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.

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