Screw Design

Selecting the Best Metering Screw for Your Resin

Choosing the correct screw design is essential for consistent melt quality, cycle efficiency, and part performance. The right design minimizes shear, prevents material degradation, and maximizes homogeneity—especially important for applications involving engineering resins, filled compounds, or recycled materials.

Why Screw Design Matters in Injection Molding & Extrusion

A properly selected screw directly affects:

• Melt homogeneity and temperature control
• Throughput and cycle time
• Material degradation risks
• Color dispersion and additive distribution

Key Variables in Screw Design

To choose the correct screw design for a resin (or resins), we need to understand metering screw design factors and their effects on material processing.​

There are five main variables to determine how a screw turns solid resin into a melt. Adjusting them, in tandem with various mixing screws and barrier options, yields a range of results. The variables are:

• Profile
• Channel Depth
• Compression Ratio
• Length/Diameter (L/D)
• Helix Angle

Screw Profile

The standard screw profile is divided into three zones:

• Feed Zone: Conveys solid pellets toward the transition zone.
• Transition Zone: Initiates melting via compression and shear.
• Metering Zone: Finalizes melting and delivers uniform melt to the mold or die.

A modified screw profile may be required for shear-sensitive materials, high-performance resins, or multi-material blends.

Channel Depth

The feed and transition zone channel depths are related to the screw's compression ratio and profile.

The meter zone's channel depth depends on the type of resin being processed.

• Deeper channels = Higher throughput, lower shear
• Shallower channels = Higher shear, better mixing

Channel depth gradually decreases from feed to metering zone. It plays a major role in shear intensity and material output.

Compression Ratio

The ratio of the screw's feed zone channel depth to the meter zone channel depth is called the compression ratio. It typically ranges from 1.5:1 to 4.5:1 for most thermoplastic materials.

The compression ratio (feed depth ÷ metering depth) typically falls between:

• 1.5:1 to 2.0:1 – for heat-sensitive or filled materials
• 2.5:1 to 3.0:1 – for general-purpose resins like PP, PE, PS
• 3.5:1 to 4.5:1 – for crystalline materials or high-viscosity resins

Most injection screws classified as "general purpose" have a compression ratio of 2.5:1 to 3.0:1.

Length-to-Diameter Ratio (L/D)

This ratio calculation is made by dividing the flighted length of the screw by its nominal diameter.

Helix Angle

The angle of a screw flight measured relative to a plane that's perpendicular to its centerline. While rarely changed, the helix angle affects:

• Material conveying rate
• Residence time
• Shear intensity

Standard screws use a square pitch (~17.7° helix angle), but variations may be necessary for specialized applications. The helix angle isn't often altered from the standard "square" pitch, since such changes can have a significant impact on processing.​

Optimizing Screw Design for Better Polymer Processing

A tailored screw design ensures better processing stability, melt quality, and product consistency. Whether you're molding high-clarity polycarbonate, extruding fiber-filled polypropylene, or processing PCR blends, understanding how each variable impacts performance is key.

Need help selecting the best screw for your resin? Our technical team works directly with processors and OEMs and ready to help!