Ask Dr. Nick: What’s an Acceptable Scrap Rate in Rotomolding?

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!

1. First of all, you need to define how you will measure your scrap rate.  Are we measuring any defective part that comes out of the molding operation, or are we ignoring parts that are initially defective but can be brought up to specification by re-work?  Personally, I would encourage the former, otherwise there is the danger that scrap rates seem to be lower than they truly are.
2. On a similar track, recycling the polymer contained in scrap parts may be “green”, but it doesn’t mean you’ve solved the problem!  Materials aren’t the only cost: think of gas, electricity, labor, factory overhead, missed delivery deadlines, reduced productivity and lost profit opportunity.
3. You should also consider whether you calculate based on number of parts or weight of parts produced.  There are arguments for both approaches.
4. In my experience of the general rotomolding market, it is not uncommon to see scrap rates of 5% and more being tolerated.  The Quality Manager of one sizeable Group once told me proudly that her plants achieved an average scrap rate of 4%. When I did the math, it seemed that this represented a cost to the business of well over $200,000!  That could represent a significant additional contribution to profit, if further measures were taken to reduce it.
5. I’ve seen a lot of well-controlled custom molding operations who regularly maintain a scrap rate below 2%.
6. I know proprietary molders making their own limited range of products who can achieve scrap rates below 0.5%.  Clearly, limited tool changes really help to keep scrap rates low.
7. Although all of the above rates still seem high, given the eye-watering consequences to the bottom line, we should recognize that rotomolding scrap rates tend to be higher than other processes.  This is partly because cycle times are long and relatively few parts will be made in a day from each mold cavity; you may have done a lot of turnin’ & burnin’ before you realize there’s a problem.  For example, consider a part made with a 60 minute total cycle time.  1 scrap part per cavity per week of 20 hour / 5 day working already represents a scrap rate of 1% on that mold.  Worse, if your processing conditions are incorrect, you may have to scrap all the parts made on that arm.  So if your operation does multiple mold changes throughout the week, investing in a mobile temperature monitoring device could demonstrate a rapid pay-back.
8. As previously mentioned, there are many different parameters that can vary and we mostly rely on our front line troops, our machine operators, to figure out what to do, in real time.  Maybe they could do with some extra attention and training?
9. I would recommend that, if you haven’t already done so, you at least start recording scrap rates and, even better, start classifying the reasons for scrap using scrap codes.  ARM covered this in some detail in our recent web-based Operator Training Program; look at Modules 3.1 & 3.2 on https://rotomolding.org/page/WebinarsOTP

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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.