Advancement of Conformal Cooling channel design technology

Understanding the Basic Concept

Introduction
As one of the world’s pioneers, OPM Laboratory Co., Ltd. has been engaged in the development of precision metal 3D printer technology since its inception in 2004. Today, metal 3D printer technology is recognized worldwide, and many equipment manufacturers have joined the field, resulting in severe competition for market share.

Since our founding, we have developed our business with a focus on service bureau functions, always considering how to apply precision metal 3D printer technology to the market. Today we have over 35 designers in our entire group, and we are the largest group of design engineers with extensive design/manufacturing experience cultivated in mass-production applications.
In this article, I would like to examine conformal cooling technology, focusing on points we have noticed in actual design work, and other points to be aware of.

1. Conformal cooling technology

“Conformal cooling channels” are an important solution using precision metal 3D printer technology. As is well known, use of these channels is being promoted by cutting-edge companies throughout the world as an effective means of shortening cooling time and improving molded part quality in molding plants.

At the same time, special-purpose modules have been developed as resin analysis systems, such as Moldex3D and MoldFlow from CoreTech System Co., Ltd., and infrastructure is being put in place to allow ordinary users to predict effectiveness beforehand. There is no doubt that applications of this technology will accelerate even further going forward.
However, cases are sometimes seen where designed water pipes lack the design enabling them to be effective, because standard indicators and criteria have not been provided for design techniques. In the following, I would like to discuss basic indicators/guidelines used for our company’s design, and through a number of case studies, provide a correct understanding of conformal cooling technology, and contribute to the advancement and establishment of this technology.

2. Definition and broad classification of conformal cooling channels

The “conformal” in “conformal cooling” means carrying out pipe design within the cavity and core in an equiangular, equidistant fashion (see Figs. 1-① and ②).

Fig. 1-①:  Problem with normal cooling channels: “Non-uniform water pipe distance” at product surface

Fig. 1-①:  Problem with normal cooling channels: “Non-uniform water pipe distance” at product surface

Fig. 1-②: Effectiveness of conformal cooling channels: “Equidistance achieved through uniform water
     pipe distance to product surface”

Fig. 1-②: Effectiveness of conformal cooling channels: “Equidistance achieved through uniform water pipe distance to product surface”

However, in actual molds the product shape is more complicated, and there are an increasing number of cases where it is necessary to segment into multiple pieces. At the same time, there are multiple elements such as pins and holes, and since it is simultaneously necessary to avoid interference with water pipes, the layout does not become equiangular and equidistant, as implied by the term “conformal.”
However, an important feature of a precision metal 3D printer is its ability to fabricate, in an integrated fashion, cavities and cores which had to be divided into multiple segments with conventional techniques, and these printers can create molds with extremely high heat exchange capacity that enable effective use of conformal cooling channels.

Design techniques for conformal cooling can be roughly classified as follows:

  • Method of providing a turbulent coolant flow by providing lattice-shaped voids in the cavity and core (see Fig. 2)
  • Method of providing coolant flow by providing streamline-shaped voids in the cavity and core (see Fig. 3)

Fig. 2: Turbulent water pipe

Fig. 2: Turbulent water pipe

Fig. 3: Streamline water pipe

Fig. 3: Streamline water pipe

However, with turbulent water pipe design, the lattice-shaped forms which allow the coolant to flow “look cool” at first, but there are issues with difficulty of design, difficulty of mold maintenance in case of problems like clogging after use in the field, increased void content on the inner surface of inserts, and also problems with reduced mechanical strength, and thus at our company this approach is still in the R&D phase.
Due in part to this background, in this article I would like to explain in more detail about conformal cooling with the streamline shape.

3. Types of practical conformal cooling channels

As shown in Fig. 4, methods of 3-dimensional pipe design with streamlines can be roughly classified into three types:

  • 1: Type with pipes arranged in a zigzag shape
  • 2: Type with pipes arrange in a spiral shape
  • 3: Type with pipes arranged in a parallel shape

Fig. 4: Three typical types of streamline water pipes

Fig. 4: Three typical types of streamline water pipes

There are three basic types, and the optimal type must be selected and used depending on the shape of the product part of the mold, and the field of the mold to be used.

4. To exhibit the maximum effect of conformal cooling

Naturally, the expected effects cannot be achieved even with conformal cooling channels if design is not done properly.
For example, if the water pipe diameter is less than φ1, and an ordinary mold temperature controller is used, then there are experimental values where pressure loss is large and the effect of passing coolant through is poor (see Table 1). Thus the effect is not exhibited if the designed diameter is not set to φ1 or higher.

Table 1: Results of evaluating flow rate for circular water pipe diameter

Table 1: Results of evaluating flow rate for circular water pipe diameter

Cooling channel (diameter α) φ0.5 φ1.0 φ1.5 φ2.0
Temperature control pressure (Mpa) 0.96 0.80 0.76 0.76
Flow rate (L/sec) 0.2 0.8 2.1 2.8

Also, if the water pipe diameter of the designed channel is constricted part way through, and there is a change in cross-sectional area, as shown in Fig. 5, there will be a large pressure loss, and, once again, the effect will not be achieved.

Fig. 5: If the water pipe diameter of the designed channel is constricted part way through, and there is a
   change in cross-sectional area

Fig. 5: If the water pipe diameter of the designed channel is constricted part way through, and there is a change in cross-sectional area

This sort of thing happens, so designers must carry out design with basic knowledge.
We would like to pursue in more detail the precautions for designing conformal cooling channels, conduct simulations with Molddex3D of a number of case studies, and carry out evaluation/analysis based also on the analysis results.

5. Basic rules for water pipe design

We would like to exhibit the maximal cooling effect, and thus the aim of the designer inevitably becomes arranging conformal cooling channels near to the product surface. However, if the design is too aggressive, the wall thickness of the water pipe inner wall surface and product surface becomes thin, and this may cause damage and water leakage.

At our company, we use CAE analysis to analyze the following in designed
conformal cooling channels (see Fig. 6):

  • Evaluation of pressure in water pipe (see Fig. 6)
  • Evaluation of maximum internal pressure applied to insert parts (see Fig. 7)
  • Evaluation of maximum stress applied to mold surface (see Fig. 7)

Fig. 6: Example of analysis of pressure in water pipe

Fig. 6: Example of analysis of pressure in water pipe

Fig. 7: Example of stress analysis

Fig. 7: Example of stress analysis

After that, we perform evaluation and review with fatigue testing measurement data, and carry out optimization. Combining these evaluation results and track record data, we recommend Fig. 8 as the basic rules for water pipe design in plastic molds.

Fig. 8: Basic design rules for conformal cooling

Fig. 8: Basic design rules for conformal cooling

6. Principles and rules for water pipe design

Figure 9 schematically indicates the basic principles for water pipes in a mold during molding, and the effects on the resin product.

Fig. 9: Principle of temperature control efficiency due to distance between the water pipe outer wall and
   product surface

Fig. 9: Principle of temperature control efficiency due to distance between the water pipe outer wall and product surface

(β is better in the case where wall thickness α in Schematic diagram A > wall thickness β in Schematic diagram B)
(If the wall thicknesses are equal, then the turbulent flow in Schematic diagram C is better)

When the metal wall thickness between the water pipe and the product surface is thin, the heat conduction efficiency is high, and if the conditions are the same, turbulent flow yields higher efficiency than laminar flow, and thus if well-grounded design is not carried out based on principles and rules, it will not be possible to achieve the expected effect, no matter how many conformal cooling channels are arranged.
In carrying out conformal cooling design, there is a need to understand important formulas relating to the cooling system during injection molding, so here these will be explained for review.

Cooling time for molded parts:

  • Varies proportionally with the square of the maximum wall thickness of the molded part, and the 1.6 power of the maximum diameter of runners
  • Varies in inverse proportion with the thermal diffusion factor of the molten resin

Cooling time can be expressed with the following formula:

Tc: Cooling time Thw: Maximum wall thickness of molded part Dr: Maximum runner diameter
α: Thermal diffusion
It is evident that, if the wall thickness doubles, the cooling time quadruples.

Thermal diffusion of the molten resin can be expressed with the following equation:

k: Thermal conductivity ρ: Density cv: Specific heat at constant volume

I imagine that any designer involved in injection molding is aware of the above equations.
Furthermore, the relation indicating the Reynolds number for laminar flow and turbulent flow is as follows:

ρ: Coolant density U: Average speed of coolant d: Diameter of cooling tube η: Dynamic viscosity of coolant

Generally speaking, values with Re>2300 are said to be turbulent flow. (This varies depending on the literature reference.)

We believe the most logical and efficient approach is to keep in mind the above basic points, and proceed with conformal cooling design while evaluating factors such as:

  • Ascertaining the thickness of molded parts
  • Prediction of mold temperature and resin temperature
  • Prediction of mold strength surface
  • Prediction of necessary water pipe diameter, prediction of necessary flow length
  • Coolant speed and flow lines
  • Roughness and residual stress of water pipe inner surface

Next time, we will examine this in greater detail, while looking at some actual cases.