Short shots happen when molten plastic fails to completely fill the mold cavity, leaving you with an incomplete part. The fix depends on the root cause, which typically falls into one of three categories: process settings that restrict flow, mechanical problems with the machine, or mold design issues that trap air or create too much resistance. Working through these systematically, starting with the easiest adjustments, will get you back to full fills faster than changing everything at once.
Why Short Shots Happen
At its core, a short shot means the plastic solidified before it reached every corner of the cavity. The unfilled areas usually show up at the point farthest from the gate, or near weld lines where two flow fronts meet. Three things can cause this: the plastic wasn’t hot enough to stay fluid, there wasn’t enough pressure to push it through, or something physically blocked its path (trapped air, a frozen gate, a plugged nozzle). Most short shots come from some combination of these factors rather than a single dramatic failure.
Adjust Process Parameters First
Process adjustments are the fastest fixes and cost nothing. Start here before pulling the mold or tearing into the machine.
Injection Pressure and Speed
If your material is high-viscosity (polycarbonate, glass-filled nylon, etc.), try increasing injection pressure by 10 to 15%. The plastic can only reach thin sections and complex geometry if the pressure behind it exceeds the resistance of the flow path. For thin-walled parts under 1 mm, injection speeds above 100 mm/s are often necessary to prevent the melt from freezing before it fills out.
For thicker parts, a multi-stage speed profile works better than a single speed setting. A slow-fast-slow curve, something like 30 to 80 to 30 mm/s, lets the melt advance past the gate without jetting, accelerate through the main cavity, then slow down at the end of fill to let air escape. This approach prevents premature solidification in thin sections while avoiding air traps that a single high speed would create.
Melt Temperature
A melt temperature set too low increases viscosity and makes the plastic sluggish. Raise it in small increments, checking part quality after each change. If you suspect a cold spot in the nozzle is causing intermittent short shots, use an infrared thermometer to verify temperature consistency along the barrel and nozzle. For soft blockages in the nozzle, heating it 10 to 15°C above the material’s melting point can clear the restriction. For heat-sensitive materials, increase the buffer temperature by just 5°C to avoid degradation.
Holding Pressure
Holding pressure compensates for the natural shrinkage that occurs as plastic cools and solidifies. If it’s set too low, the material pulls back from the cavity walls and you end up with incomplete packing, especially in thicker sections. Maintain holding pressure at 80 to 90% of your injection pressure during the packing phase. This is particularly important for parts with varying wall thickness, where thicker areas cool slower and need continued pressure to stay full.
Check for Mechanical Problems
If adjusting parameters doesn’t solve the issue, or if short shots appear intermittently, the machine itself may be the problem.
Non-Return Valve (Check Ring)
A worn check ring on the screw tip lets plastic leak backward during injection, meaning the cavity receives less material than the shot size suggests. To test this, let the screw reach its forward position after injection and watch for any forward creep. If the screw drifts forward at any point during the hold phase, your check ring or barrel is worn and allowing backflow. This is one of the most common mechanical causes of inconsistent short shots, because the leak varies from cycle to cycle.
Nozzle Condition
Inspect the nozzle for wear, cracks, or material buildup. A partially blocked nozzle restricts flow just like a kinked hose. Check the alignment between the nozzle and the mold sprue bushing with a feeler gauge. If the gap exceeds 0.05 mm, realign it. Misalignment causes material leakage and pressure loss before the plastic even enters the runner system.
Fix Venting and Trapped Air
Trapped air acts like a plug at the end of the flow path. As plastic advances, it compresses the air ahead of it. If that air has nowhere to go, it creates back-pressure that stops the flow front short of a full fill. You may also see burn marks alongside the short shot, since compressed air heats up enough to scorch the plastic.
Vents need to be deep enough to let air and gas escape but shallow enough that molten plastic doesn’t squeeze through and create flash. The exact depth depends on the resin: crystalline materials like nylon tolerate slightly deeper vents than amorphous materials like polycarbonate. Place vents at the last areas to fill, at weld line locations, and along the parting line opposite the gate.
Clamping Force and Vent Collapse
Here’s a counterintuitive fix: try reducing your clamping force. Many operators run clamp tonnage at maximum as a default, but excessive force crushes the parting line shut so tightly that the vents can’t function. In one documented case, reducing clamp force to 10 tons or less actually increased part weight because the mold could finally breathe. If the material supplier recommends 3 tons per square inch, calculate the actual projected area and use that number rather than maxing out the press. Optimizing clamp force based on part weight lets the mold vent properly, prevents over-clamping damage, and saves energy.
Address Mold Design Issues
If short shots persist despite good process settings and a healthy machine, the mold design itself may be working against you.
Gate Location and Size
Gates should follow the shortest, most balanced flow path from the runner to each cavity. Placing a gate near a thin section forces the melt through a high-resistance area early, where it’s most likely to freeze. Move gates toward thicker regions when possible, letting the plastic flow from thick to thin. If you can’t relocate the gate, enlarging it reduces the pressure drop at that restriction point.
Runner Balancing
In multi-cavity molds, unbalanced runners cause some cavities to fill before others. The cavities farthest from the sprue, or those fed by longer runner branches, see the most pressure drop and are the first to short. A balanced runner layout ensures synchronized filling across all cavities. Keep the sprue as short as possible to reduce both pressure loss and heat loss before the melt reaches the runners.
Wall Thickness Uniformity
Parts with dramatic wall thickness changes create uneven flow resistance. The melt races through thick sections and hesitates at thin ones, sometimes freezing before it can push through. Maintaining consistent wall thickness throughout the part is one of the most effective design-level prevention strategies. Where thickness transitions are unavoidable, use gradual tapers rather than abrupt steps.
Use Sensor Data to Catch Problems Early
Modern injection molding facilities use independent sensors, sometimes 40 to 50 per machine, to monitor each shot in real time. The three primary variables tracked are temperature, screw position, and cavity pressure. Pressure sensors follow the melt as it travels from the barrel into the cavity, while linear transducers track the screw’s position during injection to verify consistent shot size. If the shot size drifts even slightly, the system flags it before a short shot reaches the end of the line.
Engineering teams set tight tolerance bands for 10 to 15 critical characteristics. When any parameter drifts outside specification, the machine automatically rejects the part and signals a robot to remove it. This shot-to-shot monitoring, enabled by Industry 4.0 platforms, turns short shot detection from a visual inspection problem into an automated one. Even if you don’t have a fully sensorized cell, adding a single cavity pressure sensor at the last-to-fill location gives you an early warning system that catches incomplete fills before they pile up.
Troubleshooting Order That Saves Time
When a short shot appears, resist the urge to change multiple variables at once. A logical sequence keeps you from chasing your own adjustments:
- Verify shot size. Confirm the machine is delivering the correct volume of material. Weigh a few shots and compare to the target.
- Check the non-return valve. Run a screw-creep test to rule out backflow before adjusting anything else.
- Inspect the nozzle. Look for blockages, misalignment, or material degradation.
- Raise melt temperature. Increase in small steps (5 to 10°C) and monitor part quality.
- Increase injection pressure and speed. Bump pressure 10 to 15% for high-viscosity resins. Use staged speed profiles for complex geometry.
- Reduce clamp force. Lower it incrementally and watch for improved filling (and check for flash).
- Clean or add vents. If the short shot location is consistent and at the end of fill, venting is likely the bottleneck.
- Evaluate gate and runner design. This is a last resort since it requires mold modifications, but persistent problems in multi-cavity tools often trace back to flow imbalance.
Each step narrows the possible causes. By the time you reach mold modifications, you’ll have data confirming the mold is the problem rather than guessing.