In our blog on the injection molding process, we have dealt with PET in some way. after all, polyethylene terephthalate is a thermoplastic. If you want a closer look at injection molding of thermoplastics, we recommend this blog post about it.
We at TX Mold have already written about thermoplastics in general here. Why are thermoplastics so interesting that we write about them so often? It’s no wonder; after all, thermoplastics form the backbone of plastic injection molding, so to speak. Because thermoplastics were early discoveries, injection molding began its unstoppable career as one of the most important production processes in history.
The History of Polyethylene Terephthalate
PET, for example, was invented as early as 1941, although at first, the patent had to be kept secret due to the war. Later, fibers of this material were used worldwide, namely as Trevira, Dacron, or Diolen. many of our parents and grandparents still have to wear dresses, pants, and stockings made of this synthetic material, which is simply cheap and durable. Nevertheless, using PET as a clothing material is certainly not the ideal way to go, as it doesn’t feel as comfortable, especially when sweating.
The situation changed when further research revealed the possibility of specially treated functional underwear when quick drying was required. This is because PET hardly absorbs any moisture and allows it to evaporate quickly. But there is at least one point where this is important for its processing as plastic for injection molding, as we shall see.
Properties of PET
PET has started its journey to worldwide victory mainly in plastic bottles and packaging materials. This has also given it a very bad reputation in recent years. It is not PET’s fault, but people’s discarding behavior has caused massive pollution of the oceans, in which PET plays a major role. However, PET has such remarkable properties that it has to be actually used. It just has to be used with more caution.
- PET is food safe and resistant to many chemicals.
- PET is resistant to tearing, creasing, and weathering.
- PET is almost completely transparent
- Its existing gas permeability can be prevented by, for example, a silica coating.
PET has good tissue compatibility and has even been used as a prosthetic component for blood vessels.
However, the most important property of PET is, of course, its excellent recyclability. Pure PET can be reused almost 100% of the time if it is handled correctly during the recycling process. In this process, about 60% less CO2 is produced than when producing “fresh” PET. With its dimensional stability (which maintains its properties), it can also be recycled and used well in appliances or automobiles. But there is no light without shade. During the production of PET, acetaldehyde and antimony trioxide may be formed, and if the temperature is too high and the material is stored for too long, it can be peeled off. Then mineral water no longer tastes so good, even though scientists believe it is not harmful to health.
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Pet in Injection Molding of Products Such As Pet Bottles
In principle, the process of injection molding is simple. The pellets used are heated to softness of about 535 °F (280 °C) so that they can be liquefied into a homogeneous mass and injected into a cooled mold. Having cooled during the injection process, the PET cannot crystallize and retains its transparency. Depending on the shape the plastic is to take during injection, the mold cools the PET to a point where it reaches a stable range of properties and can be ejected. For thermoplastics, this process can be repeated as often as needed, as long as the limits are not exceeded without disrupting the order of the molecular chains. This is particularly interesting in the production of bottles.
This is because many injection molding companies now perform this process in two steps. In the first step, a compact blank is produced from plastic. The cooled blanks are sent to the customer’s plant along with thousands of other blanks. This company has its own machine, much simpler than an injection molding machine, and designed for this one purpose. They heat the PET blanks to about 230 °F (110 °C) and stretch them in all directions. To do this, the blank or “preform” is pulled over the negative mold of the bottle and then blown out with compressed air until it fills the boundaries of the mold. It is then ready to go immediately to the filling line.
Quality Control of the Product Injection Molding
It all sounds very simple. In principle, if it were a process with only a few plastic parts, it would be. However, in injection molding, cost-efficiency is of course a top priority, and this can only be achieved with high-volume production.
During production, all 200 or so different parameters of the injection molding system must not only be precisely adjusted but also constantly checked during the production of the product. This is, of course, done with the help of electronics and, in the case of complex parts, even artificial intelligence can be used.
However, there is a long list of potential errors that play an important role in quality management during the injection molding process. Here are a few of these examples so you can see how we work on a daily basis at TX Mold.
PET absorbs moisture to a very small degree. It must be removed prior to the use of pellets or the properties will change during the molding process. Drying must be done at precisely measured temperatures and takes an average of 4 hours at approximately 338 °F (170°C). Depending on the properties of the material, errors may occur during the drying process and can only be detected by precise measurements.
Among other things, these errors may occur due to excessive storage time under unfavorable conditions. This also applies to the storage of blanks or preforms. Errors in drying can also lead to changes in temperature requirements during the injection molding process. If these problems are not recognized, or if materials are not changed, crystallization, shrinkage or flying edges can occur.
Pressure and temperature deviations in the injection molding process can lead to a range of problems, from clogging of injection nozzles to scratches on the bottom of blanks and thread formation on bottles. Contamination due to poor storage or transportation problems is naturally reflected in the product as well.
Interested in injection molding? Please feel free to contact us and learn more about our services and products on our website. TX MOLD is at your service with all its experience and contacts in your industry. We look forward to getting in touch with you.