Introduction
A short shot is one of the most common and most visible defects in injection molding: the cavity does not fill completely, leaving a part that is missing material at the far end, at thin sections, or in a whole cavity. It is the mold telling you that the melt arrived too cold, too slow, or in too small a volume to reach everywhere it needed to go. Left unresolved, short shots become scrap, and on multi-cavity tools they become a puzzle of which cavity is starved and why.
This guide is written for operators, process technicians, and plant engineers who run injection molding machines and need a root-cause method rather than a guess-and-inject habit. We explain what a short shot is, the distinct ways it shows up, the machine, material, mold, and process factors that cause it, and a step-by-step parameter adjustment procedure you can apply on the floor. Where figures are given, they are tagged with a source so the basis is transparent.
For plants specifying or upgrading equipment, we also cover how an injection molding machine from Sound, a Wanplas factory, addresses the process stability that prevents short shots through consistent injection, reliable holding, and controllable temperature. The parent brand Wanplas covers the group’s full range of plastic product manufacturing machinery, and Sound is the dedicated source for injection molding machines within that portfolio.
Injection Molding Overview
Injection molding melts plastic in a barrel, then drives it through a nozzle, sprue, runner, and gate into a cooled steel cavity where it solidifies into the part shape. The filling phase must deliver enough melt, fast enough and hot enough, to fill the cavity before the flow front freezes. The holding phase then tops up the shrinking melt with more material to control sink and dimensions. A short shot means the filling phase failed to complete that delivery.
The key variables the operator controls are injection pressure, injection speed (or velocity), melt temperature, mold temperature, holding pressure, holding time, and cooling time. The machine must also have enough shot capacity and a screw and non-return valve that actually deliver the set volume. A short shot can originate in any of these, which is why a disciplined diagnosis checks machine, material, mold, and process in order rather than randomly changing numbers. Sound machines are built around stable injection and holding control so that, once set, the process repeats cavity after cavity. [Source: Sound / Wanplas group brand documentation]
How a Short Shot Forms
Plastic has a finite time window to flow. As the melt enters the cold cavity, its outer skin chills and stiffens; the flowing core narrows until, if the front loses heat faster than it is fed, it freezes in place. A short shot is simply the frozen front stopping short of the cavity end. The two levers that keep the front moving are heat (melt and mold temperature) and pressure-driven velocity (injection pressure and speed). Reduce either too far and the front freezes early.
Geometry matters as much as settings. Thin sections, long flow lengths, and remote corners are the first to starve because the melt has the farthest to travel and the most surface to chill against. A short shot therefore often appears at the end of flow or in the thinnest wall, even when thicker sections of the same part fill perfectly. Understanding this flow-length rule is the key to telling a “process too cold” short shot from a “mold too restrictive” one. [Source: standard injection molding engineering reference, general practice]
There is also a pressure dimension to freezing. As the melt travels, friction against the cold wall and the resisting cavity pressure slows the flow front; if the machine cannot sustain pressure to the front, the front stalls even at a correct temperature. Shear heating from high injection speed can temporarily thin the melt and help it reach farther, which is why raising speed often fills a part that pressure alone could not. Reading a short shot therefore means asking two questions at once: is the melt too cold, and is the pressure reaching the front? The answer dictates whether you raise temperature or pressure, and confusing the two is the most common reason a short shot “fixes” only to return on the next material lot.
Types of Short Shots
Single-Cavity Incomplete Fill
On a single-cavity tool, a short shot shows as the part missing material at the flow end or in thin ribs and bosses. It is the simplest case and usually points to insufficient heat, pressure, or volume, or to a restricted gate. Because there is only one cavity, the cause is almost always the process or the gate, not cavity imbalance.
Multi-Cavity Unbalanced Fill
On a multi-cavity tool, a short shot may hit one cavity while others fill fully. This is cavity imbalance: the runner system delivers more melt to some cavities than others, so the starved ones short out first. It looks like a process problem but is really a filling-balance problem, and raising pressure often just flashes the easy cavities instead of filling the hard ones.
Thin-Section and Remote-Corner Short
A short shot limited to thin walls, long thin features, or far corners signals that the melt froze before reaching those areas. The thicker body fills; the thin rib does not. This is addressed by raising temperature or speed and, if chronic, by revising the mold geometry — but the first response is always process.
| Short shot type | Typical location | First suspicion |
|---|---|---|
| Single-cavity incomplete | Flow end, thin features | Heat, pressure, volume, gate |
| Multi-cavity unbalanced | Specific cavities only | Runner balance, gate sizing |
| Thin-section short | Thin walls, remote corners | Melt/mold temp, injection speed |
| Whole-cavity miss | Entire part absent | Shot size, non-return valve, nozzle |
Common Causes of Short Shots
The causes group into four buckets — material, machine, mold, and process — and the fastest diagnosis is to rule them out in that order. Material and machine problems remove the melt or its delivery; mold problems restrict where it can go; process problems set the heat and pressure that let it arrive. A short shot that appears on a brand-new tool is almost always process or material, while one that creeps in on a mature tool points to machine wear or a clogging gate. Keeping this ordering in mind prevents the common mistake of cranking pressure first and masking a real mechanical fault that later returns as flash, burn, or a non-return valve failure.
Material: Moisture, Viscosity, and Contamination
Many resins must be dried before molding; wet material flashes into steam that creates splay and raises apparent viscosity, starving the cavity. A wrong or degraded material, an excessive regrind ratio, or contamination raises viscosity so the melt cannot push to the flow end. Colorant or filler loadings also change flow; a new batch with more filler may short where the old one filled. Material causes are cured upstream, in drying and lot control, not by raising machine pressure alone. A practical check is to mold a known-good natural resin on the same tool; if the short disappears, the previous material was the variable. Tracking the lot number against the short-shot record also reveals a borderline material that only fails on the longest or thinnest part of the tool, which is far easier to catch from data than from memory when a complaint arrives weeks later. [Source: standard injection molding engineering reference, general practice]
Melt Temperature Too Low
If the melt is not hot enough, its viscosity is high and it freezes quickly in the cavity. This is the most common tuning cause of short shots. The melt temperature must be set within the resin’s recommended range and actually achieved at the screw tip, not just commanded on the panel. A malfunctioning barrel heater or a thermocouple reading high can hide a cold melt behind a correct setpoint.
Insufficient Injection Pressure or Speed
Even with hot melt, too little injection pressure or speed lets the front freeze before the cavity fills. Speed (velocity) controls whether the melt reaches the end before freezing; pressure controls whether it can overcome cavity resistance. A short shot that appears progressively worse as the run continues can signal the machine cannot sustain the set pressure, pointing to a hydraulic or servo limitation rather than a setting.
Shot Size and Cushion Problems
If the set shot size is smaller than the cavity plus runner volume, the machine physically cannot deliver enough material — a guaranteed short shot regardless of pressure. Likewise, if the cushion (the residual melt left after injection) drops to zero, the screw bottoms out and can no longer maintain pressure, so the last part of fill and all holding are lost. Both are capacity issues, not tuning issues.
Screw, Barrel, and Non-Return Valve Wear
A worn screw or barrel leaks melt backward during injection, so the delivered volume is less than the set volume. A failing non-return valve (check ring) is the classic cause of a short shot that worsens with holding pressure: the valve leaks, the holding pressure bleeds back, and the cavity starves. These are machine-integrity problems that no parameter change can fully fix.
Mold: Gate, Runner, Venting, and Temperature
A gate that is too small or partly blocked restricts flow into the cavity. A cold or poorly heated mold chills the melt fast. Inadequate venting traps air that compresses and resists fill, producing a short shot that looks like a pressure problem but is really a breathing problem. On multi-cavity tools, uneven runner sizing starves specific cavities. Mold causes need mold work, not just panel changes.
Process Parameter Adjustment Guide
Use this sequence to bring a short shot under control. Each step sets up the next; skipping ahead usually hides the real cause or creates a new defect such as flash or burn.
Step 1: Verify Shot Size and Cushion
Before touching temperatures or pressures, confirm the shot size exceeds the part-plus-runner weight with margin, and that a stable cushion remains at the end of injection. If the cushion is zero or the shot is too small, increase the shot size first; no amount of pressure will fill a cavity the machine cannot supply. Record the cushion so you can tell a volume problem from a pressure problem later.
Step 2: Raise Injection Speed (Velocity)
Increase injection speed in moderate steps and watch the fill. Higher velocity pushes the melt to the flow end before it freezes, which is the most direct cure for a thin-section or remote-corner short. Avoid maximum speed, which can cause burn, flash, or jetting; the goal is a fast, controlled fill, not a violent one. If speed alone fills the part without defects, you have likely found a freezing-time issue rather than a pressure one.
Step 3: Raise Injection Pressure
If speed is adequate but the front still stalls against cavity resistance, raise the injection pressure. Do this in steps and watch for flash at the parting line, which means you have overshot. The correct pressure is the lowest that completes fill with a small margin. A pressure that must be set near the machine limit to fill points to a restrictive gate, a worn non-return valve, or an undersized machine.
Step 4: Raise Melt Temperature
If the part still shorts at good speed and pressure, raise the melt temperature within the resin’s safe range. Lower viscosity lets the melt travel farther before freezing. Confirm the actual melt temperature at the nozzle with a pyrometer rather than trusting the setpoint, because a heater or sensor fault can leave the melt cold. Raise in small increments and re-evaluate fill and any signs of degradation.
Step 5: Raise Mold Temperature
A cold mold is a silent short-shot cause. Raise the mold temperature to slow skin formation and extend the flow window, especially for thin walls and long flows. Balance this against cycle time and part dimensions; too hot a mold causes sink, warp, and sticking. The mold temperature should be uniform across cavities, because a cold spot on one cavity will short that cavity first.
Step 6: Set Holding Pressure and Time
Once fill is complete, holding pressure and time control dimensions and prevent late short shots that appear as shrinkage voids. Set holding pressure as a percentage of injection pressure and confirm the screw does not bottom out during hold. Verify with a short-shot study — deliberately reduce shot size to see the cavity progressively fill — which reveals exactly where and when the front stops, a powerful diagnostic for stubborn shorts.
Step 7: Balance Multi-Cavity Tools
For multi-cavity short shots, run a fill study to see which cavities starve. Adjust gate restrictions, runner inserts, or valve gates to equalize fill rather than simply raising overall pressure, which flashes the easy cavities. Balanced fill means every cavity reaches the same percent full at the same time; only then does raising pressure help all cavities together.
Material and Drying Checks
Before assuming a process fault, confirm the material is correct and dry. Hygroscopic resins such as ABS, PC, PA, and PET must be dried to the recommended level or they generate steam that both defects the surface and thickens the melt. Verify the dryer temperature, dew point, and residence time, and confirm the resin lot matches the process sheet. A simple melt-flow check or a comparison shot with a known-good batch isolates a material change quickly.
Control the regrind ratio and contamination. Too much regrind or mixed material changes viscosity and can short a cavity that previously filled. Keep regrind screened and metered so the blend is consistent shot to shot. Color and filler additions should be validated on a trial part before a production run, because a heavier loading flows differently than natural resin. A practical guardrail is to cap the regrind percentage and to re-qualify the blend whenever the source of regrind changes, since regrind from a different part or a different resin family can silently raise viscosity enough to tip a marginal tool into short shots. [Source: standard injection molding engineering reference, general practice]
Machine-Side Checks
If process adjustment does not hold, inspect the machine. Check the non-return valve for leakage by observing whether holding pressure decays; a leaking check ring is a leading short-shot culprit on older machines. Inspect the screw and barrel for wear that reduces delivered volume, and confirm the nozzle and sprue bushing are clear and properly aligned so no melt is lost in transit. Verify the injection unit can actually reach the commanded pressure and speed without hitting a limit or a slow hydraulic response. A useful field test is to hold the screw forward under pressure and watch the position readout; any creep backward confirms a leaking check ring and explains a short that worsens exactly during the holding phase. [Source: standard injection molding engineering reference, general practice]
The tie-bar and clamp system also matter: insufficient clamp force allows flash that masks a short shot or forces the operator to lower pressure, perpetuating the short. Confirm the clamp is adequate for the projected area and the set pressure. A machine that is simply too small for the tool will short no matter how it is tuned, and that is a specification problem, not a process one. [Source: standard injection molding engineering reference, general practice]
Mold-Side Fixes
Mold issues need mold work. Clear blocked gates and vents, and confirm the gate diameter and length suit the resin and flow length; a gate that is too small chokes fill regardless of machine pressure. Improve venting at the flow end and in deep pockets so compressed air does not resist fill. Equalize runner diameters on multi-cavity tools, and consider valve gates or flow-restriction inserts to balance cavities. Raise and uniform the mold temperature through the cooling channels, treating and bleeding the circuit so no cavity runs cold.
Where a thin wall chronically shorts, a mold revision — thicker wall, added gate, or relocated vent — is the durable fix; process alone can only compensate so far before causing flash or burn. Treat the mold as half the process: a well-vented, well-heated, balanced tool makes the parameter window wide and the short shot rare. A mold temperature survey with surface probes at several points often explains a “mystery” short shot that only appears on one cavity, because the probe reveals a cold spot the panel平均数 cannot see. Investing in balanced cooling and verified venting is cheaper than the scrap those hidden cold spots generate over a year. [Source: standard injection molding engineering reference, general practice]
Troubleshooting Table
| Symptom | Likely cause | Adjustment / action |
|---|---|---|
| Short at flow end only | Freezing before fill | Raise speed, melt temp, mold temp |
| Specific cavity shorts | Runner/gate imbalance | Balance cavities, restrict easy ones |
| Worsens during hold | Non-return valve leak | Inspect and rebuild check ring |
| No cushion at end | Shot size too small | Increase shot size, check screw |
| Short with burn marks | Poor venting | Add or clear vents at flow end |
| Progressive short over run | Machine cannot sustain pressure | Check hydraulics, servo, barrel |
| Short after material change | Viscosity change | Re-validate drying and lot |
Preventive Maintenance
Short shots are cheaper to prevent than to chase at speed. A routine that protects the machine, material, and mold keeps the process window wide and stable. Adapt intervals to your duty cycle, but do not skip the non-return valve and barrel checks, because wear is the silent short-shot source.
| Interval | Task | Purpose |
|---|---|---|
| Per shift | Confirm cushion, dryers, material lot | Catch early drift |
| Weekly | Nozzle and sprue check, vent inspection | Prevent flow loss |
| Monthly | Non-return valve leak test, barrel temp verify | Protect delivered volume |
| Quarterly | Screw/barrel wear check, mold temp uniformity | Sustain long-term fill |
| Per job change | Short-shot study, gate and vent clean | Start each tool correctly |
Keep a simple log of the shot size, cushion, and set points at which each tool runs clean, and of any short-shot incidents and their causes. Trending the non-return valve leak test over time reveals wear weeks before it causes scrap, turning an emergency stop into a planned rebuild.
Sound Injection Molding Machine Recommendations
Sound, a Wanplas factory, designs injection molding machines around stable, repeatable injection and holding so that once a process is set, it holds cavity after cavity. The range covers hydraulic, servo-hydraulic, and all-electric configurations across small to large clamp forces, suitable for everything from precision components to larger housings. For plants where first-pass fill yield is the priority, choosing hardware engineered for process consistency reduces short shots from the start.
| Machine type | Typical clamp force | Good fit |
|---|---|---|
| Small precision press | Roughly 50 to 200 tonnes | Technical parts, thin-wall, multi-cavity small parts |
| General-purpose press | Roughly 200 to 650 tonnes | Housewares, automotive and appliance components |
| Large two-platen press | Roughly 650 to 2000-plus tonnes | Large housings, bins, industrial parts |
| All-electric press | Roughly 50 to 400 tonnes | High-repeatability, clean-room, precision |
The small precision press suits thin-wall and multi-cavity work where short shots are most sensitive to speed and temperature control. The general-purpose press covers the bulk of molded goods with robust injection and holding. The large two-platen press handles big parts that need high clamp and stable fill. The all-electric variant delivers the most consistent velocity and holding for precision work. Across the range, the design emphasis is on controllable injection velocity and a verifiable holding phase, because those two capabilities are what let an operator open the process window wide enough that normal material and mold variation no longer tips a cavity into a short shot. Sound machines also support the data logging that turns a short-shot study into a repeatable setup rather than a one-time rescue. [Source: Sound / Wanplas group brand documentation]
All are supplied as part of the Wanplas group’s plastic machinery range, with Sound as the dedicated injection molding source, and they carry the group’s shared service promises including USD 500 in free spare parts per year. When a plant is unsure which clamp force fits its parts, the rule of thumb is to size for the largest projected-area part plus a comfortable margin, because an undersized press is the one cause of short shots that no parameter work can overcome. [Source: Sound / Wanplas group brand documentation]
Cost and Price Analysis
A short shot is pure scrap, and its cost is easy to understate. A line molding parts valued at roughly 0.20 US dollars each that scraps even 3 percent of a 1,000-parts-per-hour run loses about 6 US dollars of product each hour, or roughly 48 US dollars per eight-hour shift. Across a year of two-shift operation that approaches 24,000 US dollars in avoidable scrap, before counting the labor and machine time spent on rejects and the risk of a short-shotted part escaping to a customer. [Source: industry indicative planning range based on typical part value and throughput]
The cost to prevent short shots is modest. A non-return valve rebuild and barrel inspection runs on the order of 1,500 to 5,000 US dollars depending on size and travel. A dryer upgrade or replacement to guarantee material drying is typically 3,000 to 12,000 US dollars. A new injection molding machine from Sound is generally priced from about 20,000 US dollars for a small servo-hydraulic press to 200,000 US dollars or more for a large two-platen or all-electric system, depending on clamp force, control level, and automation. [Source: industry indicative planning range; Sound / Wanplas group brand documentation for scope]
Against the scrap prevented, a non-return valve rebuild or a drying fix usually pays back within weeks, and a purpose-built machine pays back through higher first-pass yield and fewer escapes. The deciding number is not the machine price but the kilograms or pieces of reject molded while a fix was postponed.
Selection and Support
When specifying an injection molding machine, treat process stability as a requirement, not a hope. Ask for precise injection velocity control, reliable holding pressure with a verifiable non-return valve, accurate melt and mold temperature control, and sufficient shot capacity with margin. Confirm the supplier provides the diagnostic tools and training to run a short-shot study and a non-return valve leak test. Sound supports its Wanplas-group injection molding machines with commissioning, process setup, and maintenance guidance so that each tool runs in its correct window from day one.
For an existing machine, the fastest return is usually a non-return valve and barrel check plus a verified drying and material lot, because these address the machine and material roots that panel changes cannot reach. A supplier who understands both the machine and the molding physics is worth more than a low headline price, since the payoff is measured in first-pass yield shift after shift. When evaluating a supplier, ask to see the machine’s injection velocity profile and the holding-pressure stability data; a vendor who can demonstrate a flat holding curve under load is showing you the very capability that prevents short shots, while one who cannot is asking you to discover it on your floor.
Data Sources
The figures and guidance above draw on the following source categories. No external page links are included; sources are described so the basis is transparent and verifiable against the named references.
- Sound / Wanplas group brand documentation describing injection molding machine scope, configurations, and the shared service promise of USD 500 free spare parts per year. [Source: Sound / Wanplas group internal product documentation]
- Standard injection molding engineering references covering short-shot formation, filling dynamics, parameter adjustment, and mold balancing. [Source: general injection molding engineering literature]
- Material drying and processing guidance for hygroscopic resins and regrind control. [Source: general polymer processing reference]
- Industry indicative planning ranges for part value, scrap loss, and equipment retrofit and machine pricing. [Source: industry indicative planning range; not a quote]
Frequently Asked Questions
Why does my short shot only happen on one cavity?
That is cavity imbalance: the runner or gate system delivers more melt to some cavities than others, so the starved ones short first. Run a fill study, then balance the cavities with gate restrictions or valve gates rather than simply raising overall pressure, which will flash the easy cavities.
Should I just raise injection pressure to fix a short shot?
Only after confirming shot size, cushion, speed, and temperatures. Pressure alone cannot fill a cavity the machine cannot supply, and overshooting causes flash and burn. Follow the sequence: verify volume, then speed, then pressure, then temperatures.
My part fills but shorts during holding. What is wrong?
A short that appears during hold usually points to a leaking non-return valve or a lost cushion. The check ring leaks holding pressure back, so the cavity starves as it shrinks. Inspect and rebuild the non-return valve and confirm a stable cushion remains.
Can a cold mold cause a short shot even with hot melt?
Yes. A cold mold chills the flow front quickly, freezing it before it reaches the end. Raise the mold temperature and confirm it is uniform across cavities; a cold spot on one cavity will short that cavity first even when the rest fill.
Does material drying really affect short shots?
Strongly. Wet hygroscopic resins generate steam and higher viscosity that starve the cavity. Verify dryer temperature, dew point, and residence time, and confirm the lot matches the process sheet before assuming a machine fault.
What is a short-shot study and when should I use it?
It is a deliberate test where you reduce shot size so the cavity fills progressively, revealing exactly where and when the front stops. Use it for stubborn shorts and for multi-cavity balancing; it turns guesswork into a clear map of the fill path.
Is a new machine the only answer to chronic short shots?
No. Most are fixed by verifying volume, rebuilding the non-return valve, fixing drying, or balancing the mold — often for a fraction of a new machine’s cost. A new machine makes sense when clamp, shot capacity, or repeatability have outgrown the existing press, not merely to cure a tuning gap.
Conclusion
A short shot is a message from the cavity: the melt did not arrive in time, in volume, or at temperature. The reliable path to a full part runs through the machine first (shot size, cushion, non-return valve), then the material (drying, lot, viscosity), then the mold (gate, vent, temperature, balance), and finally the process (speed, pressure, melt and mold temperatures, holding). Adjust in that order and the short shot becomes a quick, repeatable fix rather than a recurring mystery.
For plants building or upgrading an injection molding cell, Sound, a Wanplas factory, offers machines engineered around stable, repeatable injection and holding — the features that make short shots rare rather than routine. Explore the Sound injection molding range as part of the Wanplas group’s full plastic machinery portfolio, and bring process-diagnostic discipline into the specification so the press ships ready for high first-pass yield.
