WO2014062281A1

WO2014062281A1 - Capacitive-based mold monitoring

Info

Publication number: WO2014062281A1
Application number: PCT/US2013/054076
Authority: WO
Inventors: Brian Esser
Original assignee: Husky Injection Molding Systems Ltd.
Priority date: 2012-10-17
Filing date: 2013-08-08
Publication date: 2014-04-24

Abstract

A voltage is applied between electrically isolated components to provide an electric field within a cavity of the mold (24) of a molding machine. The resulting capacitance is measured for a time while material is introduced into the mold cavity (34). As material is introduced, the effective dielectric constant of the cavity changes such that the measured capacitance changes. If the changing capacitance falls outside pre-defined bounds, a fault condition is determined.

Description

CAPACITIVE-BASED MOLD MONITORING

TECHNICAL FIELD

This relates to the use of capacitive sensing in a mold.

BACKGROUND

Various sensors have been employed in a mold to sense operating parameters of the mold in an effort ensure a high quality molded product. For example, it is known to employ piezoelectric transducers to measure pressures in the mold cavity, and thermocouples to measure temperatures in the cavity. Piezoelectric

transducers have also been positioned behind mold ejector pins to measure the force on these pins during mold operation.

SUMMARY

The capacitance within a mold cavity is measured for a period of time while material is introduced into the mold cavity. As material is introduced, the effective dielectric constant of the cavity changes such that the measured capacitance changes. If the changing capacitance falls outside pre-defined bounds, a fault condition is determined.

According to an aspect, there is provided a method of operating a molding machine, comprising: sensing a changing capacitance within a mold cavity over a time period while a material is introduced into said mold cavity; and determining a fault condition if said changing capacitance over said time period falls outside one or more pre-defined bounds.

According to another aspect, there is provided, a molding machine comprising: a mold having a first mold portion and a second mold portion which, when said mold is closed define, at least in part, a mold cavity with at least a first wall section of said first mold portion opposite at least a second wall section of said second mold portion across said mold cavity; an electric isolator, the electric isolator configured to electrically isolate said first mold portion from said second mold portion other than at at least a portion of said mold cavity; and a controller connected for sensing capacitance between said first mold portion and said second mold portion.

According to a further aspect, there is provided a molding machine comprising: a mold, the mold defining a mold cavity; electrically isolated components arranged so that application of a voltage to said electrically isolated components provide an electric field in said mold cavity; a controller connected between said electrically isolated components and configured for: sensing a changing capacitance within said mold cavity of said mold over a time period while material is introduced into said mold cavity; and determining a fault condition if said changing capacitance over said time period falls outside pre-defined bounds.

According to a still further aspect, there is provided a controller for a molding machine, comprising: means for sensing a changing capacitance within said mold cavity over a time period while a material is introduced into said mold cavity; and means for determining a fault condition if said changing capacitance over said time period falls outside pre-defined bounds.

Other features will become apparent from the drawings in conjunction with the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

In the figures which illustrate example embodiments, FIG. 1 is a schematic view of an injection molding machine;

FIG. 2 is a schematic detail of a portion of the injection molding machine of FIG. 1 ;

FIG. 3 is a graph of time versus measured capacitance from several different cycles of the injection molding machine of FIG. 1 ;

FIG. 4 is a flow diagram illustrating operation of a controller associated with the injection molding machine; and FIG. 5 is a schematic view of another injection molding machine.

DETAILED DESCRIPTION

Turning to FIG. 1 , a non-limiting embodiment of an injection molding machine 10 is depicted. The injection molding machine 10 has a stationary platen 12 and a moving platen 14. A stationary clamp block 16 is located behind the moving platen 14, and the moving platen 14 rides on a tie bar 18 that is supported at its ends by the stationary clamp block 16 and the stationary platen 12. (Although only one tie bar 18 is shown, typically there are four such ties bars, one extending between each of the four corners of the moving platen 14, the stationary platen 12, and the stationary clamp block 16.)

A mold 24 has a first mold portion 26 attached to the stationary platen 12 and a second mold portion 28 attached to the moving platen 14. The first mold portion 26 has a first mold core piece 30 and the second mold portion 28 has a second mold core piece 32. As illustrated, the front walls of the first and second mold core pieces 30, 32 are configured such that when they are brought together they form a mold cavity 34.

A platen moving actuator 35 is connected between the moving platen 14 and the stationary clamp block 16 for moving the moving platen 14 with respect to the stationary platen 12 and thus the first mold portion 26 with respect to the second mold portion 28. A clamp actuator 36 and a clamp shutter 38 are associated with the stationary clamp block 16. The clamp shutter 38 selectively connects the clamp actuator 36 with the moving platen 14 to clamp the first and second mold portions 26, 28 together. An ejector actuator 40 functions to eject a molded part from the mold 24. A screw injector 42 has a screw 44 rotated by a screw drive 46. A hopper 48 supplies the material 50, such as granulated resin, to the screw 44. A heater assembly 52 is coupled to the mold 24. An outlet (not separately numbered) of the screw injector 42 communicates with a sprue 54 that, in turn, communicates with a runner 56 providing a passageway into the mold cavity 34 through a valved gate 58.

FIG. 2 is a schematic detail of a non-limiting embodiment of a portion of the injection molding machine 10. Referencing FIG. 2, an electric isolator 60 can be applied to second mold core piece 32 of second mold portion 28. The electric isolator 60 is configured to electrically isolate the second mold core piece 32. In some non-limiting embodiments, the electric isolator 60 is a dielectric coating.

Moreover, the dielectric coating can cover sections 62a, 62b of the front wall of second mold core piece 32 such that when the first and second mold core pieces 30, 32 are brought together to form mold cavity 34, the dielectric coating electrically isolates the second mold core piece 32 from first mold core piece 30 other than at the mold cavity 34. Notably, second wall section 62c of second mold core piece 32 bounding the mold cavity 34 is opposite first wall section 64c of first mold core piece 30 across the mold cavity 34. Rather than applying the dielectric coating to second mold core piece 32, the dielectric coating could be applied to the first mold core piece 30 of the first mold portion 26. As a further option, the coating could be applied to both mold core pieces 30, 32. A voltage generator and capacitance measurement circuitry 66 can be connected across first and second mold core pieces 30, 32 by electrical wires 72, 74. The voltage generator may be a direct current (DC) or an alternating current (AC) generator. Where the voltage generator is an AC generator, it may have a high frequency to allow accurate measurement of small capacitances. The capacitance measurement circuitry may be a bridge circuit or any other suitable circuitry (for example, in simple embodiments, the capacitance measurement circuitry could be merely a resistor or capacitor connected in series with an AC voltage generator). A controller 70 is operatively connected to the capacitance measurement circuitry. The controller 70 receives input signals on lines 76 from various sensors (not shown) including, for example, a rotation sensor (not shown), when screw injector 42 is activated during a mold cycle. The controller 70 outputs, on lines 78, control signals to, for example, the platen moving actuator 35, clamp actuator 36, clamp shutter 38, ejector actuator 40, screw drive 46, heater assembly 52, and valved gate 58.

In operation, according to a non-limiting embodiment with reference to FIGs. 1 and 2, when injection molding machine 10 is activated, heater assembly 52 is energized and the voltage generator supplies a voltage across first and second core pieces 30, 32 through the capacitance measurement circuitry. In a mold cycle, beginning from an open mold position, controller 70 activates the platen moving actuator 35 to push the moving platen 14 to close first and second mold portions 26, 28 to form mold cavity 34. The controller 70 then controls the clamp actuator 36 and clamp shutter 38 to lock moving platen 14 in this position. Next, the controller 70 activates the screw drive 46 of screw injector 42 and receives a feedback signal on lines 76 that the screw 44 of the screw injector 42 is rotating. This signals the controller 70 to output a valved gate opening signal on lines 76 and to begin to monitor the capacitance between first and second core pieces 30, 32 across mold cavity 34.

With the screw 44 operating, material 50 will be conveyed from hopper 48 by the screw injector 42. The material is injected through sprue 54 and runner 56 past valved gate 58 into the mold cavity 34. At a pre-set time, the screw injector 42 can be de-energized and the valved gate 58 closed. The material in the mold cavity 34 cools, the mold 24 is then opened and the ejector actuator 40 operates to eject the finished product from the mold 24, all under control of controller 70. During injection, the heater assembly 52 maintains the material at a desired temperature. As will be understood by those skilled in the art, with a voltage applied between first and second core pieces 30, 32, an electric field can be set up across mold cavity 34. The resulting capacitance is inversely proportional to the strength of this electric field. An increase in the effective dielectric constant of the mold cavity 34 will reduce the effective electric field. Thus, if the effective dielectric constant of the mold cavity 34 increases, the capacitance between first and second mold core pieces 30, 32 also increases. Prior to material being introduced into mold cavity 34, the effective dielectric constant of the mold cavity 34 is relatively low (as the dielectric constant of the air filling the mold cavity 34 is approximately equal to 1 ) and so the capacitance between first and second mold core pieces 30, 32 is relatively low. As material 50 is introduced into the mold cavity 34, the effective dielectric constant of the mold cavity 34 will progressively increase and the capacitance will therefore

progressively increase. The effective dielectric constant of the mold cavity 34 at any moment will result from the unique combination of air (unfilled portion of the mold cavity 34) and material 50 (filled portion of the mold cavity 34) in the mold cavity 34, giving a unique value of capacitance directly related to the amount of material 50 inside the mold cavity 34. The controller 70 continuously measures this changing capacitance.

Turning to FIG. 3, the changing capacitance measured by controller 70 may follow curve 80. The time, T _; at which capacitance begins to increase indicates when valved gate 58 opens and material 50 begins to flow into mold cavity 34. The maximum rate of change of capacitance occurs between about times T₂ and T₄ as the mold cavity 34 fills and this rate of increase in capacitance slows. The capacitance reaches a maximum value, CM, at about T₅ once the mold cavity 34 is full. Curve 80 may represent the expected curve when the injection molding machine 10 is operating normally.

If, for some reason, valved gate 58 opened late, then the measured capacitance would not begin to increase until a time after T-i . This result is shown by curve 82 where capacitance does not begin to increase until time T₃.

If, for some reason, insufficient material entered the mold cavity 34 to fill the mold cavity 34 during the mold cycle, then the final value of the capacitance across the mold cavity 34 would be lower than expected. This condition is illustrated by curve 84 where the final value of the capacitance, C_L, is lower than the expected maximum capacitance C_M. It could be, for example, that the injection pressure of material 50 was too low. This would most typically be the result of the material 50 being too cold and therefore too viscous. It could also be the result of the screw drive 46 operating too slowly. A low injection pressure would be reflected in a low maximum rate of change of capacitance, as is illustrated by curve 86.

It could also be that the initial capacitance, Ci was higher than expected. This would suggest mold cavity 34 was not empty at the beginning of the mold cycle. The controller could operate according to the simple control scheme of FIG. 4 to react to the different possible fault conditions illustrated by the different curves of FIG. 3.

Turning to a non-limiting embodiment depicted in FIG. 4, the controller 70 may start monitoring capacitance at the nominal time when material 50 begins to be injected into the mold cavity 34 (S100). Monitoring could begin, for example, the time at which the controller 70 signals the screw injector 42 to begin operating, or the time when the controller 70 signals the valved gate 58 to open. The controller 70 may store an upper bound for the initial capacitance, C|. If the initial capacitance was greater than this upper bound (S102), the controller 70 could therefore determine a fault condition and output an appropriate fault indication

(S104) and, in one non-limiting embodiment, shut down injection molding machine 10. If the initial capacitance was lower than the upper bound, the controller may report that the initial capacitance is within specification (S105).

The controller 70 may also store a lower bound for the time when capacitance begins to increase such that if capacitance does not begin to increase by this time (as illustrated by curve 82) (S106), the controller 70 can determine a fault condition and may output a signal indicative of a faulty gate and/or change the timing or initial position of the gate so that it opens earlier in the mold cycle (S108). Where, on the other hand, the lower bound is within specification, the controller may report this (S109). The controller 70 may store a threshold value (lower bound) for the maximum rate of change of capacitance during a mold cycle. If maximum rate of change of the capacitance of a measured capacitance curve did not achieve this threshold value (as illustrated by curve 86) (S110), the controller 70 can determine a fault condition. Given this circumstance, the controller 70 could simply signal the fault or could take corrective action, specifically, increase the current to the heater assembly 52 in order to increase the temperature of the material 50 (S112) or increase the speed of the screw drive 46. With a low rate of change of capacitance, the controller could also increase the temperature of other heaters (not shown) of the injection molding machine, such as manifold zone heaters (not shown). If the maximum rate of change of capacitance is within specification, the controller may report this

(S113). The controller may store a threshold value (lower bound) for the final value of the capacitance such that if the final value is below this threshold value (as illustrated by curve 84) (S114), it can determine a fault condition. Given this circumstance, the controller could simply signal the fault, or could eject the resultant molded project to a reject bin (not shown) (S116). Where the final value of the capacitance meets the threshold value, the controller may report that this parameter is within specification (S118).

At the nominal time after the material 50 ceases to be injected into the mold cavity 34, such as when the screw injector 42 is de-energized or the valved gate 58 is closed, the controller 70 may cease measuring capacitance (S120).

Even where the controller takes corrective action, fault conditions reported by the controller may be used to prescribe maintenance requirements for the injection molding machine 10.

In a more sophisticated control scheme, controller 70 could, for example, store a curve of the expected varying capacitance in an ideal mold cycle and compare this curve to actual curves measured during a mold cycle, basing action taken on variances between the ideal curve and the measured curve.

In the illustrated non-limiting embodiment, there is one pair of capacitance elements across which capacitance is measured. In alternative non-limiting embodiments, there may be several pairs of capacitive elements in order to measure with precision the capacitance at different portions of the mold cavity 34. For example, molds for products of intricate shape may have, in addition to two main mold core pieces, additional mold core pieces, with all of these pieces coming together to form the mold cavity 34. In such instance, each of these mold core pieces could be electrically isolated from each other and a voltage applied between several different pairs of these mold core pieces in order to form different capacitors having electric fields centered across different portions of the mold cavity 34. As a further alternative non-limiting embodiment, each mold core piece could be manufactured with one or more dielectric inserts which electrically partition the mold core piece into two or more capacitive elements. This would allow the mold core pieces to form multiple capacitors for measuring changing capacitance with precision at different portions of the mold cavity 34. Injection molding machine 10 is shown with one mold cavity 34. Turning to FIG. 5, in alternative non-limiting embodiments a molding machine 200 may have a mold 224 with mold portions 226 and 228 having mold core pieces 230, 232 that define multiple, typically identical, mold cavities 234a, 234b, 234c that are electrically isolated by electrical isolators 260' and 260". In such instance, voltage generator and capacitance measurement circuitry 266a, 266b, 266c may be connected across each of electrically isolated areas to a controller 270 which independently measure capacitance(s) across each mold cavity 234a, 234b, and 234c and takes appropriate action based on the capacitance curve(s) for each mold cavity 234a, 234b, and 234c. Where the mold cavities are identical, the controller can use the capacitance measurements at each cavity to determine in real time whether the fill rates of the cavities are balanced. On sensing an imbalance, the controller may indicate a fault condition and/or take corrective action, such as adjusting a zone heater. With this operation, the need for short-shot balance testing is avoided. Using the first and second mold core pieces 30, 32 as the capacitive elements allows for the capacitive surfaces to extend on opposite sides of the mold cavity 34 which allows capacitance to be measured more globally in the mold cavity 34 than would be the case with a separate capacitor at the mold cavity 34 surface.

Furthermore, use of the first and second mold core pieces 30, 32 as the capacitive elements avoids the risk of a witness mark that could result from positioning a separate capacitor at the mold cavity 34 surface. Nevertheless, useful

measurements of changing capacitance over the mold cycle could, at least for some molding machines, still be obtained by a separate capacitor at the mold cavity 34 surface, and so this is contemplated as an option for such machines.

Although valved gate 58 is shown as a valved gate, it will be appreciated by those skilled in the art that the gate may not have a valve.

While controller 70 has been described as the master controller for the injection molding machine, in alternative non-limiting embodiments, controller 70 could be a sub-controller that monitors changing capacitance and communicates with a separate master controller to signal a fault condition. Controller 70 will typically be a digital controller (operating with digital to analog and analog to digital converters where required). However, the controller 70 could also be an analog controller 70. Furthermore, as will be apparent to those skilled in the art, controller 70 could be replaced with electromechanical controls that perform the same functions as controller 70.

While voltage generator 66 has been described as a separate element, the voltage generator 66 could be part of the controller.

While the molding machine has been described as an injection molding machine, the approaches described herein can be applied to other types of molding machines, such as extrusion blow molding machines. Other modifications will be apparent to those skilled in the art and, therefore, the invention is defined in the claims.

Claims

WHAT IS CLAIMED IS:

1 . A method of operating a molding machine, comprising:

sensing a changing capacitance within a mold cavity (34) over a time period while a material is introduced into said mold cavity (34); and

determining a fault condition if said changing capacitance over said time period falls outside one or more pre-defined bounds.

2. The method of claim 1 , wherein said one or more pre-defined bounds include a first pre-defined bound, said first pre-defined bound being a lower rate bound for a rate of change of said changing capacitance.

3. The method of claim 2, wherein said one or more pre-defined bounds include a second pre-defined bound, said second pre-defined bound being a lower final value bound for a final value of said changing capacitance at an end of said time period.

4. The method of claim 3, wherein said one or more pre-defined bounds include a third pre-defined bound, said third pre-defined bound being an upper bound for a time that said changing capacitance begins to change after the beginning of said time period.

5. The method of claim 1 , further comprising:

on determining said fault condition, signalling said fault condition.

6. The method of claim 1 , further comprising:

on determining said fault condition, adjusting one or more parameters of said molding machine.

7. The method of claim 2, wherein if said determining determines said rate of change of said changing capacitance is below said lower rate bound, heating of said material introduced into said mold cavity (34) is increased.

8. The method of claim 2, wherein if said determining determines said rate of change of said changing capacitance is below said lower rate bound, increasing the speed of an injector that injects material into said mold cavity.

9. The method of claim 3, wherein if said determining determines said final value of said changing capacitance at said end of said time period falls below said lower final value bound, a rejection of a product formed by said molding machine is initiated.

10. The method of claim 4 wherein if said determining determines said time that said changing capacitance begins to change after the beginning of said time period exceeds an upper bound, adjusting control of a gate which admits material to said mold cavity.

1 1 . The method of claim 1 , wherein said time period begins at a nominal start time for commencement of introduction of said material into said mold cavity (34).

12. The method of claim 1 1 , wherein said time period ends at a nominal end time for cessation of introduction of said material into said mold cavity (34).

13. The method of claim 1 , further comprising applying a voltage between electrically isolated components so as to provide an electric field within a mold cavity (34) of said molding machine.

14. The method of claim 13, wherein said applying a voltage between electrically isolated components of said mold further includes:

applying a voltage between a plurality of electrically isolated components of said mold; and

said sensing a changing capacitance within said mold cavity includes:

sensing a changing capacitance at a plurality of different portions of said mold cavity.

15. The method of claim 13, wherein said applying a voltage between electrically isolated components of said mold further includes:

applying said voltage across said mold cavity (34).

16. The method of claim 13 wherein said applying a voltage comprises applying an alternating voltage.

17. The method of claim 1 further comprising determining that said changing capacitance falls within said pre-defined bounds and signalling that said changing capacitance falls within said pre-defined bounds.

18. The method of claim 13 wherein said mold cavity is one of a plurality of mold cavities; wherein said applying a voltage comprises applying a voltage between electrically isolated components of each mold cavity of said plurality of mold cavities; wherein said sensing a changing capacitance comprises sensing a changing capacitance within said each mold cavity over a time period while a material is introduced into said each mold cavity; and wherein said determining a fault condition determines a fault condition if said changing capacitance at any of said plurality of mold cavities over said time period falls outside pre-defined bounds.

19. The method of claim 18 further comprising, if said changing capacitance at one of said plurality of mold cavities differs from said changing capacitance at another of said plurality of mold cavities, determining an out of balance fault condition.

20. A molding machine comprising:

a mold (24) having a first mold portion (26) and a second mold portion (28) which, when said mold (24) is closed define, at least in part, a mold cavity (34) with at least a first wall section (64c) of said first mold portion (26) opposite at least a second wall section (62c) of said second mold portion (28) across said mold cavity (34); an electric isolator (60), the electric isolator (60) configured to electrically isolate said first mold portion (26) from said second mold portion (28) other than at at least a portion of said mold cavity (34);

a controller (70) configured for sensing capacitance between said first mold portion (26) and said second mold portion (28).

21 . The molding machine of claim 20, wherein said electric isolator (60) is a dielectric coating.

22. The molding machine system of claim 21 , wherein said controller (70) is configured for monitoring said capacitance over a period of time while material is introduced into said mold cavity (34).

23. The molding machine of claim 20, further comprising a voltage generator for generating a voltage between said first mold portion (26) and said second mold portion (28).

24. The molding machine of claim 23, wherein said voltage generator is configured to generate an alternating voltage.

25. The molding machine of claim 20, wherein said controller (70) includes a bridge circuit.

26. A molding machine comprising:

a mold (24), the mold (24) defining a mold cavity (34);

electrically isolated components arranged so that application of a voltage to said electrically isolated components provide an electric field in said mold cavity (34);

a controller (70) connected between said electrically isolated components and configured for:

sensing a changing capacitance within said mold cavity (34) of said mold (24) over a time period while material is introduced into said mold cavity (34); and determining a fault condition if said changing capacitance over said time period falls outside pre-defined bounds.

27. The molding machine of claim 23, wherein said controller (70) is further configured for, if said determining determines a rate of change of said changing capacitance is below a lower rate bound, controlling a heater assembly (52) in order to increase heating of said material introduced into said mold cavity (34).

28. The molding machine of claim 23, wherein said controller (70) is further configured for, if said determining determines a final value of said changing capacitance at an end of said time period falls below a lower final value bound, initiating rejection of a product formed by said mold (24).

29. The molding machine of claim 23, wherein said electrically isolated components are mold core pieces (30).

30. The molding machine of claim 26, wherein said mold core pieces (30) are electrically isolated, at least in part, by a dielectric coating.

31 . The molding machine of claim 26, wherein a first of said mold core pieces (30) has a first wall section (64c) opposite a second wall section (62c) of a second of said mold core pieces (30) across said mold cavity (34).

32. The molding machine of claim 26, further comprising a voltage generator for generating a voltage between said first mold portion (26) and said second mold portion (28).

33. The molding machine of claim 32, wherein said voltage generator is configured to generate an alternating voltage.

34. The molding machine of claim 26, wherein said controller (70) includes a bridge circuit.

35. A controller (70) for a molding machine, comprising:

means for sensing a changing capacitance within said mold cavity (34) over a time period while a material is introduced into said mold cavity (34); and

means for determining a fault condition if said changing capacitance over said time period falls outside pre-defined bounds.