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.

Stop Warping! ARM Regional Meeting in Minneapolis

What is warping and why does it occur? How do pigment applications affect warp? How can rotomolders use external means to prevent warping? We’ll address these questions and more at our How to Stop Warping! regional meetings on June 18 in Minneapolis, MN.

Here is what molders at those meetings told us:

  • “This was my first meeting and I honestly did not know what to expect. I was really impressed with everyone’s presentation and stories of struggle that we all have and suggestions on how to fix them. Thank you for having me, I would love to attend another.”
  • “I was a little concerned going in that the single topic would become redundant. However, the different POV’s from the industry made the first half of the event very interesting and applicable.”
  • “Great topic. Good job from all of the speakers. Very well planned.”

Our topics in Minneapolis will include:

  • Case studies on warp
  • Mold release application
  • How pigment affects warp
  • Preventing Warpage from the Machinery POV
  • and more

When the meeting ends at 3 PM, we’ll take a tour of Able Brewery and Seedhouse.

Speakers include:

  • Mike Haubert, Mosaic Colors
  • Rick Calrsen, Solar Plastics
  • Daven Claerbout, Dutchland Plastics
  • Ron Cooke, ExxonMobil Chemical Company
  • Adam Covington, Ferry Industries
  • Larry Whittemore, Stoner Solutions

If your company sends more than three people, you can take 25% off using the promo code warpzone

Sign up the meeting here.

South African Roto Conference: September 16 – 18


ARM members are able to attend the South African Association’s conference at their member rate this year. ARM is a member of ARMO, an affiliation of rotomolding organizations around the world.

ARMSA (Association of Rotational Moulders of Southern Africa) will be hosting the Annual ARMO Event at Sun City in South Africa from the 16th to the 18th of September 2019 and this event promises to be an event like no other.

Sun City has been specifically chosen since it caters for everyone’s needs. The Pilansberg National Park borders on the Sun City Resort which is a Game Reserve with the “Big Five” for breath-taking Safaris, the Resort also boasts two Championship Golf Courses, Casino, Waterworld (with a Beach), Crocodile Camp and Hot Air Balloon Safaris to name a few experiences!

Sun City is a luxury resort and casino, situated in the North West Province of South Africa. It is located between the Elands River and the Pilanesberg, about two hours’ drive from Johannesburg, near the city of Rustenburg.

For Rotational Molders and Suppliers alike this will be a great opportunity to network, socialize and learn in the warmth of South Africa.

Exhibitions, Demonstrations, Seminars, Safari & Socializing give you a great reason to join us in South Africa for ARMO 2019.

Visit for more information

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

President’s Message: Serving Our Members


Dru Laws

As I enter my final year as ARM’s President, I’m excited to continue to the shape the future of the Association and rotomolding. We had an amazing 2018 with the implementation of the Operator Training Program, a successful meeting in Montreal, and great technical value for members including the much anticipated Resin Guide.

2019 Goals

  • Grow Membership
  • Get Involved
  • Improve Technical Content

ARM prides itself as an association that is welcoming to new members and new ideas. We hope to continue to grow membership in 2019 to increase value for current members and grow the rotomolding industry as a whole. If you know a company that would be a good fit for ARM, contact our staff at

I hope that you will consider joining a committee this year. Committees are open to all members and require little commitment but return a great investment on your time.  If a committee isn’t your cup of lemonade, join us on a Hot Topics in HR call, troubleshooting calls, or a webinar. These are all free for members and provide value without having to leave your office. From the networking opportunities to the ability to shape the industry, I think you will enjoy getting involved with ARM in 2019.

We are happy to again have Dr. Nick Henwood as the Technical Director for ARM in 2019. Dr. Henwood provides blog posts, input on webinars, and reviews all materials for meetings. ARM is dedicated to improving technical content and continuing to be your go to source for knowledge about rotomolding. This year’s webinar series will focus on finishing training.

We are planning to update the Resin Guide and hold three regional meetings focused on How to Stop Warpage. This year’s Annual Meeting in Houston is being organized now but we know it will offer a valuable program for rotomolders at all levels and Spanish translation.

The Roy Crawford Education and Development Foundation will offer a scholarship for graduating seniors. Details are available here.

ARM exists to serve our members and we want you to feel that value.  Please consider recommending a new member, getting involved with a committee or call, and taking part in our technical programs. I’m excited for the year ahead and to make my last year as President one for the record books!

Dru Laws is the President of ARM and the Senior Vice President of Seljan Company in Lake Mills, Wisconsin.

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

Miller Time: More on Angel Hair


Rob Miller

Further to Dr Henwood’s comments on Angel Hair, I would like to add the following…

Dr Henwood is correct that it is typically formed due to high temperatures in the conveying line.

I would comment that a good conveying system design utilizing good conveying parameters can be employed to eliminate the creation of angel hair, therefore eliminating the need for any kind of angel hair trap. Traps can be helpful in collecting the angel hair that is created, but why not investigate and potentially eliminate the basis for the angel hair creation?

Most conveying in the plastics industry is known as “Dilute Phase conveying”. This typically represents that the conveying line is approximately 10-15% material concentration at any given moment.

In the designing of a conveying system, there are limited factors that can contribute to a safe or unsafe conveying situation.

Velocity is everything in conveying. Continue reading