Short shots and splay get lumped together because they both show up as “the shot isn’t right,” and because the reflex response to both is to add pressure and heat. That reflex fixes one of them sometimes and makes the other worse. They deserve to be separated, because their roots are usually different: a short shot is a delivery problem — not enough material or not enough push reached the end of the cavity — while splay is a contamination problem — gas got into the melt, and the overwhelmingly common source is moisture.
A short shot is unmistakable: the part is incomplete, missing its far corners or thin features, typically with a rounded, glossy edge where the flow stopped and froze. Splay is silver streaking that fans out along the flow direction, usually radiating from the gate, sometimes with tiny bubbles. Two different pictures, two different investigations.
Short shots: the melt never got there
A short shot means the cavity didn’t fill. The cause is somewhere on the path from hopper to cavity end, and it’s worth walking that path in order rather than jumping to pressure.
| Cause area | Specific causes | Fix |
|---|---|---|
| Material delivery | Shot size/metering too low; cushion lost; hopper bridging or starved feed; nozzle blocked by contaminant | Confirm shot size and a stable cushion; clear the nozzle; check feed |
| Check ring / screw | Worn non-return valve letting melt slip back during injection | Test cushion stability over 20–30 shots; inspect and replace the valve |
| Pressure & speed | Injection pressure too low; fill speed too slow so the front freezes en route | Raise injection pressure and speed |
| Temperature | Melt temp too low; mold temp too low, chilling the front before it arrives | Raise barrel and mold temperature to improve flow |
| Venting | Trapped air physically blocks the last-to-fill area | Add or clean vents where the shot stops |
| Mold / design | Gate or runner too small, too thin, or too long; wall too thin at the extremity | Enlarge gate/runner; review gate location and count; revisit wall thickness |
| Clamp | Excessive clamp crushing vents shut | Set clamp to requirement, not machine maximum |
Two of these are the ones experienced processors check early because they masquerade as process problems:
- A worn check ring. If cushion wanders shot to shot on an unchanged setup and shorts come and go intermittently, stop adjusting pressure — the valve isn’t sealing and melt is slipping backward during injection. That’s maintenance, not process.
- Trapped air. A cavity that can’t vent can’t fill: the air is physically in the way. If the short is always in the same blind pocket or rib tip, look at venting before pressure.
Splay: gas in the melt, and it’s usually water
Splay is silver streaking dragged along the flow. The mechanism is that gas in the melt gets smeared under the surface as the plastic flows and freezes. The dominant source is moisture — hygroscopic resin that wasn’t dried properly. Water flashes to steam at melt temperature and streaks the surface, and in resins like nylon, PC, and PET it also degrades the polymer chemically, so the part can be weakened even where it looks acceptable.
But moisture isn’t the only gas source, and knowing the alternatives keeps you from re-drying material that was already dry:
| Splay source | Tell | Fix |
|---|---|---|
| Moisture (most common) | Streaks from the gate; resin is hygroscopic; worse in humid weather or after regrind use | Dry to datasheet spec, every pass — including regrind |
| Thermal degradation | Melt too hot or resin residing too long in the barrel; often with discoloration or specks | Lower melt temp; reduce residence time/cycle; check for oversized barrel |
| Air entrainment | Excessive decompression (suck-back) pulling air into the barrel; insufficient back pressure | Reduce suck-back; raise back pressure to homogenize and vent the melt |
| Volatiles / additives | Colorants or additives decomposing at temperature | Lower melt temp; verify additive heat stability |
| Injection too fast | Shearing and gas entrainment at the gate | Reduce injection speed |
| Cold mold / poor venting | Gas can’t clear from the cavity surface | Raise mold temperature; improve venting |
The practical rule: rule out drying first, and rule it out properly. “It was dry yesterday” is not evidence — regrind in particular sits exposed to shop humidity and is frequently wetter than virgin pellets. Only after drying is verified should you spend time on residence, suck-back, and back pressure.
Why treating them alike wastes time
Here’s the trap that costs shifts. The instinctive response to a short shot is more heat and more speed — and the instinctive response to splay is often the same, because it also “looks like a fill problem.” But if the splay is from wet resin, raising melt temperature makes the hydrolysis and streaking worse, and raising injection speed can entrain more gas. You’ll be actively driving the defect while believing you’re fixing it.
| Short shot | Splay | |
|---|---|---|
| What it means | Melt didn’t reach the end | Gas got into the melt |
| Usual root | Delivery: shot size, check ring, pressure, venting, gate size | Moisture: undried hygroscopic resin |
| Helps | More pressure/speed, higher temps, better venting | Proper drying; often lower temp and speed |
| Makes it worse | Under-pressure, cold mold, blocked vents | More heat, faster fill, excessive suck-back |
The overlap is real but narrow: venting and mold temperature affect both, and severe splay can create a weak, gassy front that also shorts. Diagnose which one you actually have before choosing a direction.
A practical sequence
- Identify the defect precisely. Incomplete part with a rounded frozen edge = short. Silver streaks along flow = splay.
- For splay: verify drying first — resin type, dryer temp and time, and whether regrind was dried too.
- For a short: check delivery before pressure — shot size, cushion stability over 20–30 shots (a wandering cushion means the check ring), and nozzle clear.
- Check venting for both. Trapped air blocks fill and traps gas at the surface.
- Then adjust process, one variable at a time — noting that shorts want more heat/speed while moisture splay usually wants less.
- If the short persists at sound settings, treat it as a mold/design issue: gate and runner size, gate location, or a wall too thin to fill.
FAQs
What’s the difference between a short shot and splay?
A short shot is an incomplete part — the melt never reached the end of the cavity, leaving missing corners or features, usually with a rounded glossy edge where flow stopped. Splay is silver streaking that fans along the flow direction, typically from the gate, caused by gas in the melt. They’re different problems: a short shot is about delivery (shot size, check ring, pressure, venting, gate size), while splay is about contamination of the melt with gas — most often moisture from resin that wasn’t properly dried.
Why does raising temperature fix short shots but make splay worse?
Because they have opposite relationships to heat. A short shot often comes from melt that’s too cool or too slow to reach the extremities, so more heat and speed help it get there. Splay from wet resin is caused by moisture turning to steam in the barrel — and in hygroscopic resins, higher temperature accelerates the moisture-driven degradation, so raising heat produces more streaking, not less. That’s why diagnosing which defect you actually have matters before you touch a setting.
My short shots come and go on an unchanged setup — what’s wrong?
Intermittent shorts with a cushion that wanders shot to shot usually point to a worn non-return valve (check ring) rather than a process problem. When the valve doesn’t seal, melt slips backward during injection, so the amount actually delivered varies from shot to shot. Watch real cushion values over 20–30 consecutive shots instead of the averaged display; if it won’t hold steady, stop adjusting pressure and inspect the screw and valve. Bumping shot size to cover the shorts just masks a worsening mechanical condition.
Can splay come from something other than wet resin?
Yes, though moisture is by far the most common. Other gas sources include thermal degradation (melt too hot or residing too long in the barrel, often alongside discoloration or specks), air entrained by excessive decompression/suck-back or insufficient back pressure, decomposing colorants and additives, and shear from injecting too fast. Cold mold surfaces and poor venting also make gas harder to clear. The practical order is to verify drying properly first — including drying the regrind — and only then investigate residence time, suck-back, and back pressure.