WO1994021440A1

WO1994021440A1 - Method of molding plastic structural parts

Info

Publication number: WO1994021440A1
Application number: PCT/US1993/002550
Authority: WO
Inventors: Charles W. Bailey
Original assignee: Signet Industries
Priority date: 1993-03-16
Filing date: 1993-03-16
Publication date: 1994-09-29
Also published as: AU3926193A

Abstract

A method and apparatus are used to improve upon injection molding of a part. The method includes injecting a molten plastic material (10A) into a mold (30) having a mold cavity (32) and moving an insert (40) into the mold cavity (32) consisting of mold halves (34 and 44) to affect the structure of the part. The structure may be so affected by forming holes within the structure, densifying the material around the holes, densifying the material at a particular cross section, thinning out cross sections of the part to be molded, forming bosses out of walls of the part to be molded without causing sink marks, and forming living hinges joining thickened cross-sectional portions of an integrally manufactured plastic part. An apparatus utilizes insert pin (40) and support pin (46) actuated by camming surfaces and pistons to carry out the improved methods.

Description

METHOD OF MOLDING PLASTIC STRUCTURAL PARTS

1. Field of the Invention

The present invention relates to processes for converting plastics into products, and more particularly to improvements in injection molding processes. The present invention also relates to improved apparatus for use with the improved processes.

BACKGROUND OF THE INVENTION

Injection molding is used for making plastic parts in highly automated processes that require little finishing of the parts made. An injection molding process, analogous to metal die casting, involves heating a granular plastic material, forcing it through a heated cylinder where it is plasticized, and injecting the material through a nozzle into a mold cavity shaped to the specification of the finished plastic part. The basic steps of such a process include closing the mold containing the cavity and forcing the mold against the nozzle. The plastic material is fed into the cylinder from a hopper. A screw, plunger, or combination of screw and plunger forces the material within the heated cylinder toward the nozzle, plasticizing the material and forcing the material through the nozzle and into the mold cavity. The screw or plunger or both maintain pressure through the nozzle until the plastic material is cooled and/or set. The screw or nozzle or both then reverse to relieve pressure. The mold opens so that the plastic molded part can be removed.

For high production of plastic parts, injection molding processes have many advantages. An injection molding process is capable of high output rates of producing parts that need very little finishing. This is because of the high automation of injection molding processes. Moreover, material waste may be avoided in many instances by collecting material including material cut or machined from parts during finishing, again plasticizing the waste material, and injecting the plasticized material into the mold cavity to produce another plastic part. But injection molding processes are not without shortcomings. High equipment costs are associated with injection molding processes, including the costs of making and purchasing molds. A plastic part that is solid in a finished state has been affected by the flow patterns of the molten polymer or plasticized material in a molten state during the injection molding process. An the finished plastic parts are not well served by uneven finishing techniques and workmanship, regardless of how little finishing is needed.

For example, as illustrated in Prior Art Figs. 3A and 3B, it is known to form a hole in an injection molded plastic part by injecting molten, plasticized material 10A (for example, a polymer) around a pin 64 spanning the cross-section of the mold cavity 32 forming the cross-section of the plastic part. As the molten material 10A is forced around a pin 64, the flow pattern of the molten material 10A is along flow lines 10B which pattern stress lines that develop in the finished plastic part. Because the pin 64 restricts the flow of the molten material 10A, flow marks, weld lines, and possible cracking develop because of the molten stress.

The mechanical properties of the finished plastic part vary according to the stress patterns. The above-mentioned conditions are particularly pronounced for composite materials, such as fifty percent (50%) long glass fiber filed nylon used for high strength characteristics and used for parts that are subjected to high stress conditions. The glass fibers follow the flow lines and cause bunching or disorientation in front of the pin 64 opposite the gate through which the material flows. As the molten material flows around the pin 64 clockwise and counter-clockwise to join together, a weld line is formed. This weld line represents a line for potential cracking of the plastic part. With a part made of a composite material, where the orientation of the fibers attribute to the isotropic and anisotropic properties of the material depend on the uniform orientation of the fibers, bunched and disoriented fibers contribute to the condition for cracking along the weld line.. One means of overcoming this condition is to eliminate the hole in the molding process by cutting or machining the hole into the part after the molding process. This has the disadvantage of requiring a finishing process that loses the advantage of the high rates of production by the automation associated with injection molding. Furthermore, the quality of the finished part would be affected by the workmanship of the machining process, and parts may have holes that have burs and non-uniform edges.

Another shortcoming associated with injection molding processes is the presence of sink marks, as sink mark 72 shown in Fig. 7. These sink marks are irregularities in the wall thickness of molded plastic parts. They are unsightly and may create internal stresses. The sink marks result from not having enough of the molten material in the mold to completely fill the cavity during the cooling process, during which there is some shrinkage of the material as it solidifies. This condition is pronounced when the molten material moves from a thin cross-section of the mold cavity into a thicker cross-section as when a wall section, for example wall 22 of Fig. 7, has a boss, for example screw boss 12 of Fig. 7, formed in it. The thick cross-section across the boss cools more slowly than the thin wall section and the density of the molten material as it expands, with less pressure, into the cavity forming the boss, contribute to the creation of the sink marks and the differential shrinking in the plastic material.

A limitation of the use of injection molding processes in forming certain parts is associated with molten material flowing from thick into thin cross-sections of a mold cavity and the differential cooling that causes thinner cross-sections of the material to set, solidify, or "freeze" during the injection molding process. Disadvantageous "freezing" occurring when the material cools and sets up in the mold cavity before completely filling it. Thus, certain cross-sections cannot be successfully molded by injection molding, particularly thin transparent cross-sections of parts disposed between upstream and downstream thick cross-sections. It is for this reason that plastic tail lights are formed in various subparts that are assembled in subsequent manufacturing processes. The "freezing" phenomenon accounts for a limitation in the use of injection molding to manufacture of parts having "living hinges." Thus, a hinge joining two thickened parts, particularly along a substantial length of hinge, would likely freeze during the injection molding process. It is for this reason that living hinges are often manufactured separately from the parts joined, again requiring addition manufacturing after the parts and the hinges are molded by injection molding processes.

Yet another shortcoming of injection molding processes is the additional step required for degating finished plastic parts. The gate is the point of entry of the molten material into a mold cavity. Gates may be of any size and shape. The smaller the size of the gate, the easier it is to cut from a finished part and leave a less perceptible blemish, requiring less machining to finish the part. But smaller gates require greater pressure and, hence, energy to force the molten into the mold cavity. Thus, bigger gates requiring less pressure require less energy to manufacture parts, but require more expense to machine and finish the parts.

Thus, notwithstanding the advantages of injection molding processes, it would be useful to improve upon the processes to eliminate some of the draw-backs or shortcomings just discussed.

OBJECTS OF THE INVENTION

Accordingly, it is one object of the present invention to provide an improved process for manufacturing holes in injection molded parts.

It is another object of the present invention to provide a process for forming holes in parts manufactured by injection molding, whereby the parts are not subject to stress variances because of flow around pins during manufacturing. It is yet another advantage of the present invention to provide an improved injection molding process for manufacturing thin and thickened portions of a plastic parts without delimiting the thinness of a portion attached to a thickened portion because of "freezing" across thin cross-sections of material in the mold cavity during the injection molding process.

Yet still another advantage of the present invention is to provide an improved injection molding process for manufacturing living hinges attached to thick walled portions of a plastic part.

Still yet another advantage of the present invention is to provide an injection molding process allowing for degating without blemishes and without regard to the size of the gates involved.

It is, moreover, an object of the present invention to provide apparatus for carrying out an injection molding process that accomplishes the above objections.

SWMARY OF THE INVENTION

In accomplishing the above objects, the present invention is an injection molding that improves upon existing processes of injection molding. In general, the process includes steps of injecting a molten plastic material into a mold cavity and moving an insert into the mold cavity to produce a predetermined structure of the part being molded. The improved process of the present invention may be used to displace the molten material such as would be used in forming holes in a part to be molded, to compress portions of a part to thin out thickened walls before the material has had a chance to "freeze", and to densify the plastic material in a manner that is not unlike forging steel.

The method accomplishes hole punching by injecting the molten material into the cavity of the mold, which is unobstructed by inserts so as not to affect the flow of the molten material in the cavity. While the material is still plasticized, the insert advances through the cavity and through the material therein, punching a hole having the shape and dimension of the transverse cross-section of the insert (the cross-section transverse to the axis of the insert). When the plastic part is ejected from the mold, no machining would be required for the formed hole. Accordingly, the automated process of making the part with the hole would not require additional labor or manufacture.

To support the molten material within the die, a movable insert and a movable support, of substantially the same transverse cross-section as the insert, are disposed in neutral positions bounding the cavity at either end of a hole to be formed through the part to be molded. The molten material is injected into the cavity without encountering the insert as an obstruction to the flow of the molten material. At a predetermined time, simultaneous movement of the insert into and through the cavity and the support away from the boundary of the cavity will capture the material between the insert and the support—the material plugging the hole—and displace the material so captured out of the cavity and away from the part to be molded, thereby forming a hole in the part. So that this displaced material will not clog up the mold cavity, or other part of the mold, it is preferable that after the hole is formed or "punched," the displaced material is placed back to plug the hole by returning the insert and the support to their neutral positions bounding the cavity. The timing of this sequence would be such that the hole is punched through the plasticized material and the material freezes enough for the plug to make tacky contact with the part when the plug of material is placed back in the hole. The plug would thereby be captured by the part, and when the part is ejected, the plug may be punched out by hand, by anyone associated with either the manufacturer or the end user.

The method may be used with an intermediate step to densify the material surrounding the hole. If the hole is to be used in joining a structural component, obvious stresses will be placed on the material surrounding the hole when the part is in use. Thus, if the material around the hole can be made more dense to withstand greater stress, the part will be stronger as it is used. The method can be used to accomplish this end.

After the molten material fills the cavity, the insert may be moved without the movable support being moved. The material captured between the moving insert and the movable support is squeezed and some of the material captured between the moving insert is displaced thereby into the volume of molten material surrounding the hole, causing the material surrounding the hole to be more dense and thereby strengthened. After the insert is used to squeeze and displace some of the material that had been between the insert and movable support, the support may move with insert to displace the material remaining between them from the cavity altogether. Thereafter, the insert and support together may be moved back to replace the plug of material. The part ejected from the mold will have an easily identifiable plug which may be punched out by hand. The volume surrounding the hole will be more dense having accepted some of the displaced material from the hole. Furthermore, with regard to parts made of composite materials such as the fifty percent (50%) long glass fiber filled nylon discussed earlier, burn-off tests—tests performed by burning off the matrix material of a finished part to leave the fibers oriented as they were in the part—show that the fibers in the volume surrounding the hole become cross-liked or interlaced during displacement of the material into the volume surrounding the hole. The cross-linked or interlaced fibers strengthen the material in the volume surrounding the hole. Accordingly, the steps are used to first densify and/or cross-link fibers in the material, and then, displace the material.

This process of densification may be advantageously used to eliminate sink marks behind thickened cross-sections of a part, particularly behind a screw boss. By the process of the present invention, the material is injected into the mold cavity, creating the boss without the hole for the screw. The moving insert, in this case a core pin, may be used to move into the plasticized material along the axis of the screw hole. This creates compression and densifies the material surrounding the screw hole of the boss, particularly at the base of the screw hole. The more dense material at the base of the screw hole, which is at the base of the boss, will not shrink more than the cross-section of the plastic wall and the sink mark will not result during cooling. Also, this process avoids the short-coming already discussed in connection with the coventional forming of a hole, whereby material injected around an insert already in place in the cavity before the material is injected into the cavity causes a weld line to develop opposite the gate through which the material is injected into the cavity. This process may also be used for a boss having no screw hole by simply replacing the core pin with a compression pin which presses against the boss, rather than into the boss.

In another application of the method of the present invention, a compression pin or bar having a V-formed edge is inserted into the mold cavity to compress (and densify) the plasticized material to form a living hinge. The method has the steps of holding the compression pin in a retracted position, injecting the plasticized material into the cavity of the mold, advancing the compression pin at a predetermined time to compress the material in the hinge area as required, and retracting the compression pin to open the mold and eject the formed part having the living hinge.

Finally, another application of the method of the present invention allows for a degating process. A punch pin and support pin are in a neutral position allowing the molten material to enter through a gate to form a part in the mold cavity attached to a sprue in the channel leading to the cavity. At a predetermined time, the punch pin is advanced and the support pin is retracted to remove the gate connecting the part and the runner. The punch pin in its advanced position acts as a check or shut-off valve holding the cabin pressure and eliminating the need for injection hold time—the time necessary for continuing pressure until the molten material solidifies—and thereby reducing the injection molding cycle time. The punch and support pins are thereafter returned to their respective neutral positions, so that the gate is placed back in position where it is ejected with the part from the mold. These applications of the invention will become more apparent from a reading of the following descriptions.

BRIEF DESCRIPTION OF THE DRAWINGS

In The Drawings

Fig. 1 is perspective view of a plastic part, which is a base for an automobile seat assembly, showing a sprue, runner, and gate attached thereto.

Fig. 2 is a cross-section of an injection mold containing the plastic part therein.

Fig. 3A is a diagramatic plan view of the flow pattern in a mold cavity in accordance with the prior art.

Fig. 3B is a partial sectional view taken in the direction of arrows 3B—3B in Figure 3A.

Fig. 4A is a plan diagrammatical view in a cavity without a pin therein.

Fig. 4B is a partial sectional view taken in the direction of arrows 4B-— B in Figure 4A.

Fig. 4C is a cross-sectional view as shown in Figure 4B, but with an insertion pin or punch pin inserted into the mold cavity.

Fig. 4D is a cross-sectional view as shown in Figure 4C, but with a support pin being moved away from bordering the mold cavity and the punch pin moved through the cavity. Fig. 5 is the cross-sectional view of Figures 4B, C and D, but with the insertion pin moved into the mold cavity and the support pin moving away from the cavity without the material therebetween being compressed.

Fig. 6A is a cross-sectional view of a mold cavity with an insert or compression part aligned with one wall of the mold cavity.

Fig. 6B is a cross-sectional view as shown in Figure 6A, but with the compression part moved into the mold cavity so as to compress the plasticized material therein into a thinner cross-section.

Fig. 7 is a cross-sectional view of a screw boss formed out of one wall of a molded part showing a sink mark formed therein, in accordance with the prior art.

Fig. 8A is a cross-sectional view of a boss formed out of one wall of a screw boss within the mold cavity showing a sink mark therein.

Fig. 8B is a cross-sectional view of a mold cavity with a screw boss formed out of a wall of the part to be molded, showing an insert within the mold cavity.

Fig. 9A is a partial diagrammatical, partial cross-sectional view of a polymer flowing into a mold cavity through a gate bounded by inserts.

Fig. 9B shows an insert inserted through the mold cavity and trapping material between the insert and support.

Fig. 10A shows a cross-sectional view of plasticized material within the mold cavity and representing a hinge forming insert or hinge compression pin moving to compress the material.

Fig. 10B shows a partial perspective view of a living hinge formed by the process of Figure 10A. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Turning now to the drawings wherein like components are designated by like reference numerals throughout the various figures, Fig. 1 shows a plastic part 10 (a base for a seat assembly) manufac¬ tured by an injection molding process. The plastic part 10 is an inte¬ gral unit, that is, plastic part 10 is not assembled by joining or machining subparts to attain its structure; the subparts are molded integrally into plastic part 10. Accordingly, the subparts include a screw boss 12, holes 14, a plug 16, openings 18, a boss 20, and walls 22. Attached to the plastic part 10 during the injection molding process is a sprue 24, runners 26, and gates 28.

The sprue 24, runners 26, and gates 28 are by-products of the injection molding process that produces plastic part 10. As is known by those of ordinary skill in the injection molding art, when molten plastic material is forced through a nozzle into a mold, the molten plastic material flows through channels to get to the mold's cavity, which is shaped to yield the plastic part when the molten plastic material becomes solid in the finished state. In the molding of plastic part 10, the material that is left in these channels when the plastic is solid in a finished state is formed as the sprue 24, runners 26, and gates 28. An injection molding process not using the teachings of the present invention would require that the sprue 24, runners 26, and gates 28 be machined or cut from plastic part 10.

So as to set forth the best mode contemplated by the inventor of carrying out the steps in applications of the present invention, which improves upon existing processes of injection molding, and to enable one skilled in the art of injection molding processes to use the improvements on the present invention, the apparatus of Fig. 2 will first be explained. A design of a two-piece, one cavity injection mold 30 is shown in Fig. 2. The cross-section of what will be the plastic part 10 in the finished state is shown within a mold cavity 32 that is in the cavity half 34 of an injection mold 30. Integral subparts of the plastic part 10, including walls 22, an opening 18, and a boss 20 can be seen. Runner 26, in cross-section, also can be seen. In the apparatus according to the invention, a backup plate 36 is supported by means known to those of ordinary skill in the art. Between the backup plate 36 and the cavity half 34 of mold 30 is a punch pin carrier plate 38. Depending from the punch pin carrier plate 38, through the cavity half 34 of mold 30, is a punch pin 40. The punch pin carrier plate 38 is supported in the apparatus of Fig. 2 by hydraulic cylinders 42, which are shown to either side of the apparatus of Fig. 2. The hydraulic cylinders 42 carry pistons and piston rods that, when extended, move punch pin carrier plate 38 upwardly (in the orientation shown in Fig. 2) and, when retracted, downwardly. Movement of the punch pin carrier plate 38 will cause the punch pin 40 depending therefrom to move accordingly. Thus, a retraction of the piston rods of hydraulic cylinders 42, as would be understood by those of ordinary skill in the art, will cause punch pin 40 to drop into mold cavity 32. If plastic part 10 is not solid in the finished state, but is molten material or in an in between plasticized state, punch pin 40 can breach mold cavity 32, into and through plastic part 10.

The bottom half 44 of injection mold 30, as shown in Fig. 2, has a support pin 46 extending through it. Support pin 46, is supported by a support pin carrier plate 48. Support pin carrier plate 48 is supported on guide column 50 and on locking wedges 52. Support pin carrier plate 48 has cam following surfaces 54 which compliments camming surfaces 56 of locking wedges 52. Locking wedges 52 are attached to piston rods of hydraulic cylinders 58, in a manner known to those of ordinary skill in the art, so that when the piston rods are extended by hydraulic cylinders 58, locking wedges 52 cam support pin carrier plate 48 upwardly, and when hydraulic cylinders 58 retract piston rods attached to locking wedges 52, locking wedges 52 allow support pin carrier plate 48 to drop downwardly as came following surfaces 54 and camming surfaces 56 stay in camming contact.

Supported by a core pin carrier plate 49 is a core pin 60. Adjacent mold cavity 32, core pin 60 has an end portion that is shaped as a conventional core pin for a screw hole. When core pin carrier plate 49 moves upwardly, by the same camming mechanism for moving support pin carrier plate 48 (although those skilled in the art will recognized that a separate actuating mechanism may be construct for moving core pin carrier plate 49, with its own hydraulic cylinders, locking wedges, and camming surfaces), core pin 60 invades mold cavity 32 and into plastic part 10 when in a plasticized or molten state. As will be later explained, this action by core pin 60 is advantageous to the forming of a screw boss in plastic part 10.

Indicated in phantom in Fig. 2 is an ejector pin 62, which those of ordinary skill in the art will understand is to be used to eject the finished plastic part 10 from the mold after it reaches a solid state. Ejector pin 62 is supported by an ejector pin carrier plate 64, only a portion of which is being shown in the drawings. It is to be appreciated that other parts of the injection mold press machinery will be understood by those of ordinary skill in the art. It is only necessary here to explain the mechanisms that actuate dynamic inserts that invade the mold cavity 32 or that actuate support mechanism that cooperate with the inserts to displace material. Those of ordinary skill in the art will know that other actuating devices may be used instead of the hydraulic cylinders and locking wedges disclosed. It is the purpose of disclosing the examples of actuating mechanisms shown in Fig. 2 to teach those of , ordinary skill in the art how these dynamic parts will work to accomplish the processes now to be explained.

The invention contemplates a process by which a molten plastic material is injected into a mold cavity, for example the mold cavity 32 of Fig. 2, and by which an insert, for example punch pin 40 or compression core pin 60 of Fig. 2, is moved into the mold cavity after the molten material has filled the mold cavity 32 before it has become solid in the finished state. The insert affects the plasticized material to produce a structural configuration in addition to the configuration it obtains from the structure of the cavity 32. Accordingly, compression core pin 60 affects the plasticized material to produce a screw hole in boss 20 of Fig. 2. Also accordingly, punch pin 40 produces a hole in plastic part 10 while in a plasticized state, when punch pin 40 is inserted into and beyond cavity 32 and support pin 46 is retracted to allow the displaced material to be taken out of plastic part 10, which the material is in a plasticized state.

The particular application of the present invention in which a hole is produced plastic part 10 avoids the material flow patterns shown in Fig. 3A. Those flow patterns which result from the conventional method of using stationery insertions in the mold cavity to produce holes in the finished plastic part. With reference to Fig. 3B, that conventional method requires that a stationery pin 64 be placed in the mold cavity 32 before the molten material 10A (the polymer) is injected into the mold cavity 32. Thus, the molten material 10A, as shown in Fig. 3B flows around stationery pin 64 to the consequences discussed in the Background of the Invention, earlier in this disclosure.

With reference to Fig. 5, in the present invention punch pin 40 is withdrawn from the mold cavity 32 when the molten material 10A is injected into the mold cavity 32. In the withdrawn position, punch pin 40 forms a part of the boundary of cavity 32. Preferably, a seal is formed between the punch pin 40 and the cavity half of the mold 34 so that the molten material does not escape from the cavity 32 around punch pin 40.

Thus, with the punch pin extracted, there is no flow pattern established differently from the flow patterns across the cavity, so that stresses will be fairly uniform across the plastic part when it becomes solid in a finished state. Also, where fiber reinforced material is used, the orientation of the fibers remain isotropic in accordance with the uniform flow pattern.

Once the material has been injected into the mold cavity and the pressure is held within the cavity, punch pin 40 may be advanced (inserted) into the cavity in accordance with the dynamics explained with regard to Fig. 2, while support pin 46 is moved to allow a plug of the material 10A to be removed from the mold cavity 32. As shown in the figure, support pin 46 moves downwardly away from cavity 32 at the same speed that punch pin 40 moves away from cavity 32. Thus, a hole 14 (Fig. 1) is produced in plastic part 10 as a plug 66 of the material 10A is removed from what will be the plastic part when the material 10A solidifies. By a timing sequence set to anticipate material 10A setting up from the molten state to an almost solid finished state, but still with a tackiness, support pin 46 and punch pin 40 are moved back upwardly, in the orientation of Fig. 5, to replace plug 66 back into hole 14. When plastic part 10 is fully solid in a finished state, mold 30 opens and plastic part 10 is ejected from the mold with plug 66 in the hole 14 (Fig. 1). The plug may be removed by the manufacturer or the end user simply by pressing or punching on the plug 66. By proper timing, the part is manufacture with plug 166 so captured in the hole 14, that it may be removed by pressing on plug 66 with a finger. Rather than timing the process, gages may be installed in cavity 32 to determine the state of plasticized material 10A by temperature and/or pressure.

Fig. 4C shows a different application to that shown in Fig. 5 whereby additional advantages occur out of the process of the present invention. In accordance with Fig. 4, punch pin 40 enters cavity 32 and into molten material 10A before support pin 46 is withdrawn from the wall of cavity 32. Thus, material is squeezed into the volume of cavity 32 surrounding punch pin 40. This process, which is not unlike forging steel, is called by the inventor "densification", whereby the material is made more dense or is "densified" by the compression caused by punch pin 40 entering cavity 32. Timing, in accordance with emperical results and/or/temperature and/or/pressure determinates will be used to judge when support pin 46 is retracted to allow a thinner, more dense plug 66A to be displaced from cavity 32 as shown in Fig. 4D. Again, the material is allowed to set up and solidify to a state where at it is tacky and the plug 66A is returned to hole 14 to be later punched out by the manufacturer or end user.

Thus, this application uses the process of first positioning punch pin 40 and support pin 46 in neutral positions, whereat they present no obstruction to the plastic flow in cavity 32 of injection mold 30. Molten material is then injected into cavity 32, filing the mold cavity 32. At a pre-determined time, punch pin 40 is advanced into mold cavity and support pin 46 retracts to cause the required hole or shape to be incorporated into the plastic part 10 being molded. As a final step, upon completion of the punching operation, punch pin 40 and support pin 42 are reset into their neutral positions and a plug 66 or 66A is captured in the punched hole. Plastic part 10 is then ejected from the mold with the plug 66 or 66A still captured in the hole 14.

Another application of the present invention involves the use of a punch pin or compression part or punch part 68 as shown in Figs. 6A and 6B. Punch part 68 operates as punch pin 40 to enter the mold cavity 32 when the molten material 10A has filled mold cavity 32. Punch part 68 may be shaped with a punch face 70 that is circular, rectangular, or any other desired shape. As punch part 68 moves downwardly into molten material 10A within cavity 32, the material is pressed out from or squeezed from under punch face 70 into the area surrounding punch part 68, thereby densifying that material. Perhaps of more importance to this application, however, is that the material beneath punch face 70 is thinned to a pre-determined specification. This process avoids injecting the molten material into a thin cross-section of the mold cavity 32 where it might, because of the pressure and temperature differentials, set-up, solidify, or "freeze" before it completely fills that volume of the cavity or before it goes through the thin section of the cavity 32 into a thicker section of the cavity 32. In accordance with this process, plastic part 10 may have certain portions of such thin cross-section . that they may be transparent. Of course, if this is the desired structure, clear plastics without reinforcement would be used.

As shown in Figs. 6A and 6B, the cross-section steps from a thick portion to a thin portion that is a valley on one side of the plastic part 10. It should be understood that punch part 68 may be duplicated in the bottom half of mold 30 so that the thinned area may be situated along the cross-sectional axis of the wall 22, as one punch part moves upwardly while the other moves downwardly. The method just described may be modified and used in a timing sequence in which the punch part 68 is positioned so that the material may be squeezed through a thin cross-section extending into the cavity bounded by punch part 68. As the material flows beneath the punch part 68, it may be retracted to draw the material into a thickened part with the temperature and pressure remaining constant.

Reference is now made to Figs. 8A and 8B. Molten material 10A is injected into the mold cavity 32 and a boss 20 is formed in the mold cavity 32. Compression core pin 60 is in a neutral position out of cavity 32. As is known by those of ordinary skill in the art, as the molten material moves from a relatively cross-section into a thickened cross-section, for example at boss 20, the density of the thickened portion is lessened. Accordingly, a sink mark 72 is produced in the thickened portion. In accordance with the invention, core pin 60, at a pre-determined time, is inserted into the molten material 10A within the cavity 32, squeezing the material surrounding the core pin 60 and between the core pin 60 and the wall of mold 30. This causes the material to denεify and pushes material into the volume of the sink mark. At a pre-determined time core pin 60 is retracted from the boss 20, leaving a screw hole within boss 20. Accordingly, plastic part 10, when ejected from mold 30 will not have a sink hole beneath the boss. The configuration resulting from this process is the screw boss 12 of Fig. 1.

The process just described involves first placing the core pin 60 in a neutral, that is, a retracted position. The plastic material 10A is injected into the mold cavity 32. At a pre-determined time, the compression core pin is advanced to compress material 10A in the location of boss 20. The core pin is retracted, again at a pre-determined time, the mold opens and plastic part 10 with a screw boss 12 is rejected from the mold 30.

References now made to Figs. 9A and 9B. Yet another application of the invention is in a method that the inventor calls "degating". By this method a gate punch pin 40A is used in combination with a support pin. The pins form a gate through which the molten material or polymer flows along the channel that forms a runner, for example runner 26 of Fig. 1, through the gate that forms a gate, for example gate 28 of Fig. 1, and into the mold cavity 32 forming a plastic part, for example, plastic part 10 of Fig. 1. At a pre-determined time, gate punch pin 40A moves with support pin 46 (or if densification is needed in the area immediately adjacent the gate, gate punch pin 40 may compress material into the surrounding area before support pin 46 is retracted) to punch out a portion of runner 26 at gate 28. At a pre-determined time, this portion is replaced and provides a tack hold between the runner and plastic part 10. When the plastic part is ejected from the mold, the gate 28, runner 26, and sprue 24 are ejected also, but now the end user or the manufacturer may snap the by-products from the finished plastic part without need of cutters or machining.

Thus, the operation of degating involves first placing the gate punch pin 40A and the support pin 46 in neutral positions. Plastic molten material 10A is injected into the mold cavity 32. At a predetermined time, the gate punch pin is advanced and the support pin is retracted, removing the gate connecting the plastic part 10 and the runner 26. The punch pin 40A and the support pin 46 are returned to neutral positions, the mold is opened, and the part is ejected. The gate 28, the runner 26, and the sprue 24, can be broken off the finished plastic part by hand, leaving a finished plastic part 10 without a need for machining.

A final application of the invention to be disclosed herein involves creating a living hinge by operation of the dynamic inserts. Reference will be made to Figs. 10A and 10B. Molten material 10A is injected into a mold cavity and forms a hinge "boss" in the area in which a living hinge is desired. The hinge boss is formed by a hinge compression pin 76 in a neutral position. When the hinge compression pin is advanced into the molten material 10A within cavity 32, the material is squeezed into the area surrounding the hinge area, thereby densifying the material. The hinge punch pin 76, with its wedge face, 78 stops short of the opposite wall of cavity 32, forming the living hinge. This process has the advantage over squeezing material through a thin portion from a thick portion with the associated problems discussed earlier. When the plastic part becomes solid in a finished state, hinge pin 76 is retracted and the plastic part is ejected from the mold. As can be seen in Fig. 10B, the living hinge 80 is formed within the plastic part 10.

While particular embodiments of the invention have been shown, it should be understood, of course, that the invention is not limited thereto, since many modifications may be made. It is, therefore, contemplated to cover by the present applications any such modifications as fall within the true scope spirit and scope of the appended claims.

Claims

1. A method for injection molding a plastic part in a mold having a mold cavity, the method comprising the steps of: injecting a plasticized material into a mold cavity; and advancing an insert into the mold cavity, while said plasticized material is therein, to affect the structure of the plastic part.

2. The method of claim 1, wherein before the step of injecting said plasticized material into said mold cavity, said insert is in a neutral position whereat said insert is not within said cavity and forms a part of the boundary of said mold cavity.

3. The method of claim 2, wherein before the step of injecting said plasticized material into said mold cavity, a support part that is movable in said mold and that, before the step of injecting said plasticized material into said mold cavity, occupies a neutral position whereat said support part forms a part of the boundary of said mold cavity.

4. The method of claim 3, wherein said insert moves along an axis into said mold cavity and said support part is aligned with said insert to also move along said axis.

5. The method of claim 4, further comprising the step of moving said movable support part away from said mold cavity leaving a passageway in said mold for said insert to follow, some of said plasticized material being trapped between said insert and said support part and wherein said structure of said plastic part is affected by having a hole formed in it by said insert.

6. The method of claim 5, further comprising the step of returning said insert and said support part to their respective neutral positions, releasing said plasticized material trapped between said insert and said support part so that said returned material is a plug in said hole formed in said plastic part.

7. The method of claim 6, further comprising the step of punching said plug of plastic material from said hole after said plasticized material has solidified and said plastic part has been ejected from said mold.

8. The method of claim 1, wherein said structure of said plastic part is affected by said insert's compressing said plasticized material to density it.

9. The method of claim 1, wherein said structure of said plastic part is affected by said insert's forming a hole in said plastic part.

10. The method of claim 8, wherein said structure of said plastic part is further affected by said insert's forming a thin section in said plastic part.

11. The method of claim 1, wherein said mold cavity is shaped to form a boss and said insert is a core pin which when inserted in said mold cavity is inserted into said plasticized material formed as a boss by said cavity and wherein said plastic part is affected by said core pin forming a screw hole in said boss.

12. The method of claim 1, wherein said mold cavity is shaped to form a hinge boss and said insert is a hinge pin which when inserted in said mold cavity is inserted into said plasticized material formed as a hinge boss by said cavity and wherein said plastic part is affected by said hinge pin forming a living hinge said boss.

13. An improved apparatus for injection molding comprising an injection molding mold having at least one cavity; at least one movable insert movable into and out of said cavity, said insert having a forward end and said insert having a neutral position at which the forward end bounds the cavity without being within the cavity and an advanced position at which said insert is within the cavity.

14. The apparatus of claim 13, further comprising a support part that is movable into and out of said mold, that has a forward end, and that is movable from a neutral position at which the forward end bounds the cavity without being within the cavity and a retracted position at which the forward end is away from the cavity and within the mold, having left a passageway in the mold from the cavity to the forward end.

15. The apparatus of claim 14, wherein said insert is movable along an axis into said mold cavity and said support part is aligned with said insert to also move along said axis.

16. The apparatus of claim 15, wherein said insert is movable into said passageway to follow said support part when said support part is moved from its neutral position to its retracted position.