​RHCM MOLD TEMPERATURE CONTROLLING MACHINE HOW TO SOLVE WELD LINES, SINK MARKS

For RHCM (Rapid Heat Cycle Molding) process, the core strategy for addressing weld lines and sink marks is to leverage its dynamic mold temperature control capability: maintain high temperatures during the filling stage to eliminate weld lines, and rapidly cool during the packing/holding and cooling stages to control sink marks while shortening cycle time.

Below are the specific optimization directions and measures.

1.For Weld Lines: Improve Flowability and Fusion Quality

Weld lines occur when two melt fronts meet and fail to fully fuse due to insufficient temperature. The core countermeasure in RHCM is to ensure the melt temperature remains high enough before the fronts converge.

Increase and optimize mold heating temperature: Studies show that maximizing the mold surface temperature at the end of heating is the key factor in reducing weld lines. Higher mold temperatures allow the plastic melt to flow more smoothly and maintain better fusion when meeting.

Adopt localized heating strategies: Conventional RHCM heats the entire mold, leading to longer cooling times. An effective alternative is heater-assisted RHCM, which applies heat only to localized areas prone to weld lines. This effectively eliminates weld lines while shortening cycle times.

Optimize heating system design: For electrically heated RHCM molds, optimizing the heat flux distribution and layout of heating cartridges can make cavity surface temperature distribution more uniform. After optimization, the maximum surface temperature difference can be reduced by 63.4%, effectively suppressing both weld lines and sink marks, with surface roughness (Ra) improved from 320 nm down to 118 nm.

2.For Sink Marks: Ensure Adequate Packing and Rapid Solidification

Sink marks result from insufficient packing pressure compensation during plastic shrinkage upon cooling. RHCM addresses this through precise control of the cooling and packing stages.

Optimize cooling process parameters: After injection and packing are completed, RHCM rapidly switches to cooling media (such as water or chilled water) to quickly cool the mold, allowing the plastic to solidify and set rapidly, thereby reducing sink marks and warpage. Cooling time and cooling temperature should be optimized based on product wall thickness.

Use CAE simulation for multi-parameter optimization: Because RHCM process parameters (heating/cooling temperatures, timing, packing pressure, etc.) are interactive, it is highly beneficial to use simulation software like Moldflow for optimization. Research often employs algorithms such as sequential approximate optimization to find the best balance among three objectives: reducing weld lines, minimizing sink marks/warpage, and shortening cycle time.

Trade-offs and Summary

In RHCM, eliminating weld lines typically requires higher mold temperatures, but this prolongs cooling time, increases the risk of sink marks and warpage, and extends the overall molding cycle. Therefore, addressing these two defects is fundamentally a multi-objective optimization problem. The typical approach includes:

Pre-molding simulation: Use mold flow analysis software (e.g., Moldflow) to perform transient analysis of the entire filling, packing, and cooling process to predict defect risks in advance.

Parameter optimization: Based on simulations or design of experiments (e.g., Taguchi methods), find an optimal set of process parameters (heating temperature, cooling temperature, switchover timing, packing pressure, etc.) that achieves the best balance between production efficiency and product quality.