Optimum Drying of Plastic Resins - 4/24/2006
A majority of the plastic resins are hygroscopic. Meaning they absorb moisture when exposed to a humid environment. This moisture interferes during the molding process and can result in various molding defects. The moisture turns in to steam, mixes with the melt and shows up as streaks called splay. On thicker parts, the moisture could accumulate in pockets near the surface. When the part is ejected heat can still be retained inside the part. The moisture which is now steam exerts pressure on the sides to create bubbles on the part. On a molecular level, chain scission can occur to break down the polymer chains resulting in the degradation of the properties and therefore the under performance of the part. It is therefore very important to remove the moisture before the plastic enters the barrel on the molding machine. This is accomplished with the help of dryers. All resin manufacturers recommend a drying time and a drying temperature for their resins. Resin s dried at the proper times and temperatures will have minimum moisture and will be suitable for processing. The water forms a weak chemical bond with the plastic. So when the plastic is subjected to the drying temperature for the recommended time, the bond breaks and releases the water molecules making the resin ready for processing.
The authors’ research into drying times for nylons and polyesters has show nd that overdrying of these resins can lead to various issues. It was found that in case of the polyester there was a loss in physical properties in the parts molded with the resin dried over twelve hours. Drying for extended periods of time did not change the rheology of the resin, leading to the conclusion that there was no polymer chain scission and therefore there was a loss in the low molecular weight additives. Low molecular weight additives are added to resins to improve certain properties. In case of the nylon, it was found that the rheology of the resin changed with increased drying times. Nylon is a condensation polymer and water is a byproduct of the polymerization reaction. The moisture does not harm the chain length but acts as a melt viscosity reducer. Increased drying times removes the moisture and therefore increases the viscosity causing a shift in the molding process. A part that was 99% full during the injection phase of the molding process filled to only 62% under the same molding conditions with over dried resin.
It is clear from the above discussion that it is important not only to dry resins but also to control the drying process. Always dry the resin for the recommended time and temperature mentioned by the resin manufacturer. Calculate the pounds per hour usage of resin and size the dryer accordingly. For example, if the molding machine uses 10 pounds of resin per hour and if the recommended drying time is 4 hours, use at least a 40 pound hopper. Using a 200 pound hopper, will result in a residence time of 20 hours which could result in defective parts and/or continuous readjusting of the process. Often times the process technicians and engineers start molding making good product and after a few hours they have to readjust the process. Over dried resin could be one of the reasons for this process readjustment. Using a moisture analyzer is a must since it will give you an indication of the dryness of the resin. Unfortunately, there is no good way of determining if a resin has been overdried.