Problem during TPE processing

[greg8811]

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!

[Pilot]

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

[greg8811]

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

[Pilot]

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. :-)

[nemanja]

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.

[greg8811]

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.