Author Archives: rotomoldingblog

In 2022, bring your operators to Intro to Rotomolding

Dru Laws hosts an Introduction to Rotational Molding seminar at the Doubletree Cincinnati Airport in Hebron, Kentucky on August 29. Register here.

The seminar covers the key elements of the rotomolding process. This includes available materials, the main types of equipment used, processing parameters, process control methods to optimize part properties and key design guidelines. Attendees will have plenty of opportunities to ask questions and explore areas of particular interest.

Course Outline

Molding Cycle Overview & Temperature Profile

  • Heating and Cooling
  • Particle Shape and Size
  • Venting and Pressure
  • Rotation Speeds and Ratios
  • Static and Humidity
  • Other variables

Basic Quality Testing

  • Pre-Process Testing
  • Post-Process Testing

Wall thickness

  • Simulated Data
  • Test Data
  • Comparing to Other Processes
  • Skin Mapping
  • Design Considerations

Impact testing

  • Definitions
  • Layer Evolution
  • Impact Maturation


  • When it occurs.
  • Where it happens.
  • Why it matters.

What Happens in the Oven

  • Oven Heating Types
  • Oven Shapes
  • Anatomy of an Oven
  • Dissecting the Temperature Profile
  • Rotation Reversal

Process Control

  • What can it do?
  • What could we all be doing?
  • Why do you care?

The Rule of Three

  • Basic Machine Layout
  • Temperature Profile in the Oven Station
  • Temperature Profile in the Cooling Station
  • The Basic Steps in the Service Station
  • Powder-to-Part Density Reduction
  • Energy Consumed by the Polyethylene
  • Simple Quality Confirmations

Dru Laws

Dru Laws has shared his experience and expertise with other rotomolders endlessly through his involvement with ARM. He has frequently organized and spoken at rotomolding conferences large and small. He has presented at multiple international conferences. For many years he has presented ARM’s Introduction to Rotomolding seminar which is an entry into rotomolding for many members. When the Association needs a volunteer, he is often the first to say yes.

Laws is ARM’s immediate Past President. He has chaired Committees and written ARM’s Rotational Molding Foam Process Guide. He graduated with distinction from the Queens University of Belfast in Northern Ireland with an MSc in Polymer Engineering, emphasizing in Rotational Foam Molding. He began his experience in rotational molding at Mity-Lite and is now an executive at Tango Manufacturing.

HYDROGEN – an exciting new opportunity… or a distraction?

Over the last year, I’ve heard a number of talks and seen a number of presentations relating to a new possible application for rotomolding – hydrogen storage tanks.  These may be stationary (for intermediate gas storage) or mobile (on hydrogen powered vehicles and other equipment).

ARM Technical Director Nick Henwood

Like any other member of our community, a significant new application for rotomolding is something that I would celebrate.  However, further thought and consideration has raised some questions, which I thought that I should share with you.

First of all, some background.  Hydrogen is seen as the ultimate “green” fuel because, when you burn it, it only produces water vapour.  No carbon dioxide produced- what’s not to like?  In principle, it could be a possible substitute for both natural gas (heating) and gasoline (transport).

Unfortunately, as with most things in life, there are some big BUTS to contend with…

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Ask Dr. Nick: Using thinner while flame treating parts

Member Question: In order to finish our parts we are using thinner along with gas flaming. Our process is not very safe and we have had a few events of rags catching on fire due to flame and thinner. Is there a standard way to finish rotomold parts (PE), maybe using non-flammable chemicals?

Dr. Nick responds:

Surface flaming of rotomolded parts is a technique that is commonly practiced.  It may be done for several reasons, primarily:

  • Preparation of a surface to make it more responsive to adhesives or paints.  Untreated polyethylene (PE) is highly inert chemically and is therefore not an ideal surface for adhesives or paints to bond to.  Flame treatment, combined with surface abrasion (e.g. rubbing with 60-80 grit sandpaper) will promote a better, if not perfect, substrate.  From a technical aspect, the flame treatment oxidizes the surface and creates polar groups which improve wettability and adherability.  The abrasion roughens the surface and provides more potential for a mechanical bond.  Providing both improved chemical and mechanical adhesion will optimize the surface.
  • Improving the visual appearance of the surface and introducing a more shiny look.  In this case, the flame treatment re-melts a thin layer of PE on the surface of the part and degrades it which creates, in effect, a higher Melt Index (MI) polyethylene layer that will flow better than the original material.  If the surface already contains defects (e.g. scratches or pinholes) flame treatment will not necessarily rectify all faults; it may even make them worse.  The normal procedure is to ensure the surface is clean, by wiping it with a cleaner, then to wash a flame over the surface.  A typical treatment would be to hold the flame approx. 12 inches from the part and wash for 3 seconds.

From the details in the member’s question, it appears that they are flame treating to create a superior surface effect.  The difficulties are created by using a “thinner” as the pre-treatment cleaning fluid.

The generally accepted definition of a “thinner” is a volatile solvent used to dilute or extend oil-based paints.  Typical products used include: mineral spirit, denatured alcohol, turpentine, acetone, naphtha, toluene, MEK and xylene.  OSHA considers thinners to be flammable liquids with a Flashpoint of approx. 70-140ºF.

Generally speaking, the lower the Flashpoint, the more likely there are to be unintended fire incidents.  So one approach to the member’s problems could be to research alternative products that have higher Flashpoints, e.g. water-based cleaners or “142” solvent.  Note that I’m not specifically recommending these products for the application, I’m just logging that they have higher flashpoints than the usual thinners.  The down-sides may be increased unit cost, plus a longer wait between application and full evaporation of the solvent.  With a well thought-out procedure for the process, these down-sides may still result in a more robust result, with less hazards.  As in many instances with rotomolding, improved house-keeping can make a big difference!

An additional procedure modification could be simply to separate the cleaning and flaming operations, in both space and time, so that the probability of accidental fires is reduced.  Obviously, the time separation can’t be so long as to mean that the surface picks up fresh contamination between cleaning and flaming.

Yet another approach could be to mold the part in a higher Melt Index grade than the one currently used; the difference in surface appearance between a General Purpose grade (MI approx. 4 g/10min) and a high-flow or “Toy” grade (MI approx. 7 g/10min) can be significant.  On the other hand, higher MI grades may exhibit some reduction in impact strength.  Whether this is worth looking at depends on how much of the part surface is flamed; if you need to treat the whole part, changing material could be a good option.

The other general caution I would make regards all types of manual procedures.  These can be highly operator-dependent; different people will do the procedure in different ways, with different results.  It always pays to establish the best way of doing a job and then use formal procedures and training to ensure that everyone does it in the same way.

Dr Nick Henwood serves as the Technical Director for the Association of Rotational Molders. He has 30 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.

Everything ARM did in Q1 2022

  • Premiered three safety webinars on energy, machinery, and hand tools.
  • Translated our operator training webinar on measurement into Spanish.
  • Presented a short video on removing parts with injector pins.
  • Shared What’s New in 2022 from our rotomolding suppliers.
  • Presented our Intro to Roto seminar in California and sold out our Minnesota meeting. We’ll be in Utah June 10 and Illinois in July 15.
  • Discussed how Prop 65 is affecting rotomolders.
  • Published five quick fixes for reducing your gas usage.
  • Hosted our What’s Your Problem? troubleshooting discussion on Zoom.
  • Welcomed new members: S.R.SmithRotogalVita Plast CyprusXcelerant Growth Partners.
  • Learned what’s new in the third edition of Practical Rotational Molding.
  • Met with 85 registrants at our first Executive Forum since 2019!
  • Plus our Board and committees are working hard behind the scenes to continue to provide value to our members.

Five quick fixes to reduce your gas usage

Dr. Nick Henwood – ARM Technical Director

In a previous Blog, I talked about the possibility of a gas-free future for rotomolders.   Since then, we’ve witnessed the situation in Ukraine, which has added to the pressures on natural gas supply in many European countries.  Even if you are based in a country with a secure gas supply, it’s highly likely that your unit cost of gas (natural or LNG) will be rising.  The supply of oil and gas is a global business.

Conventional rotomolding, using gas ovens, is not an economical process, as far as energy utilization is concerned.  Rising gas prices will create an extra headache that rotomolders could certainly do without.

There are a number of far-reaching strategies that rotomoulders can apply, to build a more energy-secure future.  However, these don’t help much in the short term.

Is there anything helpful that you can do immediately?

YES!  Here are 5 “quick fixes” that could help.

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What’s new in the third edition of Practical Guide to Rotational Moulding?

Mark P. Kearn

It’s been almost 20 years since Professor Roy Crawford and I published our Practical Guide to Rotational Moulding book. Developed and based around Queen’s University’s popular ‘Hands-On Rotational Moulding Seminars’, a second updated version was published in 2012.

A third, revised and updated edition has just been published (by Elsevier). It continues to be very encouraging to see the way in which first two editions of this book have proved popular with readers throughout the world. The practical approach, with the extensive use of photographs to illustrate key points, has enabled a wide range of people to get access to simple and advanced technologies available within the rotational moulding industry today.

Since the publication of the first edition of this book, it has been very pleasing to note the continuing evolution of the industry worldwide. New market sectors continue to be developed, new types of products have emerged, and new and improved technologies and materials have become available to ensure better quality products can be offered to customers.

This third edition provides a step-by-step approach to rotomoulding, covering applications, moulds, machinery, materials, and design. The edition has been thoroughly revised to include the latest advances, including novel materials and moulds, new products, and automation.

The book begins with a chapter that introduces the rotational moulding process, analyses advantages and disadvantages, and explores common applications for rotomoulded products. The subsequent chapters provide detailed, methodical coverage of moulds, machinery, materials, and design for functionality, supported by clear illustrations and diagrams. Finally, challenges and future developments are discussed.

This hands-on technical guide helps engineers, designers and practitioners to understand all aspects of rotomoulding, with the aim of producing performant end products and parts, with uniform wall thickness and potentially in complex shapes. The book is also of great interest to professionals across the plastics industry, as well as researchers and advanced students in plastics engineering, industrial design, mechanical engineering, chemical engineering, and materials science and engineering.

I have also looked into the crystal ball again to predict some of the developments that will become available in the future – for example, greater levels of automation, increased use of robotics, etc.  I am very grateful to people throughout the global rotomoulding industry who have had input to this third edition – by providing advice, photographs, data and encouragement. I hope that the third edition will continue to be useful to those who are new to rotomoulding, as well as those experienced in the industry and are striving to push back the boundaries of this extremely versatile manufacturing technology.

Lastly, I would like to thank the family of Prof. Roy Crawford for the opportunity, within this publication, to continue his legacy of education in rotational moulding. The book is available via Elsevier and at a range of online bookstores: Amazon, Google, etc .

Mark P. Kearns is the Senior Technology Manager of the Advanced Manufacturing Innovation Centre, Queen’s University, Belfast and Rotational Moulding Research Manager at Queen’s University, Belfast. A Chartered Chemical Engineer and Fellow of the Institution of Chemical Engineers, he has been active in rotational moulding research, industrial support and innovation projects for over 25 years. Mark has co-authored two textbooks, over 60 papers and has presented advanced rotational moulding technology seminars and keynote presentations worldwide.

Welcome from Matt Bushman, ARM’s new President

First, I want to wish everyone a very Happy New Year!  After a couple of challenging years, we are all  looking forward to a better 2022. I am pleased that there are very encouraging signs that we are emerging from the worst.

As I reflect on the past two years, I am extremely proud of our industry response and successful navigation throughout these uncertainties.  The perseverance and professional commitment of the ARM staff, directors, and membership has been quite impressive and greatly appreciated.

Our theme for 2022 is Creating Efficiencies.  Meetings, seminars, and webinars will embrace this theme throughout the year.  Our industry as a whole is very healthy even though labor and supply chain issues persist.  

Efficiency objectives are achievable through: strategic planning, scheduling, automation, and recruiting. Many of these topics will be addressed at our Executive Forum in Amelia Island Florida, March 14-16. Learning about our members and their operations can be a great way to enrich our own processes and facilitate creative thought.

Please seriously consider participating in ARM’s webinars.  They provide immense value and knowledge without the necessity of leaving your office.  ARM is here to serve our members and interact through sharing  ideas and experience.  There’s a listing of our webinar library available here.

Lastly, thank you for your continued support.  I look forward to a prosperous and positive 2022.

We’re moving on!

Matt Bushman
2022 President, Association of Rotational Molding
President, Plasticraft Corporation