Short Shots in Plastic Injection Molded Parts

Short Shots in Injection Molded Parts: Causes and Comprehensive Solutions

Short shots, also known as incomplete filling or underfilling, refer to the incomplete formation of plastic parts at the end of the melt flow path or in specific cavities of multi-cavity molds—especially in thin-walled areas or the end of flow paths. This defect is a common challenge in manufacturing custom plastic molding parts, as it directly impairs product functionality and appearance. The root cause of short shots is the premature solidification of molten plastic before the mold cavity is fully filled, typically due to excessive flow resistance. Key factors affecting melt flow length include part wall thickness, mold temperature, injection pressure, melt temperature, and material composition; improper control of these factors can lead to short shots in custom plastic molding parts. This article systematically analyzes the causes of short shots and provides targeted solutions to ensure the quality of custom plastic molding parts.

1. Improper Equipment Selection

Selecting the right injection molding machine is the foundational step in preventing short shots for custom plastic molding parts.

Ensure the machine’s maximum injection volume exceeds the total weight of the plastic part and sprue.
The total injection weight should not exceed 85% of the machine’s plasticizing capacity to avoid insufficient material supply caused by mismatched equipment specifications.

2. Insufficient Material Feeding

Volume-based feeding is the most common method in production, but it is prone to feeding issues that affect custom plastic molding parts:

Uneven particle sizes of raw materials or "bridging" at the bottom of the hopper can block material flow.
Excessively high temperatures at the hopper inlet can also cause poor material feeding.
Solutions: Unclog and cool the hopper inlet; use uniform particle size raw materials to prevent bridging.

3. Poor Raw Material Fluidity

Poor melt fluidity is a major cause of short shots, especially for custom plastic molding parts with complex structures:

Mold Optimization: Improve stagnation in the gating system by rational runner layout, increasing the size of gates, runners, and sprue, and using large nozzles.
Material Modification: Add appropriate additives to the raw material formula to enhance resin fluidity; reduce the proportion of regrind material (excessive regrind impairs fluidity).

4. Excessive Lubricant Dosage

Excessive lubricants in the raw material formula can exacerbate melt backflow in the barrel (if there is wear between the check ring and barrel), leading to insufficient feeding for custom plastic molding parts:

Reduce lubricant dosage appropriately.
Adjust the clearance between the barrel, screw, and check ring; repair worn components to prevent melt backflow.

5. Cold Slug and Impurity Blockages

Blockages in nozzles, gates, or runners by cold slugs or impurities disrupt melt flow, causing short shots in custom plastic molding parts:

Disassemble and thoroughly clean the nozzle to remove impurities and cold slugs.
Enlarge the cold slug well and runner cross-section in the mold to ensure smooth material flow.

6. Irrational Gating System Design

A poorly designed gating system is a key factor in short shots for multi-cavity custom plastic molding parts:

Gate Balancing: Ensure gate balance in multi-cavity molds, with gate size proportional to the part weight in each cavity; place gates at thick wall sections of parts or use balanced runner layouts.
Runner/Gate Sizing: Small, thin, or long runners/gates cause excessive pressure loss. Enlarge their cross-sections; adopt multi-point gating for large or complex custom plastic molding parts if necessary.

7. Poor Mold Venting

Trapped air in the mold cavity generates high pressure under melt extrusion, preventing full cavity filling and causing short shots in custom plastic molding parts:

Vent Design Optimization: Add vent grooves or holes at underfilled areas of deep cavities; cut vent grooves (0.02–0.04mm deep, 5–10mm wide) on the mold parting surface, with vents at the final filling points of the cavity.
Raw Material Drying: Dry raw materials to reduce moisture and volatile content (excess moisture/volatiles generate gas that impairs venting).
Process Adjustments: Increase mold temperature, reduce injection speed, decrease gating system flow resistance, lower clamping force, and slightly increase mold clearance to assist venting.

8. Improper Mold Temperature Control

Low mold temperatures cause rapid melt solidification, preventing full cavity filling for custom plastic molding parts:

Preheat the mold to the required process temperature before startup; reduce cooling water flow in the mold during initial production cycles.
Inspect the mold cooling system design if mold temperature is difficult to raise, ensuring rational cooling channel layout.

9. Low Melt Temperature

Melt temperature directly affects fluidity; low temperatures shorten the filling length for custom plastic molding parts:

Check and repair the barrel heater; heat the barrel to the specified temperature and hold it for a period to ensure stability before startup.
For low-temperature injection (to prevent melt decomposition), extend the injection cycle to compensate for underfilling; for screw-type machines, increase the temperature of the front barrel section to improve melt fluidity.

10. Low Nozzle Temperature

Frequent contact between the nozzle and the cooler mold lowers nozzle temperature, causing melt solidification and blockages for custom plastic molding parts:

Separate the nozzle from the mold during mold opening to reduce heat transfer.
Inspect and repair the nozzle heater; raise nozzle temperature to avoid excessive pressure loss from solidified melt.

11. Insufficient Injection Pressure or Holding Pressure

Injection pressure is proportional to filling length; insufficient pressure leads to short shots in custom plastic molding parts:

Slow down the injection advance speed and extend injection time to increase pressure; if pressure reaches the upper limit, raise melt temperature to reduce viscosity (avoid overheating to prevent thermal decomposition).
Control holding pressure time rationally: short holding time causes underfilling, while excessive time leads to other defects (adjust based on part structure).

12. Slow Injection Speed

Slow injection speed causes melt cooling during filling, reducing fluidity and leading to short shots in custom plastic molding parts:

Increase injection speed appropriately (balance speed to avoid other defects such as flash or burn marks).

13. Irrational Part Structure Design

Poor structural design (e.g., disproportionate wall thickness/length, overly complex shapes, large molding areas) hinders melt flow in thin-walled sections for custom plastic mold manufacturing:

Consider the relationship between wall thickness and the maximum melt flow length during part design; follow recommended thickness ranges (1–3mm for standard parts, 3–6mm for large parts).
Avoid extreme thicknesses (＜0.5mm or ＞8mm), as they impair mold filling and part quality (e.g., PE minimum thickness: 0.5mm; cellulose plastics minimum thickness: 0.7mm).

14. Conclusion

Preventing short shots in custom plastic molding parts is a systematic project involving equipment selection, material management, mold design, and process optimization. By addressing the root causes outlined above and implementing targeted solutions—such as optimizing mold venting, adjusting injection parameters, and improving part structural design—manufacturers can effectively eliminate short shots. Continuous monitoring and refinement of production processes are essential to ensuring the consistency and quality of custom plastic molding parts, meeting the diverse requirements of customers in various industries.

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