HDPE sticking? copied from north-america post

hello! i'm at a workspace in NC that's a self-starter run by one lady (and myself as a temp assistant) and we're both learning as we go. We're having issues with melting shredded HDPE and it sticking to our mold. The mold is rectangular, 14.3 x 59.6 in. and 0.75 in. thickness, and it's made of aluminum. We've been melting the plastic at 360 F for around 35 min and then letting it cool over at least 5 hours. we're testing apel mold release lubricant today, but if anyone has better advice i'd be so glad to hear about alternatives!

We do injection molding in aluminum with HDPE. With way smaller parts, on a pneumatic desktop molder. We use MacLube 1725E, it's been working out for us great so far.

7:35 PM

I assume you have draft angles modelled into the mold?

we don't, we have regular 90 degree angles in our mold

8:12 PM

thank you for the MacLube suggestion though! we're making planks for a bench, and so the larger mold is required for our work

Any chance this is in CLT?

yes!

i'd reccomend this https://www.amazon.com/CRC-Silicone-Mold-Release/dp/B0013IZSDM?th=1

CRC Silicone Mold Release, 11.5 Wt Oz, Non-Staining, Non-Corrosive,...

CRC General Purpose Silicone Mold Release is suitable for most applications where post-mold painting is not required. This product has a medium duty silicone content (3-1/2 percent) and is a rapid dry, anti-stick mold release agent suitable for cold and hot molds. CRC General Purpose Silicone Mol...

annadorant

hello! i'm at a workspace in NC that's a self-starter run by one lady (and myself as a temp assistant) and we're both learning as we go. We're having issues with melting shredded HDPE and it sticking to our mold. The mold is rectangular, 14.3 x 59.6 in. and 0.75 in. thickness, and it's made of aluminum. We've been melting the plastic at 360 F for around 35 min and then letting it cool over at least 5 hours. we're testing apel mold release lubricant today, but if anyone has better advice i'd be so glad to hear about alternatives!

Is the mold smooth or the surface is rough? Smoothness helps in reducing sticking tendencies. Also lower temperatures can help, especially if you are melting injection type HDPE (it becomes very thin compared to blow molding one). A releasing agent is useful in every case, I'm using various kind of silicone oils or grease, some spray (commercial silicone lubricant for plastic parts) but also silicone grease is ok, applied with a rag, and it is cheaper.

1

thanks for all the suggestions! We'll definitely give this a try

Draft angles make a huge difference for us. I mold parts with around that thicknesss. We have one part that is basically a long stick with ribs on the sides for better grip. Those small ribs don't have a draft angle, and cause the entire part to stick. I need to pry it off by the runners with a screwdriver that's driven underneath the plastic. It scores the mold, but who cares - we can make more, and it only marks the runners so no cosmetics issue. If possible, I'd recommend a 2 degree draft angle at least

Maalus

Draft angles make a huge difference for us. I mold parts with around that thicknesss. We have one part that is basically a long stick with ribs on the sides for better grip. Those small ribs don't have a draft angle, and cause the entire part to stick. I need to pry it off by the runners with a screwdriver that's driven underneath the plastic. It scores the mold, but who cares - we can make more, and it only marks the runners so no cosmetics issue. If possible, I'd recommend a 2 degree draft angle at least

If I'm understanding well, the part is a simple sheet, so no draft angle is needed.

Davide->PreciousPlasticRomagna

If I'm understanding well, the part is a simple sheet, so no draft angle is needed.

Given a simple, polished mold and careful temperature control, do you reckon HDPE might be pressed without recourse to a mold release? At ambient temperatures, HDPE is famously hard to adhere to- but when heated, it seems to not enjoy that quality to the same extent- is this (mostly) a sign of overheating? To what degree might sticking be mitigated by adhering (heh!) to certain melting temperatures?

Christopher - Unmake Plastic

Given a simple, polished mold and careful temperature control, do you reckon HDPE might be pressed without recourse to a mold release? At ambient temperatures, HDPE is famously hard to adhere to- but when heated, it seems to not enjoy that quality to the same extent- is this (mostly) a sign of overheating? To what degree might sticking be mitigated by adhering (heh!) to certain melting temperatures?

From my experience, temperatures have to be kept as low as possible, not only to reduce sticking. The rule, at least for injection molding is to keep temperatures as low as possible and inject fast. I try to keep the temperatures of plastic and molds as low as possible and I can inject fast enough at least for small parts, and I don't use a lot of mold release. I use it rarely (something like a small spray for 1000 parts) for the seahorse keyring mold, and more often on the vase mold, but we are speaking of a mold wirh 1° taper for the inner and straight on the external part. If the mold is cold enough to freeze the plastic surface in a short time, you have no or low sticking issues. That's why you need to inject fast. If you can't inject fast enough, molds and/or plastic have to be hotter, to avoid the plastic freezing too early during the process. That smaller ∆t between mold and plastic temperature is what helps plastic to "bond together " and stick.

1

thanks so much for everyone's help! sticking issues have gotten a lot better with lower temperatures. hoping we can resolve our warping and air bubble issues with lower temps as well

annadorant

thanks so much for everyone's help! sticking issues have gotten a lot better with lower temperatures. hoping we can resolve our warping and air bubble issues with lower temps as well

Good to hear! Hey- don't forget to post some pics of your press and moulds.

unfortunately, my boss doesn't want these pictures circulated on here. i disagree with her, seeing as PP is open-source and a community for sharing ideas and helping, but it's not up to me to disrespect her wishes

1

4:10 PM

but what i can say is the mould is 4 t-shirt presses all pushed together, heating both the top and bottom in order to create one large 57 x 14 inch sheet. I wouldn't advise this method personally (for anyone looking for a new mould setup/to get started)

2

annadorant

hello! i'm at a workspace in NC that's a self-starter run by one lady (and myself as a temp assistant) and we're both learning as we go. We're having issues with melting shredded HDPE and it sticking to our mold. The mold is rectangular, 14.3 x 59.6 in. and 0.75 in. thickness, and it's made of aluminum. We've been melting the plastic at 360 F for around 35 min and then letting it cool over at least 5 hours. we're testing apel mold release lubricant today, but if anyone has better advice i'd be so glad to hear about alternatives!

https://www.youtube.com/watch?v=xC7FJnk3Mik https://www.youtube.com/watch?v=uGc6EnyaIlQ

Sam Wechsler

3D Print HDPE to Cold PP Sheet - Best Method

Sam Wechsler

3D Printing with HDPE - Successful Print Using Cold HDPE Build Plate