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Why Does Polypropylene Warp?

04/08/2009 11:12 PM

Why does polypropylene warp and what types of countermeasures should I implement to?

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#1

Re: Warp Causes

04/09/2009 4:27 AM

Hello,

The very simple lay-mans answer is that is it is the nature of the polymer to warp.

The reasons are many and varied. I will touch on a few and that might spur you to give a reasonable background to the question and then we can give a reasonable answer.

A brief list first:

Part design.

1. Wall section variations of greater than 40-60%.

2. Intersections of faces and feature differing 40-60%.

3. Fillets or radii that violate the rules above in (2. & 3.).

4. Thick Wall sections

Mold Design

1. Type of gate, Edge, tunnel gate, modified tunnel gate, flash gate, Hot-tip, Hot Drop

Fan gate etc.

2. Gate size.

3. Gate location, does material impinge on a wall close to the gate? or streak across the cavity and back fill trapping air, causing Knit-lines etc.

4.Venting. Isit adequate. Think of hooking up you garden hose to a soda bottle. You cannot fill a sealed container.

5. Mold cooling. Is it even, and adequate?

6. Are there long flow-lengths.

7. Is there ample surface area for 'ejection of the part'.

8. Are there undercuts or incorrectly timed lifters.

9. Is the texture on the mold rough.

10. Is there a speaker grille or other feature with a large surface area of wall and little opportunity for ejection surface.

Molding Problems and post molding cooling:

1. Is the injection cycle to ,slow or fast?

2. Is the cooling cycle to short.

3. Is packing to much ,little, to late etc.

Picture this part being molded:

Assuming that we are injection molding your said warped article, say, it is a flat rectangular 'tupperware' type lid for a food container.

It will typically be filled with one gate ( where plastic enters the part) in the center of the part. The part will fill in less than .6 of a second. It will fill the short wall side first and then the long sidewall, (W)

As the hot melt enters the cold mold it forms a cool layer on the upper and lower mold faces of the mold, a "skin" and on constantly changing flow front. It forms a molten core between these two 'insulating' skins through which the still molten plastic rushes via the line of least resistance, to fill the mold cavity.

At the point in the molding cycle where the cavity volume is filled with hot and therefore expanded plastic, the molder changes to a holding or packing and holding phase. This is also the beginning of the cooling phase. If the mold is under-packed the result when cooled and shrunken will be a stressed part, more susceptible warping. (W). Parts are typically ejected as soon as the skin sets up with enough integrity to withstand the ejector pins and lifters pushing the part out of the mold without marring the ejection surface. The part most likely is not stable and futher shrinkage will occur , mainly in the next 24-72 hours.

On the other hand if the part is over-packed we can have stress.(W) This may be due to density differences in the wall section of the part and the heat transfer during cooling.

The plastic pushes the air that was in the 'cavity' (mold shape) ahead of the flow front. This air must escape as quickly as the plastic enters. Not doing so can cause stress and (W) warp and burn marks.

If the molding surfaces (called the cavity and core surfaces) are not evenly cooled this will cause stress.(W)

If the part is not completely cooled it must be kept flat or unstressed. (W)

Uneven cooling and "Post Molding Shrinkege" due to bad part design, even if wall sections are sized perfectly cause warp. (W). There are endless reasons.

So, as you get the picture you can see why we need more info.

Conclusion: Thick wall sections can be filled using gas-assisted molding. Simpler again is to use a blowing agent which expands, nullifying the effect of shrinkage and reducing stress and warpage. Larger gates leading to lower injection pressures can help. The single biggest factor is part shape and design.

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#2

Re: Why Does Polypropylene Warp?

04/10/2009 2:16 AM

2tinker has given a comprehensive critique of the process variables.

Here's a comment on the polymer: polypropylene is a partially crystalline thermoplastic. Its density depends upon the crystallinity and the crystallinity depends very much upon the cooling rate. Areas that cool more slowly will develop a higher crystallinity, therefore a higher density (lower volume per unit mass) than areas that cool fast.

Density variation through a molding cross-section that's asymmetric (or which cools asymmetrically due to mold mass distribution, cooling circuit asymmetry, etc) will cause warping.

Generally, the component designer should understand those truths. Rarely does he/she.

Generally, the mold designer should work to minimize cooling asymmetries, whether due to local mold mass or the cooling circuit layout. Rarely does he/she.

In practical cases, the mold void space is simply a shrinkage adjusted representation of the component CAD image, which makes warp management quite tough.

Choosing a nucleated grade of polypropylene can take differential cooling rates out of the equation, causing the whole polymer mass to approach the same crystallinity level, despite component design issues and mold design issues. Alternatively, stiffening the polymer with a mineral filler is a common practice, especially in the automotive arena.

I'd be looking into either a mineral-filled PP or especially a nucleated PP.

Mark Bingham
Relativity PL

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#3

Re: Why Does Polypropylene Warp?

04/10/2009 8:17 AM

Make the polypropylene 30% glass and you will reduce the deformation characteristics of polypropylene.

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#4

Re: Why Does Polypropylene Warp?

04/10/2009 11:06 AM

Thanks for the feedback. I'm a Quality Engineer trying to help my Design Engineers with warping solutions on battery housings. They know so much that they don't have any available "room" to think differently...

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#5
In reply to #4

Re: Why Does Polypropylene Warp?

04/10/2009 10:29 PM

Hello there,

I have worked on a battery-pack for the old Motorola brick phone. The designer needed to reduce wall section to .030" or less in some areas. The material lacked all integrity at those thicknesses and warped with the heating during recharging.

Thinking of your quest for the last hour or so, can I suggest the following experiment. I am assuming that it is important to have PP as the contact surface with the battery innards .

1. Mold an ultra-thin substrate of an engineering material to provide stiffness and integrity.

2. Using this substrate, mold an ultra-thin layer of PP over it.

3. Test different wall thicknesses of each material until you optimize the process.

4. I have molded glass filled Ultem for short walls flow lengths and measuring only .010" thick. The plastic, sans glass, actually extruded into the wall section blocked by the glass at the intersection of the thick to thin wall section. Using less or more glass filling can help, as can using highflow materials.

5. Make sure that the materials are chemically compatible.

I think this would be worthwhile experimentation and you can use simple experimental tooling to try out the concept.

This can be made of aluminium or P-20. If I knew the shape I might even have some suitable molds to screw around with. It will have to withstand pretty high injection pressures but I have molded reasonably long flow lengths as small as .008-.010" in PP and even styrene and many other material while experimenting with thinwall containers. Good Luck

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#6

Re: Why Does Polypropylene Warp?

04/19/2009 10:05 PM

I am not sure what size item you are talking about but I have used shrinkage jigs with great success with both poly Prop. and poly Ethylene.

Some of these items were lead/acid batteries cases for large forklift trucks. The items were were removed from the mould and placed into the shrinkage jig and submerged in water for about 5 minutes then stacked for a few hours.

Wall thicknes was 6mm (1/4") and 750mm high (29 1/2"). Thick section and long runs gave lots of room for warpage but the jigs controlled it through to setting.

BAB

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