Incomplete mouldings

Poly(vinyl alcohol) is employed for a variety of purposes. Film cast from aqueous alcohol solution is an important release agent in the manufacture of reinforced plastics. Incompletely hydrolysed grades have been developed for water-soluble packages for bath salts, bleaches, insecticides and disinfectants. Techniques for making tubular blown film, similar to that used with polyethylene, have been developed for this purpose. Moulded and extruded products which combine oil resistance with toughness and flexibility are produced in the United States but have never become popular in Europe. 

Flow cracks. Incorrect shape and size of blank, incomplete mould flow high viscosity stock and excessive mould lubricant. Modify the shape, reduce temperature, modify curing system, peptize and pre warm the stock. 

In both cases, the mould is incompletely filled - a short shot - to leave room for the foam expansion to occur. This is responsible for many of the advantages, already listed, in the products. 

Such applications are widely used across all fields which use silicone rubbers. The technique is quite simple. In the first step a tubing or a moulding is cured to an incomplete degree of curing. This is achieved by curing at a lower temperature or too short a curing time for complete vulcanisation. These parts are then inserted into another silicone cavity (HTV or LR) and coated with the silicone rubber. It is advisable to make silicone-silicone composites from silicone rubbers with only one curing system. In other words, a peroxide... 

FIGURE 2.16 The effect of minimum film-formation temperature (MFFT) of a dispersion on its drying behaviour. Primal (Rohm Haas) dispersions were allowed to dry at room temperature, =21 °C, in a silicone rubber mould. An MFFT above room temperature results in incomplete coalescence of particles and poor film formation. The cracks and distortion that occur in the films result from the movement of water and shrinkage during drying, (a) AC-34 MFFT, 12 °C. (b) AC-73 MFFT, 37 °C. (c) B-85 MFFT, 90 °C. 

For the first 15 minutes of operation a watch should be kept over whether there are any melt leaks in the manifold space. Leaks may be indicated by shorts (incomplete filling of the mould) with a previously set specific batching capacity, a fall in injection pressure or gases escaping from the manifold space, etc. 

Incomplete mouldings are caused hy a failure to fill the cavities during injection. The way to proceed depends on whether difficulties occurred at start-up, or whether the fault appeared while production was going on. Table 11.5 shows some of the causes and remedies of incomplete mouldings. 

Knit lines and incomplete fill are said to occur when the mould cavity remains only partially filled and/or the moulding is not homogeneous, with grooves appearing in the pieceparts. Knit lines are at best cosmetically objectionable and at worst sites for the entrapment of moisture and various ionic contaminants that can lead to early failure of a device due to corrosion. A typical example of non-filling is shown in Fig. 5.13. 

Short shot term used in injection moulding when insufficient material is injected into the mould, which results in an incomplete moulding... 

Injection moulded parts with incompletely developed outer profiles are called incompletely filled parts (or short shots). An example is shown in Figure 9.28. This kind of defect often appears far from the gate if there are long flow distances, or on thin walls (e.g., ribs as shown in Figure 9.29). Due to insufficient mould venting, this defect can also occur in other areas. 

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