A major factor we need to consider during the injection molding process of plastic parts is the temperature of the mold. It can be said that the temperature of the mold is related to the quality of the entire product and affects the effectiveness of the entire injection molding process. So how does the temperature of the plastic mold affect the quality of the plastic parts? Today, the injection molding engineer of Huanke Precision will take you to understand the factors.
1. The influence of mold temperature on appearance
First, if the mold temperature is too low, the melt fluidity will be reduced, and under-injection may occur. The mold temperature affects the crystallinity of the plastic. For ABS, if the mold temperature is too low, the product finish will be low. Compared with fillers, plastics are more likely to migrate to the surface when the temperature is high. Therefore, when the temperature of the injection mold is high, the plastic components will be closer to the surface of the injection mold, the filling will be better, and the brightness and gloss will be higher.
However, the temperature of the injection mold should not be too high. If it is too high, it will easily stick to the mold, and obvious bright spots will appear in some parts of the plastic part. If the temperature of the injection mold is too low, it will also cause the plastic part to hold the mold too tightly, and it is easy to strain the plastic part during demoulding, especially the pattern on the surface of the plastic part.
Multi-stage injection molding can solve positional problems. For example, if the product has air marks when injecting glue, segmented injection molding can be used. In the injection molding industry, for glossy products, the higher the temperature of the mold, the higher the glossiness of the surface of the product. On the contrary, if the temperature is low, the glossiness of the surface will be relatively low. However, for products made of sun-textured PP materials, the higher the temperature, the lower the glossiness of the product surface. The lower the glossiness, the higher the color difference. Gloss and color difference are inversely proportional.
Therefore, the most common problem caused by mold temperature is a rough surface finish of the molded part, which is usually caused by the mold surface temperature being too low.
Mold shrinkage and post-mold shrinkage of semi-crystalline polymers depend primarily on mold temperature and part wall thickness. Uneven temperature distribution in the mold will lead to differential shrinkage, making it impossible to guarantee that the part will meet the specified tolerances. The worst-case scenario is that shrinkage exceeds the correctable value regardless of whether unreinforced or reinforced resin is processed.
2. Impact on product size
If the mold temperature is too high, the melt will thermally decompose. After the product comes out, the shrinkage rate in the air will increase, and the product size will become smaller.
When the mold is used in low temperature conditions, if the size of the parts becomes larger, it is generally due to the fact that the surface temperature of the mold is too low. This is because the surface temperature of the mold is too low, and the product shrinks less in the air, so the size is larger! Low mold temperature accelerates the “freeze orientation” of molecules, which increases the thickness of the frozen layer of melt in the mold cavity. At the same time, low mold temperature hinders the growth of crystals, thereby reducing the molding shrinkage of the product.
On the contrary, if the mold temperature is high, the melt will cool slowly, the relaxation time will be long, the orientation level will be low, and it will be conducive to crystallization, and the actual shrinkage of the product will be larger.
If the start-up process takes too long before the dimensions stabilize, this indicates poor mold temperature control, as the mold takes longer to reach thermal equilibrium. Uneven heat dispersion in certain parts of the mold will greatly extend the production cycle, thereby increasing molding costs! The mold temperature is constant, which can reduce the fluctuation of molding shrinkage and improve dimensional stability.
For crystalline plastics, high mold temperature is conducive to the crystallization process. Fully crystallized plastic parts will not change in size during storage or use; however, high crystallinity will cause large shrinkage.
For softer plastics, low mold temperature should be used during molding, which is beneficial to dimensional stability.
For any material, constant mold temperature and consistent shrinkage are beneficial to improving dimensional accuracy!
3. Effect of mold temperature on deformation
If the cooling system design of the mold is unreasonable or the mold temperature is improperly controlled, insufficient cooling of the plastic parts will cause warping and deformation of the plastic parts.
For the control of mold temperature, the temperature difference between the front mold and the rear mold, the mold core and the mold wall, the mold wall and the insert should be determined according to the structural characteristics of the product, so as to control the difference in cooling shrinkage speed of each part of the molding, and the plastic parts After demoulding, it tends to bend toward the side with higher temperature in the pulling direction to offset the difference in orientation shrinkage and prevent the plastic parts from warping and deforming according to the orientation rules.
For plastic parts with completely symmetrical body structure, the mold temperature should be kept consistent to ensure balanced cooling of all parts of the plastic part. The mold temperature is stable and the cooling is balanced, which can reduce the deformation of plastic parts.
Excessive mold temperature difference will cause uneven cooling and inconsistent shrinkage of the plastic parts, which will generate stress and cause warping and deformation of the plastic parts, especially plastic parts with uneven wall thickness and complex shapes.
4. Effect of mold temperature on mechanical properties (internal stress)
If the mold temperature is low, the weld marks of the plastic parts will be obvious, and the product strength will be reduced; for crystalline plastics, the higher the crystallinity, the greater the tendency of stress cracking of the plastic parts; in order to reduce stress, the mold temperature should not be too high (PP, PE). For high-viscosity amorphous plastics such as PC, the stress cracking is related to the internal stress of the plastic part. Increasing the mold temperature is conducive to reducing the internal stress and reducing the tendency of stress cracking.
The internal stress is expressed in the form of obvious stress marks! The reason is: the formation of internal stress during molding is basically caused by different thermal shrinkage rates during cooling. When the product is molded, its cooling gradually extends from the surface to the inside. The surface first shrinks and hardens, and then gradually reaches the inside. In this process Internal stress is generated due to the difference in shrinkage speed.
When the residual internal stress in the plastic part is higher than the elastic limit of the resin, or when it is corroded by a certain chemical environment, cracks will occur on the surface of the plastic part.
Research on PC and PMMA transparent resins shows that the residual internal stress is in the form of compression in the surface layer and in the form of extension in the inner layer. The surface compressive stress depends on the surface cooling condition. A cold mold rapidly cools down the molten resin, resulting in high residual internal stress in the molded product.
Mold temperature is the most basic condition for controlling internal stress. A slight change in mold temperature will greatly change its residual internal stress.
Generally speaking, each product and resin has its minimum mold temperature limit for acceptable internal stress. When molding thin walls or long flow distances, the mold temperature should be higher than the minimum limit for general molding.
On the side of the mold with higher temperature, after the product cools down, the deformation direction must be towards the side with higher mold temperature! It is recommended that the front and rear mold temperatures be reasonably selected according to needs. Please refer to the physical property tables of various materials for the mold temperature!
5. Affect the heat distortion temperature of products
Especially for crystalline plastics, if the product is molded at a lower mold temperature, the orientation and crystallization of the molecules are instantly frozen. When a higher temperature use environment or secondary processing conditions are used, the molecular chains will be partially rearranged. And the process of crystallization causes the product to deform even well below the heat distortion temperature (HDT) of the material.
The correct approach is to produce at a recommended mold temperature close to its crystallization temperature, so that the product can be fully crystallized during the injection molding stage and avoid post-crystallization and post-shrinkage in a high-temperature environment.
In short, mold temperature is one of the most basic control parameters in the injection molding process, and it is also the primary consideration in mold design.