Multi-shot

Kawaguchi, M., Y. Ishii, N. Sakui, N. Okanouchi, R. Ito, K. Inoue, K. Saito, and H. Nakazawa. 2004. Stir bar sorptive extraction with in situ derivatization and thermal desorption-gas chromatography-mass spectrometry in the multi-shot mode for determination of estrogens in river water samples. J. Chromatogr. A 1049 1-8. 

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. 

Figure 1. a) Polarised extinction spectra of samples with Ag nanoparticles original sample and irradiated at 400 nm in the multi-shot regime (100 pulses in single spot), peak pulse intensity was 0.42 TW/cm2 b) TEM image of deformed nanoparticles after irradiation. The laser polarisation is shown as an arrow c) SAXS intensity distribution from the irradiated sample. The light ring in the centre is the shadow of the beam stop. 

The English company has developed three 12-gauge 5 shot lines in their non-toxic tungsten. TMX mixes iron into the tungsten matrix so it is 20 percent cheaper to produce and gives 90 percent patterns at 40 yards. Impact Multi-Shot is a 60/40 iron/tungsten matrix mix. The third line is Tin Shot - Super Speed. 

The first two of these points concern the size of the mould tool that can be physically fitted into the machine. The daylight is the maximum distance between the stationary and moving platen, obviously there must be room in the tool to open and eject the part. The tie-bars restrict the length and width of a tool that can be placed on the platen as they restrict entry. With this in mind, there are tie-bar-less machines on the market which become particularly pertinent with multi-shot moulding (see Chapter 10). 

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. 

Multi-shot can be used to describe any process whereby distinct multiple material shots are applied to produce a single final component. This includes processes whereby preforms are moulded and then transferred to different cavities on the same machine. It can also be used to describe processes where multiple shots are made into the same tool without the tool opening between shots. 

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. 

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. 

A number of moulding methods can be employed to produce a multi-shot moulding but whilst mouldings can be produced by a variety of methods, those produced from the same material but in multi-colour multi-shot enjoy the highest market share. 

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. 

As well as these applications multi-shot also finds use for power tool cases, battery cases and domestic appliances (e.g., kettles handles, vacuums, lawn mowers and electric toothbrushes). 

Multi-shot processes, as the name implies, require multiple shots of material to make a single component. For each one of these materials an injection unit is required. To mould these multiple shots also requires special tooling and equipment. Multi-shot capability can be built either into the injection tool or controlled by the injection moulding machine. To enable multi-shot, multiple injection units can be arranged to feed machines in a number of ways as the next section will explain. 

Figure 10.28 Example of clamp unit for multi-shot...

Now machinery issues have been briefly overviewed, individual technologies for multi-shot will be introduced. These are ... 

Also referred to in various literature as composite injection moulding or multi-shot. The manufacturer Battenfeld uses the trade name Combiform for this process. Core back is a tooling controlled process. 

In Figure 10.29a the insert is closed. This constrains the first injected material to this area of the cavity. When the insert is opened, as shown in Figure 10.29b, the second material can feed into the newly opened cavity area and flow into the material already injected to give the multi-shot component. In this way slides and inserts are used to block and control access to the cavity for particular materials feeds. 

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. 

Figure 10.32 Two-component (top) and three-component (bottom) multi-shot moulding...

Multi-Shot with a Single Injection Unit... 

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. 

This section will introduce material selection issues for multi-shot moulding as well as providing some introductory information on two classes of materials which may not be familiar to the non-specialist moulder namely thermoplastic elastomers (TPE) and liquid silicon rubbers (LSR). This section will begin, however, with consideration of adhesion. 

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