Transfer position set at ~30% of a full part

All,

One of the processing engineers at my company was sampling the part. After he sampled the part we had some issues with parts sticking so I went over to evaluate his process parameters.

The first thing I noticed is that he was transferring to pack/hold at roughly 30% of a full part (using velocity to fill 30% of the part and pack/hold to fill the rest). When I went over there to explain that we needed to adjust shot size and transfer position he got defensive and said there was nothing wrong with molding that way.

What are the negative consequences of not filling the part out 95%-99% of the way during first stage?

It goes against molding dogma!

What kind of a Part? Wall thickness? Kind of machine? Resin? Barrel capacity/shot size%
Try it at 96.8889% full and see how it runs!


KOM

brent

By filling at 30% you're counting on screw-momentum and 'melt-flow' to finish filling the mold. Issue with this is when the process is introduced to variation, such as viscosity changes from different material lot's etc, the momentum of the screw and 'melt-flow' won't always be the same. I've experienced this issue usually with high-injection speeds..
I might recommend two things:

1) Ramp injection speed - so something like 10in/s to 3in/s to 1.5in/s - this way you're always in control and you can somewhat mimic what the screw's momentum does.. (really you could graph screw-speed at 30% fill and match it's graph by ramping injection speed)

2) Keep pack and hold on and make short shorts and slowly creep up until the part is jusst filled-out and go a littttle further.. This will get you right where you need to be by using momentum, and that "little further" is accounting for variation down the road. This method will also use less machine energy than option 1 above

All products are different though, some work well with strange methods...

I would think transferring that early would cause some cosmetic issues as the pressure is set and the speed varies. Not to mention I would think the process would be inconsistent shot to shot. Sounds like he is being defensive because he goofed it up and he knows it (at least I hope he does).

Let me go back: it's one thing to transfer at 30% and use screw momentum take you to 98%, but it's a whole different story to transfer at 30% and just use pressure to pack your part.. This is a bad idea.