Preloading support pillars. 3

[jerzy]

Hello forum members,

How much preload do you recommend for support pillars in a mold?

Jerzy

 

[ornerynorsk]

I've never made them longer than the ejector box siderails. If your press is in good condition, and your process parameters and pressures are all dialed in, I can't think of an instance where you would want them longer.

Are you having trouble with an existing mold, or planning for a new tool?

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

 

[patprimmer]

I have never discussed preload on them and always just presumed they where a neat fit for length.

I have though discussed their bearing area and calculated the tonns load they must bear without distorting enough to allow some parts of the mould to flash.

Yes moulds can and sometimes do bend under injection pressure.

I believe bearing area and steel grade are the answer. Preload is a poor Bandaid.

Regards
Pat
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[jerzy]

Thank you for the reply.

The question arose regarding an existing mold that a client was
having some problems with. Their toolmaker claimed to use .008 inch as a standard preload for support pillars. I have always thought that zero to .002 would be commonly used.

While researching the topic, I found that for some severe thin-wall applications a preload of .004 to .005 on the center pillars
might be appropriate, but this would not be normal practice for most molds.

The application in question is not a thin-wall case.

Thanks again for your valuable contribution.

Jerzy

 

[ornerynorsk]

.008 seems like an awful lot, Pat referred to it rightly as a Bandaid. Something else is amiss. Besides bearing area and grade of steel, both in the mold frame and in the pillars, is the B side platen of the press itself in good condition? (IE has it been checked) Is the mold size such that the press might be operating close to its clamp limit? Have you checked stretch on each tiebar to make sure the clamp is closing evenly?

These are just a few of a hundred variables that come to mind immediately.



It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

 

[patprimmer]

Presuming a good blank off, typically, insufficient clamp for whatever reason results in reasonably evenly distributed flash.

If wear on the tie bars allows the mould to rock, that leaves the flash down one side. If the projected area is not uniformly distributed about the axis of the platen, the flash will always be on the side where the most projected area is located.

If the flash is thick near the gate and thin near the edges, the mould is bending.

Either the platen itself is not true or is bending or the cavity is bending relative to the back plates.

If the cavity is bending it is because there is to much space between pillars or the pillars are compressing or being driven into a back plate that is compressing locally only.

Without seeing examples of the actual problem moulding and the mould and being able to take measurements, I really can't say any more.

Regards
Pat
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[jerzy]

Hello Onerynorsk,

Flashing had not been a problem: the preload question came up because there was some hobbing of the support plate and pillars.

The problem was galling of a moving core on a near-vertical shutoff. We were looking for distortion of plates.

Jerzy

 

[ornerynorsk]

You could actually be inducing distortion with support pillars that are too long, that would also cause the hobbing or fretting of the support plate, as well. Every time the clamp closes it will be unnaturally "working" the material.

Getting any rub spots on the leader pins or bushings that would indicate lateral movement?



It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

 

[patprimmer]

In my view ornery is spot on the mark with the above comment.

Regards
Pat
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[jerzy]

There is some scoring on a pin that actuates a stripper plate.

Besides distortion of plates,there might be a metallurgical problem: the galling surfaces do not seem to differ much in type or hardness.

Thanks for the helpful comments.

Jerzy

 

[klank]

I normally do a calculation on how much to preload support pillars, as the compression of the support pillars are very independent of many factors such as diameter and length of the support pillars, injection pressure, number of cavities etc.

I have uploaded a PDF showing how to do the calculations.

 

[petman]

Built moulds for over 25 years and .002" is the usual standard.

 

[jerzy]

Thank you all for the contributions. We are going to reduce the preload and see how the mold runs.

Jerzy