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Recycle HIPS in compression molding (Bubbles)

This topic contains 12 replies, has 4 voices, and was last updated by  Marius 2 years ago.

Marius mariusbr

Recycle HIPS in compression molding (Bubbles)

18/09/2019 at 15:38

Hi everyone, I’m Marius. I currently have a problem with creating High Impact Polystyrene (HIPS) samples without bubbles forming in the polymer structure.

I work at a university project that tries to recycle HIPS from different stuff like laptop cases. In the university I stay, they organized a heated press  and an aluminium moulde to make some tests (pictures following). It is the first time we are working with the machines and the material and so no one has much experience..

The Problem:
Every sample has a lot of bubbles inside and we don’t exactly know why. The material is  clean and the size is about 3×3,5mm. We also dry the material over night at ~70 °C and preheat the moulde to ~ 100 °C. After heating up the moulde, I use some silicone spray to remove the plastic after the compression melting. Then I fill the moulde with the HDPE pellets as close as possible and heat up the open moulde to 220°C – 280°C (best try was around 230°C). After one hour I close the moulde and compress it to 9 Ton, wich is the maximum of the press. Then I continue heating for ~2 hours while holding the pressure. After that, I turn off the heating and hold the pressure over night. When checking in the next day, the plastic is full of air bubbles.

I found some information in the forum and I think in general we have 3 main problems:
1. Air between the pellets after filling
Maybe our pellets are too big and it would help to get some powder? We also tried to melt the material outside the moulde in a separate oven at ~260°C for 5 hours and put it in the moulde afterwards. This also gave us a lot of bubbles and the plastic did also not melt into a homogeneous mass.

2. Gas is created
It can also be, that Gas is created while the procedure, wich forms the bubbles. Therfore, we tried at Temperatures between 210 °C and 280 °C and with/without silicon spray. Like I said, the polymer is also dryed and cleand before. Maybe it is also a problem that the gas can’t escape.

3. Polymer is reducing its size when cooling down
While cooling down the plastic is shrinking down, so we keep the pressure the whole night. But I am not sure if this solves the problem of shrinking completely.

I also tested to increase the pressure stepwise, like +1Ton every 15 mins.

Sadly, none of the ideas I had or found here in the forum helped to get a solid plastic.
Maybe some of you have already more experience in compression melting of HIPS and can help 🙂

I am happy about any idea and help! 🙂
Best wishes, Marius

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18/09/2019 at 22:06

We at @plasticoinfinito use the ps in the compression machine with the hexagonal bowl, put 500 gr of ps inside the mold in the oven for 60 minutes at 200 ° C, then compress for 10 minutes outside the oven and place it in water for another 10 minutes we unmold and cut surpluses.

in some occasions there are bubbles and in other occasions it comes out perfect, you need to put a little more material, when the pressure is made it spills a little and removes the bubbles.

We do not use sandpaper for the final finish.

I hope it helps 😉

19/09/2019 at 14:02

more photos

19/09/2019 at 14:01

Hey y’all !
I totally agree with @sneyk 😀 that’s exactly the same process we have hehe
i just took some photos of some of our PS products, and when transparent PS is used, we see the “air?” bubbles you talk about, but the surface is smooth for us. You should really be sure to be excessive amount of plastic (and have an exit point for that excess) in order for the mould to be totally filled up 🙂
Cheers !

18/09/2019 at 20:09

Have you seen this thread http://onearmy.world/community/forums/topic/polystyrene-transparency/ , especially the post by @imuh ? The image he posted is a lot closer to bubble free, perhaps he can comment on the process details.

25/09/2019 at 19:49

Small Update:
We had definetly too less material in the mould. After opening the mould after 20mins of melting we put more material inside. But with the material we also put air inside, which also creates bubbles.
We made also an Temperature test with the result, that in our setting the mould needs about 20mins to reach the temperature of the heater. So we heat up the mould 30 mins before we fill it up and put more material inside. More tests will follow…

The press is made by PHD Equipamentos para laboratorios:
3kW, max 15 Ton

19/09/2019 at 21:11

One question is how the strength changes with repeated recycling.

Your data will be of interest to the community, hope you can post the methods and results.

20/09/2019 at 00:04

Good luck. While you are getting rid of the bubbles, if your team comes up with a repeatable way to determine internal void volume (perhaps just a careful density measurement) that would be a great step.

19/09/2019 at 21:58

@s2019 great Idea! As soon as we are able to produce a solid sample we can probably start with the tests.

19/09/2019 at 20:27

@s2019 at the moment it is our only mould but for the past we are thinking to add some other materials (like waste of aluminium production or fiber) to the polymer and check how the tensile strenght is changing.

19/09/2019 at 20:14

@mariusbr , The parts you are making look like test coupons. I’m curious what the test plan might be if that is the case.

19/09/2019 at 20:14

@sneyk Do you think it is better to cool down the plastic fast? I thought it would be a lot of stress for the material, but on the other side I heard that iron is cooled down fast to make it harder.
Your bowls are looking great btw!

19/09/2019 at 19:59

Wow thank you for the fast help! Thank you @s2019, I did not know this amazing thread by @gujonon.
And also thanks to @imuh, you did some really nice pieces 🙂
Today I will try to open the mould after 20 mins of compression heating to fill the mould totally. I think you are right and we don´t get the right amount of plastic in the mould..
And also that there is no point of excess for the plastic, maybe I try to drill a hole at the side, but the aluminium moulds is so expensive. Then you could also try to use it with the injection.

Nice help so far! 🙂

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