Tag Archives: drnick

Ask Dr. Nick

Dr. Nick Henwood

Technical Director Nick Henwood’s Ask Dr. Nick series allows ARM a way to share his feedback on interesting technical questions we receive at ARM. Here’s a handy list of the ten posts we’ve published so far.

If these articles are helpful, access Dr. Henwood’s many more technical articles written for ARM in the last four years in the complete archive of his blog posts.

Long-term properties of roto materials: Should I worry?

Dr. Nick Henwood

ARM recently held its Annual Meeting as a virtual event and this new format seemed to be a great success.  Obviously, we all missed the normal human contact; the chance to meet contacts, former colleagues and friends, in person.  However, as a way of presenting information and opening up discussion, the on-line format certainly seemed to work.

As part of the main event, we organized a technical session with presentations around a common theme, which was “Long-term Properties of Rotomolding Materials.”  There was an introductory keynote, followed by four groups of expert speakers, plus a Q&A session.

I would encourage any of you who missed out on this session to revisit it via the ARM website, where we now have recordings of all parts of the session.  Some aspects of the same subject were also recently covered during our Design Webinar Series (Module 5), which is also available in recorded format.

Does Long-term Properties seem a subject which is rather esoteric and strictly for the “techies” amongst us?  Possibly, but it really shouldn’t!  There are several good reasons why any rotomolder should be aware of the basics of this subject, if not all the detail.

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A designer’s questions on rotomolding properties

Recently ARM presented Class 5 of its Design Webinar Series, which was focused on material properties, as they relate to product design.  The webinar consisted of an initial presentation by ARM’s Technical Director Dr Nick Henwood, followed by a series of questions from Michael Paloian of Integrated Design Systems, the regular presenter of the Design Series.

As a highly experienced designer, Michael’s questions were extremely searching and got into some interesting and relevant detail, so we thought that it would be useful to fully reproduce the conversation between him and Nick below.

For those who missed the live event, a recording of Nick’s complete presentation is now available in the Members’ Area of the ARM website along with Michael’s entire series on rotational molding design.

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ARM Leak Testing Summary

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Dr. Nick Henwood

On Thursday April 30, 2020 ARM held a discussion on Leak Testing, moderated by Education Committee Chairman Ron Cooke (ExxonMobil)  and Sandy Scaccia (Norstar Aluminum Molds).  The session was very well attended and the discussion between everyone on the call was excellent.  There was so much useful information exchanged that we decided to try to capture all the salient points.

We began by talking about the methods that are commonly used for leak testing of rotomolded parts, then discussed some more unusual methods that had been tried.  Then we talked about whether leaks can be rectified and what can be done to stop the leaks happening in the first place.  This led us into a very interesting detailed discussion about issues with metal inserts, which is one of the most common causes of leak problems. Continue reading

Ask Dr. Nick: What’s the difference between hexene and butene?

Question:  I have been discussing rotomolding resin made from butene versus hexene and have received mixed feedback.  Is there a real difference between the two and are there applications where one is preferred over the other?  Or is it just a question of price and availability, when it comes to using prime material in natural or color?

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Dr. Nick Henwood

This is a question that I’m asked quite frequently.  There isn’t a simple “bad / good / better” answer, it’s a bit more complicated than that.  In the description below, I’ll try to concentrate on what molders really need to know, so my description could be viewed by some polymer chemists as a bit superficial.  Others may think it’s over-complicated… I’ve tried to steer a middle way!

Butene and hexene (strictly speaking: butene-1 and hexene-1) are examples of comonomers that are used during the production of different polyethylene (PE) grades, including roto grades.  They do essentially the same thing for PE, but their presence can cause some differences in grade performance. Continue reading

Ask Dr. Nick: XLPE v. PE heating and cooling cycles

Question: What are typical heating cooling cycles compared between XLPE and PE? 

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Dr. Nick Henwood

Dr. Nick: Crosslinkable polyethylene (XLPE) rotomolding grades work in a different way to standard linear low density polyethylene (LLDPE) and high density polyethylene (HDPE) grades.

During the cook stage of rotomolding standard PE grades, two separate things need to be achieved:

  • Sintering – ensuring that powder particles melt and fuse together in a solid mass.  For standard roto grades, sintering is typically completed by the time that the Internal Air Temperature (IAT, the air temperature inside the mold) reaches approx 265 degF.
  • Consolidation – allowing sufficient time and temperature for the gases in trapped air bubbles to dissolve into the molten polymer matrix.  For standard roto grades, consolidation is typically sufficiently accomplished by the time that the IAT reaches 390 degF.

During the cook stage of rotomolding XLPE grades, the above two mechanisms need to be achieved, followed by another additional one:

  • Crosslinking – XLPE grades contain a special package of additives, based on organic peroxides, which form side links and create a network structure from the individual polymer chains.  This network structure provides improved short- and long-term physical properties.  BUT – and it’s a big but – this won’t happen unless sufficient time and temperature is provided.

The requirements for individual XLPE grades may vary, but one general recommendation that I have seen is that, during the final stages of cooking, the IAT should be above 390 degF for several minutes.  An additional processing benefit of the best XLPE grades is that over-cooking does not result in the usual catastrophic loss of impact strength due to chain scission.

I recommend that, if possible, you set up your cook cycles for XLPE using a device that can measure IAT and that you follow the guidelines above, in the absence of anything more specific from your material supplier.  A more rough-and-ready guideline might be to add two or three minutes on to the cook cycle you would use for standard PE,

However –

Depending on the formulation, over-cooking XLPE can result in some undesirable effects, that you will want to avoid.  One common effect is known as coining – the appearance of a locally depressed area on the surface of the part, as though a large coin had been pressed into the surface while the polymer was soft.  Reducing the oven temperature is the usual expedient to eliminate such defects, but then you may affect crosslinking.  Hence my main recommendation, to use available control tools to achieve as much precision in set-up as you possibly can.

Hope that helps; happy rotomolding!

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

“The Seven Stages of Rotomolding” Questions and Comments

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Dr. Nick Henwood

As part of ARM’s on-going commitment to member service, we are ramping up our webinar program for 2020.  The latest webinar was presented on March 19 by Dr Gareth McDowell of 493K.  I thoroughly recommend that, if you missed the live show, you catch up with it on the ARM website.

Many of you will be familiar with Dr Gareth’s lively presentation style from his many presentations at ARM conferences. He was able to bring this approach to the very different format of an on-line event and, as a result, we saw a high level of reaction from the live audience, in terms of comments and questions.

Unfortunately, we simply ran out of time to address everyone’s needs, so I’m doing a wrap-up via my Technical Director’s Blog.  The length of my Blog reflects the number of questions, but feel free to dip in and out of it, if you don’t have time for a long read!

Thanks again to Gareth for a really excellent webinar.   Continue reading

Ask Dr. Nick: Warpage in a polypropylene tank

Question: In a cylindrical tank made of PP powder, we have experienced a problem of warpage (internal and external waves). I wonder if you could give me your technical opinion. The inside part of the mold is welded with an additional metal stripe and in this part of the mold we are facing warpage in the molded part. The warpage area is focused in the middle part of the welded metal stripe. In the warped area, the wall thickness is between 7.5 – 8.5 mm. In order to eliminate the warpage problem, our customer has added externally a metal plaque to prevent overheating. The part is cooled up to 80-85 degC (176 – 185 degF) inside the mold. Then, the part is moved from the mold and is left for cooling in the environmental temperature.

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Dr. Nick Henwood

Dr: Nick:  I’ll try to give an opinion on this problem. 

  1.       I’ve previously seen warpage problems when molding a particular grade of PP.  The shape I was molding was a simple cylinder. The material supplier told me that PP has “natural lubricity”, by which I understood that something in the polymer migrates to the mold surface and provides what amounts to an internal release agent.  However, I have successfully molded many other grades of PP, without seeing the problem.
  2.       Generally, PP shrinks less than PE, so you would expect that warpage problems (which are caused by unequal shrinkage rates in different sections of the molded part) would be less.
  3.       Warpage effects tend to occur more often with thick parts; at 7.5 – 8.5 mm, I would consider your part to be pretty thick.
  4.       The area of the mold containing the welded metal stripe may result in a different heating condition compared to the rest of the mold surface.  This may result in a lesser or greater wall thickness building up at the stripe. It’s not clear from your description which it is, although the fact that the problem was fixed by reducing the heat to the stripe area (by adding the external metal plaque) indicates that the stripe area was previously heating up more than the rest.  Did you measure the wall thickness of the part in this area, compared to the rest of the part?  In any case, thickness variation around the part is another cause of warpage.
  5.       You’ve not mentioned anything about mold release agent (mra); your choice and level of application may be a factor.  If the PP grade you’re using has this natural lubricity (see note 1 above), then reduce the level of mra applied. You can immediately reduce the release properties of an existing surface (ie one which already has mra applied) by gently abrading with a scotch pad or similar non-metallic product.
  6.       Slower cooling can reduce warpage; you don’t specify how you cooled or the cooling rate.  In extremis, don’t apply any external cooling and allow the mold to cool naturally in ambient conditions.  Worth trying, just to see if it helps, even if this is not practical in production.

I hope the above list gives you some pointers to the problem.  Whilst the root causes of warpage are similar across production, the way these causes come together to manifest a particular warpage problem can be complicated.

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: 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.