CHIMEI (Chi Mei) ABS PA-757K is a high-gloss acrylonitrile-butadiene-styrene copolymer formulated for injection moulding of dimensionally stable components requiring an intermediate balance of impact resistance, stiffness, and processability. The grade is commonly specified for appliance bezels, electrical enclosures, and interior automotive trim where moulded articles must survive low-velocity impact events without the excessive flexural modulus penalty associated with glass-filled systems. Supplier-published data sheets report a typical melt mass-flow rate in the medium-flow band of 1.8–2.4 g/10 min when tested at 200 °C under a 5 kg load in accordance with ASTM D1238-20, a density of 1.04–1.05 g/cm³ per ISO 1183-1:2019, a tensile stress at yield of 44–48 MPa per ISO 527-2:2012, and a flexural modulus of 2.2–2.4 GPa per ISO 178:2019. Notched Izod impact strength at 23 °C is generally listed in the 20–26 kJ/m² range under ISO 180/1A, while the heat deflection temperature under a 1.82 MPa flexural stress is 86–90 °C per ISO 75-2:2013. These values place PA-757K in the medium-high impact ABS segment rather than in the ultra-high-impact or flame-retardant ABS categories.

The mechanical response of PA-757K depends on the degree of rubber-phase crosslinking and the graft density of styrene-acrylonitrile onto the polybutadiene core. Under tensile loading at 23 °C and 50 mm/min crosshead speed per ISO 527-2:2012, the material yields before cold-drawing; at -30 °C, the notched Izod impact falls below 8 kJ/m² because the polybutadiene phase passes through its glass-transition region. Moulded-in orientation from high injection speeds raises tensile stress at yield in the flow direction by up to 5 MPa and reduces cross-flow elongation at break to 10 % or lower. This anisotropy is reversible only by annealing above 90 °C for 2 h, which compromises cycle economics but stabilizes dimensional tolerance in tight-bearing bosses.

What distinguishes PA-757K from the broader CHIMEI ABS product line?

Positioning within the CHIMEI ABS family is defined by the rubber-phase level and melt-flow response rather than by a single mechanical property. Relative to grades with higher polybutadiene content, PA-757K exhibits a lower notched Izod impact value at 23 °C and a higher flexural modulus, which favours thin-wall parts needing reduced creep under load. In contrast to extrusion-oriented ABS used in sheet and profile lines, the melt viscosity of PA-757K is designed for injection moulding; it does not provide the high melt strength required for hang-back sheet calendering. The K suffix is associated in supplier technical literature with a controlled rheology variant, although published data for the specific molecular-weight distribution are limited. This designation should not be used to infer flame-retardant performance or UV-stabilised outdoor weatherability.

Hot-air Drying, Melt Residence Time, and Back-Pressure Settings

Pre-drying is mandatory when the material has been exposed to ambient relative humidity above 60 % or to bulk storage exceeding 24 h. Residual moisture above 0.05 % hydrolyses the SAN continuous phase during plastication, appearing as splay, streaks, and local delamination at the gate. Desiccant drying at 80 °C for 2–4 h is conventional; hopper-mounted hot-air units are acceptable only when the inlet-air dew point is -20 °C or lower. Barrel temperatures from feed to nozzle are typically set at 180–220 °C for low-lubricity screws and 190–230 °C for high-shear barrier screws; melt residence times exceeding 10 min above 240 °C may initiate thermal oxidation of the polybutadiene-graft-SAN rubber phase, producing yellowing and a measurable loss in notched Izod impact. The supplier recommends maintaining melt temperature below 240 °C and residence time below 10 min; published kinetic data for this specific grade are limited. Back pressure should remain in the 0.3–0.7 MPa range, sufficient to prevent screw slip but below the threshold at which shear heating closes the processing window. Mould surface temperatures below 40 °C reduce interface weld-line strength; above 80 °C extend cycle times without proportionate gloss improvement.

On high-cavitation appliance-bezel moulds equipped with valve-gated hot runners, the medium-flow rheology of PA-757K permits fill pressures in the 60–100 MPa range at the gate, provided the runner diameter exceeds 4.0 mm and land lengths are held below 1.0 mm. A 200-tonne hydraulic clamp is typically sufficient for four-cavity tools producing 600 cm² projected-area bezels with wall thicknesses of 2.0–2.5 mm. The material exhibits a mould shrinkage of 0.4–0.7 % when measured according to ISO 294-4:2018; the higher end of this range is encountered at melt temperatures above 230 °C and mould temperatures above 70 °C. Gate freeze-off occurs rapidly with unheated sprue-bush tools, so secondary gate diameters below 2.0 mm should be avoided when the flow length exceeds 200 mm.

When hot-runner gating replaces cold-runner edge gates in PA-757K moulding

When a valve-gated hot-runner system is substituted for a conventional cold-runner edge gate, the pressure drop across the manifold is added to the part-filling pressure requirement, and the shear history of the polymer melt is extended. Hot-runner manifolds with an internal surface roughness exceeding 0.8 µm Ra can generate plate-out and yellow micro-specks in light-coloured PA-757K parts, particularly at melt temperatures near 240 °C. Channel diameters should maintain a local shear rate below 5,000 s⁻¹ to avoid viscous heating beyond the terminal zone setpoint. Sequential valve-gate actuation requires injection velocities above 100 mm/s to prevent weld lines at the melt-front convergence; slower velocities produce visible jetting on the opposite wall of the cavity. Capillary rheometry performed at 220 °C, 230 °C, and 240 °C under shear rates from 100 s⁻¹ to 10,000 s⁻¹ supplies Cross-WLF viscosity coefficients to injection-moulding simulation software. The zero-shear viscosity of PA-757K at 230 °C falls in the 1,200–1,800 Pa·s range in typical ABS simulations, while the power-law index under high shear is approximately 0.3–0.4; published data for this specific configuration are limited and should be verified with a slit die on the target moulding machine.

Which moulding defects indicate a departure from the recommended PA-757K processing envelope?

Splay, silver streaking, and gate blush are typically moisture-related or shear-related rather than inherent resin flaws. In an adequately dried melt, silver streaks near the gate usually indicate shear rates above 15,000 s⁻¹ at the land entrance, causing local temperature spikes above 250 °C. Brown stripes or black specks in transparent or white colour compounds indicate dead zones in the hot-runner manifold or a cracked screw tip, not pellet contamination alone. Warpage of flat rectangular enclosures with length-to-thickness ratios above 50:1 is controlled by mould cooling rather than by melt rheology; differential shrink from gate-to-vent orientation shifts by 0.05–0.15 % and produces visible bow if the cooling-line layout is unbalanced. Gate location should be selected to place weld lines in low-stress regions, and for bosses with a height-to-diameter ratio above 2:1, vent depths of 0.02–0.03 mm are required to prevent diesel-effect ignition of volatile additives.

Electroplating-grade ABS requires a well-dispersed polybutadiene phase with particle sizes suitable for chromic acid etching. PA-757K is often used as a plating substrate when a medium-impact surface grade is acceptable, because its intermediate rubber-phase content supports uniform micro-roughness after etching in a 65–70 °C chromic acid-sulfuric acid bath for 4–8 min. Rinsing between the etch and activation steps must avoid carry-over of chromate ions above 10 mg/L, because residual hexavalent chromium inhibits palladium-tin nucleation. Adhesion of subsequent electrolytic copper or electroless nickel deposits is governed by mechanical interlocking rather than chemical bonding; tensile adhesion values measured by pull-off testing according to ISO 4624 are generally 0.8–1.5 MPa on properly etched ABS plaques, but published data for PA-757K in this specific configuration are limited.

Thermal-stability limits during regrind reincorporation and pellet storage

Regrind from PA-757K runners and rejected parts can be reincorporated into virgin material for non-appearance parts at concentrations up to 20 wt% without a statistically significant shift in flexural modulus, provided the regrind was not exposed to melt temperatures above 240 °C or to multiple heat histories. For appearance-critical components, regrind concentrations above 10 wt% may produce silver streaks and gloss reduction from degraded polybutadiene domains. Pellet storage should remain below 60–70 % relative humidity and below 35 °C to suppress moisture absorption and oxidative embrittlement of the unsaturated rubber phase. A storage period beyond 12 months does not alter the glass-transition temperature of the SAN phase but may increase the yellowness index by more than 1.0 under ASTM E313-20 if exposed to sunlight or fluorescent ultraviolet sources. Avoid combination with amine-based additives or copper-containing stabilisers unless compatibility has been verified, because such additions can alter the thermal stabilisation package and accelerate rubber-phase degradation.

In applications where medium-impact ABS is weighed against high-impact polystyrene and polycarbonate/ABS blends, PA-757K occupies the intermediate cost-performance window. High-impact polystyrene has lower density but also lower tensile strength and lower heat resistance; PC/ABS alloys provide higher notched Izod impact and heat deflection temperature but impose higher drying requirements and higher melt temperatures. The tensile stress at yield of typical PC/ABS is commonly above 50 MPa, and heat deflection temperature at 1.82 MPa can exceed 95 °C, whereas PA-757K remains suitable only for service environments below its heat deflection temperature. The use of PA-757K in contact with food or potable water is not covered by a broad FDA 21 CFR 177.1020 certification unless a specific lot is validated; it is not a transparent or high-clarity resin and is not recommended for outdoor weatherability without UV-stabilised or coextruded cap-stock protection.

Compliance status and analytical certification requirements

Regulatory documentation for PA-757K typically references RoHS 2011/65/EU as amended by (EU) 2015/863 for the restriction of lead, mercury, cadmium, hexavalent chromium, PBB, PBDE, and four phthalates. The grade is generally supplied with a letter of conformance stating that cadmium and lead are below the maximum concentration values of 100 mg/kg and 1,000 mg/kg, respectively. REACH compliance under EC No 1907/2006 is limited to the absence of included SVHCs above 0.1 wt% in the as-supplied pellet, and does not include SVHC migration from post-moulding reactions. Flammability certification is typically UL 94 HB at 1.6 mm; no V-0 rating is implied for thicknesses below 3.0 mm. End users must verify that colour concentrates, flame-retardant masterbatches, or lubricants added at the press do not invalidate these certifications, and that the final part complies with applicable electrical insulation or consumer safety standards relevant to the intended market.