Processing
troubleshooting guide

What you see

A fine notch - or scratch-like line where two advancing melt fronts have met and fused. It shows up most on dark, transparent or highly polished surfaces, on glass-fibre reinforced grades, and in materials carrying effect pigments. Where the fronts meet too cold and at too low a pressure, the chilled skin no longer presses against the cavity wall and leaves a visible groove. The same cold-front condition also reduces local mechanical strength, because polymer chains and fibres do not bridge the joint.

Why it happens

• Two or more melt fronts converge - around holes, cores and inserts, or where feeds arrive from opposite sides
• Differences in flow length, or a change in wall thickness, steer the fronts together in a visible area
• Temperature and pressure at the meeting point are too low for the fronts to properly re-fuse
• Air trapped at the convergence point prevents the fronts from pressing together

What you see

Surface layers peel or flake away as thin, detachable skins. The bond between polymer layers has failed, normally because of excessive shear in a relatively cold melt combined with an aggressively cooled mould. In semi-crystalline grades this produces layers with different crystal structures; in amorphous grades it shows as de-mixing of the melt / additive / pigment blend. Contamination with an incompatible polymer produces an identical-looking result, so rule that out first. Most often seen on unreinforced semi-crystalline materials.

Why it happens

• Excessive shear during cavity filling
• Machine and possibly the hot runner contaminated with foreign material
• An incompatible masterbatch - wrong carrier polymer for the base resin
• Air drawn into the melt during metering

What you see

Localised black discolouration at the end of the flow path, at weld lines, or at the top of ribs - often with the part not completely formed at that point. The advancing melt compresses the air ahead of it; if that air cannot escape it is compressed adiabatically and ignites, exactly as in a diesel cylinder, charring the polymer. Look at the tool as well as the part: the same locations frequently show plate-out or corrosion on the mould surface.

Why it happens

• Injection speed too high - the air has no time to escape
• Venting channels contaminated or clogged by plate-out
• Air cannot escape through the parting surface of the mould
• Mould insufficiently vented in that region
• Several melt fronts converge and seal off a pocket of air

What you see

Dark inclusions on the surface, or - in transparent grades - suspended through the wall. There are two distinct origins and they need different fixes: foreign contamination arriving with the granules or through the conveying system, and the polymer's own thermal degradation products breaking loose from dead spots in the melt path. If specks appear after a standstill or a colour change, suspect the second.

Why it happens

• Material contaminated with foreign matter
• Melt temperature too high
• Material degrading in dead spots in the plasticising unit and/or hot runner
• Metallic particles breaking loose from a worn plasticising unit or mould
• Residence time of the melt too long in the barrel and/or hot runner
• Heating and cooling times too long, or extended machine standstills

What you see

Local depressions in the surface, almost always directly opposite a rib, boss or other increase in wall section. The thick section shrinks more in volume as it cools and pulls the skin inward. They sometimes appear only after ejection, when the still-molten core reheats the frozen skin and lets it yield. On some parts they are subtle enough that you notice only a difference in gloss rather than an obvious dip - check under raking light.

Why it happens

• Volume contraction during cooling is not compensated by holding pressure
• Part design unsuited to the material - wall-thickness increases, abrupt changes along the flow path
• No melt cushion remaining at the end of holding
• Large pressure loss in the machine nozzle and/or gating system
• Part gated into a thin wall, so the gate freezes before the thick section is packed

What you see

The part is deformed by ejection forces that are too high or badly directed. Ejectors leave distinct marks - indentations, raised bosses, gloss differences or whitish discolouration (stress whitening). In the worst cases the pins puncture the part, or it cracks outright. A sprue that stays in the cavity plate belongs to the same family of problems.

Why it happens

• Undercuts or scratches on the part and/or gating system
• Too few ejector pins, badly positioned, or with too little contact area
• Draft angles too small, or the surface texture depth not matched to the draft
• Cavity overpacked during filling because holding pressure is too high
• Excessive shrinkage gripping the core
• Cooling time too short - the part is still soft when the ejectors move
• Actual mould temperature too low
• A vacuum forming between the part and the cavity surface as the mould opens

What you see

A snake-like, often rough or matt strand of material running from the gate across the surface. When melt passes from a small gate into a much larger cavity at high velocity with nothing to impinge on, it squirts in as a free jet, folds on itself and touches the wall intermittently. The surface of that strand has already cooled, so it never properly bonds to the melt that arrives behind it - which costs both appearance and local strength.

Why it happens

• Gate positioned so the emerging melt meets no obstacle
• Unsuitable gate design or too small a gate cross section
• Unfavourable filling conditions - velocity through the gate too high
• Part design unsuited - an abrupt wall-thickness change just past the gate

What you see

Different levels of gloss across a surface that was machined to a single finish. Gloss is simply how faithfully the frozen skin replicates the cavity. A polished tool reflects light directionally and reads glossy when replication is good; a textured tool scatters light and reads matt when replication is good. Anything that changes how the skin forms, or how hard it is pressed against the wall, shows up as a band of different gloss.

Why it happens

• Sudden wall-thickness changes at ribs or bosses - the resulting sink marks read as gloss differences
• Changed melt orientation and flow conditions in the area of weld lines
• Mould wall temperature, melt temperature, injection speed or switchover point poorly chosen
• Holding pressure level and holding time inadequate
• Insufficient melt homogeneity

What you see

A fine excess of material projecting from the part where the melt has forced its way into gaps in the mould's parting surfaces, slides or ejector clearances. Flash is a pressure-versus-clamp balance problem: either the cavity pressure is higher than it should be, the melt is thinner than it should be, or the tool is not holding shut. Work out which of the three before reaching for the clamp tonnage.

Why it happens

• Clamping force set too low, or the machine is simply undersized for the projected area
• Fit tolerance between the mould halves, or of slides in the mould, too great
• Sealing surfaces in the parting line damaged
• Cavity pressure too high during the filling and holding phases
• Melt viscosity too low - melt temperature too high, or degradation from excessive residence time or wet material
• Mould deflecting under filling pressure

What you see

Material that solidified in the machine nozzle or hot runner before the mould filled, then got injected with the following shot. On thin-walled or transparent parts it reads as a comet-tail marking near the gate or as locally concentrated streaks. A slug that does not re-melt can obstruct the flow channel enough to split the melt and create a weld line, and in extreme cases it blocks the gate entirely and leaves the cavity short. Because it never fuses properly with the surrounding melt, local mechanical properties suffer too.

Why it happens

• Machine nozzle temperature too low; nozzle unheated or heater-band output insufficient
• Material drooling from the nozzle or hot runner, cooling, then being injected on the next shot
• The nozzle bore has no draft, so the frozen skin is dragged in rather than held
• The tip of a tunnel gate breaks off during ejection and stays in the cavity for the next shot

What you see

The appearance depends entirely on the mechanism. If the melt flows around a pocket of air, you get anything from a short shot to surface blemishes to full dieseling. If air is folded into the melt, you get bubbles - fine to coarse, often revealing the flow direction. The diagnostic that separates this from voids: entrapped-air bubbles can sit close to the outer wall, whereas voids only ever form in the molten core.

Why it happens

• Part design and gating create a filling pattern that surrounds and seals off air
• Screw decompression too long or too fast after plasticising, drawing air in ahead of the screw or into the hot runner
• Air drawn in through the feed section during plasticising - unsuitable settings or the wrong screw for the material

What you see

Hollow spaces - microcells or bubbles - formed inside the wall during or after cooling. Voids always sit in the molten core, normally mid-wall and in thick sections, which is what distinguishes them from gas bubbles. A quick field test: drill into the part while holding it under water. Water drawn in indicates a void under vacuum; bubbles escaping indicate gas. Voids share their root cause with sink marks - a void forms instead of a sink mark when the skin has already frozen hard enough to resist the contraction pulling on it from inside.

Why it happens

• Volume contraction during cooling not sufficiently compensated by holding pressure
• Part design unsuited - material accumulations, sudden wall-thickness changes along the flow path
• No melt cushion
• Large pressure loss in the machine nozzle and/or gating system
• Part gated into a thin wall

What you see

Halo-like matt patches, most often forming a ring around the gate, sometimes appearing just downstream of a cross-section change. Two things cause it: high local shear stress at these points, and reduced adhesion of the melt to the cavity wall as it negotiates the transition. Both leave the skin unable to replicate the tool finish.

Why it happens

• Injection speed too high
• Gate not optimally configured - too small, or with sharp transitions
• Melt temperature too low
• An abrupt cross-section transition in the part
• Part gated at an unsuitable place

What you see

Markings running perpendicular to the flow direction, forming a concentric or parallel fluted structure like the grooves of a vinyl record. The convex melt front cools too much at a reduced flow rate - possibly only intermittently. The frozen skin thickens until it reaches parts of the front that have not yet touched the wall, and those regions can no longer be pressed flat against the cavity even as filling continues and pressure rises.

Why it happens

• Mould temperature, melt temperature and injection speed all set too low
• Part design unsuited - thicker walls at the end of the flow path, or a sudden wall-thickness change
• Large pressure loss in the gating system starving the front of pressure

What you see

Differences in colour across the surface - sometimes over a large area, near the gate or well away from it, frequently at weld lines and occasionally behind sharp edges. The pigment, whether from a compounded colour or a masterbatch, is unevenly distributed and has formed agglomerates. Note that the fix depends on whether you are colouring at the press or running pre-coloured compound - the two cases pull back pressure in opposite directions, so identify which you have before adjusting.

Why it happens

• Self-colouring: unfavourable processing conditions selected
• Self-colouring: plasticising unit unsuitable - inadequate dispersive action
• Pre-coloured compound: pigments damaged by excessive thermal and/or mechanical load during plasticising or filling
• Pre-coloured compound: foreign material accumulating in dead spots in the plasticising unit or hot runner

What you see

Elongated, parabolic streaks on the surface whose points always face in the direction of flow. Water in the granules flashes to steam in the barrel; the vapour bubbles burst at the advancing flow front and are smeared along the skin. There is a quick confirmatory test - purge melt into the air: if moisture is the cause, the melt foams and shows bubbles. This matters because thermal (shear) silver streaking looks similar but needs the opposite corrections - see fault 18.

Why it happens

• Residual moisture in the granules above the limit for the grade
• A water-cooled mould leaking into the cavity
• Feed-hopper temperature control set too low, so condensation forms on the granules

What you see

In glass-fibre reinforced or mineral-filled grades: rough, spotty, irregular surface areas, or uniform blemishes whose shape partly follows the flow front in that region. Depending on how the light falls they read anywhere from matt to metallically glossy. They appear mainly after openings, diversions and at weld lines - wherever the resin-rich skin that normally covers the fibres fails to form.

Why it happens

• Injection speed too low
• Mould surface temperature too low - the skin freezes before resin can cover the fibre
• Melt temperature not high enough
• Fibre orientation changing at material accumulations - thickness steps, ribs and mounting bosses

What you see

Charred streaks in silver, light-brown or very dark shades. Severe thermal damage to the melt generates gas bubbles that reach the surface during injection. The colour tells you which mechanism you have: light-brown to very dark discolouration indicates real thermal damage from oxidation or degradation - too hot, or too long in the barrel. Silver streaks normally result from excessive friction, usually localised at a small flow-channel cross section or a sudden diversion around an obstacle.

Why it happens

• Melt damaged by an excessively high temperature
• Melt damaged by an excessively long residence time
• High shear during the mould filling procedure

What you see

Alternating glossy and matt stripes running perpendicular to the flow direction, resembling the coat of a tiger. They are caused by a pulsating, unstable melt flow - the front alternately sticks and slips at the wall. This is characteristic of multi-phase systems and blends, where the phases behave differently at the melt front. Almost always a pressure-loss problem in disguise.

Why it happens

• Gate, sprue and runner cross sections too small
• Machine nozzle bore too small
• Part wall thickness insufficient
• Large pressure loss along the flow path
• Tool temperature too cold

What you see

External or internal cracks appearing at stresses well below the material's ultimate tensile strength. Locked-in internal stress from processing is a major contributor; the crack itself is then initiated by an external tensile load and/or contact with an aggressive solvent or chemical. Because the process-induced portion is the part you can control, deliberate solvent exposure is a standard way to reveal how much stress a moulding is carrying. Mould temperature is usually the dominant variable.

Why it happens

• Material overloaded because holding pressure is too high, or the mould construction is inadequate
• Mould temperature too low - the melt freezes under stress rather than relaxing
• Cracks formed during ejection by forced removal from undercuts - insufficient draft, too few or badly positioned ejectors
• Molecular weight of the chosen grade too low for the chemical environment it will meet in service

What you see

In semi-crystalline thermoplastics, granules that have not fully melted pass through the screw and end up in the part. On thin walls they raise a visible contour or elevation on the surface. In unreinforced, uncoloured grades they appear as inclusions of greater transparency than the surrounding material. This is a plasticising problem, not a mould problem - nothing you do to the tool will fix it.

Why it happens

• Unfavourable plasticising conditions selected
• Residence time of the material in the cylinder too short
• Screw geometry unsuited to the material

What you see

The part is not completely formed - most often in the areas furthest from the gate or in the thinnest sections, because the cavity never fully filled. Worth noting: a short shot is not always a volume problem. Trapped air with nowhere to go will stop a flow front just as effectively as insufficient material, so check venting before you simply add shot weight.

Why it happens

• Not enough melt metered; no melt cushion
• Melt temperature and/or mould temperature too low
• Injection speed too low
• Mould not adequately vented - trapped air blocks the front
• Too much pressure loss in the gating system
• Wall designed too thin, or the flow path too long for the grade
• A thin-walled area near the gate freezing off during filling
• Flowability of the grade insufficient even after process optimisation

What you see

The part deviates from its intended shape - twisting, bowing, wavy surfaces, angles out of tolerance. The driver is always differential shrinkage: different regions shrink by different amounts because they were packed differently or because molecular and fibre orientation varies from place to place. In glass-fibre reinforced grades this is dominated by fibre orientation, which is why shrinkage along the flow differs from shrinkage across it - and why warpage in reinforced parts is largely designed in, not dialled out.

Why it happens

• Unfavourable mould temperatures, or an unfavourable difference between the two halves
• Part has differing wall thicknesses
• Pressure gradient along the flow path too high
• Cooling time too short - the part is ejected too hot and deforms afterwards
• Unfavourable material choice for a warp-critical geometry
• Fibre orientation changing across the part - from flow diversions, end-of-flow effects, weld lines and gates

What you see

Deposits accumulating on the cavity surface, in the parting line and inside the venting system. The residue can be polymer breakdown products, additives or flame retardants that volatilise from the melt and condense on the cooler tool. It matters because it is self-accelerating: deposit blocks the vents, blocked vents cause dieseling and burning, and the resulting degradation generates more deposit.

Why it happens

• Melt temperature too high
• Injection speed too high during filling
• Material insufficiently dried
• Mould not adequately vented
• Residence time too long in the plasticising unit and/or hot runner
• Melt exposed to severe shear along the flow path