Tag Archives: drnick

Effects of pigments in dry mixing: What REALLY happens to physical properties?

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Dr. Nick Henwood

Many parts of the North American roto industry still rely on using dry color materials.  The main reasons for this are reduced cost and operational convenience. However, it is generally recognized that using dry color, rather than fully compounded pre-color, can result in a significant loss of material properties.  

If you’ve sat through as many ARM meetings as I have, you’ll have heard many different opinions voiced on the negative effects of using dry color and whether these effects can be mitigated.  As a scientist, my normal response to strongly held opinions is: “Do you have any data that supports this?” Unfortunately, when it comes to questions of dry color, there seems to be a dearth of hard data available to support us in making sensible decisions. Continue reading

Thickness Control – What’s Possible in Rotomolding?

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Dr. Nick Henwood

As we all know, a significant benefit offered by rotomolding is that we can achieve even wall thickness, compared to other processes.  One question that rotomolders often ask me is: how much thickness control can I actually achieve, in normal practice?

I pondered this question recently, while I was preparing resource materials for the ARM Operator Training Program.  I wondered whether I could get some data from a real-world example, then I remembered a product development that I was involved in a few years ago.  As part of this project, I carried out an assessment on the variation in wall thickness of a medium sized rotomolded part. Continue reading

Removing Stuck-on Debris from a Mold

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Dr. Nick Henwood

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

As part of a recent consultancy assignment, I needed to make a long run of parts in one of my hexagonal test tools.  At the end of the job, the two ends of the tool were coated with a lumpy deposit of overcooked polyethylene (PE).  I was aware that this deposit was building up as I continued to mold parts, but I was under severe time pressure, needed to keep the job going and didn’t have time to clean the mess off every cycle.  This type of thing shouldn’t happen, but it sometimes does, even in a well-regulated molding operation.

So, at the end of the job, I was left with a nasty mess to clean up.  When you repeatedly cook and cool PE, it can have a tendency to crosslink and the result is an extremely tough polymer layer which is extremely hard to remove.  This was certainly the case on this occasion and my attempts to remove it with a plastic scraper were totally ineffective.  I didn’t want to take a metal scraper to it and I really didn’t want to send my tool away for sandblasting, so I decided to try a trick that an experienced molder friend had told me about. Continue reading

Rotomolding Styrenic Polymers – is it feasible?

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Dr. Nick Henwood

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 project but comments should be on technical matters raised. Dr. Henwood’s responses are solely his responsibility and not ARM’s.

In the world of commodity plastics, it’s hard to ignore styrenics.  These plastics form a family of relatively low cost polymers and rival polyolefins (ie polyethylene and polypropylene) in the volume consumed in processes like injection molding.

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The patriarch of the styrenics family is polystyrene (aka GPPS – general purpose polystyrene) which is a rigid, hard, transparent homopolymer.  Despite these useful properties, it is too brittle for some applications, so a product called HIPS (high impact polystyrene) is also available.  This is a copolymer of styrene and butadiene; the inclusion of butadiene should increase the impact strength by a factor of about three, albeit from a relatively low base figure.

If you make a copolymer of styrene and acrylonitrile, you get a material with significantly higher heat resistance, called SAN (styrene acrylonitrile).  Impact-wise, SAN sits somewhere between GPPS and HIPS.

We rotomold lots and lots of polyethylene (PE), plus increasing amounts of polypropylene (PP), so why can’t we rotomold styrenics? Continue reading

At last! A non-PE polymer that I can rotomold!

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Dr. Nick Henwood

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 project but comments should be on technical matters raised. Dr. Henwood’s responses are solely his responsibility and not ARM’s.

I hope that regular readers to this blog series are not getting too depressed.  So far, I’ve looked at two polymers (ABS and PC) that both have – let’s be kind – “challenges” when you come to mold them.

This time I’ll talk about two related polymers that you definitely CAN rotomold.   Continue reading

Life’s just too short to rotomold polycarbonate!

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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 Continue reading

Why can’t I rotomold ABS?

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Dr. Nick Henwood – ARM Technical Director

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.

If you’re looking for a polymer that’s superior to polyethylene (PE) in terms of properties, ABS (acrylonitrile butadiene styrene terpolymer) is a good candidate.  It’s a very popular choice made by injection molders who want to make great looking parts that are heat resistant and tough.

So can you rotomold it?

Continue reading

Why do I have to use polyethylene for everything?

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Dr. Nick Henwood – ARM Technical Director

I’m sure that, at some time, you must have pondered this question.  Having spent the first twenty years of my roto career selling the stuff, lots of molders have asked me that question, along with – “why can’t you guys develop something with better properties than PE?”

I should immediately declare an interest: I love PE.  It paid for my three kids to get through university and it bought me my first house.  However, even an ardent PE fan like me has to admit that PE has its limitations.  Sometimes it’s not stiff enough, it scratches really easily, you can’t paint it, it goes seriously floppy at temperatures above 100°F and it doesn’t contain gasoline very well.

So why is our industry totally addicted to it?  Continue reading