Multi-shot techniques

Hadamard spectroscopic imaging (HSI) is a technique to obtain localized spectroscopic information from n regions of interest in n scans [Boll, Hafl, Goel, Goe2, Goe4, MU14]. It is a straightforward extension of the multi-frequency selective-pulse technique 

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The Battenfeld solution to three channel moulding is shown in Figure 10.7. This utilises machine configurations also used in multi-shot techniques as will be described in later sections. The vertical unit can be used to feed the third material into a eombined nozzle system based on their two layer technology described in Section 10.8.2.3. 

Multi-shot techniques produce not only multi-colour but also multi-material mouldings. The most common methods are the use of tool rotation, the core back technique and transfer tools. Transfer tools can be used to move shots from one cavity to another, this technique is very similar to over-moulding. These processes will be described in detail in later sections. Some applications are listed in Table 10.3. 

The ability to complete the process without mould opening or preform transport are the main advantages to this method when compared to other multi-shot techniques. However, increasing the number of components beyond two will certainly significantly increase the cost of tooling due to the increased intricacy required. The machine must also have the necessary means to actuate all the slides in the tooling. 

As in all multi-material injection, attention must be paid to the compatibility of the melts. The use of the core back technique enables greater bond strengths to be achieved than in other multi-shot techniques as the time between injection of the first melt and injection of the second material can be optimised. However, the sequence of injection of the first material then the second material is longer than in other multi-shot techniques, which proceed in parallel. Therefore in components whose design lends it to both techniques, a detailed analysis of the economic implications of the process routes may be required to determine the most appropriate method of production. 

Shaped and composite pulses are devoid of free precession periods between periods of rf excitation. Pulse sequences combine both free precession and rf pulses. Clearly, given the manifold of shaped and composite pulses, there is an infinite number of pulse sequences which can be constructed from them. A variety of such sequences is used for volume localization. This topic is discussed in Section 10.2. Here two methods relevant to the acquisition of NMR images are reviewed. These are the DANTE technique for single-shot localization, and the Hadamard technique as an example of multi-shot localization. 

Various terminologies are used in multi-material moulding to describe particular process routes. These have been split into three categories here multi-component, multi-shot and over-mould. Assisted moulding techniques include both gas and water. A brief description of each of these technologies and some of the names commonly used for them in the literature now follows. 

Moulding in this manner gives a shorter cycle time than core back moulding and it is a method of achieving the usual requirements of multi-shot such as multi-colour or hard/soft combinations. The knit line is also stronger due to the higher temperature at the interface when the flows meet. However, the materials do not maintain good separation and definition at the interface, which can be a problem in potential applications for this technique. 

Multi-shot moulding techniques are well established, their growth being pushed by the development of thermoplastic elastomer (TPE) materials, enabling rigid and flexible material combinations such as those described previously to be employed. These can also be seen in a variety of other applications from automotive seals to bras. 

The Mono-Sandwich technique for co-injection moulding was described in Section 10.8.2.3 to which the reader should refer for machinery details. This technique can also be used for over-moulding by using the core back technique, again described earlier. In this technique termed the monosandwich 5 process, an additional valve is required in the runner system that ean release different valves as necessary . Once the melt is layered, the first component is injeeted. The valve is switched within the mould to expand the cavity and then the rest of the shot is injected to ereate a multi-shot component. 

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