A rotomolder member of ARM asked about recommended actions in relation to fires in rotomolding ovens. These fires are typically caused by powder spilling out of a mold during the cooking process. This can be due to:
Incorrectly closed clamps, or clamps that fail, causing the mold to open as it rotates
A missing vent pipe, either because the operator forgot to insert it or because it was inadequately fixed and fell out during mold rotation
Missing vent fill media, either because the operator forgot to insert it or because it was incorrectly inserted and fell out during mold rotation
A damaged parting line that is leaking powder
Once the powder is released from the mold, most of it will fall to the oven floor where, as a minimum, it will melt and causes a mess. With prolonged heating, it may catch fire spontaneously and create smoke. Some of the powder (the finer sized fractions) may be drawn into the burner duct by the circulation fan. This finely divided, combustible, material will then come into contact with the burner flame and may create an explosion. Alternatively, repeated spillages may coat the sides of the burner duct and create an on-going fire hazard.
As a representative of ARM, I was recently asked in a discussion with some other ARMO groups about whether electromagnetic induction heating could be suitable for rotomolding.
The issue was raised because of how the Emissions Reduction Plan of New Zealand may limit rotomolders’ ability to use gas to power their rotomolding machines. Today there is increased pressure, around the world, to limit carbon dioxide emissions. Therefore alternative heating systems for rotomolding machines is becoming a pressing subject; it’s worth thinking about now because, in the future, manufacturers in other territories may face similar limitations to the ones being proposed in New Zealand.
When I train machine operators, I tell them that, most likely, venting problems are the biggest cause of scrap parts they will come across. This statement was originally based on a survey conducted by the ARM many years ago. After 30 years in this business, the statement still holds true, as far as I’m concerned.
Creating scrap parts because of bad venting is totally avoidable. This is one aspect of rotomolding that absolutely isn’t rocket science (not much is, actually).
If you have inadequate venting, or if your vents block during the process, you will get parts with defects.
During the cooking stage, inadequate venting will create a build-up of pressure in the mold, as the air inside heats up and cannot expand through the vent. At some point in the cycle, molten plastic will be forced into the parting line (or any other potential emergency air release passage). The result will be the creation of flash along the parting line, which will need to be trimmed off.
Worse, inadequate venting in the cooling stage will create a vacuum in the mold, as the air inside cools down and cannot draw more air in through the vent to equalize the pressure. At some point in the cycle, air from outside will penetrate the parting line. The result will sometimes be bubbles along the parting line (on the inside of the part) or, more likely, a small hole where a bubble has formed and then burst. Either way, you’ve got a part that is either scrap, or else requires re-work.
There’s a photograph below that illustrates both issues. This was made in a test mould I have that actually has no vent. You can clearly see the flash that was formed during cooking and the bubble that was created during cooling.
Horrible!. So how can you avoid this and, specifically, what constitutes adequate venting?
In the first instance, an “adequate” vent (or vents) has an opening that is large enough to allow adequate flow of air from inside to outside the mould, at all points in the cycle.
So far, I’ve not been able to find any sources of primary research into this, but there are a number of authorities that state rules of thumb relating vent diameter (note this is the inside diameter) to mold volume. These rules of thumb express themselves in various units, but the most useful way I’ve seen of expressing it is in terms of vent open cross sectional area (CSA) per mold volume. A typical number is:
0.01 in2 of vent open CSA per ft3 of mould volume
I’ve used this figure to calculate what this means, for various volumes of mold (see table). Note that this data relates to vents without packing. If you do use packing (wire wool, mineral wool, rolled-up scotch pads etc), I have seen recommendations that you double the CSA of the vent provided.
How can you estimate the mold volume?
If your mold has been designed using computer aided design (CAD), the mold cavity volume should be directly available from the design file.
If your part is a relatively simple geometric shape (eg a cylinder or box), you can roughly calculate it.
Another idea would be to fill a finished part with water and weigh it.
Either way, this doesn’t need to be exact; it’s a guide only.
Have a look at some of your existing molds and see whether, according to this table, you already have adequate venting. You may be surprised at what you find!
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.
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.
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.
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.
We shared this schematic of the rotational molding process (drawn by ARM Technical Director Nick Henwood and spruced up by Anthony Kozelka of Quantum Polymers) with our members last week. We asked them which part of the process they saw the most opportunity in automating. See the responses below and let us know your opinion in the comments.
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 →
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?
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 →
Question: What are typical heating cooling cycles compared between XLPE and PE?
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,
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.