Injected EVA foam density. 2

[waynethepain]

Hi,

I am designing a product with injections moulded EVA foam. Weight is a real issue with the design the density is 0.941g/cm3 but will the density change as the foam expands?

I ask because my CAD model tell me my design is nearly 1KG but when I double checked it against ABS it was not far.

Thanks for the help,
Wayne.

 

[Demon3]

Yes, as you foam the product more and more the density decreases.

Chris DeArmitt PhD FRSC CChem

 

[waynethepain]

Cheers Demon Do you know how to work it out?

Cheers,
wayne.

 

[Demon3]

Hi Wayne,

Here's a really nice online calculator that allows you to work out the density of a plastic with e.g. filler particles or air bubbles in it. Or, you can input the density you find and it will tell you the volume % bubbles there. I have an Excel sheet that does the same but this is far easier to use.

http://www.netcomposites.com/Composite_Tools_Calculators.asp?calcid=41

Cheers,

Chris

Chris DeArmitt PhD FRSC CChem

 

[waynethepain]

Cheers again Chris,

I am being a little thick I only have a BSc after my name. I have no idea how to work it .

I know the area is 355544 mm^2 and the volume is 950153 mm^3 at a density of 0.941 which give me a weight of 893g. Like you said before I knew it would get lighter but not sure by how much.

Any chance you could give me a little more advice?

Cheers,
Wayne.

 

[Demon3]

What are you giving me? The density of the foam or of the starting EVA. The density you gave is close to EVA unfoamed. Foamed EVA has a lower density.

Chris DeArmitt PhD FRSC CChem

 

[waynethepain]

What I'm trying to do is workout how heavy my product will be and the piece price.

Logic told me that the product would be lighter as the weight numbers did not make sense. I am presuming that EVA foam has less plastic injected that solid EVA thus less weight and cost.

I need to workout how the difference between the two.

Any more help would be appreciated I am sorry to ask again.

Cheers,
Wayne.

 

[patprimmer]

If you foam a plastic as you injection mould the density of the resultant moulding will depend on:-

1) There being enough blowing agent and suitable conditions to expand the plastic to fill the cavity.

2) The final weight will depend on the shot size. If you over pack the mould at high injection pressure then leave the moulding in the mould untilthe moulding freezes solid, then the foam cannot work.

You must control the weight with shot size then let the foaming action fill the mould.

To obtain consistent shot weight you need to accurately control screw back and barrel recharge, use a blowing agent and conditions to avoid foaming in the barrel then inject all the contents with no flow back. this requires a very good check valve in the barrel.

Regards
Pat
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[waynethepain]

Thanks Pat

Do you know if there is a percentage/rough estimate that the density/weight will decrease by?

Cheers,
Wayne

 

[patprimmer]

It depends on the shot size.

If the moulder just fills the cavity with zero cushion and no shorts or voids at the end of the shot then next shot is 10% less stroke of the barrel, then the in theory the density will be down by 10%. It actually varies a bit in reality due to elasticity in the melt and precision of some controls, so the moulder will fine tune the shot size to get the required density.

So the real answer is not what the shot weight will be, but what shot weight do you want, within a realistic range dependent on the blowing agent used and what surface finish you require.

The moulder and the supplier of the bowing agent should be able to advise realistic limits and influence on shrinkage and surface finish and other properties.

Regards
Pat
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