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Way Cool: How Hot Plastic Takes Shape

July 27, 2018

Materials Scientist Looking Into MicroscopeWe take it for granted in our industry: molten plastic cools and crystallizes into some preordained shape in a mold under a predetermined pressure in a process called plastic injection molding.

If only it were that easy.

The story begins with melted or “molten” plastics.  All these polymer molecules look relatively the same at the microscopic level — sort of like a bowl of spaghetti noodles. As they start to cool,  these molecules begin to crystallize into a structure, just like water crystallizes into ice as it cools down.  The rate of cooling these plastic polymers experience as they crystallize has a huge impact on the finished product in terms of durability, strength and certain other characteristics.

The object of injection molding is to coax plastic polymers into crystallizing in a very specific way.  It’s not as easy as simply heating and cooling the plastic material; rather, it requires just the right amount of pressure and temperature to make individual molecules align themselves with each other, in just the right order.

This is easier said than done: today’s plastic injection molding processes are complicated enough that it requires the use of  computer software to control different parameters. Unfortunately, that software is based on data from decades ago that urgently needs to be updated. For example, when the data was published years ago, plastic cooled at a base rate of 10 degrees per minute. Today’s polymers, by contrast, cool much faster — anywhere from 10 – 1,000 degrees per second. 

The rate of heat transfer, either into or out of a polymer, makes an amazing difference in how a polymer reacts once it cools down. It’s similar to cooking:  Bacon fried in a pan tastes far different than bacon heated up in an oven. This type of research is part of a scientific discipline known as rheology, or the study of  how fluids behave while flowing.

Better molding through science

Today, material scientists take a tiny sample of plastic in solid form, heat it to just above its melting point, then apply a force to it that  simulates how it flows into an injection mold. Once the sample has been allowed to cool, they study it.

Scientists rely on theories of how polymers should behave under different temperatures and pressures to design experiments. Many times these experiments are simply computer simulations that recreate physics equations for each molecule, i.e. showing where each individual molecule is likely to move in relation to neighboring molecules, and how its movement impacts them.  This gives scientists an idea of how plastic molecules are likely to be oriented once they cool into a solid shape.

Scientists have discovered some interesting things about molten plastic during this type of research.  For example, if a polymer is melted only for a short time, it seems as if  it “recalls” its former solid molecular state when it cools down again.  However, if kept in a molten state any longer, it seems to “forget” what shape it was in as a solid, and reforms into a brand new form.

What will these materials scientists discover next? No one knows but you can be sure it will be really “cool” for our industry and could certainly “heat” up profits eventually!


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