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Tech Tip October 1997

Is Injection Speed what you set?:


Have you seen a variation in part quality and fight a battle with lot to lot material variation or even box to box? Recently, I have come across this in a few plants, and it was really the result of not having a robust enough process. The result, the plastic was the driving force in cycle repeatability.

When I talk robust, I want a process where the machine controls the process and not the plastic. This means that if the machine does not have enough pressure to fill the part in the allotted time or speed, then the plastic controls the fill and we will see variation. The easiest way to see this on a new machine is that the speed is in inches or centimeters per second. Am I actually filling in the time? What I'm saying is, if I travel at two inches per second, then for two inches of stroke, it takes one second of time. Recently, I had variation. The speed/velocity was set at two inches per second, and there was approximately four inches of stroke. The fill time was varying from 3.8 seconds to 4.8 seconds. As I calculated, four inches of stroke divided by two inches per second should equal two seconds of fill time that I did not have. Result, the plastic was controlling the process and each box or lot of material could bring different quality to the parts. When I saw that I was not at the correct speed, the fill time should have been two seconds, I investigated the pressure. The pressure was set at 600 psi and was actually 689 psi. This is what we call a pressure limited situation.

When the pressure was increased enough to control at the speed set, I was able to get repeatability and good parts. Basically, this is a check to see if you are in control, or if the machine is doing what you ask of it to do.



Thanks for the time.

Steven L Silvey
Sr. Technical Service
General Polymers