Over-moulding

Co-injection and over-moulding involve several processing options ... 

In the thermoforming process, heat and pressure are applied to thermoplastic sheets which are placed over moulds to adopt their shapes (Figure 3.35). Pressure can be applied using air, compression or vacuum (vacuum forming). Compression is a low cost process because the moulds used are inexpensive and automation allows a production cycle of less than 2 seconds. Details are... 

When used in cable and connector over-moulding, it offers a finger grip, strain relief, flex control and anti-vibration all-in-one, to prevent damage to cables during use (Fig. 14.12). 

Vingaard M, Christiansen J (2012) Sealing of polymer micro-structures by over-moulding. Int J Adv Manuf Technol 61 161-170... 

Sepe and co-workers [13] found that automotive components consisting of a metal insert over-moulded with impact modified glass fibre reinforced PET cracked adjacent to the insert following thermal shock testing (100 cycles at -40 to 180 °C). The melt flow index of the material increased by 140% on drying prior to moulding. 

LPM is primarily a one-step process to encapsulate, seal and protect electronic assemblies. The process is also referred to as over-moulding or hot-melt moulding. The electronic assembly is placed in an aluminium mould which is then closed and a thermoplastic compound injected into the mould cavity encapsulating the assembly. The moulds are cooled by chilled water circulating through the mould platens to approximately 20 °C. When the part cools, it is removed from the mould. Cycle... 

Traditionally the steering wheel has comprised a metal skeleton covered in plastic. For some 40 years the coating was cellulose acetate butyrate (CAB) hard, scratch resistant - and slippery in warm weather. Increasing safety awareness led to the dished hub, and to over-moulding in PP and PVC. More recently, in meeting the requirements of ECE 21 and FMVSS 203, improved performance opposite head and body impact requirements has been achieved by the use of integral-skinned PUR foam for wheel covers and hub padding (Fig. 4.17). 

DuPont s Zenite 7130, an LCP reinforced by 30% glass fibre which features a HDT of 289 and low creep at high temperatures, is used by Epcos, for coil bobbins in its range of transformers, chokes and other surface mount devices. Another LCP glass reinforced compound, Zenite 6130L, is used by the Swiss Sonceboz company for coil bobbins and the over-moulding of the stator of its 4000-Series of torque motors. 

The market for TPEs has also benefited from advanced processing techniques such as over-moulding (see Chapter 10). The ability to use TPEs either alone or in combination with other materials has enabled them to be used in applications such as... 

Whilst injection moulding can be considered a relatively young manufacturing technology, many variants of the basic process have been developed. Examples include processes such as injection-compression, gas assisted, water injection, co-injection and over-moulding. Each of these processes are suitable for specific market applications. An overview of the technologies covered in this chapter is shown in Figure 10.1. 

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. 

This technique is where components are placed in an injection mould and are then moulded over with another material. The term over-moulding covers both insert moulding and lost core moulding. This technique is not confined to plastics and over-moulding of metal inserts, such as to produce scissor handles and plastics on ceramics is commonplace. 

A method must be found to mechanically interlock them. In the case of over-moulding and multishot this can be done with clever usage of material properties and tool design. 

Certainly in the case of many co-injection techniques, differences in mould shrinkage and thermal expansion can lead to problems such as sink marks, warpage and residual stresses. With over-moulding techniques, differences in shrinkage or the coefficient of linear thermal expansion (CLTE) can produce high stresses between restrained materials. The result in both cases can be the same, premature failure. 

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 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. 

Insert moulding with plastics is a two-step process whereby a first preform component is placed into the open mould cavity. Injection then proceeds as with traditional moulding methods with injection of a molten plastic onto the preform. This process is not limited to two material components and the resultant mouldings can be transferred in this way until the required number of layers is achieved. Inserts can be loaded by hand or by the use of robots. Inserts must be accurate in both their dimension and their placement into the over-moulding tool to prevent tool damage and provide accurate registration of one material on another. A means must also exist to hold them in place within the tool. In this way it has similar requirements to that of in mould lamination techniques commonly used to decorate plastics with films or foils, details of which can be found in a specialist Rapra Review Report [4] and will not be covered further here. 

---