Transfer moulding compounds

Larger sacle testing of inhibitors in transfer moulding compounds prior to practical use of the idea. 

Transfer moulding compounds, injection moulding compounds, casting, potting, dip and powder coating compounds, die attach adhesives. Photoresists. 

Transfer moulding compounds, coatings, potting compounds, coil impregnants, wire insulation. 

Further processing in which appropriate fillers, catalysts, pigments and other additives are incorporated leads to silicone greases, elastomers and resin-based transfer moulding compounds. A broad list of final silicone products is given in Table 3.1. 

Table 3.2 lists some of the most common of curing reactions. In the first of these (a) the reaction of hydroxyl (OH) groups extends and crosslinks the Si-O-Si chain and is characteristic of many silicone resins, including the resin constituent in silicone transfer moulding compounds which are described later. Usually this reaction is induced by heating the uncured resin which contains some Si-OH groups, in... 

However, silicone transfer moulding compounds have only been used to a limited extent for packaging integrated circuits. This is due to the less effective lead seal of the silicone resins compared with epoxies, a factor which is more important for a multi-lead IC than for a discrete device. This lead-seal deficiency can be overcome by a technique... 

Fig. 3.7. Power resistors encapsulated in a silicone transfer moulding compound. (C.G.S. Resistors.)...

A range of silicone transfer moulding compounds is available, but to illustrate their general performance Table 3.4 shows the properties of... 

Table 3.5 compares the typical ingredients of silicone and organic transfer moulding compounds. Because the silicone is naturally fire retardant, no additive for this purpose is needed, nor does the silicone require as much release agent as an organic compound. Silicone... 

Davis, J. and Jones, G. M., The role of silicone transfer moulding compounds in the packaging of semiconductors and passive electronic components. Proceedings of International Macroelectronic Conference, Electronica, Munich, 1982. 

Another extender occasionally used in epoxide formulations is clay. Clay is a hydrated alumino silicate mineral, also known as kaolin, which is available in a calcined, anhydrous form. The hydrated form is non-abrasive, chemically inert and has a large surface area. It is naturally acidic and this must be taken into account when formulating for applications where corrosion of metallic conductors may be encountered. Hydrated clay disperses readily in most formulations, particularly when dispersants or surfactants are used, and is utilised in epoxide transfer moulding compounds to allow control of flow. 

EVALUATION OF EPOXIDE-BASED TRANSFER MOULDING COMPOUNDS... 

The method of manufacture of solid rubber products by simultaneous shaping and vulcanising. An uncured blank of rubber compound is placed in the bottom half of a mould, the top half of the mould placed in position and the mould placed in a heated hydraulic press. The press performs the dual function of both closing and heating the mould. See Transfer Moulding and Injection Moulding. 

In transfer moulding, the channels in the mould which transfer mbber compound to the actual moulding cavities. 

Transfer moulding uses an additional pot/ram section built into the top of the mould. A pad of rubber compound sufficient to fill the cavities, plus an allowance for the transfer pad remaining in the transfer pot, is put into the pot and the mass transferred into the individual cavities. 

In commercial use are a wide variety of machines which transfer fixed volumes of compound to clamped moulds, by the operation of a ram in a cylinder fed with pre-heated slugs. These machines are capable of high speed semi-automatic cycling and are a great advance over the use of loose transfer moulds in conventional presses. The components which are produced are to a large degree flash-free, it only being necessary, in the majority of cases, to remove the injection feed and runner system. 

Transfer moulding involves the distribution of the uncured stock from one part of the mould called as pot into the actual mould cavity. This process permits the moulding of intricate shapes or the introduction of inserts like metals in many composite products. These procedures are difficult in compression moulds. Although these moulds are relatively more expensive than compression moulds, the actual process permits shorter cure times through the use of higher temperatures and better heat transfer which is obtained due to higher pressure applied to force the compound into the mould. 

Three types of moulding processes are available - compression, transfer and injection moulding. In compression moulding the rubber blank is placed directly into the cavity of the mould where it is heated by conduction which causes rubber flow by application of pressure. Transfer moulding uses prewarmed rubber which is heated during transfer and forced through small orifices into the mould cavities in a three-part mould. In the injection moulding process, the rubber compound is pushed under pressure from an injection head where it has been heated and plasticized into a closed heated mould where cure is completed. 

Saeki and Kaneda (1988) examined the apparent mean viscosity, minimum viscosity and apparent gel time of an epoxy-resin moulding compound during the transfer-moulding operation by using an instrumented transfer mould. 

Transfer moulding In transfer moulding the uncured rubber compound is transferred from the loading chamber (pot) of the transfer mould into the actual mould cavity. This technique is suitable for moulding products of a complicated shape, and for anchoring delicate inserts in mouldings. 

ASTM Dl896-93. Practice for transfer moulding test specimens of thermosetting compounds and extrusion materials, American Society for Testing Materials Publication. Philadelphia. 1993. 

Transfer molding is used for the manufacture of small components and is particularly useful with multi-cavity tools and where small inserts are to be moulded in. Materials used are polyester and epoxide dough moulding compounds, although a new liquid resin injection technique is reported. 

The type and amount of filler have an effect on shrinkage and sink marks more filler reduces both, but at the same time increases the mixture viscosity, which is critical when working with moulding compounds. A combination of coarse and fine particles produces the optimum results. Fillers, such as clay, calcium carbonate and wollastonite, judiciously selected and in relatively high concentrations, can also impart flame retardancy and serve as a stress transfer medium, as well as reduce the total material cost. It should be pointed out that fillers strongly influence the flow characteristics of moulding compounds. 

Closed mould (or matched-die moulding) processes are used in the composites industry for fabrication and manufacturing of three dimensional compounds and products. There are different closed moulding processes. Within these, there are transfer moulding, compression moulding, resin injection moulding, injection moulding, pultrusion and extrusion. 

In the case of moulding thermosets, the uncured (initial) compound is usually placed first in the cavity of the mould, or the transfer pot of a transfer moulding process, and heated (to about 150 °C) to provide sufficient flow for mould filling. Pressure (approximately 13 MPa) is then applied for sufficient time to allow the resin to cure. 

Uncured thermoset resin/glass fibre premix for injection or transfer moulding, also known as dough moulding compound (DMC). (See also DMC Dough moulding compound)... 

Polyester/resin fibre premix, for injection or transfer moulding. It is also called bulk moulding compound (BMC). 

Because of their high crosslink density and very brittle behaviour, phenolic moulding compounds are invariably filled with reinforcing fillers such as wood floui wollastonite, mica, mineral wool flour and glass fibres. Solid moulding compounds are normally novolac based resins for transfer moulding (RTM) and other liquid laminating processes are resols. 

A vulcanizing mould consists essentially of two or more plates that ean be brought together and separated by hydraulic pressure. The plates are usually heated by steam or eleetrieity. The rubber compounds are vulcanized in various moulds between the heated plates under pressure. The principal moulding processes for rubber are eompression moulding, transfer moulding and injection moulding. 

Transfer moulding, injection moulding, compression moulding compounds. 

As a consequence of these and continuing advances being made in moulding compounds, the packaging of components by transfer moulding is increasing in popularity and will remain by far the most important packaging technique in terms of volume for the foreseeable future. 

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