Thin-Wall vs Thick-Wall ABS: Picking Melt Index for the Mould requires separating the low-shear melt index reported on a grade data sheet from the pressure-limited flow behaviour that governs cavity filling. ABS is a two-phase system of grafted polybutadiene particles dispersed in a styrene–acrylonitrile copolymer matrix; melt index generally reflects SAN phase molecular weight, rubber content, and rubber particle packing. The most common ABS melt index test is ISO 1133-1:2022 with a melt temperature of 220 °C and a rated load of 10 kg; the North American alternative is ASTM D1238-23, condition 230 °C/3.8 kg. These two values are not numerically interchangeable for ABS. A grade labelled as 20 g/10 min under ISO 1133-1:2022 may report a different figure under ASTM D1238-23 because temperature and load differ.

Selection of melt index for thin-wall versus thick-wall ABS should rest on flow path ratio, gate geometry, hot-runner pressure capacity, and the impact retention demand after moulding. The melt index is a single-point measurement at comparatively low shear and does not by itself define the full viscosity curve. Thin-wall moulding pushes shear rates above 10 000 s^-1, where ABS exhibits pronounced shear thinning, while the melt index is measured at shear rates near 10² s^-1. Melt volume rate may be the more useful ISO 1133-1 parameter because it removes melt density assumptions, but most ABS processing discussions continue to report melt flow rate in g/10 min.

Does Melt Index Alone Predict Thin-Wall Cavity Filling?

Thin-wall ABS grades are typically formulated in the range 18–30 g/10 min under ISO 1133-1:2022. The high flow is usually achieved by reducing the molecular weight of the SAN phase and, in some grades, by lowering the rubber content; this provides short-cycle filling but sacrifices notched Izod impact and tensile elongation. Published datasheet comparisons for injection moulding grades show that moving from 14 g/10 min to 22 g/10 min at 220 °C/10 kg reduces fill pressure by roughly 20–30 % for a 1.0 mm wall spiral flow length of 300 mm, but notched Izod impact per ISO 180/A may fall from approximately 18 kJ/m² to 10–12 kJ/m².

For a Newtonian approximation, pressure drop in a strip cavity scales with apparent viscosity and flow length and inversely with the cube of wall thickness. ABS is shear thinning, so the pressure benefit of a higher melt index is smaller at high injection speeds than the low-shear melt index suggests. Capillary rheometry per ISO 11443:2021 is the correct measurement tool; a grade with melt index 20 g/10 min may have the same apparent viscosity at 1 000 s^-1 as a grade with melt index 15 g/10 min if the latter has a broader molecular weight distribution. One production-scale bottleneck on a 1200 kN injection moulding machine with a 25 mm reciprocating screw occurs when the screw charging time for high-flow ABS is shorter than the cooling time. If the screw plasticises at a higher rate than the mould opens, residence time increases and the melt begins to degrade even when the temperature is within the recommended profile. Melt index selection must therefore match screw recovery capacity; a high-flow grade below 18 g/10 min may be insufficient, while a grade above 25 g/10 min can cause metering instability if the screw is not designed for low-viscosity thermoplastics.

For a 0.65 mm wall connector housing with a flow path length of 120 mm, a melt index below 12 g/10 min at 220 °C/10 kg generally pushes fill pressure above 100 MPa in a single-cavity cold-runner tool. Pre-drying is mandatory because ABS resins at high melt index tend to develop splay at moisture contents above 0.05 %; 80 °C for 2–4 h in a desiccant dryer is typical. Barrel settings should remain between 230 °C and 250 °C, and mould temperature should be 40–60 °C to delay gate freeze without sacrificing weld-line strength. With a 1.0 mm pin gate, a high-flow ABS at 22 g/10 min can fill the part at 85–95 MPa, whereas a 10 g/10 min grade may require 110–125 MPa and still produce short shots at the last rib. The gate diameter should not be below 0.8 mm for high-flow ABS when the wall is under 1.0 mm; smaller gates freeze before packing and generate sink despite sufficient injection pressure.

When Wall Stock Exceeds 3 mm, Melt Index Must Move Downward

At wall thickness above 3 mm, cavity filling pressure decreases but cooling time and volumetric shrinkage dominate. High-flow ABS grades with melt index above 20 g/10 min have lower SAN molecular weight, which reduces tensile elongation at yield and notched impact. A thick section does not require high melt index, so moving to 6–12 g/10 min under ISO 1133-1:2022 preserves impact and long-term stress-crack resistance. General-purpose thick-wall ABS is often specified at 6–10 g/10 min at 220 °C/10 kg, with notched Izod impact per ISO 180/A higher than thin-wall high-flow grades. However, lowering melt index below 5 g/10 min without raising melt temperature may create filling defects in long flow paths or textured surfaces.

The governing issue in thick sections is not short shot but packing and gate freeze. With a wall thickness above 4 mm, pack pressure must be held long enough to compensate for volumetric shrinkage; excessive melt index is associated with gate blush, jetting, and sink because the gate does not freeze quickly enough to prevent backflow. Mould temperature and pack pressure have a stronger influence on thick-wall quality than melt index once the mould fills, but melt index still sets the molecular weight of the fused melt and therefore the notched impact of the solidified part. Representative supplier datasheet ranges for unfilled ABS are summarised in Table 1; these values are not a substitute for grade-specific shear viscosity data.

Automotive interior trim panels with a 2.5 mm nominal wall typically require a melt index of 14 g/10 min at 220 °C/10 kg to balance grained-surface replication and impact retention. Textured surfaces demand higher melt temperature and fail to reproduce grain when the melt index is below 8 g/10 min or the mould temperature is below 30 °C. Vicat softening temperature per ISO 306/B50 is usually around 100 °C for general-purpose ABS and may be slightly lower for high-flow grades; published data for this specific configuration is limited when heat resistance above 105 °C is required. Heat-resistant ABS grades usually require a higher SAN molecular weight, so their melt index is intentionally lower, and thin-wall filling must rely on elevated melt temperature rather than higher melt index.

Hot Runner Pressure Drop and Gate Freeze Constraints

In a hot-runner tool, manifold pressure drop scales inversely with melt index but also depends strongly on runner diameter. Increasing ABS melt index from 14 g/10 min to 22 g/10 min reduces manifold pressure drop by roughly 15–25 % in a balanced 8-drop system with 6 mm runner bores, according to hot-runner supplier pressure drop charts for amorphous thermoplastics. The gate remains the limiting feature. For a 0.9 mm wall, an open gate of 0.8 mm diameter freezes in less than 0.5 s with melt at 240 °C; injection velocity must therefore exceed 150 mm/s. High melt index alone cannot compensate for gate freeze because the melt must enter the cavity before the gate solidifies. Valve-gate hot runners are preferred with high-flow ABS when wall thickness is below 1 mm and the flow path ratio exceeds 100:1.

Hot-runner manifolds with long residence zones also degrade high-flow ABS before visible yellowing occurs. Accumulation of low-volatile degradation products on valve-gate stems increases wear and gate sticking. The melt index selected for a hot-runner thin-wall tool should therefore be validated with a melt temperature profile that does not exceed 260 °C at the manifold and with a purge programme that clears degraded material after any interruption longer than 10 min.

Electronic enclosure moulding with wall thickness transitions from 1.2 mm to 0.9 mm often fails when the melt index is selected only for the nominal wall. A grade change from 18 g/10 min to 15 g/10 min at 220 °C/10 kg can increase short-shot rates at the end-of-fill ribs unless melt temperature is raised from 240 °C to 250 °C and injection velocity is increased. Thin ribs solidify before pack pressure acts; therefore the thinner section controls grade selection, not the thicker shell. This is a common production bottleneck on multi-cavity tools with 16 cavities and one cold runner.

Thermal Stability Limits Add a Ceiling to Melt Index Choice

General-purpose ABS starved in a screw at barrel temperatures above 270 °C for more than 5 min yellows and loses impact because the polybutadiene phase degrades. High-flow thin-wall grades are not exempt; their lower SAN molecular weight can accelerate chain scission under repeated heat histories. The melt index of regrind after three heat histories may increase by 10–15 % relative to virgin material because chain scission dominates, while notched Izod impact decreases. If regrind content exceeds 30 %, a melt index at the lower end of the thin-wall band may be preferred to keep the final mixture within the original grade specification.

Thermal stability also places a ceiling on how high the melt index can be pushed for a given part. A 25 g/10 min ABS may fill a 0.7 mm wall at lower pressure, but if the shot uses only 30 % of the barrel capacity, residence time rises and the melt degrades before the next shot. The result is brown streaks, weld-line weakness, and batch-to-batch variance in notched Izod values. Published data for this specific configuration is limited, but the failure mode is widely reported in ABS processing literature.

For a 4 mm thick equipment cover moulded in ABS, a melt index of 7 g/10 min at 220 °C/10 kg is usually sufficient if the flow path ratio is below 70:1. Mould temperature at 60 °C and pack pressure at 60 MPa minimize sink marks; cooling time scales with the square of wall thickness, so increasing from 2 mm to 4 mm raises conductive cooling time by roughly 4×. Thick-wall ABS should not be selected from thin-wall high-flow grades merely because inventory is available; the lower molecular weight and reduced rubber content create lower impact retention and higher visible sink at the same packing pressure. High-flow ABS above 25 g/10 min is generally incompatible with solvent bonding and chemical welding because lower molecular weight reduces stress-crack resistance; published data for this specific configuration is limited.