Closed mould processes

Resin Transfer Moulding (RTM)—A closed mould process in which dry reinforcement in the form of mat or cloth is placed into a matched mould. Resin is then injected in to fill the cavity and flows through the fibres to fill the mould space. 

RTM is a closed mould process in which the component is formed between a matched pair of moulds. The moulds are often themselves made from FRP and are therefore relatively low cost. The process typically has a cycle time of 15 to 30 minutes. 

Harper A, Closed mould processing. Reinforced Plastics, 28-34, Feb 2001. 

Fibres can also be prepared in the form of a mat consisting of randomly oriented short fibres held loosely together hy a chemical hinder, sometimes in a carrier fabric, as a continuous thin flat sheet. Mats are commercially available as blankets of various weights, thickness, and widths, which can he cut and shaped for use as preforms in some closed-mould processes and in hand lay-up, press moulding, bag moulding, autoclave moulding, and in various continuous impregnating processes. 

Open mould processes are operated manually, with one exception (filament winding) while closed mould processes are either semi-automatic or completely automatic (Figure 9.4). 

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. 

An unaccelerated, non-thixotropic orthophthalic resin designed for closed mould processing such as RTM. e. 5/25 i. 81 C... 

Resin transfer moulding (RTM), is a prominent type of liquid injection moulding, defined as a closed moulding process of composite preparation in which catalyzed resin is transferred into an enclosed mold cavity to impregnate a pre-positioned fibrous reinforcement (preform), as presented in the Figure 13.1 [1],... 

In recent years rotational casting methods have made the slush moulding process virtually obsolete. In these processes an amount of material equal to the weight of the finished product is poured into a mould. The mould is then closed and rotated slowly about two axes so that the paste flows easily over the cavity walls in an oven at about 200-250°C. When the compound has gelled, the moulds are cooled and the moulding removed. Compared with the slush moulding process there is no wastage of material, little flash, and more even wall thickness. Completely enclosed hollow articles such as playballs are most conveniently made. 

The convention extrusion blow moulding process may be continuous or intermittent. In the former method the extruder continuously supplies molten polymer through the annular die. In most cases the mould assembly moves relative to the die. When the mould has closed around the parison, a hot knife separates the latter from the extruder and the mould moves away for inflation, cooling and ejection of the moulding. Meanwhile the next parison will have been produced and this mould may move back to collect it or, in multi-mould systems, this would have been picked up by another mould. Alternatively in some machines the mould assembly is fixed and the required length of parison is cut off and transported to the mould by a robot arm. 

The application of instrumentation to the automatic control of a sequence of operations, e.g., injection moulding processes. Once the mould has been loaded with inserts (assuming a rubber to metal bonded part) into the press, the operation of a push-button starts the controller which closes the press, injects the rubber, controls the cure cycle, recharges the injection unit, opens the press, operates the ejectors and presents the mould for cleaning and loading of inserts. 

The injection-compression process is an important variation in which rubber is injected into a partly closed mould under low pressure used for moulding-on of rubber soles to footwear uppers. Rubber is injected into a gap between the shoe upper and the sole plate, which is held 3-4 mm wider than it would be in its final position. After injection at the first mould station of a multistation machine the sole plate is closed in the manner of compression moulding at the second mould station and rubber is pressed along the sole and over the toe cap. The importance of the process is that it becomes possible to mould without using normal high injection pressures which tear and distort canvas, leather or synthetic uppers. 

In the case of paints and printing inks, the initial preparations will be in the semi-solid state because solvents are needed both in the process of dispersing the pigment in the paint or ink medium and for application purposes. These solvents dry out after the paint or ink is applied. When making coloured plastic articles, both heating and solvents may be used to aid dispersion in the plastic medium as part of the moulding process. However, from the viewpoint of the optical properties in all of these pigment uses, what is most important is that each of these media has a refractive index close to 1.5. 

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. 

The new method of plastisol processing called, low-pressure moulding has been developed during recent years. The procedure involves the filling of closed moulds... 

Injection moulding can be applied to two-component thermosets in a process known as Reaction Injection Moulding. The two liquid components are mixed and then injected into a closed mould. In the case of epoxies and polyurethanes, heat is produced on mixing due to the exothermic reaction between the two components (polyol and isocyanates in the case of polyurethanes), so no external source of heat is required. 

A further development of the use of vacuum to improve processing is RTM. The process is shown in Figure 3.5. A key reason for using the RTM route as a manufacturing route is to reduce costs. The simple definition of RTM is the injection of liquid resin at low viscosity, under vacuum or low pressure, into prearranged reinforcement materials or preform, which are contained in a closed mould die [15]. Potter (1999) [18] provides a history of the RTM process over the last 60 years and identifies the developments that have taken place over this period. The process is described in detail by [ 19]. 

A widely used method of laminate construction is by the contact moulding process or hand lay-up . In this, a layer of catalysed unreinforced resin, known as the gelcoat , is brushed or sprayed onto the mould and allowed to cure and then layers of reinforcing-fibre impregnated with resin are built up behind (Fig. 6.3). Variations of this include resin injection and vacuum-assisted resin injection in closed moulds. After removal of the cured moulding from the mould, the gelcoat forms the outer skin and as such performs the two functions of decoration and protection ... 

Phenolics are now growing into the FRP composite areas of hand lay up, open and closed mould techniques, pultrusion and filament winding. The driving force behind this recent growth is the concern over flammability and smoke generation of materials used in transportation and in certain types of construction. Use of phenolics is limited to those fabrication processes and sizes of structure which allow high temperature curing. 

Therefore, this research work aims to produce and characterize low-density cellular materials based on dry natural rubber in order, on the one hand, to reduce the density and consequently the price of this raw material, and on the other hand to provide further knowledge about the topic. A production process, which combines the crosslinking by sulfur, commonly called vulcanization, and the foaming of NR by decomposing a chemical blowing agent inside a closed mould, was developed. Finally a study of the correlation between density, cellular structure and mechanical and acoustic absorption properties of the foams produced has been carried out. 

There are close similarities between the moulding process and the formation of an adhesive joint. In both cases, one material (the polymer or adhesive), usually in liquid or semi-liquid form, is brought into close contact with another (the mould or substrate) and then sets. The phenomenon of mould release is in many respects the same as the phenomenon of adhesion a moulding is almost like an adhesive bond where very low, even zero, adhesion is desired. See Mould adhesion - measurement. 

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