PLASTIC MOULDING TOLERANCE SPECIFICATIONS 5

[flipp313]

DOES ANY BODY HAVE ANY INFORMATION REGARDING PLASTIC INJECTION MOULDED COMPONENT TOLERANCE SPECIFICATIONS
THIS WILL HELP ME WITH DETERMINING WITH MY MOULD SUPPLIERS THE TOLERANCE LEVELS WE CAN WORK WITH

 

[Rocko]

Your best bet is to contact the manufacturers of the components ie , runners ect...

 

[CorBlimeyLimey]

The tolerances would be entireley dependant on the type of product you are making. If it is a "standalone" part then the tolerance can be fairly wide. If it is apart which has to fit closely to other parts, or slide, or have a molded insert the tolerance would have to be adjusted accordingly.

One company I worked for produced some parts which had to be +/- .001" ... another produced parts which were +/- 1/32" ... yet another company did not even set tolerances.

Without knowing what you are producing, & its purpose, it is impossible to give you a definitive answer.

from (the City of) Barrie, Ontario.

If you choke a smurf, what color does it turn?

 

[Heckler]

Flipp313,

Most casting houses I've worked with offer their "cook book" standards which are based on a number variables....such as material being used, and processes. Then you have to compare those against your design intent....it has been somewhat of an iterative process. Best of luck

 

[Theophilus]

You spec your own tolerances for whatever you want.

Different plastics will mold with different dimensional stability and shrink rates. Therefore, you need to make sure you stick with the plastic you specify after the tooling has been cut (unless your shrink rates are identical).

I normally get parts to hold within +/-0.005" on all dimensions just be giving my vendors a solid file. If I need more control, I'll spec it in a drawing--only the non-conforming dimensions.




Jeff Mowry
Industrial Designhaus, LLC

http://www.industrialdesignhaus.com

 

[macPT]

Try the french standard NF T 58-000. There you will find tolerances separeted in categories (according to plastic type), direction (according to mold opening) and tolerance class.

But if you supply some big company (like a car manufacturer), they have their own tolerances specs.

Regards

 

[Heckler]

Flipp,

Here is another resource:

http://www.engineersedge.com/plastic_design_menu.shtml

 

[StiLLiving]

Check this one out....:

http://www.engineersedge.com/plastic_design_menu.shtml

 

[werks]

Check this out:

http://www.socplas.org/business/literatu.htm#AQ-102

The Society of Plastics Industry publishes many books and guidelines that are used as standards in the plastics industry. I highly recommend this particular handbook. Good luck.

 

[RonNagle]

I know in what we do we have to conform to ASTM standards. It all depends on what industry you are in.

Regards

 

[patprimmer]

Dimensional control of plastic parts is a broad subject, and cannot simply be answered in the manner you ask, as is evident by the number of various answers you received.

Close tolerances can be achieved by careful selection of material, lots of trials and small alterations, and tight control of conditions, but these all cost, and should only be specified if really required.

Set the tolerances as wide as you can comfortably accommodate, then get quotes for tighter and loser, and decide where the best acceptable cost/performance point lies. Also, wider tolerances opens up your choice of material, so you may be able to specify a material that offers a better overall balance of all other properties.

Details to be aware of are:-

Plastics typically have a co-efficient thermal of expansion in an order of 10 times that of typical for metals.

Many plastics absorb water or common chemicals to some extent and expand as a result of this.

If the plastic is glass fibre reinforced, you will get anisotropic properties re dimensional stability.

Many plastics are semi-crystalline, and degree of crystalinity, and the nature of the crystals will effect dimensional stability.

The degree of crystallisation can be significantly effected by moulding conditions, after treatments, conditions of storage, additives and colour pigments used, and the passage of time.

The co-efficient of expansion is not linear vs temperature, and quite dramatic bumps in the graph can occur when at a second order transition temperature. Plastics often pass through a second order transition at a temperature within the range that might be considered ambient, like most plastics will go through a second order change between -20 and 100 deg C.

Plastics are not perfect liquids nor solids in the classic sense. Plastics can be slightly compressed in there molten form, so injection pressure and hold time can have a significant effect on shrinkage.


Regards
pat pprimmer@acay.com.au
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.

 

[MouseTrap]

flipp,

Patprimmer gets a star for his response - lots of valuable insight in his post.

Two things I would like to add...

1. I recommend you speak personally to a couple different MATERIAL RESIN MANUFACTURERS regarding tolerances expected of the (generic) material you wish to use and interpolate from those responses a set of reasonable tolerances for your part.

2. No one has mentioned that large parts need larger tolerance zones than smaller parts. The material resin manufacturers should be able to help you with this as posted above.

Good Luck - Mousetrap.