A better steel for Plastic shattering blades?

[sustacimetin]

Hello dear eng-tip users,

Our company produces blades for plastic recycling machines. Steel we use for that is 50CrV4. With heating process we bring it's rockwell degree to 40-44 HRC. Although this material works properly we are in search for better materials. The problem we face is that the blades pointy edge gets circular easily with the usage. We want a harder steel compound, but it should have some elasticity. One reason for that, when users try to recycle plastic bottles etc, sometimes they accidentally throw metal nails and such into the machine. When blade hits that iron or metal, if steel blade is so much hardened, whole blade breaks and hits everything in the machine, which causes the other nine blades to break and with the torque it can even throw the blades out from the machine dangerously.

I am still in search for materials. Best we came up with was h500 and h900 kind of materials and I'm clueless about both. Do you have any other thing in mind that can work properly for us? Or materials I mentioned are logical to use? Im not an engineer, nor I have science degree. So anything you write can be a big of help. Thanks for reading.

 

[tbuelna]

Can you attach the blades so that they have a bit of compliance when they strike something dense (like a lag hinge in a helicopter rotor blade). Or how about adding a separator ahead of the blades that diverts dense or magnetic materials. A low speed, high torque processing section ahead of the high-speed chopping section might also help.

Unfortunately, I don't think that making the blades from a harder steel will resolve the problem

 

[sustacimetin]

The machines we produce blades for works with the same principle. You put six to ten blades on rotor, we call them mobile blades. Also one or two blades placed on to machine so when rotor turns, rotor blades hit these two baby to shatter the plastic completely, we call them stationary blades.

I like the magnet idea, but process of that may be costly compared to efficiency. Also there are some machines with magnet rails so it seperates the iron before shattering. But most of the recyclers dont have that and also that's not efficient so much.

Producing blade from a harder steel wouldnt completely solve the problem, you are right. But it will be more durable on prolonging the life of the blade. So the manifacturer can recycle 50 tons of plastic instead of 40.

 

[Compositepro]

Many blades like this have very hard cutting edges with a tough back-up alloy. The hardened edges can be done by localized heat treatment, carburizing or by brazing on tungsten carbide edge.

 

[tbuelna]

sustacimetin-

I do appreciate that you are looking for a solution to your problem that won't involve a costly design change. As you noted, the problem with making the steel blade harder and less ductile is that it will have a greater tendency to fracture rather than deform when it impacts a dense object like a nail. From the nice images of the blades you provided it appears like they are designed to be resharpened on a regular basis. Using a case hardening treatment on the cutting faces would give a combination of a hard surface supported by a tough core, but the case hardening would require constant rework from sharpening the blade. Using a thru hardened steel permits the blade to be repeatedly sharpened simply by machining the cutting faces, but the level of hardness used for the blade steel is a difficult compromise between durability and damage tolerance.

One thought I had that you might look into is modifying the profile of your blade cutting edge. Currently your blade edges are straight and follow a circumferential path about an axis normal to the direction the material is fed into the processing section. My proposal is to use a serrated profile where the cutting edges are at an angle to the direction the blade travels (see the attached sketch). Having the cutting faces at an angle to the direction of travel should reduce the impact force when it strikes a small dense object like a nail. The blunt blade tips shown in the sketch would make them tolerant to impact damage. The blunt tip traveling at high speed should still have sufficient force to punch through the thin wall of a plastic container, and the angled cutting surfaces behind the tip should easily be able to shear the remaining material. Getting the blade configuration right will require some design optimization studies, but I think you could get it to work. This serrated design might even be able to use brazed carbide cutting edges as Compositepro suggested. The primary drawback is that the blades would be more expensive to produce and would be more difficult to re-sharpen.

Hope that helps.
Terry