CHIMEI (Chi Mei) ABS PA-779, listed in supplier technical dossiers as POLYLAC PA-779, is an acrylonitrile-butadiene-styrene terpolymer developed for injection-moulding operations in which melt flow and surface gloss are the primary screening criteria. The material is supplied in natural pellet form and is typically evaluated for thin-wall appliance housings, consumer-electronics enclosures, cosmetic closures, desk accessories and decorative handles. Representative supplier-published values for natural and black lots are summarised in Table 1; they are typical values rather than specification limits.

The single-point melt flow rate of 55 g/10 min under ISO 1133-1:2022 is the primary rheological differentiator. In hot-runner systems, this reduces nozzle-pressure demand but also narrows the allowable residence-time envelope. Because high-flow ABS grades are sensitive to uncontrolled shear heating, the material should not be treated as a drop-in replacement for a lower-MFR grade without verifying the complete viscosity curve. The full Cross-WLF viscosity model coefficients for PA-779 are not reproduced in the public supplier bulletin; simulation input should be obtained from the regional technical service center or from the approved material dossier. Apparent viscosity variation with shear rate, temperature, and pressure is essential for predicting gate pressure drop. Published data for this specific configuration is limited.

How Does the Grade Sit Between Standard and High-Impact Polylac Materials?

Within the Chi Mei ABS range, PA-779 is positioned above the standard general-purpose PA-757 in melt flow and surface gloss, while its notched Izod impact of 20 kJ/m² at 23°C is below the values published for high-impact grades such as PA-747S. This differentiation makes PA-779 suitable for thin-wall components that tolerate moderate impact loads but require low injection pressure and excellent visual appearance. Published data for exact property differences among colour-matched lots is limited; a direct substitution from PA-747S to PA-779 should be accompanied by a notched Izod comparison on the production tool because weld-line impact retention in high-flow ABS is often more sensitive to mould temperature than the single-point impact value indicates.

When a moulder examines the POLYLAC data set for PA-779 against PA-757 and PA-747S, the high-flow grade has the highest MFR and among the lowest notched Izod values. This inverse relationship is a designed trade-off because the same molecular adjustments that increase flow reduce the energy dissipation capacity of the polybutadiene phase. A material substitution without tooling modification is therefore not neutral: the lower viscosity reduces filling pressure, but the lower rubber-phase concentration may require higher mould temperature to maintain weld-line impact.

Pre-drying at 80°C to 85°C for 2 h to 4 h in a desiccant dryer with a return-air dew point of −30°C or lower is recommended before processing. The moisture content should be held below 0.05% to prevent splay and gloss defects. If material is exposed to ambient air at relative humidity above 60%, pre-drying becomes mandatory. Moisture content above 0.05% can generate splay, silver streaks, and surface roughness at the melt front. Dryer hoppers should be stainless steel and purged to avoid cross-contamination with hygroscopic resins. Overdrying above 90°C can fuse pellets and cause bridge formation at the hopper throat.

Barrel profiles are typically set from 210°C at the feed throat to 240°C – 250°C at the nozzle, with mould temperatures maintained between 40°C and 70°C. A general-purpose screw with an L/D ratio of 20:1 to 24:1 and a compression ratio of 2.0:1 to 2.5:1 is adequate. Back pressure should be 0.3 MPa to 0.7 MPa, and metering position should be selected so that the shot size occupies 50% to 80% of the rated barrel capacity. These conditions are consistent with established ABS processing practice; machine-specific optimization is required because hot-runner pressure drop and screw recovery time depend on the manifold design and check-ring condition.

When Wall Thickness Decreases Below 1.2 mm in Multi-Cavity Tooling

Thin-wall moulding with PA-779 exploits the high melt flow at 220°C to 250°C, but the process window becomes constrained by two competing failure modes. At nominal wall thickness of 1.0 mm to 1.2 mm, injection pressures of 70 MPa to 110 MPa are typically sufficient on machines with clamp force of 0.35 ton/cm² to 0.55 ton/cm² of projected area. When gate-size constraints force filling shear rates above 100,000 s⁻¹, the polybutadiene phase may undergo shear-induced orientation and surface gloss loss; if gate freeze time is shorter than 0.5 s, packing underflow results in sink marks and dimensional variation. Below 0.8 mm wall thickness, mould temperature should be raised toward 70°C and melt temperature should be held below 250°C to avoid thermal yellowing. Published spiral-flow data for PA-779 at injection speeds above 200 mm/s is limited; cavity-filling studies should therefore be performed on the production tool rather than extrapolated from general-purpose ABS databases.

The thermal degradation pathway in ABS at elevated temperature is dominated by oxidation of the polybutadiene phase, followed by chain scission of the SAN matrix. In PA-779, the narrow melt-temperature window is therefore not controlled by the SAN melting point but by the stabiliser package and rubber-phase thermal history. Production lines with hot-runner systems above 250°C should be monitored for gloss loss and yellowing; hot-runner nozzle thermocouple profiling is recommended. Published activation-energy data for PA-779 degradation is limited.

Packing pressure is typically 70% to 90% of filling pressure, with hold time set by gate-seal studies. For a nominal wall thickness of 1.5 mm, gate-seal time commonly falls between 2 s and 4 s; thinner walls require shorter hold times. Published gate-seal data for PA-779 is limited, so a cavity-pressure transducer is recommended for establishing the correct hold-time window on the production tool. Mould temperatures below 40°C reduce surface gloss and increase internal stress. For high-gloss parts, mould temperature is typically 60°C to 70°C, accepting a longer cooling time. The cooling circuit should maintain turbulent flow with a Reynolds number above 10,000 to stabilise mould surface temperature.

Because regrind incorporation alters rubber-phase crosslinking and melt-flow stability, regrind levels up to 20% are typical for non-appearance parts; higher levels may reduce gloss and notched Izod impact. For parts requiring electroplating or high-gloss first surface, regrind should be excluded or limited to lot-specific verification. The effect of regrind on PA-779 has not been systematically published; both visual inspection and notched Izod retention testing should accompany any increase in regrind level.

In electroplating and decorative paint systems, PA-779 is selected for high-gloss unpainted surfaces and for plating-on-plastic after chromic-acid etching of the surface butadiene phase. Electroplated copper-nickel-chromium adhesion is commonly evaluated by thermal cycling from −20°C to 80°C or by ISO 2819; published adhesive-loss data for PA-779 specifically is limited. Acrylic or urethane coatings are preferred. Solvent-based inks or cleaners containing ketones or esters can craze the ABS surface unless annealing or low solvent exposure is demonstrated. Mould release agents based on silicone can interfere with paint adhesion and should be avoided without post-mould cleaning.

Regulatory Boundaries, Flammability Classification, and Additive Constraints

Regulatory compliance for PA-779 must be verified against the specific lot certificate. EU RoHS Directive 2011/65/EU, including amendment (EU) 2015/863, typically requires restricted substances to be below 0.1 wt% in homogeneous material. REACH Regulation (EC) No 1907/2006 requires evaluation against the Candidate List of substances of very high concern as updated by ECHA; the absence of intentionally added SVHCs does not release the downstream user from Article 33 communication duties. The grade is classified UL 94 HB at 1.5 mm; re-evaluation is required for thinner walls because burn rate and flame dripping are thickness-dependent. Food-contact status under US FDA 21 CFR 177.1020 or Commission Regulation (EU) 10/2011 is not automatic and must be supported by lot-specific certification and end-use migration testing.

Because PA-779 is controlled for high gloss and high flow, moulded-in stress is a significant quality variable. Annealing at 70°C to 80°C for 1 h to 2 h is used when dimensional stability at elevated service temperature is required. Accidental blending with polyamide or polycarbonate without compatibiliser is not recommended; phase separation can create splay, delamination, and low weld-line strength. Metal stearate lubricants should be limited to 0.3% or less to reduce mould plate-out and to maintain electroplating adhesion. Avoid PVC-based colour carriers because dehydrochlorination can corrode mould surfaces and degrade the polymer.