Re: Rapid Tooling

From: Peter H. Gien (
Date: Tue Feb 20 1996 - 02:27:05 EET

Carl Deckard responded to the general tooling question as follows:

>>2) Is plus or minus 0.0002 inch realistic for a production injection mold?
>>No. The accuracy of CNC is generally overstated and even if the tool were
>>perfect, you would have to control the temperature to plus or minus 1.7
>>degrees C to maintain plus or minus 0.0002 inch on a ten inch dimension.<<

I agree except for one fine point: The parting line. In general, most dimensions
of the tool can be quite gross, but the parting line had *better* be a tight
match of around 0.005". Why? If not, the plastic comes squirting out of the gap
and you end up with lots of flash to remove from the part.

The required tolerance of the parting line depends on several things:

1) The viscosity of the material being injected.
2) The injection pressure
3) The clamping pressure of the mold halves
4) The temperature of the plastic when it reaches the parting line
5) Planar or 3D parting line

If the parting line lies in a single plane, then it is fairly easy to adjust the
tool with secondary polishing and finishing operations. If the parting line is a
3D curve in space (have a good look at the handle of a pair of Fiskars scissors
for a hairy example), then matching the mold halves becomes more of an art than
anything else. A good tool and die maker is a highly skilled and highly paid
individual. A good tool and die shop can work miracles with a block of steel.

Carl is right when he states that NC tolerance is over rated. You need the magic
touch of a tool and die maker to eke out the last few drops of tolerance!

Peter H. Gien
POGO International, Inc.


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