Mold base deflection?

[dioiao]

Hi,

we're designing a 4 cavity thin-wall cup, but we face the problem of mold base deflection. the middle of mold plate(male side) will sink by injection pressure and cause uneven wall-thickness. When mold base sink, mold cores will move toward the center.

we have to put few thin copper sheet (maybe 0.1~0.15mm) in the middle of male side plate to adjust the plate deflection. Is there any better to avoid plate deflection.

Thanks

 

[patprimmer]

Use larger support plates and pillar blocks.

Design the mould taking into account that hydraulic pressure applied to the cavity surface which can be as much as 2000 bar in a thin wall application.

Regards
Pat
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[dioiao]

Dear Pat,

yes, the plate thickness is very important. Some of our mold core support plate can be over 120mm....

attached is another sample of mold. you can see mold deflection in the middle area.

we can adjust by machining the support plate or by putting some thin copper sheet in the middle of plate. but i just want to find out if there's another way to solve the problem.

Thanks

 

[Pud]

It may be the machine platen flexing and allowing the tool to flex too.

One reason not to buy cheapo moulding machines...

H

http://www.tynevalleyplastics.co.uk

 

[765923]

I agree, you may need better moulding machine with higher clamping force.

 

[patprimmer]

It does not need more clamp as such. It needs more rigid structure. On some moulds in some machines that can also depend on where you bolt the mould to the platen. ie with a long skinny mould you can bolt between the toggle attachments to the platen or across them by rotating the mould 90 deg.

Regards
Pat
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[Pud]

Pat: Indeed so.

With very thin wall "packaging" products, the moving half of the tool cores are not inserted in the bolster - they are all machined from solid. This costs $$$!.

In the op's case, with the image shown of a plastic spoon array, imho the injection speed/pressure is too great* or the machine platen is flexing. (or the moulding conditions are a) rubbish! or b) the tooling is mild steel instead of what you actually specified - another story altogether)

"Give me a tool and I can make it flash"

H

* I am assuming the parts are HIPS - increase melt temp = increase MFI = lower pressures - melt temp makes virtually no difference to cycle time (contrary to popular belief)

(Last post this thread)

http://www.tynevalleyplastics.co.uk

 

[dioiao]

Dear Pud,the material is PS. Mold core steel is H13,HRC 50.

Dear Pat, you're right for long thin mold, this kind of deflection happens all the time. It also happens on muti-cavity thin-wall container. ex. a 4 cavity thin-wall cup, mold core will always tilt toward the center of mold plate.

for the spoon mold, you can image the injection force in the middle of sprue will make the steel bend slightly, our experience will be 0.05~0.1mm.

Sometimes to adjust this situation is very difficult. Even we have to add the wall-thickness to full-fill the cavity.

 

[patprimmer]

You can also improve fill by adding a flow promoter down the centre of the cavity.

Harry is also correct re temperatures. A rule of thumb is

Flashed near gate and short equals material to cold.

Mould is accurately blanked off but flashes only in some places equals mould bending.

Regards
Pat
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[bobslo]

Hi

Some info, hope it will be useful:

Step 1: Technology
Too large clamping force will increase stress/strain on the mould. You should reduce the clamping force till this burr has same thickness as you have now or. smaller.

Step 2esign

a) Deflection in the middle of moulde base is normally solved with supporting pillars (surplus 0.01 mm)

or

b)cavity/supporting plates should have multiple dowel pin connections. This will transfer much of plates bending stress to shear stress on pin/plate connection there-fore reducing deformation.

Best regards