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Old 01-21-2009, 09:50 PM
yawanur yawanur is offline
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Thumbs Up Melting/forming HDPE

So I really want to work with plastics, cause they'd be super good for building all the random bullshit I make. HDPE is particularly intriguing to me because it's free [milk jugs], food safe, relatively strong [+ high density], and slippery! No, I am not making sex toys you sick bastard. The first thing I'm thinking of making is an ipod case, but I'd really like to just mess around with it. It's melting point is something like 266 [looked it up yesterday, I think that's it...], so I was thinking if I can get it to that temp I should be able to consolidate a bunch of scraps cut up from milk jugs into a big molten hunk of plastic.

Questions- Is HDPE one of those plastics that can only be remelted/formed a limited number of times before degrading and shitting out? I don't know the scientific proper way to say this.
What would be the best way to get just to the melting point and not much hotter, so as to avoid burning? Possibly a heat gun? Boiling water in a pressurized container [I could probably figure out the chem to find the pressure that makes water boil at the right temp if I needed to, or just ask a junior in chem, or ask lab tips...I'd have to ask my old chem teacher about pressurizing the container]?
Do you think I could get it to a clay-like consistency so I could do basic molding before it cools, then machine it to the final product?
Should I be looking into other plastics instead?
Is plastic welding cheap/easy enough for me to consider?
What is your favorite color?

Thanks!
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Old 01-21-2009, 10:42 PM
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Default Re: Melting/forming HDPE

My favorite color is blue. That is my contribution to the thread.
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  #3  
Old 01-26-2009, 02:01 PM
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Default Re: Melting/forming HDPE

HDPE is a thermo-softening plastic, so yes, it can be melted and reformed many times (although exactly how many times I'm not sure). It probably won't 'char' until a much higher temperature than the melting point.

I'd use a household oven to get it that hot (most go to 300), and I'd do it outside too. You might be able to get an old oven that someone's replacing for very cheap or free. I'm not sure what I would use to make the crucible to melt it in out of - if you can get cheap metal containers that would be best (old tins might work so long as they are steel rather than aluminium, which could eventually deform due to the repeated exposure to high temperates (this is called 'creep')).

I don't know if the properties of the HDPE would change if you varied the heat or cooling time, this is something you're better off talking to your chemistry or technology teacher about.

Another problem you might have is what to use as a mold - most products made from HDPE use incredibly well made machined metal molds with an incredibly smooth surface to stop the plastic sticking. Unless you've got access to a good machine shop you won't be able to make decent molds that can deal with completely molten plastic. If you do, you might want to look up information on 'hobby injection moulding' for plans on how to make your own devices.

An easier way to make things is through vacuum or sheet forming, which only requires the plastic to be soft and tacky. The plastic needs to be in a thin sheet (3-10mm thick), you might be able to do this by melting some in an oven and pouring it out to cool on an old baking tray. Then it's heated uniformly until it's soft - you can do this easily at home with heating elements out of a toaster, or if you're careful in an oven - and forced onto a mold which is the negative of the shape that you want to make.

You can force it onto the mold either by applying a vacuum through a grille to 'suck' the plastic down onto it, or by covering it in wet heavy sand with tin foil to separate, or if the mold is flat by simply placing a weight onto it. Because the plastic doesn't get very hot, your molds can be made from wood, plastic or even fired clay or fibreglass.

The problem with making things in this way is that it's difficult to get a lot of fine detail, that the molds have to be quite 'shallow' (3-4 inches high at most unles you have industrial equipment) and the molds need to have draft angles (as in, if you look at the mold side-on, all the vertical lines must be angled slightly inwards by about 3 degrees, so that the mold can be released from the plastic).

You could make an ipod case quite easily like this. You'd have to make it in two halves and glue it together though.

http://en.wikipedia.org/wiki/Vacuum_forming

Here's a good video on the method (although his molds are really crap and the results aren't as good as they could be because of that)
http://uk.youtube.com/watch?v=yhajk_...eature=related
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Old 01-26-2009, 04:51 PM
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Default Re: Melting/forming HDPE

http://www.youtube.com/watch?v=VWodZ...eature=related

Wow. DIY tail and head lamps. Do want.
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Old 01-26-2009, 10:06 PM
yawanur yawanur is offline
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Thumbs Up Re: Melting/forming HDPE

Thank you very much for all of that, I know what track to go on now. I'll start hunting free stuff on craigslist for stoves immediately! Vacuum forming seems to be the way to go for my purposes, as I have no way to make high quality molds. This opens tons of oppurtunity for making protective cases; 10mm is almost 1/2", which is plenty thick for protecting electronics and such. I may even vacuum form a case for my glass piece. 6mm sheet over each side of the piece, then just cut out two halves leaving a small flange and hinge them together with wire. Thanks again!

Note- I suppose I'll announce it here, even though this is a DIY thread- I got a job working at an autobody shop and I had my first day today. Sanded me some bumpers for painting, good stuff.
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Old 01-28-2009, 01:32 AM
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Thumbs Up Re: Melting/forming HDPE

Test results:



So first things first this was impossible to get out of my stainless steel bowl "crucible". That shit was stuck in there incredibly tight, and I thought I'd never get it out. Then, nature saved my ass. I remembered ice cracking boulders in half from camp, and I cracked that HDPE right out of there with just one freeze thaw cycle and a forceful slam onto my concrete basement floor.

I was trying to get it to melt into a more uniform, level surface and become completely liquid, but it started charring. I cut the hunk up to show it's solid and to get rid of some burn marks.

This worked out better than expected. I'm thinking to make sheets for vacuum forming, I'll lay out a bunch of the plastic shreds on a cookie sheet and hit it with a heat gun. They probably won't be even sheets with a smooth surface, but for protective cases it won't really matter.

Once I have my sheets, I will follow the basic procedure of vacuum forming from that video, wrapping my objects in aluminum foil first to avoid permanently encasing my ipod in hard plastic.

I was also thinking if I had some sort of neutral heat resistant play-dough I could cover the object with that, put on a layer of plastic and heat. But I don't have that clay,so that wouldn't really work. Fuck that one, forget I mentioned it.

So for the actual cases- This plastic is about 3/4" thick and rock fucking solid, so I'm thinking a 1/4" case for a camera or my little spoon would be a good balance between strong and slim. I plan on charging kids for custom plastic cases for their pieces if this works well, sooooooo many kids would go for that. And I'll make it seem like it's a really difficult process that takes hours and hours but I really like doing it, so that's why my price is totally cheap and their getting a good deal. Haha, maybe just for kids who are dicks.


Questions-
Anybody have a better idea to get the plastic out of the tin besides ice? I'm not too confident ice will work on a 12"X12" sheet 1/4" thick in a flat bottomed pan. Maybe aluminum foil? Wax paper? Grease? It has to resist temps up to maybe 350
And what for the object, will foil work to protect stuff from the soft plastic?

Anyways, thanks for the help, I'll post more results if this works!
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Old 01-28-2009, 04:16 PM
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Default Re: Melting/forming HDPE

A bit of aluminium foil won't stop the heat transferring to the object, it's too thin. I don't know what kind of temperature the plastic has to be at for vacuum forming - every time I've done it, I used ABS and it has been in really thin sheets, it was so cool that you could vacuum form your own hand without too much discomfort. The HDPE will probably have to be a bit hotter and it'll be thicker too, so basically, I don't really know. You could make a copy of your ipod or camera or whatever in clay or wood though.

As for getting it out of the baking tray, I'd line it with really thick aluminium foil or grease paper, because it's strong enough not to tear easily when you peel it off the plastic. I'd imagine that a baking tray is flexible enough that you can just 'pop' the plastic out if you twist it.
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Old 01-28-2009, 04:41 PM
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Default Re: Melting/forming HDPE

The aluminum foil around the object was mainly to stop the plastic from sticking to the object; I suppose heat could be a problem too and I'll have to run tests on cheap objects before I try it on electronics that could possibly be damaged by heat.

Is grease paper the same as wax paper? I have that, and some restaurant grade super thick foil as well, I'll give both a try. Thanks again!
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Old 01-28-2009, 04:51 PM
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Default Re: Melting/forming HDPE

No probs, show us how it works out, I'm pretty interested in doing stuff like this myself.
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Old 02-02-2009, 02:22 AM
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Thumbs Up Re: Melting/forming HDPE

I've done two more tests- I melted a crushed milk jug on a sheet of aluminum just to see what would happen- it became relatively flat and quite sturdy. I'm not sure what temp I did it at though, I thought it was 300F but I just did another test flattening one jug's worth of scraps into a sheet roughly the size of a sheet of paper [8.5"X11"] and the layers didn't fully fuse in all the spots. I cooked the sheet for 40 min, no idea how long I did the other one. Seems like if the temp stays in the low 300s the plastic is safe from charring, but I'll have to toy around with that more to be sure. I want to get it to the highest temp possible without charring so everything is fully fused. I've noticed that even though the melting temp is 266, it seems impossible to get this plastic into a liquid, so I'll have to settle for making it as soft as possible.

The sheet is 1-3 layers of jug plastic thick, although the jug plastic tends to be thicker towards the bottom of the jug, so a "layer of jug plastic" is not a definite measurement. The sheet is quite flexible but also and strong, and easily maintains it's shape [no sag if held at either end]. It could be easily vacuum molded if there weren't holes in a few spots, so I'm thinking of melting more plastic over it or just folding it in half and re melting it.

Oh yeah, to make it I just cut up the jug into relatively flat pieces, arranged them like nachos in a piece of folded-in-half aluminum foil, sandwiched that between 2 cookie sheets, put it in the oven, then put a cast iron skillet on top of it all. 40 minutes @ 300.

I'll post pics if you really want them, but seriously, this took 10 minutes to set up and 40 minutes to bake, so give it a shot.

What are you interested in making with plastic?

Last edited by yawanur; 02-02-2009 at 02:25 AM.
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Old 02-02-2009, 02:29 AM
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Default Re: Melting/forming HDPE

Try talcum powder as a releasing agent. I would say to lightly oil the metal surfaces, but you would need to figure out which oils wouldn't dissolve into the plastic.

I've been plenty interested in forming thermoplastics as well for display chasses. How flexible are the final products?
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Old 02-02-2009, 02:45 AM
yawanur yawanur is offline
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Default Re: Melting/forming HDPE

I haven't had any sticking problems so far with aluminum foil, but I'll keep that in mind if any issues pop up.

Do you guys know exactly what happens to Al if you heat it too many times? If I just use the same piece of foil over and over again [it's super thick, heavy duty foil] and deforming is not really a problem, so 'creep' won't matter, right?

AP2K- Chasses is plural of chassis, like a car chassis? I've found this sheet to be a great balance between flexibility and strength; I can bend it over on itself to form a circle, but I probably wouldn't go any further as the layers aren't fused too well and it might crack. But HDPE also tends to bend instead of breaking, so I'm not sure. The flexibility is kind of like that really thin aluminum flashing, but really it's incredibly easy to make, try it for yourself.
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Old 02-04-2009, 04:25 AM
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Default Re: Melting/forming HDPE

Quote:
Originally Posted by Runaway_Stapler View Post
I haven't had any sticking problems so far with aluminum foil, but I'll keep that in mind if any issues pop up.

Do you guys know exactly what happens to Al if you heat it too many times? If I just use the same piece of foil over and over again [it's super thick, heavy duty foil] and deforming is not really a problem, so 'creep' won't matter, right?

AP2K- Chasses is plural of chassis, like a car chassis? I've found this sheet to be a great balance between flexibility and strength; I can bend it over on itself to form a circle, but I probably wouldn't go any further as the layers aren't fused too well and it might crack. But HDPE also tends to bend instead of breaking, so I'm not sure. The flexibility is kind of like that really thin aluminum flashing, but really it's incredibly easy to make, try it for yourself.
Are you using the aluminum foil as a mold? Creep will happen due to physically working the metal, but heat cycling shouldn't do anything as far as I understand it.

Chasses, I think is the correct work for the plural of chassis. I know that HDPE is extremely tough stuff, but I was checking up on how to make custom chasses for dash clusters for cars a while back, so they need to be rigid as well. If I were to be able to make custom ones I wouldn't be forced to design around the stock plastic chassis.
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Old 02-05-2009, 03:55 AM
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Default Re: Melting/forming HDPE

Quote:
Originally Posted by ballin' View Post
could this method be used to make like champane glass holders? for my car bro, bitches would love that shit.
www.reprap.org

Design it and then build it. HDPE is a very common thermoplastic that would go great with any reprap project.
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Old 02-06-2009, 03:35 AM
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Quote:
Originally Posted by Vargus View Post
Are you using the aluminum foil as a mold? Creep will happen due to physically working the metal, but heat cycling shouldn't do anything as far as I understand it.

Chasses, I think is the correct work for the plural of chassis. I know that HDPE is extremely tough stuff, but I was checking up on how to make custom chasses for dash clusters for cars a while back, so they need to be rigid as well. If I were to be able to make custom ones I wouldn't be forced to design around the stock plastic chassis.
big post just got erased, I'll come back to this later.....
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Old 03-09-2009, 06:48 AM
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Default Re: Melting/forming HDPE

So how are things going?

I just packed a Thermold AR-15 mag full of Play-Doh to try to get the inside featuresimg]https://www.mainemilitary.com/productcart/pc/catalog/therm420.jpg[/img]

I will be looking at shrinkage as it dries. Unfortunately since it is only open at two ends I worry that drying time will be a very long time. I might employ the oven at low temperature to speed up drying time. Basically what I'm trying to do is make a mold for my own mags because I want a lot of them but don't have much money. (Not to mention their outrageous pricings nowadays.)

Once I get the molds figured out I will see if I can cobble up a mold injector with a heater in it, probably out of plumbingware.

EDIT: I think that it would be easier to carve large sprues into the mold, pour in molten plastic into the preheated mold, and keep it heated and agitated for some time for the plastic to reach all of the small details. Perhaps even better would be to carve the gates so that the mold fills from the bottom and have a large sprue at the top to allow for air to escape.
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Last edited by Vargus; 03-09-2009 at 07:41 AM.
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Old 03-09-2009, 01:47 PM
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Default Re: Melting/forming HDPE

I've been following the progress of RepRap since forever. Darwin, Mendele, all that shit. They're still extremely unrefined . I haven't seen much progress in the last 6 or so months at all. I had high hopes a head full of dreams.
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Old 03-09-2009, 05:16 PM
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Default Re: Melting/forming HDPE

So, between now and when I last posted the bottom and top of the playdoh has begun to harden on the surface. It is still slightly squishy at the surface, but definitely squishy in the middle. Interestingly, it has actually expanded, as opposed to shrinking. Expandage has been about 1/16" over ~5", which would be about 1.25%.

T+7 hours, play-doh still squishy, but surface is dry.
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Old 03-11-2009, 03:41 AM
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Default Re: Melting/forming HDPE

Do a search for 'Rotational molding'.

Thats when they make a big steel mold & throw in (a measured amount of) HDPE pellets & start to rotate the mold & then start up a bunch of gas burners under it.
The HDPE melts & forms a layer on the inside of the mold.

Have a look at HDPE row boats or plastic water tanks.

With the boats they actually have two molds facing each other & make two boats at the same time.

You will probably have to make your HDPE into pellets or such, I'd recommend putting them thru a large shredder (as I have one that would do the job!)

Theres probably a chance of making some money by using free HDPE bottles & making a mold of something yourself.
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Old 03-11-2009, 07:28 PM
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Thumbs Up Re: Melting/forming HDPE

Quote:
Originally Posted by Vargus View Post
Are you using the aluminum foil as a mold? Creep will happen due to physically working the metal, but heat cycling shouldn't do anything as far as I understand it.

Chasses, I think is the correct work for the plural of chassis. I know that HDPE is extremely tough stuff, but I was checking up on how to make custom chasses for dash clusters for cars a while back, so they need to be rigid as well. If I were to be able to make custom ones I wouldn't be forced to design around the stock plastic chassis.
The aluminum is just to prevent the plastic from sticking to the object, so I guess I'm good.

And that would be ridiculous on a car. I always figured the dash facade was supported by a couple bars going across the frame, but yeah if it's a sort of frame underneath there you could definitely fabricate something. This stuff is very strong, milk jugs are just paper thin.

Quote:
Originally Posted by Vargus View Post
So how are things going?

I just packed a Thermold AR-15 mag full of Play-Doh to try to get the inside featuresimg]https://www.mainemilitary.com/productcart/pc/catalog/therm420.jpg[/img]

I will be looking at shrinkage as it dries. Unfortunately since it is only open at two ends I worry that drying time will be a very long time. I might employ the oven at low temperature to speed up drying time. Basically what I'm trying to do is make a mold for my own mags because I want a lot of them but don't have much money. (Not to mention their outrageous pricings nowadays.)

Once I get the molds figured out I will see if I can cobble up a mold injector with a heater in it, probably out of plumbingware.

EDIT: I think that it would be easier to carve large sprues into the mold, pour in molten plastic into the preheated mold, and keep it heated and agitated for some time for the plastic to reach all of the small details. Perhaps even better would be to carve the gates so that the mold fills from the bottom and have a large sprue at the top to allow for air to escape.
Are the insides of the mags simple enough that a crude casting will suffice? I assume the actual ammo is held with a clip, so I suppose as long as the mag locks into the rifle nice and solid and the ammo is in an accurate enough position to feed into the receiver, good to go? That's pretty cool; perfect example of something you could sell for a nice profit.

Keep in mind that HDPE isn't quite liquid when it's molten, at least in my experiments I've only gotten it down to a sticky state where things can mold together. Like on that first plastic nugget thing, look at the top. I had that in the oven for over an hour, and it started out as a 95% air bowl of plastic shreds. So if you don't want to get into maybe using a neutral gas in the oven to prevent combustion and stuff, I wouldn't put too much faith in injecting into a mold. Do you think that would work if you got rid of oxygen by running a CO2 tank or something? Then just crank the heat up to 500 on the oven and it should pour right through...

In that case, if you wanted high quality mags, definitely find someone in ceramics and get a more accurate casting with clay that won't shrink and a finer "tooth" for lack of a better word. Does play doh ever turn rock hard?

Eesakiwi- That rotational mold thing is sweet, but I don't think I have the motivation/funds to follow through with something like that. Maybe on a small scale, graft a toaster oven with a coffee can and an old AC motor, but probably not. If we're lucky DIY will like this thread and it will take off, then with all of us combined things will progress quickly. I guess we'll see.
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Old 03-11-2009, 10:45 PM
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Default Re: Melting/forming HDPE

Consider Shredding the shit out of the plastic with whatever means? Grind it down to <1/4" bits and it should melt a lot better.

RDP
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Old 03-12-2009, 03:17 PM
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Default Re: Melting/forming HDPE

Making the interior mold with Play-Doh is massive fail. Shit was dry on the outside which made a moisture seal. It just don't dry like it used to.

I'll try to make the next one out of plaster.

Quote:
Are the insides of the mags simple enough that a crude casting will suffice? I assume the actual ammo is held with a clip, so I suppose as long as the mag locks into the rifle nice and solid and the ammo is in an accurate enough position to feed into the receiver, good to go? That's pretty cool; perfect example of something you could sell for a nice profit.

Keep in mind that HDPE isn't quite liquid when it's molten, at least in my experiments I've only gotten it down to a sticky state where things can mold together. Like on that first plastic nugget thing, look at the top. I had that in the oven for over an hour, and it started out as a 95% air bowl of plastic shreds. So if you don't want to get into maybe using a neutral gas in the oven to prevent combustion and stuff, I wouldn't put too much faith in injecting into a mold. Do you think that would work if you got rid of oxygen by running a CO2 tank or something? Then just crank the heat up to 500 on the oven and it should pour right through...

In that case, if you wanted high quality mags, definitely find someone in ceramics and get a more accurate casting with clay that won't shrink and a finer "tooth" for lack of a better word. Does play doh ever turn rock hard?
The cartridges just push down into the mag. The only times clips are used is for speedloading internal mags and for some types of removable mags.

I should also look into epoxy or thermosetting plastics, perhaps, as opposed to viscous thermoplastics? That is, since I can just go around having to make an injector.

Also, I wouldn't try to heat plastic over its autoignition temperature unless you knew what you were doing. The plastic would most likely break down at those temperatures on its own.
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Old 03-13-2009, 07:16 AM
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Default Re: Melting/forming HDPE

So what sorts of ideas do you guys have for taking a mold of the inside of a plastic container? Play-Doh doesn't work, since they seem to have changed the formula and now instead of water it's some sort of oil that doesn't dry easilly at all. (Water also makes it very nasty. I remember making cups that held water years ago that didn't get soft. )

Another thing that comes to mind is ice. I tried using electrical tape (which is all I have right now) to seal the bottom, but no matter how much I put, it always leaks. I was able to get a small amount of ice to form. I was apprehensive at first because I figured the ice would not break free easilly, but it did and seemed to have a good impression of the ramp inside the mag case. Sealing the thing without any leaks would be my biggest problem here. After I get an ice mold, I can use play-doh or something like it to make the negative, and then use that to cast a positive out of plaster.

Another way would be to use wax, but I wonder if the wax I have would melt at a temperature that is much below the melting point of the plastic that makes the mag. From what I understand, LEOs had trouble with the Thermold mags like this because after sustained fire the top of them started to melt. (I take it that was fairly uncharacteristic)
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Old 01-20-2010, 12:11 AM
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Default Re: Melting/forming HDPE

I've been doing a lot of the same kind of experimenting. It looks like the OP would be satisfied with the products sold here. They've got all sorts of molding and casting products.

http://www.smooth-on.com/

unfortunately they don't have what I'm looking for. I'd like a spray-on plastic - basically something that I can apply to frabric to make it rigid. They do have something like that but it is pretty expensive (tools and materials).

They told me to try their 'shell shock' brush on, and I might, but it sounds difficult to apply an even layer.

I did just sign up so that I could post that, but I'm in no way affiliated with them and have absolutely no exposure to their products beyond that web page so I can't speak intelligently about quality or suitability...
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