Stainless Steel Extrusion, Blow Molding and Injection Screws
The most common base material that extrusion, blow molding and injection screws are manufactured out of is 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 is 4340 HT Steel at a 28-32 Rc, which is approximately 10% stronger than 4140 and it is highly recommended to be used for high torque applications where fraction melt HDPE, PP and other viscous resins are being processed.
But the material that is best for small screws up through about 2.5” in diameter and also for applications where both FPVC and RPVC is being processed 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 does cost about twice that of 4140 and it is a bit more difficult to machine, but for small screws it is an ideal base material to eliminate the possible of breaking or twisting a small screw. I always recommend that any screw under the size of 1.5” or 35mm be made of 17-4 ph stainless.
Also, for application where PVC is being processed, typically screws are made of 4140 HT and chrome plated .002” thick. This works fine but after a while when the screw has been pulled and cleaned a few times the screw will wear away in the channel radius and the raw 4140 base material is exposed and corrosion starts to occur. 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 opinion and have your screw supplier quote you a screw made of 17-4 ph stainless steel. I think you will be quite pleased.