Tag Archives: drnick

Why is there (what looks like) orange contamination in my powder?


Dr. Nick Henwood

Bizarre as it may seem, in the past few weeks, I’ve had two different consultancy customers report powders with this same problem.  They sent me samples and parts but, even before they arrived, I suspected that the problem was gas fading.

Some of you may have experienced this phenomenon before, and wondered why it happens:

The problem: A coloration (usually either orange or pink colored) that you can see clearly in your powder.  I’ve included a photograph below, to illustrate the point.  This material was actually compounded white, but it can show in natural material as well…


Despite appearances, this is not just a gross contamination of the powder, it’s something else.  So – what is it? Continue reading

Ask Dr. Nick: Why does the same mold need different cook times in a different rotomolding machine?


Dr. Nick Henwood

Rotomolders who have multiple machines often find that, if they move a mold from one machine to another, an adjustment in cooking conditions is invariably required.  The differences between machine performance can be considerable. Whilst this may be expected when moving from one style of machine to another, an adjustment may even be required when moving between machines of the same type or model.

Whilst most rotomolding grades of polyethylene are actually quite forgiving of processing variations, the issue becomes especially relevant when molding materials with a narrower processing window (eg repro, foams, polypropylene or crosslink).

Why can there be such a big difference?

The first thing to understand is that the temperature showing on the control panel of your machine is, almost certainly, not the actual temperature in the oven.

The oven requires a control signal that will call on the burner, when required.  This signal is a temperature, measured by a thermocouple located in the burner duct.  The burner duct is a passage external to the main oven, which contains a circulating fan and the burner itself.  The action of the circulation fan draws air out of the main oven, raises its temperature (if necessary) by switching on the burner, then sends the air back into the main oven at a different place.

The position of the control thermocouple in the burner duct will make a significant difference to the temperature it reads.

In many North American machines, the control thermocouple is located upstream of the burner.  In this case, the temperature measured will be less than the temperature in the main oven, because heat will already have been taken out of the air stream by the action of warming the contents of the oven (ie the arm, plate, molds and mold contents).

In some other well-known brands, the control thermocouple is located downstream of the burner.  In this case, the temperature registered will be more than the temperature in the main oven, because heat will not yet have been absorbed by the contents of the oven.

So, the temperature showing on the machine control panel is most unlikely to be the same as (or even similar to) the temperature in the oven.  Its purpose is simply to act as a control variable, to operate the burner. Clearly, its value is related to the oven temperature, but it will not be the same.

In many ovens, the difference can be significant.  In addition, the difference will vary depending on the Actual Oven Temperature.

To illustrate the point, I have shown data from my gas-fired laboratory machine.  This is laid out in the same way as larger roto ovens, with a burner duct containing a circulation fan, the burner itself and a control thermocouple.

Using a K-PAQ that I have permanently installed on the arm of my machine, I measured the Actual Oven Temperature achieved after the system had reached equilibrium.  I then varied the Set Point Temperature (ie the temperature showing on the control panel), waited for the oven to reach equilibrium and recorded the Actual Oven Temperature again.  I repeated this procedure for a number of Set Point Temperatures and produced the Oven Characterization Curve shown below.

Screen Shot 2019-04-09 at 4.50.37 PM

You can see from the graph that, for my oven, the Set Point Temperatures were consistently lower than the Actual Oven Temperatures.  For example, at 300°F Set Point, the Actual was 370°F (70°F difference). At 375°F Set Point, the Actual was 460°F (85°F difference).  At 450°F Set Point, the Actual was 545°F (95°F difference).

So, even the numerical difference between Set Point and Actual is not fixed.  To fully understand the relationship between these two temperatures, you need to perform a characterization exercise across your normal oven operating range.  Then you will know what Set Point Temperature on Machine A is equivalent (in terms of Actual Oven Temperatures) to a certain Set Point Temperature on Machine B.  You need to characterize and compare all the ovens in your shop.

Of course, if you constantly use in-mold temperature measurement to control your process, you don’t need to worry with any of this.  However, for the 99% of moulders who don’t do this, characterizing your ovens will be a good start to achieving better process control and more operational flexibility.

With a bit of ingenuity, you can do a characterization with a hand-held thermocouple.  Alternatively, you could get someone with a K-PAQ (or similar device) to come and do it for you.  Once this exercise is done, you will be set up well for future operations.

Happy rotomolding!

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.

Ask Dr. Nick: How does cold weather affect the rotomolding process?


Dr. Nick Henwood

You may have heard that in England, when we’re not obsessing about Brexit, we often discuss the weather.  This is odd, when you consider that, other than a lot of rain, we don’t see extremes of weather that often.

Recently, we’ve experienced what we Brits would regard as some cold weather, although it’s been nothing like as bad as that experienced by my friends in the Midwest.  I recently phoned Adam Webb in Chicago, on the day when outside temperatures sank to minus 50°F.

My rotomolding lab in UK is a 1,500 sq ft industrial unit, of pretty standard construction (mainly precast concrete panel).  On a recent morning, I experienced an ambient temperature of 30°F, when I opened up the molding area. This compares to an ambient temperature in the range 60-80°C during mid-summer and even higher after a heavy day of rotomolding.

I’m molding all year round, for my various R&D  and consultancy projects, and I know it’s important to maintain consistency.  The question is: how big an effect will ambient temperature have on what I produce and how I produce it? Continue reading

Ask Dr. Nick: Can I fix gaps in a parting line?


Dr. Nick Henwood

A badly fitting parting line is a regular pain in the neck, for a number of reasons.  The most notable annoyance is that, as the mold rotates in the initial stages of heating, powder spills out from any gaps that exist.  This wastage of powder can cause an under-weight part and, even if the spillage is small, the powder burns, makes a mess in your oven and creates a nasty smell.  Better to avoid the problem, if you can!

Recently I was given an old steel test mold from another lab; it was a hexagonal cylinder used to make 5 inch square plaques for the ARM Low Temperature Impact Test. (The procedure for this important test is available on the ARM website.)  The first time I put the mold on my machine, I noticed that I had a small powder spill from the parting line area.

gap edited

By good fortune, the next day I participated in one of ARM’s Troubleshooting Calls; we run these every month, as a free-to-member service.  One of the regular moderators is Sandy Scaccia of Norstar, who is one of our industry’s top mold experts.

During the call, I asked Sandy for some advice about what I could do to reduce, or hopefully eliminate, the parting line gaps.  He told me of a procedure he had used for aluminum molds: heat up the affected area and use an exterior clamp to squeeze the parting line shut while it is still hot.  He expressed some doubt that this would be as effective for a steel mold, but I thought it was worth a try. Continue reading

Ask Dr. Nick: Avoiding “Angel Hairs”


Nick Henwood

On ARM’s most recent What’s Your Problem? teleconference, there was a question asked about powder piping systems, especially about avoiding and dealing with “angel hairs”. I provided some follow up to the molder after the call, which we’re happy to share here for everyone’s use.

My first recommendation was to contact ARM’s mainstream pulverizer supplier members, who should be able to offer good advice. They are the real experts in this area.

In the meantime, I also recommended installing simple traps for angel hair in your lines.  Pulverizer systems have these in place.  The grid has something like ½ inch gaps and the sideplate can be opened for manual removal of accumulated debris.  The pulverizer folks will have proper drawings of this; please excuse my rudimentary draftsmanship!

trap for angel hair

As far as I’m concerned, angel hair production gets bad when ambient heat is sufficient to start to soften powder particles.  A particle momentarily trapped on an obstruction will then get stretched and elongated by fast air flow around it and a hair gets formed.  So keeping temperatures down (say below 100°F), or not generating elevated temperatures in the first place (correct pipe sizing and avoidance of sharp bends), should help a lot.

Note from staff: ARM offers What’s Your Problem? teleconferences — an audio version of our popular troubleshooting workshop — to our members every six weeks as a free benefit of membership.

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.

Ask Dr. Nick: What’s an Acceptable Scrap Rate in Rotomolding?


Dr. Nick Henwood

Recently an ARM member from Latin America phoned in with a technical question and we got into a conversation about the vexed subject of scrap.  I appreciate that it’s a bit of a prickly issue and probably not something individual rotomolders would be keen to discuss openly.

Not being a molder, I was probably more comfortable than most to tell our colleague what I had observed myself, having worked with hundreds of rotomolding companies over my 30 years in this business.

I thought that my observations might be useful, if only to reinforce how important this issue can be commercially.  We have to live in the real world, so some scrap is almost inevitable. Rotomolding is simultaneously fascinating and frustrating, because there are so many variables at play and not all the variables are easy to control (eg the weather / ambient conditions).

It seems to me that the main trick is to stay vigilant and bear down on scrap and the reasons why we may make scrap.

Anyway, here are some of my thoughts.  If any of you molders out there would comment, that would be fantastic!  If you think I’m talking nonsense, feel free to “roast” me! Continue reading

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


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?


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


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?


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.


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