Manufacturing Process Explained by a Nitrided Screw Manufacturer

In the high-pressure world of plastic extrusion and injection molding, the screw is the component that bears the brunt of every cycle. As a Nitrided Screw Manufacturer, we often get asked: “What exactly happens in the furnace to make steel this tough?” Understanding the manufacturing process isn't just for engineers; it’s for every manufacturer who wants to ensure their machinery doesn't fail when the production schedule is tight.

Step 1: Selection of High-Grade Alloys

The journey of a high-performance screw begins with the raw material. Most nitrided screws are crafted from specialized alloys like EN41B (Musco) or 38CrMoAlA. These steels are chosen because they contain aluminum, chromium, and molybdenum—elements that have a high affinity for nitrogen.

While a Hardened Screw Manufacturer might focus on through-hardening tool steels like D2 or H13 (which are hard throughout), we focus on materials that allow for a tough, flexible core and an incredibly hard exterior "case." This balance ensures the screw can withstand high torque without snapping, while the surface resists abrasive wear.

Step 2: Precision Machining and Threading

Before any hardening happens, the screw must be shaped. Using advanced CNC turning and thread-milling machines, the alloy bar is carved into its specific flight geometry. This stage is critical because nitriding is a "low-temperature" process, meaning there is minimal dimensional distortion. However, any error in the initial machining will be "locked in" once the surface is hardened.

The screw is usually stress-relieved between machining stages to ensure that when it enters the nitriding furnace, it remains perfectly straight. Even a deviation of a few microns can lead to uneven wear against the barrel walls.

Step 3: The Nitriding Cycle: Creating the "Case"

This is where the magic—or rather, the chemistry—happens. The machined screw is placed in a sealed furnace. As a Nitrided Screw Manufacturer, we typically use Gas Nitriding, which involves heating the screw to between 500°C and 550°C in the presence of anhydrous ammonia ($NH_3$).

At these temperatures, the ammonia dissociates at the steel surface according to the following reaction:

$$2NH_3 \rightarrow 2N + 3H_2$$

The atomic nitrogen ($N$) diffuses into the surface of the steel, reacting with the alloying elements to form a layer of hard nitrides. The depth of this layer ($d$) is generally proportional to the square root of the time ($t$) spent in the furnace:

$$d = k\sqrt{t}$$

Typically, a screw will stay in the furnace for 60 to 90 hours to achieve a case depth of 0.4mm to 0.7mm with a surface hardness of 60-70 HRC (1000 HV). This creates a "white layer" on the very surface that is exceptionally resistant to adhesive wear.

Step 4: Comparing Technologies: When to Upgrade?

It’s important to know that while nitriding is excellent for general-purpose plastics (like PP or PE), it has its limits.

  • Nitrided vs. Bimetallic: If you are processing 30% glass-filled nylon, the thin nitrided layer will eventually wear through. This is when you would consult a Bimetallic Screw Manufacturer. Bimetallic screws feature a thick alloy inlay (often 1.5mm to 2mm) cast onto the flights, offering significantly longer life in abrasive environments.

  • Nitrided vs. Hardened: A Hardened Screw Manufacturer provides a screw that is hard all the way to the core. These are great for high-torque applications where surface wear isn't the only concern, but they can be more brittle than the "soft core, hard shell" design of a nitrided screw.

Final Inspection and Polishing

After the screw emerges from the furnace, it undergoes a final "lapping" or polishing process to remove any surface oxides and achieve a mirror finish. This ensures smooth material flow and prevents the polymer from sticking to the flights.

By understanding this process, you can see that a nitrided screw is a piece of precision-engineered armor. It’s the cost-effective "workhorse" of the industry, provided it’s manufactured with the patience and metallurgical expertise it deserves.

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