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Max/Full Crystallinity in Injection Molding: What, How & Why?
As custom injection molders of coil bobbins, we at Plasti-Coil are often required to produce parts to “max” or “full” crystallinity. What exactly does that mean? Well, it doesn’t mean that we are required to produce parts that are 100% crystalline by weight. That’s not possible and even if it were, it wouldn’t be desirable.
What “max” or “full” really means, in this case, is that we’re expected to realize 95% of the material’s potential crystallinity. That’s the standard that we’re held to. At this point you may be thinking, “All right, how do you do that and how can you tell when you’ve done it?” I’ll address that in a moment but first, let me explain what is meant by crystallinity.
What is crystallinity in injection molding?
Crystallinity is basically the ordered alignment and structure of the molecular chains that make up the resin we are using to produce parts. If we are using a material that is semi-crystalline (has the potential for crystallinity) then some crystallinity will surely be present. We cannot completely prevent it nor can we produce a part that is completely crystalline. All parts produced from semi-crystalline material will have both regions of crystallinity (order) and regions that are amorphous (contains randomly distributed molecular chains).
However, we can, within limits, influence how much of each develop. It should be noted that in this case, we’re talking about maximizing crystallinity. That will result in parts that are harder, more thermally stable, more rigid (and brittle), and more chemically resistant than would otherwise be the case. If these characteristics are not important or are undesirable then we would not want to maximize crystallinity.
Why would we want crystallinity in custom parts?
Maximum crystallinity would be desirable if the characteristics mentioned above are important for the application. For example, many of our bobbins are used in high-temperature applications so the increased thermal stability associated with higher crystallinity is necessary. Some of our bobbins are wound under high tension and then molded over with high temp materials so the increased rigidity and thermal stability both contribute to the structural integrity of the bobbin.
How do we maximize crystallinity?
As mentioned earlier, all semi-crystalline materials will develop some degree of crystallinity in normal processing. It’s the nature of the material to do this, at least to the degree we allow it to. If we want to maximize it then we need to understand the conditions in which it develops. For a given semi-crystalline material, there is a temperature range within which crystallinity develops. During the molding process, the material will pass through this temperature range twice. Once on the way up to the melting temperature, and then once again during cooling after injection. It is during cooling that we develop and retain whatever degree of crystallinity we end up with.
If we want to maximize crystallinity then we have to provide the material enough time within the target temperature range, as it is cooling, to develop maximum crystallinity. The results of DSC analysis on finished parts allow us to develop processes that consistently produce the desired results during the molding process, without having to resort to secondary, post-molding operations such as annealing, and that helps keep the cost down.
Can Plasti-Coil do it for you?
Definitely. We’ve been doing this for a long time and have developed a level of expertise that is unsurpassed. Give us a call. We’ll be happy to discuss your application, roll up our sleeves, and get to work with you.