This course is 3 Days in duration.
Why traditional DOE approaches will not work well for Plastic Injection Molding.
The injection molding process does not comply with the standard DOE type rules very well at all. For example randomizing the trials is not practical whenever temperatures are used as a factor. Also changing one parameter/factor on the injection molding machine affects many things and can violate our fundamental processing rules. Everything is an interaction.
Most often it seems people use a DOE type study to find what parameters to use to “make the right size part”. Often this results in a “bad” process which does not comply with the DeCoupled Molding rules and is not as stable. DOE should not be used to develop a process. It should only be used on a stable process and never sacrificing the Six Sigma (DeCoupled) methodology.
Examples of Problems:
#1 - Changing the injection speed also changes the material’s viscosity, how full the part is at transfer, the melt temperature, mold temperature, pressure gradient, and ejection temperature and cycle time.
#2 - Changing the melt temperature changes the viscosity, fullness at transfer, pressure gradient, mold temperature and ejection temperature. There are many of these types of interactions and need to be sure we all understand this situation fully and define the best way to handle it all.
Example Tips for Success:
#1 - If you adjust anything that might change the 95% (“fill only”) part, DeCouple the process and restore the 95% part. This refers to things such as injection speed, back pressure, rpm, temperatures, …. The shot size and transfer relationship should be adjusted to maintain the 95% fill only part. You always need to maintain a proper size cushion.
#2 - Do not adjust anything that might change the process from having Gate Seal to not having it, or the other way around. If you are running gate seal, make sure that does not change. If you are not running gate seal try and maintain the same amount of discharge.
#3 - Watch out for unplanned changes such as if you extend hold time you might have extended cycle time too accidentally. If you try different injection speeds, this will change your cycle time unless you adjust for it. If your machine has an injection forward timer and you slow down the injection speed, you just used up some of the time used for holding.