Screw Design & Plasticizing Capacity in Injection Molding – Calculations and Optimization

Introduction to Injection Screw Design

The injection screw is the heart of every injection molding machine – it's the component that melts the plastic, mixes it, and pumps it into the mold under pressure. Proper screw design directly impacts performance, part quality, energy consumption, and machine durability.

This guide covers plasticizing capacity, screw geometry, compression ratio, recovery time, pressure generation, and optimization strategies for various materials and applications.

Plasticizing Capacity Fundamentals

What is Plasticizing Capacity?

Plasticizing capacity is the amount of plastic material that a screw can melt and homogenize per unit time (typically kilograms per hour).

• Unit: kg/h (kilograms per hour)
• Empirical formula: Pc ≈ 0.5 × D² × rpm × ρ, where D = screw diameter (cm), rpm = revolutions per minute, ρ = bulk density
• Theoretical capacity: maximum speed under ideal conditions (never achieved in practice)
• Practical capacity: 60-70% of theoretical capacity (due to slippage and heat losses)
• Effective capacity: actual output accounting for material properties and process conditions

Practical significance: An injection machine with 50 kg/h capacity can produce a maximum of 50 kg of material per hour; if a part weighs 100g, then maximum 500 parts/hour (not accounting for cycle time).

Capacity vs Cycle Time

Plasticizing capacity must be coordinated with cycle time:

• Too low capacity: screw cannot melt material quickly enough, cycle time increases
• Too high capacity: excessive thermal energy, material degradation, wear
• Optimal capacity: 50-60% of maximum capacity for the given material and cycle

Screw Geometry and Elements

Three Main Zones of the Screw

Every injection screw has three functional zones:

• Feed Zone:

  • Length: 5-10% of total screw length
  • Function: receives pellets from the hopper, pushes them forward
  • Full flight depth
  • No compression

• Compression Zone:

  • Length: 50-70% of total length
  • Function: gradually compresses and melts material, mixing and homogenization
  • Progressively decreasing flight depth
  • Increasing pressure

• Metering Zone:

  • Length: 20-30% of total length
  • Function: maintains a constant amount of material in the chamber, regulates flow to the outlet
  • Minimal flight depth (0.5-1.5 mm typically)
  • Maximum pressure

Geometric Parameters

Key screw parameters affecting performance:

• Screw diameter (D): 20-100 mm for typical machines; larger diameter = higher capacity
• Screw length (L): typically L/D = 18-24 (length/diameter ratio); L/D = 18 for fast cycles, L/D = 24 for better mixing
• Flight pitch (Pitch): distance between flights; standard = 1 × screw diameter
• Helix angle (Flight Angle): 17-18° (standard for injection); larger angle = higher flow, smaller = higher shear

Compression Ratio

Definition and Calculation

The compression ratio is the ratio of flight depth in the feed zone to flight depth in the metering zone:

• Formula: CR = H_feed / H_meter
• H_feed: flight depth in feed zone (typically 5-10 mm)
• H_meter: flight depth in metering zone (typically 1-2 mm)
• Typical CR: 3:1 to 5:1 (three to five times compression)

Example: If H_feed = 9 mm and H_meter = 2 mm, then CR = 9/2 = 4.5:1

Impact of Compression Ratio on Process

• Low CR (2:1 to 3:1):

  • Faster material flow
  • Less mixing
  • Lower discharge pressure
  • Application: temperature-sensitive materials (PVC, ABS)

• Medium CR (3:1 to 4.5:1):

  • Balanced flow and mixing
  • Moderate pressure
  • Standard for most applications
  • Application: PP, PE, PS, PA (polyamides)

• High CR (4.5:1 to 8:1):

  • Excellent mixing and homogenization
  • Higher discharge pressure
  • Higher material shear, potential degradation
  • Application: filled materials, masterbatches

Recovery Time and Screw Rotation

What is Recovery Time?

Recovery time is the time required to melt and equalize a new charge of material in the metering zone, ready for the next injection.

• Unit: seconds (s)
• Formula: t_recovery ≈ (V_meter × ρ) / Pc, where V = metering zone volume, ρ = density, Pc = plasticizing capacity
• Typical time: 5-30 seconds (depends on material and parameters)

Relationship Between Cycle Time and Recovery Time

• Recovery time < cycle time: ideal - screw has time to prepare material before next injection
• Recovery time = cycle time: critical - injection may be delayed if process doesn't run smoothly
• Recovery time > cycle time: problem - machine cannot melt material fast enough, cycle time increases

Practical calculation: If cycle time is 20 seconds and recovery time is 25 seconds, the machine has a 5-second bottleneck – parameters or screw must be changed.

Pressure Generation in the Screw

Compression and Pressure Mechanics

Pressure in the screw is generated by compressing material in the metering zone:

• Pressure build-up: increasing flight depth in the compression zone
• Backpressure: resistance that material encounters as the screw rotates
• Head resistance: resistance in the discharge chamber and nozzle
• Typical pressure: 50-100 MPa (500-1000 bar) in the injection chamber

Backpressure and Its Role

Backpressure is deliberate pressure introduced to prevent material leakage from the nozzle during screw rotation:

• Typical ranges: 10-30 MPa (100-300 bar)

• Functions:

  • Prevents material leakage (dribbling)
  • Promotes better mixing
  • Reduces gas content in material
  • Increases temperature uniformity
• Too low backpressure: material leaks, poor mixing
• Too high backpressure: wasted energy, screw wear, material degradation

Heat Balance and Plasticizing Temperature

Heat Sources

Material is heated from two sources in the screw:

• Jacket heat: 20-40% of total heat; from heaters surrounding the screw
• Frictional heat: 60-80% of total heat; from shearing and friction of material against screw and barrel

Calculating Required Heat

Total energy needed to heat material:

• Q = m × c × ΔT, where m = mass, c = heat capacity, ΔT = temperature change
• Example: Heating 100g PP from 20°C to 220°C requires ~100g × 2.3 kJ/kg·K × 200K ≈ 46 kJ energy
• Power: Heater power: P = Q/t (to heat in 30 seconds, P = 46 kJ / 30s ≈ 1.5 kW)

Material Flow and Turbulence

Flow Type

Flow in the screw can be laminar or turbulent:

• Laminar flow: layer by layer, typically for viscous materials, slow rotation
• Turbulent flow: chaotic mixing, better homogenization, higher temperature
• Reynolds number: Re = ρ × v × D / η determines flow type (Re < 2300 laminar, Re > 4000 turbulent)

Screw Surface Effect

Rough screw surface increases friction and mixing:

• Polished screw: lower friction, faster flow, less heat
• Rough screw: higher friction, better mixing, more heat
• Special coatings: hardfacing to increase durability

Screw Wear and Problem Diagnosis

Types of Wear

• Corrosion: material chemistry or water entrapment causes rust on screw
• Abrasive wear: gradual friction wear of flight after thousands of hours
• Fatigue cracks: when stresses cycle (rotating load)
• Tip damage: when pellets jam or don't melt sufficiently

Diagnosis and Repairs

• Excessive motor power: worn screw (increased friction) - replace
• Uneven temperature: worn screw, uneven flow - replace
• Variable injection pressure: screw slipping, worn check ring - replace ring or screw
• Brittle/broken parts: possible material overheating - adjust parameters (reduce rpm or backpressure)

Performance Optimization Strategies

To Increase Throughput (kg/h):

• Increase screw rpm - but not beyond material limits (typically 100-200 rpm)
• Slightly increase backpressure (improves homogenization)
• Lower feed zone temperature (material flows more smoothly)

For Better Mixing and Homogenization:

• Increase compression ratio (if possible)
• Increase backpressure (greater shearing)
• Slightly decrease screw rpm (more time for mixing)
• Increase material temperature (lower viscosity, better flow)

To Reduce Energy Consumption:

• Lower backpressure (if quality allows)
• Decrease screw rpm (if recovery time allows)
• Lower heater temperatures (more heat from friction)

Screw Selection Guide by Material

For PP (Polypropylene):

• CR: 3:1 to 4:1 (standard)
• L/D: 20:1
• Backpressure: 15-20 MPa
• RPM: 100-150

For PA (Polyamides):

• CR: 4:1 to 5:1 (better mixing due to higher viscosity)
• L/D: 22:1
• Backpressure: 20-30 MPa
• RPM: 80-120

For PVC:

• CR: 2:1 to 3:1 (low, as PVC is temperature-sensitive)
• L/D: 16:1 to 18:1
• Backpressure: 10-15 MPa
• RPM: 50-100 (slow due to degradation risk)

Summary

Injection screw design is a combination of engineering, experience, and optimization for specific materials. Key points:

• Plasticizing capacity: determines maximum output, must be coordinated with cycle time
• Compression ratio: 3:1 to 5:1 standard; higher for better mixing, lower for fast flow
• Recovery time: must be below cycle time to avoid limiting output
• Pressure generation: backpressure of 15-30 MPa is optimal for most materials
• Heat balance: 60-80% of heat comes from friction, only 20-40% from heaters
• Material flow: turbulent flow provides better mixing, laminar faster speeds
• Wear: regularly check motor power, temperature, pressure - worn screw requires replacement
• Optimization: for each material there is an optimal combination of rpm, backpressure, temperature

Mastery of injection screw design and optimization opens the door to higher output, better quality, and lower energy costs. The right screw for your material and application is the foundation of modern injection molding.