TEREZ NatureGran PV 6930 is an impact-modified polylactic acid injection moulding compound. The model designation PV 6930 identifies a specific impact-modifier architecture and melt-flow balance within the NatureGran portfolio; the polymer matrix is PLA, the processing route is injection moulding, and the modification is intended to reduce room-temperature notch sensitivity relative to unmodified PLA. The grade is supplied as cylindrical pellets and is processed on conventional three-zone screw injection moulding machines with non-return valves and general-purpose screws having an L/D of at least 20:1. Published lot-independent datasheet values for this specific configuration are limited, so downstream qualification should rely on a lot-specific certificate of analysis and first-article inspection against the intended part geometry.

A complete incoming-material inspection protocol for this product should reference the following property-reporting matrix. Unless otherwise stated, test specimens are conditioned at 23 °C and 50 % relative humidity for at least 40 h according to ISO 291. Impact-modified PLA grades in this class commonly shift the tensile failure mode from brittle fracture toward ductile yielding, but the exact lot values must be obtained from the certificate of analysis.

Which Processing Parameters Govern Sink-Mark Formation and Weld-Line Strength?

Impact-modified PLA compounds exhibit shear-thinning behaviour at injection moulding shear rates. Melt viscosity at the flow front controls the pressure required to pack out ribs, bosses, and thin-wall sections before gate freeze. Sink-mark formation is governed by local volumetric shrinkage, hold pressure, and gate seal time; therefore machine pressure settings alone are insufficient. Cavity pressure sensors located near the gate and before the end of fill are used to determine whether the gate remains open during the packing phase. If gate freeze occurs before cavity pressure decays to a stable level, sink marks deepen and part mass variability increases. On production-scale toggle-clamp machines with clamp forces between 500 kN and 2000 kN, the typical response is to extend the hold-pressure time or enlarge the gate diameter. Weld-line strength in impact-modified PLA is especially sensitive to flow-front temperature. When two melt fronts meet after flowing around a core pin, the impact-modifier phase can become oriented parallel to the weld plane, producing a local notch-sensitive plane. Venting near the weld line, elevated mould temperature, and gate repositioning are used to raise the flow-front temperature without exceeding the degradation limit of the compound.

For snap-fit closures and threaded caps, the practical processing target is to maintain melt temperature at the weld line above the crystalline onset temperature of the PLA phase. Infrared thermography at the parting line is one available method; cavity pressure sensors in the knit area are another. The hold-pressure profile should be derived from pressure-time data rather than generic machine settings. A switch-over from injection to hold that occurs too late can produce flash and mould damage, while switch-over that occurs too early can produce short shots or excessive shrinkage. The processing window is therefore established on the specific machine and mould, not transferred from a generic data sheet.

Thermal Degradation Pathways in Impact-Modified PLA Compounds

Thermal degradation of PLA proceeds through random chain scission, unzipping to lactide, and hydrolytic chain cleavage if moisture is present. At melt temperatures above 220 °C, the rate of molecular weight loss increases sharply. The resident melt in the barrel should be kept below the manufacturer’s maximum melt temperature, and screw recovery should be matched to the cycle so that the melt cushion is not re-plasticised for long periods. The presence of an impact-modifier phase may alter the degradation profile depending on the modifier’s thermal stability and residual acid content. Alkaline processing aids should be avoided because they catalyse PLA ester hydrolysis. Hot-runner systems with dead spots or overly long residence times can produce yellowing and plate-out; free-flow hot-runner geometries with direct-gated nozzles are preferred.

Experimental lot evaluations on reciprocating-screw machines show that melt residence times above 5 min at melt temperature can reduce molecular weight and shift notched Charpy impact strength even when visual degradation is absent. The recommended control is to record melt temperature, cycle time, cushion size, and screw recovery speed for each lot. A falling melt viscosity or rising melt mass-flow rate after repeated drying cycles may indicate progressive hydrolytic or thermal damage rather than normal lot-to-lot variation. The use of regrind from impact-modified PLA parts should be controlled by blending ratios and verified by ISO 1133-1:2022 melt flow testing and ISO 179-1/1eA impact testing, because multiple heat histories accelerate molecular weight reduction.

Comparative Failure Mode Analysis: Unmodified PLA, Impact-Modified PLA, and ABS

Unmodified PLA tested according to ISO 527-2 typically fails in a brittle mode with low elongation at break. Impact-modified PLA reduces notch sensitivity and increases ductility, but the trade-off is a reduction in tensile modulus and tensile strength. The exact balance for TEREZ NatureGran PV 6930 is specific to the certificate of analysis, but the expected difference from unmodified PLA is a measurable increase in notched Charpy impact energy and a lower modulus. Heat deflection temperature may also be lower than that of a neat PLA grade because the impact-modifier phase contributes compliance without increasing the load-bearing network.

Compared with ABS, the impact-modified PLA retains a higher biobased carbon content but has a density near 1.24–1.28 g/cm³, while amorphous ABS is typically 1.04–1.07 g/cm³. This creates a weight penalty for PLA mouldings. ABS also generally provides higher notched impact strength and better retention of stiffness at elevated service temperatures. PLA-based impact-modified compounds should therefore be limited to non-load-bearing, non-safety-critical mouldings where service temperatures remain below the Vicat softening point. Compared with other impact-modified PLA formulations, the PV 6930 grade should be differentiated by its melt mass-flow rate, notched Charpy impact strength at 23 °C, shrinkage anisotropy, and the specific modifier chemistry. High-viscosity impact modifiers may raise injection pressure requirements, while low-viscosity grades may sacrifice impact strength or create flash on worn platens. Tool design should not be finalised without lot-specific shrinkage data acquired according to ISO 294-4.

When Moisture Uptake Exceeds 0.25 wt%, Pre-Drying Becomes the Critical Unit Operation

PLA hydrolyses at melt temperatures, so residual moisture must be kept below 250 ppm before processing. If pellets are exposed to ambient air at relative humidity above 60 %, moisture uptake can exceed the threshold within hours. Pre-drying in a desiccant dryer with a dew point below -40 °C and an inlet air temperature of 80 °C for at least 4 h is the standard procedure. Residual moisture is measured by ISO 15512:2019 Karl Fischer titration or an equivalent calibrated loss-on-drying instrument. If moisture exceeds 250 ppm, the melt mass-flow rate increases, impact strength falls, and splay marks appear on the part surface. Production-scale experience with recirculating dryers shows that a dew-point sensor failure can raise moisture and produce viscosity drift even when barrel temperatures remain unchanged; therefore melt viscosity or MFR should be monitored after each drying cycle.

For non-load-bearing injection-moulded articles such as cosmetic packaging, office equipment housings, and consumer electronic enclosures, TEREZ NatureGran PV 6930 is evaluated on drop-weight impact, moulded-in stress, and dimensional stability after conditioning. Food-contact applications require verification of impact-modifier compliance under EU Regulation 10/2011 or FDA 21 CFR 177.1520; the presence of an impact-modifier phase may exclude use under those clearances unless the specific grade is explicitly listed. REACH and RoHS documentation should be requested from the manufacturer. Continuous service above 50–55 °C may produce creep and loss of interference fit in press-fit assemblies, so elevated-temperature performance must be verified with the final moulded part rather than inferred from the polymer family. The material is not intended for safety-critical or load-bearing structural components.