Core back

Fig. 10 Core-back injection molding of surface coating with SCCO2. (From Refill)...

Interface Reactive Two-Component Injection Moulding (Core-Back Process).183... 

Partitioning into overlying partial melts. The ultra-low velocity zone at the coremantle boundary may reflect the presence of mantle melt (see Garnero 2000 Ohtani and Maeda 2001), and partitioning from the core back into the overlying mantle may occur if conditions are favorable. However, a flux cannot be easily calculated without constraints on partition coefficients and the volume and residence time of melts at the core-mantle boundary. 

Reflectors are used to return neutrons leaving the reactor core back to the core. 

Figure 11.1 Core-back technology, which combines MuCell process with a secondary expansion process. SCF laden plastic is injected with high pressure and minimal initial cell growth. The mold volume is then rapidly increased to initiate uniform cell growth.

In bi-injection, materials are processed simultaneously at different points into a tool. In this maimer it has features of co-injection (simultaneous injection of two materials but through one nozzle) and core back moulding (where injection is at two points but in sequence). This method is discussed in detail... 

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 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. 

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. 

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. 

However for some parts, other methods may be inappropriate. This could be due to either tooling costs, especially on large parts, or to the nature of the material. For example robotic transfer of a very flexible material may be difficult. Other important factors may be cycle times, for example the cooling times may be very short on a thin moulding. In cases such as these, a core back tooling system may be the best option. 

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. 

The melting and softening characteristics of materials have been mentioned already. They also affect the potential process order. Since the material injected first is already well on its way to cooling when the second material is injected, it is better to mould the material with the lowest melting temperature first. This is where one technique may become preferable over another for certain material combinations. For example, core back moulding can use less than a full cycle between material injection. This means that injection of the second component can occur before full cooling has taken place, therefore increasing the potential to achieve adhesion in some cases. This is not possible with the rotary or transfer methods. 

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