Short Shots and Weld Lines in ABS: When Flow Length Runs Out is a cavity-filling boundary condition, not a single material parameter. Acrylonitrile Butadiene Styrene (ABS) injection grades exhibit a practical flow length limit governed by melt temperature, mold wall temperature, local wall thickness, and the pressure drop between the screw tip and the last point of fill. Published processing guides for medium-flow ABS injection grades report a flow-length-to-wall-thickness ratio of roughly 100:1 to 150:1 at injection pressures between 80 MPa and 120 MPa. These ratios apply only when the melt temperature is maintained between 210 °C and 250 °C, the mold surface temperature is above 40 °C, and the cavity is adequately vented. If the flow front freezes before the cavity fills, the result is a short shot; if two flow fronts converge but the frozen skin layers do not interdiffuse, the result is a weld line. In multi-cavity tools, the cavity with the longest flow length from the sprue usually fails first, while shorter cavities may pack normally. The diagnostic question is therefore not whether ABS can fill a specific geometry, but where the flow length runs out under the available pressure and thermal conditions.

The pressure at the gate must exceed the integrated pressure loss along the sprue, runner, and cavity. In cold-runner tools for ABS, a standard round runner diameter of 4 mm to 6 mm for a 2 mm wall is often necessary because the runner freezes at nearly the same rate as the cavity. A subgated or tunnel gate with a diameter below 1.0 mm can restrict flow and become the first freeze point. When the gate freezes before the packing phase, the cavity may be visually full but the weld line remains weak because additional melt cannot be packed into the converging front region.

Why Does the Melt Front Freeze Before the End of Fill?

Short shot formation in ABS is both a thermal and a rheological failure. Melt viscosity is non-Newtonian and shear thinning; however, the available shear heating is smaller than the heat lost to the mold steel. Capillary rheometry data for injection-molding ABS tested per ISO 11443:2021 show apparent viscosity at 230 °C and 1000 s⁻¹ typically between 80 Pa·s and 200 Pa·s for medium-flow grades. As the melt enters the cavity, a frozen skin grows from the mold wall. The effective channel thickness decreases, and the pressure gradient required to maintain a given volumetric flow rate rises steeply. For a simplified plane-channel Newtonian approximation with constant viscosity, the pressure gradient scales inversely with the cube of the effective thickness; a local thickness reduction of 25% more than doubles the required pressure for the same flow. ABS is amorphous, so there is no sharp crystallization temperature to restart flow; instead, flow ceases when the melt temperature approaches the no-flow temperature, which material suppliers typically report between 150 °C and 180 °C for standard grades. Mold temperatures below 40 °C can prematurely pinch off thin sections before the packing phase begins, especially when the wall thickness falls below 1.5 mm and the flow length exceeds 120 mm.

A short shot may also be velocity-limited. At low screw advance speeds, the ABS melt front advances slowly and loses heat. Raising injection velocity from 50 mm/s to 120 mm/s can increase shear heating and reduce short shot frequency, but above a material-dependent threshold, the pressure demand can exceed the machine hydraulic limit or produce jetting. The hydraulic pressure at the screw tip is usually lower than the displayed injection pressure because of non-return valve losses and runner pressure drop.

A systematic short shot study on a new tool is performed by reducing shot size in increments of 5% and recording the position of the flow front. For ABS, the flow front should advance uniformly across each fill stage; if one area consistently stops at the same location while others continue, the cause is usually a wall thickness reduction, insufficient venting, or a gate that freezes early. The short shot series also reveals the true flow path and the last point of fill, which is the required location for a vent or an overflow tab.

Material preparation affects both short shot and weld line severity. ABS must be dried in a desiccant dryer with a dew point below −30 °C to keep residual moisture below 0.1% by weight. At temperatures above 80 °C or drying times beyond 4 h, some heat-stabilized grades may yellow; at moisture levels above 0.1%, steam generated at the melt front can collapse pressure and increase weld line depth. Moisture analysis per ISO 15512:2019 should be part of the short shot investigation when the molding floor humidity exceeds 60% RH.

Weld Line Tensile Failure in ABS Versus Unwelded Stock

Weld-line strength loss in ABS is driven by the fountain-flow orientation of two advancing fronts. The outer skin layers are highly oriented and are carried to the weld zone, where they meet at an angle. The result is a V-shaped notch on the cosmetic surface and a boundary region where interdiffusion is incomplete. Tensile bars molded with a single edge gate and with two opposing edge gates can be compared per ISO 527-2 or ASTM D638-14. Specimen preparation should follow ISO 294-1 so that the flow direction and gate location are controlled. Published comparative data for ABS show weld-line tensile strength retention between 40% and 70% relative to unwelded stock, depending on melt temperature, mold temperature, and rubber content. Notched impact retention per ISO 179-1 or ASTM D256 is commonly lower, often in the 20% to 50% range, because the weld line creates a crack-like defect. High-impact ABS grades with higher polybutadiene content may show more severe retention losses because the rubber particles are oriented or depleted at the weld interface.

At mold temperatures from 40 °C to 60 °C, the frozen skin is thick and the weld line is deep. Raising the mold temperature to 80 °C improves interdiffusion and can raise tensile strength retention by 10 to 20 percentage points, but it increases cooling time and may cause sticking on textured surfaces. Melt temperature exerts the largest direct influence on weld-line strength. At melt temperatures below 220 °C, the viscosity is high and the fronts have less time to interdiffuse. At melt temperatures above 260 °C, degradation of the butadiene phase may generate free radicals that crosslink the rubber and produce brown streaks. The practical operating window for most ABS grades is 230 °C to 250 °C at the nozzle, with the hot runner manifold kept within ±5 °C of the nozzle set point.

When Gate Placement Relocates a Weld Line

Weld line position is determined by melt front convergence and can be moved by changing gate location, runner balance, or wall thickness. A single gate may create one weld line where the flow splits around a core; a second gate creates a weld line where the two fronts meet. Relocating a gate from the geometric center to an edge moves the weld line from a structural boss to a low-stress side wall. Sequential valve gating eliminates multiple fronts by opening hot runner drops in sequence, replacing the weld line with a single flow boundary. Flow leaders and deflectors can accelerate or delay a front, but they alter pressure drop and may shift the short shot location. Mold-filling simulation tools compute weld line location and temperature using the Cross-WLF viscosity model and the no-flow temperature; however, the predicted weld-line position is sensitive to the assigned no-flow temperature and to mesh resolution at the meeting angle.

An overflow tab placed at the end of a weld line moves the notch outside the component; the tab is later trimmed. This method is effective when the weld line would otherwise intersect a mounting hole or a sharp internal corner. For ABS, the tab should be located so that the weld line exits through the tab rather than ending at a side wall.

Venting at the last point of fill is mandatory. Supplier mold design guides for amorphous materials recommend vent depth of 0.025 mm to 0.040 mm for ABS. If a vent is blocked or undersized, compressed air ahead of the melt front can stop filling or force the fronts together under gas pressure, producing a short shot that resembles an electrical discharge machine burn mark. On polished P20 cavity steel, vents should be located along the parting line at the outermost fill points and at thin section ends. Vacuum-assisted venting is applied only when the cavity cannot be vented normally; published comparative data for ABS vacuum level settings are limited.

Values are typical ranges compiled from supplier datasheets and published comparative studies; they are not single-grade specifications.

Short Shot Diagnosis on a 1300 kN Hydraulic Machine

Production-scale short shot analysis on a 1300 kN hydraulic toggle machine with a 40 mm screw and 20:1 L/D begins with the non-return valve. A worn check ring allows melt to slip backward during injection, producing stable barrel pressure but progressively lower shot weight. The cushion may fall below 2 mm before the screw can transfer to packing, and the cavity farthest from the sprue shows a rounded, incomplete edge. In a four-cavity tool with an H-type runner, cavity-to-cavity weight differences above 5% indicate runner imbalance, gate wear, or uneven venting rather than a machine fault. The screw forward time is often maintained by the controller, but the actual delivered volume is reduced because the check valve closes slowly. Replacing the check ring and seat restores shot weight if the screw diameter is not worn beyond the manufacturer’s limit.

Peak hydraulic pressure is not always a reliable indicator of short shot because the injection phase may end on position, not pressure. On a machine with closed-loop control, the transfer from velocity control to pressure control occurs when the screw reaches the V-P switchover position. If the cavity has not filled by that position, the injection pressure cannot recover; a late switchover can create a short shot even though the peak pressure set point is high. For ABS thin-wall parts, the V-P switchover is usually set at 90% to 98% of full shot volume, and the final cushion must be verified after three consecutive cycles.

Barrel residence time at 240 °C should not exceed 5 min for standard ABS. Longer residence reduces melt viscosity and may reduce short shots temporarily, but weld line strength falls because the styrene-acrylonitrile matrix degrades and the butadiene phase crosslinks. The short shot investigation should record screw forward time, transfer position, cushion, peak hydraulic pressure, and screw decompression; decompression settings above 5 mm can draw air into the barrel and produce a false short shot on the next cycle.

Flame-retardant ABS and laser-markable grades may require mold temperatures above 80 °C to maintain weld-line strength, but published data for specific formulations is limited. Glass-fiber-reinforced ABS compounds exhibit shorter flow lengths than unfilled grades because fibers increase viscosity and reduce the effective flow-length-to-thickness ratio. In hot runner systems, internal heater zones should be independently tuned; if the nozzle tip temperature falls below 220 °C, the cold slug at the gate can generate a short shot that mimics a cavity fill issue. When moving from a 2.5 mm wall to a 1.8 mm wall, the flow length may drop disproportionately, and weld line strength retention can fall below the unfilled ABS range cited above. Each tooling change should be verified with a short shot study, not with a single full-shot sample.