Plastic molds

General Composites Discussions Composites Talk
#1

Hello,

I came across a large quantity of polypropylene plastic pellets. I thought to myself that I can attempt to use them for either mold making or casting. I put some of the pellets into the oven and at about 300 degrees F, I had a clear consistent soup of plastic. The only problem was that it was far to thick to pour or brush on anything. The fast cooling of the plastic out of the oven dosent help it either.

Is there any kind of plastic thinner one can add to help the plastic reach a pourable stage, or am I stuck with having to mold or cast parts in the oven?

Thanks for the help!

#2

If you want to use plastic pellets, buy an injection machine.
Brushing melted PP will go nowhere.:smiley:

#3

How about this idea?

Here’s a link to one set up.
http://www.solsylva.com/cnc/rotomolder.shtml
Sure - you’d need to make one of metal - but you could put the entire apparatus in the oven - pellets inside a mold and turn it on. Bring it up to temp while spinning - then after a bit - shut off the oven - still running and when it cools down Voila! A molded part - Some experiment eh? May be easier than compression/injection molding.
Guess nothing comes for free!
Cheers - Jim

#4

Ha thanks, that looks pretty cool; but im looking to make molds with the plastic, not parts. Im just trying to find out if there is some kind of thinner I can add to hot plastic to help it flow better.

#5

don’t put thinner in an oven

#6

maybe try a heat gun to localize the heat after you have done a cook in the oven?

#7

I got it to about 350 degrees F and it started smoke, I guess any hotter in the oven or the heat gun will cause it to burn. Theres still no real change in viscosity with an increase in temp.

I did find this stuff though, I may give it a try. Although I might as well use epoxy resin and glass to make molds if im going to wait for this plastic stuff to cure.

http://www.smooth-on.com/Urethane-Plastic-a/c5_1120_1165/index.html

#8

smooth-on is a good special effects company, not a composite company. There are plenty of distributors out there with much more suitable products. If your doing something in composites most of the time it’s been done before, and there are products for that process. And if you’re doing weird things like sticking injection molding plastic in an oven you’re flat out doing it wrong.

#9

PP does not thin with most solvents. Many TPU’s like Kraton SBS’s will thin out, but I have not had luck with it.
Thermoplastics are just that…heat.
If you want to cast something by using pellets, you will need a compression mold, that is heated to the correct temp. I have 55lbs bags of MANY kinds of plastics, and the only thing I can use it for is making film…plastics without an injection molding system are very boring!!!
Good luck!

Have you tried using silicone?

#10

Thank you for your input, but I make a living by doing “wierd things.” I am an inventor and hold several US Patents. I actually have since found a use for this plastic, and it does involve sticking injection molding plastic in an oven. You may think that it is the wrong way to do things, but somewhere, there is a mind to find a way to make it a right way :slight_smile:

#11

Thanks for the input, I am going to go with silicone for molding, the plastic wont work for my molding needs, but at least I have a use for it!

#12

Are your patents available to look up? What are the numbers? Just a bit curious.

#13

eh, heat is heat as long as you don’t get too high, or have bad control!

#14

if you want a solid shell, you can put the PP pellets between 2 metal plates, with a weight on top, and melt it. If your new platic sheet is good quality, you can then thermoform it around the silicone mold outside. This way the silicone mold will have a harder backing, and might perform better (won’t distort as much, or even handleing/storage of the mold.)

check out Polytek’s website. They have some good advice for silicone/UR molds.

#15

…I hope you are not also using that oven for food too! :eek: