In precision injection molding, the phenomenon of incomplete cavity filling is known in the industry as Short Shot (or “underfill”). This defect results in incomplete part geometry, compromising structural integrity and production yield. A systematic diagnosis of equipment, process, and mold factors is essential to resolve this issue.
Primary Causes and Diagnostic Protocol
- Insufficient Melt Supply: The screw’s plasticizing capacity or shot size setting is inadequate to fill the mold cavity volume. Gradually increase the shot size while monitoring residual melt at the screw tip and real-time part fill pattern. If full filling is still unattainable after maximum adjustment, consider upgrading to a higher-tonnage machine.
- Check Valve Failure: The non-return valve (check valve) at the screw tip may be worn or stuck, allowing molten plastic to flow backward during plasticizing. This reduces actual injection volume despite increased shot size. If increasing shot size yields no improvement in fill, inspect and replace the check valve assembly immediately.
- Systemic Leakage: Leaks may occur at the barrel-nozzle interface or between the nozzle and the mold sprue bushing. Before proceeding with other diagnostics, shut off injection and visually inspect for melt extrusion under pressure. Only after confirming no leakage should further checks be conducted.
These three mechanical and process-related causes must be eliminated in sequence before investigating secondary factors such as poor mold venting, undersized gates, low material flowability, or suboptimal mold temperature.
Frequently Asked Questions (FAQ)
Q1: Is a short shot in injection molding always caused by an injection machine malfunction?
A: No. While equipment issues (e.g., leakage, check valve failure) are primary, mold design flaws (e.g., blocked vents, narrow gates) or material conditions (e.g., high moisture content, excessive regrind ratio) can also cause short shot. A holistic evaluation is required.
Q2: How can I distinguish between insufficient material and leakage?
A: If increasing shot size improves part fill proportionally, the issue is insufficient material. If shot size increases but fill remains unchanged despite stable injection pressure, leakage at the check valve or nozzle is likely. Pressure sensors are recommended for diagnostic precision.
Q3: What advantages do local enterprises in Dongguan offer for resolving the shortage?
A: Leveraging Dongguan’s status as “China’s Mold Capital,” Yize Mould provides rapid on-site support, including machine calibration, component replacement, and process optimization. Emergency service is available within 24 hours across key manufacturing zones including Chang’an, Humen, and Houjie.
