Processing Method

[chrisprocess]

Has anyone heard of anything like this?

195

This was recently introduced to me.

Thought is to use multiple injection profiles and adjust screw-speeds and screw-positions to achieve a consistent melt pressure throughout injection phase. (Not pressure limited, still 98% fill)
Said to compensate for part geometery..

Has anyone ever had experience with this or heard of such?

[Suhas]

Hi Chris,

Food for thought but I am not able to get the concept. High pressure could mean the plastic is very viscous, or cold or thin section etc., Let me think but why would the person who introduced this to you thinks it works? - Let us think, he could have a point but what? Flowrate is very important and so why would pressure matter?
Anyone else?

Suhas

[JayDub]

How is this any different from running pressure limited with max pressure set wherever you want it and velocity set to maximum? Effect would surely be identical with a lot less fuss. Why you might want to do it and what it would achieve is entirely another question.

[chrisprocess]

Thanks for the responses. Again, I'm not 100% clear on this approach but wanted to see if anyone else was, or if it even make sense..

Similar to a viscosity curve, the ideal pressure is determined by Pressure v Time graph (by setting injection speed to maximum and incrementally lowering pressure and recording fill time)
196


The assumption here is that change in pressure will minimally effect flow rate. So with this information one could argue the optimal pressure in this graph in approx 900 bar.

Thought is to hit this pressure throughout the entire injection stage by increasing/decreasing screw speeds and profile positions. (compensating for part geometry).

As part geometry changes, so will injection pressure. E.g. If there's a thin-wall during fill, pressure would typically increase, right? But with this method you could decrease screw speed until your desired injection pressure is reached.

[chrisprocess]

Hi Chris,
Flowrate is very important and so why would pressure matter?

Suhas

I'm not sure. Would there even be an advantage to controlling melt pressure? Scientifically speaking, could it help with shear and other variables?

[rickbatey]

I thinks it's all folly. I can't think of a single press brand that would inject a tool with that profile. I could see the pressure profile running inverse to your proposed velocity curve but no press will need to max pressure out at the start of the mold filling. Again this is still pressure limited!? I wonder who thought this up and what part it was and did this really solve the issue?
Rick.

[chrisprocess]

I thinks it's all folly. I can't think of a single press brand that would inject a tool with that profile. I could see the pressure profile running inverse to your proposed velocity curve but no press will need to max pressure out at the start of the mold filling. Again this is still pressure limited!? I wonder who thought this up and what part it was and did this really solve the issue?
Rick.

Thanks Rick. Not pressure limited, just shows an 'ideal' curve. Fast injection speed in the very beginning so pressure 'ideally' goes straight-up. Then slowing screw down towards end of fill for control. Thought is to maintain the melt pressure as much as possible..

In reality the curve looks something like this:

198

There are a number of injection profiles in this graph. The 'blip' at the end of Actual Melt Pressure is what tells you part is completely filled.
It's a little odd - to pack parts out more is done by screw position, not pack pressure.

This method came from a molding group in Mexico and was introduced to us..

Apparently they argue it is better in a number of ways, I'm curious to see if anyone else has done anything like this.
Half our guys are using this method; the other half use standard scientific methods.

[JayDub]

As presented, there are a couple of huge holes in the argument:
• Pressure is resistance to flow, so it’s an output not an input.
• The only pressure you are controlling is pressure in the barrel – strictly speaking, immediately ahead of the screw. Plastic pressure at the flow front, until end of fill and assuming adequate venting, is always and inevitably zero psig.
So why would you be so concerned to control pressure in the barrel that you would abandon any attempt to control flow in the cavity?

[chrisprocess]

As presented, there are a couple of huge holes in the argument:
• Pressure is resistance to flow, so it’s an output not an input.
• The only pressure you are controlling is pressure in the barrel – strictly speaking, immediately ahead of the screw. Plastic pressure at the flow front, until end of fill and assuming adequate venting, is always and inevitably zero psig.
So why would you be so concerned to control pressure in the barrel that you would abandon any attempt to control flow in the cavity?

Well placed concerns, however

Pressure IS an output, but it can be monitored and manipulating by updating screw speeds and screw positions, which are inputs.
I agree the flow-front itself may be close be 1 ATM, or something close (0 psi would be a perfect vacuum). But the bulk of the melt itself is under pressure.

I know this because the melt compresses during injection. And again when part geometery changes.
It makes sense to 'smooths' out some of the dynamic pressure within the part.

I neither agree nor disagree with this method. Though it does have some valid theory (in my mind).
It's good to hear peoples thoughts.
And guys that are using it are making quick/reliable processes with it.

[MTUHusky]

Hey Chris,

I have heard of transferring on a set pressure (instead of a set position), but I have never seen someone attempt to maintain a pressure throughout the entire shot. Now pressure differentials in the plastic can lead to warp but this is taking it to an extreme in my opinion. By running this way you are setting yourself up for a couple things.

- First, your molecular chain orientation is going to be different from fill stage to fill stage, which can lead to some unpredictable warp. Higher fill speeds equals higher chain orientation, slower fill speeds equals lower orientation. Differences in orientation lead to warp.
- You are also going to have a thicker frozen layer in the areas of the slower fill stages, which will lead to warp. Not to mention those areas with a thicker frozen layer section will be cooled while the earlier thinner frozen layer section will still be cooling. This differential cooling is a killer when talking warp.
- Also, in the slower fill stages, as mentioned in the previous point, these areas will have thicker frozen layers and will cool quicker which will make it harder to pack out, which you guessed it... leads to warp.

From a processing side, if you have a process setup that way and you change lots of material which may have a slightly higher molecular weight or a lower MFI, then I would no longer trust the process that was shown above. If the MFI went down then the material is harder to push, you might be fighting shorts this week. Next week when you get a new lot and the MFI is higher, then you might be fighting shutoff issues and flash.

With all of that being said, I am not saying that there isn't a part that couldn't benefit from this. I just haven't seen one.

[brentb]

Hogwash!

KOM

brent

[Joel JS]

This is psi limited, and I have had some limited success with such, but only as a last resort. I always start giving the fill profile more pressure than it actually needs. And, actually, I almost always as having the psi set about 5-10% higher than required.

But back to the original question..... this is psi limited during the fill stage. ... I cant' see how it's not, and I am not sure why it would be recommended.. of course a strange mold can come along to question our methodology.

Joel

[MTUHusky]

Joel,

I think you are absolutely right. It is pressure limited 1st stage.

If we were to profile a process as suggested in the original post, and then run a single injection speed with the pressure limit being set to the pressure achieved for the profile in the original post, then I think the two processes would react the same way provided the maximum injection time was set high enough to allow it.

If we run a process with a pressure limit and a high max injection time limit, the machine will hit the max injection pressure and compensate by slowing down the injection speed until it is under the limit again. I bring up the max injection time because if the timer was set too low and the machine was trying to keep itself under the pressure limit, then it could prematurely switch to 2nd stage (the timer will end and the machine will switch to stage 2 regardless of position). Looking at it this way I wouldn't recommend this at all unless you have tried everything else under the sun and are desperate.

[chrisprocess]

From your responses, I think you're right. In a way it mimics a pressure limited profile. One could easily argue to achieve a curve like this you could set injection pressure at say, 20,000 psi and set injection speed to max. The result of this would be some unknown injection profile with mixed speeds and distances of the screw. If you were to view Screw Stroke it would probably be wavy, all over the place.

A pressure limited system is a red flag for obvious reasons in that you have no control of your screw.

However going back to this graph 200

You can see the system is not pressure limited. If I were to show Screw Stroke it would appear wavy, and random. However the system is not random, it's controlled by screw speed.
At no point does this injection profile go out of control. And yes, as a rule of thumb injection pressure is set approx. 10% higher than what the system uses.

Since part geometry is dynamic, wouldn't it make sense to have a dynamic fill profile?

Say you were molding a cone. Wall thickness might be the same throughout, but the cross-sectional area is larger at the base of the cone than at the tip. If you were to use 1 injection profile, Flow would be slower at the base of the cone (larger cross-sectional area) and speed up towards the tip (smaller cross sectional area). This would be a change in flow-rate.

However Flow could be controlled by starting injection profile Fast and then slowing it down towards end of fill.

Anyways, it makes some sense to me. Maybe it's a little difficult to explain considering the steps in the method. I can sorta see where these guys are coming from. From everyone's responses it's clear it's not an industry norm of any sort..
And Hey, who wants to program 10 fill profiles when you can program 1 :p

[rickbatey]

I think that graph is incorrect. No way you would hit that high of an injection pressure with a mold that has good vents, correctly sized melt channels and gate(s) so much early while filling an empty cavity! Now I've slowed fill rates due to knit lines, poor venting, or even varying wall stock areas that created shear induced splay. But only because I had to. Now last week we moved a mold from a 2850t press to a 2000t press and I had to profile pack / hold pressure to keep parting line flash off part but reach previous part weight. But I had to do that or I would have used a flat pack / hold profile.
I still don't see the use BUT tomorrow I'll find that tool in the press.
Heck in reality you 'should' run varying screw rpm's and back pressure based on the constant loss in screw length. But how many have ever done it or had to?
Rick.

[Joel JS]

Ok.. I see that it is different than what we call PSI limited... I have not seen it, nor can I envision it being useful....... but perhaps on some parts.

(Pressure in the cavity is always more at the gate then end of fill is one argument against.)

Joel

[MTUHusky]

I missed that second photo. So it's different from a PSI limit because you have given it an extra 10% PSI to avoid the limit? And the goal is to have uniform pressure distribution throughout the shot?

I think the problem with the theory is it only works consistently for the 1st stage. You may be able to reach a constant pressure during the first stage consistently with this setup but I think the issue is going to be during pack. With this setup, you always have to have your later fill stages at a slower speeds. With some molds, especially thin-walled molds, you are going to have the issue of trying to pack out the end of the part that is already starting to solidify, where the gate and beginning of the part is still molten.

Might be worth trying but make sure you have your constant pressure high enough that your flow front isnt freezing off.

[chrisprocess]

I missed that second photo. So it's different from a PSI limit because you have given it an extra 10% PSI to avoid the limit? And the goal is to have uniform pressure distribution throughout the shot?

Yes 10% more or less - just enough to ensure cavities are filled, But not high enough to blow open the mold if part gets stuck in the cavity.
And yes, uniform pressure throughout the shot. Or at least during fill.. As long as fill time is consistent and machine is in good working order it should fill fast enough (and have full control) before melt freezes off.




I think the problem with the theory is it only works consistently for the 1st stage. You may be able to reach a constant pressure during the first stage consistently with this setup but I think the issue is going to be during pack. With this setup, you always have to have your later fill stages at a slower speeds. With some molds, especially thin-walled molds, you are going to have the issue of trying to pack out the end of the part that is already starting to solidify, where the gate and beginning of the part is still molten.

Might be worth trying but make sure you have your constant pressure high enough that your flow front isnt freezing off.

Hard to say for sure.. I'd argue slowing the screw down is ideal because it gives control..

I've seen processes where once it hits switch-over the inertia/momentum of the screw 'slams' plastic into the cavity and sometimes can even bounce back.
I've also seen processes that switch-over at approx 50% part volume because the momentum of the screw carries the melt to 98% fill when running fill-only parts. (clearly a red flag!)

I mean the screw's gotta stop right!? Where's all that momentum and moving-force going to?
And even so The screw's speed eventually will slow down on it's own - Doesn't it make sense to control this

[MTUHusky]

As long as fill time is consistent and machine is in good working order it should fill fast enough (and have full control) before melt freezes off.
This will be true for 1st stage fill, but the pack stage is where the colder melt is going to hurt you, especially with certain part geometries. If we take a simple dog bone part that is thinner in the middle and just look at 1st stage fill, we would see a much thicker frozen layer in the thin area and towards the end of fill as I have illustrated in diagram one. 201 I realize this is a bad design and an exaggerated senario but the lower speed fill stages are going to have thicker frozen layers and in certain part geometries this is a 2nd stage nightmare as your packing pressure will be very high by the gate and low at end of fill instead of going hydrostatic like it should and creating a uniform pack pressure throughout the cavity.




Hard to say for sure.. I'd argue slowing the screw down is ideal because it gives control..
True, it will give you control over pressure through 1st stage, but you give up control over viscosity and shear rate as these will both change with each change in fill speed, as well as losing control of your packing pressure.



I've seen processes where once it hits switch-over the inertia/momentum of the screw 'slams' plastic into the cavity and sometimes can even bounce back.
I've also seen processes that switch-over at approx 50% part volume because the momentum of the screw carries the melt to 98% fill when running fill-only parts. (clearly a red flag!)

I have also seen this, you can definitely bounce the melt off of the end of the cavity if you are shooting too fast. This is why I encourage recording the cushion when you are doing a doe fill study just to see the effects of the momentum. If the fill study is done correctly then the fill speed obtained should not cause this issue but monitoring your cushion during the test will give you a heads up if something is off.


I mean the screw's gotta stop right!? Where's all that momentum and moving-force going to?
And even so The screw's speed eventually will slow down on it's own - Doesn't it make sense to control this

In my experience the ideal situation is that the 1st stage ends at 95%-98% and then pack begins at roughly 75% of the transfer pressure. At stage 2 the screw will drop pressure until the pack pressure is reached. This is where the screw slows down, packs out the cavity and stops and the desired cushion. Keep in mind that most polymers are compressible (some very compressible) and that compression will eat up a lot of that momentum provided you are not slamming the plastic in there and causing a rebound.
It is important to keep in mind that 1st stage is a velocity controlled process and 2nd stage is a pressure controlled process. In other words, we want a consistent fill from shot to shot and a consistent pack pressure from shot to shot.

[chrisprocess]

Thanks Husky. Honestly not too thrilled about the method myself either. But apparently this is the route some guys want to go. I'll keep you updated if I have any success with it

[Joel JS]

Chris,

Just wanted to say thanks for bringing this up, and creating some lively conversation. It's good for the brain to hear new ideas. Let us know if you come across a part it works good with.

Joel