Mold Tech

Five major effects of mold temperature on injection molded parts

• 2024-09-23

Mold temperature refers to the surface temperature of the cavity in contact with the product during the injection molding process. It directly affects the cooling rate of the product in the cavity, thereby greatly influencing the internal performance and external appearance of the product. This article briefly discusses 5 effects of mold temperature on quality control of injection molded parts, providing information for reference.

Mold

Industrial production uses various molds and tools for injection molding, blow molding, extrusion, die casting, or forging to obtain the required products. In simple terms, molds are tools used to produce molded items, consisting of various components. Different molds are composed of different parts. They mainly achieve the processing of product shapes by changing the physical state of the molding material.

01 Impact of mold temperature on product appearance

Higher temperatures can improve the flowability of resin, usually resulting in a smooth and glossy surface of the product, especially enhancing the surface aesthetics of glass fiber-reinforced resin products. It also improves the strength and appearance of weld lines.

For textured surfaces, if the mold temperature is low, the melt may have difficulty filling into the roots of the texture, resulting in a glossy appearance on the product surface and inability to replicate the true texture of the mold surface. Increasing mold temperature and material temperature can achieve the desired textured effect on the product surface.

02 Impact on internal stress of the product

The formation of internal stress during molding is mainly due to different thermal shrinkage rates during cooling. After the product is molded, its cooling progresses from the surface to the interior. The surface first shrinks and hardens, followed by the interior, resulting in internal stress due to differences in shrinkage rates.

When the residual internal stress in the plastic part exceeds the elastic limit of the resin or under certain chemical environments, cracks may appear on the surface. Studies on PC and PMMA transparent resins show that residual internal stress is compressive at the surface layer and tensile in the inner layer.

Surface compressive stress depends on the surface cooling conditions, with a cold mold causing rapid cooling of the molten resin, resulting in higher residual internal stress in the molded product. Mold temperature is a basic condition for controlling internal stress, and slight changes in mold temperature can significantly alter residual internal stress. Generally, each type of product and resin has a lower limit for acceptable internal stress. For thin-walled or long-flow distance products, the mold temperature should be higher than the lower limit used in general molding.

03 Product warpage

If the mold's cooling system is designed improperly or if mold temperature control is inadequate, insufficient cooling of the plastic part can cause warpage deformation.

For mold temperature control, temperature differences between core and cavity, core and mold wall, and mold wall and inserts should be determined based on the structural characteristics of the product. This utilizes different cooling shrinkage rates in different parts of the mold to counteract directional shrinkage differences and avoid warpage deformation according to directional patterns after demolding.

For plastic parts with completely symmetrical structures, mold temperature should be consistent to ensure balanced cooling of different parts of the plastic part.

04 Impact on product shrinkage

Low mold temperature accelerates molecular "frozen orientation," increasing the thickness of the frozen layer of melt inside the cavity and hindering crystalline growth, thereby reducing the molding shrinkage of the product. Conversely, higher mold temperatures result in slower melt cooling, longer relaxation time, lower orientation levels, and are conducive to crystallization, leading to a higher actual shrinkage rate of the product.

05 Impact on product heat deflection temperature

Especially for crystalline plastics, if products are molded at a low mold temperature, molecular orientation and crystallization are instantly frozen. When subjected to a higher temperature environment or secondary processing conditions, molecular chains will partially rearrange and undergo a crystallization process, causing deformation even at temperatures far below the material's heat deflection temperature (HDT).

The correct approach is to produce products at a mold temperature close to their recommended crystallization temperature during injection molding, ensuring sufficient crystallization during production and avoiding subsequent crystallization and shrinkage at high temperatures.

In conclusion, mold temperature is a fundamental control parameter in injection molding processes and also a consideration in mold design. Its impact on product molding, secondary processing, and use cannot be underestimated.

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