In thermoplastic injection molding, part quality and cycle time depend strongly on the cooling stage. in this case we study some alternative cooling devices for the injection mold cooling design for core, the expected result is an improvement of the part quality in terms of shrinkage and warpage.
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 above, 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 very homogeneous temperature distribution. Another logical development of baffles is single- or double-flight spiral cores, as shown above.
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 above. 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.
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 Upmold Cooling analysis.
A thermal pin is an alternative to baffles and bubblers. It is a sealed cylinder filled with fluid. The fluid vaporizes as it draws heat from the tool steel and condenses as it releases the heat to the coolant, as shown above. 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.
Cooling for 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 above. This procedure, of course, does not permit maintaining an exact mold temperature.
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 above. 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.
Cooling for 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 a core and insert helically to the outlet, as shown in above. This design weakens the core significantly.
Cooling for cylinder cores
Cooling of cylinder cores and other round parts should be done with a double helix, as shown above. 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.