On Fri, 15 Aug 2008 18:16:12 -0700, mpate wrote:

I am building the Gingery plastic injection molding machine right now
and have a few questions for people who have already built them.

The heater block assy is CRS and you are supposed to drill and ream a
0.501 inch bore.
The plunger is CRS and is 0.5 inch rod, which we have measured at 0.5009
inch diameter.
What is a reasonable tolerance for the 0.501 inch bore, so that it will
inject the plastic (pmma and polycarbonate mainly) into the mold and not
leak around the plunger and squirt or shoot out the top?

I used aluminum rather than CRS for the heater block, and 3/4" stainless
for the plunger. Drilled a hole through the block, then bored it out
to size on a lathe. Ended up with a sloppy fit (the bore might be as much
as .001" oversize) but the viscosity of the various common thermoplastics
I've molded with is high enough that leakage around the plunger isn't much
of a problem. Given the small amount of plastic molding I do (a few dozen
parts per year) it's not enough of a problem at present to be worth fixing.

I have the plunger attached to an arbor press rather than to the lever
shown by Gingery. Driven by the same force, your .5" plunger would develop
more than twice as much pressure as my .75" plunger does. Anyhow, at
higher pressure you will see more leakage. When I see leakage, it means
the mold is full or it wasn't hot enough and the plastic congealed before
filling it.

I am concerned about similar metals and very close diameter to bore
clearance and having things gall up when I try to use the parts
together( it was usually aluminum not CRS), but I don't want the hot
plastic leaking around a loose gap.

I don't have much info for you re your other questions -- mold release
spray / nickle plate on aluminum mold / sources of polycarb or pmma
pellets -- since I haven't used any release spray, and have reused
old plastic, cut up on a bandsaw, rather than buying pellets. The
parts I mold are small and relatively easy to remove from the mold.
Some ebay sellers have yellow Acetal/Delrin at about $1/#, and Dupont
Hytrel and Surlyn at $.25-$.40/#.

-jiw