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.

As rotomolders, we’re invariably in the business of making durable products.  Our customers expect that rotomolded parts will be made to last for years, if not many decades.  Therefore we need to be confident that our parts will meet service expectations over an extended period.

Most rotomolded products are ultimately used in an outdoor environment, so the weatherability of the polymers we use and the stability of the colors we create are both very important.  Whilst the choice of additives and pigment can generally provide adequate protection, we shouldn’t ever assume that this is a “given”.

The most popular material used in roto is polyethylene (PE), a material that we all know and love.  Whilst PE has many excellent properties, some of its long-term properties are less than optimal.

As a semi-crystalline polymer, PE has relatively poor resistance to creep.  When PE is subjected to load, it stretches and this behavior is often described using tensile and flexural data.  However, bear in mind that the usual tests stated on Data Sheets are only measuring these effects in the short-term.  If the load is sustained for an extended period, PE continues to stretch; this is what is known as creep.

Another important issue, which is slowly gaining recognition in rotomolding, is crack propagation.  Practical rotomolded parts will contain micro-defects (eg air bubbles and pigment clusters); under conditions of continuous or cyclic loading, these defects may act as sites for the growth of micro-cracks that can lead to sudden part failure after several years of use.

Tests exist that will help us evaluate our materials by measuring long-term data.  In principle, we should be able to build this into our Finite Element Analysis programs, or we can recommend Factors of Safety that should be applied.  However, our industry does not yet have agreed testing protocols that have been proved to be reliable and robust in a rotomolding context.

One of my missions, as ARM Technical Director, is to encourage and support the kind of research and development that will answer real problems and address practical issues.  Fortunately, rotomolding has many talented and committed technical professionals that are willing to work together to help.  This has always been a characteristic of our industry, as evidenced by the success of organizations such as ARM.

“Knowledge is power,” so the first step is to raise awareness of the issues involved and their practical importance.  Look out for future initiatives by ARM in this area and, if you haven’t already done so, I recommend that you make use of the existing educational resources that already exist.

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.

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