As Designed verses As Moulded

[DJBLINX]

I asked the following question in the Drafting Standards forum and received a number of replies, but thought I would try on this forum as well. Any insights will be appreciated.

When designing injection moulded parts, tolerances are added to ensure the parts will go together (GD&T). When the parts are moulded they fit together, but do not necessarily meet the drawing specification. How do other companies manage this? Do you modify the drawings/models to match the actual parts (could be difficult for complex parts), insist the supplier modifies the mould tools (expensive), or do you do something completely different?

Thanks

David.

 

[MacGyverS2000]

I don't understand the issue... if the parts do not meet the drawing spec, it's irrelevant if they fit together. This assumes, of course, the drawing specs for different parts line up with each other.

If the molder is unable to meet the spec, you have two choices... change the spec, or change the molder.

Dan - Owner

http://www.Hi-TecDesigns.com

 

[jmw]

Or is it a case of not make due allowance in the drawing for the various factors that affect the finished component?
But if you add tolerances, do you not include these on the drawing and if so how is the part not to drawing?

JMW

http://www.ViscoAnalyser.com

 

[patprimmer]

I have seen a situation where a part was made to drawing. It did not quite work, the moulder and designer fine tuned the mould until good working parts where obtained and these where accepted for years.

Many years later a new QA person was appointed. He started failing parts unless they met the drawing. They then had failures in use. The obvious answer was to change the drawing, but sometimes fresh young hotshots cannot see the obvious.

Another case was for nylon. It met size specs when measured dry as moulded, but failed after moisture conditioning, however if used dry the part failed, so the moulder started supplying them conditioned. QC then started failing them. Sometimes common sense should prevail, but often it's not that common.

The purpose of QC is to assist in meeting suitability for purpose at minimum cost. If that is not the outcome of QCs actions, QC is doing it wrong.

Regards
Pat
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[bobslo]

All the weapon of QC guys are design specification. If these are not tuned with real life, then sooner or later some QC hotsot will start making noise.
If parts are functional, put simple remark on the drawing or extend tolerance area.

 

[Richar24]

What I have seen in industry is to check fit and function; Plastic parts are usually out of round, not flat, etc.

The best way to handle this is to determine what the most variable dimensions are on the part that is easy to inspect. Usually the longest length and largest diameter. Use these dimensions and maybe a few dimensions that are critical to function on the Inspection instructions. On diameters specify dimension on the gate side or opposite the gate side, do a gage R and R to ensure that you get good data. Do the same with your longest dimensions. Center the nominal dimension to ensure a CPK of at least one to ensure a processing window.

If you change all of the dimensions on the print, and the mold is retooled to the new dimensions and it shrinks the same your part will be different.

Suggestions: Use a good mold maker, Ensure that you have a good part design, proper draft, uniform wall sections, etc. and you will see a lot of these dimensional discrepancies go away