Polybutylene Terephthalate Moulding Start Up Dryer Barrel And First Off Checks
Polybutylene terephthalate moulding start-up: dryer, barrel and first-off checks addresses the start-up window in which a semicrystalline polyester can transition from dried pellet to dimensionally stable moulding without undergoing hydrolytic degradation. PBT contains ester linkages that undergo random chain scission when residual moisture is present above the crystalline melt interval of 223–225 °C. A start-up sequence that does not control dryer exit dew point, barrel temperature profile and first-shot inspection can produce mouldings with acceptable visual appearance but reduced tensile elongation, notched impact strength and melt-viscosity consistency. The following checks define the critical start-up boundary for unfilled, glass-fibre-reinforced and flame-retarded PBT grades.
What Are the Dryer Exit Conditions That Prevent Hydrolytic Chain Scission Before the Feed Throat?
Drying is the first critical boundary in PBT start-up. Desiccant-wheel dryers are specified with a return-air dew point no higher than -30 °C, and preferably -40 °C, because the vapour pressure differential between the desiccant air and the pellet surface controls the final moisture level. For general-purpose unfilled PBT, the hopper is held at 120 °C for 3–4 h. Glass-fibre-reinforced grades may require 130 °C for 4 h when regrind or sizing residues increase the local moisture load. The dryer outlet target is no greater than 0.02 wt% when tested by ISO 15512 using Karl Fischer coulometry. At moisture levels above 0.02 wt%, the barrel heats the wet melt above 240 °C and the ester bond is attacked; the process signal is a measurable fall in injection peak pressure and screw torque against the dried-shot reference. Dryer hopper outlet temperature is monitored with a thermocouple, and flexible hoses between the dryer and feed throat are insulated. At ambient relative humidity above 60%, drying time is extended or the hopper is blanketed with dry air because pellet moisture pickup during transfer becomes a start-up variable that the dryer cannot recover once the feed throat is hot.
Before heating zones are enabled, the feed throat is confirmed below 50 °C to avoid bridging and moisture adhesion at the screw root. The barrel is ramped from a soak of 150 °C to setpoint in 20–30 K increments, with a minimum of 20 min soak at each increment to prevent thermal overshoot. A typical barrel profile for PBT is shown below; grade-specific supplier settings take precedence, especially for flame-retarded systems.
| Grade family | Feed zone | Compression zone | Metering zone | Nozzle | Melt temperature | Mould temperature |
|---|---|---|---|---|---|---|
| Unfilled PBT | 230–240 °C | 235–245 °C | 240–250 °C | 240–250 °C | 230–250 °C | 60–90 °C |
| 30% glass-fibre PBT | 240–250 °C | 245–255 °C | 250–260 °C | 250–260 °C | 245–265 °C | 80–110 °C |
| Flame-retarded PBT | 220–235 °C | 225–240 °C | 230–245 °C | 230–245 °C | 225–245 °C | 60–85 °C |
Flame-retarded PBT grades typically require melt temperatures 10–20 K lower than unfilled grades; halogenated flame-retardant packages can release acidic decomposition species above 250 °C. Published data for specific flame-retardant package stability at prolonged residence is limited and should be obtained from the compound manufacturer.
Barrel Set-Points, Screw Speed and Residence-Time Distribution
The barrel profile must be managed with screw recovery time because viscous dissipation can raise melt temperature beyond the rear-zone setpoint. Screws for PBT are commonly specified with a compression ratio of 2.5:1 to 3.0:1 and an L/D of 20:1 to 24:1. Screw speed is set so that recovery completes within the cooling period but does not exceed 70% of the shortest cooling time; this limits unnecessary shear heating. Back pressure for unfilled PBT is maintained at 0.3–0.7 MPa, while glass-fibre-reinforced PBT may require 0.5–1.0 MPa to homogenise fibre distribution. A melt cushion of 1.5–3.5 mm is held to maintain consistent hold-pressure transfer. Screw decompression is limited to 2–5 mm; excessive suck-back can draw air into the melt stream and create splay or oxidation defects during the first shots.
Residence time at melt temperature is a defined boundary during start-up. For unfilled PBT, total residence time above 240 °C should not exceed 10 min; for flame-retarded grades, residence time above 230 °C is limited to 5 min. If the start-up is interrupted for more than 10 min, the barrel is purged with a low-viscosity purge compound rather than allowing the melt to remain at elevated temperature. During purge, nozzle temperature is reduced by 10–15 K to avoid drooling and premature decomposition at the nozzle tip. The first shots after purging are discarded until the purge compound is cleared and the melt cushion is stable.
When a Cold Mould Masks Slow-Fill Defects During the First Shots
First-off inspection begins only after the nozzle melt temperature has been verified with an immersion pyrometer and at least five full shots have passed through the melt cushion. A cold mould can hide short-shot and flow-line defects because rapid skin formation freezes the surface before the cavity fully packs. For unfilled PBT, the mould is stabilised at 60–90 °C; for glass-fibre-reinforced PBT, the mould is stabilised at 80–110 °C using turbulent-flow mould temperature controllers. The first 20 shots after start-up are treated as process-stabilisation shots and are not used for dimensional release.
First-off visual checks are performed under 10× magnification with oblique illumination. Silver streaks or gate-area splay are interpreted as moisture or trapped volatile defects, even when the dryer dew point appears acceptable. Brown streaks or yellowing are interpreted as residence-time degradation or local hot-spot formation in the barrel. Surface delamination in glass-fibre-reinforced PBT is checked for glass-fibre orientation defects linked to injection speed or mould-temperature imbalance. If splay is present despite a dryer dew point of -40 °C, the dryer return-air filter and hopper seal are inspected; air leaks in the dryer circuit are a common field cause of elevated dew point that does not appear on the controller display.
The first-off melt-viscosity check is an indirect but rapid diagnostic. Injection peak pressure and screw torque are recorded for the first 20 consecutive shots. A downward drift in peak pressure greater than 5–10% against the dried-shot reference indicates ongoing hydrolytic chain scission or a feed-bridging event. Mass variation is assessed by weighing 20 consecutive shots on a balance with 0.01 g resolution; a coefficient of variation above 0.3–0.5% indicates cushion instability, check-ring leakage, or irregular screw recovery. Where mechanical data are required, first-off plaques or tensile specimens are prepared in accordance with ISO 294-1 and compared with the supplier datasheet using ISO 527-2 or ISO 179-1 as applicable. Dimensional verification is performed only after the mould has reached steady-state surface temperature and the first 20–30 shots have stabilised the melt cushion.
Shrinkage verification on first-off mouldings uses the supplier datasheet values indexed to ISO 294-3. For unfilled PBT, mould shrinkage is typically 1.5–2.1%; for 30% glass-fibre PBT, mould shrinkage ranges from 0.3% to 1.1% depending on flow direction. Directional shrinkage is checked in both parallel and perpendicular flow directions, and the first-off dimensions are recorded after the mould has operated for at least 1 h with stable coolant temperature. A first-off audit is complete only when the dryer outlet moisture is below 0.02 wt%, the barrel temperature profile is verified with an immersion probe, the melt cushion is stable, and the first-off mass, visual and dimensional data fall within the defined start-up acceptance window.
