Injection moulding Tolerances

[mrlizzard]

I’m designing some small injection moulded parts made from POM for a new hand held product Im getting manufactured in China. I’m a cad designer but have never dealt with the higher tolerances of injection molding before, rather vac forms, GRP and rottery mouldings etc. I don’t want to over tolerance the drawing/3D model for obvious reasons but don’t want to leave myself open for problems. I will put GD&T on the critical areas of the drawing but wanted to know of any general tolerances I could use and what realistic tolerances can be achieved.

I know it’s fairly ambiguous question but any help would be appreciated

 

[patprimmer]

The mould shrinkage of POM is very dependant on many different parameters.

Until you know the shrinkage, you will not know the tolerance.

Also you need to ensure your moulding parameters can be accurately controlled and maintained in the middle of the processing window and that window changes with design.

Typically you need a mould temperature of about 40 to 60 deg C maximum injection pressure and hold up until the gate freezes.

The gate must be big enough to allow good pack.

Many will say POM does not need drying, but if you dry it at 80 deg C for several hours you will get a better finish and more consistent moulding.

Different colours might quite possibly give different sizes.

Also it will show some post moulding shrinkage and may take several days to settle down.


Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Pud]

If you want high precision parts which will retain their sizes after moulding, tool temps should be in the 90 -100C mark to ensure maximum crystallization (hence shrinkage) during processing. What sort of tolerance are you thinking of? E.g. we make a part which a 10mm dimension is controlled to better than .01mm - then our customer hammers it into hole!

These used to do some excellent design guides for POM - try this for a start:

http://www.ticona.com/redesign/index/tech/design/general_design.htm

Cheers

Harry

 

[mrlizzard]

Thanks, When Im home from work I will post a picture of the model for comment.

 

[dwightdixon]

The two major acetel material suppliers have excellant design handbooks with suggested tolerances for free. A lot also depends on the quality of the tool builder and processor you pick. I would tolerance the areas you need and leave the rest off for the time being and call in a couple of the tech support people in from the suppliers and ask for their recommendations as to whether the part is reasonable and get their suggestions on other tolerances. Adding glass to the material reduces the shrinkages and increases the consistancy of the sizes dramatically.

 

[bvanhiel]

+/-.005in/in generally. Another rule of thumb is that the tightest a dimension can be held is ~1/3 of the shrink.

-b

 

[patprimmer]

Harry

I agree in theory, but many years ago I got a tour of a moulding shop in Japan who was a major supplier of acetal mouldings to Sony. They used 40 deg C as their standard set up temperature for precision, non aesthetic acetal co-polymer parts. Polyplastics had me there for training. This was their showcase moulder in Osaka, so I did not argue much. They claimed that

I agree, the colder the mould, the more the post moulding shrinkage.

Dwight.

Adding glass fibre dramatically reduces shrinkage in direction of flow and to a lesser extent across the flow. In simple parts with parallel one direction of flow, this might increase stability substantially, but it also introduces a potential for warpage once there is a complication or change of direction of flow in the moulding. Mineral or bead type fillers are best for stable accurate dimensions, but they reduce most physicals substantially.



Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[esqrkim]

mrlizzard:

My advise to part designers are these:

1) +/- 0.002 to 0.005 is fine. +/-0.001 is possible for smaller parts, but it's still pushing it.

2) Don't break your neck trying to second guess the moldmakers or the molders on how they will meet your design needs. Just give them what you really need in terms of tolerance. In the end, whoever takes the job will be responsible to produce. If three or more molders reject your requirements, then think again; but don't let one or two rejections frighten you.

3) Focus more on designing the part so that it can be molded properly. Consistent wall thickness, good gate locations, good ejection surfaces, etc.



**My comments are my opinions, and results are not guaranteed**
Eugene Kim

http://www.eeksolutions.com

 

[mrlizzard]

Thanks. I will put 0.2mm (0.007) as a general tolerance for linear dims and my critical dims need to be 0.1mm (0.003), so sounds like it should be reasonable then. I will be talking materials etc when I make contact with a potential toolmaker.

One other thing, should I specify the injector pin locations and the envisaged parting lines on the component detail drawing or leave this to toolmaker/toolmaker drawings?

Thanks again

 

[dgowans]

What you specify on the detail drawing depends entirely on what you need to control. I typically do not specify ejector pin locations except in the case where I know I don't want any ejector pins (a surface that will mate with a gasket, for example).

I also very rarely identify where parting lines will be on a part drawing because I model the parts complete with the draft I can accommodate in the design. As such, the parting lines are defined by the model that goes to the mold maker.

 

[esqrkim]

Exactly as dgowans said. In general, pay more attention to what you as a product designer need rather than any other thing. If there are conflicting issues, the toolmaker/molder will let you know. Then you can address the issue at that time. In some instances a toolmaker/molder will find creative solutions to the issue that will meet your needs as well as their needs.


**My comments are my opinions, and results are not guaranteed**
Eugene Kim

http://www.eeksolutions.com

 

[patprimmer]

If any doubt exists try to set up a meeting involving all parties, including the end user, designer, moulder, toolmaker and raw material supplier as all will spot consequential problems from offered solutions.

Such a meeting needs the man paying the bills there to keep the focus on his needs and all working toward an overall workable solution.

It is really surprising how flexible a firm requirement can become when the real cost and the consequential compromises are made clear.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[sizzer33]

hi Mr Lizzard guy,

You can start putting basic tolerances on plastic by using the DIN16901 guide. I have a hard copy here and its in Dutch. But it is basically a table which gives you the tolerances depending on the material, dimension you're controlling and how the part is shaped. I know i kinda understand it halfway too. BUt when i want to know what tolerance is good for a POM with 14mm, i would jsut see the table and get the highest tolerance to start with.

 

[mrlizzard]

I think this will help me in the future before I get involved with the toolmakers.

I had a look on internet to see if I could find any free extracts of this table from the DIN standard but it seems I will prob have to buy it if I get enough work to justify it. Thanks Sizzer33

 

[patprimmer]

No matter what is said in tables, unless they are very complex and consider a large number of variables, they will not be dependable.

If you take a best case scenario, acetal can be reasonably precise, but if there are any difficult to control parameters, then the attainable tolerance will be a lot wider.

Acetal has a VERY wide range of shrinkages and potential warpage depending on design, tooling and moulding conditions.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Pud]

Pat,

You will not believe this:

Had a customer turn up with a tool in his car yesterday - an acetal part with a hexagonal bore which is fitted (transition fit) onto a hex steel shaft.
To get the bore size correct, had to run the tool at 40C!! (Around 0.2mm undersize at 90C) Rubbish surface finish but customer v. happy, and that's what counts!!

I do like these URGENT jobs..

Cheers

Harry

 

[mrlizzard]

I do appreciate the wide variety of variables but I think you guys have answered my question. I only need to acquire 0.2mm in critical area so sound like I’m not asking too much, so the drawings won’t raise unnecessary alarm bells and increased cost. I will be talking to suppliers after getting ball park costs from the drawings.

I must say that I’m looking forward to being involved in the injection molding world as the recent freelance jobs I’m getting are asking for these components.

I am only working on designs for my client at the moment so hopefully will be engaged in the manufacture of the product and liasing with toolmaker, so I cant unfairly get any toolmakers to involved at this point – only for ball park costing!

PUD, most modern keyboards are fitted with a JUST button for those urgent jobs. Lol

 

[getcoles]

As a mold maker we are asked to hold our steel tolerances to 1/2 of the tolerance on the drawing.
DIN 16901 (Automotive) gives a complete list of tolerances expected in that industry. Google DIN 16901

Also this site will give you a rough idea of the impact of tolerances on the cost of injection molds.
www.flowtekengineering.com