## 4 Discussion  
Extraction operations were typified by different authors, depending on the lesser or greater difficulty of the operation for the extracted geometry part. A good attempt to summarize the various situations was made by [PERSON] (1973), but the cases reported do not include the needs brought by multi-material injections. Regarding the impact on costs, [PERSON] (1973) made a good attempt to describe all of the factors affecting the overall cost of the molds, including the maintenance and non-quality costs. Yet, the problems caused by complex and lengthy extraction devices into the mold, with implications for the selection of the injection machine were not reported. [PERSON] [PERSON] and [PERSON] (1973) addressed a problem similar to that discussed in this work, considering the cost implications of the mold regarding the introduction of hot runners. However, in that case, the analysis was carried out in terms of the cost of the mold, while in the case of the present work the implications are addressed for two factors: direct cost of the mold and cost of operating the mold, since it implies the use of a greater capacity, which also leads to the consumption of a greater volume of energy, thus being less sustainable in service [PERSON] (1974). [PERSON] (1974) published a book regarding the multi-material injection molding, where the core back molding and the rotating molding techniques were described, but the problems related to the eventual collisions and space occupied by the extractors, together with the preparation system for the change of cavity between the first and the second injection, were not discussed. Thus, after a deep search throughout the literature, the subject discussed and developed in this work seems very scarce. This reinforces the novelty of this approach. The conventional molds that are used to obtain parts by bi-injection have a simple structure, as shown in Figure 18. As previously mentioned, the movements required to operate both injections are usually performed by the employment of lengthy and complex mechanical devices (Figure 1). The use of these components in the molding area causes the dimensions of the mold to increase (to accommodate these components). However, in the case of the developed mold, this occupied space is reduced, because the movements are produced by hydraulic cylinders, which can be placed externally to the main structure of the mold. Indeed, the hydraulic cylinders produce their actuation in the wedge (90\({}^{\circ}\)), which avoids the necessity of the same height of the mold. Moreover, the cylinders can be connected externally to the main structure of the mold, which does not consume the volume of the mold, implying a higher volume. The great success of this mold is reflected in the way it performs bi-injection parts using a relatively simple, yet extremely effective, hybrid mechanism combining mechanical and hydraulic movements to move the cavity from the first to the second injection, thus creating the necessary space for the second material to be injected over the first one. This can be performed without changing the external structure of the mold, which remains close to the conventional structure observed in Figure 18.  
The developed mechanism consists of a horizontal hydraulic movement, which activates a mechanical device in a direction perpendicular to itself (Figure 13, Type-1 cylinder). The device is used to make the cavity movement occupy less space than usual (when compared to the conventionally used extraction devices), allowing for the remaining free space to be used by the extractors/lifters to perform the movement needed between the first and second injection. Because the hydraulic cylinders can be positioned vertically relative to the mold movement (parallel to the base of the mold), they do not require a higher distance between the injection machine plates (by reducing mold height) and can be easily accommodated among the remaining devices needed for extraction.