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Plastics/Transfer position to Cushion Position


My question does it matter how much of a difference you have between you transfer position and  Cushion. The part I am running is a Cover/lid and it has tabs all around it to help hold it onto the Can that is molded separately.

We have a issue with the small tabs tearing so we need to run the cool time 2 times higher than normal so they don't tear at mold open.  I do know that the mold is not in perfect balance, Mold is Valve gated,  Material is Polypropylene, shot size is 1.47 Transfer is .69 Cushion is .18. transfer and cushion repeat consistently.  Hold Psi 700 Hold time 1.0sec cool time is 8.0.  In my mind if the mold is filling about 95% at transfer, how can the cushion get to .18, could the valve gates not be holding, parts are good. Cool time used to run about 4 sec, Engineer thinks its all a steel problem.

I just cant in my mind grasp that at transfer I am at .69 with a cushion of .18, Seems that the press is pushing more plastic than it should be. With a valve gated mold would doing a gate freeze type study make any sense? and if plastic is getting by the valve gates should I not see some stringing or gate extension of some kind? I sincerely thank you for any response, Good day sir.

If in fact you are utilizing a valve gate once the valve gate closes there should be no more material entering the cavity. This can be checked by weighing the parts that you are running now vrs. what they weighed previously when the parts ran well.
What did the cushion used to be when the process was running correctly making good parts and the cooling time was four seconds?
I would ask that you review your math and determine exactly how much of the shot size you think you are using at transfer.What you describe @ 0.69 transfer is a 46.9% fill at transfer and 12.2% @ cushion.
In my mind the problem you describe is not in the mold or the process even though they both may need some refinement. If there hasn't been any change to the mold steel it should still be able to produce good parts.
If you have the following information related to these feedback actual values produced during the process it would be beneficial to determining if the problem resides inside the barrel.
Fill Time:
Peek pressure during fill:
Pressure @ transfer:
If the cushion now is significantly lower then it was during previous runs then most likely the non-return valve (check assembly) is leaking or the I.D. of the barrel is word and allowing material to leak by and cause less material to be injected into the mold.
This reduction in the amount of material that is packed into the mold will directly effect how the tabs are filled and packed out potentially causing them to tear.
Rik Keeley 440@840@4212 Give a call if you have more questions.


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Richard W. Keeley


Hands on training of technical and operational personnel in all aspects of machine operations, quality monitoring, and process capability studies in a molding machine environment. Complete operation of production line facilities from concept (PPAP runoffs) to full production run statistics. Capable of a wide range of competencies on various computers systems and computer software. (RJG cavity and press monitoring, scientific molding methods and 8D problem solving tools) An ability to use continuous improvement techniques to maintain quality of products, scrap reduction and operating expenses in a manufacturing environment (TQM) Ability to trouble shoot Programmable Logic Control Systems (PLC’s)


A strong commitment to achieving sustained customer satisfaction through the use of quality systems. Met with customers on a regular basis to develop machine specifications and solve operational problems. Conducted customer training and troubleshooting on all types of machine operations throughout the United States and Central America. Experienced in the set-up and entertaining of customers at plastics industry expositions and trade shows.

Society of Plastics Engineers Society of Plastics Industry

Ferris State College, Big Rapids, MI 1983. Associate Degree in Plastics Engineering Technology. Member of Society of Plastics Engineers (SPE) Active participation in professional development workshops and seminars

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