Cyclic Cushion variation every 35 or 40 shots.I have noticed the following phenomenon which I cannot explain. I am running a job which exhibits an incremental change in cushion each cycle for about 20 shots and then begins to decrease and return to nominal. This is then repeated..every 35 to 40 shots. Moulding is in a 100 Ton JSW EIII hydraulic machine.

My first suspicion was that it may be resulting from the regrind from the sprue, however the same result occurs with 100% virgin materials.

I’d be interested in hearing from anyone who has a theory.

One potential cause could be cycling of the mold temperature. If the temperature controller (water temp control) is cycling up and down significantly, and the mold temp is following it, the cushion could be affected. I have seen this temperature cycling occur fairly often, especially if the water flow is restricted or the controller is large relative to the mold and flow passages. This seems to me the most likely cause of the cushion variation.

As part of the investigation process, I would also verify the following to see if they are varying over time: hold pressure, shot size, hydraulic oil temperature(could affect pressures and speeds), temperatures in the injection barrel and hot runner, if applicable, and cooling water to the feed throat.

There are of course many interacting variables in the molding process, several of which could cause this phenomenon.

Once you complete your investigation, I would be interested in knowing what you find.
Jeff Boggs

You could be exhibiting a worn screw and barrel. Just 0.010" to 0.020" of wear on your screw and barrel can cause an inconsistant cushion. You might also have a worn check ring at the screw tip. This would give the same problem. You didn't indicate the material being processed or the shot size. If it is an abrasive material [glass reinforcement or mineral filler] and/or your shot size is 10% to 20% of your shot capacity and your cycle is 8-10 seconds, your wear on these components, over time will be more dramatic than expected.

Does the machine have automatic or self correcting cushion control? If so, what are the correction settings?

On the screw recovery is it always recovering to the same point? Also, is the part weight changing with the cushion? If the part weight is not changing it is likely a bad check valve or worn barrel.

I think that the worn screw, barrel, or check ring is probably not the cause. I would expect these factors to cause random variability in the cushion. But, I don't think they would cause the regular pattern of gradual up and down variation which was described.

I agree with jeff..if its really a pattern, sounds like thermal cycling. What is the resin? (i'm gonna guess polycarb..with large barrel for a small shot weight..)

Also...re: cushion control.. I don't know anyone that has ever used it, don't recommend it, and don't understand why its even an option....

I've seen this when running a regrind/virgin mix and it is layered in the hopper and not properly blended. Add a mixer just above the screw on the feedthroat. I've also added more back pressure to eliminate or reduce this problem. Worn barrel or screw will also cause this as will improper heat profile. Check the zones with a pyrometer to make sure that one or more zones aren't way off screwing up your profile. Also, if you are using a tiny amount of the available shot and not getting adequate turns of the screw, you will get this problem. Also with tiny shot your decompression may be more than the shot and that can cause this but usually every other shot, not every 35 - 40 shots. Hope this helps.