Multi-cavity injection molding is a complex engineering task whose core objective is to ensure that parts produced from all cavities are highly consistent in dimensions, weight, appearance, and performance. Achieving this goal requires attention to multiple critical aspects throughout the entire process.
I. Mold Design Stage
This is the most critical phase determining the success of a multi-cavity mold.
Runner System Balance
Naturally Balanced Runner: Also known as a “fishbone” or “H-shaped” runner. It uses symmetrical geometry to ensure the melt reaches each gate simultaneously via identical flow paths and resistance. This is the preferred solution, especially for high-precision parts.
Artificially Balanced Runner: Used when the number of cavities is too high or the mold layout restricts a natural balance. It involves calculating and adjusting the diameter and length of different runner branches to make the melt reach all gates at the same time. This demands extensive experience from the designer.
Hot Runner System
Thermal Balance: Ensuring uniform temperature for each hot nozzle is fundamental for consistent filling. It requires selecting high-quality hot runner systems with excellent thermal homogeneity.
Gate Type Selection:
Open Hot Nozzles: Lower cost, but may introduce minor filling variations.
Valve-Gated Hot Nozzles: Highly recommended for demanding multi-cavity molds. Through Sequential Valve Gating (SVG) technology, it controls the opening and closing sequence of individual gates, there solving runner balance issues, eliminating weld lines, and reducing injection pressure.
Cooling System
Uniform Cooling: Each cavity must have independent, symmetrical, and balanced cooling circuits. This ensures identical cooling rates and effects for every cavity. Uneven cooling leads to part warpage, inconsistent shrinkage, and longer cycle times.
Cooling Efficiency: Multi-cavity molds concentrate heat, requiring highly efficient cooling designs (e.g., beryllium copper inserts, turbulent flow cooling) to maintain production cycle times.
Cavities & Venting
Cavity Consistency: All cavities must be machined using the same precision equipment (e.g., slow wire EDM, mirror finish EDM) under identical settings to ensure micro-level consistency in their dimensions and surface roughness.
Venting System: The depth, location, and size of venting channels must be consistent and sufficient for each cavity. Poor venting can cause short shots, burns, trapped air, and other defects, which may manifest differently across cavities.
II. Injection Molding Process Stage
Even with a perfect mold design, process tuning is crucial.
Process Window
Find a stable and robust process window. This means part quality from all cavities remains consistent despite minor parameter fluctuations.
Employ Scientific Molding principles, using methods like V/P switchover, to ensure process repeatability.
Material & Drying
Batch-to-batch variations in material viscosity affect flow. For multi-cavity molds, it is advisable to use consistent, high-quality brand-name materials.
Materials must be thoroughly dried. Even trace amounts of moisture can cause issues (e.g., splay, hydrolysis degradation) in one or more cavities.
III. Production Maintenance & Monitoring
Preventive Maintenance
Regularly clean and maintain the hot runner system to prevent gate blockage from degraded material.
Periodically inspect the condition of mold vents, ejector pins, sliders, and other moving components to ensure they are clear and function smoothly.
Process Monitoring
Once stable production is achieved, weigh the parts from each cavity individually. This is the “gold standard” for verifying mold balance.
Monitor key parameters each cycle – such as peak pressure, V/P switchover position, screw recovery time – and set alarm limits to detect abnormalities immediately.