When cooling channels directly related to part shape are used in an injection mould, part quality goes up and cycle time goes down. Optimum placement of the cooling channels cannot, however, be achieved by drilling into conventional (i.e. one-piece) mould components. Innova Zug Engineering GmbH, Menden, Germany, a firm that concentrates its activities solely on optimizing the thermal conditions in injection moulds, has developed the Contura system to achieve this optimization.
The three images in Figure 1 contrast different thermal conditions 10 seconds after volumetric filling of the cavity for a box-shaped part. Using conventional drilling techniques, only nonuniform cooling (top) results. Even if placement of the channels is optimized (center) with the aid of a computer-aided mould cooling program, cooling is still irregular. With the Contura system (bottom), the cooling channels follow the part shape and ensure a uniform cavity-wall temperature.
Here's how it works. A mould insert is produced with cooling channels in close proximity to the cavity wall. These follow the part shape, assuring good heat removal and thus shorter cycle times. In addition, the cooling channels are not restricted to being circular in cross section.
Why? Because the mould inserts are made from two or more parts; cooling channels are machined into the mating surfaces. With the aid of a special joining technique, these parts are subsequently bonded to form a one-piece mould insert.
More uniform cavity-wall temperatures result in more uniform shrinkage in the moulded parts and, in general, more uniform surface quality. A further benefit during operation of the mould inserts is that there is no need to replace O-rings anywhere along the cooling channels.
A mould for a polypropylene emergency stop switch serves as an example. This mould was designed and built by Hasco, Ludenscheid, Germany. It has been run for high-volume production at the Dausenau plant of Klockner-Moeller in Germany since 1996 and, compared to the predecessor mould for the same part, exhibits a 20 percent reduction in cycle time.
The switch is hemispherical in shape and has a diameter of 100 mm. The achievable cycle time depends on how close to the cavity and core surfaces the cooling channels can be placed in the mould.
Figure 3 shows the mould in its original conventional cooling configuration. Here, the cycle time of 33 seconds is determined by the high mould temperature in the region around the hot runner nozzle (75).
Redesigning the cooling as shown in Figure 4 results in a noticeable improvement in productivity. The two mould cores (22, 61) and the cavity (60) have been designed using the Contura system: the cooling channels conform to the part shape in close proximity to the cavity wall and extend to the region around the gate.
Heat is removed considerably faster and more uniformly than before. (The two core halves are shown as one in Figure 4, since they are bonded together at their mating surfaces to form a single component.)
With the use of this cooling arrangement, the mould temperature near the gate was reduced from 90øC to 50øC. The cycle time was shortened by more than 20 percent. The part is filled via a hot runner nozzle with pneumatically actuated needle shutoff. The shutoff needle forms a small dimple in the surface of the part in order to prevent injury from any protruding gate vestige. The ejector plates (8, 9) are connected to the stripper plate (4), together with the stripper ring (62) and the stripper sleeve (21). A taper fit is used between the stripper ring (62) and core (61) to eliminate wear and prevent the formation of flash on the moulded part. Upon actuation of the ejector rod (66), the part is ejected by the stripper ring (62) and stripper sleeve (21)
While the cost of the mould using this system is higher, costs can quickly amortize, thanks to a shorter cycle time.