Why Two-Section Alloy Liner for Extruder Screw and Barrel

Extruder Screw and Barrel Description

For extrusion machine, screw and barrel are very important parts. Many customers care about the material, hardness and service life. But actually, the inside structure of barrel is also very important for long time production.

For our new screw and barrel system, Starsplas uses two-section SKD11 alloy liner inside the barrel, instead of normal three-section design.

Why we make it into two sections?

The reason is not complicated. Less connection points can make the barrel more stable and also reduce material staying inside.

Why Two Sections Instead of Three Sections?

Three-section alloy liner is easier for manufacturing. Because every section is shorter, it is easier for machining and installation.

But there is also one problem.

Three sections means there are more connection points inside the barrel.

During extrusion, barrel will have heating and cooling again and again. After long time running, very small gap may happen at the connection position.

For normal material maybe it is not very obvious. But for PVC material, we need to pay more attention to this point.

PVC material may stay inside the small gap. If it stays there for long time under high temperature, the material may become burnt and stick inside the barrel.

This can influence the material flow and also final product quality.

So our idea is very simple:

If we can make less connection points, we make less connection points.

Less Connection Points, Less Material Staying

For three-section design, there are more joints inside.

Even the machining accuracy is good, after installation there may still have very small difference between each section.

Maybe you cannot see it directly, but after machine runs for long time, material can slowly stay at this position.

For our two-section design, we use longer SKD11 alloy liner.

The machining is more difficult for us, but the benefit is for customer.

There are less joints inside the barrel, so the inner surface can be more continuous. Material can pass through more smoothly and it is not easy to stay at the connection position.

Especially for PVC extrusion line, this is important.

Better Straightness and Concentricity

Another reason is the straightness.

The screw is always rotating inside the barrel. So the screw and barrel should keep good concentricity.

If we use many short alloy sections, every connection will bring one more tolerance.

It is just like connecting several short parts together. Short parts are easier to make and easier to install, but more connections also mean more possibility for small position difference.

Two longer sections have less connection points.

So it is better for us to control the straightness and concentricity of the complete barrel.

For customer, this can help the screw run more stable inside and reduce unnecessary wear after long time production.

Heating and Cooling Is Also Important

Extruder barrel is not always at one temperature.

When machine starts, temperature goes up. When machine stops, temperature goes down.

So the barrel and alloy liner will have heating and cooling many times during its service life.

If there are more connection points, there are also more positions which may be influenced by this temperature change.

After long time, a very small gap may appear between the alloy sections.

This is also one reason why we prefer two sections.

We cannot say there will never have any problem, but reducing the connection points can reduce this kind of risk.

For customer, stable production is more important than making the part easier for us.

38CrMoAl Screw with Alloy Surface

For screw material, we use good quality 38CrMoAl steel.

After rough machining, the screw will have quenching and tempering treatment, and then nitriding treatment.

The nitriding depth is around 0.4–0.6 mm, and surface hardness is more than HV900. The nitriding brittleness is controlled within Grade 2.

We also make alloy treatment on the screw surface to improve wear resistance.

For the barrel, the nitriding depth is around 0.4–0.7 mm, with surface hardness around HV940–1100. Inside the barrel, we use SKD11 alloy liner for the wear area.

These treatments are mainly for one purpose: make the screw and barrel suitable for long time production.

Two Sections Are More Difficult to Make, But Better for Long-Term Use

Some customers may ask: if two sections are better, why three-section design is still used?

Actually, three sections are easier to manufacture.

Each alloy liner is shorter, so machining and assembly are easier. If there is any adjustment during production, short sections are also easier to handle.

For manufacturer, this way is more convenient.

But we think more from customer’s production side.

Machine will run every day in customer’s factory. Maybe 8 hours, 12 hours or even 24 hours.

After several months or several years, a small difference inside the barrel may become a bigger production problem.

So we prefer to make the manufacturing work a little more difficult on our side, and make the machine more stable on customer’s side.

Small Design Difference, But Important for Production

From outside, customer cannot see much difference between two-section and three-section alloy liner.

But inside the barrel, the difference is there.

With less connection points, we can have better control of straightness and concentricity, and also reduce the possibility of material staying at the joints.

Especially for PVC and SPC production, we think this design is more suitable for long-time running.

This is also the idea of Starsplas when we improve our extrusion equipment.

We don’t only consider whether the machine can run today. We also consider how it will run after one year, two years and longer time.

For us, the two-section SKD11 alloy liner is not a big change from outside.

But for customer, it can mean less material staying, less wear, easier maintenance and more stable production.