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In injection molding production, short shots are the most "basic yet fatal" defect–directly leading to product scrap, delivery delays, and even customer complaints. Its essence is that melt flow resistance exceeds injection pressure, or there is insufficient melt supply, failing to completely fill mold cavity. This article, combining typical cases from automotive and 3C industries, guides you through a systematic approach to overcoming this common trial molding obstacle, covering four modules: flow mechanism, five-dimensional troubleshooting, precise solutions, and long-term prevention.

 

Core contradiction of short shots is that during filling process, melt encounters excessive flow resistance or insufficient available pressure/melt, preventing it from reaching end of mold cavity. This requires analyzing underlying logic from four dimensions: material flowability, process driving force, mold flow characteristics, and equipment supply capacity.
1. Material Characteristics: "Inherent Limitations" of Flowability and Viscosity
Flow performance of material directly determines difficulty of filling:
- Low-flow materials (e.g., PC, PMMA, high-viscosity PP): Low melt flow index (e.g., PC melt index < 10g/10min), high melt viscosity, and high flow resistance. In a trial molding of an automotive headlight cover (PC material), short shots occurred at the end because PC melt index was only 8g/10min (standard requires ≥12g/10min), making it difficult for melt to flow through long, narrow runner.
- Hygroscopic materials (e.g., PA, PET): When not dried, moisture reduces melt viscosity (e.g., PA viscosity decreases by 30% at 0.2% moisture content), but vaporization of moisture creates air resistance (bubbles hinder melt flow). In a case of short shots in a home appliance connector (PA66), testing revealed a material moisture content of 0.18% (standard ≤0.1%). After drying, air resistance disappeared, and filling was complete.
- Material degradation: Excessive melt temperature leads to molecular chain breakage (e.g., PVC decomposes above 190℃), resulting in a sudden drop in melt viscosity but insufficient strength, easily causing "flow interruption" in thin-walled areas.
2. Process Parameters: Insufficient Driving Force from Pressure and Speed
Process settings directly determine whether melt can "reach" end of mold cavity:
- Insufficient injection pressure: Unable to overcome resistance of runner/cavity. In a mobile phone frame (PC+ABS) with short shots in thin-walled area, increasing injection pressure from 100 MPa to 120 MPa increased melt pressure by 20%, resulting in successful filling.
- Insufficient injection speed: Melt cools prematurely, forming a "solidified layer" at the front that hinders flow. In a toy casing with short shots at the end of a long runner, increasing injection speed (from 40% to 70%) resulted in more continuous melt flow and eliminated short shots.
- Insufficient barrel temperature: High melt viscosity (e.g., for ABS, decreasing temperature from 230℃ to 210℃ increases viscosity by 50%). In a tool handle with short shots, increasing melt temperature back to 240℃ improved melt fluidity and resulted in complete filling.
3. Mold Design: Acquired Defects in Flow Guidance and Space
Mold structure determines "path" and "space" of melt flow:
- Improper gate location/size: Gate is far from thin-walled area (e.g., gate is located at thick-walled end of product's thin-walled end), requiring melt to flow a long distance, resulting in significant pressure loss. In a car trim panel with short shots in thin-walled area at the end, changing gate from thick-walled side to center of thin-walled area reduced flow distance by 50%, solving short shot problem.
- Runner too thin/too long: Main runner diameter < 6mm or runner length > 200mm (standard ≤ 150mm), resulting in a sharp increase in melt flow resistance. In a medical device casing with short shots, increasing runner diameter from 5mm to 6mm reduced pressure loss by 30%, resulting in complete filling.
- Poor venting: Gas is not vented from end of mold cavity (e.g., no vent groove at the bottom of a deep cavity), and compressed gas forms a "gas wall" that hinders melt. In an electronic device casing with short shots, mold disassembly revealed no vent groove at the end. After adding a 0.02mm deep vent groove, melt flowed smoothly.
4. Equipment Capability: Injection Volume and Screw "Supply Shortage"
Injection molding machine's injection capacity is "basic guarantee":
- Insufficient injection volume: The total weight of product exceeds 80% of injection molding machine's maximum injection volume (e.g., a 100-ton machine with an injection volume of 120g, and a product weight of 100g, representing 83%). A small part experienced material shortage; after switching to a 150-ton machine (injection volume 180g), supply was sufficient and filling was complete.
- Screw wear: Excessive clearance between screw and barrel (e.g., a Φ45mm screw with clearance increasing from 0.1mm to 0.2mm) leads to melt backflow and a reduction in actual injection volume. A precision gear experienced material shortage; screw clearance was found to be excessive, and after replacing screw, injection volume stabilized.

Material shortage is easily confused with flashing and trapped air, requiring a five-step troubleshooting process combining defect location, process conditions, and equipment status:
Step 1: Observe location and morphology of material shortage (3-minute quick assessment)
- Material shortage at the end/thin-walled area: Often due to high flow resistance (far from gate, narrow runner) or poor material fluidity (e.g., PC with long runners).
- Randomly dispersed material shortage: Often due to undried material (air bubbles obstructing flow) or process fluctuations (unstable pressure/temperature).
- Material shortage near gate: May be due to premature gate freezing (premature holding pressure switch) or low material temperature (premature solidification of melt).
Step 2: Verify material and drying (5-minute test)
- Measure melt flow index: Compare with standard melt flow index of material (e.g., PC requires ≥12g/10min). If it is too low, material needs to be replaced or material temperature increased.
- Check moisture content: Hygroscopic materials (PA, PET) are prone to air obstruction if not dried. Use a rapid moisture analyzer to test (e.g., PA ≤0.1%). A PA connector experienced material shortage; moisture content was 0.15%, and after drying, air obstruction disappeared.
- Material change verification: Test molding with same brand of dried material. If material shortage is reduced, material problem is ruled out.
Step 3: Adjust Process Parameters (10 minutes of trial and error)
- Increase pressure/speed: Prioritize increasing injection pressure (increase by 10 MPa each time) and speed (increase by 10% each time). For a mobile phone case with material shortage, increasing injection pressure from 90 MPa to 110 MPa and speed from 50% to 70% resulted in complete filling at the end.
- Increase material/mold temperature: Increase barrel temperature (e.g., ABS from 220℃ to 240℃, viscosity decreases by 25%) or mold temperature (e.g., PC from 80℃ to 100℃, improving melt flowability). A tool handle with material shortage was resolved after increasing material temperature.
- Extend injection time: Extend injection time (e.g., from 2s to 3s) to ensure melt has enough time to fill mold. A thin-walled part with material shortage was completely filled after increasing injection time by 50%.
Step 4: Check Mold Design (30 minutes - 1 hour)
- Measure gate/runner: Use a projector to measure gate diameter (e.g., ABS requires Φ6-8mm) and runner length (≤150mm). A home appliance casing had an excessively long runner (220mm), which was shortened to 180mm, reducing pressure loss by 25%.
- Check venting: Use a feeler gauge to measure depth of vent groove (e.g., ABS requires 0.02-0.03mm), or observe if there is scorching at material shortage location (due to air resistance causing local high temperature). A material shortage in an electronic device casing was resolved after adding vent grooves.
- CAE mold flow analysis: Use Moldflow to simulate filling process and visually identify pressure loss points (e.g., if pressure drop in a certain area is >50MPa, runner needs to be optimized).
Step 5: Confirm Equipment Capability (10 minutes of testing)
- Verify injection volume: Calculate product weight + runner weight, ensuring it does not exceed 80% of injection molding machine's maximum injection volume. A part with a total weight of 100g on a 100-ton machine had an injection volume of 120g (83%), which was resolved by switching to a 150-ton machine.
- Check screw wear: Use a micrometer to measure gap between screw and barrel (standard ≤0.1mm). If it exceeds standard, screw needs to be replaced. A precision gear was experiencing short shots due to material shortage; screw clearance was 0.18mm, and injection volume stabilized after replacement.

 

If main cause is process parameters:
- Optimize injection curve: Use a "slow-fast-slow" segmented injection (e.g., 50% speed in filling stage → 80% speed in holding stage → 30% speed at the end) to balance pressure and flow.
- Increase mold temperature/material temperature: For high-viscosity materials (PC, PMMA), increasing mold temperature by 10℃ improves melt fluidity by approximately 15%, reducing risk of short shots.
If main cause is mold problems:
- Adjust gate/runner: Move gate to center of thin-walled area (e.g., for short shots in automotive trim panels, add a submerged gate at the end); or increase runner diameter (from 5mm to 6mm).
- Add venting: Machine 0.02-0.03mm deep vent grooves at short shot location (e.g., bottom of a deep cavity), or embed porous steel (e.g., PM-35) to accelerate venting.
If main cause is equipment capability:
- Replace/modify equipment: If injection volume is insufficient, upgrade to a larger tonnage injection molding machine; if screw is worn, replace it with a new screw (clearance ≤ 0.1mm).
- Increase auxiliary injection: For large molds, use "sequential valve injection" to compensate for shrinkage in different areas and reduce pressure loss in main runner.

1. Product Design Stage: Avoiding Short Shot Risks
- Control wall thickness uniformity: Avoid abrupt changes in wall thickness (e.g., R≥1mm transition) to reduce concentrated flow resistance. A home appliance casing originally had an abrupt wall thickness change from 2mm to 6mm, resulting in severe short shots; after mold modification, it was changed to 2mm→4mm→6mm (R=1.5mm), resulting in smooth filling.
- Optimize gate layout: For long, narrow products, use multiple gates (e.g., one gate every 50mm) to shorten flow distance.
2. Mold Development Stage: Ensuring Flow from Source
- High-precision runner machining: Runner diameter tolerance controlled to ±0.05mm to avoid increased resistance due to machining errors.
- Reserved venting space: Reserve venting grooves in deep cavities and thin-walled areas (depth = material flow resistance * 0.01mm), or design "venting wells" (diameter Φ5mm, depth 10mm).
3. Mold Trial and Verification Stage: Establishing a "Material Shortage Risk Profile"
- Recording critical parameters: For each product, record parameters such as "minimum injection pressure without material shortage" and "shortest injection time," which can be directly used in subsequent production.
- Material shortage sensitivity testing: During mold trials, intentionally reduce pressure/speed (e.g., pressure -10%, speed -10%) to observe whether material shortage occurs, quantifying mold/process's resistance to material shortage.
4. Production Management Stage: Dynamic Monitoring and Regular Maintenance
- Real-time monitoring of injection pressure: Install pressure sensors; an alarm is triggered when injection pressure fluctuation > 10% (e.g., if injection pressure of a certain injection molding machine drops from 110MPa to 99MPa, a warning is triggered to prevent material shortage).
- Regular maintenance of screw/barrel: Check screw wear monthly (gap ≤ 0.1mm), and clean residual material from barrel quarterly (to prevent material degradation from affecting fluidity).

Essence of material shortage is "flow resistance > driving pressure" or "insufficient melt supply." Troubleshooting requires a five-dimensional approach: material (fluidity/drying), process (pressure/speed), mold (gate/venting), and equipment (injection volume/screw). Prioritize adjusting process (low cost, quick results), then optimize mold (to solve structural problems), and finally upgrade equipment (to address fundamental limitations). The key to long-term error prevention is "design prediction + parameter solidification + dynamic monitoring," ultimately achieving "zero material shortage in molds and stable production."

 

Core Mantra: Material shortage first check thin end, flow resistance is the most common; increase pressure and speed to test process, thicken mold gate; design to avoid sudden changes in wall thickness, archive data to prevent recurrence.