Injection Molding Screw and Plasticizing Unit Design – Comprehensive Guide 2026

Introduction – The Role of the Screw in Injection Molding

The injection molding screw is the heart of every injection molding machine – it is responsible for transporting, melting, homogenizing, and metering the plastic material into the mold. Plasticizing quality directly determines part quality: thermal uniformity, shot-to-shot repeatability, and the absence of streaks, voids, and material degradation. Although the screw accounts for only 2–3% of the machine cost, it influences over 60% of the quality parameters of the finished part.

In modern injection molding machines such as the Tederic NEO-T and D-Series, the plasticizing unit is engineered with the latest advances in screw geometry, wear-resistant materials, and precision temperature control. This article provides a complete engineering guide to the design, selection, and optimization of the plasticizing unit.

Plasticizing Fundamentals – How the Plasticizing Unit Works

Plasticizing is the process of converting plastic pellets into a homogeneous melt of controlled temperature and viscosity. The plasticizing unit of an injection molding machine consists of three main components: the screw, the barrel, and the non-return valve.

Energy Sources in the Plasticizing Process

Melting of the material in the barrel originates from two energy sources:

• Frictional (shear) heat – generated by the rotating screw; accounts for 60–80% of the total energy required to melt the resin. Shear intensity depends on screw speed, channel depth, and melt viscosity.
• Conducted heat – supplied by band heaters on the barrel; accounts for 20–40% of the energy. Serves a compensating and regulating function, ensuring a precise temperature profile.

The ratio of these energy sources depends on the resin type. High-viscosity materials (PC, PMMA) generate more shear heat, while semi-crystalline resins with low melt viscosity (PP, PE) require a greater contribution from external heating.

The Plasticizing Cycle

During each injection cycle the screw performs two key functions:

• Plasticizing (recovery) phase – the screw rotates, transporting, melting, and homogenizing the material. Melt accumulates ahead of the screw tip, pushing the screw rearward (screw retraction). Typical recovery time: 5–15 seconds depending on shot size and material.
• Injection phase – the screw moves axially forward like a plunger, forcing the melt into the mold through the nozzle. Axial velocity: 50–200 mm/s, injection pressure: 800–2500 bar.

Screw Geometry – Key Design Parameters

Injection screw geometry defines plasticizing output, melt quality, and unit service life. The most important design parameters are described below.

L/D Ratio (Length to Diameter)

The L/D ratio is the single most important parameter describing an injection molding screw. It expresses the ratio of the effective working length of the screw to its nominal diameter.

• L/D 18:1 – 20:1 – short screws used in older machines; limited homogenization, adequate for commodity resins (PP, PE).
• L/D 22:1 – 24:1 – the current industry standard; a good balance between homogenization and residence time. Most commonly used in modern general-purpose injection molding machines.
• L/D 25:1 – 28:1 – extended screws for engineering resins (PA, POM, PC) and filled materials; provide better mixing and degassing.
• L/D 30:1+ – special screws for coloring, masterbatch blending, and processing fiber-reinforced composites.

Tederic D-Series machines feature a standard L/D ratio of 24:1, with an upgrade option to L/D 26:1 for demanding applications.

Compression Ratio

The compression ratio is the ratio of the volume of one flight in the feed zone to the volume of one flight in the metering zone. It determines the intensity of the mechanical work applied to the resin.

Resin	Compression Ratio	Rationale
PE-HD, PP	2.5:1 – 3.0:1	Fast melting, high crystallinity – requires moderate shear
PS, ABS	2.0:1 – 2.5:1	Amorphous, melt easily – lower shear is sufficient
PA (nylon)	3.0:1 – 3.5:1	Highly crystalline, narrow melting range – requires intensive shear
PC, PMMA	2.0:1 – 2.3:1	Shear-sensitive – low compression ratio prevents degradation
PVC	1.8:1 – 2.2:1	Highly temperature-sensitive – minimal compression ratio required
PET	2.8:1 – 3.2:1	Highly crystalline, fast cooling – requires efficient melting
TPE, TPU	2.0:1 – 2.5:1	Thermoplastic elastomers – moderate shear, gentle plasticizing

Flight Geometry

Additional geometric parameters of the screw include:

• Flight width – typically 0.08–0.12 × D; narrower flights increase throughput but accelerate wear.
• Helix angle – standard 17.66° (pitch = 1D); modification affects conveying and residence time.
• Channel depth in the feed zone (h₁) – typically 0.12–0.18 × D; deeper channels increase output but may cause uneven conveying.
• Channel depth in the metering zone (h₂) – typically 0.03–0.06 × D; shallower channels improve homogenization at the expense of throughput.
• Screw-to-barrel radial clearance – typically 0.05–0.15 mm; excessive clearance causes melt backflow, insufficient clearance causes excessive wear.

Three Screw Zones: Feed, Compression, Metering

Every injection molding screw is divided into three functional zones, each serving a distinct role in the plasticizing process.

Feed Zone

The feed zone typically accounts for 50–60% of the screw's working length. Its main tasks are:

• Receiving pellets from the hopper
• Conveying solid material toward the compression zone
• Pre-heating the pellets through contact with the hot barrel wall
• Compacting the material and expelling air from between pellets

Channel depth in this zone is the greatest (h₁) and remains constant along its entire length. Conveying efficiency depends on the coefficient of friction between the pellets and the barrel wall (which should be high) and between the pellets and the screw surface (which should be low). For this reason, barrels have a grooved or nitrided inner surface, while screws are polished.

Compression Zone (Transition Zone)

The compression zone typically accounts for 20–30% of the screw length. In this zone:

• Channel depth gradually decreases (from h₁ to h₂)
• The material is compressed, intensifying contact with the hot barrel
• Frictional shear heat increases sharply
• Pellet melting occurs – a film of molten material forms at the barrel wall
• Residual air is squeezed rearward (toward the hopper)

The transition profile can be linear (gradual) or abrupt (steep). Semi-crystalline resins (PA, PET) with a narrow melting range require a more abrupt compression, while amorphous resins (PS, ABS) tolerate a gradual transition.

Metering Zone

The metering zone typically accounts for 20–25% of the screw length. Its functions are:

• Melt homogenization – equalizing temperature and viscosity
• Generating the pressure needed to overcome nozzle and mold resistance
• Precise metering of material ahead of the screw tip
• Final mixing of colorants and additives

Channel depth in this zone is at its minimum (h₂) and remains constant. A channel that is too shallow causes excessive shear and thermal degradation. A channel that is too deep results in inadequate homogenization and unstable metering.

Screw Types: General Purpose, Barrier, Mixing, Special

General Purpose Screw

The standard three-zone general purpose screw is the most widely used design, found in 70–80% of all injection molding machines. It features simple geometry with a single flight and gradual compression.

• Advantages: versatility, low cost, easy maintenance, availability
• Disadvantages: limited homogenization for sensitive materials, no dedicated mixing section
• Applications: PP, PE, PS, ABS – commodity resins

Barrier Screw

The barrier screw features an additional flight (barrier) in the compression zone that physically separates solid material from the melt. Molten resin passes over the barrier into the melt channel, while unmelted pellets remain in the solids channel.

• Advantages: higher plasticizing output (15–30% more kg/h), better melt thermal uniformity (±2°C vs. ±5°C for a GP screw), reduced risk of unmelted pellets reaching the metering zone
• Disadvantages: higher cost (30–50% more expensive), more difficult refurbishment, not suitable for abrasive-filled resins
• Applications: PA, POM, PC – engineering resins with narrow melting ranges

Screws with Mixing Elements

Screws with mixing elements feature special sections at the end of the metering zone that intensify homogenization. The most common designs are:

• Maddock mixer (fluted mixer) – a series of longitudinal grooves with barriers; provides distributive mixing without excessive shear
• Spiral mixer (Saxton) – a spiral element with multiple channels; effective for colorants and masterbatches
• Pin mixer – cylindrical pins on the screw flight; intensive dispersive mixing for pigments and fillers
• Pineapple mixer – diamond-shaped cuts; gentle mixing for shear-sensitive materials

Special Screws

• PVC screw – low compression ratio (1.8–2.2:1), no sharp edges, short compression zone; prevents thermal degradation
• LSR (liquid silicone rubber) screw – short (L/D 14–18:1), smooth surface, cooled barrel; prevents premature crosslinking
• Fiber-reinforced materials screw – deep channels, low compression ratio (2.0–2.5:1), large clearance; minimizes fiber breakage
• Regrind/recyclate screw – degassing zones with a vent port; removes moisture and volatiles from recycled material

Non-Return Valve – Design and Impact on Quality

The non-return valve (check valve) mounted at the screw tip prevents melt from flowing backward during the injection and packing phases. It is a critical component affecting shot-weight repeatability and process stability.

Non-Return Valve Types

• Ring check valve – the most commonly used type; a ring slides axially to open or close the flow path. Simple, reliable, and easy to service.
• Ball check valve – a ball seals the flow passage; faster closing, better suited for small shot sizes and precision applications.
• Poppet check valve – a mushroom-shaped seal element; highest precision, used in micro-injection molding.

Impact of Valve Wear on the Process

A worn non-return valve causes:

• Unstable part weight (variation of ±2–5% instead of ±0.5%)
• Inability to maintain packing pressure
• Streaks and short shots
• Extended cycle time due to the need to compensate for leakage

Recommended non-return valve replacement interval: every 500,000–1,000,000 cycles, or when part weight variation exceeds ±1%.

Plasticizing Barrel – Materials and Configuration

The plasticizing barrel works in conjunction with the screw, providing material heating and pressure containment. Barrel quality directly affects unit longevity and plasticizing performance.

Barrel Materials

• Nitrided steel – the standard solution; surface hardness 60–65 HRC; good wear resistance for commodity resins (PP, PE, ABS)
• Bimetallic barrel – inner lining of nickel-boron or cobalt-chromium alloy; hardness 55–70 HRC; wear and corrosion resistance; recommended for resins with mineral fillers and glass fibers
• Tungsten carbide barrel – highest wear resistance (80+ HRC); used when processing highly abrasive materials (ceramics, carbon fibers, metals in MIM)

Barrel Heating Zones

Modern injection molding machines divide the barrel into 3–7 independent heating zones, each with its own PID controller. The temperature profile is critical for plasticizing quality:

• Feed throat zone – water-cooled (30–60°C); prevents premature melting and pellet bridging
• Barrel zones – ascending temperature profile from the feed zone to the metering zone; typical gradient: 180°C → 200°C → 220°C → 240°C for a general-purpose resin
• Nozzle zone – highest temperature; compensates for heat loss through contact with the mold

Screw Selection by Resin Type

Proper screw selection for the resin being processed is critical for output and quality. The table below presents recommended configurations.

Resin	L/D	Compression Ratio	Screw Type	Mixing Elements	Screw Material
PP, PE-HD	22–24:1	2.5–3.0:1	General purpose	Optional Maddock	Nitrided / chrome-plated
PS, SAN	20–22:1	2.0–2.5:1	General purpose	Not required	Nitrided
ABS	22–24:1	2.0–2.5:1	General purpose / barrier	Maddock recommended	Nitrided
PA 6, PA 66	24–26:1	3.0–3.5:1	Barrier	Spiral mixer	Bimetallic
PC	24–26:1	2.0–2.3:1	Barrier	Pineapple mixer	Bimetallic
POM	22–24:1	2.5–3.0:1	Barrier	Maddock	Chrome-plated / bimetallic
PVC	18–20:1	1.8–2.2:1	Special PVC	Not recommended	Chrome-plated (HCl-resistant)
PA-GF30	24–26:1	2.0–2.5:1	Fiber-reinforced	Not recommended	Tungsten carbide / bimetallic
PET (preforms)	24–28:1	2.8–3.2:1	Barrier	Spiral mixer	Bimetallic / CPM
LSR (silicone)	14–18:1	1.0:1	Special LSR	Static mixer	Chrome-plated / nitrided

Plasticizing Process Optimization

Proper plasticizing optimization can reduce cycle time, improve part quality, and lower energy consumption.

Screw Rotational Speed

Screw rotational speed (RPM) affects plasticizing output and melt quality:

• Peripheral speed – the key parameter, not RPM; recommended range: 0.1–0.3 m/s for most resins
• Calculation: v = π × D × n / 60 [m/s], where D = screw diameter [m], n = speed [RPM]
• Speed too low – extends recovery time, reduces output
• Speed too high – excessive shear, thermal degradation, uneven melting

Back Pressure

Back pressure is the hydraulic pressure applied to the screw during the recovery phase. Typical range: 50–150 bar (5–15 MPa).

• Low back pressure (50–80 bar) – faster recovery, less shear; used for shear-sensitive materials (PVC, PC)
• Medium back pressure (80–120 bar) – optimal compromise; standard for most resins
• High back pressure (120–200 bar) – intensive colorant mixing, better homogenization; used when coloring with masterbatch

Decompression (Suck-Back)

After recovery is complete, the screw retracts 2–5 mm, reducing pressure in the barrel. This prevents melt from leaking out of the nozzle and drooling. Excessive decompression causes air ingestion and voids in the molded part.

Wear and Diagnostics of the Plasticizing Unit

Wear diagnostics of the screw and barrel are critical for maintaining production quality and planning maintenance.

Typical Wear Patterns

• Adhesive wear – metal-to-metal contact when the melt film is insufficient; manifests as scoring on the screw flight lands
• Abrasive wear – dominant when processing filled materials (GF, minerals, TiO₂ pigments); visible as flight-land diameter loss
• Corrosive wear – caused by aggressive gases (HCl from PVC, acids from PA hydrolysis); discoloration and pitting on the surface
• Erosive wear – in the compression zone, where high-velocity melt impinges on the surface; typical for semi-crystalline resins

Diagnostic Methods

• Screw-to-barrel clearance measurement – new clearance: 0.05–0.15 mm; replace when >0.3 mm. Measure every 6 months or every 500,000 cycles.
• Plasticizing output test – compare current output (kg/h) to the nominal value; a drop >15% indicates significant wear.
• Part weight analysis – monitor part weight standard deviation; an increase >2× indicates non-return valve wear.
• Visual inspection – an industrial endoscope allows evaluation of screw and barrel surface condition without disassembly.
• Back pressure analysis – an increase in the pressure needed to maintain the same screw speed indicates wear.

Troubleshooting Plasticizing Problems

Problem	Possible Causes	Solution
Unmelted particles in the part	L/D too low, temperature too low, screw speed too high, worn compression zone	Increase barrel zone 2–3 temperatures, reduce RPM, consider a barrier screw
Streaks and discoloration	Insufficient mixing, dead spots in the barrel, material degradation	Add a mixing element, increase back pressure, purge the barrel
Part weight variation	Worn non-return valve, unstable metering, hopper bridging	Replace the non-return valve, stabilize back pressure, check the hopper
Voids and splay marks	Wet material, excessive decompression, air ingestion	Dry the material, reduce decompression to 2–3 mm, check nozzle seal
Thermal degradation (burn marks)	Temperature too high, residence time too long, excessive shear	Lower temperatures, reduce shot size (min. 20% of barrel capacity), reduce RPM
Extended recovery time	Worn screw, back pressure too low, temperatures too low	Measure screw-to-barrel clearance, increase back pressure, raise temperatures
Nozzle drool	Insufficient decompression, nozzle temperature too high, worn nozzle	Increase decompression, lower nozzle temperature, inspect/replace the nozzle

Summary and Recommendations

The plasticizing unit is the injection molding machine component with the greatest impact on part quality and process efficiency. Proper selection and maintenance of the screw, barrel, and non-return valve determine a molding plant's competitiveness.

Key takeaways from this guide:

• An L/D ratio of 22–24:1 is the industry standard; extended screws of L/D 25–28:1 are necessary for engineering resins and composites
• The compression ratio must be matched to the resin – from 1.8:1 for PVC to 3.5:1 for PA
• Barrier screws increase plasticizing output by 15–30% and improve melt thermal uniformity
• Mixing elements (Maddock, spiral, pin) are essential when coloring and blending masterbatches
• The non-return valve should be replaced every 500,000–1,000,000 cycles; its wear directly affects shot-weight repeatability
• Wear diagnostics should include screw-to-barrel clearance measurement every 6 months and part weight deviation monitoring
• Plasticizing parameter optimization (RPM, back pressure, temperature profile) can reduce cycle time by 5–15% without sacrificing quality

Tederic injection molding machines feature advanced plasticizing units with precision servo control, configurable screws, and online diagnostics systems. To select the optimal configuration for your production, contact the TEDESolutions experts.