In order to understand the proper way to fix small core shots or other base inconsistencies, it helps to know about the materials you are working with. Just like it is important in woodworking to understand the wood’s oil and moisture content, grain characteristics, figure, and so on, when repairing your bases, it is helpful to know a bit about P-Tex. Understanding its properties will help you further understand the reasons why the above techniques are effective.
P-Tex is a type of polyethylene, the most widely produced plastic in the world. It is used in everything from Ziploc bags to cartilage analogues in joint replacements. The main difference among the wide array of applications of polyethylene, however, is its chemical structure.
Polyethylene is a polymer, meaning it is made up of many (“poly”) long chains of repeating molecules (“mers”); in this case, ethylene repeats.
In Ziploc bags, the ethylene chains are highly cross-linked, which makes the material less dense and therefore easily pliable. P-Tex, on the other hand, is made of an ultra-high-molecular-weight version of polyethylene (UHMW for short). This means that each chain of ethylene molecules is very, very long: hundreds of thousands of repeat units long, in fact. These long chains make the material very resistant to impact (i.e., hard).
P-Tex is also a highly crystalline polymer that’s generally formed by sintering (basically, a process of heating and compressing a powder to make it a solid). The high degree of crystallinity is what gives P-Tex its strength, while the sintering process causes the polymer chains to line up in both a uniform structure and a grain pattern. This crystallinity, along with the UHMW polyethylene chains, make P-Tex hard (resistant to impacts) yet very porous and able to accept wax.
The strongest P-Tex fix will integrate seamlessly into the existing base material. In order for this to happen, one needs to be cognizant of the changes made to the properties of P-Tex during a repair, and minimize them. The main goal is to keep the crystal structure and the long chain length of the P-Tex intact, which means keeping the P-Tex candle at the lowest possible temperature when lit.
If the P-Tex candle gets too hot, the long polyethylene strands begin to lose their crystalline structure. Because the crystalline structure requires very tight packing of the molecule chains, any deviation from this tight packing will result in changes to P-Tex’s physical properties. The two main properties that will be affected by heating are its toughness and density. In other words, the more it is heated, the less resistant to impact (the more brittle) the P-Tex becomes, and the less mass per unit volume it will have (which will make the P-Tex more susceptible to thermal expansion).
As described above, two ways to minimize the temperature of the P-Tex are to keep the lit candle as close to the base as possible, and spin the candle to cool it. Doing so will keep the change in the P-Tex’s density and physical properties to a minimum, ensuring your repair will hold longer and stronger.