Mold wall

Slip casting of metal powders closely follows ceramic slip casting techniques (see Ceramics). SHp, which is a viscous Hquid containing finely divided metal particles in a stable suspension, is poured into a plaster-of-Paris mold of the shape desired. As the Hquid is absorbed by the mold, the metal particles are carried to the wall and deposited there. This occurs equally in all directions and equally for metal particles of all sizes which gives a uniformly thick layer of powder deposited at the mold wall. 

Although the continuous casting of steel appears deceptively simple in principle, many difficulties are inherent to the process. When molten steel comes into contact with a water-cooled mold, a thin soHd skin forms on the wall (Eig. 10). However, because of the physical characteristics of steel, and because thermal contraction causes the skin to separate from the mold wall shortly after solidification, the rate of heat abstraction from the casting is low enough that molten steel persists within the interior of the section for some distance below the bottom of the mold. The thickness of the skin increases because the action of the water sprays as the casting moves downward and, eventually, the whole section solidifies. 

Most PET botties are produced by injection blow mol ding (71) the resin over a steel-core rod. The neck of the bottie is formed with the proper shape to receive closures and resin is provided around the temperature-conditioned rod for the blowing step. The rod with the resin is indexed to the mold, and the resin is blown away from the rod against the mold walls, where it cools to form the transparent bottie. The finished bottie is ejected and the rod is moved again to the injection-molding station. This process is favored for single cylindrical botties, but cannot be used for more complex shapes such as botties with handles. 

The work [40] deals with the redistribution of filler particles in the process of injection molding. In this case nonuniform distribution may occur both in the cross-section of a sample and along its length. Both kinds of nonuniformity are linked together if particle moves away from the mold walls it enters the zone of high velocity flow, therefore, a deficit of particles near the walls should be accompanied with a surplus of them far from the inlet. It should be noted that all the works mentioned consider spherical particles there are no theoretical or experimental studies of the redistribution of particles of other shapes, say, fibers or bars. 

Some draft is required in side walls to facilitate the easy removal of the product from the mold. Female molds require less draft since products tend to pull away from mold walls as they shrink during cooling. With female or male tooling, for most plastics the draft on each side wall should be at least 1 degree (Fig. 3-39). 

Some combinations of materials are not feasible with this method. For instance, after molding the first layer against the mold wall, the second material cannot have a higher melt temperature, which, of course, would melt the first layer, probably causing them to mix. 

Freezing action Because of the heat exchange between the flowing TP melt and the mold walls, the flow may freeze (solidify) before the product is completely filled. Products that have alternate sections with thick and then thin walls can cause problems in flow and cooling that make them difficult to fill. In some cases the plastics that have been selected for the end use requirement are too viscous to flow properly in a mold cavity, and this makes the manufacture difficult. 

Residual stress There is a condition that develops, particularly in products with thin walls. This is a frozen-in stress, a condition that results from the filling process. The TP flowing along the walls of the mold is chilled by heat transferring to the cold mold walls and the material is essentially set (approaching solidification). The material between the two chilled skins formed continues to flow and, as a result, it will stretch the chilled skins of plastics and subject them to tensile stresses. When the flow ceases, the skins of the product are in tension and the core material is in compression that results in a frozen-in stress condition. This stress level is added to any externally applied load so that a product with the frozen-in stress condition is subject to failure at reduced load levels. 

The heat transfer problem which must be solved in order to calculate the temperature profiles has been posed by Lee and Macosko(lO) as a coupled unsteady state heat conduction problem in the adjoining domains of the reaction mixture and of the nonadiabatic, nonisothermal mold wall. Figure 5 shows the geometry of interest. The following assumptions were made 1) no flow in the reaction mixture (typical molds fill in <2 sec.) ... 

Figure 6. Temperature profile in the mold for nonisothermal mold wall temperature (steel mold), Vc = 10 cm/sec and (Tp)max = 413°K at 7 sec...

These results have been fit to experimental data obtained for the reaction between a diisocyanate and a trifunctional polyester polyol, catalyzed by dibutyltindilaurate, in our laboratory RIM machine (Figure 2). No phase separation occurs during this reaction. Reaction order, n, activation energy, Ea, and the preexponential factor. A, were taken as adjustable parameters to fit adiabatic temperature rise data. Typical comparison between the experimental and numerical results are shown in Figure 7. The fit is quite satisfactory and gives reasonable values for the fit parameters. Figure 8 shows how fractional conversion of diisocyanate is predicted to vary as a function of time at the centerline and at the mold wall (remember that molecular diffusion has been assumed to be negligible). 

Figure 8, Conversion vs, time (predicted) at mold center (y = 1,0) and mold wall...

Once the polymer has cooled to its solid state, the molding is ejected. This is accomplished with the aid of ejector pins that protrude from the mold walls as it opens. Small items typically drop directly into a catch pan or onto a conveyor belt below the mold. Larger items are removed manually. 

Birefringence can also be used to analyze polymer samples after melt processing. As we will see in the next chapter, the shear produced in certain molding techniques, such as injection molding, can orient polymer chains in certain parts of the mold, especially near the mold walls, whereas the chains in low-shear regions, such as in the middle of the mold, are not as oriented. Figure 6.104 shows the variation in birefringence, as... 

The flow characteristics can be determined using the model shown in Figure 7.79. During the time interval nnder consideration, it is assumed that the polymer has been heated to a uniform temperatnre, T , that is equivalent to the mold wall temperature. As long as the preform radius, R, is less than the radius of the outer wall of the mold cavity, Ro, we can treat the problem as an isothermal radial flow of an incompressible power law flnid flowing between two disks that approach each other at a constant rate, h. In this way, the velocity field, Vr z, r, t), the pressnre distribution in the mold, P z, r, t), and the plnnger force, Fn z, r, t) can be obtained as follows ... 

Consider the vacuum forming of a polymer sheet into a conical mold as shown in Figure 7.84. We want to derive an expression for the thickness distribution of the final, conical-shaped product. The sheet has an initial uniform thickness of ho and is isothermal. It is assumed that the polymer is incompressible, and it deforms as an elastic solid (rather than a viscous liquid as in previous analyses) the free bubble is uniform in thickness and has a spherical shape the free bubble remains isothermal, but the sheet solidifies upon confacf wifh fhe mold wall fhere is no slip on fhe walls, and fhe bubble fhickness is very small compared fo ifs size. The presenf analysis holds for fhermoforming processes when fhe free bubble is less than hemispherical, since beyond this point the thickness cannot be assumed as constant. 

Figure 19.25 Shape of heterogeneous nucleus (ABCD) for solidification formed at the root of the crack in the mold wall, (a) Oblique view, (b) Cross section through the nucleus midplane. The crack extends normal to the page.

After mixing of several streams, the reactive mixture goes into a mold where polymerization or solidification takes place. The behavior of materials in a mold depends substantially on the chemical nature of the initial raw materials, on mixture viscosity, on shrinkage during polymerization and solidification, on adhesion to the mold walls, and on temperature. The main feature of free casting is that the mold is either unpressurized or the pressure is no higher than 0.5 MPa if a mold is... 

After filling a mold in the form of a thin rectangular parallel pipe, a reactive mass can be represented by a layer of width H and length L (H < < L). This layer is positioned between the mold wall at temperature Tm, and the surface of the item is at temperature Ti (Fig. 4.14). 

In rotational molding of anionically polymerized E-caprolactam one can produce vessels up to 10 m3, fuel tanks of various shapes, and other seamless large-sized items. As a rule, the mold rotates with different speeds about the two axes in such a way that the reactive mixture flows to the mold walls with minimal net centrifugal force. When molding long items, a greater difference in rotation speeds is required than when molding round items. 

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