What Limits Direct Sheet Extrusion Above 190 °C?

Bionolle Starcla™ 10S 40% Bio-Based Polylactic Acid/Starch Compostable Compound is converted into sheet between 0.30 mm and 1.20 mm on a vented twin-screw extruder with an L/D ratio of 36:1 to 40:1. The starch-rich secondary phase imposes a narrow thermal processing window: above 200 °C, starch undergoes thermal browning while the PLA phase loses molecular weight through hydrolysis when residual moisture exceeds 0.05 wt%. Production-scale edge tear at the die lips, vacuum port blockage, and melt pressure oscillation are observed when pellet drying falls below 4 h at 70–80 °C in a desiccant dryer with a -40 °C dew point. The extruder barrel profile from rear to die is set at 160 °C / 170 °C / 180 °C / 185 °C / 185 °C. Die temperature is held at 185 °C. Melt temperature measured at the die should not exceed 195 °C. A vacuum vent of -0.08 MPa is applied after the first mixing section to remove moisture and low-molecular-weight volatiles. Die pressure typically falls between 8 MPa and 14 MPa; die pressure variation of more than ±0.5 MPa over 10 min indicates inconsistent feeding or moisture-induced viscosity drift. Melt mass-flow rate is checked by ISO 1133-1:2022 at 190 °C and 2.16 kg; this compound class generally sits between 3 g/10 min and 8 g/10 min. Sheet is quenched on a three-roll stack set at 35–55 °C. The lower roll nip is closed only to gauge the sheet; excessive nip pressure creates internal stress that later appears as corner cracking during thermoforming. Regrind from edge trim may be added up to 15 wt% for non-food-contact layers or closed-loop sheet, but higher regrind raises gel counts and lowers dart impact. Compliance for foodservice sheet is evaluated under Regulation (EU) No 10/2011 and relevant national schemes; U.S. FDA status must be established through a Food Contact Notification or applicable 21 CFR clearance, not inferred from the resin alone. Tensile yield of the sheet is measured by ASTM D638-14; elongation at break is lower than unmodified PLA and should be checked because starch-rich surfaces initiate surface crazing after repeated flexing. The following starting parameters are line-confirmed values, not specification limits.

Die lip build-up of hydrolysed starch is controlled by purging with a PLA-based purge compound at the end of each run; purging with polyolefin-based purge leaves a contamination layer that causes delamination in subsequent sheet runs. The screw should be inspected after every 200 h of operation. Screw tip wear accelerates when free starch dust is present; metal detection upstream is recommended. Sheet quality is not granted solely by drying; ambient relative humidity above 60% may require a hopper dryer and side feeders to maintain stable starve-fed gravimetric dosing. A gravimetric feeder accuracy of ±0.25% is needed because minor pellet mass variation changes output rate and screw fill.

Cutlery knives, forks, and spoons are moulded in multi-cavity cold-runner tools with clamp force between 150 t and 200 t for 32-cavity configurations. The pellets are pre-dried at 70–80 °C for 4–6 h to below 0.05 wt% moisture and fed to a reciprocating screw with a 20:1 to 24:1 L/D ratio. Barrel temperatures are set between 165 °C and 185 °C; the nozzle is held at 180 °C. Injection speed is limited to 60–120 mm/s. High shear rates at gates produce flow lines and possible starch burn marks; a positive shut-off nozzle prevents drool but should not raise melt residence time above 8 min. Holding pressure is set at 50–80 MPa and back pressure at 5–10 bar. Mould coolant temperature is kept at 25–45 °C. The low elongation at break of starch-containing PLA requires cutlery to be designed with ribbed handles and a minimum wall thickness not below 1.8 mm; thinner sections snap at pack-out stations. Colour masterbatch is added at 2–3 wt% using a PLA-compatible carrier; polyolefin-based colour masterbatch is incompatible and reduces weld-line strength. Screw recovery time increases by 10–15% when pellet temperature drifts upward because lower melt viscosity reduces plasticating efficiency. Mould release is limited to food-grade non-amine agents; amine-based additives and alkaline cleaning residues cause polyester aminolysis and reduce melt stability. The finished cutlery is tested under EN 13432 for compostability and under ISO 527-2 for tensile properties. Flexural strength is checked by ISO 178; cutlery must tolerate a bending load of at least 15 N at fork tine roots in processor internal specifications, though published data for this specific grade is limited. Batch-to-batch variance is visible as gloss variation at the gate zone; shipment samples should be checked for melt flow rate by ISO 1133-1:2022 before full production. Regrind incorporation up to 20 wt% is used for dark-coloured cutlery; clear or translucent items must use virgin material to avoid yellowing and black specks from starch decomposition.

Thermoforming Window Narrowing at Starch-Rich Surfaces

Deli cups, portion trays, and lids formed from extruded sheet require plug-assisted thermoforming with aluminium tooling maintained at 30–50 °C. The sheet surface must reach 70–90 °C before forming. Zonal IR ceramic heaters are used because starch-rich areas absorb radiant heat at a different rate than PLA-rich areas; a ±5 °C surface variation causes local thinning at plug contact lines. The plug assist speed is limited to 200–400 mm/s. High plug friction tears the sheet surface; plugs made from low-thermal-conductivity syntactic foam improve wall distribution and reduce surface marking. Hole formation occurs when the sheet surface exceeds 95 °C, while cracking at the lip radius occurs below 65 °C. The forming air pressure is set between 0.4 MPa and 0.6 MPa. Wall thickness at the corner is measured by ultrasonic gauging; a corner wall below 0.18 mm fails stacking strength in foodservice distribution. Scrap regrind from trim is reintroduced at a maximum of 15 wt% into sheet extrusion; thermoformed scrap from starch-rich sheet shows lower elongation at break by ASTM D638-14. Compliance is evaluated against EU 94/62/EC for packaging waste concentration limits and EN 13432 for organic recovery. In North America, compostability claims must reference ASTM D6400 or ISO 17088 certification of the finished article, not only the resin certificate. The material is not appropriate for hot-fill above 60 °C because heat distortion under load drops sharply at starch-rich surfaces; heat distortion temperature is measured by ASTM D648 under 0.455 MPa and should be verified on the specific gauge. Published data for this specific grade is limited; trials on the target tool are required to establish a robust forming window.

Cast film from the compound is processed on single-screw extruders with grooved feed zones and a 24:1 to 30:1 L/D screw. The compound has lower melt strength than pure PLA; therefore the air gap is shortened to 10–15 cm and the chill roll is set at 15–25 °C. Die temperature is held at 175–185 °C; melt temperature above 190 °C causes film yellowing and edge instability. The die gap is set 10–20% wider than the target film thickness because the melt draws down rapidly and starch-rich domains cause die swell variation. Film thickness below 25 µm is not recommended on standard cast lines due to melt curtain resonance and edge thinning; the curtain breaks at line speeds above 150 m/min in starch-rich formulations. Film gauge is controlled by beta gauge feedback to the extruder screw speed. Dart impact testing is performed by ASTM D1709; starch-rich films show lower puncture resistance than unmodified PLA and are not suitable for high puncture-risk packaging. Seal initiation temperature is reached at 85–105 °C, but sealing dwell time must be extended because starch-rich surface roughness delays interfacial wetting. The film is used for compostable produce bags, garment bags, and lamination layers. The film cannot be run as a high-stalk blown bubble; if a blown line is used, blow-up ratio is limited to 2:1 or less and bubble cooling must be reduced to prevent frost line instability. Compliance for flexible packaging is evaluated under Regulation (EU) No 10/2011, REACH (EC) No 1907/2006, and RoHS Directive 2011/65/EU for heavy metals. The film is not a barrier to oxygen or moisture in the sense of EVOH or HDPE; it is used where compostability carries the functional requirement and shelf life is short. Published data for this specific configuration is limited; line trials are necessary because starch phase dispersion varies with extruder screw shear history.

When Compostable Screw Closures Replace Polypropylene in Cold-Fill Packaging

Injection moulded caps and screw closures for cold-fill bottles are attempted when oxygen ingress is not a limiting criterion and when the filling temperature remains below 30 °C. The melt is injected at 180–195 °C into 8-cavity cold runners with valve gates. Mould temperature is held at 15–30 °C. Cycle time is set at 18–24 s for a closure weight of 2–4 g. Gate drool is controlled by positive shut-off valve gate pins; any stringing indicates melt temperature above 195 °C or moisture above 0.05 wt%. Core and cavity tooling is designed with increased draft angles because starch-rich PLA shrinks less than polypropylene but has higher stick tendency on polished cores. Removal torque and reseal performance are verified by ASTM D2063. Leakage of vacuum-packed or carbonated products is tested by ASTM D3078. Thread designs with sharp root radii should be avoided because notch sensitivity in starch-rich PLA leads to stress cracking at the thread engagement line. The compound cannot be used for hot-fill or pasteurisation above 60 °C; thread deformation occurs and removal torque drops below consumer acceptance thresholds. It also cannot withstand repeated high-torque closure events exceeding 1.5 N·m without thread surface scuffing. Amine-based torque modifiers and alkaline cleaners are incompatible because they attack the PLA ester backbone; a food-grade non-amine processing aid at 0.2–0.5 wt% is used where flow length is limited. Compliance is tested under Regulation (EU) No 10/2011 for migration and under EN 13432 for the finished closure. The closure thread surface after moulding should not be flame treated; flame treatment degrades starch domains and creates dust. Published data for this specific closure configuration is limited; closure torque retention trials should be run on the actual bottle finish because PLA/starch compound is more sensitive to finish geometry than polypropylene.

Horticultural plant clips, nursery tags, and vine clips are injection moulded from the compound in a manner similar to cutlery, but with thinner walls and faster cycle times. The melt temperature is set at 170–190 °C; mould temperature is held at 20–40 °C. Living hinge designs are avoided because elongation at break measured by ISO 527-2 is below 15% in typical starch-containing formulations; clip arms open through flexural deflection rather than hinge motion. The part must tolerate flexural strain in the field; flexural modulus is checked by ISO 178. In industrial compost, disintegration begins after 12 weeks at 58 °C; the final article is certified under EN 13432 or ISO 17088 when soil degradability claims are not made. Degradation in soil is strongly moisture- and temperature-dependent and is not guaranteed by industrial compostability certification. UV exposure causes surface chalking and molecular weight loss; unstabilised parts become brittle after one season in high-sunlight areas. If UV stabilisation is needed, the chosen stabiliser must be assessed for compostability scheme approval. Recycled content or regrind from rejected clips is not reintroduced above 10 wt% because outdoor exposure of regrind already reduces molecular weight before re-processing. Printing on tags is performed with compostable inks; solvent-based inks may soften the starch-rich surface and should be avoided. Published data for this specific grade in outdoor horticultural applications is limited; actual exposure trials should be conducted before making biodegradation claims on-pack.