MOULD FILLING

Hiebcr, C, A. and Shen, S.F., 1980. A finite element/finite difference simulation of the injection-moulding filling process. J. Non-Newtonian Fluid Mech. 7, 1-32. 

One such process is the TAP process, the basic patent being held by Dow. It was developed in Japan by Asahi in conjuction with Toshiba. Foam expansion after mould filling is made possible by use of retractable mould cores. Because of the difficulty of allowing expansion in more than one direction this process has been largely limited to the production of flat products. Efficient gas sealing systems are also vital and the process needs close control. For this reason it has not been widely used in either Europe or North America. 

The early 1980s saw considerable interest in a new form of silicone materials, namely the liquid silicone mbbers. These may be considered as a development from the addition-cured RTV silicone rubbers but with a better pot life and improved physical properties, including heat stability similar to that of conventional peroxide-cured elastomers. The ability to process such liquid raw materials leads to a number of economic benefits such as lower production costs, increased ouput and reduced capital investment compared with more conventional rubbers. Liquid silicone rubbers are low-viscosity materials which range from a flow consistency to a paste consistency. They are usually supplied as a two-pack system which requires simple blending before use. The materials cure rapidly above 110°C and when injection moulded at high temperatures (200-250°C) cure times as low as a few seconds are possible for small parts. Because of the rapid mould filling, scorch is rarely a problem and, furthermore, post-curing is usually unnecessary. 

It is now necessary to derive an expression for the pressure loss in the cavity. Since the mould fills very quickly it may be assumed that effects due to freezing-off of the melt may be ignored. In Section 3.4(b) it was shown that for the flow of a power law fluid between parallel plates... 

Apart from the injection step itself, which is identical to common injection moulding, there are three steps for mould filling and gas introduction ... 

A study was made of the thermal degradation behaviour of PS foams used as patterns in the lost foam process for metal casting. The data indicated that the thermal degradation of the pattern had a strong effect on mould filling and casting quality. The use of other types of foam such as PE, PP, PMMA and styrene-methyl methacrylate copolymers to overcome problems encountered with PS foam patterns is discussed. 7 refs. 

Design Technique for the Process of Mould Filling and Gelatination of Plastisols. 

Design Technique for Mould Filling in Considering Plastisol Gelatination 123... 

The question arises whether the studied thyxtropic phenomena should be taken into account in mould filling with plastisols of Type II, i.e. to assume medium instantly reacting to loading like plastisols I. This is supported by experimental data used in Fig. 4. As is seen, the stress responds instantly both on its fall and rise. Hence,... 

Now let s summarize the data on the viscosity of PVC plastisols. First, as concerning injection moulding (filling of thin moulds), both the plastisols (those showing and those not showing thyxotropic phenomena) may be considered as non-linear liquids with no characteristic times provided that the filling time is much larger than the characteristic time of the medium. 

Equation (4.9) provides a possibility to determine all important parameters (from the technological point of view) of mould filling ... 

Let s note that assumptions (1) and (2) are fulfilled only at some time interval after mould filling. 

Equation (4.50) implies that, under the conditions of Eq. (4.50), the mould filling time is given by ... 

Analysis of the Final Stages of Injection Moulding of Plastisols 5.1 Pressure Equalization upon Mould Filling... 

At this stage, one has to take into account the volume compressibility of the material, since upon feed-up the hold-on time of material under pressure is determined by compressibility and slow viscous flow. If the pressure of injection PQ is sufficiently high, then at this stage a liquid may be considered to be Newtonian with viscosity q ,. Keeping this in mind, we may state that the calculation given below will be applicable to various plastisols (of types I and II) with the only difference that for plastisol I q = const, while for plastisol II q = q. For the sake of simplicity, the analysis will be performed for the case of a flat mould filled through a slit runner (Fig. 10 a). 

Mould filling. By closely tracing the derivation of Eq. (4.8) in Sect. 4, it becomes clear that Eq. (4.8) holds also in the presence of Eq. (3.8)—(3.10) when viscosity increases with time. Then we have ... 

Low-pressure moulding (filling moulds in making long shaped articles) may be performed using screw extruders as sources of thermoplastic melt. 

The lack of experimental data impose difficulties for modelling the processes of low-pressure moulding of thermoplastics. From this point of view, it is of interest to refer to 85> containing a wide scope of experimental material. The role played by energy dissipation as applied to flow in capillaries of viscosimeters was studied in 86>. To check the predictions of theory and to elucidate the applicability of one or another plastication unit, we have measured the pressure dynamics in the course of mould filling. Theory gives the following expression for pressure as a function of time at the head of an extrusion plasticator ... 

The process of mould filling should not be considered as completely understood and controlled 98>, though some important features of gas-filled system flow were established by Fridman and co-workers 99 100) who have explained the formation of laminated structures in low-pressure moulding of foaming melts and found the correlation between moulding conditions and morphologic macro-structure and properties of foam articles. 

In conclusion it may be noted that in spite of serious theoretical and experimental studies carried out in the field, the problem is still far from clearly understood. Let us formulate the most important questions to be solved. The first is the complete formulation and solution of the problem for mould filling either for the general case, or for individual processing procedures (aiming at shortening a moulding... 

G. A. A. V. Haagh and F. N. V. D. Vosse, Simulation of Three-dimensional Polymer Mould Filling Processes Using a Pseudo-concentration Method, Int. J. Numer. Meth. Fluids, 28, 1355-1369 (1998). 

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