Variation Law of Thermal Expansion for Seal Clearance of Plastic Injection Molds
When plastic molds operate continuously under production conditions, the seal clearance measured at room temperature will change with temperature rise. Steel expansion compresses the matching gap, which is one of the common root causes of flash, mold jamming and seal failure during mass production. Mastering the variation rules of thermal expansion of seal clearance can provide reliable reference for mold design, trial mold and daily maintenance, especially for thin-walled parts such as wire grommets with annular sealing structure.
Basic Logic of Steel Thermal Expansion
The core mold inserts and sealing surfaces of injection molds are mostly made of mold steel. When heated, the material expands in all directions. The higher the temperature, the greater the expansion value. Common pre-hardened steel and quenched steel have similar thermal expansion coefficients. As temperature rises, the length and width of the sealing surface expand synchronously, which reduces the reserved seal clearance. The clearance measured in cold mold state is only applicable to shutdown status, and the actual working clearance during molding will shrink after the mold heats up.
Change Rule of Seal Clearance During Heating Stage
In the process of heating the mold from room temperature to the set molding temperature, the seal clearance keeps shrinking. In the initial heating period, the surface temperature of the mold insert rises faster than the inner part, forming a temperature gradient. The surface expansion is larger than the core part, resulting in uneven reduction of seal clearance, and local sealing surfaces are prone to interference. When the mold reaches thermal equilibrium, the overall temperature of mold inserts is stable, the expansion amount tends to be fixed, and the seal clearance stays at the minimum value, which is the real working clearance of injection molding. If the reserved clearance at room temperature is too small, the gap will disappear after thermal balance, leading to mold surface crush and insert jamming. If the cold clearance is too large, flash will be generated easily even after heating.
Differentiated Change Rules of Seal Clearance at Different Positions
For flat parting surface sealing areas, expansion extends along the plane direction, and the change of seal clearance is relatively uniform. The shrinkage of clearance can be calculated according to the steel thermal expansion coefficient. The clearance of vertical shut-off sealing is more sensitive. Both core and cavity expand outward when heated, and the bidirectional expansion superposition makes the clearance shrink more obviously than flat parting surface. For annular sealing structure of thin wire grommet, radial expansion changes the sealing gap directly. The inner core expands outward while the cavity shrinks inward, squeezing the seal clearance from two sides. Separate calculation of radial expansion compensation is required in design.
Variation Law During Mold Cooling Process
When the mold shuts down and cools down, the seal clearance gradually increases in reverse. The surface of mold dissipates heat faster and contracts first, while the internal temperature drops slowly. In a short period, the shrinkage of different parts is inconsistent, leading to uneven clearance. After fully cooling to room temperature, the seal clearance returns to the initial assembly value. In the repeated heating and cooling cycle of mass production, the sealing surface suffers from fatigue wear, and the clearance will increase irreversibly with the increase of molding shots, raising the risk of flash.
Superposition Effect of Multi-component Combined Sealing
A complete mold consists of mold inserts, inserts, sliders and lifters. If different parts adopt steel grades with different thermal expansion coefficients, their expansion amounts differ after heating, resulting in local offset of seal clearance. Even for the same steel material, thin inserts expand faster than thick plates due to different thickness, causing misalignment of sealing surfaces. This superposition effect frequently occurs in multi-insert wire grommet molds, and it is an important reason for sudden local flash in the middle of mass production.
Design and Inspection Principles for Thermal Expansion
The design of seal clearance cannot only rely on cold mold measurement. The clearance under thermal equilibrium state shall be taken as the design benchmark. For flat parting seal, the reserved cold clearance needs to deduct the shrinkage caused by thermal expansion. For shut-off and radial annular seal, extra compensation allowance should be reserved to prevent hot jamming. Mold trial evaluation of sealing effect shall be carried out after the mold temperature stabilizes. During regular maintenance, inspect the matching status under hot condition, distinguish temporary clearance change caused by thermal expansion and permanent clearance increase caused by wear, and carry out targeted mold repair.
