In injection mold industry, there are often new industry inquiries: Why temperature of injection mold is high and gloss of plastic parts produced is high? Now let's explain this phenomenon in plain language and how to choose mold temperature reasonably.

 

First of all, if mold temperature is too low, melt fluidity will be reduced, and under-injection may occur; mold temperature affects crystallinity of plastic. For ABS, if mold temperature is too low, product finish is low. Plastics and fillers are more likely to migrate to surface when temperature is high. Therefore, when temperature of injection mold is high, plastic component is closer to surface of injection mold, filling will be better, brightness and gloss will be higher. But temperature of injection mold should not be too high. If too high, it is easy to stick mold, and there will be obvious bright spots in plastic parts. If temperature of injection mold is too low, it will also cause plastic part to hold mold too tight, and it is easy to damage plastic part when demolding, especially pattern on the surface of plastic part.
Multi-segment injection molding can solve problem of location. For example, if product has air lines when it is injected, it can be taken as a section injection molding method. In injection molding industry, the higher temperature of glossy product and mold, the higher gloss of product surface. On the contrary, the lower temperature, the lower gloss of surface. But for products with sun-printed PP material, the higher temperature, the lower gloss of product surface. The lower gloss, the higher color difference, gloss and color difference are inversely proportional.
Therefore, the most common problem caused by mold temperature is rough surface finish of molded parts, which is usually caused by mold surface temperature being too low.
Molding shrinkage and post-molding shrinkage of semi-crystalline polymers mainly depend on temperature of mold and wall thickness of part. Uneven temperature distribution in mold will cause different shrinkages, which cannot guarantee that parts meet specified tolerances. In the worst case, whether processed resin is unreinforced or reinforced, shrinkage exceeds correctable value.

 

If mold temperature is too high, melt will thermally decompose. After product comes out, shrinkage rate in the air will increase, and product size will become smaller. When mold is used at low temperature, if part size becomes larger, it is generally caused by surface temperature of mold being too low. This is because mold surface temperature is too low, product shrinks in the air also, so size is larger! Reason is: low mold temperature accelerates "freezing orientation" of molecules, which increases thickness of frozen layer of melt in mold cavity. At the same time, low mold temperature hinders growth of crystals, thereby reducing molding shrinkage of product. On the contrary, if mold temperature is high, melt cools slowly, relaxation time is long, orientation level is low, and it is conducive to crystallization, actual shrinkage of product is large.
If start-up process is too long before size stabilizes, this indicates that mold temperature control is not good, because mold takes a long time to reach thermal equilibrium.
Uneven heat dissipation in certain parts of mold will lead to greatly prolonged production cycles, thereby increasing cost of molding! Constant mold temperature can reduce fluctuations in molding shrinkage and improve dimensional stability. For crystalline plastics, high mold temperature is conducive to crystallization process. Fully crystallized plastic parts will not change in size during storage or use; however, crystallinity is high and shrinkage is large. For softer plastics, low mold temperature should be used in forming, which is conducive to dimensional stability. Any kind of material, mold temperature is constant, and shrinkage is consistent, which is conducive to improving dimensional accuracy!

 

If cooling system of mold is not designed properly or temperature of mold is not properly controlled, insufficient cooling of plastic parts will cause warpage and deformation of plastic parts. For temperature control of mold, temperature difference between front mold and rear mold, core and mold wall, mold wall and insert should be determined according to structural characteristics of product, so as to control difference in cooling shrinkage speed of each part of mold. After demoulding, plastic part tends to bend in the direction of higher temperature to compensate for difference in orientation shrinkage, prevent plastic parts from warping and deforming according to orientation law.
For plastic parts with completely symmetrical body structure, mold temperature should be kept consistent, so that cooling of each part of plastic parts is balanced. Stable mold temperature and balanced cooling can reduce deformation of plastic parts. If temperature difference of mold is too large, cooling of plastic parts will be uneven and shrinkage will be inconsistent, which will cause stress, warpage and deformation of plastic parts, especially plastic parts with uneven wall thickness and complex shapes are more prominent. On the side with high mold temperature, after product cools, direction of deformation must be toward side with high mold temperature! It is recommended that temperature of front and rear molds be selected reasonably according to needs. See physical properties of various materials for mold temperature!

 

Mold temperature is low, and welding marks of plastic parts are obvious, which reduces product strength; crystalline plastics, the higher crystallinity, the greater stress cracking tendency of plastic parts. To reduce stress, mold temperature should not be too high (PP, PE). For PC-like high-viscosity non-crystalline plastics, stress cracking is related to internal stress of plastic parts. Increasing mold temperature is helpful to reduce internal stress and reduce stress cracking tendency.
Expression of internal stress is that stress marks are obvious! Reason is: formation of internal stress during molding is basically caused by different thermal shrinkage rates during cooling. When product is molded, its cooling is gradually extended from surface to inside. Surface shrinks and hardens first, and then gradually to inside, during which internal stress is generated due to difference in shrinkage speed. When residual internal stress in plastic part is higher than elastic limit of resin, or under erosion of a certain chemical environment, surface of plastic part will crack. Studies on PC and PMMA transparent resins show that residual internal stress is in compressed form on surface layer and stretched form in inner layer.
Surface compressive stress depends on surface cooling condition. Cold mold rapidly cools molten resin, resulting in higher residual internal stress in 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. In general, acceptable internal stress of each product and resin has its lowest mold temperature limit. When forming thin walls or longer flow distances, mold temperature should be higher than minimum for general molding.

 

Especially for crystalline plastics, if product is molded at a lower mold temperature, molecular orientation and crystallization are instantly frozen. When a higher temperature environment or secondary processing conditions are used, molecular chains will be partially rearranged and crystallized, causing product to deform even below material's heat distortion temperature (HDT).
Correct approach is to use recommended mold temperature close to its crystallization temperature, so that product is fully crystallized during injection molding stage to avoid such post-crystallization and post-shrinkage in high-temperature environments. In short, mold temperature is one of the most basic control parameters in injection molding process, and it is also primary consideration in mold design.

Now that mold has become more and more complex, it has become increasingly difficult to create suitable conditions to effectively control molding temperature. In addition to simple parts, molding temperature control systems are often a compromise. Therefore, following suggestions are only a rough guide.
In mold design stage, temperature control of appearance of processed parts must be considered.
If designing a mold with a low injection volume and a large molding size, it is important to consider heat transfer performance.
There should be a margin when designing cross-sectional dimensions of fluid flowing through mold and feed pipe. Do not use fittings, otherwise it will cause serious obstacles to fluid flow controlled by mold temperature.
If possible, use pressurized water as temperature control medium. Use ductile pipes and manifolds that are resistant to high pressure and high temperature.
Give a detailed description of performance of temperature control equipment that matches mold. Data sheet given by mold manufacturer should provide some necessary figures regarding flow rate.
Use insulating plates at the junction of mold and machine mold plate.
Use different temperature control systems for moving and fixed mold.
On either side and center, use an isolated temperature control system, which will allow different starting temperatures during molding process.
Different temperature control system circuits should be connected in series, not in parallel. If circuits are connected in parallel, difference in resistance will cause volumetric flow rate of temperature-controlled medium to be different, resulting in a greater temperature change than in the case of a circuit in series. (Operation is only good when temperature difference between inlet and outlet of mold is less than 5°C when series circuit is connected)
It is an advantage to have display of supply temperature and return temperature on mold temperature control equipment.
Purpose of process control is to add a temperature sensor to mold, so that temperature change can be detected in actual production.
Thermal balance is established in mold through multiple injections throughout production cycle. Generally, there should be at least 10 injections. Actual temperature in reaching thermal equilibrium is affected by many factors. Actual temperature of mold surface in contact with plastic can be measured with a thermocouple inside mold (reading 2 mm from surface). The more common method is to measure it by holding a pyrometer, probe of pyrometer must respond quickly. Determination of mold temperature should measure many points, not a single point or one side temperature. Then it can be corrected according to set temperature control standard. Adjust mold temperature to an appropriate value. Recommended mold temperature is given in the list of different raw materials. These recommendations are usually given considering optimal configuration among factors such as high surface finish, mechanical properties, shrinkage, and processing cycle.
For molds that process precision components and meet strict requirements for appearance conditions or parts with certain safety standards, higher mold temperatures are usually used (post-mold shrinkage is lower, surface is brighter, and performance is more consistent). For parts with low technical requirements and production costs as low as possible, lower processing temperatures can be used for molding. However, manufacturers should understand shortcomings of this option and carefully check parts to ensure that parts produced can still meet customer requirements.