SPI tolerancing standards for drawings

[cityjack]

Morning all,

First time poster here. I am a new employee here at my company and will be visiting a new mold house to go over plastic part design and moldflow with them. I have been asked by my company to make sure they can and will adhere to SPI tolerancing for our part prints once they receive them from us. In my experience from other lives, we just give them a .stp, igs, or a 3D CAD file and they go from there developing the tool. As far as SPI tolerancing on the print we would just tolerance part prints according to the material we are using and the knowledge of the material and shrink rates etc. Maybe include in the notes a volume and weight of the part given to us by the CAD software. Can someone please fill me in on a generic plastic part SPI tolerance or maybe even a DIN tolerance for plastic parts?

 

[MikeHalloran]

Applying a generic or standard tolerance by contract or agreement to a theoretically perfect CAD model seems insane to me. I know it's done all the time. ... and it causes or catalyzes a lot of disagreement and confusion.

What should govern the finished product's dimension limits is its function and fit within the end product. The tolerances necessary to make the finished product work properly must be conveyed to the mold maker and the molder. So far, CAD models don't do that, so you need a part print, and it must govern for moldmaking and inspection.

Conveying tolerance information is just one of the many ways that CAD modeling has fallen far short of its alleged potential.



Mike Halloran
Pembroke Pines, FL, USA

 

[dgallup]

http://www2.basf.us//PLASTICSWEB/displayanyfile?id=0901a5e180004879

page 12

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The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

 

[cityjack]

Thanks Guys. I like the BASF link. I think the SPI "Standards and practices for molders" literature will do based on the material chosen. Not sure what that will be yet. We are due to visit the mold house on Friday this coming. I am fairly well versed in good plastic part design practices, but its been a bit since I have dimensioned a plastic part print with GD&T and am not familiar with how SPI likes things done.

Thanks all.

Sid

 

[Pud]

We often check parts to CAD files. The good thing is, our CMM machine will do a "best fit" to the part. Lovely - all green dots on the report....

I (as a moulder), like CAD files, for as well as containing all the drawing errors, they contain all the design errors too!!

H

www.tynevalleyplastics.co.uk

It's ok to soar like an eagle, but weasels don't get sucked into jet engines.

 

[cityjack]

Let me ask you all something if I may please. I have what amounts to a rectangular bucket that is about 8" deep. The whole bucket inside and out has a 1 degree draft. Inside the bucket I have a couple of walls that stand off the inside walls that are half way up the main walls. These walls are drafted at a half degree of draft. This is not my part. I inherited this project and parts and have been asked to look it over before we go to visit the molder tomorrow. In past lives I really do not remember seeing unequal drafts in such a simple structure. Is it desireable to have all draft angles equal whenever possibly from a molders standpoint? Is it any additional work for the molder or is it just a process of making the electrode and burning or just machining? I know if the walls were the same height the part would come off the larger angle draft first.

Thanks all.

Sid

 

[mcgyvr]

There is really nothing wrong with unequal drafts as long as the part can be molded AND release easily from the mold.
The unequal drafts could be part of the design too and need to be that way to allow it to fit somewhere or whatever.

And as someone who HATES doing 2d drawings I can't wait for the day that 100% design intent/tolerances/design notes,etc.. can all be incorporated into the 3d model. Its getting there but not fast enough IMO.

 

[HDS]

It sounds like you need to read the SPI spec. The SPI specs I have used were for cosmetics and mold construction. They were very general documents that did a good job of defining terms and techniques. However, they still required us to specify what part of the standard applied to our part. For example which surfaces were class A and which ones were class B. Did we want the best mold or the cheapest? I expect the spec for tolerances is similar.

 

[cityjack]

Thank you sir. I am going through the SPI spec for ABS specific parts as I type here. Let me ask you if I may, What are some generic notes you might see in the corner of your drawings for parts that are plastic. It past lives we have put like the volume and or weight of the part. Maybe material if it is not in the material block. Flash notes. Parting line notes and some additional that I cannot remember. I am making some new drawings today and was hoping for some hints. Some companies even have borders or templates that are plastic or metal specific. Any help would be greatly appreciated.

Thank you

Sid

 

[HDS]

% regrind allowed
cosmetics
possibly packaging
cleanliness (mold release?, dust, dirt)
manufacturer and grade of material
UL requirements ect.
color

 

[APDplastmold]

Sid,

What we suggest to our clients is this: generate an e.g. pdf file with ALL design notes (the more the better) and tolerance requirements, attach it to the 3D design and indicate the latter must not be used w/o the former. The tolerancing issue is rather simple if we refer to (attach) the DIN 16901 standard (din_16901_eng_plastic_tolerance.pdf that has been around since the 60's(?))that takes into consideration the material, required part quality levels and the dimension itself (longer dimensions require wider tolerances and v.v.). If anybody finds a good replacement for the DIN 16901, please let us know. As to very high precision applications tested computer programs have been available for decades that can determine the attainable part tolerances at +/-x.xxxx inch accuracy for critical dimensional by taking into consideration apx. 25 variables (material, design(wall thickness, fiber/molecular orientation, etc), gate number/size, location(s), molding and part inspection environment, mold temp. distribution, molding/packing conditions just to name a few) and for the analyzed dimensions, compute precision shrinkage also at x.xxxx in/in accuracy.


G. Markus

http://www.robustmolding.com

 

[cityjack]

Thank you sir.

 

[APDplastmold]

You are very welcome. Please let us know how it worked out for you.

G. Markus

http://www.robustmolding.com