1.The influence of mold temperature on appearance:
Firstly, the mold temperature is too low, which will reduce the melt fluidity and may cause underinjection. Mold temperature affects plastic crystallinity, for ABS, mold temperature is too low, the product finish is low. Compared with fillers, plastics migrate more easily to the surface when the temperature is high. So when the temperature of the injection mold is high, the plastic composition is closer to the surface of the injection mold, the filling will be better, and the brightness and luster will be higher. But the injection mold temperature can not be too high, too high easy to stick to the mold, but also in the plastic parts in the local place of obvious bright spots. And the injection mold temperature is too low, will also cause plastic parts too tight, demoulding when easy to pull plastic parts, especially the surface of plastic parts.
Multi-section injection molding can solve the problem of position, such as the product into the rubber when the air lines can take the way of sectional injection molding. Injection molding industry, smooth products, the higher the temperature of the mold, the higher the gloss of the product surface, on the contrary, the low temperature, the gloss of the surface is also relatively low. But for the sunline PP material products, the higher the temperature, the product surface gloss will be relatively low, the lower the gloss, the higher the color difference, gloss and color difference is inversely proportional.
Therefore, the most common problem caused by mold temperature is the rough surface finish of molded parts, which is usually caused by the mold surface temperature is too low.
Moulding shrinkage and post-moulding shrinkage of semi-crystalline polymers depend mainly on mould temperature and part wall thickness. Uneven temperature distribution in the die will lead to different contractions, which will not ensure that the part meets the specified tolerances. In the worst case, shrinkage exceeds the repairable value regardless of whether the resin is processed as unreinforced or reinforced.
Impact on product size:
2.Impact on product size:
Mold temperature is too high, will make melt thermal decomposition, products out of the air after the shrinkage rate increases, the size of the product will become smaller, mold in low temperature conditions, if the size of the parts becomes larger, generally due to the mold surface temperature is too low. This is because the mold surface temperature is too low, the product in the air shrinkage is also low, so the size is larger! The REASON IS THAT THE LOW MOLD TEMPERATURE ACCELERATES THE “FREEZING ORIENTATION” OF MOLECULES AND INCREASES THE THICKNESS OF THE FROZEN LAYER OF MELT IN THE MOLD cavity. At the same time, the low mold temperature hinders the growth of crystallization, thus reducing the molding shrinkage rate of products. On the contrary, high die temperature, melt cooling slow, long relaxation time, low orientation level, and is conducive to crystallization, the actual shrinkage of the product is larger.
If the starting process is too long before the dimensions have stabilized, this indicates that the mold temperature is not controlled well, because the mold takes a long time to reach thermal equilibrium.
Uneven heat dispersion in some parts of the mold will greatly prolong the production cycle, thus increasing the cost of molding! The mold temperature is constant, which can reduce the fluctuation of the molding shrinkage rate and improve the dimensional stability. Crystalline plastic, high mold temperature is conducive to the crystallization process, fully crystallized plastic parts, in storage or use will not change the size; But the crystallinity is high and the shrinkage is large. For soft plastics, low die temperature should be used in forming, which is conducive to size stability. Any kind of material, mold temperature constant, shrinkage consistent, are conducive to improve dimensional accuracy!
3.Influence of mold temperature on deformation:
If the cooling system design of the mold is not reasonable or the temperature of the mold is not controlled properly, the plastic parts are not cooled enough, which will cause the warping deformation of the plastic parts. For mold temperature control, should according to the structure characteristics of products to determine before and after mold, mold core and mold wall, temperature difference between the die wall and insert, and using the control of molding parts, cooling contraction speed, plastics mold release more tend towards higher temperature side of traction after bending, the characteristics of differential shrinkage to offset orientation, avoid the parts according to the orientation rule of warping deformation.
For the plastic parts with completely symmetric structure, the mold temperature should be consistent accordingly, so that the cooling of each part of the plastic parts is balanced. Mold temperature stability, cooling equilibrium, can reduce the deformation of plastic parts. Mold temperature difference is too large, will make the plastic parts cooling uneven, inconsistent shrinkage, resulting in stress caused by plastic parts warping deformation, especially the uneven wall thickness and complex shape of plastic parts more prominent. The high temperature side of the mold, after the product is cooled, the deformation direction must be to the high temperature side of the mold deformation! It is suggested that the temperature of the front and rear die should be reasonably selected according to the needs. Mold temperature see all kinds of material properties table!
4.Influence of mold temperature on mechanical properties (internal stress) :
The mold temperature is low, the plastic parts welding mark is obvious, reduces the product strength; For crystalline plastics, the higher the crystallinity, the greater the stress cracking tendency. To reduce the stress, the mold temperature should not be too high (PP, PE). For a high viscosity amorphous plastic, the stress cracking is related to the internal stress of plastic parts. Increasing the mold temperature is conducive to reducing the internal stress and stress cracking trend.
The expression of internal stress is obvious stress marks! The reason is that the formation of forming internal stress is basically due to the different thermal shrinkage rate caused by cooling, when the product molding, its cooling is gradually extended from the surface to the interior, the surface first shrinkage hardening, and then gradually to the interior, in this process due to the shrinkage of the difference between the internal stress. When the residual internal stress in the plastic is higher than the elastic limit of the resin, or under the erosion of a certain chemical environment, the plastic surface will crack. The study of PC and PMMA transparent resins shows that the residual internal stress in the surface layer is in the form of compression, and the inner layer is in the form of stretching.
The surface compressive stress depends on its surface cooling condition. The cold mold makes the molten resin cool down rapidly, so that the molding product produces higher residual internal stress. The mold temperature is the most basic condition to control the internal stress. The residual internal stress will be greatly changed if the mold temperature is changed slightly. In general, each product and resin has its lowest mold temperature limit for acceptable internal stress. When forming thin wall or longer flow distance, the mold temperature should be higher than the minimum of the general molding.
Influence the thermal deformation temperature of the product:
Especially for crystalline plastics, if the product forming under the lower mold temperature, molecular orientation and crystallization were instantly freeze, when the use of a relatively high temperature environment or under the condition of secondary processing, its molecular chain will be partially rearranged and crystallization process, make product in even far below the material deformation under thermal deformation temperature (HDT).
The CORRECT PRACTICE is to USE THE recommended mold temperature close to the crystallization temperature, so that the product is fully crystallized at the injection molding stage, to avoid such post-crystallization and post-shrinkage at high temperatures. In a word, mold temperature is one of the most basic control parameters in 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, so it is more and more difficult to create the right conditions to effectively control the molding temperature. Molded temperature control systems are often a compromise, except for simple parts. Therefore, the following recommendations are only a general guide.
The temperature control of the shape of the machined parts must be considered in the mold design stage.
If the design of low injection volume, large molding size mold, it is important to consider the heat transfer performance.
The cross section dimensions for flow through the die and feed pipe should be designed with a margin. Do not use connectors or this will cause a serious obstacle to the flow of fluid controlled by mold temperature.
If possible, use pressurized water as the temperature control medium. Use ductile pipes and manifolds that are resistant to high pressure and temperature.
A detailed description of the performance of the temperature control equipment matching the mold is given. The data sheet given by the die manufacturer should provide some necessary figures on the flow rate.
Use insulation boards at the joint between the mold and the machine template.
Different temperature control systems are used for moving and fixed molds
Use ISOLATED temperature control systems on either side and center to allow different starting temperatures during molding.
Different temperature control system circuits should be connected in series, not in parallel. If the circuit is CONNECTED in parallel, the difference IN resistance will cause the volume flow rate of the temperature controlled medium to be different, resulting in a greater temperature change than in the case of the circuit in series. (The operation is good only if the series circuit is connected to the die inlet and outlet temperature difference is less than 5°C)
It is an advantage to have a display of supply temperature and return temperature on the mold temperature control equipment.
The purpose of process control is to include a temperature sensor in the mold, so that the temperature changes can be detected in the actual production.
Thermal balance is established in the mold by multiple injections throughout the production cycle, generally at least 10 injections. The actual temperature at which thermal equilibrium is reached is affected by many factors. The actual temperature of the surface of the die in contact with the plastic can be measured using a thermocouple inside the die (reading at 2mm from the surface). The more common method is to hold a pyrometer to measure, the probe of the pyrometer to respond quickly.
The mold temperature is determined by measuring many points, not the temperature of a single point or side. Then it can be modified according to the set temperature control standard. Adjust mold temperature to appropriate value. Recommended die temperatures are given in the list of different raw materials. These recommendations are usually based on the optimal configuration for high surface finish, mechanical properties, shrinkage, and machining cycles.
For molds that need to process precision components and those that need to meet strict appearance conditions or certain safety standards, higher mold temperatures are usually used (which can make the post-molding shrinkage lower, the surface brighter, and the performance more consistent). For parts with low technical requirements and production costs as low as possible, lower processing temperatures can be used in molding. However, the manufacturer should be aware of the disadvantages of this option and carefully inspect the parts to ensure that the parts produced can still meet the customer’s requirements.