In this series we delve into a discerning look back into the history of our industry and how we all got here.
In this series we’ll delve into a discerning look back into the history of our industry and how we all got here.
PAI is a material capable of achieving levels of performance that even semi-crystalline engineering polymers like PEEK, PPS, and PPA cannot. But you have to be willing to work for that performance, and the annealing process is an essential part of this.
TPUs form physical, rather than chemical, crosslinks that can be broken by reheating. Formation of these crosslinks can be sped up by annealing, which improves a variety of properties.
As with semi-crystalline thermoplastics, annealing can be used in thermosetting polymers to obtain a level of crosslinking that may not be possible within the molding cycle.
You can forgo the elevated mold temperatures normally recommended for high-performance semi-crystalline materials. But it’s risky and likely to yield parts that under-perform expectations … assuming that they emerge from the annealing process looking anything like the drawing.
For these polymers, annealing is done to establish a level of crystallinity that cannot be practically obtained within the parameters of a normal molding cycle. Here’s some guidance on setting annealing time and temperature.
In amorphous polymers, annealing is performed to draw down the internal stresses to a level not achievable within the conditions of a normal molding process. But a few parameters are important to achieving the desired results.
Relatively rapid cooling rates in processing introduce internal stress. If functional problems in use result, annealing may draw down the stress to levels that may not be achievable during processing.
Here is how to tell the difference.
To properly understand the differences in performance between PET and PBT we need to compare apples to apples—the semi-crystalline forms of each polymer.
Injection molders of small fuel tanks learned the hard way that a very small difference in density—0.6%—could make a large difference in PE stress-crack resistance.
PE parts can fail when an inappropriate density is selected. Let’s look at some examples and examine what happened and why.
PE properties can be adjusted either by changing the molecular weight or by altering the density. While this increases the possible combinations of properties, it also requires that the specification for the material be precise.
Don’t be afraid to increase mold temperature to improve part quality when making PC parts. Take a look at a few examples here.
An often overlooked factor in optimizing the ductility of PC is the rate at which the polymer is cooled in the mold.
In the final part of this five-installment series, we bring DMA to a practical level using the results for glass-filled nylon.
The presentation of DMA data can sometimes frustrate users and discourage its adoption. Those unfamiliar with DMA should insist on receiving data relevant to the manner in which the part they are developing will be used.
DMA provides a rich and detailed picture of polymer performance, and it can be challenging to understand all the things that we can and should know about our materials. But the fact that it may be hard does not mean that it should not be attempted.