Heavy-gauge sheet for high-heat food-service trays is produced on a 33:1 L/D single-screw extruder equipped with a barrier feed section, a metering zone fluid-coupled to a gear pump, and a vacuum vent port maintained at −0.08 MPa. Prior to extrusion, PURAPOL L175 is dried in a desiccant-wheel hopper dryer with a supply-air dew point not warmer than −40 °C and an inlet temperature of 80 °C for a minimum of 4 h, reducing residual pellet moisture below 250 ppm as determined by ISO 15512:2019 Karl Fischer oven method. Moisture values above 300 ppm at the feed throat typically produce a sharp viscosity loss during plastication because ester-bond hydrolysis is autocatalytic once free carboxyl end groups accumulate; the practical consequence on a production line is uncontrolled fluctuation in die pressure and visible irregular gloss bands in the sheet. Where plant ambient relative humidity exceeds 60 %, dried pellets are transferred to the extruder feed throat through a closed desiccated-air conveying line, because exposure to ambient air for more than 20 min can raise moisture above 300 ppm. A reverse-temperature barrel profile of 190 °C in zone 1, 200 °C in zone 2, 205 °C in zone 3, and 210 °C in the metering zone keeps the melt temperature near 212 °C at the gear pump inlet, while the adapter and flat die are held at 205 °C to 210 °C. The chrome-polished three-roll stack operates with roll temperatures of 45 °C, 50 °C, and 40 °C from top to bottom; this creates an amorphous, low-crystallinity sheet that reheats uniformly. For sheet thickness between 0.80 mm and 1.50 mm, line speeds are adjusted so that the material reaching the haul-off does not exceed 55 °C surface temperature, because early cooling below the glass transition near 60 °C locks in unintended stress that later appears as corner splitting during thermoforming. Solid-state pressure forming of this sheet uses contact plate preheating followed by infrared pyrometry in the 9 µm to 14 µm band; the sheet surface is brought to 112 °C to 118 °C before forming. The processing window at the upper forming plateau is narrow: at 120 °C, dwell sag increases rapidly, and at 108 °C, the sheet fails to replicate sharp radii and creates microcracks at the plug-assist contact point. A ±5 °C deviation is therefore controlled by proportional closed-loop IR heating zones, and the plug material is high-density syntactic foam held at 100 °C to reduce chill marks. Food-contact documentation for this downstream segment requires a compliance matrix combining European Union Regulation (EU) No 10/2011, United States Food Contact Notification coverage where applicable, and REACH Regulation (EC) No 1907/2006 Article 33 declarations for substances of very high concern. PLA is not directly covered by 21 CFR 177.1520, because that regulation addresses olefin polymers; therefore, a converter must confirm the grade-specific FCN or equivalent no-objection letter from the supplier before using reworked offcuts in direct food contact.

What happens when PURAPOL L175 is run through a hot-runner injection moulding cell?

Injection moulding of thin-wall high-heat serviceware from high-viscosity PLLA differs from standard PLA in that melt pressure at the nozzle can reach 1400 bar to 1600 bar during injection when wall sections drop below 0.80 mm, even though barrel temperatures are kept at 190 °C to 215 °C. The screw utilized is a general-purpose 20:1 L/D design with a compression ratio of 2.0:1 to 2.5:1, and a non-return ring with a clear melt channel is specified because high-viscosity PLLA can generate excess shear heating in a restrictive check valve. The mould is a two-plate, valve-gated hot-runner system with manifold temperature set points of 200 °C to 210 °C and valve tips held near 170 °C. High melt viscosity at the low end of the barrel creates a risk of screw recovery time exceeding cooling time; therefore shot weight is maintained at 60 % to 70 % of barrel capacity to avoid melt residence times beyond 5 min. A residence time longer than 10 min at 215 °C leads to a measurable decline in average molar mass and a drop in dynamic viscosity, which in production appears as short shots on the next cycle after a dwell period. Melt is injected with a velocity profile rising from 50 mm/s to 180 mm/s in the first 0.3 s, followed by holding pressure at 600 bar to 800 bar for 1.0 s to 1.5 s; these values are determined by short-shot studies per ISO 294-1:2017 and are not transferable to standard low-viscosity PLA because the pressure drop across the gate is higher. Mould temperature is maintained at 25 °C to 35 °C for a cold-cavity surface; when hot-fill performance is required, parts are annealed after demoulding rather than using a high-temperature mould, because mould temperatures above 60 °C increase cycle time without fully eliminating amorphous skin formation. Warpage is quantified by pressing the part against a datum plate and measuring gap height with an optical comparator; parts produced with a 1.2 mm wall have shown deviation below 0.35 mm after annealing when the gate location is placed opposite the thickest section. Hot-runner valve gates reduce exposed gate vestige, but the valve pin seat clearance must be below 0.010 mm to prevent stringing from high melt elasticity. Material degradation in the hot runner is monitored by recording injection pressure at a fixed screw transfer position; a drop greater than 35 bar across an 8 h run indicates hydrolysis or a worn check ring. ISO 1133-1:2022 melt mass-flow rate testing at 190 °C and 2.16 kg is used for incoming lot release, but the high molecular weight of this grade means that MFR is less sensitive to moderate chain scission than an intrinsic viscosity measurement per ISO 1628-3:2010. The dominant process conflict is the interaction between hot-runner residence time, screw recovery, and shear heating; these variables must be rebalanced whenever the wall thickness changes by more than 0.2 mm.

After pellet drying to below 200 ppm moisture, melt spinning of high-viscosity PLLA homopolymer into multifilament yarn is carried out at spinneret temperatures of 210 °C to 220 °C, with gear pump output stabilized at a setpoint determined by denier target and take-up speed. The high molecular weight of PURAPOL L175 improves melt strength, allowing low-speed spin-draw runs between 500 m/min and 1500 m/min without filament breakage; high-speed winding above 2500 m/min is generally avoided because spinline stress can exceed the tensile strength of the partially quenched filament. Quenching air is supplied in a crossflow cabinet at 15 °C to 20 °C and 0.3 m/s to 0.6 m/s, and the threadline is drawn to a ratio of 2.5 to 4.0 at a hot godet temperature of 80 °C to 100 °C. Drawn PLLA filaments acquire orientation-induced crystallinity, which is measured by differential scanning calorimetry using ISO 11357-3:2018; non-isothermal heating at 10 K/min typically shows a first glass transition near 60 °C, a cold-crystallization peak near 95 °C, and a melting endotherm near 175 °C. Tensile properties of conditioned monofilament are measured according to ASTM D2256-21; a tenacity of 3.0 cN/dtex to 4.5 cN/dtex and elongation at break from 20 % to 40 % are typical targets in textile-grade PLLA, while the exact range for PURAPOL L175 depends on draw ratio and winder tension. For nonwoven applications, spunbonded PLLA web consolidation uses calendar bonding at 120 °C to 130 °C with an engraved roll nip pressure of 20 N/mm to 50 N/mm, but the resulting web strength is sensitive to fiber diameter distribution. Online diameter monitoring with a laser gauge set to 0.01 mm resolution is required because diameter variance above 8 % creates weak spots that propagate as tears in carded webs. Pellet moisture above 200 ppm lowers spinline viscosity and causes capillary fouling at the spinneret face. The operational boundary for this downstream segment is that high-viscosity PLLA is less suited to high-speed POY production where low viscosity and high throughput dominate; in such cases a lower-viscosity grade is preferable.

Filament extrusion for fused filament fabrication with high-temperature bed adhesion

In fused filament fabrication, filament produced from PURAPOL L175 is converted into 1.75 mm or 2.85 mm diameter filament with a tolerance of ±0.05 mm and maximum ovality of 0.03 mm, using a single-screw extruder of 24:1 L/D with a gear pump for melt pressure stability. The die is a constant-pressure, single-hole extrusion head held at 200 °C to 205 °C, and the filament is pulled through a two-stage water-cooling trough with a first bath at 30 °C and a second bath at 20 °C. Laser diameter gauges placed after the second cooling bath provide closed-loop feedback to the puller speed; this loop is essential because the high melt viscosity of PLLA creates a time lag between screw speed changes and filament diameter response. Filament is wound on spools under 0.5 N to 1.0 N tension; winding tension beyond 1.5 N introduces amorphous orientation that later relaxes as spool-set curvature during storage. Before printing, the filament is dried at 60 °C for 6 h in a forced-air dryer to reduce moisture below 300 ppm, because printing wet filament generates steam bubbles, filament popping, and inconsistent extrusion at the nozzle. High-heat PLLA filament requires a nozzle temperature of 210 °C to 230 °C, a bed temperature of 60 °C to 80 °C, and an enclosed build chamber at 40 °C to 50 °C to limit warping in parts exceeding 80 mm in length. Mechanical properties of printed specimens are evaluated with ASTM D638-14 for tensile strength and ASTM D790-17 for flexural modulus; resulting values are orientation- and raster-dependent, so comparison to injection moulded data without specifying print direction and infill is invalid. Annealing printed parts at 100 °C to 110 °C for 2 h in a circulating air oven increases the degree of crystallinity and lifts the heat deflection temperature, but it also produces anisotropic shrinkage of 0.5 % to 2.0 % in the X-Y plane. The operational boundary is that this high-viscosity grade can clog 0.25 mm nozzles if the hot-end internal surface roughness exceeds Ra 0.8 µm; polished stainless-steel heat breaks and hardened steel nozzles are specified. Filament produced from PURAPOL L175 is therefore directed at printers that can maintain stable melt flow and accept longer purge times when switching from lower-viscosity PLA grades.

When chemical foaming agents are dispersed in high-viscosity PLLA for tray extrusion

Because the high melt elasticity of PURAPOL L175 stabilizes bubble growth, chemical foam extrusion is performed by metering 1.0 wt% to 2.5 wt% of an endothermic sodium bicarbonate/citric acid foaming agent into the feed throat through a gravimetric side feeder, while the main screw is a 30:1 L/D single-screw configuration with a gas injection port for optional physical blowing agent. The chemical foaming agent decomposes in the melt at approximately 150 °C to 200 °C, releasing carbon dioxide that dissolves into the PLLA melt; the high viscosity of this grade provides the melt strength necessary to stabilize cell growth when the melt exits the annular or flat die. Die temperature is set at 180 °C to 190 °C, which is 20 K to 30 K lower than the melt temperature setting for compact sheet, because premature melt cooling at the die restricts cell coalescence and collapse. A melt pump is positioned after the extruder to maintain die inlet pressure at 80 bar to 120 bar, and downstream pressure release occurs over a calculated die land length so that the pressure drop rate is kept below 5 bar/mm; uncontrolled nucleation can produce bimodal cell size distributions with large voids that reduce local flexural stiffness. The foamed sheet is calibrated in a horizontal three-roll stack at 40 °C to 50 °C, producing an expansion ratio of 1.5 to 2.5 and apparent density in the range of 0.50 g/cm³ to 0.80 g/cm³ as measured by ISO 845:2009. Flexural modulus and stress at break of the foam are tested according to ISO 178:2019 and compared to the unfoamed substrate; the foamed structure reduces weight but also lowers modulus nonlinearly, so a converter cannot assume that mechanical behavior scales linearly with density. The processing window for chemical foaming with PURAPOL L175 is narrow at the upper end because cell stabilization depends on extensional viscosity of the melt; above 210 °C, viscosity drops steeply and the foam collapses, while below 178 °C blowing agent decomposition is incomplete and residual acid residues can remain in the polymer. A 5 °C deviation in die temperature therefore determines whether the cell structure is stable or whether the sheet surface shows gas pinholes and collapse lines. Production-scale foaming of this grade has shown that regrind from foam scrap at levels above 15 wt% lowers melt strength, because cell walls contain degraded low-molar-mass species; therefore, closed-loop regrind is limited to 10 wt% to 15 wt% unless the foam density is below 0.70 g/cm³. The resulting foam trays are used in high-heat takeaway applications only when the formed part is subsequently annealed to increase crystallinity; this application is not interchangeable with expanded polystyrene because the PLLA foam has a higher moisture vapor transmission and different set recovery behavior.

For hot-fill containers and dual-ovenable trays, the downstream route that determines end-use heat resistance is not the primary shaping process but the subsequent solid-state annealing operation. Parts produced from PURAPOL L175 are annealed in a forced-circulation oven at 100 °C to 120 °C for 30 min to 120 min, with the specific dwell time selected by part wall thickness and required HDT B performance. The annealing step increases semicrystalline order by cold-crystallization; differential scanning calorimetry per ISO 11357-3:2018 is used to quantify crystallinity increase by comparing the cold-crystallization enthalpy of the as-formed part against that of the annealed part. A part annealed at 110 °C for 60 min typically shows a heat deflection temperature at 0.45 MPa of 85 °C to 105 °C when tested to ISO 75-2:2013 method B, whereas the unannealed amorphous part is limited to approximately 55 °C to 60 °C; published data for this specific grade is limited, so the converter must validate the exact HDT on parts with the actual wall thickness and moulded-in stress history. Dimensional change during annealing is a major operational constraint: flat trays can exhibit 1.0 % to 2.5 % planar shrinkage, and injection moulded cups can show uneven diameter change because of frozen-in orientation from the gate. Parts are therefore nested on steel mandrels or constrained between flat quartz plates to maintain critical dimensions while allowing through-thickness air circulation. Heating ramp rate is maintained at 1 K/min to 3 K/min up to annealing temperature; faster ramps produce transient temperature gradients and cause warpage at the transition between thick and thin sections. Airflow within the oven is set to 0.5 m/s to 1.0 m/s to avoid dead zones, and temperature uniformity is verified with a multi-point thermocouple fixture placed in the same loading pattern as production. After annealing, parts are cooled to below 50 °C before removal from the fixture; hot demoulding from the annealing fixture tends to recapture amorphous skins if the part remains above the glass transition while unrestricted. Hot-fill performance is evaluated by filling formed containers at 90 °C to 95 °C and measuring sidewall deflection and volume change after 90 s; the pass criterion is typically less than 3 % volume change and no visual distortion. Dual-ovenable suitability is assessed by subjecting annealed trays to 30 min at 180 °C in a convection oven and comparing the shape against a calibrated contour gauge; PLLA crystallizes in this regime, but dimensional recovery can be sensitive to residual stress. This annealing route is not universally applicable to all PLLA parts because a thick section may require an uneconomically long annealing time; parts with wall thickness beyond 2.5 mm are better served by nucleated grades rather than post-processing.