Injection Unit for EVA

[Rojan]

Hello,

I wanted to ask for help those familiar with processing of EVA. I came across with an issue on parts that are made of natural color EVA. The problem is that with every start-up there is a lot of scrap in the form of black specs. We decided to pull the screw out of the barrel and what I saw frightened me. The screw is corroded, or at least it seems to me like this. I think the injection unit is not suitable for processing of EVA. I already asked the OEM what resins can be processed, but have not got answers yet. I also found out that this machine had processed natural PA6 for 4 months which could also damage the screw. What are your experiences with this kind of issue? Is it possible to polish the screw and barrel and then coat it with something to resist the corrosive environment?

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Thank you for your input.
Rojan

[JayDub]

I’m not familiar with EVA, but your screw looks fine to me. Clean it with a brass brush. I don’t think EVA should be corrosive and nylon certainly isn’t (although glass-filled grades are obviously abrasive and will cause screw wear). Your screw appears to have some sort of mixing geometry, which may be adding shear and contributing to the black speck problem. If you have a standard square-pitch screw, use that instead. Otherwise, try lowering your back pressure and/or screw rpm on recovery.

[rickbatey]

The screw looks pretty good to me as well. It just needs to be properly cleaned. I also see the secondary screw flight so
I suspect it's maybe a barrier screw so it requires a specific barrel heat profile to work its best. You really should purge the barrel down with something more heat stable than EVA. Never ran it though I've used some colors made with it that showed low heat preference and residence time issues.
Rick

[Rojan]

Thank you for replies. Yes, it is a barrier screw. What methods do you recommend to clean it. My maintenance department do not do this so I'm asking. Do you use something else after brass brush?

[rickbatey]

No but the copper gauze is better and faster. Clean the barrel and all parts before reassembly.
Rick.