The screw and barrel are the core components of an injection molding machine, responsible for melting, mixing, plasticizing, and injecting plastic material into a mold. Their design and material quality directly affect product precision, stability, cycle time, and overall machine efficiency.

In this article, Zhoushan Juyuan Plastic Machinery Co., Ltd. explains the structure of injection molding screws and barrels, how they work, and how to choose the best configuration for your production needs.

If you want to learn about extrusion screw barrels, please read our related guide:
👉 Extruder Screw Barrel Guide (Internal link)

1. Structure of Injection Molding Screw

Injection molding screws typically have a three-section design:

1) Feed Zone

• Large channel depth
• Feeds plastic pellets into the screw
• Begins pre-heating through friction

2) Compression Zone

• Gradual reduction in screw channel depth
• Material begins melting
• Pressure and shear increase
• Additives and colors start to mix

3) Metering Zone

• Final melting and homogenization
• Stabilizes melt temperature
• Provides consistent melt before injection

Optional Structures

• Barrier screw (for higher output & faster melting)
• Mixing head (Maddock / spiral mixer)
• Vent screw (for moisture-rich recycled materials)

2. Injection Screw Additional Components

Injection molding screws include extra parts not typically found in extrusion:

1) Check Ring (Non-Return Valve)

Ensures the melt flows forward only; prevents backflow during injection.
Improves shot consistency and product accuracy.

2) Screw Tip / Nozzle Head

Provides final pressure and controls flow into the nozzle.

3) Screw Shank / Drive End

Connects the screw to the machine’s drive system.
Must withstand very high torsional loads.

3. Injection Barrel (Cylinder)

The barrel houses the screw and provides controlled heating.

Features

• Multiple heating zones
• Precise temperature control
• Internal wear-resistant layer
• Optional bimetallic alloy for long life

A high-quality barrel ensures stable melting and reduces carbonization or burning.

4. Materials Used for Screw & Barrel

Choosing the correct material is critical for long life and stable performance.

Screw Materials

• 38CrMoAlA (nitrided steel)
• SKD61 (H13 hot-work tool steel)
• DC53 high-wear steel
• GH4169 (Inconel alloy for high-temperature applications)

Barrel Materials

• 38CrMoAlA nitrided barrel
• Bimetallic barrel (HCR alloy)
• Nickel-based alloy liner

Bimetallic Barrel Advantages

• Excellent wear resistance
• Strong corrosion resistance
• Ideal for:
  • Glass fiber
  • CaCO₃ filler
  • Recycled material
  • Flame-retardant grades

5. How Injection Screw & Barrel Work

Step 1: Feeding

Pellets enter the feed zone through the hopper.

Step 2: Melting

Heat + shear melt the pellets in the compression zone.

Step 3: Metering

Melt becomes uniform and well-mixed.

Step 4: Injection

The screw moves forward, injecting molten plastic into the mold cavity.

Step 5: Plasticizing Again

The screw retracts and prepares the next shot.

6. Common Problems and Solutions

1) Black Spots / Burning

• Temperature too high
• Material stagnation
• Worn screw/barrel
  Solution: Use bimetallic barrel + optimized screw.

2) Short Shot

• Low back pressure
• Melt not fully plasticized
  Solution: Increase compression ratio or mixing section.

3) Color Inconsistency

• Poor mixing
  Solution: Maddock or spiral mixing section.

4) Glass Fiber Wear

• Severe abrasion on barrel
  Solution: Use tungsten carbide alloy or HCR bimetallic barrel.

7. How to Choose an Injection Screw & Barrel

Choose based on:

✓ Resin Type

PP, ABS, PC, PA6, PA66, PET, PVC, recycled plastics.

✓ Fillers & Additives

CaCO₃, talc, fiberglass, flame retardants.

✓ Machine Model

DEMAG, Haitian, Toshiba, JSW, Engel, KraussMaffei, etc.

✓ Cycle Time Requirement

Faster cycles may require barrier or mixing screws.

✓ Wear Resistance Level Needed

Bimetallic barrel + hardened screw for high-filler or glass-fiber materials.

Why Choose Juyuan for Injection Screws & Barrels?

✓ 20+ years of experience

✓ High-precision CNC machining

✓ Bimetallic wear-resistant barrels

✓ High-strength screws for tough materials

✓ OEM solutions for all injection molding brands

✓ Fast delivery & worldwide shipping

Compatible with Haitian, DEMAG, ENGEL, KraussMaffei, JSW, Toshiba, Fanuc and many others.

📞 Contact Us

For customized injection molding screw and barrel solutions, contact us anytime:

Zhoushan Juyuan Plastic Machinery Co., Ltd.
🌐 Website: https://juyuan-machinery.com/
📧 Email: cxj.job@gmail.com
📱 WhatsApp / WeChat / Phone: +86 13905808474