First-Shot POM Moulding Failed: Reading the Part and the Process

Polyoxymethylene (POM per ISO 1043-1) is a semicrystalline thermoplastic with a density of 1.41 g/cm³ to 1.42 g/cm³ when measured by ISO 1183-1. The crystalline melting peak determined by differential scanning calorimetry to ISO 11357-3 lies near 165 °C for copolymer grades and near 175 °C for homopolymer grades. Melt volume-flow rate for general-purpose injection grades is typically 8 cm³/10 min to 13 cm³/10 min at 190 °C under 2.16 kg load per ISO 1133-1. These values establish the material boundaries for reading a first-shot failure, because the first shot is not moulded under the same thermal and pressure conditions as the tenth or hundredth shot.

A first-shot POM moulding failed when the part is read as a record of start-up disturbances rather than as a stable material property. The observable failures—short shot, splay, gate streak, burn mark, cold slug at the gate—can be separated only by process signature. A short shot with a fully formed gate and a smooth flow front indicates pressure transmission loss in the injection unit. A short shot with a rough, bubbled flow front indicates gas or moisture. A first-shot-only splay pattern points to condensation or a cold nozzle tip, whereas persistent splay points to wet granulate. This distinction is required because changing dryer settings cannot correct mould condensation, and changing melt temperature cannot correct a non-return valve leak.

What Does the First Shot Reveal About Melt Cushion and Non-Return Valve Leakage?

On a first shot, the check ring may not seat if the melt in the ring groove has cooled or if the ring is held open by rotation after screw recovery. The result is that the screw moves forward to its set stroke but polymer flows backward across the ring instead of entering the cavity. The part may be short at the last-filled edge even when the machine reports full stroke. Cushion position after the first shot is a direct indicator. For unfilled POM processed on a 500 kN to 1,200 kN clamp-force machine, the cushion after shot should remain between 3 mm and 5 mm; a first-shot cushion below 2 mm means that shot volume is not controlled. The pressure-time trace on the injection ram shows a rounded peak and slow pressure build, and a cavity pressure sensor near the gate commonly does not exceed 30 MPa during filling when steady-state shots reach 40 MPa to 70 MPa. Published production records for this specific configuration are limited; therefore the 30 MPa first-shot boundary is an instrumented-tool interpretation, not an absolute material specification. Sectioning the short part at the last-filled edge and comparing wall thickness against a steady-shot part confirms incomplete packing.

Injection unit inspection should include the non-return ring seat, screw tip, and nozzle body. Ring-type non-return valves on general-purpose screws of 18:1 to 22:1 L/D are sensitive to first-shot temperature gradients. If the nozzle tip heater is 10 °C below the barrel set point, a cold plug can form at the tip and produce a similar first-shot short fill. The cold slug well in the runner should extend at least 1.5 times the runner diameter; for a 3 mm runner, the cold slug well is therefore 4.5 mm to 6 mm beyond the runner intersection. A first-shot part with a cold plug at the gate indicates the slug well was undersized or the nozzle tip was not heated to the set temperature.

Surface condensation on a cold cavity face produces first-shot-dominant splay. If the moulding cell is at 22 °C and 50% RH, the dew point is approximately 11 °C. A mould surface below that temperature will condense a water film before the first shot. When molten POM enters the cavity, this film flashes to steam and disrupts the flow front, creating silver streaks oriented from the gate. The streaks disappear as the tool surface rises above the dew point within the first 3–5 shots. Bulk granulate moisture does not disappear after a few shots unless the dryer is on-line. For POM injection grades, the moisture limit measured by Karl Fischer titration is generally 0.20 wt% per ISO 15512 Method A; drying at 80 °C for 3 h with a desiccant bed air dew point of -40 °C is sufficient for most unfilled grades. If material has been exposed to relative humidity above 60% for more than 1 h, surface moisture can appear in the first shot before the dryer reaches steady state.

When the First Shot Displays Splay but Subsequent Shots Are Sound

This condition is assigned to a transient thermal interface, not to a material moisture fault. The nozzle tip is the most common interface. If the nozzle body set point is 10 °C below the melt temperature set point, resin at the nozzle tip can solidify before the first shot. The following shot shears the solidified plug and delivers it to the gate as splay or a visible cold slug. A check of the nozzle tip surface with a contact thermocouple, followed by a purge shot before the first production shot, resolves the condition. It is also necessary to verify that the screw decompression after recovery is not excessive. A decompression of 2 mm to 5 mm of screw displacement is typical for POM to prevent drooling. Above 8 mm, air is drawn into the melt at the nozzle tip and becomes a first-shot splay source. The same first-shot-only splay can appear if the mould surface at the gate is below 60 °C, because the cold gate freezes the flow front and restricts molecular orientation near the part surface.

Gate and runner geometry also determine whether a first-shot defect becomes visible. For unfilled POM, a cold runner diameter should generally be at least 3 mm and at least 1.5 times the maximum part wall thickness; long runners require larger diameters or heated systems. If the runner pressure drop is marginal, the first shot becomes the indicator of an under-designed cold runner. A short shot that disappears after several shots is not automatically a material fault; it may be a cold-runner start-up effect that the process can mask but the runner cannot permanently tolerate.

Thermal Degradation Signature in First-Shot POM

When the first shot is rejected for brown or black streaks at the gate and a sharp odour of formaldehyde, the reading is thermal degradation in the barrel, hot runner, or nozzle, not ordinary surface contamination. Polyoxymethylene degrades primarily by thermal unzipping and oxidative attack; the rate accelerates above 230 °C and in oxygen-rich dead zones. In a hot runner manifold held at production temperature during a pause, POM can remain in the drop for the whole idle period. The first shot then acts as a purge of degraded, lower-molar-mass material. The part records this as dark streaking concentrated at the gate. If subsequent shots are visually clean, the start-up thermal history is still non-representative. A first-shot moulded tensile bar should not be used for conformance testing to ISO 527-2 because the heat-load history of the first shot is not equivalent to steady-state material. Start-up procedure should reduce hot runner and barrel zones to 10 °C to 20 °C below production set point during idle, and discard 3–5 shots after any idle period longer than 10 min.

Barrel temperature profile and heater band function must be verified with an immersion melt-temperature probe because viscous dissipation can raise the actual melt temperature 10 °C to 15 °C above the set point. If the first-shot colour shift disappears after 5–10 shots, the cause is start-up residence time. If the shift persists after 20 shots, the barrel set point, heater band calibration, or screw speed and back pressure should be checked. Hydraulic back pressure above 1.0 MPa can increase the melt temperature contribution of viscous heat and accelerate POM depolymerisation.

First-shot burn marks at the last point of fill are often misclassified as material degradation. On the first shot, the cavity is cold, the vents may be partially coated with mould protection oil, and the air path has not stabilised. The melt compresses the trapped gas ahead of the flow front, raising gas temperature and oxidising the polymer surface to brown or black deposits at the end-of-fill. For POM, parting-line vent depth is typically 0.015 mm to 0.025 mm, with land length 1 mm to 2 mm and vent width 5 mm to 10 mm. If the first-shot burn occurs only at the last-filled edge and the vent is stained with oil or metallic powder, the tool condition is the dominant variable. A change in melt temperature cannot correct a mechanical vent obstruction and may introduce thermal degradation without eliminating the burn mark. Cleaning of vents and verification of clamp tonnage against projected area should be completed before changing POM process set points.

Process Boundaries for a Repeatable First Shot When the Mould Is Cold

For the first shot to be acceptable, the start-up settings must compensate for the cold tool without overcompensating the melt temperature. If the mould surface is at 60 °C and the melt temperature is 200 °C, a thin-walled section of 1.5 mm should be filled rapidly to prevent the flow front from freezing before pack. Injection speed should be set to fill the cavity within 0.5 s to 1.5 s; slower filling produces visible flow marks and a loss of tensile elongation measured by ISO 527-2. The first shot should not be used to establish process capability because the cavity surface has not reached thermal equilibrium. The process parameters can be qualified only after the mould surface temperature has been verified at the gate, mid-cavity, and last-filled edge. A mould temperature controller set to 80 °C does not guarantee that all cavity surfaces are above 60 °C in the first shot; the heat transfer lag through tool steel must be measured with surface thermocouples or infrared pyrometry.

Jetting is a separate first-shot risk if the melt enters a cold cavity without contacting a runner wall. The gate land length for unfilled POM is generally 0.5 mm to 1.0 mm. A gate that is too short can create a high-velocity melt front that folds behind itself and produces low-strength weld lines in the first shot. If the part shows a sinuous strand near the gate and glassy surface ripples, the first-shot injection speed was too high for the gate dimensions. The fix is to adjust the gate land or to reduce first-shot fill speed until the flow front advances as a stable radial front; this is confirmed by short-shot progression studies performed at 5–10% increments of shot volume.

After a first-shot failure, the mould, nozzle, and barrel must be checked for residues and cross-contamination. A degraded first shot can deposit a thin film of formaldehyde-derived material in the gate or vent channel, changing the apparent vent depth and gate roughness. A neutral pH aqueous cleaner followed by dry air is appropriate. Halogenated cleaning agents must be fully evaporated before POM is moulded because residual chlorinated species can catalyse acid hydrolysis of polyoxymethylene at processing temperatures. POM is also incompatible with strong bases and oxidising agents. If a previous resin was processed in the same barrel, a dedicated POM purge or a low-melt-index polyethylene purge is required before the first production shot. A first shot after material change is not suitable for melt flow rate testing by ISO 1133-1 because it may contain a mixture of purge polymer and partially degraded POM.