Injection Moulded Parts - As Designed v As Moulded 2

[DJBLINX]

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.

 

[DeanD3W]

What I like to say is that "You can process control your way, or robustly design your way, to reduced need, or even no need, for GD&T".

For assemblies of complex molded plastics parts (most of my experience is with ink jet printers, which fall into that category for sure), I think it would be fair to say that the best GD&T that time allow, with the best process control that the organization will enable, and the most robust design that design resources can manage may all three be needed to make the most profitable product.

Dean

http://www.d3w-engineering.com

 

[pmarc]

Frank,
I did not say that at the beginning, but we also stated in the table that the acceptable dimensional deviations were applying for the specific tool-no. only.

The logic behind was that if a second mold (with different no.) was going to be built for the same component after the first one had been worn down, the acceptable deviations for the first tool would not necessarily stay functional for the next generation tool, because in the same time a mold for corresponding mating part could have been also changed. After each mold replacement, fit and function analysis was repeated and new tables with the acceptable dimensional deviations for new tools were added to the prints.

Complicated, but at that company it worked sufficiently.

 

[fsincox]

pmarc,
I see so you are saying the other "mating" part may be shifted too, I had not thought of that, wow. What a tangled web we weave!
Well I agree guys it ain't pretty!

Dean,
I would love to know what you said there?
Frank

 

[MechNorth]

Deformable parts, while unpleasant to deal with from a design/tolerance perspective, are a large segment of manufacturing. Going back and modifying the dimensions & tolerances on the original drawing to reflect the as-built results is a no-win situation. Designers don't have enough information over the process being used, and therefore usually don't select effective tolerances; when they do, tolerance stack-ups become a nightmare because the values can be so large. Processes and tooling will create a shifting target over time; do you update the drawing every time? Some processes are so poorly controlled for environment and such that literally the results change depending on the weather outside.
I favor tying the adjustments into the PO rather than the original drawing if there is ongoing variance. In that way, each batch/run is qualified based on the original intent rather than based on the last run.

Jim Sykes, P.Eng, GDTP-S
Profile Services

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[MikeHalloran]

I have adjusted plastic part dimensions when the first articles came in a little different than I expected, due to other than predicted shrinkage, or due to different shrinkage when a material change was forced.

I have never made adjustments to cover batch variations after the first lot in a given material. ... but I have almost never had enough production volume to wear out a mold or otherwise force generation of a second cavity for the same part.

As an accidental specialist in low to medium volume parts, I have had to deal with forced material changes on quite a few occasions. If you don't buy railcar loads of resin, your supplier may one day decide to just stop making the stuff you need, because his primary customer changed materials, and the least awful substitute you can find is sure to be different.


Mike Halloran
Pembroke Pines, FL, USA

 

[drawoh]

pmarc,

I have designed a plastic part that slid over an aluminium boss, with an interference fit. My design objective was to have zero clearance for accurate centreing. I worked out the stresses and forces due to the interference fit and friction. It was possible to slide the two components together by hand. This would not have been possible with metal components.

If you are trying to open up tolerances, you can work out maximum clearance between your components, and the maximum allowable interference. In the case of a lot of plastics, this could be quite a bit.

JHG