Eutectic cores 1

[bikswahla]

Hello guys I am not sure if any body can help me in this, I am looking for material like wax which melts at 300 deg F and can be reuse again and again. I want use it is for making hollow silicone rubber parts, my idea is to mold the hot molten wax in the required shape and wrap silicone rubber preps around the wax, cure the rubber at 250 deg F, drill some holes in the molded part and drain the wax by heating the part in the oven. Any ideas guys where can I find high temp wax ????

 

[btrueblood]

That's getting pretty hot for waxes. Keeping the wax from oxidizing will be a problem. Dunno how soft the wax might get either, as your cure temp and core-removal temp are pretty close. Here's some possibilities

http://www.frankbross.com/sww.htm

A better idea might be to find base metal casting alloys (indium, bismuth, tin, etc. mixtures) with melt points in the right range. Same problem might occur with softening as with wax.

see

http://www.mcmaster.com/#catalog/116/3600/=7gzyfs

A possible alternative, if the core needs no real strength, is a salt and sand mixture, formed and baked into a solid "cake". After molding, you would squirt water thru the hole you drilled, and slowly dissolve/wash out the core.

 

[bikswahla]

thanks trueblood

Well I had tried tin bismuth alloy but it leave the residue inside the part which may cause problems, I had also tried ceramic sand mixture which still left residue after washing, currently I am trying draw salt ( mixture of potassium nitrate and soadium nitrites) which works good but my part is not cured from inside

 

[btrueblood]

Not clear on what the problem with the draw salt is - is the salt somehow interfering with the cure? Or is the salt contracting and leaving voids? The latter problem might be counteracted by making sure the core is heated above 100 C to drive off all the water from the salts before laying up the rubber.

Is the residue left by the first two processes due to adhesion of the rubber to a relatively porous (sand cast?) core, and could it be improved by a coating of wax to fill pores in the core materials.

 

[bikswahla]

Well with the salt I think you may be right it is leaving a trail of moisture on the surface and when it comes in contact with rubber it wont cure, I am trying to post cure the cores at 100 F for 10-15 min and then lay rubber and cure the part

For first two processes I used mold release agents applied to the cores but still left the residue, I can try a coat of wax what type of wax I should try.

 

[btrueblood]

No, the salt will hold water (as hydrated salt) all the way to 212 F (100 C). You need to bake that water out, or it will come out during your cure phase as steam, and make all kinds of trouble. After the cores are baked at 225 or so, you need to then immediately place them into a sealed, dry container so they won't absorb water as they cool. Then, limit the time you expose them to ambient air and humidity as you wrap them (i.e. don't pull them out the dry box until you are ready to use them).

For the sand or metal core, my thought was to use any old wax, sealing wax from a stationery store might be the best bet. But the high temp waxes from the site I gave you would be my best guess at something that would work for your temperatures. The idea would be to try and seal the pores in the cores, and get a smoother surface. Even though the wax will soften and melt, it will still be working as a filler as the parts heat up. I'd hope.

 

[bikswahla]

Excellent buddy, will try the ideas

 

[btrueblood]

Good luck, biks. Holler back and let me know how it works out.

 

[Compositepro]

Have you considered using liquid rubber and roto-molding?

 

[bikswahla]

I am not sure what you are saying?

 

[btrueblood]

bikswahla,

His idea is to pour liquid silicone 2-part rubber (room temperature vulcanizing or RTV) into an open mold that forms the outer contour of your part. Then close the mold, and place into/onto a machine that continuously rotates (tumbles, really) the part around at least two axes as the part cures. Lots of polyethylene plastic hollow structures, tanks, etc. are made this way, using heated molds. The problem is, there is no way to put compression on the inside surface during curing. Well, there is, but not the 1,000 psi or so that is typical in compression molding for rubber.

Compositepro, I think (ok, am guessing that biks is using a formulation that has a reinforcement cloth imbedded in the rubber, requiring a layup procedure, e.g. the way hose & gloves are made. Or, just needs the higher tensile properties that heat+pressure cured silicone can provide vs. the RTV compounds.

 

[bikswahla]

Yeah I got this, but my preps have to have fabric for reinforce and also most the silicone I will be using is FR