As we all know, a significant benefit offered by rotomolding is that we can achieve even wall thickness, compared to other processes. One question that rotomolders often ask me is: how much thickness control can I actually achieve, in normal practice?
I pondered this question recently, while I was preparing resource materials for the ARM Operator Training Program. I wondered whether I could get some data from a real-world example, then I remembered a product development that I was involved in a few years ago. As part of this project, I carried out an assessment on the variation in wall thickness of a medium sized rotomolded part.
The product was developed by my friend Bob Slee of Palm Equipment International, who is in the kayak business and is one of the best rotomolders I know. In this case, his new product was not a kayak, although it served a related area. It was a system known as RAPIDBLOCS, which has subsequently become the leading product in its field. RAPIDBLOCS is a system of modular rotomolded blocks, which can be connected together in a range of configurations, to create water features and obstacles in white water and slalom courses. These courses are used by kayakers for training, practice and competition.
Since it was launched, the RAPIDBLOCS system has been used in a string of World Championship and Olympic courses.
The basic block unit is a relatively simple shape (see plan view, below), which is an ideal rotomolded product.
During the early development of the product, I did some static and dynamic load testing. As a precursor to the main tests, I measured the actual thickness of the cross section in 26 different places, which you can see labelled alphabetically in the sketch.
In the chart below I’ve shown the variation of thickness around the part, relative to the average wall thickness:
You can see clearly how much variation occurred in the wall thickness of the part, which was made using a steel fabricated tool. Ignoring a couple of outliers, +/- 20% seems to be the normal range. If you consult other authorities (e.g. Glenn Beall’s excellent book, Rotational Molding: Design, Materials, Tooling, and Processing, Hanser, 1998), they state a similar range for “commercial” rotomolding. For the RAPIDBLOCS product, this amount of variation was acceptable, as it is for many standard industrial products.
However, for a process that promotes itself as being able to produce “even” wall thickness, this amount of variation is quite surprising. If you need a tighter tolerance than this, you’ll need to start taking additional measures to control heat to the tool locally. I’ll give you some ideas about how to do this in a future blog.
By the way, I mentioned the ARM Operator Training Program earlier. If you’re an ARM Member and you haven’t accessed this valuable FREE training tool, I really would encourage you to do so. You’ll find it in the Members’ Area of the new ARM website.
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.