Technically PTFE as Teflon (tm) is a trade name and tends to cost a bit
more.

In casting and injection I make primarily aluminum molds. I do some
steel press dies, and other things, but mostly I make aluminum molds.
The cost for low to medium production is very desirable.

I have customers who cast lead, tin, and bismuth. (Tin sticks to
aluminum). I'm playing with some aluminum die casting designs for steel
molds, but that's personal.

I also make a lot of low pressure injection molds for a product called
plastisol. Its a heat activated PVC product that comes as a liquid and
cures as a rubber. Due to its nature it shrinks a great deal when
cooling from a liquid to its semi sold state. Depending on the exact
mold this can result in either drawing air into the core resulting in an
unintentional hollow end product, or it can pull away from the cavity
surfaces resulting in a surface dent in the finished injection.

The general process is to fill the mold with a hand injector that looks
like a gigantic aluminum syringe. Well for most of my customers. There
are machine processes also.

I often design larger body items with an excess capacity runner so the
body cools it can draw from the reservoir this excessive size runner
creates. It still can often result in a hollow from the sprue all the
way thru the core of the body.

The general reason is that aluminum conducts heat away from the
plastisol very quickly. This is desirable for cycle times allowing
demolding rather quickly compared to other materials which may just
build heat making each successive injection take longer than the last.
Yes, secondary cooling is an option as well, but the extra cost is not
"worth-it" for most of my market segment.

My thought is to use a piece of teflon tube inside the runner to reduce
the cooling rate of only the material in the runner. This should allow
more of it to remain liquid (syrup really) and fall inwards from gravity
to prevent just a molten core of flowing and rather than becoming an
undesirable hollow injection.

PTFE is a decent insulator, has a high degree of lubricity, and a
relatively high working temperature. Plastisol runs from about 300-400
depending on the exact formula and working process to get good fills in
a particular mold.

--
Bob La Londe
Proffessional Hack, Hobbyist, Wannabe, Shade Tree, Button Pushing, Not a
real machinist

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