Careful Heat-Treatment of Engineering Plastics
Machined engineering plastics components often perform critical functions where other materials such as steel and aluminium are not suitable, but far greater care is needed in processing them. Suitable heat treatment is required at a number of stages in the manufacturing process to ensure a stable and accurate final product, and these can vary greatly with plastic type. A poor heat treatment regime is often behind issues such as cracking, crazing, dimensional in-accuracy, de-lamination and stability issues and this is especially evident with precision machined or bonded parts. Two of the more sensitive engineering plastics are Arcylic/Plexiglas (PMMA) and Ultem (PEI).
Arguably, the most critical heat treatment step is to first normalise the material before performing any machining operation. Care needs to be taken on the choice of normalisation temperature, which is typically close to the glass transition temperature (Tg), as materials such as Acrylic come in many different commercial grades with different Tg’s. Depending on the thickness of the material, this normalisation can take between a few hours and several days, requiring planning and significant oven capacity and control if dealing with multiple types and thicknesses. This normalisation process addresses two critical areas. Firstly it pre-shrinks the material in X and Y (and expands in Z) which is particularly important when using the highest-grade materials such as Cell cast Acrylic. Without this step, you would not be able to hold accurate nominal dimensions after machining and future heat treatment, let along fine tolerances. The second reason is that it de-stresses the material, making it far less prone to cracking and crazing, which is often seen in poorly made parts, either hours or years after production.
When the part has undergone any significant machining processes, it is important to anneal the part to remove the inherent stress that the machining processes have imparted into the product. Machining plastics is far more challenging that metal, with lower cutting speeds and many more passes being required, however even with careful machining, some stress is still built up in the part. Annealing is performed at a lower temperature than normalising, and usually for less time.
High Accuracy Diffusion Bonding (HADB)
Carville have a unique technique called High Accuracy Diffusion Bonding (HADB) in which certain plastic materials can be bonded together at a molecular level, without the use of any cements or adhesives, to very high degrees of accuracy. For this process to be highly repeatable, accurate and produce very long lasting parts for critical applications, it is vital that the pre and post-bonded parts have a strict regime of heat treatment. This avoids any potential for cracking, crazing, de-lamination and helps maintaining the high tolerances required, including 150 micron channels. Some of the more complex diffusion bonded manifolds, with multiple layers, can have more than five separate heat treatment stages.
Carville have developed and optimised a vapour polishing process called “super finishing” which when used in stages during the manufacturing of bonded manifold layers, produces optically clear channels. This is especially important in certain life science and medical device applications where fluid samples are being optically analysed or need to be clearly visible to users. The nature of this vapour polishing also imparts some stress into the plastic, so an annealing regime around this process is also important to avoid cracking.
Carville engineering plastic products are used in many critical applications including class III medical life support systems, medical implants, point of care diagnostics, industrial safety systems and aircraft. The requirement for long-term, reliable operation is vital and the heat treatment processes is key part of ensuring these requirements are met. It is also important to record all of the individual heat treatment profiles of time and temperature and have full part traceability as part of the quality management system.November 7, 2017