Interpretation of Plus/Minus Draft - Injection Molded Parts

M

MaryMalin

#1
Hello All,
First let me apologize for my ignorance - I've not previously posted a question.
I need some help and clarification regarding inspection of injection molded parts. We have engineering drawings which state a dimension and either plus draft or minus draft. The question pertains to how the inspection dept is to provide (or report) results. For example, if a feature is called out a diameter with "minus draft", should the inspection report state the smallest or largest diameter measured?
 
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NikkiQSM

Quite Involved in Discussions
#3
I work for a medical plastics company, where we provide compounded plastic pellets.

Our pellet size has a deviation of "+/- X.XXX" Plus would be the maximum, and minus would be the minimum.

I would say your report should state the smallest diameter if it states "minus draft".
 
M

MaryMalin

#5
Thank you. The other interesting aspect is that several of our Engineers are of the understanding that the physical placement of the dimension on the drawing dictates the direction of the draft. For example, if the dimension is placed on the left side of a feature and states minus draft - the understanding is that the farthest point of the feature (away from the dim) would be the smallest.
 

RSEGRIGGY

Involved In Discussions
#6
Are you measuring an outside diameter or an inside diameter?

An inside diameter that has a size 4.000 +-.035 plus draft I would interpret as the largest measurement to be less than 4.035. The additional draft adds more material, so the hole could be smaller than the tolerance as the draft increases and the hole closes up.

If you were measuring an outside diameter of 4.000 +-.035 plus draft, I would interpret the smallest measurement must be at least 3.965. Extra material on the exterior could increase the diameter beyond the tolerance.

Do you have any tolerance on draft? Degrees or additional material perhaps?

I'm not familiar with the physical placement of the dimension on the print having the connotation that you mention.
 
M

MaryMalin

#7
We are measuring an outside diameter. Unfortunately, no draft angle or draft tolerance is stated on the drawing.
 

RSEGRIGGY

Involved In Discussions
#8
What does your call out say exactly? Does it reference a standard like Y14.5?

If you get an outside diameter requirement of "4.000 minus draft", it sounds like the intent is the outside has to be at least 4.000 no matter where you measure it. But how much draft is too much? If the part is 5.000 all around, is that acceptable?

Based on the given information, it sounds like you can make a statement of either "It meets the minimum" or "It does not exceed the maximum."

One way is to just state the range you measure. Report your results but do not interpret as to good or bad. Example: "OD is 3.590 to 4.015."
 
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