Injection molding of multilayer articles with post-pullback pressure monitoring

1 . An injection molding machine for molding a multilayer article, comprising:

a coinjection nozzle having a surface layer material channel and an internal layer material channel 426 ;

a surface layer material injection unit 104 ;

an internal layer material injection unit 106 ;

a mold cavity 800 ; and

a controller 108 ′configured to:

cause the surface layer material injection unit to commence injecting a surface layer material 455 into the mold cavity 800 via the surface layer material channel;

then cause the internal layer material injection unit to commence injecting an internal layer material 466 into the mold cavity via the internal layer material channel; then during application by the surface layer material injection unit of a hold pressure upon the surface layer material in the surface layer material channel and with the surface layer material injection unit and the internal layer material injection unit in fluid communication with one another via the surface layer material channel and internal layer material channel, cause a pullback stroke to occur at the internal layer material injection unit for reducing a pressure of the internal layer material; then after a delay interval, detect at least one physical parameter indicative of a post-pullback pressure of the internal layer material at the internal layer material injection unit; and generate an indicator of pullback effectiveness based on a relationship of the post-pullback pressure indicated by the at least one physical parameter and a threshold pressure PT.

2 . The injection molding machine of claim 1 wherein the controller is configured to perform the detecting of the at least one physical parameter indicative of the post-pullback pressure of the internal layer material at the internal layer material injection unit when the surface layer material injection unit and the internal layer material injection unit are in fluid communication with one another via the surface layer material channel and the internal layer material channel.

3 . The injection molding machine of claim 1 further comprising an injection actuator 114 for actuating a driving element 110 of the internal layer material injection unit, the injection actuator having first and second sensors 122 , 124 for sensing a bore-side pressure PB and a rod-side pressure PR respectively of the injection actuator, and wherein the detecting of the at least one physical parameter comprises detecting the bore-side pressure and the rod-side pressure of the injection actuator using the first and second sensors respectively while the driving element is stationary; and wherein the controller is further configured to compute the post-pullback pressure based in part on the bore-side pressure and the rod-side pressure.

4 . The injection molding machine of claim 1 wherein the controller is configured to generate a negative indicator indicating pullback ineffectiveness based on the post-pullback pressure being below the threshold pressure.

5 . The injection molding machine of claim 1 wherein the controller is configured to automatically trigger at least one remedial action for increasing the post-pullback pressure based on the post-pullback pressure being below the threshold pressure, the at least one remedial action being one or more of the following remedial actions: (a) increasing the post-pullback delay interval at the internal layer material injection unit; (b) increasing a length of the pullback stroke at the internal layer material injection unit; (c) increasing either or both of a pressure of the surface layer material at the surface layer material injection unit and a hold time of the surface layer material by the surface layer material injection unit; (d) increasing a speed of the pullback stroke at the internal layer material injection unit.

6 . The injection molding machine of claim 5 wherein the at least one remedial action comprises a plurality of the remedial actions (a) to (d).

7 . The injection molding machine of claim 6 wherein the triggering of the plurality of the remediation actions (a) to (d) comprises: triggering one of the remedial actions (a) to (d); waiting for the injection molding machine to effect at least one injection molding cycle then repeating the detecting of the physical parameter indicative of the post-pullback pressure of the internal layer material in the internal layer injection unit resulting from the latest pullback stroke; and upon determining that the post-pullback pressure indicated by the at least one physical parameter is below the threshold pressure, triggering another one of the remedial actions (a) to (d) and repeating the waiting and the detecting until the determining determines that the post-pullback pressure of the internal layer material meets or exceeds the predetermined threshold.

8 . The injection molding machine of claim 6 wherein the triggering of the plurality of the remedial actions (a) to (d) is performed in the sequence (a) to (d).

9 . The injection molding machine of claim 1 wherein the surface layer material channel is a primary channel for conveying surface layer material, wherein the coinjection nozzle also has a secondary channel for conveying surface layer material from the surface layer material injection unit towards the mold cavity, and wherein the controller is configured to increase the post-pullback pressure by automatically increasing a speed at which the secondary channel is opened upon initiation of the pullback stroke based on the post-pullback pressure being below a threshold pressure.