Design of mold cooling system is a key factor in success of mold design, which directly affects quality and production efficiency of plastic products. In injection molding process, cooling time of plastic products in cavity accounts for 50% to 60% of entire molding cycle, cooling speed and uniformity directly affect quality of product. If design of cooling system is unreasonable, it will result in a long production cycle and high cost. On the other hand, uneven cooling will also cause warping deformation of product due to thermal stress, thereby affecting product quality.
In addition, problems caused by poor cooling design of mold are not as easy to find as casting system. For example, appearance of product can be inspected or short shot sample can be observed to find possible sticking.

Injection mold cooling system include cooling channels and devices for cavity and core - such as baffles (see Figure 1), fountains (bubblers, see Figure 2), thermal tubes (see Figure 3) and so on.

 

Figure 1

 

Figure 2

 

Figure 3

Mold cooling system also includes means and piping for supplying, recovering a coolant, such as mold temperature control units, pipelines, manifolds, hoses, etc. Temperature controller adjusts temperature of coolant to a set temperature range, pipe is connected to mold temperature machine and mold hanging on injection molding machine, manifold diverts or collects coolant, hose is connected to manifold and mold or different water paths in mold.

Uneven cooling causes curing of each part of plastic part to be successive. Post-cured plastic has a large tensile stress, surface gloss is bright. Vice versa, which is dull, resulting in formation of male and female face, hurt appearance quality.
Figure 4 is an automotive interior part made of PP+EPDM with a large difference in surface gloss that makes part unacceptable. Behind two glossy areas, there is an insert with no cooling channels and devices, but there are cooling channels and devices in mold core surrounding insert. Plastic shrinks unevenly due to uneven cooling, resulting in a difference in surface gloss.

 

Figure 4

Mold temperature is high and uniform, residual stress is small, and surface gloss is high for opaque member.
Figure 5 shows ABS plastic part of Microcellular Injection Molding. Plastic parts below are formed by a high-modulus temperature mold which is uniformly heated by steam, surface of plastic part corresponding to polished surface of cavity can be distinguished, and surface of plastic parts corresponding to bite surface of cavity is evenly soft. Upper plastic parts is formed by a conventionally low and uneven mold temperature, which is not only eclipsed, but also uneven in gloss. Mold temperature is high and uniform, transparency is good for transparent member.

 

Figure 5

 

Figure 6

Figure 6 is a photoelastic diagram of a transparent member corresponding to 100 C, 120 C, and 140 C mold temperature molding. It can be seen that the higher mold temperature, the slower and more uniform color change, indicating that residual stress is low and transparency is good.

Filling stage of plastic injection molding adopts high mold temperature, which can ensure quality of product (such as high light without trace). Low filling temperature is used in post-filling stage, which can shorten cycle time. Technique of balancing high, low mold temperature with product quality and production efficiency is called high and low mold temperature conversion technology.
Use of this method to produce high-gloss, spray-free products is often plagued by problem of deformation. Reason is often that female and male mold adopt high and low mold temperature in filling stage, respectively, heat dissipation of female and male mold sides is asymmetric with center plane of cavity, increasing residual stress of center plane asymmetry, so that bending moment of product deformation is increased, and product deformation is relatively large.
Figure 7 is a 46-inch TV front frame of a PC/ABC injection molded using high and low mold temperature conversion technology. Original female and male mold temperature were 120C and 50C in filling stage and 50C in post filling stage. As a result, front frame of TV was concave toward cavity side.

 

Figure 7

Through Moldflow simulation, mold temperature of female and male mold is changed to 145C in filling stage, and 10C in post-filling stage, deformation is reduced by 7/8, cooling time is basically unchanged. Since mold temperature used in filling stage is glass transition temperature of PC, highlight effect on surface of product is better than before.
Figure 8 shows relay part of PBT-GF30. Side wall of rectangular box-shaped product has a convex inward condition. This is often caused by insufficient cooling of four inner corners, resulting in uneven cooling, shrinkage inside and outside box. from perspective of Moldflow deformation reasons, we call it corner effect.

 

Figure 8

Cooling balance should consider that cooling of plastic parts on core and cavity sides should be symmetrical with center surface of plastic parts, and also consider cooling symmetry of plastic part on one side - core and cavity sides - otherwise it will lead to uneven shrinkage of plastic, high residual stress and large deformation.

Cooling time accounts for about 60% of molding cycle time, and if cooling time is long, molding cycle time is difficult to shorten.
Uneven cooling also lengthens molding cycle. This is because uneven cooling causes plastic to shrink unevenly, leaving a high residual stress after plastic parts is solidified. If mold is opened too early and plastic parts is ejected, plastic parts is too soft and deformed greatly under high residual stress. A common countermeasure is often to extend cooling time so that molded part forced in cavity is cooled to a strength sufficient to restrain residual stress therein, so that molding cycle becomes longer due to extension of cooling time.
Figure 9 is a car light distribution product. Original mold flow path is designed to be 12mm. Product cooling takes 25 seconds, but flow channel solidification of 50% can take 57 seconds to eject. Molding cycle is too long. After Moldflow optimization analysis, cold runner will be used. Diameter is changed to 9mm, which meets molding requirements, and cooling time of runner is only 35 seconds, which shortens molding cycle by 22 seconds.

 

Figure 9

Cooling of injection molding molds and common problems such as gloss differences, color differences, transparency, warpage, long molding cycle, etc., cooling design must focus on balance and efficiency, Moldflow can help us optimize design of waterway and help us Increase yield and productivity.