Universal, or General Purpose screw designs are more or less the standard modus operandi in resin processing – particularly in the injection molding sector. In theory, this sounds like the ultimate solution to the ultimate problem, however the possibility of such a design is relatively impossible from polymer to polymer, especially when comparing polymers of vastly different properties and physical attributes. Remember, it is also crucial to consider the other vital pieces of your system. The melt stream components (screw, barrel, check ring, nozzle, etc.), die or mold, and downstream equipment are all parts of the production line that must work interdependently, with their own unique requirements.

While there is no true “GP” or Universal screw design, there certainly are no equivalents in the mold or die department either, and for good reason. The mold or die being used in any application is determined by the final molded part being produced. The product’s required geometry primarily determines the mold and die design. Moreover, the final molded part’s physical characteristics and end use determines the specific resin to be used during molding. There is no “general purpose” or “universal” molds or dies – this is abundantly apparent. Most downstream components are specific to the end user’s particular application, similar to the differences between the properties of the different polymers. With this in mind, it is critical to consider the quality and consistency of the extrudate supplied to the downstream components. A mold or die can only utilize what is being supplied to it, and if the process is started with poor quality, or contaminated material, no amount of altering the process or design changes are going to change that outcome. Simply put, you cannot solve downstream problems without considering upstream causes.

It is important to note that no one particular part of a system can single handedly change the results of a less than desirable outcome; the most important part of a system is the synchronicity of all elements of the system. As a rule of thumb, when it comes to melting efficiency, the screw does roughly 80% of the work, while the external barrel heat bridges the gap. As previously mentioned, the downstream components and the feedstock being used is largely predetermined by the final molded part. With the screw being the workhorse of the system that supplies the quality and output required by these predetermined parameters, an “off-the-shelf” approach for screw design is far from ideal. Different resins have different processing requirements. GP screw designs are largely characterized as being square pitch screws with no mixing elements. The lead, flight width, pressure profile, channel depths, etc. are all critical design elements that can significantly alter the performance of a given screw. More importantly, the lack of adequate mixing greatly reduces the potential melt quality and stability. Again, different polymers have different melting requirements. Just as we wouldn’t use a mold designed to make cutlery to make car bumpers, we cannot reliably use a screw designed for HDPE on Rigid PVC. If we are to be serious and competitive with part quality, scrap rates, and overall efficiency, all aspects of the system must be investigated.

All of these things considered, a good process monitoring system, good quality feedstock, a screw design based on your particular application, proper education and practices with your employees, and an overall investment in the infrastructure of your system are the keys to success. No one thing is more important than the other when it comes to a polymer processing system, whether your product is molded or extruded. Each piece of a given system should be carefully considered. Instead of looking for a Universal or “GP” screw design, we should all consider the return on investment of looking into the finer and more specific things that make our product lines so vital to our modern civilization. To be competitive in the global marketplace, all aspects of the plasticating process must be taken into account.