Stiffening moulds

Poly(vinyl acetal) itself is now of little commercial importance. The material may be injection moulded but has no particular properties which merit its use. It is occasionally used in conjunction with nitrocellulose in lacquers, as a vehicle for wash primers and as a stiffener for fabrics. 

The easiest way to stiffen injection mouldings is to use ribs on the hidden surface (Fig. 13.3a). Ribs correspond to slots in the mould, so straight ribs are easier to machine (using a milling wheel) than those with corners. The rib thickness r should be less than 2/3 of the thickness t of the surface that they support, so they complete solidification first, thereby avoiding sink marks appearing on the product surface (Chapter 6). 

Toughness (consistency near plastic limit) After removing particles larger than the No. 40 sieve size, a specimen of soil about 1 cm in size, is moulded to the consistency of putty. If too dry, water must be added and if sticky, the specimen should be spread out in a thin layer and allowed to lose some moisture by evaporation. Then the specimen is rolled out by hand on a smooth surface or between the palms into a thread about 3 mm in diameter. The thread is then folded and re-rolled repeatedly. During this manipulation, the moisture content is gradully reduced and fhe specimen stiffens, finally loses its plasticity, and crumbles when the plastic limit is reached. 

A set of tools is required for the manufacture of a turbine blade these include the two half shells in a pair of moulds, the web stiffeners and the spar cap. Further moulds may be associated with blade edges and root ends. 

Plate 18 The window panels of this coach are made from cold-press moulded GRP. They are stiffened with both aluminium and steel formers which are bonded directly onto the panel with a toughened acrylic adhesive. Material Permabond F241. (Photograph courtesy British Rail Engineering Ltd.)... 

The Mark 9 clip has a centrally positioned stiffening rib, which aligns the stress correctly in service, and, in production, provides a good flow channel for the molten polymer during injection moulding. The stress now develops principally in the web section (B in Figure 5.2), and this can be calculated for various service conditions. 

Absence of sink marks enables thick sections to be produced with large bosses, webs, stiffening ribs, etc. on the reverse side of a moulding. Noryl SF computerized cash registers make a good example of this kind of exploitation in a demanding application, i.e. sink marks would be totally unacceptable on the surface of the product. 

The minimum section thickness of SF parts is approximately 5-6 mm since below this level the foam has no space in which to expand and solid plastic sections are produced. Above 6 mm thickness the cooling time increases dramatically. This, it was decided by the PDD of BIP to adopt a general wall thickness of 6 mm for the trolley platform. From aesthetic and mechanical requirements the depth of the platform was put at 60 mm and since the smallest platform area to be moulded was 1000 x 600 mm it was evident that some stiffening ribs would have to be incorporated. 

Immediately the moulding is removed from the press it is fully cooled by total immersion in a bath of cold water for a few minutes. This stiffens the moulding and prevents any tendency to after blow - reverse effect of sink marks. The runners are then removed by clipping off at the gates and a final trim of the gate scar is done with a knife. 

The most important technique to consider for reinforced thermoplastic composites is injection moulding. Since the melt viscosity is higher whenever fibres are included in the system, the injection pressures necessary are higher than their unreinforced thermoplastic counterparts. In addition, the product is much stiffen Although the cycle times are less for reinforced thermoplastics, increased stiffnesses can certainly effect the ejection, hence the mould design needs to be modified considerably. 

Some part geometries, such as stiffener-reinforced panels, require preparing sub-structures (preforms) prior to lay-up of the part. These could, for example, consist of a C-channel stiffener made from several prepreg plies. Such a preform can be manufactured by the same lay-up techniques as described above, e.g. by depositing ply by ply into a C-shaped mould. As an alternative, preforms can also be made by the process of hot forming. 

A spar cap, web stiffeners and other details are installed in one of the halves of the blade mould and adhesive is applied to the exposed bonding edges of these units. The seeond half of the mould, containing the half-shell, is then turned over and lowered onto the first the adhesive is then allowed to enre, thus joining the two halves of the wind turbine blade together. The steel support frames are hinged together at one side to enable the mould to... 

Soft-mud Process. A process for the shaping of building bricks from clay at a water content of about 35%. The prepared wet clay is fed into sanded moulds which are then shaken or jolted until the clay fills the mould because of its THIXOTROPY (q.v.), after the jolting ceases the clay stiffens sufficiently for the bricks to maintain their shape. The process can form the basis of handmaking or, more commonly, it can be mechanized as in the berry machine (Berry Son, Southend-on-Sea, England) or in the aberson machine (Aberson, Olst. Holland). 

---