Mould geometry

A widely used test for mouldability evaluation is the spiral flow test (Fig. 24.4). In this test the mould has the form of a spiral the polymer melt flows into this mould under pressure and freezes in the spiral, the length of the polymer spiral being the test result. Mould geometry, temperature and pressure are standardised. 

This equation is used alternately with the flow equation, to update the melt temperature distribution. If the injection pressure is below the limit of the machine, the new position of the melt front is computed. The programmes output the melt front shape and melt pressure isobars at various times, hence predict whether a mould can be filled satisfactorily. If not, modifications can be made to the CAD file for the mould geometry, and the analysis repeated, before the mould cavity is machined. Figure 5.27 shows the predicted flow fronts for an instrument panel moulding. 

The progression of the resin flow front is highly dependent on the mould geometry and the flow properties of the reinforcement placement of the inlet and vent ports is therefore crucial to obtain a good-quality manufactured part. Defects or material variation in the preform can also greatly affect the flow of resin. The inclusion of sensors built into the mould can help track the resin progression, and the use of multiple inlets and vents enables some control over the shape and advancement of the flow front. 

The mechanical properties of the polymer-based composites are dependent on the short fibres orientation, and, on its turn, this one is influenced by the characteristics of melt flow, injection conditions, and mould geometry [1254, 1256]. 

Usually, the fibres are not unidirectional oriented in composite materials. Fibres orientation varies in function of the pieces thickness, processing method, and mould geometry. Accepting a random orientation of fibres, or a known and measured orientation, the laminates theory was appealed in order to describe the behaviour of this type of composites. The method allows the determination of the weakest layers, which will be firstly fractured [1285]. The Figure 3.456 illustrates a good agreement between the theory and practice, considering, for comparison, the data furnished by P.E. Chen [1285] and J.K. Lees [1286]. 

As regards the influence of the mould geometry, the following is generally valid the flow resistance should be kept as low as possible, e g., by avoiding sharp edges in the component (pressure losses due to abrupt turning by the compound flow). 

Restrictions with regard to mould geometry - parts with sharp comers, changes in wall thickness, ribs or bosses are difficult with co-injection moulding due to the problems of distributing skin and core materials. 

The flow process in an injection mould is complicated by the fact that the mould cavity walls are below the freezing point of the polymer melt. In these circumstances the technologist is generally more concerned with the ability to fill the cavity rather than with the magnitude of the melt viscosity. In one analysis made of the injection moulding situation, Barrie showed that it was possible to calculate a mouldability index (p.) for a melt which was a function of the flow parameters K and the thermal diffusivity and the relevant processing temperatures (melt temperature and mould temperature) but which was independent of the geometry of the cavity and the flow pattern within the cavity. 

In any particular material, the flexural stiffness will be defined by the second moment of area, /, for the cross-section. As with a property such as area, the second moment of area is independent of the material - it is purely a function of geometry. If we consider a variety of cross-sections as follows, we can easily see the benefits of choosing carefully the cross-sectional geometry of a moulded plastic component. 

Extrusion blow moulding is continually developing to be capable of producing even more complex shapes. These include unsymmetrical geometries and double wall mouldings. In recent years there have also been considerable... 

In practice the clamping pressure will also depend on the geometry of the cavity. In particular the flow ratio (flow length/channel lateral dimension) is important. Fig. 4.42 illustrates typical variations in the Mean Effective Pressure in the cavity for different thicknesses and flow ratios. The data used here is typical for easy flow materials such as polyethylene, polypropylene and polystyrene. To calculate the clamp force, simply multiply the appropriate Mean Effective Pressure by the projected area of the moulding. In practice it is... 

Consider the thermoforming of a plastic sheet of thickness, Ao, into a conical mould as shown in Fig. 4.55(a). At this moment in time, t, the plastic is in contact with the mould for a distance, 5, and the remainder of the sheet is in the form of a spherical dome of radius, R, and thickness, h. From the geometry of the mould the radius is given by... 

Plastics differ because of their morphology, structure, rheology, etc. The geometry of a part and the complexity of the corresponding mould vary. Consequently, the tolerances of a part depend on ... 

CONSTRAINED GEOMETRY CATALYST, 154 CONTACT ANGLE, 262 CONTACT MOULDING, 246 CONTINUOUS EXTRUSION, 314 CONTINUOUS MOULDING, 312 CONTINUOUS... 

ISO 694310 for fatigue in tension specifies two different types of test pieces, rings and dumb-bells, which correspond to the geometries used on commercially available apparatus. There is, in principle, little difference between the two forms of test piece but dumb-bells are necessary for studying directional effects. They are also easier than rings to cut from sheet, but normally a specially moulded sheet is required such that the dumb-bells... 

For spherical particle geometry, as in the case of a microbial floe, a pellet of mould or a bead of gel-entrapped enzyme, the expression for the effectiveness factor can again be derived by a procedure similar to that used in Chapter 3 for a spherical pellet of conventional catalyst. A material balance for the substrate across an elementary shell of radius r and thickness dr within the pellet will yield ... 

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