Splay Marks and Silver Streaks in Injection Molding — Moisture, Shear, and Material Degradation

A quality engineer at a medical device contract manufacturer is holding a polyphenylsulfone (PPSU) surgical instrument handle — a transparent amber part, 120 mm long, with a uniform 3.5 mm wall, molded in a single-cavity tool with a direct sprue gate. The part surface shows a fan-shaped pattern of fine silvery-white streaks radiating from the gate area — approximately 25 mm long, visible on both sides of the part, and most pronounced within a 15 mm radius of the gate. Under the customer’s 1,200-lux inspection standard, the streaks fail the cosmetic specification. The problem began three days ago, after a material change to a new lot of PPSU from the same supplier and grade. The previous lot had run for six months without a single cosmetic reject. Nothing else changed: same mold, same machine, same process parameters.

The engineer checks the dryer: setpoint is 150°C with a −40°C dew point, drying time of 5 hours — all within the material supplier’s recommended range. The dryer’s dew point display reads −38°C. The material was loaded into the hopper this morning. Everything appears correct.

The root cause is moisture — but not from inadequate drying. The new lot of PPSU had been in warehouse storage for 14 months at the supplier, and the sealed bag — while technically unopened — had a pinhole in the aluminized foil layer, invisible to casual inspection. The material had absorbed approximately 0.15% moisture over 14 months in a non-climate-controlled warehouse in Guangdong, where the average humidity is 75–85%. The standard 5-hour drying cycle at 150°C reduces moisture from 0.15% to approximately 0.04% — above the 0.02% maximum for PPSU, and enough to produce visible splay. The solution was not to change the dryer settings or the process parameters. It was to extend the drying time to 8 hours for this specific lot, reducing the moisture to the target 0.02%, and to add incoming moisture verification for all future material lots.

Splay marks are the most frequently misdiagnosed injection molding defect because they all look similar — silvery or whitish streaks — but have four independent root causes. Treating thermal-degradation splay by increasing the dryer temperature makes the problem worse. Treating moisture splay by reducing the melt temperature increases the melt viscosity and degrades filling. The corrective action must match the root cause, and the root cause can be identified by the splay pattern if you know what to look for.

What Splay Is — and What the Four Types Look Like

Splay (also called silver streaks, silvering, or splash marks) is a surface defect consisting of fine, elongated streaks — typically silvery-white, sometimes brown-tinted — that radiate from the gate or appear in isolated patches on the part surface. The streaks are oriented in the flow direction, because the gas or degraded material that causes them is elongated by the fountain flow as the melt fills the cavity.

Splay is fundamentally a gas-driven defect. Gas bubbles entrained in the melt stream — whether water vapor, decomposition gases, or entrapped air — are stretched into elongated streaks as the melt flows through the cavity, and the streaks are frozen into the surface when the melt contacts the cavity wall and solidifies. The four root causes differ in where the gas comes from and how it gets into the melt:

The diagnostic sequence: determine which of the four gas sources is producing the splay, then address that source. Addressing the wrong source may make the splay worse or trade one type of splay for another.

Type 1: Moisture Splay — Water Vapor in the Melt

Moisture is the most common cause of splay, and the first thing to check when splay appears — especially if the splay began after a material lot change, a dryer maintenance event, or a seasonal humidity change.

Most engineering thermoplastics are hygroscopic — they absorb moisture from the air. The equilibrium moisture content depends on the material, the ambient humidity, and the exposure time. ABS reaches approximately 0.3–0.5% moisture at 50% relative humidity in 24 hours. PA6 reaches 1.5–2.5% at 50% RH — the highest of common injection molding materials — and can reach 3–4% at 80% RH. PPSU reaches 0.4–0.6% under similar conditions.

When moist material enters the barrel, the water vaporizes at melt temperature (200–350°C depending on the material). The volume expansion from liquid water to steam is approximately 1,700:1 at atmospheric pressure. A single pellet of PA6 containing 2% moisture carries enough water to produce approximately 0.3 cm³ of steam when heated to 280°C — and a single 50-gram shot of PA6 at 2% moisture carries approximately 1 gram of water, producing 1.7 liters of steam at atmospheric pressure. In the confined space of the barrel and cavity, this steam is compressed, entrained in the melt, stretched by the flow, and frozen into the part surface as splay.

Material drying requirements — the target moisture content below which splay does not occur:

Drying time must be measured from when the material reaches the drying temperature at the pellet core — not from when the dryer is turned on. A hopper dryer loaded with 50 kg of cold pellets takes 30–60 minutes for the pellet bed to reach the setpoint temperature. The 4-hour drying time starts when the pellets reach temperature, not when the dryer power is switched on.

Diagnostic confirmation: If moisture splay is suspected, take a moisture analyzer reading of the material at the feed throat — not from the dryer hopper, but from the machine hopper where the material actually enters the barrel. A material that measures 0.03% at the dryer outlet may have picked up 0.01–0.02% moisture during transfer through unheated hoses or an unsealed machine hopper. The reading at the feed throat is the moisture content the barrel actually sees.

Type 1 Corrective Actions

1. Increase drying time. If the material has absorbed moisture beyond the normal range — from long storage, damaged packaging, or high-humidity exposure — the standard drying time is insufficient. Double the drying time for material known to have been exposed to humidity beyond normal warehouse conditions, and verify with a moisture analyzer before approving for production.
2. Increase drying temperature within the supplier’s recommended range. A 10°C increase in drying temperature roughly doubles the moisture diffusion rate in most thermoplastics. The upper limit is the material’s softening or degradation temperature.
3. Verify dryer performance. A dryer with saturated desiccant delivers air at a −10°C to −20°C dew point instead of the specified −40°C. The equilibrium moisture content of the material is determined by the dew point of the drying air — not the dryer’s temperature. A dryer at the correct temperature with a failed desiccant bed at −15°C dew point will dry PA6 to approximately 0.15–0.20% moisture, not the <0.10% target. Replace desiccant, clean filters, and check the regeneration cycle.
4. Seal the material transfer path. From the dryer outlet to the machine feed throat, the dried material must travel through sealed hoses or tubes — not open to ambient air. A dried PA6 pellet exposed to 70% RH ambient air re-absorbs approximately 0.02–0.03% moisture in 10–15 minutes — enough to degrade from <0.10% to >0.12%.

Type 2: Thermal Degradation Splay — The Polymer Is Cooking

Thermal degradation splay occurs when the polymer undergoes chain scission or oxidative degradation from excessive temperature or residence time in the barrel. The degradation produces low-molecular-weight fragments — some of which are gaseous at melt temperature — and the gases are entrained in the melt and stretched into streaks at the part surface.

Unlike moisture splay, which is silvery-white, thermal degradation splay has a characteristic brown-to-black tint and is often accompanied by a yellow shift in the base color of the material. In transparent materials (PC, PMMA, PPSU), the degraded material appears as a yellow-brown haze around the gate area. In opaque materials, the degradation produces dark brown or black streaks — difficult to distinguish from burn marks except by location (degradation splay is near the gate, burn marks are at the last-filled region).

Thermal degradation has three sub-causes:

Sub-cause A: Excessive melt temperature. The material supplier specifies a melt temperature range — typically 260–300°C for PC, 230–260°C for ABS, 280–310°C for PA66. Operating at or above the upper limit degrades the polymer. The degradation rate approximately doubles for every 10°C increase above the recommended range. A PC processed at 310°C degrades approximately twice as fast as the same PC at 300°C.

Sub-cause B: Excessive residence time. Even at the recommended melt temperature, polymer degrades if it sits in the barrel too long. The maximum recommended residence time varies by material: 5–8 minutes for PC, 8–12 minutes for ABS, 4–6 minutes for POM, 10–15 minutes for PE and PP. The residence time is calculated as:

Residence time (min) = Barrel capacity (g) / Shot weight (g) × Cycle time (min)

For a barrel capacity of 500 g, a shot weight of 100 g, and a cycle time of 30 seconds (0.5 min), the residence time is 500/100 × 0.5 = 2.5 minutes — well within most materials’ limits. For a barrel capacity of 500 g, a shot weight of 25 g, and a cycle time of 45 seconds (0.75 min), the residence time is 500/25 × 0.75 = 15 minutes — too long for most engineering thermoplastics. The barrel is too large for the shot size.

Sub-cause C: Hot spots in the barrel or hot runner. A failed heater band, a thermocouple reading incorrectly, or a hot runner manifold with a localized hot spot can overheat a small fraction of the melt. The overheated material degrades, produces gas, and creates splay — but only the degraded fraction shows the defect, so the splay may be intermittent (one streak every few shots) rather than consistent.

Diagnostic confirmation for thermal degradation: Purge the barrel and hot runner, collect a purge sample, and inspect it for color shift compared to virgin material. An air-shot (purge shot into open air, not into the mold) that shows brown tint or bubbles confirms degradation in the barrel. If the air-shot is clean but the molded part shows splay, the degradation is likely at the gate (shear overheating) or in a hot runner dead spot.

Type 2 Corrective Actions

1. Reduce melt temperature — target the center of the supplier’s recommended range, not the upper limit. If the material processes at 260–300°C, start at 280°C and adjust upward only if filling problems demand it.
2. Reduce residence time — use a barrel sized for the shot weight (shot weight should be 20–60% of barrel capacity). If the barrel is oversized, consider a smaller screw and barrel set.
3. Purge at shutdown. Never leave heat-sensitive materials (PC, POM, PBT, PPSU) in a hot barrel during downtime. Purge with a stable purging compound or the next production material before shutting down the barrel heat.
4. Check heater bands and thermocouples. A heater band that is running 30–50°C above its setpoint due to a failed thermocouple coupling is a localized degradation source. Verify barrel temperatures with an independent probe thermometer — not the machine’s display, which reads the thermocouple that may be faulty.

Type 3: Shear Overheating Splay — Too Fast Through Too Small

Shear overheating splay occurs when the melt experiences a shear rate that exceeds the material’s critical shear rate at a flow restriction — typically the gate, a sharp corner in the runner, or a thin wall section. At shear rates above the critical limit, the polymer chains undergo mechanical scission: the shear forces literally tear the polymer chains apart, producing low-molecular-weight fragments and gaseous decomposition products.

The critical shear rate is material-specific:

The shear rate through a gate is calculated as:

Shear rate (1/s) = 4 × Volumetric flow rate (cm³/s) / (π × Gate radius³ (cm))

For a PC part with a volumetric flow rate of 80 cm³/s through a submarine gate of 1.2 mm diameter (0.06 cm radius):

Shear rate = 4 × 80 / (π × 0.06³) ≈ 320 / 0.000678 ≈ 472,000 1/s

This far exceeds PC’s critical shear rate of 20,000–40,000 1/s. The gate is too small for the flow rate. The melt is being mechanically shredded as it passes through the gate, producing shear splay at the gate area.

The gate size test: If the splay appears at the gate and nowhere else, and it disappears when the injection speed is reduced by 30–40%, the root cause is shear overheating — the injection speed (and therefore the flow rate) is too high for the gate diameter. The permanent fix is to increase the gate diameter or change the gate type to one with a larger cross-section (e.g., submarine gate → edge gate, or edge gate → fan gate).

Gate land length also matters. A long gate land (the parallel section at the gate entrance) extends the time the melt spends under high shear. Shortening the gate land from 2.0 mm to 1.0 mm reduces the shear exposure time by 50% for the same flow rate.

Type 3 Corrective Actions

1. Reduce injection speed — the immediate process fix. A 30% reduction in injection speed reduces the volumetric flow rate and correspondingly reduces the shear rate by 30%. If this eliminates the splay, the diagnosis is confirmed: shear overheating.
2. Increase gate diameter — the permanent design fix. The shear rate scales with 1/r³ (gate radius cubed), so a small increase in gate diameter produces a large reduction in shear rate. Increasing a submarine gate from 1.2 mm to 1.5 mm diameter reduces the shear rate by approximately 50%.
3. Increase melt temperature (counterintuitive but correct for shear splay). A hotter melt has lower viscosity, which reduces the shear stress for a given shear rate. A 10–15°C increase in melt temperature can drop the viscosity by 15–25% and reduce shear heating proportionally. This is the opposite of the corrective action for thermal degradation splay — which is why diagnosing the root cause before adjusting parameters is critical.
4. Change gate type. A submarine gate with its sharp entry angle concentrates shear. Replacing it with an edge gate of larger cross-section or a fan gate for wide parts reduces both the peak shear rate and the shear exposure time.

Type 4: Entrapped Air Splay — Air in the Melt Stream

Entrapped air splay occurs when air is drawn into the melt stream during plastication (screw rotation and recovery) and is carried into the cavity with the melt. Unlike moisture splay, which produces silvery-white streaks, and degradation splay, which produces brown-black streaks, entrapped-air splay produces patchy, irregular silver marks that are not consistently located and not necessarily radiating from the gate.

Air can enter the melt stream through four pathways during plastication:

1. Screw retraction (pull-back/suck-back) too fast or too large. After the screw reaches its shot size, most machines apply a small screw retraction (1–3 mm, sometimes called decompression or suck-back) to prevent drool at the nozzle. If the retraction distance is too large (>5 mm) or the speed is too fast, air is drawn into the nozzle from the mold side, and this air is mixed into the next shot’s melt during the subsequent plastication cycle.
2. Insufficient back pressure. Back pressure during screw rotation compacts the melt in front of the screw and forces entrained air backward along the screw flights to escape through the hopper. Without adequate back pressure (typically 5–15 bar hydraulic for general-purpose materials, 10–25 bar for filled materials), air bubbles remain in the melt and are carried forward into the next shot.
3. Hopper bridging. When pellets bridge in the hopper throat — forming an arch that blocks material flow into the barrel — the screw starves, and air is drawn into the barrel through the gaps in the bridged pellets. The intermittent material flow produces inconsistent melt density with air pockets.
4. Screw design. A screw with a worn compression zone or an incorrect compression ratio for the material may not compact the melt adequately, leaving entrained air in the melt stream. A general-purpose screw (compression ratio 2.0–2.5:1) running a semicrystalline material that requires a 2.5–3.5:1 compression ratio may produce air-entrapment splay.

Diagnostic confirmation: Watch the screw recovery phase. An inconsistent plastication time (±0.5 seconds or more) suggests air entrainment or hopper bridging. Listen for a squealing or hissing sound during screw rotation — this is air being forced backward past the screw flights. Check the nozzle tip for bubbles in the melt during an air-shot purge: a purge shot that spits, pops, or shows bubbles confirms air in the melt.

Type 4 Corrective Actions

1. Reduce screw decompression distance. Target 1–2 mm of pull-back — just enough to relieve the pressure at the nozzle tip and prevent drool. For materials with very low melt viscosity (PA6, PPS, LCP), use 0.5–1.0 mm or eliminate decompression entirely and rely on nozzle shut-off valves instead.

2. Increase back pressure. Start at 10 bar and increase in 5 bar increments until the plastication time stabilizes (consistent shot-to-shot within ±0.2 seconds) and the air-shot purge is bubble-free. The trade-off is increased melt temperature from the additional shear work during plastication — monitor the melt temperature and adjust the barrel temperature profile if needed.

3. Check the hopper and feed throat. Ensure the hopper is not bridged (sticky pellets, humid conditions, fines accumulation). Clean the feed throat — material fines and dust can accumulate and restrict pellet flow into the barrel. Consider a hopper vibrator or agitator for materials prone to bridging (regrind, fine powder, regranulate).

4. Reduce screw RPM. A screw speed that is too high for the material can entrain air by throwing pellets away from the screw rather than drawing them into the flights. The maximum screw RPM for a given material is approximately:

   Max RPM = 15,000 / Screw diameter (mm)

   For a 50 mm screw: max RPM ≈ 300. A 60 mm screw: max RPM ≈ 250. Running above this speed risks air entrainment regardless of back pressure.

Diagnostic Table: Which Splay Type Is This?

Use this table in sequence: check material moisture first (most common root cause), then check air-shot appearance (distinguishes barrel degradation from air entrainment), then run the injection speed reduction test (isolates shear overheating). The root cause is the one whose diagnostic test produces a significant improvement — if none of the individual tests clears the splay, two root causes may be operating simultaneously.

The medical device quality engineer who started with moisture splay at the PPSU surgical handle eventually traced the problem to the material, not the process. A moisture analyzer reading of the material at the feed throat showed 0.05% — well above the 0.02% maximum for PPSU. The material in the dryer hopper tested at 0.025%, but the material at the machine hopper — after falling through a 1.5-meter unheated transfer tube exposed to ambient air at 28°C and 78% RH — picked up an additional 0.025% moisture in the 20 minutes between the dryer and the feed throat.

The corrective actions: the drying time for that lot was extended to 8 hours to reduce the core moisture of the unusually wet pellets; the unheated transfer tube was replaced with an insulated, sealed tube connected directly from the dryer outlet to the machine hopper; and incoming material moisture testing was added to the receiving inspection procedure — every new lot is tested for moisture content at receiving, and any lot above 0.05% moisture is flagged for extended drying before release to production. The splay did not reappear.

Splay is not one defect with one fix. It is four defects that share a similar visual signature. The diagnostic skill is knowing which of the four gas sources is producing the streaks — because drying material that is already dry does nothing, and lowering the melt temperature for shear-induced splay makes it worse. Diagnose first, then act.

Discuss a splay issue on your production line →