Melt cushion variation- short shot - checkring?

[processtech]

Hello,
Having some difficulty with a product and melt cushion variation.Spent a long time working on this today, and at stages thought i had a run-able process, only for the problem to return after 5 or 6 shots
Material is TPE melt temperature around 170°C
Whats i think is happening, is the checkring inst closing off every shot.

Running fill only shots, everything seems ok, no variation in the following:

plasticizing stroke
melt pressure at changeover point
changeover point
melt cushion

The one thing i couldn't eliminate at this stage was overshoot at changover point, for example, changeover set at 10mm, cushion of 9mm, also the volume of fill varies slightly shot to shot, 90 to 95%
Plasticising time is also varying up to 0.4sec

When i apply hold pressure, some shots are ok, then the next shot the melt cushion will drop, not an awful lot but still causing shorts

To try and resolve this:
I have increased suckback, suckback speed, back pressure, all to no effect

increased melt temperarure, even added a fast speed stage to the process towards changeover point to try and get the checkring to seal off,


What do you guys on here recomend that i look at now, or that may cause this issue
Thanks
Processtech

[Suhas]

Hi,

> How about back pressure? Will increasing it make a difference?

> Do you have a second slow down speed before you hit transfer? (You should always have one, it is like slowing down before you come to the red traffic light)

> If your cushion is good for a few shots and suddenly drops then it could be the check ring. Will having very low hold pressures give you a consistent cushion? You can try (very carefully) the check ring test. This is good only for a cold runner mold with a sprue. Take a shot and as soon as the screw recovery starts put the machine in manual. Wait till you think the sprue is fully frozen. Now, with plastic pressures of about 7500 psi, keeping the nozzle forward inject the plastic. If it bottoms out then the check ring and/or the barrel are bad. Of course, check for leakage around the nozzle tip.

Let us know.
Suhas

[rickbatey]

I'd perform a check ring test. Fill only for 5 parts, delay charge time for approximately the same thing me it takes the gate to seal off. The five shots should weight within 10% of one another. If not check ring performance issues. Lastly you could add 10-20 mm to all screw strokes. If the process is stable your life may have wear in the front of the barrel. BTW- what's the residence time and percentage of screw stroke used? You may need to adjust the barrel temp settings because I've had an issue once on an Engel when we were using only 19% of the screw stroke. It took some serious temp reductions to get everything working 100%.
Rick.

[processtech]

Thank you Suhas and Rick,

I forgot to include it in my first post, but this is a hot runner mould
I increased back pressure and still had the same variation afterwards
I hadnt a second slow down speed in this process, the way i was running was a single speed to fill the mould, and then transfering to hold 220 bar, with 2700psi eoc peak.
This ran ok with no shorts, yet looking back through data had the same melt cushion variation, so im a bit confused as to why i wasnt getting shorts.

What got me looking at this was an attempt to run at lower peak pressure, due to having hydraulic side cores locked very tight, in order to eliminate flash, this we think was the reason for bolts breaking in the cylinder mounting flanges


Residence time around 7 seconds
25% of barrel usage

I checked variation of fill only shots and a result of 4.5% was achieved, also adding a slow speed profile towards switch over position, gave a variation of 1.4%, also overshoot of switch over point reduced to about 0.4mm, where as it was this was about 1mm

The real confusing thing for me here is this indicates a good checkring, but yet its hold pressure that seems to cause the variation/or at least this is when i see the variation in cushion, unless there is something in the checkring that is intermittently not allowing it to seal.

To give an update on how i got on, reducing melt temps by 10°C with increased back pressure gave the most consistent results, im still not overly happy with it though, back pressure is high at 90bar, variation still around a millimeter, but no shorts

Just to add this is a new machine


Thanks for the pointers, and look forward to your replies

[rickbatey]

The small usage of the barrel is the issue. What exactly are your barrel settings? I'd expect from the center back to be much colder due to the 25% utilization of the barrel. Most likely-15 or -20*C to reduce the residence heating. Also soft resins need short hold times with modest pressure. Too much of either and you can pack cold slugs into the part or cause sticking. Also understand that such a larger screw has so little stroke from non-fill to flash that control may never be what you hope for. On the Engel I had to tune the decompression and initial fill rate commands on the press as they caused much of the inconsistency. Many machines start the injection from an internal chart that states that x amount of flow is equal to x amount of velocity. Then after the command to get the screw accelerated and moving, the press begins to adjust the pump or valve flow to lock in the fill velocity. With very small suck back and injection strokes, the press makes some over corrections and you see the results in the data collected.
BR,
Rick

[Jendalf]

hi,
first check producer recomendations for TPE (temp. and temperature profil in barrel). I think that you have low melting temperature - it should be around 210°C for TPE. Also adjust the hot runners temperature close to this. Than you don´t need so high backpreassure.
The low residence times togather with low temp can cause some of material not to be fuly melted. This can influence your check ring.
J

[rickbatey]

Jendalf
His issue that he's only using 25% of the available screw stroke, not 95%. His barrel temp settings will require serious adjustment to allow the resin to run but survive the long residence time. Besides I use at least 100 psi (6.9B) hydraulic back pressure unless I really need the fastest screw recovery. This has allowed me to run much more consistent compared to other processes created.
Rick.

[processtech]

Jendalf
His issue that he's only using 25% of the available screw stroke, not 95%. His barrel temp settings will require serious adjustment to allow the resin to run but survive the long residence time. Besides I use at least 100 psi (6.9B) hydraulic back pressure unless I really need the fastest screw recovery. This has allowed me to run much more consistent compared to other processes created.
Rick.

When you say adjustment of the temperatures,is this reducing them?

What i cant understand is, i have very consistent fill only shots, less than 2% variation, but yet when i apply hold pressure the variation comes into play, and also, increasing hold pressure by around 50 bar helps to eliminate the shorts, i still have the variation in cushion, but besides the 25% barrel usage, i wonder was the issue of shorts caused by too low hold?

[rickbatey]

When you say adjustment of the temperatures,is this reducing them?

What i cant understand is, i have very consistent fill only shots, less than 2% variation, but yet when i apply hold pressure the variation comes into play, and also, increasing hold pressure by around 50 bar helps to eliminate the shorts, i still have the variation in cushion, but besides the 25% barrel usage, i wonder was the issue of shorts caused by too low hold?
Yes I mean reducing the heat profile to about -20*C from center of barrel back toward the feed throat. Remember that soft resins are compressible and can be over packed with high psi and/or time. The issue is due to resolution of the control system to precisely control the injection rate and pressure. If you weren't using so small an amount of the screw stroke I'd say add 10-20 mm to all strokes and see what happens.
Rick.

[rickbatey]

I prefer to use the 60% peak fill psi for hold psi unless the mold, resin, or part shows me that it isn't the right thing to do.
What does the resin OEM tell you to do?
Rick.

[brentb]

Is the hot runner valve gated?

KOM

brent

[Valentin Leung]

I would like to know a few things too, we process TPE in our factory on overmolded parts (PP as a substrate, the TPE is overmolded on it).
What I found out, TPE does not work well with suck back, we used to have a rule of thumb in the factory to always add suck back to recovery in a bid to be sure that the check ring will close, but my cushion is less stable with ANY suckback than without. I am using 31% of the total screw stroke available. My back pressure is very low, 2 bar only and we have consistent shot, I am within +/-0.1mm (out of a 60mm stroke) cushion variation. Also one thing weird is that I don't seem to be able to trip my machine backpressure limit, because even with no material, the screw can still consider it can shoot material without the "empty hopper" alarm ringing, this might be my machine...

From what I understood of processing, in order to reduce your injection pressure, you have to reduce the speed of your fill, but this can only be reduced to a certain extend, you can't go indefinitely down (as viscosity goes up, the pressure will go back up, so that's your lower limit). At any moment, I should not be pressure limited, the time I start to be limited, the overflow gate is open and I am not in control of the melt anymore and allowing plastic to backflow (<-please correct me if I am wrong).

Finally, if your side action hydraulic actuator have issues holding the injection pressure and bolts are breaking, then, it's not a process issue to me, but you should have the tool design review so that your factor of safety is higher, you have a reliability issue there. Lowering Pressure is one thing, but you are just increasing the number of cycle that the bolts can hold, not pushing their load to a bearable state that will increase significantly increase their lifetime.

[pjhall]

When only using 25% of the barrel capacity you might see one shot with a cushion and the next with no cushion and it will alternate late that. This can be due to "bounce back" after the hold time is over. I've had a shot size of 10mm and a cut-off of 5mm and the screw would bounce back to 8 and only recover the remaining 2mm. I find adding some recovery delay time and screw decompression helps.

[JA_CINE]

@ProcessTech If you're still seeing flash post-production that you want to remove, my company Nitrofreeze provides batch and piece by piece cryogenic deflashing solutions that clean up TPE nicely. If this seems like a process that could add value to your production, give our website a look (http://www.nitrofreeze.us/cryogenic-deflashing/). Best of luck with this project.

[Ten Fingers]

Is the mold valve gated or a hot tip?