Design options with plastics

[jmw]

At the risk of being shot down for not googling properly, I have a query way outside my field.
I am trying to discover the penalties of poor product design.

I have a fairly simple component that is to be moulded and the final component should meet very precise dimensional properties and should not distort.
But when designing the part I have two options.
For simplicity suppose we have a hollow cylinder where the target internal and external diameters must be met with very precise tolerances. The cylinder also has slot through the wall extending along the entire length.
It is a mass produced item.

The question is to know if there are any penalties if I make the part with a constant diameters along its length or if I should introduce a slight taper to both the internal and external dimensions.
Is there a cost penalty?
Does it affect the choice of plastics and/or fillers?
Does it affect any aspect of post moulding management?

JMW

http://www.ViscoAnalyser.com

 

[Pud]

listen to my instructor

Kenat: ask him how screw-on milk bottle tops are made. (No, they're not unscrewed either. Takes too much time)

http://www.tynevalleyplastics.co.uk

Why be happy when you can be normal?

 

[MacGyverS2000]

Pud,

If I'm not careful, a wrong tug on the "safety strap" on those milk caps show just how easily they can be removed without unscrewing ;-)

Dan - Owner

http://www.Hi-TecDesigns.com

 

[patprimmer]

Yes, lots of caps including milk bottle and aerosol can caps are jumped off the core. Unscrewing moulds are somewhat bulkier and more expensive than one that jumps the core over the undercut as well.

Notice the change from no draft to actual undercuts.

This is somewhat design and material sensitive though. PC does not jump of undercuts very well at all, but PP is quite good at it.

A ramp angle on the undercut helps a lot.

Of course the core has to be out far enough so there is no external constraint on the moulding.

Undercuts in cavities are a lot harder than undercuts on cores.

Mouldings that jump over undercuts tend to spray themselves all over the factory floor and unreachable regions of the machine.



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

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for site rules

 

[bobslo]

Perhaps a suggestion for new generation design:
If you use two different materials with different shrinkage and with NO ADHESION you could mold as follows:
- first shot piston (lower shrinkage material)
- second shot disc (with piston as insert). Material for disc should have higher shrinkage.
Such process exclude assembly process tolerances.

Perhaps this sounds a little-bit futuristic but (some) new throttle bodies with vane inside are done in this way.

 

[Richar24]

Just a few quick observations.
1. Parts with a "C" cross section are very difficult to mold and hold tight tolerances. The issue is that depending on where the gate is located the slot will either open up or close down. If the baffel is molded in place, the bore will be a different diameter (probably smaller at the open ends). Inserting the baffels as a second operation will avoid this issue.

2 Draft is always better, Without draft on the core, you may have to retract the core out of the cavity with the mold closed (This increases your mold cost). The standing piece of steel that produces the slot will require either a slide or draft to allow for a shut off. These options increase the cost of the mold .

3. If you dont need the slot on the side the full length of the part it may simplfy the mold.

4. You can mold the baffel seperately and adhesive them in place or sonic weld them in if this is an option.
A single baffel plate can be molded in place, and the second baffel plate inserted ito the bore and fixed in place.

5 A really cheap mold can be made that has the core, cavity and slot all cut in the solid, but will likely prove difficult to mold. This will be a decision you will have to make based on your part volumes and mold costs and part cost. Their are a number of ways this part can be produced