modeling an injection modling part 1

[Ksplice]

is it better to model an injection molded part with equal wall thicknesses and equal draft and then round off the width numbers on the drawing. or would you dimension the theoretical sharp corners of the molded part? How does everyone model a molded part?

thanks

 

[MacGyverS2000]

If the drawing is for creation of a mold, you don't put items on the drawing that won't be molded into the part. Dimension to the faces, with notes specifying draft angles, filet radii, etc.

Dan - Owner

http://www.Hi-TecDesigns.com

 

[patprimmer]

I agree with Dan.

Regards
Pat
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[KENAT]

If the model is going to be used to generate the mold directly then rounding off dimensions on drawings is to be avoided. It's generally bad practice anyway but worse when the model data is going straight to getting parts made from.

Dimensioning to theoretical sharps (annotated on the drawing as need be - not actually modeled as sharp corners) may be appropriate depending on tolerance scheme.

If you could clarify your question and maybe mention what dimensioning standards etc' you typically use that may influence the answer.

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[Ksplice]

I have a model from a customer where the model is drawn with equal wall thicknesses and all the walls are drafted at 1 degree. I also have a drawing from them as well where the dimensions are to certain walls which are drafted and have raddii on the corners. Becasue of that the engineer who did the drawing rounded off the numbers..the numbers on the model are within tenths of the nominal numbers on the drawing and the tolerance on these numbers is ±.010". Also only the important dimensions are called out on the drawing, the rest of the features are controled to the model. My boss is asking me to redraw their model to get all the numbers to nominal and add the missing dimensions to the drawing off their model. So the model is going to be used to make the mold and the dimensioning standard they are using is ASME Y14.5M-1994. Is there a proper way to handle complicated molded parts? Do you put the important dimensions on the drawing and then control the model with a standard profile tollerance? I'm just starting out in my career, fresh out of college, and would really apprieciete any advice on how to properly handle a molded part drawing and model so that the tool maker is able to make a quality part. Thanks

 

[KENAT]

Going to Model Based Dimensioning is one way to go about it, essentially making the model geometry 'basic' and applying a surface profile tolerance to it and having only a partial drawing primarily for showing tolerances (and other non geometric information) rather than geometry dimensions. ASME Y14.41 starts to attempt to address this.

However, it's still relatively immature, and a lot of people don't have the understanding of GD&T to support it. There have been threads on this over in GD&T and elsewhere.

It seems your boss may be going about it a bit backward, I'd suspect that the model may represent what they really want/need and that poor drafting created a drawing that didn't really define what they want.

If you then create a new model that meets their drawing, it may not work in reality. Now legally you should be OK because your part meets their explicitly stated requirements, however from a customer service point of view for repeat business etc. it may not be good.

Has your boss gone back to the customer pointing out the discrepancy and got clarification on what's really needed?

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[Ksplice]

ok cool I'll check out the gd&t forums and see if I can find some more info. to answer your question no he has not gone back to them and got clarification on what is really needed, I'll bring that up to him and see if he'll go for it. It seems a little backwards to me as well. Also this is a product we have been making for 4 years with the current model and drawing. What we are doing now is building a new tool, what I'm doing seems kind of unessary.

 

[KENAT]

So the real task is perhaps document what is actually being made on the existing tool and has been accepted by the customer for 4 years?

I'm getting a horrible feeling this could be one of our suppliers, just tell my you don't work for Doug or Dan.

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[Ksplice]

Nope don't work for those guys

 

[patprimmer]

There are very occasional exceptions, but as a rule, if you have a part that works well but does not meet spe, you do not change the part, you change the spec.

I have seen to many in field failures as a result of an over zealous QC guy insisting on changing working parts to spec, when the original parts failed and where modified to the current part, but no one updated the drawing and spec.

If you have a current working part, I would think the starting point for a new mould would be to digitise the existing mould.

The big caution when doing this, especially with semi crystalline of fibre reinforced mouldings would be that a change in temperature profile within the running mould or a change in gate position or size or type might change the dimensions of the finished part.

Regards
Pat
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