Heat moulding

For mass production purposes matched metal moulding techniques are employed. Here the preform or mat is placed in a heated mould and the resin poured on. The press is closed and light pressure ( 501bf/in ) applied. Curing schedules are usually about three minutes at 120°C. It is possible to produce laminates using less resin with pressure moulding than with hand lay-up techniques and this results in better mechanical properties. 

Compression moulding is one of the most common methods used to produce articles from thermosetting plastics. The process can also be used for thermoplastics but this is less conunon - the most familiar example is the production of LP records. The moulding operation as used for thermosets is illustrated in Fig. 4.62. A pre-weighed charge of partially polymerised thermoset is placed in the lower half of a heated mould and the upper half is then forced down. This causes the material to be squeezed out to take the shape of the mould. The application of the heat and pressure accelerates the polymerisation of the... 

Heating can be achieved by discontinuous processes, such as heated moulds, ovens, or by continuous processes such as hot tubes for wire curing or hot tunnels. 

The required quantity of thermoplastic is compressed at high pressure into the hot cavity space between the two parts of the heated 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. 

Briefly, a measured amount of the thermosetting compound is placed in the cavity of a heated mould which is attached to the platens of an hydraulic press. The temperature of the mould may be (as an instance) 150°C. When the press is closed and heat and pressure applied the material flows and fills the cavity excess (known as flash ) escapes from the mould. Heat applied causes cross-linking and hardening of the material the moulding sets in the shape of the cavity and after a pre-determined time (perhaps three minutes) can be removed. Pre-heating the material (to, say, 70°C) can be employed as a method of shortening cycle times. 

Here the transfer-moulding process where by the epoxy-silica charge is transferred to the mould to encapsulate the integrated circuit is shown. Note that in this case the fragile integrated-circuit pattern is placed inside the heated mould (typically at about 170-180 °C)... 

Glenn and Orts [153] describe another technique for making starch-based foams using a compression-explosion process. Starch feedstock is compressed in a heated mould at 230 °C under a 3.5 MPa clamping force for 10 s, which is then... 

A sp>ecial injection mould has been designed such that samples could be cooled at a known cooling rate and imder a known pressure (Brucato et al., 2002, Brucato et al., 2000, La Carrubba, 2001). This heated mould consists of a conical cavity (the sprue), which is located in the fixed platen of the injection molding machine, coupled to a "diaphragm". The front of the cavity is sealed with a high tensile, high thermal conductivity copp>er-beryllium... 

Injection Sprue Thermocouple Heated Mould Cavity... 

A common type of isothermally heated mould was used to determine the flowability of formulations with each of the catalyst candidates. This maze mould is shown in Figure 1.1. 

Fibre reinforcement and resin are premixed, and other additives are added prior to compression moulding in a heated mould which triggers the hardening of the resin. Many forms of premix are available, making a variety of fibre arrangements possible ... 

Savings in cost most often accrue from the injection moulding of thermosets (such as polyesters), instead of thermoplastics, because thermoset polymer is usually cheaper than thermoplastic and the cycle times are sometimes shorter (the material cross-links in the mould so that cooling is not necessary before the part is removed). The purpose of the screw is to plasticize and homogenize the precursor material (which may contain short fibres) in preparation for injection into a heated mould in which cure takes place. The basic machine is similar to that described above for thermoplastics, but there are important differeiKes in detail (see Figure 7.40) ... 

The plastidzed precursor material is injected into a heated mould. The mould temperature depends on the pofymer tem, but is, as a rule, considerabty higher ( 100 K) than the barrel temperature. 

In the heated mould, the liquid precursor quickly cross-links and solidifies the mould is opened and the part is removed whilst hot. 

This was the first technique used for forming plastics in production quantities. The mould comprises a matched pair of male and female dies. A measured quantity of partially cross-linked polymer (this technique is not now used for thermoplastic polymers) is placed between the two halves of the mould. The upper die is then lowered and the polymer compressed it is simultaneously heated by heat transfer from the heated mould. Before setting, the hot polymer completely fills the mould. It is then left to cure. The mould is finally opened and the part removed. The metering of the appropriate amount of polymer, and also some preheating, is being increasingly performed by screw machinery. This can increase by up to 400%... 

Molding compound is transferred in the cold state by pressure from the material hopper into the main injection chamber. Here it can be preheated before injection into the heated mould tool. Injection, through a special nozzle, can be either by ram or screw pressure. If screw feed is used, the screw must be of the type designed to process thermosets as opposed to thermoplastics. 

In moulding sperm candleSy as those produced from spermaceti are termed, the material is first melted and heated to a temperature approaching that of boiling water it is then run into the previously heated moulds, and in order to preserve its transparency, it should be cooled as... 

For thermoset polymeric materials, the chemical reaction involved in their preparation is not complete when producing the moulding powder (which is still thermoplastic). By applying heat and pressure, the powder can be readily pressed into a mould to a desired shape. When the heated mould reaches the proper temperature, the chemical reaction is completed, with the creation of an infusible, crosslinked product. Being crosslinked, recycling of the material is not possible. 

Figure 9.9 Transfer moulding. Molten resin transferred to the heated mould (left) followed by closing the mould and curing (right).

Some specific RTM methods, such as Assisted RTM (ARTM) and Thermal Expansion RTM (TERTN) are recognised as cost-effective techniques. In RTM, the compound is placed in a heated mould - where the temperature is between 120 and 150 °C - or a cold three-dimensional mould. When the mould is closed and pressure is applied (between 100 kPa and 15 MPa), the compound flows and fills the mould cavity(ies), while curing is taking place. Application of heat increases the rate and the extent of the cure. 

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