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HDPE compression molding

This topic contains 7 replies, has 4 voices, and was last updated by  Klaus 1 year ago.

inge kvivik verkstednord

HDPE compression molding

11/08/2018 at 15:14

Were experimenting with compression molding HDPE at the moment, and struggle to find the right temp and time in oven.

Seems it is either too hot (220-260C) and stick to the molds, or not melting/sticking together properly, even if it is in oven for 2-3 hours, see photo

Thankful for any thoughts, peps 🙂

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29/08/2018 at 09:35

From your picture, it seems the button part melted alright but the top didn’t.

Can you show the oven heating coil position?

How is it that you can’t control the temperature, no PID CONTROLLER?

29/08/2018 at 10:28

Hello @verkstednord !

Seems like your temperature is VERY high !
Melting point for HDPE is between 115°C & 135°C.

Do you know how you HDPE was sourced?
HDPE is often used to package toxic  liquids (but in order for the HDPE to resist the toxic liquids, differents adjuvants are added to the HDPE, changing it’s chemical & melting properties)

as @dplasto mentions, the bottom part of your product seems to have melted properly, could you send us a photo of the inside of your oven?

Peace !

31/08/2018 at 15:00

@dplasto and @imuh much thanks for the reply.

I apologize for not explaining very accurate. Relevant to know is that the item is melted upside down, so it is the bottom of it that has not melted well. Further, the mold is placed directly over the heating coils, so it does not make very much sense to me. (Sorry, havent got a picture of oven here but will post one later, it is a weld rod oven, same specs like this one but bigger than the one in the foto: https://www.electroheat.com/product/large-re-drying-rod-oven-edo400/ ).

Furthermore, we do have a PID/temp controller. I have experimented with different temperatures from 130C and upwards, but it does not melt until we are well over normal melt temps for HDPE, like 195C. My best results is on 200C for 2 hours – then it melts like object in the pic, usually.  Going any higher the mold start to stick very bad.

The HDPE is mostly sorced from semi-transparent windshield washer fluid cans.

01/09/2018 at 20:04

That oven must have its heating elements at the top only and that is why the top of the plastic melts while the bottom doesn’t.

Try melting a flat layer of plastic to see how that plays.

I don’t think the oven is suitable for your purpose though, the heat is not surrounding the plastic mould, thats why it melts unevenly.

01/09/2018 at 21:20

The pic of the oven is not accurate, check scetch. So the melt closest to the heat coils is the problematic part. Anyway, Ive even tried putting the mold upside down. to even out any temp difference. Its a bit of a puzzle.

I do not think it is the oven. But I do have a lower compartment where I might be able to place the mold with heat coils both under and over, should def try that.

02/09/2018 at 15:59

Those stralght lines on which the electrodes are kept, is that what you call the heat coils?

Do they glow when the oven lights up?

IIf the do, then you can try the lower portion you spoke about.

02/09/2018 at 17:47

Did you verify the temperature with an external thermometre? Just to be sure….

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