The part is almost complete. Missing a corner, a rib, a thin section. The machine has enough capacity, the material is correct, but something prevents complete fill. Short shots are among the most frustrating defects because the solution seems obvious, just add more material, yet adding more material often doesn’t fix the problem or creates new problems in the attempt.
Short shots result from a race between flow and solidification that the material lost. Understanding why it lost that race, whether due to pressure limits, flow restrictions, or venting problems, directs troubleshooting toward solutions that actually work.
What Creates Short Shots
Every injection molding shot is a race against time. Molten plastic enters the cavity at several hundred degrees, while the mold sits at perhaps 100 to 150 degrees. Heat transfers from plastic to mold, and the flow front progressively loses temperature as it advances. If the material freezes before reaching all areas of the cavity, the result is a short shot.
Flow length is the distance material must travel from gate to the farthest point of the cavity. Longer flow lengths increase the challenge because material has more time to cool and more surface area to lose heat.
Wall thickness dramatically affects flow capability. Thin walls conduct heat to the mold faster, accelerating freeze-off. The relationship is non-linear: halving wall thickness far more than doubles the filling challenge.
Material properties determine how quickly the flow front loses fluidity. High-viscosity materials fill slowly and resist flow. Materials with narrow processing windows freeze quickly once temperature drops below a threshold.
Process conditions provide the force and thermal conditions that either enable or prevent complete fill. Injection pressure, speed, and temperature all affect whether material reaches every corner before freezing.
Pressure-Related Causes
Pressure drives flow. When pressure is inadequate, material stops moving even though the cavity isn’t full.
Insufficient injection pressure may simply mean the machine can’t push hard enough. Every machine has a maximum injection pressure, and some applications approach or exceed that limit. Checking actual pressure readings during fill indicates whether the machine is pressure-limited.
Pressure drop through the system consumes available pressure before it reaches the cavity. Pressure drops through the sprue, runners, and gates. Long runners, small cross-sections, and sharp turns all increase pressure drop. By the time material reaches the cavity, available pressure for filling may be marginal.
Gate restriction is a common pressure-loss point. Undersized gates create excessive pressure drop. Gate freeze during pack phase prevents pressure transmission even if the gate was adequate during fill. Larger gates reduce pressure drop but may leave larger vestiges.
Machine capacity may be exceeded even though the machine seems adequately sized. Check shot capacity and injection rate specifications, not just tonnage. A machine might have adequate clamp force but insufficient injection speed or pressure for the application.
Diagnosing pressure limits involves monitoring actual cavity pressure if sensors are available, or observing whether increased injection pressure produces longer fill. If filling extends with more pressure, the process is pressure-limited. If filling doesn’t respond to pressure increases, another factor limits fill.
Flow-Related Causes
Flow-related short shots occur when material viscosity or flow path geometry prevents adequate filling regardless of available pressure.
Material too viscous resists flow and freezes quickly. Low melt temperature is the usual cause, creating material that won’t flow fast enough before solidifying. Material degradation from extended residence time can also increase viscosity.
Material too cold is different from viscous material but has similar effects. If the barrel isn’t maintaining temperature or if material residence time is too short, the melt never reaches proper processing temperature.
Flow path too long challenges some geometries regardless of material condition. Very long flow lengths relative to wall thickness may be beyond the material’s capability. Flow length to wall thickness ratios above 150:1 typically require special attention; above 250:1 enters thin-wall molding territory.
Hesitation effects occur when flow pauses in thin sections while continuing through adjacent thicker sections. The thin section cools during the hesitation and may not fill even though thicker areas fill completely. These short shots occur in specific locations related to wall thickness variations.
Injection speed too slow allows excessive cooling during fill. Higher injection speeds maintain material temperature by filling faster, reducing heat loss time. Some geometries require high injection speeds to fill before freezing, particularly thin-wall applications.
| Flow Limitation | Diagnosis Clue | Typical Solution |
|---|---|---|
| Low melt temperature | Material strings, poor flow length | Increase barrel temperatures |
| Slow injection | Fill time >2 seconds for thin walls | Increase injection speed |
| Long flow path | Consistent short at distant features | Multiple gates, sequential valve gates |
| Hesitation | Short in thin sections | Increase wall thickness or reroute flow |
| Gate restriction | Fill stalls early in cycle | Enlarge gate |
Venting-Related Causes
Air occupies the cavity before plastic enters. That air must escape somewhere as material fills the cavity. If it can’t escape, pressure builds up and stops flow.
Inadequate venting traps air in last-to-fill locations. As material advances, it compresses the air ahead of it. Eventually, back-pressure from compressed air equals injection pressure, and flow stops. The short shot occurs at vent locations.
Vent location determines whether air can escape. Vents must be positioned at the end of flow, which depends on gate location and part geometry. Incorrectly located vents may vent air from some areas while allowing traps in others.
Vent capacity must match fill rate. Fast injection requires more venting than slow injection because air must escape faster. A process change to faster fill may create venting problems that didn’t exist at slower speeds.
Plugged vents from process residue, mold release, or material deposits reduce effective venting over time. Venting that was adequate when the mold was new may become inadequate as deposits accumulate. Regular vent cleaning restores capacity.
Diagnosing vent problems involves looking for burn marks at short shot locations (burned material indicates compression heating of trapped air) and checking whether reduced injection speed reduces short shots (slower fill gives air more time to escape through marginal venting).
Systematic Diagnosis
Short shot troubleshooting benefits from systematic investigation rather than random adjustment.
Perform a short shot study. Progressively increase shot size while observing fill pattern. The pattern reveals how material flows through the cavity and where it stops.
Identify the limiting factor. Does fill extend with more pressure (pressure-limited)? Does fill extend with higher temperature or speed (flow-limited)? Does fill stop abruptly at specific locations that correspond to poor venting (vent-limited)?
Check for changes. If short shots started suddenly, what changed? New material lot? Temperature drift? Maintenance performed? Different operator? Something changed to create a new problem; finding what changed often identifies the solution.
Inspect the mold. Gate condition, runner condition, and vent condition all affect filling. Physical obstruction from cold slugs, contamination, or buildup can cause short shots that no process adjustment fixes.
Solutions by Root Cause
Solutions must match causes. Applying the wrong solution wastes time and may create new problems.
For pressure limitations:
Increase injection pressure if machine capacity allows. Enlarge gates to reduce pressure drop. Shorten runners or increase runner diameter. Add gates to reduce flow length from any single gate. If machine is at capacity, consider a larger machine.
For flow limitations:
Increase melt temperature to improve fluidity. Increase mold temperature to slow cooling. Increase injection speed to fill before freezing. Select a higher-flow material grade. For severe hesitation, modify part geometry to improve flow balance.
For venting limitations:
Add vents at last-to-fill locations. Deepen existing vents to increase capacity (staying within material limits). Clean plugged vents. Reduce injection speed to give air time to escape. Vacuum venting for severe cases removes air actively rather than relying on displacement.
For machine capacity:
If the machine lacks pressure, speed, or shot capacity for the application, no amount of process optimization compensates. Running the mold in an appropriate machine is the only solution.
The goal of systematic diagnosis is identifying which category limits fill, then applying solutions that address that category. Increasing pack pressure doesn’t fix vent problems. Cleaning vents doesn’t fix pressure limitations. Matching solution to cause produces efficient troubleshooting.
Short shots have identifiable causes. Systematic diagnosis leads to solutions; random pressure increases usually make other problems worse while failing to address the actual limitation.
Sources
- Rosato, Donald V. “Injection Molding Handbook.” Springer.
- RJG Inc. “Short Shot Analysis.” https://rjginc.com/
- Beaumont, John P. “Runner and Gating Design Handbook.” Hanser, 2004.
- Plastics Technology. “Troubleshooting Short Shots.” https://www.ptonline.com/
- Kazmer, David O. “Injection Mold Design Engineering.” Hanser, 2007.