2 to 0.4 sec. to minimize orientation and stresses due to flow through thick frozen layers. The gate is then shut and the part is coined or the injection pressure is profiled to reduce overpacking stresses. The outer 0 edges and central gate areas of the parts are highly stressed due to rapid cooling. These areas do not have the low birefringence required for optical disc use which limits the area of the disc available for data storage. According to this simulation, it takes 4.6 5 seconds for the center of the part to cool to the solidification or glass transition temperature where it is solid. The very large temperature gradients through the part from the time the plastic first enters the mold until the center cools below the solidification 0 temperature with this conventional injection mold and injection molding method are also evident from Figs. 12 and 13.

In contrast, according to the present invention, a

0.012 inch mold cavity surface layer of nickel in the 5 injection mold was backed up by an insulating layer 0.050 inch thick of Vespel. The mold surface temperature and plastic behavioral patterns for polycarbonate amorphous plastic are shown in Fig. 14. Similar mold surface temperatures and plastic 5 behavioral patterns are obtained for polyethylene, crystalline plastic. The significantly reduced temperature gradients and also the raising of the mold surface temperature to a temperature above the solidification temperature of the plastic upon -03 iπtrodirction of the molten plastic into the mold cavity, which are evident from Fig. 14, provide the aforementioned advantages of the present invention as compared with the conventional injection molding whereby birefringence can be substantially reduced or 5 eliminated in the optical discs.

Another thermal analysis of a mold according to the present invention was compared with that for a conventional metal mold for molding a double convex lens with polycarbonate plastic. The lens had a 25 mm 0 diameter with 25 mm spherical surface radius at both faces. The edge thickness was 1.3 mm and the center thickness 8mm. Constant thermal conductivity and temperature variable specific heat for optical grade Lexan OQ 1010-111 made by GE were used for the 5 analysis. The solidification (glass transition) temperature for this material is about 305°F. The mold 28 in Fig. 6 was used for simulation of the invention with the surface layers 16 and 17 made of quartz glass and the tailoring heat sinks 26 and 27 made of nickel. 0 The insulating layers and edges 14 and 15 were Xydar SRT-300 and the metal inserts 9 and 10 beryllium copper. Other significant dimensions are shown in Fig. 15.

Computer simulations were performed for 5^* polycarbonate injected at 600°F into a 200°F all metal mold with surface temperature of 240°F, and for polycarbonate at 575°F injected into a 180°F mold 28, Fig. 6, with a surface temperature of 240°F. For efficiency of analysis, symmetry was taken into account 5 and only one quarter of the cross-sectional area and a 0.008 radian pie slice thickness was modeled.

The results of the analysis are shown in Figs. 16A, 16B and 17A and 17B. These figures show lines of constant temperature (isotherms) in the rot: plastic lens at specific times after injection of the plastic. Figure 16A shows the plastic in the all metal mold (known method) and the results with the method and mold according to the present invention are depicted in Fig. 16B about the time that a portion of the lenses

15 start to solidify. The maximum temperature difference in the plastic in the known method is about 295°F with very steep temperature gradients at the surface as evidenced by the closeness of the lines which are 37.8°F apart. The maximum temperature difference in 0 the plastic in the improved method of the invention is only 165°F and the temperature gradients are much more gradual as seen by the relatively even spacing between lines which are 18.6°F apart.

Figures 17A and 17B show the plastic lens at about 5 the time that it has completely solidified in the case of each mold. The maximum temperature difference in the lens in the all metal mold is 110°F. For the mold according to the invention, the maximum temperature difference in the lens is 63°F. The temperature 0 gradients are less steep in the improved method of the invention as shown by the closeness of isotherms which are 13°F apart in the all metal mold and 7.8°F apart in the mold according to the invention. From the above description of the invention, it can be seen that parts can be injection molded according to the method of the invention using the injection mold of the invention so as to have a 5 homogeneous structure. Further, because the polymer is held near or above the solidification temperature, e.g., the melting or glass transition temperature, for an extended period while the cavity fills and the center of the part cools, flow-induced and temperature

1303 .. gradient induced orientations and stresses are minimized. Packing will be very even throughout the part because the polymer will still be in the melt stage when all locations in the part and all cavities of multicavity molds are filling. Strong weld lines

15 are also formed with parts molded according to the invention because converging melt fronts can easily be kept near the same temperature. Heavy wall parts can also be molded void free for the same reasons. Molecular orientation is minimized by first forming

20 frozen skins of minimal thickness than remelting the skin so that molecules can relax. The injection mold of the invention controls the temperature gradients and rate of cooling during molding so that the polymer crystallization will be extensive and uniform thus

25 producing high quality parts and greatly improved tolerance control. This is done without requiring the provision of heating means in the mold. Since birefringence is linearly related to molecular orientation, minimal or no orientation is possible in

30 optical parts such as lenses and optical discs which are injection molded in the injection mold according to the method of the invention to net shape with excellent optical, physical and mechanical properties.

While I have shown only three embodiments in

3_5 accordance with the present invention, it is understood that the same is not limited thereto, but is susceptible to numerous changes and modifications as known to those skilled in the art. For example, it is envisioned that the method of the invention is applicable to injection molding other plastics than those specifically referred to herein and that the injection mold of the invention can also have other forms than those illustrated herein. In particular, the method and mold of the invention could be used for compression molding plastic in which case the gate in the mold would not be necessary as a slug of plastic would be placed between mold halves and then compressed therein. Therefore, I do not wish to be limited to the details shown and described herein, but intend to cover all such changes and modifications as are encompassed by the scope of the amended claims.

Claims

1. In a method of molding plastic to form molded parts, comprising the steps of providing a mold having at least one mold cavity in the form of a part to be

5 molded, introducing molten plastic into said mold cavity, allowing at least a portion of the molten plastic in the mold cavity to cool until it freezes and removing the part from the mold cavity, the improvement comprising causing at least a substantial portion of ID-. he: surface of the mold cavity to have a temperature above the solidification temperature of the molten plastic during molding of said part.

2. A method according to claim 1, further comprising maintaining the temperature of at least a

15 substantial portion of the surface of the mold cavity near or above the solidification temperature^" of the molten plastic from an extended period of time during molding while the mold cavity is filled with molten plastic and the center of molten plastic in the mold

20 cavity cools from a temperature above the solidification temperature toward the solidification temperature thereby reducing flow-induced and temperature gradient induced orientation stresses in the solidified plastic of the molded part.

25 3. A method according to claim 2, wherein the temperature difference between the outer surface and the center of the plastic in the mold cavity does not exceed 0.5°F x part thickness in mils when the outer surface starts to solidify after at least a substantial

30 portion of the surface of the mold cavity has been caused to have a temperature above the solidification temperature of the molten plastic.

4. A method according to claim 1, further comprising insulating the mold cavity so that the heat to cause at least a substantial portion of the surface of the mold cavity to have a temperature above the solidification temperature of the molten plastic during molding comes from the heat of the molten plastic introduced into the mold cavity.

5. A method according to claim 4, wherein said step of insulating the mold cavity comprises providing a layer of material having a low thermal diffusivity at or near the surface of the mold cavity.

6. A method according to claim 5, including providing a layer of insulating material at or near the entire surface of said mold cavity that contacts molten plastic during said molding.

7. A method according to claim 5, wherein said layer of insulating material forms at least a substantial portion of the surface of the mold cavity.

8. A method according to claim 5, wherein said layer of insulating material is located beneath a layer or layers of other materials which form the surface of the mold cavity and also act as a heat sink to draw off excess heat to minimize cooling time and to cause more even temperature throughout the part during molding.

9. A method according to claim 1, wherein said mold includes a gate or gates through which molten plastic is injected into said mold cavity during molding.

10. A method according to claim 1, wherein the entire surface of the mold cavity is caused to have a temperature above the solidification temperature of the molten plastic during molding of said part.

11. A method according to claim 1, wherein the temperature of the surface of said mold cavity is colder than the solidification temperature of the molten plastic when the molten plastic first contacts t e; mold surface during said molding so that a layer of the plastic contacting the surface of the mold cavity freezes, said frozen layer being remelted during said molding when the at least substantial portion of the surface of said mold cavity is caused to have a temperature above the solidification temperature of the molten plastic.

12. A method according to claim 1, wherein said molded part is an optical part such as a lens or optical disc which has little or no flow induced molecular orientation, residual stress and birefringence.

13. A method according to claim 1, wherein said mold has a plurality of cavities and all cavities of said mold are filled with molten plastic during said molding.

14. A method according to claim 1, wherein said molten plastic is compression molded in said mold to form the molded part.

15. An injection mold for molding plastic to form molded parts, said injection mold comprising means to defining at least one mold cavity in the form of a part to be molded, at least one gate in said mold through which molten plastic can be forced into said mold cavity, and means for causing at least a substantial portion of the surface of the mold cavity to have a temperature above the solidification temperature of the molten plastic injected into said mold cavity during molding.

16. An injection mold according to claim 15, wherein said means for causing at least a substantial portion of the surface of the mold cavity to have a temperature above the solidification temperature of the molten plastic maintains the temperature of the at least substantial portion of the surface of the mold cavity near or above the solidification temperature of the molten plastic for an extended period of time during molding while the mold cavity is filled with molten plastic and while the center of molten plastic in the mold cavity cools from a temperature above the solidification temperature toward the solidification temperature whereby flow-induced and temperature gradient induced stresses in a solidified mold material of the molded part are minimized or avoided.

17. An injection mold according to claim 15, wherein said means for causing a temperature above the solidification temperature comprises insulation provided above the mold cavity.

18. An injection mold according to claim 17, wherein said insulation is in the form of a layer of material having a low thermal diffusivity located at or near the surface of the mold cavity.

19. An injection mold according to claim 18, wherein said layer forms at least a substantial portion of the surface of the mold cavity.

20. An injection mold according to claim 18, wherein said layer is located beneath a layer or layers of other materials which form the surface of the mold cavity and provide a heat sink to draw off excess heat to minimize cooling time and to cause more even temperature throughout the part during molding.

21. An injection mold according to claim 20, wherein said layer of material forming the surface of the mold cavity is a metal.

22. An injection mold according to claim 18, wherein said layer of insulation material is located between a metal insert of the mold and the mold cavity, and wherein means are provided for cooling said metal insert.

23. An injection mold according to claim 18, wherein said layer of insulation material is a liquid crystal polymer.

24. An injection mold according to claim 18, wherein the thickness of the insulation layer is selected so that it stores sufficient heat from the molten plastic injected into the mold cavity to raise the temperature of the surface of the mold cavity above the solidification temperature of the plastic being molded.

25. An injection mold according to claim 17, wherein said insulation is also provided about the gate of said mold.

26. An injection mold according to claim 15, wherein said means for causing a temperature above the solidification temperature causes the entire surface of the mold cavity to have a temperature above the solidification temperature of the molten plastic during molding.

27. An injection mold according to claim 15, wherein said mold cavity is configured for forming an optical part such as a lens or optical disc as the molded part.

28. An injection mold according to claim 15, wherein said means defining at least one mold cavity defines a plurality of mold cavities.

29. A method of injection molding plastic to form molded parts, comprising the steps of providing a mold defining at least one mold cavity in the form of a part to be molded, said mold including heat sink means spaced from said mold cavity and an intermediate medium located between the mold cavity and said heat sink means and means for injecting molten plastic into said mold cavity, said intermediate means having a low thermal diffusivity and appropriate thickness such that the surface of the mold cavity rapidly increases in temperature to a temperature above the solidification temperature of the molten plastic after injection of the molten plastic into the mold cavity, and said intermediate medium having a thermal resistance high enough to allow a controlled release of heat from the molten plastic in the mold cavity to the heat sink means to retard melt cool-down and minimize melt internal temperature gradients.

30. A mold for molding plastic to form molded parts comprising means defining at least one mold 5 cavity in the form of a part to be molded, heat sink means spaced from said mold cavity for removing heat from molten plastic introduced into said mold cavity, and an intermediate medium located between said mold ca ±fcy and said heat sink means, said intermediate

ID medium having a low thermal diffusivity and appropriate thickness such that the surface of the mold cavity rapidly increases in temperature to a temperature above the solidification temperature of the molten plastic upon introduction of the molten plastic into the mold

15 cavity, and said intermediate medium having a thermal resistance high enough to allow a controlled release of heat from the molten plastic in the mold cavity to the heat sink means to retard melt cool-down and minimize melt internal temperature gradients.

20 31. A mold according to claim 30, wherein at least one material is provided between the cavity and the intermediate medium to form the cavity surface, said at least one material being configured to have the required geometry and thermal capacitance to draw off

25 excess heat and to cause more uniform temperature throughout the plastic.