Melt moulding

The polymer is not easy to process and in injection moulding melt temperatures of 300°C are employed. In order to prevent excess embrittlement by shock cooling of the melt, mould temperatures as high as 150°C may be used. The polymer may also be compression moulded at temperatures of 250-260°C. 

The hot-melt microencapsulation process to produce microspheres is analogous to the melt moulding process to form flat devices. In the hot-melt microencapsulation... 

LPM is primarily a one-step process to encapsulate, seal and protect electronic assemblies. The process is also referred to as over-moulding or hot-melt moulding. The electronic assembly is placed in an aluminium mould which is then closed and a thermoplastic compound injected into the mould cavity encapsulating the assembly. The moulds are cooled by chilled water circulating through the mould platens to approximately 20 °C. When the part cools, it is removed from the mould. Cycle... 

Melt moulding Independent controllability of porosity and pore size High temperature required for nonamorphous polymer... 

Porous 3D polymeric scaffolds with HAp for bone regeneration are fabricated by solvent casting and particulate leaching, melt moulding, emulsion freeze drying, gas foaming, electrospinning, thermaily induced phase separation or solid freeform fabrication (SFF) (Table 3). 

Melt moulding, an alternative method of constructing 3D polymer/HAp scaffolds, has many advantages that are offered by modern processing technologies, such as reproducibility, shapeability, homogeneous distribution of filler and low cost. 

Melt moulding PE/HAp Compounding in the extruder, granulating and injection moulding [71-74, 76-84]... 

Polymers owe much of their attractiveness to their ease of processing. In many important teclmiques, such as injection moulding, fibre spinning and film fonnation, polymers are processed in the melt, so that their flow behaviour is of paramount importance. Because of the viscoelastic properties of polymers, their flow behaviour is much more complex than that of Newtonian liquids for which the viscosity is the only essential parameter. In polymer melts, the recoverable shear compliance, which relates to the elastic forces, is used in addition to the viscosity in the description of flow [48]. 

Creep of polymers is a major design problem. The glass temperature Tq, for a polymer, is a criterion of creep-resistance, in much the way that is for a metal or a ceramic. For most polymers, is close to room temperature. Well below Tq, the polymer is a glass (often containing crystalline regions - Chapter 5) and is a brittle, elastic solid -rubber, cooled in liquid nitrogen, is an example. Above Tq the Van der Waals bonds within the polymer melt, and it becomes a rubber (if the polymer chains are cross-linked) or a viscous liquid (if they are not). Thermoplastics, which can be moulded when hot, are a simple example well below Tq they are elastic well above, they are viscous liquids, and flow like treacle. 

These super-alloys are remarkable materials. They resist creep so well that they can be used at 850°C - and since they melt at 1280°C, this is 0.72 of their (absolute) melting point. They are so hard that they cannot be machined easily by normal methods, and must be precision-cast to their final shape. This is done by investment casting a precise wax model of the blade is embedded in an alumina paste which is then fired the wax bums out leaving an accurate mould from which one blade can be made by pouring liquid super-alloy into it (Fig. 20.4). Because the blades have to be made by this one-off method, they are expensive. One blade costs about UK 250 or US 375, of which only UK 20 (US 30) is materials the total cost of a rotor of 102 blades is UK 25,000 or US 38,000. 

At yet higher temperatures (>1.4T ) the secondary bonds melt completely and even the entanglement points slip. This is the regime in which thermoplastics are moulded linear polymers become viscous liquids. The viscosity is always defined (and usually measured) in shear if a shear stress o produces a rate of shear 7 then the viscosity (Chapter 19) is... 

Deformation of a polymer melt—either thermoplastic or thermosetting. Processes operating in this way include extrusion, injection moulding and calendering, and form, in tonnage terms, the most important processing class. 

The principles of thermoplastic melt processing can perhaps best be illustrated by reference to Figure 8.1 illustrating extrusion, injection moulding, bottle blowing and calendering operations. In order to realise the full potential of the process it is necessary to consider the following factors ... 

Table 8.1 Heat required (enthalpy required) to raise polymers to their processing temperatures from an ambient temperature of 20°C and the heat required to be removed in cooling a polymer from the melt to mould temperature...

Polymer Melt temperature i°C) Mould temperature (X) SG Specific heat (Jkg- K ) Heat required to melt Heat removed on cooling ... 

The meli and mould temperatures and the value of the heal removed per gram on cooling are taken from the paper by Whelan and Goff. The values for the amount of heat required to raise the temperature to the melting point and the heat requirements per unit volume (both for heating and cooling) have been calculated from these data by the author. 

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. 

Some typical data for this mouldability index are given in Figure 8.8. One limitation of these data is that they do not explicitly show whether or not a mould will fill in an injection moulding operation. This will clearly depend on the thickness of the moulding, the flow distances required and operational parameters such as melt and mould temperatures. One very crude estimate that is widely used is the flow path ratio, the ratio of flow distance to section thickness. The assumption is that if this is greater than the ratio (distance from gate to furthest point from gate)/section thickness, then the mould will fill. Whilst... 

The difference between the temperature of the melt on injection into the mould (Tj) and the mould temperature (7),). 

Some data on the amount of heat required to reduce the temperature of a polymer from a typical melt temperature to the temperature of the mould (in terms of both per unit mass and per unit volume) are given in Table 8.1. Whilst a moulding will usually by withdrawn at some temperature above the mould temperature, the data do provide some comparison of the different heat requirements of different polymers. It will be noticed that there is a more than 7-fold difference between the top and bottom polymers in the table. 

The time available for disorientation as the melt cools from Tp to T. This will depend on the value of Tp-T where is the temperature of the environment (the mould temperature in injection moulding) since this will with the specific heat determine the rate of cooling. The time will also depend on Tp-T since this will determine the extent of cooling. 

---