Proper melt preparation is critical to the injection molding process. To achieve good melt takes a plasticating screw that is designed and manufactured for your specific needs. Do not settle for anything less. At Md Plastics, we have the experience, technical knowledge, and software programs that can provide you with plasticating screws that are optimized for your specific situations. Give us a call today. You may be surprised at what a conversation with us can reveal about the inadequacies of your current plasticating screws.
Posi-Melt™ Multipurpose Screw
When a screw design is considered for a particular polymeric family, the rule-of-thumb has been; Amorphous screws process better with low compression designs, while Semi-Crystalline materials prefer high compression designs. Since Semi-Crystalline materials have a sharp melting point and are either in the solid state or melt state, higher compression designs with shallower metering sections have done a better job of assuring that each pellet is melted. Whereas high compression designs with shallow metering sections have the tendency to over-shear and heat Amorphous materials. The Industry Standard GP screw, is a square pitch design that has a compression between 2.3-2.7:1 and a 50/25/25 feed, transition, metering length ratio. The GP geometry does an adequate job at best with both polymer groups within a narrow process window. Since Injection applications entail a broad range of polymer groups, residence times, and swept volumes, a more advanced design is needed to assure a more uniform melt homogeneity.
The Posi-Melt geometry is a core change to the geometry of a compression screw. We have separated the coordinate systems of the root and the flight pitch to force the pellets through a channel that changes pressure and velocity while exposing 55-60% more pellets to the heated barrel wall for increased conductive heat transfer. The "stepped" root geometry exposes the polymer to a change in velocity and extensional strain that enables us to lower the volumetric compression for low shear heating while establishing a high "linear depth ratio" between the feed depth and the metering depth. The lower volumetric compression works well with Amorphous polymers by lowering the shear between the pellets in the transition zone, and the high "linear depth ratio" assures a shallow meter depth for efficient melting of the Semi-crystalline polymers. The Posi-Melt screw has won acclaim from resin suppliers, OEM's and our customers/fabricators on both ends of the crystalline spectrum from LCP to Rigid PVC and the improved melt quality broadens the process window for custom molding shops that process a variety of materials. The Multi-Purpose Posi-Melt geometry delivers a more thoroughly melted polymer with a higher level of melt homogeneity that is able to produce higher quality parts over a broader spectrum of residence time and swept volumes.
Posi-Melt "MB-XLA" Barrier/Mixer Screw
Our proprietary "MB" barrier forces the polymer between the barrier flight and the barrel ID for added extensional strain to assure complete melting. We can couple the "XLA" mixer to the Barrier for processing a variety of resin groups, including PP, HDPE, LDPE, PS, PET, ABS, and other groups delivering the Highest Outputs and Homogenization available. All Posi-Melt screws are designed to have an adequate feed depth to promote solids conveying to build pressure for moving the polymer through the transition and metering zones. Additional care is taken to design-in the proper leading and trailing edge radii to avoid low-velocity sections in the channel and to maximize the pressure. The benefits of our design are: Color and Material change times are reduced compared to our competition because there is no need for Two separate channels to achieve thorough melting and good dispersion…our channel is 100% pressurized throughout for Solids Conveying, Melting and Pumping.
Posi-Melt "XLA" Mixing Screw
In order to add a higher degree of homogenization to the melt pool, we developed a very effective distributive/dispersive mixer that works by forcing the polymer to change direction numerous times over a short axial distance and by adding extensional strain to the polymer by forcing the material to cross over a shallower land multiple times for added shear. A positive pumping angle and major flight OD wiping action assures a low-pressure drop for high throughput yet pressurizes the channels for positive displacement for melt cleaning purposes. The design is very versatile, rows can be added, the number of slots determined, and the clearance between the barrel ID and the Mixing Land can be changed to suit the polymer family. The mixer is used for virtually all resin groups when precise color matching is needed or the residence time and swept volume dictate.