Twenty five years ago hardly anyoneheard of
infrared (I/R) ovens. Now most O&P labs have one or more
I/Rs that they rely on every day to cook plastic.
For the most part, I/R ovens are more expensive than what had been
considered the standard in the industry up to that point – the
“pre-owned” pizza oven. I’m not sure why someone would think
that an oven no longer suited to baking pizzas would be what you’d want to
cook plastic used in medical devices, but that’s another story.
Industry standard
After the use of thermoplastics caught on, the majority of practices and
manufacturers used a variety of
convection ovens to cook their plastic. They employed the
pizza ovens, the Despatch oven, a variety of pre-owned baking ovens and the
Grieve oven. These ovens all work to one degree or another, especially after
you get used to associated quirks.
The biggest problem with these ovens is that they were not designed for
O&P use. Standard production ovens did not have the capacity to handle our
sheet plastic without going into the much larger, industrial versions, which
were too expensive and much too large. In fact, just moving a moderately sized
pizza oven requires either its partial disassembly or a forklift. What we
needed was an oven designed for use by O&P.
Something new
At a time when everyone was using a convection oven to heat
thermo-formable plastics one man dared to try something new and built our
industry’s first I/R oven. His name is Wally Beitl and the first of these
ovens was custom-made for a now defunct central fabrication facility known as
Dobi-Symplex.
Beitl’s design was based on what he observed in the plastics
industry and it was basic compared to today’s models, but it worked on the
same principles. Light waves are generated with special elements and used to
excite the molecules in the plastic, resulting in it heating up. Dobi-Symplex
used these I/R ovens for a while before another central fabrication facility
copied this design for its own use. This copy evolved into what we know as the
PDQ, which was then copied by the now defunct Martek and the Witzel, a German
import.
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| Small convection ovens (top) an I/R ovens (bottom) can meet different needs within the same lab. |
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| Image: Steve Hill |
The advantages that an industry-designed I/R oven provides are numerous.
Even an industry-designed convection oven would be a blessing, but I/R adds
unmatched speed, efficiency, weight and size advantages.
Speed and efficiency
I/R heats plastic quicker and with less energy than convection ovens.
The I/R elements begin to emit cooking temperatures in microseconds and heat
the inside and outside of the plastic simultaneously. Convection heating coils
– known as calrods – not only take time to come to temperature but
they must then heat the air within the confines of the oven box before it
starts heating the plastic. The larger the oven box the more this effect
becomes apparent.
Since it’s energy is concentrated only on the plastic, I/R uses
less energy overall. For these same reasons, it also does not heat up the
surrounding area as much as a convection oven. That has obvious benefits during
the summer when the air conditioner is not locked in a perpetual struggle for
dominance.
Another efficiency benefit in a properly tuned and adjusted I/R oven is
that you should be able to leave the heated plastic in the oven long after its
forming temperature has been reached. Because they do not overshoot the cook
temperature like convection ovens do, I/R ovens are capable of keeping your
plastic at forming temperature for a surprising length of time, assuming it is
set to the forming temperature. When in a busy lab, that is a handy feature to
have.
Weight and size
Not-so-apparent every day are considerations like “how much does it
weigh”? It is not until you want to rearrange or expand your workspace
that you find yourself in a bit of a quandary as to how to move this
monstrosity. Pizza ovens are the easiest, all you have to do is remove all of
the fire brick inside and then get six strong guys to yank it across the shop
and then reassemble it. If you have one of the larger and much heavier
industrial ovens then you have a problem on your hands.
Most models that are big enough to do O&P technicians any good are
so heavy that they require a forklift to safely move it. If your shop is
located in a manufacturing plant or a large institution, then you probably have
a choice of the handy little vehicles, but if you are like the vast majority of
practices then you might find yourself devoting an entire day to move it across
the building.
Because of the extra weight, they are frequently larger than similar
capacity I/R ovens. There needs to be room inside for fire brick and thick
insulation. Many of them are designed to be explosion-proof, a level of safety
required for heat-curing some adhesives and various other industry laboratory
standards.
A purpose for everything
You might believe that I favor I/R over convection – but if you
thought that you would be wrong. Each has its own place in our industry.
For cooking single pieces of large sheet plastic, I/R is definitely the
way to go. If what you want to do is to heat multiple sheets of plastic, you
might be better served by a tall, forced air convection oven. This allows you
to stack multiple trays one on top of the other and cycle through them for high
production. In this case it doesn’t matter that it takes the plastic 15
minutes longer to cook since you are probably busy vacuum forming 10 molds, one
after another.
Most practices have a small convection oven for heating small pieces of
foam, cork and other low temperature materials used to adjust our
patient’s devices and fabricate foam arch supports. These are almost
always inexpensive toaster ovens that serve their purposes perfectly. You can
buy them for $20 or so and they will work for years. Even if you bought one
every other month it would take several lifetimes to equal the cost of a
similar I/R oven.
Options
I’ve seen satellite offices that might cook one or two small,
AFO-sized sheets of thermoplastic a week. Many of them simply use a standard
home range, and let the plastic cook slowly to avoid scorching it with calrods
that are too close to the plastic. It is not the best way to heat plastic, but
it is the least expensive.
Also, there are practices that do not have the cash on hand to afford a
high-end oven. You can probably pick up a used oven for $100 or, if you are
lucky, get one for free. Then in a few years, after you have made some money,
you can upgrade to an I/R oven for sheet plastic. It has become the industry
standard and, over time it will make your lab more profitable in more ways than
you might realize.
The real answer is to get both. You can use both types, each in its
proper role. Make an assessment of what you need in your individual practice
and apply that to your oven budget. A new tool does not have to completely
replace the old. A wise craftsman will use each for its specific purpose.
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