As for difffculty and complexity, the stack mould is similar to the multi-component injection mould. The stack mould has two split lines which are in principle two injection moulds built one above the other. With this type of mould plastic parts are manufactured in two stacks. Therefore it is called a stack mould. A simpliffed stack mould is presented in Figure 2.35: the two moulds, the split lines, the guiding and the gear for a smooth opening and closing.

Figure 2.35 Schematic side view of a stack mould

2.5.1 Material CombinationsWith a stack mould multiple combinations of plastic parts can be produced.ƒ In the simplest version, the same parts come out of both split line faces, like in our example: from each half 35 containers from hard plastic, for example ABS, are ejected.ƒ A further variation is that one sort of parts made out of the same material comes out of every split line. For this variation there are 35 containers and 35 covers, also made out of hard ABS.The third variation is that different parts made out of different materials come out of the two split lines. For our example, it could be that 35 containers from hard ABS come out of the leff half and 35 covers from a soffer PP come out of the right half.

2.5.2 Hot RunnerThe stack mould can only be used with a hot runner (see Section 3.9.10). We go on with the ffrst variation in which all parts are made from the same plastic. The ffrst hot runner is in the mould on the half which faces the sprue bush of the injection moulding machine (moving half 2 in Figure 2.35). This hot runner is only the feeder for the plastic. In the middle section the hot runner with the distributor which fflls the entire cavity with plastic is located. In the split line between ffxed half 2 and the moving half 2 (see Figure 2.35) there is a transfer point for the plastic. Through the opening and closing of the mould this transfer point between the two hot runners is separated and connected again once in every cycle. In Figure 2.36, a hot runner system has been integrated in the mould. The transfer point can be seen in the split line 2 (see Figure 2.35). The plastic is injected centrally through the machine nozzle. 

Figure 2.36 Stack mould with hot runner

2.5.3 Opening and Closing

The mould is clamped as usual to the injection moulding machine: with a clamping plate on the leff to the moving half machine plate and with a clamping plate on the right on the ffxed half machine plate. The central part of the mould is held in the centre by the guiding between the two moving halves. For this special case, in some machines there are sliding guides on the machine base and the central part is supported with a guide shoe.In Figure 2.37 the injection mould is clamped on the machine. The central part is supported on both sides by the guide shoe.

Figure 2.37 Injection mould installed on the machine

When opening and closing, the machine or the injection mould covers a certain distance, in our example 400 mm. The central part is controlled through the gear and travels half the distance, that is, 200 mm, and the opening width is the same for both split lines. Figure 2.38 illustrates the situation for an open mould. The machine has travelled 400 mm and the opening width of each split line is 200 mm. Therefore there is enough space for the ffnished plastic pieces to fall from the mould. For an even better visualisation of this concept, a three-dimensional view of the open injection mould with gear and external guiding is displayed in Figure 2.39.

Figure 2.38 The situation for the 400 mm open machine

Figure 2.39 Three-dimensional view with gear

2.5.4 Toggle LeverA further possibility for the motion sequence of a stack mould is the toggle lever. It is installed in the middle of the central part. The two lever arms at the top and the bottom have the same length and are installed at the same distance from the centre. When turning the middle lever the two lever arms move simultaneously with it. As with the gear, half the distance is covered per side. The mould has been equipped with round guiding elements here due to lack of space. In Figure 2.40 the situation of the mould opened via a toggle lever is displayed. The distance is also halved here. In this version the central part is guided and supported through the round guides in the mould and the guide shoes on the machine base.

Figure 2.40 Mould with toggle leverTo better imagine this mould Figure 2.41 shows a three-dimensional view of the stack mould. The mould is moved via a toggle lever and the round guides are installed in the mould.

Figure 2.41 Three-dimensional view with toggle lever

2.5.5 EjectionThe plastic pieces are ejected centrally via an ejector retaining plate like for the classic mould. The ejectors in half 1 in Figure 2.40 can be controlled via the ejector unit in the machine. However, in half 2 an ejector unit must be installed which is moved hydraulically via cylinders.2.5.6„General Information on the Stack MouldThe closing force of the injection mould is deffned by the injection pressure and the projected area. If the plastic is injected in two stacks, the closing force is not higher despite the doubled number of plastic parts. The projected area does not add up. The closing force is an important criterion for purchasing an injection moulding machine. 

The total costs for a stack mould are only about 30 to 40 % higher in contrast to the doubled number of plastic parts. Nevertheless, a stack mould only makes sense when it is continuously used for high quantity production.

It should also be noted that the stack mould has a higher installation height than a normal, simple injection mould. Therefore the opening path of the machine for a stack mould can be much larger.