Moulding PBT GF30: Barrel Temperatures, Screw Wear and the Abrasion Question

Polybutylene terephthalate reinforced with 30% short glass fibre (PBT GF30) is processed as a semi-crystalline thermoplastic with a crystalline melting point near 224°C. Injection moulding melt temperatures are normally held between 245°C and 270°C; outside this range, either excessive melt viscosity or hydrolytic and thermal degradation kinetics dominate. Moisture content must be reduced to below 0.02% by weight before the polymer enters the screw. Drying at 120°C for 3 h to 4 h in a desiccant dryer with a −40°C dew point is the minimum practical production condition. Residual moisture is commonly verified by ISO 15512:2019. Hydrolytic degradation in the barrel generates carboxylic acid end groups and molecular weight loss; these appear as splay, part embrittlement, melt-pressure fluctuation and increased variation in shot weight. Under ISO 1133-1:2022, PBT GF30 injection moulding grades typically show a melt volume-flow rate between 10 cm³/10 min and 20 cm³/10 min at 250°C and 2.16 kg. Low-flow grades retain longer glass fibres but raise injection pressure; high-flow grades reduce barrel pressure but can permit more screw-channel slip and non-return valve leakage.

The feed throat is water-cooled to prevent bridging and to keep the rear barrel seal stable. Feed throat temperature is normally maintained below 70°C. If the throat warms beyond this bound, PBT GF30 granules soften and stick, causing screw starvation and accelerated wear on the screw root in the feed zone. The feed pocket geometry and screw root diameter must remain free of resin hang-up because PBT GF30 is sensitive to residence time and stagnant melt layers.

Why barrel profile flattening occurs in GF30 melts

Barrel temperature settings for PBT GF30 are usually flattened or run as a near-flat profile because excessive rear-zone heat causes premature melting in the feed pocket, while excessive front-zone heat accelerates ester cleavage. General-purpose unfilled PBT may tolerate a wider rear-to-nozzle gradient; the 30% glass-filled material generates additional frictional heat from fibre-solid contact, so the rear zone is kept relatively cool. A practical profile for a medium-sized hydraulic machine is shown in Table 1. The upper melt limit of 270°C is valid only when residence time is below approximately 8 min; longer residence at the upper limit produces black specks, acrid odour and carbonised deposits on the screw root. Barrel zone-to-zone uniformity should be verified with an immersion thermocouple because air-shot measurements alone do not reflect the melt temperature at the non-return valve.

On a single-screw injection moulding machine with a 40 mm diameter screw and 20:1 L/D ratio, screw speed for PBT GF30 is commonly limited to 40–80 rpm. Back pressure is held between 0.3 MPa and 0.7 MPa; higher values increase plastication work and glass fibre breakage without improving melt quality. Injection speed for medium-walled parts typically falls between 80 mm/s and 120 mm/s, while thin-wall sections may require 150–250 mm/s. These ranges are not universal; cavity-pressure sensors and short-shot studies provide more reliable limits than general machine setpoint tables. Screw recovery time is normally set to less than the cooling time; recovery time that drifts upward at constant screw speed and back pressure is an early warning of wear or material change.

Screw Wear Mechanisms in Glass-Reinforced PBT

Glass fibre is abrasive, but the wear pattern is not uniform. In PBT GF30, the transition and metering sections of the screw, the non-return valve seat, and the barrel section immediately upstream of the check ring show the earliest measurable dimensional loss. Fibres are not fully wetted at the point where solids enter the compression zone; exposed E-glass ends plough the flight lands. The wear mechanism is low-stress abrasion combined with fibre impingement on the screw flight surfaces. E-glass fibre has a typical tensile modulus of 72–76 GPa and a density of 2.54–2.62 g/cm³; this stiffness means the fibre does not deform away from metal contact points. General-purpose nitrided screws show visible flight land wear after sustained production, and the non-return valve wears fastest because PBT GF30 passes through it as a high-pressure, low-viscosity melt containing oriented fibre bundles. Laboratory abrasion tests such as ASTM G65-16 or ASTM G132-96 are not directly predictive of screw wear because they do not duplicate polymer melt viscosity, fibre wetting, or acidic degradation products; they rank substrate hardness but do not capture melt-service wear rates.

Hydrolytic degradation products increase the polar character of the melt and can contribute to corrosive wear of unprotected screw surfaces. The combination of mechanical abrasion and acid attack is more damaging than either mechanism alone; this is a known failure mode when drying of PBT is inconsistent. The screw surface must therefore resist both abrasion and corrosion. For short-run production, a nitrided screw can function, but continuous PBT GF30 moulding exposes the nitrided case to a combined wear regime that standard case depth may not withstand.

Screw suppliers modify geometry for glass-filled PBT by using a compression ratio between 2.2:1 and 2.8:1 and a gradual transition section rather than a short abrupt step. A barrier screw with a main flight and a barrier flight separates the solid glass bed from the melt film; this reduces fibre bundle jamming and local shear heating in the transition zone. The metering depth is chosen to keep the shear rate in the screw channel below the threshold for severe fibre attrition, but not so low that screw recovery time increases. On many medium-tonnage machines, a 45 mm diameter screw with a metering depth of 3.5 mm to 4.0 mm is used. Metering depths below 3.0 mm are associated with rapid temperature overshoot and faster flight and barrel wear because frictional energy is concentrated in a smaller channel volume.

When fibre bundle orientation accelerates flight land wear

In the melt, glass fibres align with the local flow direction. Along the barrel wall, the shear field orients most fibres nearly parallel to the screw channel; however, at the flight land, fibres are forced across the narrow land gap and can become perpendicular to the sliding surface. That orientation increases contact stress at the fibre tip and produces a cutting or ploughing action across the flight land. The condition is more severe when the barrel is hot enough to soften the metal surface; a barrel zone set above 260°C can reduce the surface hardness of nitrided layers and accelerate wear even though melt viscosity decreases. A hotter barrel therefore does not necessarily reduce abrasion; it lowers melt viscosity but can increase thermal softening of the screw and barrel surface. The abrasion question is not whether 30% glass-filled PBT is abrasive, but whether the plasticising unit can operate within a temperature and velocity window where wear remains economically acceptable.

Standard nitrided screws and barrels may be adequate for short runs, but continuous production of PBT GF30 usually justifies enhanced metal protection. Table 2 lists common wear routes. Hardness values are typically measured by ISO 6507-1:2018 or ISO 6508-1:2016. A bimetallic barrel with a nickel-chromium-boron hard alloy liner provides a more wear-resistant surface than conventional nitrided steel; liner hardness is commonly 60–62 HRC. Screw flight lands can be protected with a nickel-based hardfacing applied to the flight O.D.; the non-return valve seat and ring are produced from D2 or CPM 10V tool steel at 58–60 HRC. Ceramic or tungsten carbide coatings are available, but they are less common because of chipping risk at sharp flight edges and higher machining cost. Flame-hardened flight lands typically reach 50–55 HRC to a depth of 2–3 mm, while a nitrided case is often only 0.3–0.5 mm deep. Once the hard case is breached, wear accelerates rapidly.

The abrasion question cannot be separated from machine condition

Barrel temperature control and screw wear are coupled. A worn screw lengthens plastication time because the effective metering depth is reduced; machine operators may respond by raising barrel temperature, which increases thermal degradation risk and metal surface softening. Instead, shot-to-shot cushion stability and screw recovery time should be tracked. On production lines, a gradual increase in recovery time at constant screw speed and back pressure indicates barrel or screw wear; a drop in measured melt temperature despite unchanged barrel set points indicates that frictional heating has been replaced by extended residence time. The non-return valve is the first functional site of concern. A worn check ring permits melt slip during injection, producing shot weight variation, cushion collapse and moulding defects. On machines processing PBT GF30, non-return valve clearance should be checked whenever the cushion variation exceeds ±0.5 mm.

Published data on long-term wear rates for PBT GF30 over 10,000 h campaigns are limited because machine size, screw design, barrel metallurgy, regrind content and maintenance quality vary widely between production sites. When a mould shop runs 30% glass-filled PBT continuously, the screw and non-return valve should be inspected after the first 3,000–5,000 h of service; this interval is not a guaranteed service life but a practical inspection point derived from equipment manufacturer service bulletins. Glass fibre length distribution can be checked before and after moulding by ISO 22314:2006; significant fibre attrition creates shorter mean fibre length and lower mechanical properties. When the measured mean fibre length after plastication drops below the resin supplier’s release limit, the plasticising unit is either running too aggressively or the screw clearances have already exceeded the allowable wear band.