Marlex PP HD168MO is a polypropylene homopolymer grade positioned for injection-moulded rigid articles that require high stiffness, moderate heat resistance, and predictable post-mould shrinkage. The grade designation belongs to the Marlex polypropylene product line; available datasheets classify it as a high-crystallinity homopolymer, which separates it from propylene-ethylene random copolymers by a markedly higher flexural modulus and from impact copolymers by the absence of a deliberate rubber phase. Published data for this specific grade are not uniformly available in public literature; therefore, the numerical ranges below are representative of high-crystallinity Marlex PP homopolymers in the same melt-flow classification and should be verified against the current lot certificate.

The grade is supplied as pelletised feedstock with density between 0.900 g/cm³ and 0.910 g/cm³ when measured under ISO 1183-1:2019 or ASTM D792-20. Melt flow rate, determined at 230 °C with a 2.16 kg load under ISO 1133-1:2022, is the primary processing discriminator. For HD168MO-class material, the melt flow rate typically falls between 8 g/10 min and 20 g/10 min; the exact value must be taken from the certificate because a shift of 2 g/10 min can alter shortest fill time and cavity pressure transmission. The product converts by open-loop injection moulding rather than profile extrusion or blow moulding. Typical end-use categories include rigid caps and closures, appliance housings, storage containers, and industrial packaging where side-wall deflection under load must be minimised.

What Processing Parameters Prevent Short-Shot Formation in HD168MO?

On production-scale hydraulic injection machines with clamp force capacity of 0.5 tonnes/cm² to 0.8 tonnes/cm² of projected area, the barrel should be profiled so that the melt temperature at the nozzle stays between 200 °C and 230 °C. The feed throat is maintained below 60 °C to prevent pellet bridging. If the nozzle falls below 190 °C, short shots occur in walls below 1.5 mm because the crystalline fraction is not fully disengaged and the apparent viscosity remains too high for thin-section filling. If the nozzle remains above 230 °C for more than 5 min, thermal-oxidative chain scission accelerates, producing free radicals that reduce molecular weight and shift melt flow rate upward. This shift can create flash at the parting line and lower notched impact force, even if the visual surface appears unchanged.

Pre-drying is not required when storage relative humidity remains below 60%. When condensation is visible, a desiccant dryer set to 80 °C for 2 h to 3 h is sufficient; drying above 90 °C can cause pellet agglomeration and feed-throat blockage. The screw should have an L/D ratio between 20:1 and 24:1 and a compression ratio between 2.5:1 and 3.5:1. Back pressure is normally set at 0.5 MPa to 1.5 MPa to homogenise melt temperature without excessive shear heating. Injection speed should be profiled: a slow initial advance of 30 mm/s to 60 mm/s at the gate, followed by 100 mm/s to 150 mm/s during cavity filling, reduces jetting and improves weld-line integrity. Hold pressure is set to 40% to 60% of peak injection pressure; hold time is commonly 3 s/mm to 8 s/mm of nominal wall thickness. Lower hold pressure produces sink marks because crystallisation shrinkage of 1.2% to 1.6% in the flow direction must be fed from the gate until seal. Higher hold pressure can increase residual stress and gate stress whitening.

Short-term mechanical response follows the general pattern of a high-crystallinity homopolymer: tensile yield strength is typically 34 MPa to 38 MPa when tested at 50 mm/min under ISO 527-2:2012. Flexural modulus, measured at 2 mm/min under ISO 178:2019, falls between 1.5 GPa and 1.8 GPa. This modulus envelope is materially higher than that of a random copolymer with similar flow, which typically shows 0.9 GPa to 1.2 GPa. Notched Izod impact resistance by ISO 180:2023 is 1.5 kJ/m² to 3.5 kJ/m² at 23 °C and 1.0 kJ/m² to 2.0 kJ/m² at 0 °C. The product therefore cannot blunt cracks effectively and is not selected for sub-zero impact duty; its selection is driven by static rigidity and dimensional stability. Heat deflection temperature at 0.45 MPa under ISO 75-2:2013 generally lies between 95 °C and 110 °C, placing it above common random copolymers that may deflect between 75 °C and 90 °C under the same stress. Rockwell hardness by ISO 2039-2:1987 or ASTM D785-23 is on the 90 to 100 R-scale, contributing to surface scratch resistance but also increasing gate-trim brittleness.

Lot-to-lot melt flow variation of ±1.5 g/10 min can require adjustment of injection speed or hold pressure. On machines without closed-loop melt-pressure control, converters should hold a cushion of 2 mm to 5 mm and monitor screw recovery time. A recovery-time increase of more than 5% from the qualification baseline often indicates higher viscosity, a feed blockage, or a mis-set rear zone. A recovery-time decrease of the same magnitude may indicate chain scission or contamination with a higher-flow resin.

Thermal Response, Crystallisation Kinetics, and Shrinkage Anisotropy

Differential scanning calorimetry under ISO 11357-3:2018 places the main crystalline melting peak between 160 °C and 165 °C, with a recrystallisation exotherm during cooling between 110 °C and 120 °C. The crystalline weight fraction is usually 50% to 60%, and the heat of fusion is typically 145 J/g to 160 J/g. This heat must be removed in the mould, so cooling time scales with wall thickness squared. For a 2 mm wall at a mould surface temperature of 30 °C, cooling times of 8 s to 14 s are common. Raising mould temperature to 50 °C to 60 °C improves gloss and reduces weld-line visibility but increases cycle time and post-mould shrinkage. Lower mould temperatures below 20 °C can freeze the gate too early and increase moulded-in stress.

Mould shrinkage measured on 60 mm × 60 mm × 2 mm plaques under ISO 294-4:2018 is 1.2% to 1.6% in the flow direction and 1.3% to 1.7% in the transverse direction. The difference arises from flow-induced molecular orientation and subsequent oriented crystallisation. Differential shrinkage is the main driver of warpage in flat lids and rectangular containers. Tool design should create a linear flow front and maintain packing pressure until gate seal. Hold pressure below 40% of peak injection pressure often produces underpacked central regions; above 60%, residual stress increases and may produce stress whitening at the gate. The practical melt-temperature window is narrow: below 190 °C, screw recovery torque can rise sharply because pellets are not fully plasticised; above 240 °C, degradation can shift melt flow rate by more than 10% over 30 min residence. Older machines with poor barrel-zone control should operate in a narrower profile of 200 °C to 220 °C to limit lot-to-lot variability.

The grade is not a polyethylene blend. Admixture of high-density polyethylene above 2 wt% can reduce flexural modulus by 10% to 20% and produce iridescent flow lines from phase separation. It should not be dry-blended with amine-based additive masterbatches that can bloom in high-humidity service. Standard phenolic-phosphite stabilisation is typical, but the converter should confirm the stabiliser package with the supplier for heated or long-duration end-use.

For material substitution studies, the following property envelope compares the HD168MO-class homopolymer with a random copolymer and a high-flow polypropylene homopolymer. All values are typical screen ranges and do not replace lot certification.

When Food-Contact or Household Chemical Resistance Is Required

Compliance statements must be derived from grade-specific certification, not from generic polypropylene clearances. Polypropylene homopolymers may be cleared for food-contact use under 21 CFR 177.1520 if the finished article meets the applicable extractive limits. Under EU Regulation No 10/2011, overall migration into the specified food simulants must not exceed 10 mg/dm² for most single-use applications; repeated-use articles must also satisfy specific migration limits for the stabiliser package. The HD168MO designation should not be treated as a medical-grade identifier. Unless the supplier has issued an affirmative statement under ISO 10993-1 or a related evaluation, the grade is not appropriate for medical devices or pharmaceutical packaging.

For household chemical exposure, the homopolymer exhibits limited stress-crack resistance to chlorinated solvents and aromatic hydrocarbons. Short-term contact with dilute acids and alkalis at 23 °C is generally acceptable, but oxidative environments such as hot chlorinated water or high-temperature bleach solutions can produce surface crazing. Long-term outdoor exposure requires a stabilised grade or a UV-absorber masterbatch. Unstabilised polypropylene homopolymer can lose more than 50% of tensile elongation after 2000 h to 4000 h of accelerated QUV-A weathering under ISO 4892-3:2024.

The following checklist summarises the regulatory and handling boundaries that should be confirmed with the lot certificate.

On a 120-tonne hydraulic moulding machine with a 40 mm screw and a two-cavity cold-runner tool, comparable Marlex PP homopolymer classes have shown that setting the rear barrel zone to 180 °C reduces screw motor load but increases melt-pressure variation at the transfer point. A flat profile of rear 200 °C, centre 210 °C, front 220 °C, and nozzle 220 °C provides more stable recovery. If the cycle is recovery-limited rather than cooling-limited, back pressure should be raised within the 0.5 MPa to 1.5 MPa range before increasing barrel temperature.

When regrind is introduced into non-critical packaging, the fraction is typically held between 10% and 30%. Regrind must be free of fines because fines accelerate thermal oxidation and create black specks. The grade can be sorted in PP streams by density separation at 0.96 g/cm³ to 1.00 g/cm³ or near-infrared resin identification. Contamination from polycarbonate or PET above 2 wt% can cause delamination and surface defects. In a multi-cavity tool, batch-to-batch melt flow variation should be controlled so that the cushion does not fall below 2 mm; loss of cushion causes pressure loss at the gate and produces sink marks in the thickest section of the article.