Advancement of Conformal Cooling channel design technology

Applied Technology 01

7: Let’s develop an understanding of the basics relating to the importance of the
 cooling process when performing injection molding

In designing optimal conformal cooling channels, the designer needs to have a basic understanding of troubles due to inadequate cooling and factors produced by the cooling time.
The following three items can be listed as typical problems for the cooling process:

  • Sink marks
  • Warping
  • Long molding time (long cooling time)

(See Fig. 10 and Fig. 11.)

Fig. 10: Conceptual illustration of sink mark and warp phenomena

Fig. 10: Conceptual illustration of sink mark and warp phenomena

Fig. 11: Cooling time as a fraction of the molding cycle

Fig. 11: Cooling time as a fraction of the molding cycle

The sink marks and warp which have a conspicuous effect on quality of the molded article are frequently due to uneven volumetric contraction, and a crucial perspective is to predict the following problems at the design stage, and take measures to resolve them beforehand:

  • Differences in dwelling force
  • Differences in mold temperature distribution = heat removal capacity
  • Differences in fiber orientation

Also, long molding times (long cooling times) results in lower actual mass production performance, and have an adverse effect on production cost. Thus, it is necessary to proceed with placing mold design while always keeping in mind maximization of the cooling design in the mold.

8: Learning from actual cases of conformal cooling channel design

Up through Section 7, I described the basic content, but in this section I would like to consider this topic together with all of you based on an actual case of design.
In this case, there are limitations on reduction of cooling time with the conventional method, and an attempt was made to achieve improvement in heat accumulation near the gate for a PET bottle preform by arranging conformal cooling channels. (See Fig. 12.)

Fig. 12: Actual example of PET bottle preform

Fig. 12: Actual example of PET bottle preform

Gate bush inserts using the conventional method have also been fabricated with the part segmentation system of the screw fastening type. Water pipes like those in Fig. 13A have already been arranged, and this is the start of evaluation based on the situation where they are working to achieve maximal cooling on the customer side.

Fig. 13: Comparison of three types of water pipes in a PET bottle preform gate bush

Fig. 13: Comparison of three types of water pipes in a PET bottle preform gate bush

Our company designed plans B and C shown in Fig. 13 from the following perspectives.
In Plan B, design was carried out by using conventional type water pipes, and bringing water pipes closer to the gate.
In Plan C, design was carried out by using a streamline system, and bringing water pipes closer to the gate in the same way.
Now, I'd like to ask you, the reader: can you clearly explain which of the two plans, B or C, is effective, and how it is effective?
I would like to discuss and further explore this based on the results of analysis and evaluation.
As indicated in the table of molding conditions in Fig. 14, the conventional method enables mass production of preform molded articles with satisfactory molding quality in a cooling time of 2.2 seconds.
The solidification rate of the heat accumulating part near the gate at that time is 50.4% (see Fig. 16), and the average temperature is 215.1°C (see Fig. 17).

Fig. 14: Analysis conditions for PET bottle preform

Fig. 14: Analysis conditions for PET bottle preform

Fig. 15: Shapes of water pipes used for analysis

Fig. 15: Shapes of water pipes used for analysis

Fig. 16: Solidification layer rate near gate at same 2.2 seconds as conventional method

Fig. 16: Solidification layer rate near gate at same 2.2 seconds as conventional method

Fig. 17: Average temperature near the gate at the same 2.2 seconds as the conventional method

Fig. 17: Average temperature near the gate at the same 2.2 seconds as the conventional method

In contrast, the calculated analysis results show that the solidification rate for Plan B is 50.7% (see Fig. 16), and its average temperature is 213.2°C (See Fig. 17), while the solidification rate for Plan C is 53.5% (see Fig. 16), and its average temperature is 210.2°C (see Fig. 17).
I imagine many readers thought that perhaps Plan B has the best cooling efficiency, but when the cooling speed vectors in Fig. 18 are compared, it is evident that Plan C is better than the conventional type A and Plan B.

Fig. 18: Comparison of cooling speed vectors

Fig. 18: Comparison of cooling speed vectors

Even if the cooling circuit is expanded from the conventional type A to near the gate as in Plan B, the flow of coolant is not as good as one might expect. To put it another way, the best approach is to guide the coolant itself to near the heat accumulating part in order to ensure more efficient flow without lowering the speed vectors (i.e., while eliminating points of stagnation as far as possible), and the correct answer is C.

Incidentally, if the flow lines of coolant in Fig. 19 are compared, it is evident that Plan C will achieve flow through points closest to the gate.

Fig. 19: Comparison of flow lines

Fig. 19: Comparison of flow lines

Based on this, in contrast to the 2.2-second cooling time where molded article quality is guaranteed with the conventional method, if the cooling time with Plan B and Plan C is predicted for an average temperature of 215.1°C near the gate heat accumulating part, then it is predicted that, as indicated by the graph in Fig. 20, Plan B will achieve a reduction of 0.2 sections to 2.0 seconds, and Plan C will achieve a reduction of 0.5 seconds to 1.7 seconds. However, the culture at our company OPM Lab is to never rest satisfied and always continue pursuit of further improvements, and in the next section I will examine, together with the reader, whether even further reduction can be achieved.