Problem during TPE processing

Greetings everybody,

I have a returning problem during TPE processing (2K overmold), what I couldn’t solve yet.
TPE-SEBS overmolded on PP with talcum additive.

The tool is an index rotary plate mold. The tool is a valve gated 4 cavity tool. The valves controlled by hydraulic.

The production is runs well, the parameters are stable, the part quality is fine. But after a few weeks of production, contamination appears on the TPE material. The injection pressure increasing and also the fill time. The cavity balance starts to became wrong. One cavity starts to overfill, while other cavities are underfilled.

In the past, after this issue, the toolshop disassemled the tool and we see the TPE material burned on the valve pins, and inside the manifold (which we can’t check.)
We checked the hydraulic valve pins and there is no oil leakage. The pins moves at the same time and same speed. They open at the beginning of the injection phase with a delay. (-0,5 s)
At the end of the production we use a purging compound to clean the barrel and the hot-runner system. We also ue it when there is a longer stillstand of the machine.

In the material datasheet the range of the melt temperature is given beetween 160 – 240°C. We use 200 °C.
I checked the residence time of the material, and it’s 1,7 minute. The maximum residence time is 5 minute for this material.
We use 50% of the barrel capacity.

I’m new to this TPE processing and I don’t have any ideas how to solve this issue.

Thanks for the advice!

hi, which TPE-SEBS material are you useing? Brand name please :-)

Hi,
Brand name and type: LIFOFLEX XGEA17 UV 252404-5 NATUR

HI, I do no know this material exactly, but generally.... The processing temp 200 °C looks good, if it is also on hot runers. The issue can be overheating of TPE, which can bring a odour and vaporisation .... and some of this dirty can stick on the barrel, hot runner etc.
Please check the material temperature directly after purging. My opinion 220°C max. Try to reduce backpressure and screw temp profile also down.
Also the issue can be hot runner controler which can overheat the material during start up. Also all sharp edges in material flow way are dangerous.
The residence time is pointing on temp. sensitive material. :-)

Hi,
Do you use virgin material? Is it possible that you can mix materials? Is drying necessary and if, how stable process is.
I didnt work with this material, but i had similar problems with TPE materials.
Also maybie is good idea to clean hot-runner when is "open" (remove valve pins to get biggest possible diameter on hot-runner exit).
I had situation with building up burnt material on walls of hot-runner and after some time pieces of that material end up in gate.

Thanks for the replies!

I was busy on this week. I will check the temperature of the melt after purging.

@nemanja
Yes, we use virgin material. We store the material separated, I think there is no possibility to mix it with other TPE material. Drying is not necessary for this material (according to the datasheet). The process is stable. As I wrote, after few weeks of production the injection pressure increases (for example from 410 bar to 455 bar).
The fill time is stable 0,78 s, but when the pressure increases the fill time also increases to 0,81 s
I'm sure the TPE contamination decreases the diameter in the hot-runner, this issue cause higher pressure and longer fill time.

So the promblem is a little bit complex. It's hard to investigate the flow in the hot-runner manifolds or in the hot runner tips.

In the past the machine maintenance checked the screw and the barrel. There was no burned TPE in the screw threads.

@Pilot
As You mentioned, the material can burn at the start-up of the machine. We use purging compound after the machine or the production stops. So there is no TPE in the hot-runner, the purging compound can withstand higher temperature. After the hot-runner reaches the set temperature, the production starts. There is no longer stillstand time between heat-up and start of production. The cooling of the hot-runner turns on after the heating turns on (machine setting). At start-up, the PP zones starts to heat-up, and after these zones reaches 80% of set heat the TPE zones starts to heat (Pre-heating of TPE zones). Maybe the heating-up is too fast for the TPE zones. I will investigate this.

As Pilot mentioned above, sharp edges in the melt flow path can cause burning. Check that when the gates are open, the valve pins are retracted out of the flow channel. If not, material flowing past the end of the pins may see excess shear and burn on to the pins. If that is what is happening, see if you can use shorter pins with longer travel.

please check also nozzle tip and seat diameter if are the same. Please check hole quality if the edges are not "mushroomed" :-) without sharp.

I would lower the barrel settings slightly as I believe the barrel capacity is low enough to cause cooking out of some of the additives in the resin. Also what size are the resin purchases? I ask because I e seen other resins do this due to lot to lot variations.
Rick

please check the valve pin fitting, it seems the pin cannot be pulled in position that cause the fill pressure become higher. TPE material is easy flow material, higher melt temperature make it much easier, the gap between the bushing and valve pin are easy to be filled with material. also please check the cooling is good or bad for hot runner. if the cooling line was blocked that also can make the hot runner work incorrect.