The design of PET Thermoform Tooling: Mould Materials, Vacuum Holes and Trim Jigs begins with the reheat behavior of polyethylene terephthalate sheet. Amorphous PET rollstock entering a shuttle or rotary thermoforming line has a glass transition temperature measured under ASTM D3418 between 78 °C and 80 °C and a melt peak near 245 °C to 255 °C. Thin-gauge APET sheet-surface setpoints are generally maintained at 110 °C to 130 °C; heavy-gauge APET and CPET can require 140 °C to 170 °C depending on the crystallinity target and draw ratio. Density of the solid polymer is 1.38 g/cm³ to 1.40 g/cm³ under ISO 1183-1. Moisture management is a tooling boundary condition: sheet exposed to relative humidity above 60% is commonly pre-dried at 65 °C to 70 °C for 4 h to 6 h, because residual moisture above 0.005% produces hydrolytic degradation, splay, and loss of melt strength during heating. The tooling system therefore operates as three coupled mechanical domains: the cavity and plug-assist mould body, the vacuum-vent distribution network, and the post-form trim fixture.

What Limits Mould Material Selection When Cavity Wall Temperature Exceeds 150 °C?

Aluminium tooling plate 6061-T6 is widely used for thin-gauge APET cavity construction because its thermal conductivity of approximately 167 W/m·K allows rapid heat extraction and helps hold cavity walls below the crystallisation threshold. 7075-T6 aluminium is substituted in plug-assist and wear zones where higher hardness is required, but its conductivity falls to roughly 130 W/m·K. Both alloys carry a coefficient of linear thermal expansion near 23.6 × 10⁻⁶ K⁻¹. P20 prehardened steel by comparison exhibits thermal conductivity near 29 W/m·K and a coefficient near 12.8 × 10⁻⁶ K⁻¹. For CPET tools operating at 140 °C to 160 °C, aluminium loses compressive load-bearing capacity and can gall on sliding cores, so high-temperature tool steel or high-temperature composite is specified. Epoxy and aluminium-filled epoxy prototype moulds have thermal conductivity below 2 W/m·K; they are acceptable for low-run trials but lengthen cycle time. Cavity surfaces on appearance areas are specified below Ra 0.8 µm using ISO 4287, while machined cavity tolerances follow ISO 2768-1 class m in most production tool drawings. Hard-coat anodising to 50 µm to 75 µm thickness reduces abrasive wear on aluminium but adds interfacial thermal resistance and can alter feature dimensions.

Plug-assist materials for PET include syntactic foam, Ertalyte, and acetal where contact temperatures remain below the heat-distortion limit of the material. The plug is typically machined 0.5 mm to 1.0 mm undersized relative to the female cavity to permit controlled material distribution. Excessive plug surface roughness above Ra 0.8 µm can drag the sheet and generate thin spots. Tooling plate thickness for thin-gauge cavities is usually 15 mm to 25 mm to provide vacuum-gallery depth and flatness stability. Vacuum galleries behind the vent holes are often milled as 3 mm to 5 mm wide channels, with dowelled alignment features specified at H7/g6 fits to prevent cavity shift under clamping pressure.

Vacuum Hole Geometry and Pneumatic Evacuation Time

Vacuum vents in PET tools function as air passages that must evacuate trapped gas before the sheet surface falls below the glass transition and loses conformity. In thin-gauge APET from 0.2 mm to 0.5 mm, vent diameters are typically 0.3 mm to 0.5 mm. For sheet thicknesses of 0.6 mm to 1.0 mm, diameters may increase to 0.5 mm to 0.8 mm. Heavy-gauge CPET above 1.5 mm can use 0.8 mm to 1.2 mm vents in non-appearance zones. The governing pneumatic constraint is viscous conductance, which scales with the fourth power of diameter and inversely with channel length. Reducing a vent from 0.8 mm to 0.4 mm therefore reduces idealised flow capacity to 1/16 at equal length and pressure differential. Production tools with vent length-to-diameter ratios above 12:1 are susceptible to restricted evacuation and poor replication at rib intersections. Vacuum systems on shuttle machines typically generate pressure differentials of 0.7 bar to 0.9 bar across the sheet; the tool vent pattern should allow cavity evacuation within 0.5 s to 1.5 s for thin-gauge APET. Holes on appearance surfaces are kept at 0.3 mm to 0.4 mm to prevent dimpling. Back-drilling from a sub-surface gallery reduces effective channel length and improves conductance without increasing visible surface diameter.

Unlike cavity-side mould surfaces, trim jigs operate after the sheet has cooled and must locate a part that has already undergone anisotropic shrinkage. The fixture body is commonly machined from aluminium plate or cast polyurethane, with vacuum clamping ports of 0.5 mm to 1.0 mm diameter and contoured nests that reproduce formed surfaces to within ±0.1 mm. Steel-rule dies for thin-gauge PET use hardened rules with bevel angles of 45° to 60° and die clearances between 0.03 mm and 0.08 mm for sheet thicknesses below 0.8 mm. PET exhibits limited shear ductility; clearances outside this range create edge whitening, microcracking, or secondary burr. Matched metal trim dies for CPET and heavy-gauge sheet use die clearances of 4% to 6% of sheet thickness where edge quality is critical. Punch and stripper wear is concentrated at the trim edge; recutting or shimming intervals of 5,000 to 20,000 cycles are typical depending on filler content and glass-fibre abrasion. CNC trimming of heavy-gauge PET uses carbide single-flute upcut end mills at spindle speeds of 20,000 rpm to 35,000 rpm and feed rates of 3 m/min to 8 m/min to avoid heat build-up and edge melt. Trim fixture datums should reference formed features used in downstream filling, sealing, or lidding rather than the raw trim line alone. First-article inspection is performed on a coordinate measuring machine with volumetric accuracy of ±0.005 mm or better; locating features are toleranced under ISO 2768-1. Pilot-run shrinkage measurements are transferred to tool offsets to compensate for machine-direction and transverse-direction differences.

When Mould Temperature Control Drifts Outside the PET Processing Window

Thermal control in PET thermoforming is a closed-loop constraint rather than a static setpoint. For APET, cavity wall temperatures above 45 °C can produce visible haze or crystalline spots; walls below 15 °C can quench the sheet before complete corner conformation and increase residual stress. For CPET, cavity surfaces are held at 140 °C to 160 °C to induce spherulitic crystallisation over dwell times of 2 s to 5 s. A drift of ±5 °C in high-crystallinity CPET applications alters the crystallinity gradient enough to create flange warpage and seal-surface flatness failures. Production-line failure modes include mineral scale blockage in drilled cooling galleries, uneven flow distribution across parallel circuits, and edge overcooling where aluminium inserts meet steel bolster plates. Cooling circuits are often sized for turbulent flow with Reynolds numbers above 10,000 and pressure drop below 0.5 bar per circuit; ultrasonic flowmeters and thermocouples embedded 2 mm to 3 mm beneath the cavity surface are used to verify balance.

High-temperature steel moulds for CPET require cartridge heater zones with proportional-integral-derivative controllers and thermocouple feedback. When one heater zone fails, PET near the failed zone remains amorphous while adjacent zones crystallise; the boundary appears as haze and creates differential shrinkage. Insulating plates between the mould and press platen reduce thermal drift at the parting line. Published data for specific tooling configurations is limited, so trial runs on production shuttle machines are used to map the operating window for each sheet grade and thickness.

Trim Fixture Datums Must Compensate for Anisotropic Shrinkage

PET rollstock orientation before forming creates unequal machine-direction and transverse-direction shrinkage after cooling. Thermoformed parts can shrink 0.5% to 1.5% depending on draw ratio, sheet thickness, and mould temperature. A trim fixture datum system that uses the same reference features as the filling line reduces downstream assembly variance. In practice, fixture surfaces are machined from pilot-run scans; tool offsets are not taken from nominal sheet shrinkage tables because draw-induced orientation causes local deviations. Vacuum clamping in the fixture is supplied from a separate pump circuit to avoid pressure loss when the forming press is cycling. Seal surfaces on the fixture are verified against the part with contact marking and gap measurements below 0.05 mm.