When making 3D printing filament, there are many factors that can affect the quality of the results. Probably one of the most important of these is material moisture. We explain step by step how to dry granulate before extrusion to get high quality results without bubbles and inconsistencies.
There are two main types of polymers, polar and nonpolar. Nonpolar Materials usually don't need to be dried. Exposing nonpolar materials (HDPE/PP etc.) to water is like putting oil in water. Almost every other commercial polymer has some level of polarity and therefore can absorb a certain amount of moisture from the air. Many materials are dried only to optimize surface appearance, too much moisture can lead to cosmetic defects known as splay or silver streaking (Image 2). Other polymers are damaged if they are processed with too much moisture. These materials chemically react with the moisture (hydrolysis), resulting in a reduction in material strength. You can keep an overview which materials need to be dried with this table.
There are several different DIY methods to dry polymeres; you can use an oven, a dehydrator or a vacuum dryer. But In this How-To we will focus on a self-built drybox with silica gel, because this method delivers the best results while being cheap, easy and energy efficient. One of the problems we encountered with the oven is overheating the plastic, resulting in a big mess. We tried replacing the silica gel with a household dehydrator, but they are not designed to achieve this kind of low humidity levels needed to effectively dry plastic. We also included a temperature table for different polymers.
To make our drybox, we simply use a plastic box that we upgrade with some window seals to make it airtight. Just glue them to the rim of the box.
You might already know silica gel from a shoebox for instance. It comes in small bags in order to keep the inside dry by absorbing any moisture. Silica gel works perfectly with polymers as well so just put around 2 kilograms in the plastic box. There is no particular amount you have to use, but the more you have in the box the less often you need to take them out and dry them.
A hygrometer comes in handy to check the humidity inside the drybox. It is convenient to use a transparent box so you can keep track of the drying process at all time.
Using a microplastic bag (we use one from Guppyfriend) to place the material inside of the drybox will save you time and space. It keeps everything in place while the humidity can get out.
The simplest method to measure how much moisture has been absorbed is weighing the granulate beforehand and afterwards. Unfortunately, if you are using hot air to dry granulat, volatiles are released from the sample, also reducing the weight. Therefore the weight loss does not exactly equal the loss of moisture so the measurement is imprecise. The professional approach involves making accurate moisture measurements in the drying hopper. This is possible using measurements of dielectric properties in real time. sensors are very expensive though.
After a while, you will notice that the silica gel reaches its maximum capacity of water that it can absorb. Simply put it in the oven for one or two hours at about 100°C/212°F and it will be as good as new.
Rapid changes in the Temperature lead to surface moisture. If a cold drink placed outside in the hot summer, you will notice that water starts condensing on the surface. This also is the case for polymers. We had many problems because we took plastic from the cold, damp garage to the warm production area. This resulted in a lot of surface moisture that produced bubbles and inconsistencies in our 3D-Printing Filament.