Thermoforming and the Coefficient of Thermal Expansion

By Thom Murray, Sr. Materials Engineer

By definition, the Coefficient of Thermal Expansion (commonly referred to as CTE), is “the amount of expansion (or contraction) per unit length of a material resulting from a one degree change in temperature”. In simplest and most practical terms for the thermoformer, it can be thought of as how much a material will grow when its temperature increases.  In English units it is typically expressed in length/length/per degree Fahrenheit. For example, inch/inch/°F. In metric, it is m/m/°C.

When we think about CTE in relation to plug assists, the problem is a bit more complex.  Solid polymers, by their very nature, have a relatively high CTE.  They like to grow when heated.  For example, Delrin® has a CTE of 59 x 10-6 in/in/°F. Nylon can range from 40 – 60 x 10-6 in/in/°F.  This means they grow and change greatly during the plug assist thermoforming process as they absorb heat from the sheet during contact. With materials that are specifically designed and engineered for thermoforming, like HYTAC® syntactic foam, thermoformers can greatly reduce variation in their process.

Syntactic foams are filled with hollow glass spheres, so even though they are polymeric in nature, the stable fillers mean that the CTE is relatively low, around 17 – 20 x 10-6 in/in/°F.  When you couple this with the fact that syntactics are excellent insulators and therefore take less heat from the sheet and run at much lower operating temperatures, you have a material that maintains a much higher dimensional stability that other plug material types.  With more stable materials, the thermoforming process itself becomes much more stable.

CMT Europe Opens for Business

By Conor Carlin, International Sales Manager

On August 1, CMT Europe officially opened for business! With a full inventory of HYTAC® plug-assist materials, cut-to-size services and 2 full time employees, the new operation opened the doors and began shipping orders immediately.  At the same time, I put several thousand kilometers on the (very small) car with our new sales engineer, Luc Hoefnagels. We visited thermoformers and toolmakers in multiple countries to talk about the new operation and also to discuss technical matters.

During one visit to a large thermoformer, we spent some time with the CNC operators to talk about machining FLX material. They had recently made some plugs for a new mold and we were discussing the finer points of cutting. I showed them a ½” solid carbide, 2-flute cutting tool to illustrate the high-helix geometry that works best with syntactic foam. When using a tool specifically designed to cut syntactic foam, the intent is to take as large a chip as possible as quickly as possible, eliminating heat buildup or surface marring while controlling any chance of chip-out.

The operator asked if he could try it in his machine to re-cut a similar piece of material and compare the finish to the plug he had just made. After swapping out the end mill, he dialed in the program, increasing the speed to match the new tool geometry, and began to cut another small block of FLX. When complete, we all gathered around to compare the surface finish. Though we didn’t have a profilometer to measure the precise surface profile, there was enough of an improvement for the operator to ask if he could keep our tool! (We gladly provided it to him.)

This encounter, and many similar conversations, only proves what we at CMT have seen in tool shops and thermoforming plants around the world: knowing how to machine HYTAC® materials leads to faster throughput, better plug performance, longer tool life and happier machine operators.


2013 SPE Thermoforming Conference Preview

The 21st annual Thermoforming Conference in Atlanta is just around the corner. On Monday, September 9, CMT will participate in an exciting new workshop titled, “Mythcrushers: Separating Black Art from Science”.


Anyone who works in thermoforming will be familiar with the term “black art”. The “Mythcrushers” workshop is designed to examine thermoforming processes and, by the end of the session, allow the participants and attendees to decide whether thermoforming is a science or still a black art. CMT has long been involved in both academic and industry studies to apply rigorous testing to those elements of thermoforming where the plug assist is involved, but the nature of the process means that we are always looking at multiple variables.

There are many dynamics affecting the different types of materials used for plug assist.  During the workshop, CMT will discuss why it is important to understand the interplay of the plug material, plug geometry, tool design and sheet temperature, and not just to look at each element in isolation. While the choice of plug assist material directly impacts the plug life, machinability and ease of processing, it is also directly tied to material distribution, cycle time, start-up time and process cost.

We’ll be in booth #623 at the show. Stop by and say hello!

METAPOR® – Air Permeable Materials

By John Shpack, Process Specialist, CMT Materials

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