News & Press Releases


The Value of Heat Transfer Within Composite Curing

2013-07-22

This first posting on the News site will deal with the issue of why is heat transfer important in the curing of composite structures and the processes that produce them.

 

A successful cure depends on uniform heat energy being applied to a resin/fiber matrix to achieve an acceptable Tg. That cure temperature and the energy needed to get there must be applied to the materials and the mold uniformly so as to have the complete part cure at the same time. A part that cures at different times in different places causes molded in stresses to occur. These stresses result in twist and warp and other deformations that take the part out of the acceptable physical dimensions. More insidiously, stresses molded into the part may not result in dimensional error but will cause the part to fail prematurely.

 

The most impacting variable that determined the size of your scrap barrel is heat transfer. It is also the least easy variable to define. Most inconsistencies in materials can be discovered through simple lab QA testing. Tooling can be measured for dimensional accuracy before processing and can be checked at elevated temperature to determine if the dimensional accuracy of the tool is maintained at processing temperatures. The challenge with heat transfer during the cure segment of the molding process is that it is assumed to be a constant due to the temperature control used to maintain the process temperature set-point selected for the process. We all know that today’s temperature controls are dead accurate and their control functions hold process temperatures to within one to two degrees. In fact, the control system only holds the sensor at a temperature set point of one or two degrees, not the process.  Therein lies the problem. If the mold is not absorbing heat energy at a uniform rate and yielding that energy to the resin /fiber matrix uniformly, then cure does not occur uniformly and stress is created.

 

The challenge is to develop a method for assuring that the sensor temperature is completely indicative of the overall process temperature and that no portions of the material or mold are outside an acceptable window of temperature/time variation during the cure process.

 

In future posts we’ll get into these challenges and their resolution in various processes. I hope you will find this of interest and come back to learn how, with a little applied science and some simple modifications to tooling and the environments the tools operate in during the cure process, we can achieve a tight temperature range for the cure process using both simple and complex tools and molds while dealing with vacuum bags, autoclaves, matched metal molds, carbon tools etc.

 

Joe Ouellette

CTO - Acrolab Ltd.



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