Splay Marking and Staining
Splay marking and staining look unrelated, but both print the melt’s history onto the part surface. Splay is a process fault. Silver streaks fan from the gate where the melt front lost pressure. Staining is a material fault. A polished mold surface goes dull and discolored, and every part after it carries the haze. Different causes, different cures.
Splay marking: the melt ran out of push
Splay shows as an uneven, lined area near the gate. Sometimes it looks like grooves on a vinyl record. Sometimes it is just an irregular half-circle of streaks. The mechanism is slow pressurization of the final fill. As back pressure builds in the mold, flow through the gate slows. The melt no longer has the fluidity to copy the mold surface. The front stalls and smears instead of packing.
The first check is machine power. The mold must fill rapidly in one continuous push. If the press runs out of power before the cavity is full, the last material creeps in slowly while hydraulic pressure rebuilds. The material already in the cavity has cooled past the point of surface conformity by then. Confirm the hydraulic system can deliver the required rate in the allowed time before blaming anything else. An undersized press is a common root cause that no temperature tweak can fix.
If power is adequate and splay persists, look at the feed system. Every sharp turn, undersized runner, and long gate land steals pressure during fill. Open the sprue and runner path or make it more direct, and cut the gate land as short as the design allows. A restricted feed system mimics a weak machine. The symptom is identical, so measure fill time against the design intent before cutting steel.
Acetal deserves special attention here. Its higher melt viscosity makes it more splay-prone than most commodity resins. Small parts run on hand machines or low-power presses show the same pattern in many materials. There is simply not enough push behind the front, and the last 10% of fill crawls.
Take splay seriously beyond cosmetics. Any discontinuity in the surface finish is suspect. In aggressive chemical environments, a splayed region can crack or fail outright while the rest of the part survives. A part that looks marginal usually is marginal.
Staining: the mold surface is telling on the material
Staining appears when a highly polished mold surface suddenly turns dull and discolored, and the haze transfers to every molding. The causes are almost all material-side. An additive breaking down. A pigment blooming to the surface. Slight oxidation liberating just enough gas to react with the steel. Not enough to burn, enough to stain.
Rework is the usual suspect. Rework can carry mold release contamination, slightly degraded polymer, damp dry-color pigment, or a tail of dusty fines. Each of those stains a little differently, so work through them one at a time instead of changing everything at once. PVC has its own notorious mechanism. Decomposition frees hydrochloric acid, which attacks a polished mold surface fast. With PVC, staining is an early warning of degradation, not a cosmetic quirk. Treat it as a process alarm.
The cure follows the cause. Dry the color, cut the rework ratio, or drop the melt temperature slightly to starve the decomposition. Because staining is nearly always a material fault, process tweaks only mask it. Fix the feedstock. Note the overlap with silvering and splay from moisture. Volatiles and damp material streak the part, while staining films the steel. Different surface, same discipline: dry material, clean rework, controlled temperatures.
How to tell splay from silvering
Splay and silvering both streak from the gate, and shops confuse them constantly. The distinction decides the fix. Splay marks follow the flow direction in ridged lines and fade when fill speeds up. Silvering looks flaky or micaceous and ignores fill speed entirely. If raising injection rate clears the streaks, it was splay. If drying the resin clears them, it was silvering. Run that two-step test before changing anything else. It takes one material change and one process change, and it prevents a week of wrong adjustments.
Staining has its own tell. It films the mold steel, not just the part. When the same haze appears on consecutive shots and wipes off the steel with solvent, the material is the source. Polishing the mold treats the symptom. The stain will return with the next batch of the same resin.
Prevention checklist
- Size the press so fill completes in one rapid push, with no coasting at the end.
- Keep runners direct and generously sized. Keep gate lands short.
- Watch acetal, and small parts on low-power machines, for early splay.
- Cap the rework ratio and keep rework dry, segregated, and documented.
- Treat sudden mold staining as a material alarm, especially with PVC.
- When streaks persist across materials, run a defect root-cause analysis before polishing the steel again. Polishing a staining mold without fixing the material just schedules the next stain.
