Sink Marks and Short Shots in HDPE: A Moulder's Checklist

On a production-scale injection moulding line processing high-density polyethylene with a melt mass-flow rate between 4 g/10 min and 20 g/10 min under ISO 1133-1:2022 conditions of 190 °C and 2.16 kg, sink marks and short shots are linked by the same pressure-limited flow and solidification sequence. HDPE is a semi-crystalline polyolefin with solid density reported under ASTM D792 from 0.940 g/cm³ to 0.970 g/cm³, and its crystallisation from melt to solid produces linear mould shrinkage values measured under ISO 294-4:2018 commonly between 0.015 mm/mm and 0.040 mm/mm for unfilled grades. Sink marks arise where local wall thickness increases and the frozen gate blocks compensating melt flow before crystallisation shrinkage ends. Short shots arise where the melt front solidifies before reaching the cavity boundary. The same HDPE grade can exhibit both defect classes in a single multi-cavity tool when runner imbalance, venting restriction or a worn non-return valve narrows the processing window.

In a 16-cavity cold-runner tool producing cylindrical closures with main wall 1.2 mm and rib bases 2.0 mm, sink marks appeared on the outer surface opposite the rib when the screw cushion fell below 2.0 mm, while short shots occurred in the last cavity when injection velocity was reduced by 15 mm/s due to a pressure-limiting setpoint of 120 bar. The defect distribution was therefore interpreted as a machine stability issue first and a material limitation second.

What Distinguishes Sink Marks from Voids and Short Shots in HDPE?

Sink marks are visible surface depressions formed when the solidified skin of a thick section collapses inward under atmospheric pressure as the core material shrinks. A void is the same internal volumetric deficit formed when the outer layers are stiff enough to resist collapse; the defect is not visible but can reduce local mechanical strength. A short shot is not a shrinkage event but a flow-length or venting event: the melt front cools below its no-flow temperature before cavity packing can begin. Quantitative acceptance thresholds for visible HDPE surfaces are often stated as sink depth not exceeding 0.010 mm to 0.020 mm on high-gloss regions, but published data for this specific configuration is limited; texture depth, pigment and surface gloss can change visual detection by more than the measured depth value. Dimensional measurements of sink depth can be made with a dial indicator positioned over the rib for trial parts; repeatability across 3 cycles should be within ±0.005 mm if the process is stable.

The distinction between a sink mark and a void is therefore a skin stiffness condition: a sink mark forms when the surface layer buckles inward, while a void forms when the surface layer resists deformation. If a void is detected by sectioning or X-ray computed tomography, the corrective action is not simply to increase packing pressure, because increasing packing pressure after the skin has already consolidated may chase the void to a different location. A short shot is differentiated by the presence of an incomplete part edge, a frozen flow front, or low part weight; the part weight from a short shot is typically lower than the nominal shot weight by the mass of the missing cavity volume.

A worn non-return valve on a hydraulic injection unit with screw diameter 35 mm may show cushion variation from 4.0 mm to 0.5 mm over 10 cycles; that variation produces intermittent sink marks because some shots lose packing before gate freeze. The checklist therefore begins with screw cushion stability, non-return valve response and barrel temperature zone balance. HDPE does not require desiccant drying for typical closed packaging; surface condensation may occur at hopper inlet when pellets stored below 10 °C are charged into a room at 25 °C and 70 % relative humidity. In such cases, a hopper cover or room-conditioning adjustment avoids surface splay and intermittent short shots from condensate entering the melt stream.

Melt temperature at the nozzle measured with a needle pyrometer should be within ±5 °C of the setpoint before flow adjustments are made. If the nozzle melt temperature is below 190 °C, short shots may appear because viscosity rises rapidly with decreasing temperature; if the melt temperature exceeds 230 °C, thermal degradation can shift the molecular weight distribution and alter both MFR and sink-mark behaviour. The injection unit should be purged after a material change, and the screw recovery speed should be set so that the screw completes rotation within 60 % to 80 % of the cooling time to avoid melt temperature variation from shear heating.

Gate Freeze-Off, Packing Decay and Crystallization Shrinkage

Gate freeze time is the limiting packing variable for sink marks in HDPE cold-runner tools. A circular edge gate of diameter 1.0 mm can freeze in 3 s to 5 s at melt temperature 210 °C and mould temperature 25 °C, while the crystallisation shrinkage of a 3.0 mm boss continues beyond 10 s. Packing pressure must therefore act before the gate solidifies; after freeze, no additional flow enters the cavity. Holding pressure decay from 70 MPa to 30 MPa before gate freeze can restore some cushion but reduces the internal cavity pressure below the level required to suppress surface collapse. Semi-crystalline HDPE has a sharp specific-volume change during crystallisation; packing time should extend past the crystallisation plateau, not merely to the point of visible gate freeze.

On machines with hydraulic pressure-controlled pack, transition from injection to packing should be set to occur at 95 % to 98 % of the cavity fill volume, not by timer alone, to avoid a pressure drop during the switchover. Field data from polyolefin moulding suggests that cavity pressure at gate freeze below 20 MPa is associated with visible sink marks, while values above 30 MPa generally suppress them. These values are material- and geometry-dependent and should be verified with cavity pressure sensors. If the non-return valve leaks, the screw cushion decays and the effective packing pressure is lower than the setpoint; replacing the non-return valve can reduce sink depth without changing the process parameters.

When Melt Temperature and Injection Velocity Fail Flow-Length Requirements

When the flow-to-thickness ratio of a HDPE part exceeds 120:1 for a standard moulding grade with MFR 8 g/10 min, short shots become likely unless the melt temperature is raised into the upper processing range from 210 °C to 230 °C and injection velocity is increased from 60 mm/s to 100 mm/s or more. HDPE has thermal conductivity of approximately 0.40 W/m·K to 0.50 W/m·K, which is higher than amorphous thermoplastics; high-flow grades with MFR above 20 g/10 min can fill thin sections, but the same grades may exhibit lower tensile modulus and greater post-mould shrinkage under ISO 527-2:2012.

If the short shot occurs at the end of the flow path despite sufficient melt temperature, the venting system should be inspected before increasing injection speed further, because trapped gas can create a false pressure rise and stall the flow front. Vent depths for HDPE are commonly specified between 0.010 mm and 0.030 mm with land lengths 1.0 mm to 2.0 mm; depths above 0.030 mm can flash on low-viscosity HDPE grades. For a uniform wall thickness of 1.0 mm, a standard HDPE with MFR 12 g/10 min can fill a flow length of approximately 120 mm to 150 mm at 210 °C; increasing wall thickness to 1.5 mm raises the fill length before freeze. These values are geometry-dependent and should be confirmed by pilot tool trials.

A Moulder's Checklist for Defect Resolution on the Shop Floor

The following checklist sequences the adjustments from machine stability to mould modification. The order is deliberate: moving flow parameters before stabilising the non-return valve can hide the defect while increasing part weight variation.

When a defect appears in a multi-cavity tool, the first step is to compare shot weight from each cavity; a cavity-to-cavity weight range above 0.05 g indicates runner imbalance and requires runner adjustment or valve-gate timing changes. Process capability indices such as Cp and Cpk should be calculated from at least 25 consecutive shots if available; a Cpk below 1.33 on cushion or shot weight indicates that the process cannot hold the defect control window.

Why Venting Depth, Land Length and Gate Size Shift the Processing Window

Venting failure is frequently misdiagnosed as short shot from low injection pressure. If the pressure at transfer is 80 MPa but the cavity does not fill, the vent may be blocked by carbonised resin or insufficient depth. For HDPE, vent land length reduction from 2.0 mm to 1.0 mm improves air evacuation without increasing flash risk as long as vent depth remains below 0.030 mm. Sub-gate diameter below 0.8 mm can freeze before packing; increasing to 1.5 mm or changing to an edge gate with thickness 60 % to 80 % of the nominal wall reduces sink marks. In an 8-cavity mould with naturally balanced runner, cavity-to-cavity fill imbalance of 0.10 g to 0.15 g was traced to vent land wear. The current tool-maintenance interval for vent cleaning on polyolefin moulds is often 50 000 to 100 000 cycles.

Shear rate in gates should be kept below 10 000 s^-1 for HDPE to avoid melt fracture and gate blush; injection pressure drop across the gate can be calculated from capillary rheometry data under ISO 11443:2021. A gate located into a thick section allows packing pressure to reach the sink-prone region, whereas a gate on a thin wall adjacent to a thick boss may freeze before the boss has received sufficient packing. Gate relocation is therefore the final corrective action after machine and process settings have been verified stable.

Laboratory correlation of sink depth with mould shrinkage measurements under ISO 294-4:2018 should use plaques moulded at the same processing conditions as the production cavity. A round-robin comparison between the production process and laboratory plaque conditions can separate material shrinkage from tool-related flow restriction. The following standards are relevant for HDPE moulding qualification.

The effect of regrind addition on MFR and shrinkage should be verified by testing granulated HDPE at 100 %, 30 %, and 0 % addition; changes in MFR above 1 g/10 min may shift the short-shot threshold. Published data for this specific configuration is limited, but batch-to-batch variation in MFR and pellet size can exceed the range needed to maintain a fixed process window.