Air trap in PMMA 4

[morphi]

HI,
after inject the mold , the parts has not air trap But outside the mold during get cooling , the air trap will apear ,the picture is attached.
part weight=170gr
thickness=20 mm
height=100 mm
width= 70mm

 

[patprimmer]

It is not an air trap, it is a contraction void.

Think about it, how can air leak through the plastic into the bubble.

You overcome the problem by packing more material into the mould. This can be by increasing injection speed, injection pressure, back pressue, shot size, hold up time, hold up pressure or delaying witch over point or by increasing the gate size if the above does not work.

Make sure it holds a cushion for the full hold up time.

Make sure the pellets are really dry.

Regards

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[MacGyverS2000]

Damn, Pat, how can you see any sinkholes with that pic?! I do see a number of air pockets in there (such as the red ring in the middle of the pic), but I'm with you on saying they're not happening after the fact.

Dan - Owner

http://www.Hi-TecDesigns.com

 

[Pud]

Dan,

They are not air pockets - they are vacuum pockets, caused by the outer surfaces cooling much before the material core.

Very common in alloy castings where they call them shrinkage voids.

It's a problem with all thick section moulding/casting - it's just that mostly they are not visible as usually opaque material.

Pat is suggesting that higher hold pressure (given a big enough gate) will elastically compress the inner material enough to compensate for the thermal shrinkage during cooling.

I think...

Cheers

Harry

 

[patprimmer]

I didn't even look at the pic. The bubble forming after being ejected was proof of a void, not an air trap.

Like Pud says, they form due to the contraction of the material as it cools and freezes.

Plastics shrink at an elevated rate as the material solidifies or freezes. When this happens in the centre of the thick section after the surface has already frozen solid it causes a void to form. If the surface was still hot enough, it would show as a sink mark or surface depression. he void forms more easily if there are volatiles in the plastic which boil off into the void.

Also like Pud says, they are rectified by packing the cavity with enough material to slightly compress the liquid and by continuing to pack material into the mould to replace lost volume due to cooling. The larger the gate, the longer the packing can continue before the gate freezes.

The methods I mentioned can all increase the pack or expel volatiles, and which to use depends on existing parameters. If some are already optimised, you need to try others.

You can test gate size by holding up longer with each shot then weighing the part. Once there is no weight gain, there is no point in increasing hold time unless you also increase gate size



Regards

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[morphi]

Hi

Thanks everybody!
could we control the contraction with boiling water ?
I mean , after molding , drop the part into boiling water ,
so the temperature of surface will be decreased slowly ,
does it work?

Regards

 

[patprimmer]

No. Boiling water is quite cold compared to the freezing temp of nylon

Regards

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[Pud]

Or even PMMA....

 

[Pud]

Any change of a section view through top to bottom so we can see the section thickness at the base (where I assume it's being fed)?

H

 

[patprimmer]

Opps. Wrong thread re the resin

Regards

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[morphi]

so sorry!
what do you mean " Opps,Wrong thread re the resin"

Regards

 

[patprimmer]

I said nylon, not PMMA a few posts back as I had also recently been posting about nylon in another thread.

Regards

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[morphi]

Thanks for your attention!