Acrylonitrile butadiene styrene (ABS) is a two-phase amorphous terpolymer classified under ISO 2580-1 and ASTM D4673. The styrene-acrylonitrile matrix exhibits a glass transition near 105 °C; the dispersed polybutadiene rubber phase has a glass transition near -80 °C. In injection moulding, First-Shot ABS Moulding Failure: Reading Short Shots, Bubbles and Sink Marks begins with the recognition that these defects occur in different phases of the moulding cycle. Short shots are a filling-phase event, bubbles are a plasticating or venting event, and sink marks are a packing and cooling event. Typical melt temperatures for ABS are 220–260 °C, mould temperatures 40–80 °C, injection pressures 70–140 MPa, and residual moisture below 0.1 wt%. These values are measured according to ISO 294-1 for specimen moulding and ISO 1133-1:2022 for melt flow rate. First-shot instability is commonly caused by cold steel, unsaturated melt cushion, accumulated moisture, and residual gas in the hot runner or nozzle after a material change or weekend shutdown. The diagnostic distinction is not cosmetic; it determines whether the correction belongs to the injection unit, the mould venting, or the packing profile.

Why Does the First Shot Freeze Before the Impression Fills?

Short shots in first-shot ABS moulding are normally the result of flow-front arrest at the thinnest rib, boss, or end-of-fill location. The defect appears as an incomplete part with rounded edges and no burn mark, indicating that the melt cooled below its practical flow limit before the cavity filled. ABS is not a crystalline material, so there is no sharp solidification temperature; the flow front stops when the apparent viscosity rises sharply as the melt loses heat to the mould wall. At 240 °C and a shear rate of 1000 s⁻¹, capillary rheometry according to ISO 11443 typically reports apparent viscosity between 90 Pa·s and 200 Pa·s, but a first-shot mould temperature below 40 °C can double wall-frozen-layer growth compared with a stabilised mould at 60 °C. On a reciprocating screw with an L/D of 20:1, the first shot after start-up may also have an unsaturated melt cushion; a cushion below 2 mm prevents the screw from transmitting pressure to the flow front. Injection velocity should be set between 50 mm/s and 150 mm/s. Velocities below 50 mm/s extend fill time and allow gate freeze before fill completion, while velocities above 150 mm/s can cause jetting and gas entrapment. Gate geometry is a first-order variable: an edge gate thickness of 50–75% of the nominal wall is typical; a gate below 0.75 mm on a 2.0 mm wall can seal before packing. Cavity pressure transducers located near the last fill point should show a rapid pressure rise at fill completion; a maximum cavity pressure below 20–30 MPa suggests the gate or flow front froze before transfer. The corrective sequence is to raise the melt temperature to 230–250 °C, raise the mould temperature to 60 °C, increase injection velocity to 80–120 mm/s, and verify that the screw cushion remains at 3–8 mm. The melt temperature must not exceed 260 °C for residence times beyond 5 minutes, because the butadiene phase degrades and generates volatile species that convert a short shot into a bubble defect.

Bubble formation in first-shot ABS cannot be attributed to a single source without examining the void geometry, location, and surrounding streaks. Moisture splay appears as elliptical voids or silver streaks, often spread across the surface; it occurs when pellet moisture exceeds 0.1 wt%. ABS can absorb 0.2–0.5 wt% water after 24 h at 23 °C and 50% RH when measured by ISO 62 or ASTM D570. A desiccant dryer with a dew point of -40 °C or lower should be operated at 80–90 °C for 2–4 h; residual moisture is verified by Karl Fischer titration according to ISO 15512:2019. Trapped air produces spherical voids, typically at blind pockets, sharp transitions, or the last area to fill. Vent depth for ABS in production tooling is normally 0.02–0.04 mm, with a land length of 0.5–1.5 mm; insufficient vent depth or dirty vents raise the local gas pressure and can leave a bubble at the end of fill. Degradation gas is distinguished by yellowing, brown streaks, or a sharp acrid note; it arises from overheated nozzle bodies, screw speeds above 120 rpm, or back pressures below 0.2 MPa that allow air to be dragged into the melt. Screw speed for ABS should be maintained between 60 rpm and 100 rpm, with back pressure between 0.3 MPa and 0.7 MPa. On a first shot after material change, the previous polymer may remain in the check ring or hot runner; purging with 5–10 shots at the target melt temperature removes incompatible residue before diagnostics are trusted.

If a first-shot short shot appears only after a grade change, the melt flow rate and rubber content of the new grade should be compared with the previous grade before the tool is adjusted. High-flow ABS grades with a melt volume-flow rate above 20 cm³/10 min at 220 °C and 10 kg according to ISO 1133-1:2022 reduce short-shot pressure demand but may increase sink mark depth because lower molecular weight reduces the modulus of the packing phase. Low-gloss or high-heat ABS grades with α-methylstyrene comonomer may require melt temperatures near the upper limit of 260 °C. The butadiene rubber phase content, typically 5–30 wt%, affects impact resistance and melt elasticity; higher rubber content increases viscosity and may shift the apparent viscosity at 240 °C and 1000 s⁻¹ toward the upper end of the 90–200 Pa·s range. The injection pressure required for a given cavity geometry scales with apparent viscosity; a grade change from 10 cm³/10 min to 5 cm³/10 min may require a 10–20 °C increase in melt temperature, not just higher injection velocity.

Sink Mark Formation Kinetics in Unfilled ABS Rib-Boss Junctions

In unfilled ABS, sink marks are surface depressions above thermally massive features such as ribs, bosses, and screw bosses. The mechanism is differential volumetric shrinkage. ABS moulding compounds exhibit a nominal mould shrinkage of 0.4–0.9% when measured according to ISO 294-4. In a rib-boss junction, the thick core cools more slowly than the adjacent skin; if the gate seals before the local cavity pressure compensates for specific-volume loss, the surface collapses. The standard design limit is a rib root thickness of 50–60% of the adjacent wall and a boss wall thickness of 60% of nominal; values above 70% routinely produce visible sink even with high hold pressure. On a 2.0 mm nominal wall, this means the rib root should be 1.0–1.2 mm. Process-side correction requires hold pressure between 50 MPa and 80 MPa and hold time sufficient to achieve gate seal. Gate seal is verified by cavity pressure decay after the hold phase; if pressure decays immediately after transfer, the gate has frozen before packing was completed. Increasing hold pressure after gate seal does not reduce sink mark depth. For a first-shot ABS failure, the tool is usually below 60 °C, and the melt cushion may be too small to transmit hold pressure. The machine should maintain a cushion of 3–8 mm. Quantitative sink depth is measured with a contact profilometer according to ISO 4287 or an optical surface comparator. A depression depth above 0.01 mm on a high-gloss black ABS surface is visible under factory lighting. Texture depth on a grained surface can mask 0.02–0.03 mm of sink, but it does not remove the underlying density reduction. If sink marks persist after these corrections, the moulding is violating the rib-thickness rule and must be cored out or foamed; published data for chemical foaming of ABS in thin-wall cosmetic parts is limited and requires tooling trials.

Measured cavity pressure and screw position traces from instrumented moulding cells separate short shots, bubbles, and sink marks before the part is ejected. A piezoelectric pressure sensor positioned near the last fill point records the filling pressure rise. A short shot signature shows a pressure plateau or decay without reaching the setpoint; the screw position at transfer is below the fill volume. A bubble signature may show normal fill pressure but pressure instability during plasticating, often caused by wet pellets or air entrainment in the melt. A sink mark signature shows an acceptable fill peak but rapid pressure decay after velocity-to-pressure transfer, indicating gate seal before packing. During plasticating, the injection unit load-cell signal shows pressure oscillations if the screw is dragging air or if wet pellets are flashing to steam. At 80 rpm, the melt pressure signal may oscillate at 2–4 Hz. A cavity pressure sensor near the last fill point records a short shot as a pressure trace that fails to reach the transfer setpoint, while a bubble from trapped air may record a small pressure spike as the air pocket compresses but does not dissolve. The screw position channel records a short fill volume when the screw stops before the transfer position. These traces should be archived for first-shot start-up comparison against the tenth-shot baseline; a baseline drift in the fill peak of more than 5 MPa often indicates a worn check ring or a blocked vent. Table 1 summarises the defect differentiation. Table 2 gives representative operating ranges and measurement methods for first-shot ABS moulding trials.

When Volatile Load and Melt Cushion Offset the Packing Phase

First-shot ABS failures are often mixed because volatile load and cushion decay interact. A moisture level above 0.1 wt% generates steam during plasticating; the steam displaces melt pressure in the screw channel and creates bubbles, but it also reduces the density of the melt stream and can produce local underpacking that appears as a sink mark. On a 1200 kN toggle clamp line moulding a grained ABS interior trim component with a 2.0 mm nominal wall, a 1.0 mm rib root, and a cold runner, the first-shot failure signature is commonly a short shot in a thin rib followed by a bubble at the last-fill point. The mechanical correction is to disable automatic start-up until the mould has reached 60 °C and the melt cushion is stable at 5 mm. The process is then set to a melt temperature of 240 °C, injection velocity of 100 mm/s, hold pressure of 70 MPa, hold time to gate seal, back pressure of 0.5 MPa, and screw speed of 80 rpm. Residual moisture is verified below 0.1 wt% before restart. If the bubble persists at the last-fill point, the vent depth is checked and raised only within the 0.02–0.04 mm range. If the sink mark persists above the boss after gate seal, the boss wall is cored to 60% of nominal rather than increasing hold pressure further.