Life’s just too short to rotomold polycarbonate!

henwood

Dr. Nick Henwood

In the early 2000s my company was doing some research on rotomolding polycarbonate (PC) and this was the heart-felt response of one of my technicians, who had spent nearly a month producing various scrap parts using this interesting – but very frustrating – polymer.

PC is another of those materials that would be absolutely fantastic for rotomolders – provided it was rotomoldable!  Three times as stiff as ordinary PE roto grades, excellent creep resistance, high heat resistance, extremely high impact strength and paintable (with the right paint systems).

Another really interesting aspect is that PC is inherently flame retardant; even without additives, it can pass at V-2 on the UL94 flammability test.  With some added help, a V-0 pass is definitely possible, even for a thin-walled part.

Knowledgeable readers will have noticed that I haven’t yet mentioned that PC is a totally transparent: not translucent, like polyethylene, but glass clear.  The pursuit of clarity, especially for applications such as street lighting globes, could be viewed as one of the aspects that has retarded the extended use of this material.

Put simply: you can’t get rid of the bubbles!

One feature of the rotomoulding process, is that air bubbles tend to form as the powdered polymer lays down on the surface of the mold.  Most of these bubbles will disappear as the cook process continues and, with an opaque or a translucent material, the presence of a few bubbles in the wall of the part will go largely unnoticed.  Unfortunately, with a truly transparent plastic, every single bubble is obvious and plain for all to see.

All of which brings me to an odd, frustrating, aspect that I found when I was trying to rotomold PC.

Like any experienced molder, I knew that continuing the cook cycle should result in the gases in the air bubbles eventually dissolving into the polymer matrix.  When I tried this systematically with PC, the incidence of bubbles did indeed start to reduce as I cooked the part more.  However, just as I thought I was getting on top of my bubble problem, a whole collection of new bubbles started to appear!  Somehow they looked different to the other bubbles and they got appreciably more numerous as I increased the cook time.  Weird!

In retrospect, I think what I was seeing was the effect of moisture.  PC is a hygroscopic polymer, which means that it absorbs and retains moisture from the atmosphere.  During the roto process, this moisture gets turned into steam, reacts with the polymer and forms more gas.  Having read around the subject, I’ve found that something similar can occur when thermoforming PC and drying of the raw material is often recommended.

If you can’t totally get rid of the bubbles, you can try to hide them.  For lighting globes, you can put a heavy texture on the mold, which will not impede light transmission, but will disguise the presence of bubbles in the wall of the part.

Alternatively, for applications other than lighting, you could pigment the material and render it opaque.  Problem solved, except you still need to deal with the bubbles that appear on the surface as pinholes.

In my previous researches, we ran out of time and patience and moved on to easier projects.  If that seems a bit spineless, better molders than me have struggled with the same problems.  One of ARM’s finest, Formed Plastics, fought with PC’s challenges for a number of years.  I’m grateful to Pat Long for a recent, highly informative, conversation which confirmed that my own experiences with PC were by no means unique.

Having said all this, PC remains a very attractive polymer, in terms of its unusual combination of physical properties.  There are a few molders worldwide who appear to have refined their molding techniques and produce high quality rotomolded items from PC.  For example, one of my local UK customers produced some very fine looking mannequins from PC.

PVC lady.png

A blog like this can only be a short introduction to the challenges of rotomoulding PC.  There is actually an ARM booklet “The ABC’s of Rotationally Molding Polycarbonate” that provides more information.

For molders who fancy a real challenge, why not try your luck?

As always, please do respond to my blog and give me some feedback.

I would love to hear from any brave, determined soul out there with practical experience of rotomolding PC.

Does anyone have any experience of drying hygroscopic powders?  This is a wider issue than just PC, because a number of other, potentially rotomoldable, polymers (eg nylon 6, acetal etc) would probably need drying before use.

Would the challenges presented by PC be any easier to cope with in a non-conventional rotomoulding machine?  Does anyone have an opinion or, better still, some practical experience?

ARM Technical Director, Nick Henwood, is drafting a publication for the ARM Education Committee on rotomoldable polymers other than PE, PP, and PVC. Early drafts of each chapter will be published here every two weeks.

Our legal counsel notes: Dr. Henwood speaks for himself and not for ARM as a whole. We encourage feedback on this projects but comments should be on technical matters raised. Dr. Henwood’s responses are solely his responsibility and not ARM’s.

Dr Nick Henwood serves as the Technical Director for the Association of Rotational Molders. He has 25 years-plus experience in rotomolding, specializing in the fields of materials development and process control. He operates as a consultant, researcher and educator through his own company, Rotomotive Limited, based in UK.

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