Tool Design - Mould Cooling System and Design Consideration...

                                               Mold cooling system overview

Importance of cooling system design:-Mold cooling accounts for more than two-thirds of the total cycle time in the production of injection molded thermoplastic parts. Figure 1 illustrates this point. An efficient cooling circuit design reduces the cooling time, which, in turn, increases overall productivity. Moreover, uniform cooling improves part quality by reducing residual stresses and maintaining dimensional accuracy and stability (see Figure 2).

FIGURE 1. Mold cooling accounts for more than two-thirds of the total cycle time.

FIGURE 2. Proper and efficient cooling improves part quality and productivity

Mold cooling system components

A mold cooling system typically consists of the following items:
- Temperature controlling unit
- Pump
- Supply manifold
- Hoses
- Cooling channels in the mold
- Collection manifold

The mold itself can be considered as a heat exchanger, with heat from the hot polymer melt taken away by the circulating coolant.

Figures 3 and 4 illustrate the components of a typical cooling system.

FIGURE 3. A typical cooling system for an injection molding machine.

FIGURE 4. A cooling channel assembly attached to the mold plates.

                  Cooling-channel configuration

Types of cooling channels

Cooling-channel configurations can be serial or parallel. Both configurations are illustrated in Figure 1 below.

FIGURE 1. Cooling-channel configurations

Parallel cooling channels:-

                                                 Parallel cooling channels are drilled straight through from a supply manifold to a collection manifold. Due to the flow characteristics of the parallel design, the flow rate along various  cooling channels may be different, depending on the flow resistance of each individual  cooling channel. These varying flow rates in turn cause the heat transfer efficiency of the cooling channels to vary from one to another. As a result, cooling of the mold may not be uniform with a parallel cooling-channel configuration.

                                                 Typically, the cavity and core sides of the mold each have their own system of parallel cooling channels. The number of cooling channels per system varies with the size and complexity of the mold.

Serial cooling channels

                            Cooling channels connected in a single loop from the coolant inlet to its outlet are called serial cooling channels. This type of cooling-channel configuration is the most commonly recommended and used. By design, if the cooling channels are uniform in size, the coolant can maintain its (preferably) turbulent flow rate through its entire length. Turbulent flow enables heat to be transferred more effectively. Heat transfer of coolant flow discusses this more thoroughly. However, you should take care to minimize the temperature rise of the coolant, since the coolant will collect all the heat along the entire cooling-channel path. In general, the temperature difference of the coolant at the inlet and the exit should be within 5ºC for general-purpose molds and 3ºC for precision molds. For large molds, more than one serial cooling channel may be required to assure Cooling-channel Configuration uniform coolant temperature and thus uniform mold cooling.

                                                       What do they do?

Baffles and bubblers are sections of cooling lines that divert the coolant flow into areas that would normally lack cooling. Cooling channels are typically drilled through the mold cavity and core. The mold, however, may consist of areas too far away to accommodate regular cooling channels. Alternate methods for cooling these areas uniformly with the rest of the part involve the use of Baffles, Bubblers, or Thermal pins, as shown below.

FIGURE 1. Baffle, bubbler, and thermal pin

Baffles

A baffle is actually a cooling channel drilled perpendicular to a main cooling line, with a blade that separates one cooling passage into two semi-circular channels. The coolant flows in one side of the blade from the main cooling line, turns around the tip to the other side of the baffle, then flows back to the main cooling line.

This method provides maximum cross sections for the coolant, but it is difficult to mount the divider exactly in the center. The cooling effect and with it the temperature distribution on one side of the core may differ from that on the other side. This disadvantage of an otherwise economical solution, as far as manufacturing is concerned, can be eliminated if the metal sheet forming the baffle is twisted. For example, the helix baffle, as shown in Figure 2 below, conveys the coolant to the tip and back in the form of a helix. It is useful for diameters of 12 to 50 mm, and makes for a very homogeneous temperature distribution. Another logical development of baffles are single- or double-flight spiral cores, as shown in Figure 2 below.

 

FIGURE 2. (Left) Helix baffle. (Right) Spiral baffle.

Bubblers

A bubbler is similar to a baffle except that the blade is replaced with a small tube. The coolant flows into the bottom of the tube and “bubbles” out of the top, as does a fountain. The coolant then flows down around the outside of the tube to continue its flow through the cooling channels.

The most effective cooling of slender cores is achieved with bubblers. The diameter of both must be adjusted in such a way that the flow resistance in both cross sections is equal. The condition for this is:-

Inner Diameter / Outer Diameter = 0.707

Bubblers are commercially available and are usually screwed into the core, as shown in Figure 3 below. Up to a diameter of 4 mm, the tubing should be beveled at the end to enlarge the cross section of the outlet; this technique is illustrated in Figure 3. Bubblers can be used not only for core cooling, but are also for cooling flat mold sections, which can’t be equipped with drilled or milled channels.

FIGURE 3. (Left) Bubblers screwed into core. (Right) Bubbler beveled to enlarge outlet

NOTE: Because both baffles and bubblers have narrowed flow areas, the flow resistance increases. Therefore, care should be taken in designing the size of these devices. The flow and heat transfer behavior for both baffles and bubblers can be readily modeled and analyzed by C-MOLD Cooling analysis.

Thermal pins

A thermal pin is an alternative to baffles and bubblers. It is a sealed cylinder filled with a fluid. The fluid vaporizes as it draws heat from the tool steel and condenses as it releases the heat to the coolant, as shown in Figure 4. The heat transfer efficiency of a thermal pin is almost ten times as great as a copper tube. For good heat conduction, avoid an air gap between the thermal pin and the mold, or fill it with a highly conductive sealant.

FIGURE 4. Thermal pin heat transfer efficiency 

Cooling slender cores

If the diameter or width is very small (less than 3 mm), only air cooling is feasible. Air is blown at the cores from the outside during mold opening or flows through a central hole from inside, as shown in Figure 5. This procedure, of course, does not permit maintaining an exact mold temperature.

FIGURE 5. Air cooling of a slender core

Better cooling of slender cores (those measuring less than 5 mm) is accomplished by using inserts made of materials with high thermal conductivity, such as copper or beryllium-copper materials. This technique is illustrated in Figure 6 below. Such inserts are press-fitted into the core and extend with their base, which has a cross section as large as is feasible, into a cooling channel.

FIGURE 6. Using high thermal conductivity material to cool a slender core

Cooling large cores

For large core diameters (40 mm and larger), a positive transport of coolant must be ensured. This can be done with inserts in which the coolant reaches the tip of the core through a central bore and is led through a spiral to its circumference, and between core and insert helically to the outlet, as shown in Figure 7. This design weakens the core significantly.

FIGURE 7. Use of helical baffle to cool large core

Cooling cylinder cores

Cooling of cylinder cores and other round parts should be done with a double helix, as shown below. The coolant flows to the core tip in one helix and returns in another helix. For design reasons, the wall thickness of the core should be at least 3 mm in this case.

 
  
     FIGURE 8. Double helix with center bubbler
  
                                                            Cooling time

Theoretically, cooling time is proportional to the square of the heaviest part wall thickness or the power of 1.6 for the largest runner diameter. That is:

where the thermal diffusivity of polymer melt is defined as

In other words, doubling the wall thickness quadruples the cooling time.

Reynolds number and coolant flow

Whether or not the coolant flow is turbulent can be determined by the Reynolds number (Re), as listed in Table 1. The Reynolds number is defined as:

where ρ is the density of the coolant, U is the averaged velocity of the coolant, d is the diameter of the cooling channel, and  η is the dynamic viscosity of the coolant.

TABLE 1. Coolant flow types and corresponding
Reynolds number ranges