Tip of the Day 81: Tip of the Day #81: Not So Independent Variables – Reprise

Since Tip # 14 this one-page overview of molding variables has received much feedback. So I have modified it several times. It is surely not the last word. It’s purpose is to keep before us the concept that plastics molding has many variables that interrelate with each other. It also stimulates much discussion and keeps us honest about what we are trying to say.

Example 1: What does Backpressure Affect?

A prime example is the tendency of those not trained in systematic molding to adjust backpressure to “fix” a problem. One way to use this page is to put your finger on backpressure and see where it leads. It is reputed to affect how some materials mix and melt. But what many people forget is that it makes the melt denser during plastication. When plastication is done (and decompressed) you have more material behind the shot size and available to be injected to the transfer position.

 

Example 2: Correlation Studies on Flow Rate?

Another use for the page is as a reminder when doing correlation studies. We teach correlation between the part characteristics and the plastics variables. If you want to adjust “flow” (from the plastics point of view) this may be a helpful reminder to look at other settings. If you decrease the fill speed substantially the plastic may stop flowing at fill speed earlier in the cavity. If it is only 50% full at transfer then half of its its “flow” may be just a sort of oozing in after transfer instead. The half of the flow will not be at a fixed and controllable rate unless you adjust both speed and transfer to create the same short shot.