Minimum Wall Thickness - Injection Molded PC / ABS

It should not be less than 1.5 mm due to higher MFI of PC/ABS alloy properties.

Depends how hard you want to push it. The comment on melt flow index is correct, but, I've molded straight PC down to .015in wall. Knit lines, holes, part configuration and the talent of your molder all come into play.

You don't give us much to go on. The quality of the answer is directly proportional to the information we have to work with.

What did your supplier say?

My supplier didn't give me any answer.

The pc/abs was used to manufacture bluetooth housing(complicated configuration).but we met an intractable trouble: most of the housing were cracked while insert the screw, and the crack is somewhat away from the screw position.

The cause was analyzed, and we found that the injection process parameter is reasonable, the raw material is of no reclaimed materials. so we transfer our attention to our design,Partial region of the wall thickness was designed to 0.8mm, maybe it is too thin to endure the stress, but we don't have enough truth and reason to support this presumption.

Thus the quiestion above was asked, i wonder if the original supplier has a propositional minimum wall thickness for injection moulding of PC/ABS alloy.

If all the cracks are in the same position relative to the screw location, it may be knit line failure.

Knit lines occur on the opposite side of a feature from the gate.

Do you know what a knit line is?

Yes ,i know knit line.

But the crack is not in the position of knit line, so this is the difficult problem.

We really have no other idea.

The screw insertion generates a radial deformation of the boss and bending stresses in surrounding walls. Have you had a look at:

- hole diameter before screw insert? since if it is too small the stresses increase

- with a FEA analysis what stresses appear in the wall where the cracks are when the screw is pushing outside the boss wall?

The result can be a combination of problems in the mold with a design problem.

Hello,

You have an interesting problem but you do not give enough clear detail about the crack initiation. If the crack is initiated close to or at a stress point in the plastic caused by a knit line you can change the flow. Test it by adding a temporary restriction near the gate to eliminate this as a cause. You can put a pin into the mold near the gate and remove it when testing is done.

If the plastic still cracks and evidence is pointing directly to a screw boss design or a weakness in the design, you maybe able to thicken the area locally without causing a sink mark. Make sure the local thickening does not hurt your assembling other items.

Polycarbonate and ABS blends can be inherently troublesome if the mold temperature is not high enough and if the "Blend" is not consistent and mixed correctly. Try a mixing nozzle on the molding machine. They are a little expensive but the really work.

The .8mm thickness in itself is fairly thin and if it is the only place that cracks you may have to face the reality of product and mold ECI's.

You may try to find a different material supplier or find another suitable material.

I have tested large Plaques of this material in a Lab setting. The test plaques measured ~ 6mm thick x 200mm x 300 mm.

They all cracked in the direction of flow. The material did not impinge off a wall close to the gate but filled the cavity like a coiling rope.

The gate was in the center of the 200 mm side and was about 5mm x 200mm. The material did not mix or blend correctly and its glass transition temperature was different. This caused huge stresses in the plaques and they cracked while dogbones were being prepared for tensile testing.

The material was supplied and molded by a Manufacturer of blended PC/ABS and certainly was not fit for sale to the public.

One last thing. Do not use acetone to clean the parts and if you are using a potting compound, make sure that it has no acetone or similar solvent in it.

I hope that this sheds some light on your problem

Thank you very much ,and your answer is professional and perfect.

I will try to analyze this problem as you told me .I really appreciate you!Thanks and best regards!

Can die temperature for PC/ABS blend be 120℃ ?

There is an other aspect which is of importance : the ratio length/thickness.

The mold wall has a temperature lower than the injected material. This "hot" plastic transfers heat to the wall and increases its viscosity which can lead to a bad structure and bad junction with an other flow path. Look at this too. If you are confronted often with such problems there are today soft packages to simulate the flow and its temperatures which consider the mechanical properties evolution with temperature.

It could be worth for you to buy such a programme and reduce the risk at the design of new parts.

Is it the software of mold flow analysis .

Yes, i think it will really do help!