CHIMEI (Chi Mei) ABS PA-748 is an acrylonitrile-butadiene-styrene terpolymer classified by the manufacturer as a plating-grade injection-moulding resin. The PA-748 designation denotes a controlled polybutadiene phase distribution, acrylonitrile content, and molecular weight range intended for acid etching, palladium-tin activation, and subsequent electroless deposition. Typical lot certification data are generated under ASTM D1238-23a, ASTM D638-14, ASTM D790-17, and ASTM D256-23. The resin is supplied as cylindrical pellets and is normally processed on reciprocating-screw injection moulding machines. Published grade-selector data from the manufacturer position PA-748 for plumbing fittings, automotive interior and exterior trim, cosmetic closures, appliance fascias, and decorative parts requiring metallic finishes. The datasheet should be read alongside the certificate of analysis because lot-to-lot variation in emulsion rubber coagulation and graft morphology can shift flow and impact results within the declared production window.

What Does the Plating-Grade Designation Entail for PA-748?

Plating-grade ABS differs from general-purpose ABS primarily in surface morphology after oxidative etching. In production, PA-748 surfaces are conditioned in a chromic acid-sulfuric acid bath; the acid preferentially oxidises surface polybutadiene domains, leaving a network of micro-cavities that provide mechanical anchoring for subsequent metallic deposition. The preparation sequence is governed by ASTM B727 for plastics preparation and by the plater’s internal calibration for etch rate. Etch weight loss and surface roughness are typically monitored after treatment at 60–70°C. Because the polybutadiene domain size in PA-748 is controlled for relatively uniform micro-roughness, the grade is less prone to local over-etching in gated regions than grades formulated with larger rubber particles. Surface roughness after etching can be quantified under ISO 4287; where published adhesion values for this specific substrate configuration are limited, the plater should establish a baseline using ASTM B533 peel-strength coupons before full-scale production.

The electroplating sequence for PA-748 typically comprises alkaline cleaning at 50–60°C, chromic acid-sulfuric acid etching at 60–70°C for 5–15 min, neutralisation to remove hexavalent chromium residues, palladium-tin activation at 20–30°C, acceleration, electroless nickel or copper deposition to 0.5–1.0 μm, then electrolytic copper, semi-bright nickel, bright nickel, and chromium. Etching tanks require titanium or PTFE-lined heating and agitation because the solution is strongly oxidising. Bath temperature uniformity should be maintained within ±1°C because etch rate increases sharply with temperature and varies with air agitation. For PA-748, etch weight loss is monitored per surface area; excessive etch loss above the plater’s upper control limit can create deep cavities that entrap process liquor and cause delayed blistering, while insufficient etch loss reduces mechanical interlocking. Hexavalent chromium control is required under REACH Regulation (EC) No 1907/2006 and local wastewater permits; this does not remove the etch requirement but imposes closed-loop rinsing and chromium reduction before discharge.

A closed-loop dehumidifying dryer delivering air with a dew point below -30°C is recommended before moulding. Manufacturer guidance for plating-grade ABS specifies pre-drying at 80°C for 3–4 h to reduce pellet moisture below 0.02% by weight. At ambient relative humidity above 60%, drying should not be omitted because moisture-driven hydrolysis of the styrene-acrylonitrile matrix can generate splay, weld-line porosity, and surface defects that remain visible after electroplating. PA-748 should not be blended with amine-containing masterbatches or incompatible regrind streams; such additives can shift the acid-etch response and reduce peel strength. Regrind use is limited to clean, segregated material and must remain within the percentage validated in the plating process record.

Thermal, Mechanical, and Flow Property Benchmarks Under ASTM and ISO Methodology

The following values are compiled from published Polylac PA-748 data and should be treated as nominal lot averages rather than specification minima. Where multiple methods exist, the standard designation determines specimen geometry and thermal history; direct interlaboratory comparison without method alignment is invalid.

Nominal lot averages must not be used as acceptance limits; the certificate of analysis for the shipped lot governs.

Among the flow-related defects observed during injection moulding of electroplating-grade ABS, jetting and internal weld-line depth are critical because they become visible after bright nickel-chromium plating. PA-748 exhibits a shear-thinning viscosity profile typical of medium-flow ABS; when processed at melt temperatures of 210–250°C and mould temperatures of 40–80°C, the material fills thin-wall sections without excessive packing pressure. CAE simulation may be used to position gate locations away from visible surfaces; plate adhesion is lower at weld lines where polymer chains are only partially fused. Published data for weld-line peel strength in this specific configuration is limited, so platers commonly qualify parts by thermal cycling from -40°C to 85°C with no blistering or delamination. In hot-runner systems, the manifold temperature should be profiled to avoid dead zones imposing residence times above 12 min; prolonged residence above 250°C degrades the butadiene phase and increases yellowness index, which can be measured under ASTM E313-20.

Mould shrinkage for PA-748 is typically 0.4–0.6% in the flow direction and 0.5–0.7% transverse when measured on 3.2 mm plaques under ASTM D955-21. This anisotropy must be accounted for in plated parts because bright finishes reveal sink marks and shadow lines more readily than unpainted textured surfaces. Cavities should be polished to a diamond finish rather than vapour-honed because tool surface roughness is transferred to the part and becomes visible after bright plating. If a tool is textured for an unplated version, the texture must be removed or re-qualified for the PA-748 plated version because deep texture alters acid wetting and plate coverage. Gate vestiges should be located in non-visible areas, and cold slag wells should be designed to prevent degraded resin from entering the cavity.

When PA-748 Replaces PA-757, PA-747, or PA-749 in a Production Schedule

Grade substitution within the Polylac ABS family is not a drop-in decision. PA-748 is separated from PA-757 general-purpose grade primarily by etch adhesion and melt-flow index: PA-757 is optimised for surface gloss and general mechanical balance, whereas PA-748 is optimised for micro-porous etch topography while retaining adequate flow. PA-747 high-impact grade provides higher notched Izod impact, but its larger rubber particles can produce lower interfacial adhesion after chromic acid etching and occasionally greater moulded-in residual stress. PA-749 high-flow grade can fill thin-wall geometries at lower injection pressure, but it generally sacrifices notched impact and may require higher pack pressure to prevent sink marks; for plated parts, the lower rubber content of high-flow grades also reduces the available mechanical anchoring after etching. If a moulder replaces PA-748 with PA-747 without re-qualifying etch time, the result is often under-etched or over-etched surfaces; if PA-749 is substituted, plate adhesion may fall below the internal peel-strength minimum because the surface density of etched rubber domains is lower. Grade selection should therefore be locked into the part drawing and process-validation record, and any substitution requires new qualification to ASTM B727 preparation and the plater’s adhesion protocol.

Plating Adhesion and Failure Modes in Etched PA-748 Surfaces

Adhesion failure in plated ABS is generally cohesive within the substrate rather than purely interfacial when the etch profile is correctly developed. For PA-748, peel strength in new production is governed by the depth and density of etched polybutadiene cavities, but processing variables can degrade the surface before plating. Overly long residence time, excessive injection velocity near the gate, and flash-generating clamp pressures below the required mould force can create local shear heating and surface oxidation; these regions etch differently and appear as dull patches after plating. Mould release agents should be avoided because they interfere with acid wetting and produce skip plating under ASTM B727 preparation. If the plater observes blisters after thermal cycling, the cause is often gas inclusion from moulded-in voids or moisture, not solely loss of adhesion. PA-748 should be evaluated for plating adhesion using peel coupons per ASTM B533 or pull-off testing per ISO 4624, with cross-section analysis under optical microscopy to confirm etch depth and palladium distribution.

For production-scale reciprocating-screw injection moulding machines with clamp force between 1,200 kN and 2,500 kN, PA-748 is typically processed with a compressed melt cushion of 2–4 mm and screw back pressure of 5–10 bar. Injection speed should be profiled to avoid jetting; when the melt enters the cavity, first-stage injection speed is set at 30–70% of maximum machine velocity depending on gate size. Holding pressure is set at 50–70% of peak injection pressure and held until the gate freezes; early gate freeze in cold-runner systems can produce sink marks, while late gate freeze may overpack and raise stress at the gate. Ejection should occur only after the part has cooled below the Vicat softening range, and parts should be protected from moisture and handling oils before the etching step. These processing limits are operational boundaries derived from standard ABS moulding practice; published data for this specific configuration is limited for material lot shifts, so the processor should monitor peak cavity pressure and transfer position rather than relying on a fixed machine timer.

Regulatory Compliance Does Not Confer Plating Adhesion.

Standard regulatory documents relevant to PA-748 include RoHS Directive 2011/65/EU as amended, REACH Regulation (EC) No 1907/2006, and UL 94 flammability classification. A supplier declaration of conformity may list the grade as UL 94 HB at the tested thickness; the absence of flame-retardant additives is one reason PA-748 can be used in decorative plated articles without concern for bloom from halogenated additives. REACH documentation should be requested for each production lot because residual monomer and processing-aid composition can vary with the polymerisation route. These regulatory documents address restricted substances and flammability; they do not substitute for adhesion testing after electroplating. A plated part that meets RoHS but fails ASTM B533 peel strength is not acceptable for exterior automotive or plumbing applications. The grade should therefore be specified with three distinct documents: the Polylac PA-748 datasheet, the certificate of analysis for the lot, and the plating process validation report for the specific part number.