Injection molding process includes preheating, injection, and cooling stage, where cooling and holding stage occupies more than 50% of entire molding cycle. If mold temperature is too high, shrinkage of molded plastic parts will increase, dimensional accuracy will decrease, surface will have mottling, and it will cause difficulty in demolding; if mold temperature is too low, viscosity of plastic melt will decrease, and flow resistance will increase, resulting in insufficient cross-linking and solidification of material, which affects mechanical strength of molded plastic parts, Therefor, uniformity of mold temperature field is an important indicator that affects molding quality of plastic parts.
Now use mold flow analysis software to determine factors that affect cooling effect of conformal cooling water channel by studying cooling performance of mold core during injection molding process, design conformal cooling water channel for lamp housing to enhance heat dissipation of the mold core, improve uniformity of shrinkage of plastic parts.

 

Figure 1 Two-dimensional cooling channel heat transfer model

Classic theory of cooling channel analyzes design criteria of injection mold from two aspects: on the one hand, starting from structure of cooling channel, considering cross section of cooling channel, distance between cooling channel and surface of mold parts, flow rate and velocity of cooling medium; On the other hand, from perspective of heat conduction, balance between heat output of plastic parts, heat conduction of mold, and heat convection of cooling system is mainly considered. Based on classic theory of cooling channels, scholars from Stanford University in United States divided cooling channels into two small areas with adjacent channels for analysis, proposed six design criteria for accompanying cooling channels. Based on research foundation of predecessors, heat transfer model of conformal cooling water channel shown in Fig. 1 is now used, and meaning of each symbol is shown in Table 1.

 

 

Figure 2 Experimental model

Factors that affect accompanying cooling effect mainly include diameter of cooling water channel, distance between cooling water channel and molding surface of mold core, center distance between adjacent water channels. In order to determine sequence of three factors affecting cooling effect, method of orthogonal test is now used, test results are analyzed using MoldFlow analysis software. Model used in experiment is shown in Figure 2. Model and water channel are imported into analysis software. Grid type is 3D grid, injection period is 20s, and analysis sequence is cooling (FEM). Plastic material is polypropylene, melt temperature is 240°C, and cooling medium is 25°C water. By comparing temperature difference between the highest temperature and the lowest temperature of mold parts, time for plastic parts to reach ejection temperature, influence ratio of different factors on heat dissipation effect of cooling channel is determined. Test factors and test levels are shown in Table 2.

 

Taking difference between the highest temperature and the lowest temperature of mold cavity wall surface as research object, test results are shown in Table 3.

 

It can be seen from Table 3 that distance between water channel and cavity wall surface has the largest range. From range method, distance between water channel and cavity wall surface has the greatest influence on temperature difference between core and cavity wall, that is, uniformity of cooling of water channel; Second is distance between channels, and finally channel diameter. Therefore, when designing water channel, in order to ensure cooling uniformity of accompanying cooling water channel, distance between water channel and surface of cavity wall should be selected to a smaller value.

 

Factors that examine cooling effect of conformal cooling channels are not only uniformity of cooling, but also cooling efficiency of channels. Taking time for plastic parts to reach ejection temperature as research object, test results are shown in Table 4.
It can be seen from Table 4 that the biggest factor affecting cooling efficiency of conformal cooling channels is diameter of channel, followed by center distance of channel, and finally distance between channel and molding surface of core; but extreme value of the three is relative small with respect to surface temperature of cavity wall. Based on traditional injection mold design experience, for plastic parts of lamp housing, design parameters of cooling channel are finally determined to be D=6mm, W=16mm, lm=18mm.

In addition to design dimensions of water channel, structure of water channel is also one of factors that affect cooling effect. At present, common cooling channel types of injection molds are: traditional straight-through channels, which are easy to machine and are common in traditional metal molds. In addition, processing method of 3D printing is not limited by complex structure inside mold. During gradual development of 3D printing, various new types of cooling water channels have been proposed, among which the more common ones are structural water channels based on plastic parts. After extracting geometric features of plastic parts, this kind of water channel classifies geometric features, each feature is divided into cooling units, and corresponding cooling water channels are designed for each cooling unit, then cooling water channels of different characteristics are combined to form layout design of conformal cooling water channels of entire plastic part. Because distance between this kind of water channel structure and cavity wall surface is basically same everywhere, its cooling uniformity is better.

 

A. Traditional cooling channels

 

B. Structural cooling channel

 

C. Spiral cooling water channel

 

D.) Side-by-side cooling channels
Figure 3 Cooling channels of different structures
For plastic parts of lamp housing, designed cooling water channel is shown in Figure 3. Cooling effect is compared in MoldFlow, process parameter settings are same as previous section.
Taking difference between time when plastic part reaches ejection temperature and temperature of mold cavity wall surface as research object, histograms are drawn as shown in Figure 4 and Figure 5, respectively.

 

Figure 4 Comparison of cooling channel time for plastic parts to reach ejection temperature

 

Figure 5 Comparison of cavity wall temperature difference

As can be seen from Figure 4, cooling efficiency of three conformal cooling channels is better than that of traditional cooling channels, cooling efficiency of side-by-side cooling channels is better; from Figure 5, it can be seen that conformal cooling channels can effectively improve problem of uneven cooling of traditional water channels. Among them, structural type water channel has the smallest cavity wall temperature difference, but difference from side-by-side type water channel temperature difference is only 0.1 ℃. When plastic parts reach ejection temperature, mold temperature clouds of two water channels are compared, as shown in Figure 6.

 

(A). structural waterways

 

(B) Side-by-side water channels

Figure 6 Comparison of mold temperature clouds

It can be seen from Fig. 6 that although temperature difference of cavity wall of structural water channel is small, for small structure of plastic part, cooling uniformity of small structure of plastic part is inferior to that of side-by-side water channel, which may lead to excessive warpage at small structures of plastic parts and poor dimensional consistency of molded plastic parts. For comprehensive cooling efficiency, it is advisable to use side-by-side channels for plastic parts of lamp housing.