In the injection mold industry, there are often new entrants in the industry who consult: Why does the temperature of the injection mold increase the gloss of the produced plastic parts? Now we use plain language to explain this phenomenon, and explain how to choose the mold temperature reasonably. The writing style is limited, so please advise us if it is wrong! (This chapter only discusses mold temperature, pressure and others are beyond the scope of discussion)
1. The influence of mold temperature on appearance:
First of all, if the mold temperature is too low, it will reduce the melt fluidity and undershoot 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 easier to migrate to the surface when the temperature is high. Therefore, when the temperature of the injection mold is high, the plastic component is 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 is easy to stick to the mold, and there will be obvious bright spots 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 when demolding, especially the pattern on the surface of the plastic part.
Multi-stage injection molding can solve the problem of position. For example, if the product has gas lines when the product is injected, it can be divided into segments. In the injection molding industry, for glossy products, the higher the temperature of the mold, the higher the gloss of the product surface. On the contrary, the lower the temperature, the lower the gloss of the surface. But for products made of sun-printed PP materials, the higher the temperature, the lower the gloss of the product surface, the lower the gloss, the higher the color difference, and the gloss and color difference are inversely proportional.
Therefore, the most common problem caused by mold temperature is the rough surface finish of molded parts, which is usually caused by too low mold surface temperature.
The molding shrinkage and post-molding shrinkage of semi-crystalline polymers mainly depend on the temperature of the mold and the wall thickness of the part. Uneven temperature distribution in the mold will cause different shrinkage, which makes it impossible to guarantee that the parts meet the specified tolerances. In the worst case, whether the processed resin is unreinforced or reinforced resin, the shrinkage exceeds the correctable value.
2. Impact on product size:
If the mold temperature is too high, the melt will be thermally decomposed. After the product comes out, the shrinkage rate in the air will increase, and the product size will become smaller. If the mold is used in low temperature conditions, if the size of the part becomes larger, it is generally due to the surface of the mold. The temperature is too low. This is because the mold surface temperature is too low, and the product shrinks less in the air, so the size is larger! The reason is that the low mold temperature accelerates the molecular "frozen orientation", which increases the thickness of the frozen layer of the melt in the mold cavity. At the same time, the 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 beneficial to crystallization, and the actual shrinkage of the product will be greater.
If the start-up process is too long before the size is stable, this indicates that the mold temperature is not well controlled, because the mold takes a long time to reach thermal equilibrium.
Uneven heat dispersion in certain parts of the mold will greatly extend the production cycle, thereby increasing the cost of molding! Constant mold temperature can reduce the fluctuation of molding shrinkage and improve dimensional stability. Crystalline plastic, high mold temperature is conducive to the crystallization process, fully crystallized plastic parts will not change in size during storage or use; but high crystallinity and large shrinkage. For softer plastics, low mold temperature should be used in forming, which is conducive to dimensional stability. For any material, the mold temperature is constant and the shrinkage is consistent, which is beneficial to improve the dimensional accuracy!
3. The influence of mold temperature on deformation:
If the mold cooling system is not properly designed or the mold temperature is not properly controlled, insufficient cooling of the plastic parts will cause the plastic parts to warp and deform. For the control of the mold temperature, the temperature difference between the front mold and the back mold, the mold core and the mold wall, and the mold wall and the insert should be determined according to the structural characteristics of the product, so as to control the difference in the cooling and shrinkage speed of each part of the mold. After demolding, it tends to bend in the traction direction on the higher temperature side to offset the difference in orientation shrinkage and avoid warping and deformation of the plastic part according to the orientation law.
For plastic parts with a completely symmetrical structure, the mold temperature should be kept consistent accordingly, so that the cooling of each part of the plastic part is balanced. The mold temperature is stable and the cooling is balanced, which can reduce the deformation of the plastic part. Excessive mold temperature difference will cause uneven cooling of plastic parts and inconsistent shrinkage, which will cause stress and cause warpage and deformation of plastic parts, especially plastic parts with uneven wall thickness and complex shapes. The side with high mold temperature, after the product is cooled, the direction of deformation must be toward the side with high mold temperature! It is recommended that the temperature of the front and back molds be selected reasonably according to the needs. The mold temperature is shown in the physical properties table of various materials!
4. The influence of mold temperature on mechanical properties (internal stress):
The mold temperature is low, and the weld mark of the plastic part is obvious, which reduces the strength of the product; the higher the crystallinity of the crystalline plastic, the greater the tendency of the plastic part to stress cracking; in order to reduce the stress, the mold temperature should not be too high (PP, PE). For PC and other high-viscosity amorphous plastics, 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 expression of internal stress is obvious stress marks! The reason is: the formation of internal stress in molding is basically caused by different thermal shrinkage rates during cooling. After the product is molded, its cooling gradually extends from the surface to the inside. The surface first shrinks and hardens, and then gradually goes to the inside. The internal stress is generated due to the difference in contraction speed. When the residual internal stress in the plastic part is higher than the elastic limit of the resin, or under the erosion of 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 a compressed form on the surface layer and a stretched form in the inner layer.
The surface compressive stress depends on the cooling condition of the surface. The cold mold quickly cools down the molten resin, which causes the molded product to produce higher residual internal stress. Mold temperature is the most basic condition for controlling internal stress. A slight change of mold temperature will greatly change its residual internal stress. Generally speaking, the acceptable internal stress of each product and resin has its minimum mold temperature limit. When molding thin walls or longer flow distances, the mold temperature should be higher than the minimum for general molding.
5. Affect the thermal deformation temperature of the product:
Especially for crystalline plastics, if the product is molded at a lower mold temperature, the molecular orientation and crystals are instantly frozen. When a higher temperature use environment or secondary processing conditions, the molecular chain will be partially rearranged And the process of crystallization makes the product deform at even far below the heat distortion temperature (HDT) of the material.
The correct way is to use the recommended mold temperature close to its crystallization temperature to make the product fully crystallized in the injection molding stage, avoiding this kind of 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.
Recommendations for determining the correct mold temperature:
Nowadays, molds have become more and more complex, and therefore, it has become increasingly difficult to create suitable conditions to effectively control the molding temperature. In addition to simple parts, the molding temperature control system is usually a compromise. Therefore, the following recommendations are only a rough guide.
In the mold design stage, the temperature control of the shape of the processed part must be considered.
If designing a mold with low injection volume and large molding size, it is important to consider good heat transfer.
Make allowances when designing the cross-sectional dimensions of the fluid flowing through the mold and the feed tube. Do not use joints, otherwise it will cause serious obstacles to fluid flow controlled by mold temperature.
If possible, use pressurized water as the temperature control medium. Please use ducts and manifolds that are resistant to high pressure and high temperature.
Give a detailed description of the performance of temperature control equipment matching the mold. The data sheet given by the mold manufacturer should provide some necessary figures about the flow rate.
Please use insulating plates at the overlap between the mold and the machine template.
Use different temperature control systems for dynamic and fixed molds
On any side and center, please use an isolated temperature control system, so that there are different starting temperatures during the molding process.
Different temperature control system circuits should be connected in series, not in parallel. If the circuits are connected in parallel, the difference in resistance will cause the volumetric flow rate of the temperature control medium to be different, which will cause a greater temperature change than in the case of the circuit in series. (only when the series circuit is connected to the mold inlet and outlet temperature difference is less than 5 °C, its operation is good)
It is an advantage to display the supply temperature and return temperature on the mold temperature control equipment.
The purpose of process control is to add a temperature sensor to the mold so that temperature changes can be detected in actual production.
In the whole production cycle, the heat balance is established in the mold through multiple injections. Generally, there should be at least 10 injections. The actual temperature in reaching thermal equilibrium is affected by many factors. The actual temperature of the mold surface in contact with the plastic can be measured with a thermocouple inside the mold (reading at 2mm from the surface). The more common method is to hold a pyrometer to measure, and the probe of the pyrometer should respond quickly. To determine the mold temperature, many points should be measured, not the temperature of a single point or one side. Then it can be corrected according to the set temperature control standard. Adjust the mold temperature to an appropriate value. The recommended mold temperature is given in the list of different materials. These suggestions are usually given in consideration of the best configuration among factors such as high surface finish, mechanical properties, shrinkage and processing cycles.
For molds that need to process precision components and molds that must meet strict requirements on appearance conditions or certain safety standard parts, higher mold temperatures are usually used (the post-molding shrinkage is lower, the surface is brighter, and the performance is more consistent). For parts with low technical requirements and production costs as low as possible, lower processing temperatures can be used during molding. However, the manufacturer should understand the shortcomings of this choice and carefully check the parts to ensure that the parts produced can still meet customer requirements.