The effect of cooling on mold quality. We all underestimated it
The design of the mold cooling system is a key factor in the success of the mold design. It directly affects the quality and production efficiency of plastic products. During the injection molding process, the cooling time of the plastic product in the cavity should account for 50% to 60% of the entire molding cycle, and the cooling speed and uniformity directly affect the quality of the product. If the cooling system design is not reasonable, it will cause long production cycles and high costs. On the other hand, uneven cooling will also cause the product to warp and deform due to thermal stress, which will affect the product quality.
In addition, the problems caused by the poor cooling design of the mold will not be found as easily as the pouring system. For example, the appearance of the product or the short shot sample can be inspected to find the possible crux.
Common cooling system design
The injection molding cooling system includes cooling channels and devices of the mold and Cavity and Core-such as baffles , fountain tubes, Thermal pins and so on.
The cooling system also includes a device for supplying and recovering cooling fluid (coolant) and piping, such as a mold temperature control unit, pipelines, manifolds, hoses, etc. The thermostat adjusts the coolant temperature to a set temperature range. The piping connects the mold temperature machine and the mold hanging on the injection molding machine. The manifold splits or pools the coolant, and the hose connects the manifold and the mold.) waterway.
Appearance problems due to poor cooling The uneven cooling makes the solidification of each area of the plastic parts have its own sequence, and the post-cured plastic has a large tensile stress, and its surface is bright and bright, otherwise it is dull, which results in the formation of yin and yang faces, which damages the appearance quality.
an automotive interior part made of PP + EPDM. The difference in surface gloss makes plastic parts unacceptable. Behind the two areas with different gloss, there is an insert each, and there are no cooling channels and devices therein, but there are cooling channels and devices in the mold kernel surrounding the insert. Plastics shrink unevenly due to uneven cooling, causing differences in surface gloss.
The mold temperature is high and uniform, and the residual stress is small. For opaque parts, the surface gloss is high.
the ABS plastic parts of The Microcellular Injection Molding. The following plastic parts are formed by high mold temperature molding with uniform steam heating. The surface of the plastic part corresponding to the polished surface of the mold cavity can be seen, and the surface of the plastic part corresponding to the bite surface of the mold cavity is symmetrical and soft. The upper plastic part is formed by the traditional low and uneven mold temperature, which not only tarnishes but also has uneven gloss. The mold temperature is high and uniform. For transparent parts, its transparency is good.
A photoelastic diagram of transparent parts corresponding to mold temperatures of 100 C, 120 C, and 140 C. FIG. It can be seen that the higher the mold temperature, the slower and uniform the color change, indicating that the residual stress is low and the transparency is good.
Deformation due to uneven cooling
The filling stage of plastic injection molding uses high mold temperature to ensure the quality of the product (such as high light without traces), and the use of low mold temperature in the post-filling stage can shorten the cycle time. This technology alternates between high and low mold temperatures to take into account product quality and production efficiency. It is called high and low mode temperature conversion technology.
Using this method to produce high-gloss spray-free products is often troubled by deformation problems. The reason is often that the female and male molds use high and low mold temperatures respectively during the filling stage. The heat dissipation on the female and male mold sides is asymmetric with the center plane of the cavity, which increases the asymmetric residual stress on this center plane, which makes the switch As the bending moment of product deformation increases, the product deformation becomes larger.
a 46-inch front frame of a PC / ABC injection molded using high and low mold temperature conversion technology. It turned out that the temperature of the female and male molds were 120C and 50C during the filling stage and 50C during the post-filling stage. As a result, the front frame of the TV set was concave toward the cavity side.
According to Moldflow simulation, both the mold temperature of the female and male molds were changed to 145C during the filling stage, and were changed to 10C during the post-filling stage, the deformation was reduced by 7/8, and the cooling time was basically unchanged. The mold temperature used during the filling stage was The glass transition temperature of PC, the highlight effect on the product surface is better than before.
the relay part of PBT-GF30. The side wall of the rectangular box-shaped product is convex inward. This is often caused by insufficient cooling of the four inner corners, causing the inside and outside of the box to cool. The results of unevenness and shrinkage unevenness, from the perspective of Moldflow deformation cause analysis, we call it the corner effect.
For cooling equalization, the cooling of the plastic part on the male and female sides (core and cavity sides) must be symmetrical to the center plane of the plastic part, and the cooling of the plastic part on one side-male or female side-must be considered. Otherwise, it will lead to uneven shrinkage, high residual stress and large deformation of the plastic.
Long molding cycles due to poor cooling
The cooling time accounts for about 60% of the molding cycle time. If the cooling time is long, it is difficult to shorten the molding cycle time.
Uneven cooling will also extend the molding cycle. This is because the uneven cooling causes the plastic to shrink unevenly, leaving a high residual stress after the plastic part is solidified. If the mold is opened and ejected too early, the plastic part is too soft and under high residual stress , Plastic parts deformed greatly. The usual countermeasure is often to extend the cooling time, so that the plastic parts shaped in the mold cavity are forced to cool down until their strength is sufficient to restrain the residual stress therein, so that the molding cycle will become longer due to the extension of the cooling time.
a car light distribution lens product. The original mold runner is designed to be 12mm, and the product needs 25 seconds to cool. However, 50% of the runner solidification can be ejected in 57 seconds. The molding cycle is too long. The diameter is changed to 9mm, which meets the molding requirements, and the cooling time of the runner only takes 35 seconds. After this, the molding cycle is shortened by 22 seconds.
The cooling of injection molding molds is closely related to common problems such as gloss differences, color differences, transparency, warpage deformation, long molding cycles, etc. The cooling design must focus on balance and efficiency, and Moldflow can help us optimize the design of the waterway and help us greatly Improve yield and productivity.