Tag Archives: drnick

“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.

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

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.

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.

Ask Dr. Nick: How does cold weather affect the rotomolding process?

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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

Ask Dr. Nick: Can I fix gaps in a parting line?

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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

Ask Dr. Nick: Avoiding “Angel Hairs”

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Nick Henwood

On ARM’s most recent What’s Your Problem? teleconference, there was a question asked about powder piping systems, especially about avoiding and dealing with “angel hairs”. I provided some follow up to the molder after the call, which we’re happy to share here for everyone’s use.

My first recommendation was to contact ARM’s mainstream pulverizer supplier members, who should be able to offer good advice. They are the real experts in this area.

In the meantime, I also recommended installing simple traps for angel hair in your lines.  Pulverizer systems have these in place.  The grid has something like ½ inch gaps and the sideplate can be opened for manual removal of accumulated debris.  The pulverizer folks will have proper drawings of this; please excuse my rudimentary draftsmanship!

trap for angel hair

As far as I’m concerned, angel hair production gets bad when ambient heat is sufficient to start to soften powder particles.  A particle momentarily trapped on an obstruction will then get stretched and elongated by fast air flow around it and a hair gets formed.  So keeping temperatures down (say below 100°F), or not generating elevated temperatures in the first place (correct pipe sizing and avoidance of sharp bends), should help a lot.

Note from staff: ARM offers What’s Your Problem? teleconferences — an audio version of our popular troubleshooting workshop — to our members every six weeks as a free benefit of membership.

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: What’s an Acceptable Scrap Rate in Rotomolding?

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Dr. Nick Henwood

Recently an ARM member from Latin America phoned in with a technical question and we got into a conversation about the vexed subject of scrap.  I appreciate that it’s a bit of a prickly issue and probably not something individual rotomolders would be keen to discuss openly.

Not being a molder, I was probably more comfortable than most to tell our colleague what I had observed myself, having worked with hundreds of rotomolding companies over my 30 years in this business.

I thought that my observations might be useful, if only to reinforce how important this issue can be commercially.  We have to live in the real world, so some scrap is almost inevitable. Rotomolding is simultaneously fascinating and frustrating, because there are so many variables at play and not all the variables are easy to control (eg the weather / ambient conditions).

It seems to me that the main trick is to stay vigilant and bear down on scrap and the reasons why we may make scrap.

Anyway, here are some of my thoughts.  If any of you molders out there would comment, that would be fantastic!  If you think I’m talking nonsense, feel free to “roast” me! Continue reading

Effects of pigments in dry mixing: What REALLY happens to physical properties?

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Dr. Nick Henwood

Many parts of the North American roto industry still rely on using dry color materials.  The main reasons for this are reduced cost and operational convenience. However, it is generally recognized that using dry color, rather than fully compounded pre-color, can result in a significant loss of material properties.  

If you’ve sat through as many ARM meetings as I have, you’ll have heard many different opinions voiced on the negative effects of using dry color and whether these effects can be mitigated.  As a scientist, my normal response to strongly held opinions is: “Do you have any data that supports this?” Unfortunately, when it comes to questions of dry color, there seems to be a dearth of hard data available to support us in making sensible decisions. Continue reading