Originally published in 1982 as The Introductory Guide to Designing Rotationally Molded Plastic Parts, the “green book” has long served as a foundational resource for the industry. This new edition was written by ARM Technical Director Dr. Nick Henwood with support from the ARM Education Committee and builds on the work of the many ARM members who contributed to the original publication, including Glenn Beall.

The guide is dedicated to the memory of Tom Wyszynski, who assisted with the rewrite.

The revised guide explains the rotational molding process, its advantages, part design principles, and the plastics used in rotational molding. It is intended for rotational molders, designers, buyers, and anyone looking to better understand the process and its capabilities.

Readers will notice that a few images are still missing. If you have photos or illustrations that could help fill those gaps, please contact ARM staff.

ARM also packed a ton of value for members into the first three months of the year:

• Hosted six live webinars, including
  • A three-part ARM x Sandler sales training series: Prospecting – Filling the Funnel, Sales Manager Essentials, and Creating Urgency and Keeping Deals Moving
  • Rotomolding Machine Safety, presented by Adam Covington of Ferry Industries
  • Two What’s Your Problem? troubleshooting sessions led by Hall of Fame members Dru Laws and Jon Ratzlaff
• Opened registration for the 2026 Executive Forum at Queen’s University Belfast (June 9–12), featuring tours of Kingspan, Collins Aerospace, Clarehill Plastics, and Corcoran, and a educations sessions.
• Added the six-part Rotational Molding Design Webinar Series to the Training Academy, covering the full design process from product development costs to a real-world case study. All six courses are free to members.
• Launched the Operator Training course in Portuguese, with more Portuguese-language courses on the way.
• Asked members to collaborate on two Process Optimization Committee studies: one on melt index comparison and one on mold geometry and powder distribution.
• Published a new Ask Dr. Nick blog post on what’s in rotomolding fumes..
• Built momentum for Rotoplas 2026 in New Orleans (November 16–19). The exhibitor list is growing fast.
• Welcomed Eight New Members
  • Asbury Advanced Materials
  • C-PAK
  • Diversified Plastics Inc
  • Kingspan Water & Energy
  • Momentum Chemical, LLC 
  • Ningbo Starway Roto Co., Ltd
  • PolyJohn Enterprises
  • Top Notch Molding LLC

ARM delivers tremendous value included with your company’s membership. Not a member yet? Join today.

Although the article is written from a UK perspective, I believe the substance is highly relevant to all rotomolders.

The basic message is this: don’t tolerate machine downtime for the sake of stocking a few spare parts.

A failed flame detector or spark plug is one of the most common causes of unexpected burner downtime. These components are small, relatively low-cost and are often overlooked, but without them, a burner simply won’t run. Keeping just one spare on the shelf can be the difference between a quick fix and hours (or days) of lost production.

I’d encourage you to read the whole article, it walks through the six components every site should keep on hand.

Read the article: Preventing Downtime with Smart Spare Management

Dr. Nick Henwood, Technical Director of the Association of Rotational Molders, is a 30-year expert in materials and process control. He operates Rotomotive Limited as a consultant, researcher, and educator in the UK and was inducted into the Rotational Molding Hall of Fame in 2022.

Dear friends,

Ten years ago, I wrote my president’s message about making ARM’s 40th anniversary an unforgettable celebration. We brought more than four hundred rotomolders together in New Orleans, we welcomed a new Technical Director, and we proved once again that this Roto Family of ours is something truly special.

Now here we are at our 50th. Can you believe it?

I remember writing back then that we decided not to wait for the 50th to have a party. Well, the 50th is here now, and I can promise you we will celebrate it all year long. From those 21 companies that met in cold Chicago in 1976 to the worldwide community we are today, we have built something extraordinary and every one of you has been part of that story.

But celebrating our history is only half of the equation. The other half is making sure ARM keeps growing and evolving for the next fifty years. That is why I want to share three goals I am focused on for 2026:

• Fund relevant research. Technical knowledge has always been the heart of what makes ARM valuable. We need to continue investing in research that pushes our process forward and gives our members a real competitive advantage.
• Use AI and our new website to serve you better. With the launch of our redesigned website, we are collecting better data about our members’ specialties, capabilities, and your questions. We want to use these tools, including AI, to provide more personalized service so that ARM works harder for each of you individually.
• Celebrate our 50th anniversary all year. This will be a celebration of who we are and where we are going. Every gathering will carry the spirit of this milestone.

Speaking of gatherings, I hope you are already making plans for our Belfast Meeting in June. We will arrive on June 8th for tours, sessions, and networking through June 11th, with a food and drink tour on the morning of June 12th. Belfast holds a special place in the history of rotomolding research, and I cannot think of a more meaningful location to celebrate fifty years of learning from one another.

As always, ARM is all of us. Our success comes from your willingness to share, to show up, and to treat competitors like family. That has not changed in fifty years, and I am confident it never will.

Let’s make this a year to remember.

¡Hasta la vista!

Conchita Miranda
Directora General, Miraplastek
President, Association of Rotational Molders

Conchita Miranda is General Director of Miraplastek, a leader in rotational molding with over 30 years of experience. The company exports to more than 10 countries and partners with major OEMs including Caterpillar and John Deere. She has served as a member of the Regional Economic Development and Nearshoring Advisory Council since November 2024. Miranda is President of the Association of Rotational Molders (ARM) and a member of the Rotational Molding Hall of Fame.

Join us November 16–19 in New Orleans for Rotoplas, the premier global exposition for rotational molding. Discover cutting-edge technologies, products, and services tailored to the industry. Rotoplas unites manufacturers, suppliers, designers, engineers, and researchers from around the world to exchange ideas and advance rotational molding innovation. Registration opens Spring 2026.

A standout exhibition hall is taking shape

These exhibitors have already secured priority booth space for Rotoplas 2026. More are signing up almost every day.

Put Your Brand at the Center of the Industry

Exhibiting gives you direct access to a targeted audience actively seeking new rotational molding technologies, partners, and solutions. By exhibiting at Rotoplas, you help drive industry growth while building global visibility for your brand. (See Exhibitor Rules, Regulations, and Schedule)

Question: We use polyethylene in our rotomolding process. Occasionally, we have a mold failure which results in some product leaking from the mold, causing it to degrade due to excessive heat and resulting in smoke and fume (“a smoker”). 

We wish to do some air monitoring when this occurs, but we need to fully understand the constituents in this fume, so we know exactly what we are testing for.

Dr. Nick: Polyethylene (PE) molecules are comprised solely of carbon and hydrogen atoms, linked together in long, chain-like structures.  When PE degrades, the products will be compounds of these two species plus oxygen, which is present (at about a 21% level) in air.

When PE burns (combusts) in an excess of oxygen, (ie lots of air present), the products will be carbon dioxide gas (CO₂) and water vapour (H₂O).  This will be the typical situation if you spill a small amount of PE from a vent whilst the mold is still inside an oven (especially powder).  Some of the powder will become airborne, will be sucked into the burner duct by the circulation fan, will ignite when it reaches the burner flame and will breakdown into its products of combustion.

If the spill is larger, powder may will melt on the floor of the oven and will degrade differently (see below).  It will also cause a nasty mess on the floor, which will need to be cleaned up, when safe to do so (ie when the oven is stone-cold and fully isolated from electricity and gas).

If an excessive quantity of powder flows into the burner duct, it will create an explosion hazard.  Explosions have happened in rotomolding plants in the past, with catastrophic results.  Such an incident would normally be caused by a relatively big spillage, for example tens of lbs of powder spilling out if a mold completely opens whilst inside an oven.

If there is insufficient oxygen available, the products of combustion will be different: carbon monoxide (CO), free carbon (C) and water vapour.  The free carbon appears as smoke.  CO is a poison and even low levels can be dangerous.  Hopefully, you’re monitoring CO levels in your plant already; if not, get a meter and alarm installed immediately; this is a basic Health & Safety issue.  The bigger danger of a CO release is from poor combustion in your gas burner.  Even for my small gas-fired lab machine, I have a CO detector installed at my workstation.

PE can also degrade without combustion, if it is heated to a temperature above its melting point.  This happens routinely in rotomolding, if a part is over-cooked.  In this case, the degradation products are mainly a family of more complex compounds; they are known as “carbonyls”, because they all contain a carbonyl (C=O) bond.  The most common members of this carbonyl family are aldehydes and ketones.

Aldehydes have distinctive odours and the characteristic smell that we typically experience when we approach a rotomolding machine is mainly a low-level mixture of aldehyde compounds.

I would say that detecting low levels of such complex organic compounds is outside the practical scope of most industrial facilities and would involve relatively expensive levels of instrumentation.  From a safety point of view your top priority, in my opinion, should be adequate carbon monoxide monitoring.

About a month later, I received an update:

Question:Thanks for your reply, it was very comprehensive. We have since had comprehensive CO monitoring installed. 

However, we still get the occasional “smoker”, and it always raises concerns.  What is the best way to deal with one, once we have one?  What do others do?

Dr. Nick: For dealing with an occasional occurrence, I guess you have two basic options:

1. Local high-volume extraction, close to the likely source

2. Taking additional measures to minimise the causes.

In terms of Option 2, you previously stated that there was occasional leakage of material from a mold during rotation in the oven.  For this, and other good reasons, tightly fitting mold parting lines are a sensible, and achievable, objective.  You can undertake the necessary rectification work, ie re-making any parting lines that have gaps in them.  Gaps in parting lines will also cause defects on the molded part.

The other main potential source is the escape of powder from the rotating mold via mold vents.  The direct solution to this is to use some type of media in the vent, to prevent any powder escape.

Choice of vent media is an operational choice, and you have several possibilities:

• Wire wool is a common choice but, in my opinion, not a good one.  Wire wool will heat up during its time in the oven and is more likely to clog with melted powder.  There have also been reports of hot wire wool causing in-mold explosions.

• Some form of temperature-resistant non-metallic wool is a better option.  Standard fibreglass insulation wool may not be sufficiently temperature-resistant: maximum operating temperature is 250ºC / 480ºF.  Mineral wool (eg “Rockwool”) can resist heat up to 760°C / 1,400°F, so it’s a better choice.  You’ll need very little to form a suitable plug in the vent.

• Wool media should be considered a consumable item; the wool plug should be refreshed on every cycle.  Partially blocked, or fully blocked, vents will create multiple problems.

• More permanent insert systems are available, which have high temperature silicone rubber components.  These are convenient, but potentially more expensive.  Although they will last for a number of cycles, the silicone rubber tends to embrittle eventually, and the components then need replacing.

Dr. Nick Henwood, Technical Director of the Association of Rotational Molders, is a 30-year expert in materials and process control. He operates Rotomotive Limited as a consultant, researcher, and educator in the UK and was inducted into the Rotational Molding Hall of Fame in 2022.

During our What’s Your Problem troubleshooting session last week, we discussed why some parts turn yellow when no one expects it. An attendee shared a helpful ARM paper, “Why Did My Product Turn Yellow and Other Questions You Should Never Need to Ask,” presented by Thomas Steele in 2000. The ideas in it remain useful, especially when we talk about how storage and the surrounding environment can change the color of a part long before anyone uses it.

Summary of the Insights

1. Storing hot powder leads to oxidation

Polyethylene can discolor when it is stored while still warm. Ground powder is at higher risk because more of its surface touches oxygen. Cooling powder to room temperature after grinding and using it soon after helps prevent this problem.

2. Cardboard boxes can cause yellowing

“Cardboard yellowing” happens when antioxidants used in cardboard adhesives move into the resin. This can happen even when the box is kept in the dark. A plastic liner usually prevents it, unless the liner contains its own antioxidants such as BHT that can also migrate.

3. Air quality can trigger fast discoloration

Nitrogen oxides from forklifts, ovens, heaters, or any burning fuel can react with common antioxidants in polyethylene. This often causes yellowing. If strong basic additives such as HALS are present, the color may shift toward pink or salmon. Sunlight can make the color fade for a short time, but the discoloration returns once the parts are exposed to NOx again.

The Association of Rotational Molders presented the following awards on October 1 at the Annual Meeting:

• Persico S.p.A and Rotoline were awarded Supplier of the Year.
• The Charles D. Frederick Distinguished Service Award was presented to Adam Covington from Ferry Industries.
• Excellence in Member Recruitment to Entec Polymers and Ferry Industries.
• Safety Awards were presented to Formed Plastics and Flexahopper Plastics.
• Student Design Awards were given to Jesslyn Gudeman, Bronte Kinkier, and Sofia Solano Brenes of Purdue University.
• Product of the Year was awarded to Hedstrom Plastics for the Float / Boom.
• Product Competition Awards were also presented to: 
  • WeckMethod RMT Club by Hedstrom Plastics for PVC Product
• Previously announced: Rick Carlsen and Dave Smith were inducted into the Rotational Molding Hall of Fame.

The Association of Rotational Molders is thrilled to announce the election of Rick Carlsen and Dave Smith to the Rotational Molding Hall of Fame. Their induction ceremony will take place on October 1 at the ARM Annual Meeting in Dallas.

Rick Carlsen

As a multi-term ARM Board member and Past President, Rick is known for welcoming new members, generously contributing his knowledge, and bringing humor and approachability to technical conversations. He makes people feel welcome in the industry and the organization.

During his years at Solar Plastics, Rick helped drive significant growth, innovation, and excellence across every facet of the business. His colleagues praise his blend of technical depth and strategic insight, as well as his authentic leadership style that builds trust and consistently delivers results.

He currently serves as Director of Engineering at Tank Holding Corp.

Dave Smith

Known for his leadership and spirit of collaboration, Dave Smith has dedicated his career to advancing the rotational molding industry.

A Past President of the Association of Rotational Molders and the current President of the Affiliation of Rotational Molding Organizations, Dave has demonstrated exceptional service and vision at both levels. His legacy is defined by years of service and by the personal investment he makes in others. He is widely known as a knowledgeable mentor and a tireless advocate for the next generation of molders.

At ARM meetings, Dave is often found meeting with first-time attendees to answer their questions, explain the molding process, and encourage them to see the value of ARM membership. His openness and generosity exemplify the spirit that defines the Association.

Dave led Rotational Molding Technology as President for many years.  He currently serves as Technical Advisor for Rotational Molding at Muehlstein.

The Nomination and Election Process

• Nominations for the Hall of Fame were collected from the industry throughout the Spring.
• All nominations received from 2020 – 2025 comprised the longlist.
• The ARM Executive Committee elected a Hall of Fame Selection Committee comprised of five members of the Hall of Fame.
• The Selection Committee narrowed the longlist to a shortlist of six nominees.
• All Hall of Fame members voted on a ballot to elect inductees from the shortlist.
• All Hall of Fame members reunite at the Annual Meeting Awards Dinner to induct our new members.