Most extrusion, blow molding and injection screws are manufactured out of 4140 HT Steel at a 28-32 Rc condition. This grade of 4140 has a yield strength of 95-100 kPSI. The next most common base material used for these screws is 4340 HT Steel at a 28-32 Rc. It is approximately 10% stronger than 4140 and is highly recommended for use in high-torque applications, where fraction melt HDPE, PP and other viscous resins are being processed.
But the material that is best for screws up through about 2.5-in. dia—and also for applications involving flexible and rigid PVC and RPVC processing—is 17-4 ph Stainless Steel. 17-4 ph Stainless Steel has a yield strength in the area of 175 kPSI, or almost twice the strength of 4140. This material costs about twice that of 4140, and it is a bit more difficult to machine, but for small screws it is an ideal base material because it eliminates the possibility that the screw will break or twist. I always recommend that any screw under the size of 1.5-in. (or 35 mm) be made of 17-4 ph stainless.
For PVC processing applications, screws are typically made of 4140 HT, and chrome plated 0.002-in. thick. This works fine at first, but after the screw has been pulled and cleaned a few times the chrome will begin to wear away in the channel radius, exposing the raw 4140 base material, which leads to corrosion. But if 17-4 ph Stainless steel is used, this never becomes a problem. 17-4 has 16% chromium, whereas 4140 only has about 1% and therefore has excellent corrosion resistance to PVC.
So the next time you buy a new screw, think about requesting a second option and have your screw supplier quote you a screw made of 17-4 ph stainless steel. I think you’ll be quite pleased.
Tim Womer is a recognized authority in plastics processing and machinery with a career spanning more than 35 years. He has designed thousands of screws for all types of single-screw plasticating. He now runs his own consulting company, TWWomer & Associates LLC. Contact: (724) 355-3311; firstname.lastname@example.org; twwomer.com