Extrusion: Two-Stage Screw Offers Distributive, Dissipative Mixing
New design from Davis-Standard offers outputs comparable to barrier screws at lower melt temperatures.
The new DS-Blend feedscrew from Davis-Standard (D-S) provides distributive and dissipative melt mixing. The two-stage design reportedly achieves outputs similar to those provided by barrier screws, but at ower melt temperatures and reduced energy requirements. It can be installed on new or existing single screw Davis- Standard extruders as well extruders from competing suppliers. John Christiano, D-S’ v.p. of technology, calls the new design “one of our most versatile and energy-efficient feedscrews to date.” He adds, “As with all of our feedscrew designs, we are focused on product line performance, improved outputs, material homogeneity and feedscrew longevity.”
Feedscrews are available from 3/4 to 12 in. diam. with varying L/Ds. Each screw is engineered for specific polymers and processes with customization available depending on end product. Davis- Standard feedscrews accommodate the full range of extrusion and converting applications including blown film, blow molding, cast film, compounding, elastomer, extrusion coating, fiber, laboratory, pipe, profile and tubing, reclaim, sheet and specialty systems. Testing and trials are available to fine-tune the process prior to purchase.
As part of what it calls a “start to finish” high-quality feedscrew-manufacturing process, D-S uses plasma-arc welding stations to provide wear-resistant materials such as Colmonoy 56 and 83. Three state-of-the-art whirlers are used for screw cutting and milling for screws made of stainless steel, 4340 steel or Inconel materials. These machines produce screw surfaces at very tight tolerances. After screws are milled and polished, they are treated with chrome plating or other wear-resistant coatings based on application. Screws receive a final inspection and polish before shipment.
Putting one or two vents between the feed throat and die is a good way to remove moisture, trapped air, and other volatiles from melted plastic as it moves through an extruder.
Around three dozen, mostly European, processors are pushing commercial development of high-speed single-screw extrusion. They have installed more than 100 of the small hyper-drive machines whose screws turn at up to 1500 rpm, about eight to 10 times faster than standard extruders. At least two German machine builders are working on machines that will go to 2000 rpm and even higher. The goal is to raise output without increasing extruder size.
A poorly designed profile die—one that does not permit the part to be extruded with the same dimensions from run to run—coupled with a lack of understanding of the extrusion process, is a recipe for scrap generation.