"Shoulder of Curve' for Benefit vs. Cost for Plastic Tolerances

[CraigInCali]

Hello,

I'm doing some plastic parts design for the first time and would like to know if there are any general rules of thumb for tolerances with regard to price?

My parts are a few inches in size, and do have to mate up with other parts of similar size. I expect the plastic to be PP, something like that used for plastic organizer drawers, laundry baskets, etc.

I have some cutouts that are about .04 inches in diameter, and some clearances that are about .02 inches on this part.

1) I'd be happy with .005" tolerance on those dimensions, is that a reasonable/simple tolerance given current manufacturing methods?

2) If I didn't specify anything, is there a basic tolerance that today's manufacturing methods can easily get without even making a specification?

Thanks

 

[patprimmer]

Supply the moulder with the part that it needs to fit to and ask him if he can do it. The shrinkage on PP is high, but can be controlled to some extent by conditions.

These conditions may or may not increase cycle time.

Verify that the part the plastic has to fit will be reliably on tolerance. I have seen many cases where users have complained the plastic part does not fit, only to find the metal part has varied.

The best way to get reproducible on size mouldings is by good mould design, especially with regard to flow of material in the mould and cooling.

It may be necessary to make a prototype mould or to leave extra metal on the mould then remove as necessary to get the moulding on size.

It is best to size the mould so that the part is moulded on size with the parameters set in the middle of the processing window.

How much this initial work impacts on cost is directly related to how many parts you then make from the mould.

An investment in mould tuning might increase outlay, but reduce piece part price by reduced cycle time, maybe!

Sorry it's not a straight answer, but the question is not as straightforward as you may have thought. You need to work with the moulder, the toolmaker and the raw material supplier.

A good raw materials supplier should be able to provide a lot of data for shrinkage vs various parameters like mould temperature, section thickness, flow direction and effective pack of the mould via injection pressure, flow path to thickness ratio and gate size etc.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

http://eng-tips.com/market.cfm

for site rules

 

[Demon3]

To add to the excellent advice from Pat.

What properties of the PP are critical? The reason I ask is that by adding filler you can reduce the shrinkage and adjust the mechanicals. Adding 40 weight % calcium carbonate will double your modulus, leave the strength about the same and double your unnotched impact resistance and at the same time reduce the materials cost. Suitable filler grades would be Omyacarb 2AV or Imerys Carbital 115 or 110. If you need to retain all the strength of the PP then talc, wollastonite or mica are options.

Chris DeArmitt

"Knowledge has no value except that which can be gained from its application toward some worthwhile end."
Think and Grow Rich - Napoleon Hill

 

[Pud]

If your decimal points ae in the correct position and the general wall thickness is not too thick, those tolerances would be easily maintained.

As mentioned, tuning the tooling may be required but unlikley.

Cheers

Harry

http://www.tynevalleyplastics.co.uk

 

[CraigInCali]

Thanks for the info everyone. I have some things to think about.

 

[bvanhiel]

Craig,

Basic injection molding tolerances are often given as .005" plus .005" per inch.

The rule of thumb I've used for very tight dimensions is 1/3 of the material shrink. This assumes that the tooler is tweaking this dimension and that the manufacturer is monitoring it. There should only be one of these per part.

-b

 

[patprimmer]

Depending a lot on grade and conditions and direction of flow, PP can very easily go over 2.5% shrinkage and is typically around 2%, but some grades under some conditions can go less than 1%.

It is essential to know which grade and what parameters effect it in what way. These are things any good moulder and the raw material supplier can work out between them.


Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

http://eng-tips.com/market.cfm

for site rules

 

[Demon3]

Borealis has a low shrinkage PP that uses just 9 weight % of filler added in the reactor for extra performance. Its shrinkage is the same as for a 40 weight % calcium carbonate filled PP but the lower filler amount reduces weight, important for automotive applications.

Chris DeArmitt

http://www.phantomplastics.com

consulting for the plastics industry

 

[Pud]

It does not matter what the shrinkage is - it matters whether the whole process (shrinkage/moulding) is consistent!

H

http://www.tynevalleyplastics.co.uk

 

[patprimmer]

Harry

I tend to disagree to a small extent. OK nit pickingly small extent.

While a very consistent process will give a very consistent result, in semi crystalline materials, the very high shrinkage materials are normally also much more variable for a given change in a processing variable. No process is absolutely consistent. It may be consistent within your ability to measure but there is always some small variation. This makes a bigger difference to the more sensitive materials. PP homopolymer is one of the more sensitive materials.

Having said that, 0.005" over 0.04" and 0.02" should normally be obtainable with modern, well run well designed machines, moulds and plant so long as there are no unusual aspects to the design like compromised gate position, compromised vents, compromised weld line location, excessive changes in section thickness or compromised temperature uniformity on the mould surface.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

http://eng-tips.com/market.cfm

for site rules

 

[Pud]

Pat,

We agree then...

H

http://www.tynevalleyplastics.co.uk