# melting hdpe



## shew97

What is the best way to melt hdpe?


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## JonM

Using the search function will answer that one. there are several people that are doing it :wave:


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## Rathunter

small chips + metal bowl in [email protected] C.

Pour into mold, apply pressure.

Cut out whatever.


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## parnell

The following thread will answer just about every hdpe question you may have. It is a long read, but we'll worth it. To answer you basic question cut into small pieces and melt at 350 F. http://slingshotforum.com/topic/22723-hdpe-sheet/


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## shew97

Can you use a heat gun


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## parnell

I don't think a heat gun would melt it evenly enough, but I haven't tried it. Give it a go and let us know the results.


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## ash

I prefer to use a sandwich press lined with baking paper. There are details in that big HDPE thread.


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## lexlow

hullooo ash lol, there seems to be an explosion of hdpe  awesome...... I do wonder what the rise of hdpe looks like from the point of view of people that have been working with wood etc . I get the impression some don't see it as something needing as pure a skill. In some ways they would be right though. Working with hdpe is more forgiving, and errors can be adapted easier.


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## Lacumo

+1 on a couple of the thoughts above... A heat gun won't very likely adequately heat HDPE up into a workable state. You're going to need to use an oven or a sandwich press.

HDPE doesn't really "melt", either. It gets soft, pliable and formable but it doesn't ever get liquid or drippy, which is what I picture when I hear the word "melt." I think using a heat gun might get the exact spot of the pile of pieces you're working on ready to go, but it won't get the whole pile ready to be formed into a frame. The result will be too localized to enable you to form up an entire frame.

One other thing that will probably make life a bit easier for you is using the semi-clear/translucent HDPE from gallon milk jugs. Being thinner and non-colored seems to make it a lot easier to work on and form. I've experimented with colored HDPE (in a good variety of different colors) from liquid laundry detergent jugs and my main accomplishment so far is an impressive string of failures and "close but not quite" near-successes. The stuff I've been playing with is a lot thicker than the milk jug stuff and the pigments used to color it seem to make it harder to work on and form. Getting rid of the air pockets has been my #1 ongoing problem and I still haven't entirely solved it.

You should read that HDPE thread in the tutorial section. It's a long, wandering thing but the answers to your next 2 or 3 dozen questions are all buried in there.


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## Rathunter

shew97 said:


> Can you use a heat gun


yes. You can.

It is much easier to use an oven. And it takes somewhere around half the time with half the smell.

You need a bath of fluid for the bowl with HDPE in it. synthetic motor oil should work- the kind that is supposedly very hard to ignite.

If it smokes.... don't use it.

Heat guns apply concentrated high heat. You need not-concentrated low heat.

Hence the oil bath.

basically make a double boiler with oil for the fluid, and the inside a metal bowl with the HDPE chips in it.

you can ruin it by overheating it!

It works, but be careful!

I honestly prefer an alcohol fueled camp stove I made to just about anything except an oven-but mom makes me do it outside. Ovens weigh ~100 pounds or more. Not practical to bring outside.

DO NOT try a propane torch- too hot.


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## Lacumo

That's a real interesting approach. You say that synthetic motor oil "...should work...", which makes me wonder---have you actually done HDPE this way? In all the posts about HDPE in this forum, I haven't seen anybody talk about using a double boiler type vessel with any kind of oil as the liquid between the two pots.

I'm also curious about one other thing. I've never had any smell at all when I've worked with HDPE in my kitchen oven, although I do rinse the HDPE well with clean water before I process it. What kind of smell problems have you had and what processing methods caused you to have those smell problems?


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## Rathunter

I don't always rinse HDPE chips.

Milk heated smells awful.

I have used oil-based double boilers for lots of things. Including melting HDPE(well, about 3 cubic inches of the stuff).

They get a LOT hotter than water, and when you are making, say rocket candy, over a heat source, it puts a layer of somewhat-less-flammable stuff between your heat source and your heated thing. also, it has the side effect of eliminating hot spots.

olive oil, peanut oil, etc work, but are expensive. I think it was synthetic motor oil I used... that or brake fluid... or hydraulic fluid.... I wasn't the one who had the idea- nor was it my gas stove.

TEST the oil at 200 deg C before using it

supposedly mineral oil will go past 250 C without smoking, but I doubt that- it will smoke once it gets over about 180-200 depending on purity.


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## Rathunter

BTW if it can be done in an oven it can be done in either a double boiler on a stove or an oil bath on a stove/any other heat source.

you just have to know your heat source WELL.


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## shew97

Is it better to make a mold or just a block


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## Lacumo

The mold vs. block thing has been discussed pretty extensively in the HDPE tutorial thread. You really should take the time to read it. The feelings on mold vs. block are pretty much that each has its own pluses and its own minuses and in the final analysis, it's probably a toss of the coin between the two. Some prefer the one and some prefer the other.


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## Rathunter

Depends on if you want to make more than 2-3 slingshots of the same design.

To start, a mold actually doesn't take much more time than a block to make- just take a sheet of 1" plywood, cut out your design(i use a scroll saw) so you have a fork made from plywood and a mold made from the same piece, then tack another piece of wood over the hole(it must be removable), then pour molten HDPE into hole, put another sheet of plywood on top, and push really hard on it(I stand on it). Once its cool, remove both plywood sheets and then pop out the frame.

Molds require less HDPE chips to start, but as HDPE is pretty much infinitely reusable....

Blocks are easier to make the molds for- just take a baking pan and pour the stuff into that.


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## Lacumo

I've never had HDPE get molten or pourable. The best mine ever does is getting pliable and formable, but it never gets liquid or pourable. How do you get yours to be liquid and pourable?


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## parnell

Rathunter have you been able to successfully reuse a mold? I haven't tried a specific slingshot mold since everyone else's attempts have resulted in a one time use that they have had to cut to get the frame out. If you have succeeded please let us know your process.


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## Rathunter

Lacumo said:


> I've never had HDPE get molten or pourable. The best mine ever does is getting pliable and formable, but it never gets liquid or pourable. How do you get yours to be liquid and pourable?


It's not exactly molten/pourable like water. it is like very thick dough in some ways. When its molten, you can somewhat pour/push it out of the bowl(like clay) and then put it into the mold, press, and it is swaged into the mold. I do this with clay a TON for making slinging glans.

Also it it is somewhat hotter, it tends to be more pourable and less like a colloid/solid.

If you design the mold as you do would for lead, it isn't too hard to reuse the mold.


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## parnell

You talk of clay and lead, but you haven't answered my question. Have you been able to do it successfully with hdpe?


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## Lacumo

I have to admit I've never experienced HDPE ever being pourable to any degree. I've found that it'll get soft, maybe a bit gooey, formable and workable (as long as it's still hot), but never pourable. When I take a fully heated batch out of the oven, it's soft and workable for an extremely short period of time---until it starts to cool---but it's never been pourable for me. If I tried to pour it, nothing would happen. I'd have to (quickly) scrape it out of the pan in one blob. I've pressed the baking temperature to the point where I've caused a couple batches to get singed and get lightly browned on the top surface too, so it's not like I'm not heating it up enough.


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## ash

I think the technical term we're looking for here is "pics or it didn't happen" :thumbsup:


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## you'llshootyereyeout

I use a toaster oven I picked up from Goodwill ($4) and a parchment paper lined loaf pan to heat HDPE. Also I use a sheet metal lined plywood form/press to make blocks (3/4" X 4" X up to 7"). My form maxes out at about 12oz of chips. A few bar clamps seems to exert enough force to get the job done. I have found that HDPE is limited in the number of times it my be reused. It seems to pick up debris and other contaminates when handling in its melted state resulting on a sort of grainy texture in consequent melts. I have been using a wax block to prepare my form and have had no problems with the HDPE sticking.

I have used a heat gun but only to fuse pieces together. Currently I'm testing a disassembled flat iron for the same purpose.

Good luck

Matthew


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## Rathunter

it may be the wax.

Or it might be oxidation of the plastic.

No good way to test the oxidation theory.


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