Ask Dr. Nick: How does cold weather affect the rotomolding process?

nickh

Dr. Nick Henwood

You may have heard that in England, when we’re not obsessing about Brexit, we often discuss the weather.  This is odd, when you consider that, other than a lot of rain, we don’t see extremes of weather that often.

Recently, we’ve experienced what we Brits would regard as some cold weather, although it’s been nothing like as bad as that experienced by my friends in the Midwest.  I recently phoned Adam Webb in Chicago, on the day when outside temperatures sank to minus 50°F.

My rotomolding lab in UK is a 1,500 sq ft industrial unit, of pretty standard construction (mainly precast concrete panel).  On a recent morning, I experienced an ambient temperature of 30°F, when I opened up the molding area. This compares to an ambient temperature in the range 60-80°C during mid-summer and even higher after a heavy day of rotomolding.

I’m molding all year round, for my various R&D  and consultancy projects, and I know it’s important to maintain consistency.  The question is: how big an effect will ambient temperature have on what I produce and how I produce it?

It’s well known that ambient temperature will affect cooling cycles and I intend to cover this in detail when it actually becomes an issue for most rotomolders: in the summer (ie when ambient temperatures are relatively high).

What is often less well appreciated is that low ambient temperatures can significantly affect the cooking cycle.  This is particularly true at the start of molding, after a shutdown of the machine.  It is also true if you are loading up a new mold on an arm, particularly one that has been sitting out in a cold warehouse.

I find that, even those molders who do give this some consideration, usually seriously under-estimate how much of an effect a cold mold can have on how well the material inside is processed.

Here’s a typical scenario.  I arrive on the shop floor to conduct a trial of a new material and the customer has already selected a particular mold / machine combination for me to work on.  He has got his mold mounted on the arm of a machine and the arm hasn’t been through the oven yet.

I’ll ask the operator or supervisor how they compensate for a cold mold on a cold arm and the usual answer I get is: “oh yeah, we usually add 1 or 2 minutes on to the cook time, for the first molding cycle of the day”.

In my experience, this won’t actually provide much compensation, certainly not enough.  Because I’m usually monitoring my trial by measuring in-mold temperatures (with my trusty, but nowadays rather battered, K-PAQ), I can demonstrate this directly.

When I show them the evidence, the machine operator and the supervisor are always surprised and, often, still skeptical.  How can a single mold and a few lb of material rob so much heat from the oven?

Well, first of all, it’s surprising how long both metal and polyethylene take to warm up, even in a hot oven.

To illustrate this, I did a simple experiment the other day.  I took a round bar of scrap steel and drilled a hole for a thermocouple down its length.  Then I put the steel bar into my gas-fired lab oven (which was steady at 480°F) and monitored the temperature build-up in the bar.

Screen Shot 2019-02-13 at 2.25.51 PM

It took over 8 minutes for the temperature of the steel bar to rise from ambient temperature (which was 55°F that day) to 170°F.  That represents a temperature rise of less than 15°F/min. The bar weighed about 11 lb, there was nothing else in the oven at the time and my lab oven has been deliberately designed with a relatively powerful burner, for fast temperature build-up and recovery.

Now imagine how long 200 lb of steel (or aluminum) mold is going to take to fully warm up when it starts off stone-cold.  Let alone the many hundreds of lb of steel in your arm and plate.

Once the arm and mold have been through a few cycles, their residual temperature will rise significantly.  In particular, the arm will likely be in the range 140-170°F, even when it comes out of the cooler; the fact that you wouldn’t be able to touch it without a lot of discomfort, if not actually being burned, means that it must be up in this temperature range.

OK, interesting, but why does all this matter?

The main issue is that, if you don’t compensate properly, you run the danger of seriously under-cooking your materials, for the first few cycles of the day.  Given that, in my experience, most rotomoulders have a tendency to under-cook their materials, you could easily end up with brittle parts and a rough inside surface.

You may also notice that your thickness distribution is more uneven.  This is because, in these early cycles, the material inside the mold will have seen fewer rotations where the mold surface is sufficiently hot to pick up material.

If you’re molding something that is more temperature sensitive (eg crosslink), you could get yourself in some real trouble.

Net result: scrap parts and money lost.  Bad news, however you cut it.

I always tell students on my operator training courses that being consistent is the secret to successful rotomolding.  This issue, of cold molds and arms robbing heat out of the process, is a prime example.

The first thing to do is measure something.  If you don’t want to get involved with sophisticated temperature measurement devices, try strapping a temperature-sensitive label to your vent for each of the first three cycles of the day.  Use either stainless steel wire, masking tape (for a metal vent tube) or a suitable Viton O-ring. This will at least show roughly how much you are under-shooting your target Peak Internal Air Temperature (PIAT).

You can get temperature-sensitive labels from general industrial catalogs and other sources.  I believe that 493K sell several styles of label, designed specifically for rotomolding temperature ranges: try http://www.493k.com/shop/k-label/

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.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s