Remove air bubbles

Hello everyone I am a hobby machinest I do alot of metal work at home I want to melt HDPE into a solid round cylinder for machining parts
I have tried melting milk bottles cut up small in a tin and put it in a toaster oven adding a bit at a time in between adding I ram it down but I got bubles in the middle of it this made it not good
my question is how can I make a cylinder of solid plastic eg: 65mm dia and 100mm long

Thank You All


Good question. Getting rid of voids in larger pieces isn’t always easy.
I’m not sure the toaster melt and stuff it in a form approach will get you there. The people on youtube that appear to make solid parts do a lot of hand manipulation (folding, twisting, etc.) of the hot plastic during the melt. Note in a regular toaster the temperature of the plastic is basically unknown. The surface flakes see the hot elements and are, well, toasted (also releasing bad fumes). The rest is at some other temperature. The approach using a sandwich grill/press or the air fryers is probably better controlled.
I took a shot at making some stock material in this long thread . Most of it was using a DIY injection machine with the design and approach evolving over time, but if you look at the March 22 post I heated an aluminum tube and compressed some HDPE into a cylinder. Today, I still could not guarantee a part without some voids or bubbles. In general I heat the plastic longer and apply lots of compaction presses during heating and apply pressure (clamp) during cooldown.
Good luck and post your results.

Did you see this thread? Similar configuration. I don’t know what design @dasjannis settled on.

@s2019 Thanks! Yes you’re right, I might need to beef up the bottom rails. But I think I’ll go side-by-side again, I’d like to find a thick flat plate to put each side and leave a space in the middle so I can fit long things vertically into the press. Maybe one day I’ll add an adaptation with a heated tube and plunger running down through the middle so I can use it to make an injector…

Some serious steel there. With a 20T jack though, you may bend the two angle sections on the bottom. If you have more of the rectangular tube (or can go borrow some more from the bridge you got the rest from), I would lay it along the bottom to distribute the load if needed.

That will be a serious press.

@s2019 yea I’m using a baby version of that jack… definitely will look to upgrade. Hopefully the pic will upload… you’ll see I’m not great at welding but it’ll hold 🙂

I have not done much press work. I would say it depends on the size/shape of the molds you are planning. If you upgrade, just make sure the rest or your structure can handle the higher the loads. If you have not started building, perhaps one of these 20 ton Chinese shop presses may be a cheap way of getting the basics in place. Not sure if they are available where you are. The other option could be to add a bunch of large C-clamps or clamping bolts. They can exert a lot of force.

From what I’ve been reading it seems that the main way to reduce air bubbles, aside from having the plastic clean, dry, well melted and mixed; is of course to use enormous pressure. The press I am building is currently only set up with a 2 ton jack. I am wondering if anyone can share their experiences about how much pressure is necessary? Is it worth upgrading to a 10 or even 20 ton press?

@s2019 It’s a great idea, air fryers come up cheap secondhand here in the uk because they are basically not very good for cooking as the capacity is usually quite small. For normal cooking it’s easier to bake in the oven.
I’m on the lookout for one and will definitely test the idea as soon as I can. It could be ideal for my purposes as I can put the heater directly on top of my mould and heat the plastic in it.

The air fryers have been advertised a lot lately. Not sure what they call them in the rest of the world. Here are some examples . I have not bought one yet but they look like they would be useful either to melt or to preheat for injection.

@s2019 I like the idea of an air fryer, any threads or links on this?

@ppboys very useful thanks

we had this recently for ‘SEBS’ (transparent, Silicon alternative) to evaluate the v3 injector for being suitable. Turns out this material is fantastic to test your procedure in general btw. The user needed 99% air free and it took me a day to figure out that :

– plastic flakes/pellets has to be small as possible
– barrel temperature should be around 20% above melting temperature
– you have to fill up very slowly the barrel, giving the plastic time to melt and space to release the air among the flakes.
– don’t plunge during the filling process, you are just pumping air down …

– plunge very slowly, remaining air seems to have a slight chance to escape
– more clearance between plunger and barrel seems to help too
– an additional vent on the top of the mold will help to release the air during filling. it’s more controlled that way than letting air finding it’s own way
an auger based feed system might change a lot since the filling alone took 20 mins. the bubbles in the picture below came from wrong nozzle/mold setup.

I am also interested in this subject, I am currently aiming to produce cylindrical HDPE billets or blanks, around 150mm diameter and 60mm deep. This is fairly ambitious and bubbles will probably be the main problem, particularly as I hope eventually to turn the blanks on a lathe.

I have read that making sure that the plastic is extremely dry is important, as any trace of moisture will lead to bubbles. However, the voids caused by shrinkage on cooling seem to be more difficult to address.

I am hoping to make these billets in cylindrical steel moulds made from sections of old gas bottles (the smaller diameter type such as for welding gases). The base will be attatched by bolts and the lid will slide inside the cylinder to compress the plastic. My current plan is to melt the HDPE in the open mould, in an oven with the upgraded thermocouple as used on the PP compression machine. Once melted I will remove it from the oven, insert the lid and transfer the mould to my press for compression. I will leave it under pressure to cool.

I have been thinking that it might help to reduce bubbles if I were to add an in-line spring to the lid of the mould, so that pressure can be maintained by the press as the plastic shrinks. The press compresses the spring with the plastic, but the spring should then expand as the plastic shrinks. I think the ideal type of spring would be the type used in bike frame suspension, as they are widely available as scrap, and already come with a shaft and fittings which can be easily adapted.
Sadly I am working alone on this project so it will be some time before I can share any pics.