Switching on machine pressure versus Stroke

[iautry1973]

Quick question for those more experienced than I. First a little background. We have a part that is being revived by the customer and the process that existed before switched from 1st stage to 2nd stage on machine pressure (not a cavity pressure sensor) instead of a stroke. We are having some issues with the part so I was asked to look at it. It is set to switch at 1400 PSI (Hydraulic). When I went to look at it I switched back to stroke as it seemed to be over packing the parts. Normal decoupled method had the peak pressure at about 400-450 PSI (hydraulic). The DOE I was running was to determine what was causing some issues with concentricity.

Now here is the questions. I haven't dug to deep on it and no one has given a good reason yet as to why it was originally run this way. Anyone have any insight as to why you would set up a machine to switch on machine pressure and not stroke?

[Joel JS]

No, I can't think of a reason...... (Maybe original machine would only do it that way?)... I have always changed on position as I was taught that way, and should be a more consistent part this way.

(Also, from your description, sounds like it is set up to fill And Pack on first stage, but not sure why this would be done either, sometimes a very finicky job does make us do/try unusual things! )

Joel

[chrisprocess]

I don't have a great answer for you, however I know guys will transfer on pressure with large multi-cavity tools 64+/128+ cavities.
This allows the technician to easily shutoff one or more cavities (for whatever part-quality reason) and still transfer at peak pressure when cavities are ~98% full.
The advantage here is not having to fool around with shot-size or transfer position each time a cavity goes down...
Colleague of mine did this at a previous company, he said they were chasing their tails trying to adjust shot size 'n positions every time cavitation changed!
They'd run 128 cavities for extended periods until around 80% cavitation yield, then swap in an identical mold with 100% cavitation while the other mold was being worked on in tooling.

As for transfering on pressure for a single cavity tool I'm not clear why someone would do that other than some very strange/specific problem with the machine or mold..

edit1: if you're seeing peak pressure at around 400-450 psi at 98% fill by weight, and they were transferring at 1400 psi it sounds to me like they were over-packing the part. This could cause internal stresses in the part and result in concentricity issues. The counter argument is it was running like that for a very long time.. Are you able to compare current samples to previous samples? Same inspection methods are used for both? Concentricity is one of those 'calculated' CMM measurements from my experience. Easily confused with roundness, circularity, and cylindricity. I would push back on QE to be as specific as possible

edit2:
When I went to look at it I switched back to stroke as it seemed to be over packing the parts.

Did you choose your switchover position based on the original process? (transferring at 1400psi - and choose whatever position that correlates to)
Or did you choose your switch-over position by 98% full fill-only parts?

I'm guessing if guy was over-packing parts at 1400psi transfer, the holding pressure effectively had less of an impact on the molding process. Now you're transferring at 450psi and so the holding pressure has a much larger delta than previous, which I could see impacting parts.

[iautry1973]

Thanks for the answers guys. Here is a little more info:

Joel,

It is running on the same press as it used to. It's an Engel SPEX with a CC200 controller, so not to old. I have tried a few things in the past that were not by the book, but this was a new one to me.

Chris,

It is only a 4 cavity mold so shouldn't have to use that trick, but I will keep that in mind if I have to setup one with that many cavities! I am assuming that the over packing was causing some sort of issue with either moving the core over (the part is gated on the side and shoots straight at the core) or that it had to do with some sort of stress built up in the part. The fill only parts weighed in pretty close to the packed parts before I switched it to position. They are small MIM parts and I asked my fellow process engineer why they had done that. The reason I was given was that the shot to shot consistency was not great and so they did it that way so that they kept the pressure on the part consistently. However not sure I really buy into that reasoning. Using standard decoupled methods you keep pressure on it by using hold pressure so hmmmmm. As far as switchover for my DOE I did it at 98% full by weight. Waiting for the parts to get measured and see if the lower pressure parts ended up better.

Thanks again guys!

[JayDub]

I’m not familiar with MIM so the following is pure speculation:

I can see where you might need to overpack a MIM part to get the right dimensions/density in the finished metal part. I can also see where the heat transfer of a metal-filled resin might be higher than we normally see, so gate freeze might be quite rapid. So if you were using a traditional process and transferring at 450 psi, your gate might freeze before pack pressure could get up to 1400 psi. Injecting fast and transferring on pressure would at least guarantee there’s a good amount of material shoved into the cavity.

A gate freeze study might give you a clue as to what’s going on. My guess is you are walking a fine line between getting enough metal into the cavity to get a good final part, while not getting so much plastic into the cavity that the green part warps out of spec. But like I said, I’m just shooting in the dark here, so take this all with a large grain of salt. Final suggestion – be sure to measure the weight of the finished parts for your low pressure samples so you can compare density against the old high pressure parts.

[michaelhurley87]

I have done this when there is slipping issues with the check ring or wear in the barrel. it works well but the issue behind that is the fill rate must stay constant. Adjustments to heats, fill speed, BP... all change the fill pressure at transfer. If the guys don't understand this, it will be an issue. IMO this is not the way the part should be molded on a regular basis but in the time of need... works great. (May need to adjust shot size if bottoming the screw out)

[rickbatey]

Worn barrels and inconsistent NRV functions tend to drive the decision to pressure transfer. Multiple cavities with cold slug or drool issues tend to end up at hydraulic pressure transfer. Years ago we ran a 4 ounce tumbler for kids meal set. When operators learned that excess labels (hand inserted) led to over packing and press downtime; well you know how that went. So we set the press on hydraulic pressure transfer and it couldn’t over pack and lock up any more! Funny thing was it was a toggle machine and still wouldn’t open!! So maybe for PIM this is standard and done for all parts to weight to have the correct amount of steel in them.
Rick.