Technical Points of Chinese RHCM High-gloss Zero-mark Injection Molds

RHCM rapid heat cycle molding heats and cools the cavity surface alternately. Plastic melt replicates the mirror cavity surface fully under high temperature before rapid cooling, producing glossy parts without weld lines and flow marks, eliminating secondary spraying and polishing. This process is widely used for air conditioner panels, automotive interior trims and smart home shells. Mold design directly determines whether zero-mark appearance can be achieved.

Working principle of RHCM process

Conventional molds keep constant low temperature. Melt forms a solidified skin once touching cold cavity surface and leaves weld marks. RHCM heats cavity surface above material heat distortion temperature before injection, generally 130℃ to 180℃. Melt viscosity drops and fills smoothly, and trapped air is eliminated. After filling, high-pressure cooling water is injected for fast setting. The switching speed between heating and cooling decides appearance quality and cycle time.

Heating and cooling circuit design

Alternating heating and cooling media are the core of RHCM molds. Steam or hot water is used for heating, and high-pressure cooling water is applied for cooling. The same channels inside cavity inserts switch medium through valves. Water channels are placed 8~15mm below cavity surface. Too far channels slow temperature response, while too close channels weaken mold strength. Surface temperature difference of cavity is controlled within 5~10℃ to guarantee uniform gloss.

Cavity steel and surface finishing requirement

High-polish corrosion-resistant steel such as S136 and 2316 is selected. Surface roughness reaches Ra0.02~0.05μm. Nickel plating, chrome plating or PVD coating reduces sticking and improves wear resistance. Repeated temperature alternation creates thermal fatigue stress, so steel must go through full tempering to stabilize internal structure and avoid cracks. Split lines on cavity surface are forbidden to protect the zero-mark effect.

Structural design for temperature alternation

Thermal expansion gap must be reserved for inserts. Inserts expand when heated and lock up if clearance is insufficient. Pressure test is carried out for heating channels to prevent medium leakage. Integral cavity inserts are preferred to reduce split seams that show on high-gloss surface. Parallel water channels reduce local turbulence and temperature difference.

Coordination between mold and molding process

Cavity keeps high temperature during filling. Injection speed and packing pressure are matched to achieve complete filling. Timing sequence of heating, injection, packing, cooling and ejection is synchronized between mold temperature controller and injection machine. Gates are arranged away from main cosmetic surfaces.

Defect analysis and prevention

Common RHCM defects include uneven gloss, residual weld lines, stress marks, yellowing and air lines. Uneven gloss usually comes from blocked cooling channels and inconsistent channel distance. Residual weld lines mean insufficient heating temperature or heating duration. Yellowing is caused by overheating and material degradation. Vacuum venting is used to eliminate trapped air.

Maintenance for stable mass production

Seals, switching valves and heating elements are vulnerable parts. Water channels are cleaned regularly to remove scale that reduces heat exchange efficiency. Mirror surfaces need periodic gentle polishing to avoid scratches. Temperature sensors and controllers are calibrated on schedule. Gloss and weld marks are sampled continuously during production.