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Ask Dr. Nick: A Basic Review of Foam in Rotational Molding

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Dr. Nick Henwood

ARM often receives questions about the foaming process as it relates to rotomolded parts and we thought a basic review might be useful.

 A number of different foam products have been used in conjunction with rotomolded articles, in order to impart enhanced properties.  These include:

  1. Polyurethane (PU) Foam, injected into a cavity in the final rotomolded part, with the aim of completely filling it.  Typically, a fully cooled part is contained inside a foaming fixture and the foam components (polyol and isocyanate) are mixed and injected through a special nozzle.  The creation of the PU Foam is extremely rapid, once the components are fully mixed.  PU Foams have very low density (typically 0.050 g/cm³), which gives them excellent heat insulation properties.  They are also used to add buoyancy to marine components.  There is no bond between the PU and the PE part and de-lamination of the foam is instantaneous.  The PU Foam does not impart any additional stiffness to the product.
  2. Expanded Polystyrene (EPS) Foam, created in a cavity in the final rotomolded part, with the aim of completely filling it.  Typically, a fully cooled part is contained inside a foaming fixture and pre-expanded EPS beads are poured inside the part.  Steam lances are then inserted into the part and the associated heat further expands the beads.  This process takes time (typically tens of minutes) to complete.  EPS Foams have low density (typically 0.150 g/cm³), which give them moderate heat insulation properties.  However, they are mainly used to add buoyancy to marine components.  There is no bond between the EPS and the PE part.  The EPS Foam does not impart any additional stiffness to the product.
  3. “Syntactic” Foam, created in a cavity in the final rotomolded part, with the aim of completely filling it.  These are composite materials; for rotomolded applications they usually consist of an epoxy-based polymer matrix with hollow glass spheres suspended in it.  This structure provides low density and very high stiffness / crush resistance.  The density can be adjusted over a wide range, but when used in rotomolded products, it is typically in the range 0.400-0.500 g/cm³.  The main application is for subsea flotation devices (eg flotation collars around undersea pipelines), where they impart high resistance to crushing by water pressure.  There is no bond between the PU and the PE part and de-lamination of the foam is instantaneous.
  4. PE Foam, which differs significantly from other types.  PE Foam is generally added as a second charge during the molding process, when an outside skin of standard solid PE has already been formed.  In this case, the aim is not normally to completely fill the cavity; rather, the aim is to produce a second layer of even thickness around the inside of the rotomolded part.  This imparts a degree of extra stiffness and a degree of heat and sound insulation (although significantly less that Options 1&2).  There is a full bond between the PE Foam and the outside PE skin.  The density of PE Foam can be adjusted over a limited range, the minimum practical density that can be achieved is approx. 0.200 g/cm³ and the maximum density is, theoretically, the density of the PE used in its formulation (i.e. zero foaming).

For more information, ARM’s website includes a free webinar for members on In-Process Rotational Foam Molding, conducted by Dru Laws. Late this summer and throughout 2019, ARM will conduct a series of webinars on Finishing that will go into more detail on foams.

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.

President’s Message: Investing in New Value for Members

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Dru Laws

As I write this, I have just returned from ARM’s Summer Board meeting. This is the Board’s longest meeting of the year and it is typically used for planning. As a result of the Association’s success over the last few years, we were able to spend a lot of the meeting talking about ways to invest in value for all of our members.

The various ideas for adding for members can be separated into three categories:

  • Making rotomolding more productive
  • Promoting for the rotomolding process
  • Providing research into the rotomolding market

Some of the ideas are bold long-term projects that we will study before we move forward.  One project we have acted quickly on, and I hope we can announce within the next few weeks. And the other ideas are somewhere in the middle.

One of the drivers of these discussions is a new ad hoc committee focused on Productivity Improvement, this project of ARM and the Roy Crawford Rotomolding Education Foundation has been driven by ARM Past President Corey Claussen and Vice President Rick Carlsen. The group spans our industry and supply base. Their goal is to research, experiment, document and make available methods to improve the industry’s productivity. They have wide-ranging discussions on automation, robotics, material development, and mold building methods. They’re focused on identifying areas that we can invest in advancing our process and communicating valuable productivity improvements that already exist.

Over the past year, ARM has seen an uptick in new members. This is one of the reasons we’re able to invest in new value for members. In the first five months of 2019, these companies have joined ARM:

  • Alpha Systems
  • Aquaplastica
  • Bergen International
  • CIPSA Industrias
  • Crane Resistoflex
  • Ecotank
  • Fibeco
  • Houry Design Services
  • Legend Brands Dri-Eaz
  • Progen
  • Rock Lake Robotics
  • RPS Corporation

ARM just finished our Spring regional meeting schedule. We held two regional meetings on Warping Parts in California and South Carolina. Our final regional meeting will be held in Minneapolis on June 18. Also, I personally had the pleasure of presenting a two-day hands-on rotomolding course at Penn College in Williamsport, Pennsylvania. All of these meeting attendees will tell you just how valuable face-to-face meetings can be. I hope we’ll see all of you at our Annual Meeting in Houston this October 27-30. It will be well worth your time.

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

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

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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…

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

ARM Members Access 55 On-line Training Videos

For ten years, ARM has been building a library of webinars and training videos that are available to our members online.  We currently offer 55 titles. Members can access all of these titles immediately. Non-members can join here.

Online training includes:

  • Operator Training Program (25 Videos) presented by Nick Henwood, Dru Laws, and more Click for the detailed contents
  • Burner Maintenance presented by Adam Covington
  • Considerations When Light-Weighting Rotomolded Parts presented by Henry Hay
  • Cooling Fixtures presented by Ron Joannou Jr. and Joey Morsi
  • Determining Your True Cost presented by Steve Osborn
  • EPCRA Reporting & Combustible Dust (with Roto 101 Intro) presented by EPA & OSHA
  • Finishing Parts with Robots (Case Study) presented by Dru LawsFinite Element Analysis: The Basics presented by Michael Paloian
  • Improving Rotational Molded Insert Performance presented by Glenn Larkin and Jon Ratzlaff
  • In-Process Rotational Foam Molding presented by Dru Laws
  • Infrared Thermography presented by Bill Spenceley
  • Lengthening the Life of Your Mold presented by Tony Short
  • Making the Most of Your Marketing Budget presented by Melissa Shearer
  • Mold Clamping Methods – Pros and Cons presented by Erik Adams and Bob Mueller
  • Mold Release presented by Even Silo & Scott Waterman
  • Prop 65 for Rotomolders presented by Mike Haubert
  • Proper Sampling Techniques presented by Josh Hunsberger and Dave Loeffler
  • Rebranding Your Business (Case Study) presented by Tom Innis
  • Rotational Molding & the Evolving Structural Part Processing Business presented by Dr. Peter Mooney
  • Roto 101 presented by Rick Carlsen & Ron Joannou Jr.
  • Safety in Rotomolding & OSHA Compliance presented by Dave Schwoerer
  • Save Energy & Save Money presented by Bill Spenceley
  • Selecting the Right PE Grade for Your Part presented by Nick Henwood
  • Solve Cosmetic Part Problems presented by Jon Ratzlaff
  • Stiffness Models and Options for Multi-Layered Parts presented by Dr. Nick Henwood
  • Test Method for Flowability (Dry Flow Rate) of Polyethylene Powders Using a Specified Funnel presented by Ron Cooke & Jon Ratzlaff
  • Troubleshooting Your Rotomolding Operation from a Shop Floor Perspective presented by Ron Cooke
  • Turning a 4% Bottom Line into a 10% Bottom Line presented by Al Bates
  • Understanding the Molding Cycle presented by Gareth MacDowell
  • What Does Internal Air Mold Temperature Have to do with Quality presented by Gareth McDowell
  • What’s Your Problem (2 videos) presented by Ron Cooke & Sandy Scaccia

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

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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.

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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

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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 www.armo2019.com for more information