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10/14/2012 | 1 MINUTE READ

Stainless Steel Screws

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For smaller screws up to 2.5 in. diameter, and for PVC processing applications, you might find this to be the best choice.

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; tim@twwomer.com; twwomer.com