plastic tolerancing issue...

[dgschaefer]

I spent 5 years in a tool design shop. The rule of thumb we used was
that the tolerance you can get is 20% of the shrink over a given
distance. An example:



Tolerance on a 10" dimension on a part made with ABS with a shrink of
.005" per inch. Shrinkage on the dimension is 10" * .005 = 0.050". 20%
of 0.050" = 0.010", so the tolerance you can expect is +/- 0.010". A
good molder can probably beat that, but it's a good starting point.
Same dimension in Polypropylene with a shrink of 0.020" is +/-0.040".



I've been out of the molding business for a while, maybe things have
improved since then but that was the guideline we used.



Doug Schaefer

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[ptc-2860737]

I wonder if John was referring to the CTE of different materials (not mold
shrinkage). Tolerances need to be considered carefully for materials with
significantly different CTE values if they will be used over a wide
temperature range.


Doug Barton
Mechanical Designer
Parker Hannifin Canada
Electronic Controls Division
1305 Clarence Avenue
Winnipeg, MB R3T 1T4 Canada
direct 204 453 3339 x309
fax 204 452 7156
-
www.parker.com/ecd

[jbennett]

We have started to talk about 2 separate items here. I do agree with Nate that stated that the mold shrinkage is considered in the tooling phase. That was not what I was referring to. Doug was correct and stated what I was trying to explain better than I did. Each material will act differently under the same environment. That is why I asked if both joining parts were to be made of the same material. If not, you have to consider what the materials will be and where they will be used.

Doug, thanks for the clarification.

John Bennett
Cad Business Administrator
(801)513-9001

[Lifetime_Logo_BlkWhite_Sans_email sig]

[dgschaefer]

Yes, shrinkage is taken into account in tooling, but your molder is
going to have a hard time holding tight tolerances in a material with a
greater shrink rate than in one with a small shrink rate. On critical
fits, particularly in larger dimensions, taking material shrink into
account when doing a tolerance analysis is important.



Also, as Steve pointed out, while they can tweak the process to improve
the tolerance on a given dimension, they are balancing many factors and
nailing that dimension may mean compromise in other areas. In other
words, don't ignore the shrinkage and assume the tool maker and molder
will make it all work.



I'd add that if your tool maker and molder aren't asking questions about
form, fit and function like Steve indicated and you don't go over it
with them; don't be surprised later if things don't work.



Doug Schaefer

[A.DelNegro]

Luckily the parts will be of the same material. We did discuss fit and
function for quite a while. We are just worried about how to best
capture that in writing so we don't get stuck with parts that don't work
or we have to pay extra to get fixed.

Thanx for all your advice.

Tony