Compression mold positive

Figure 19 illustrates one type of compression mold suitable for molding lip type shaft seals with a metal outside diameter (O.D.). Figure 19a shows the compound preform resting on the shelf and the mold beginning to close. The mold continues to close until it gets to the position shown in Fig. 19b. Finally, the mold opens as shown in Fig. 19c. It can be seen that this molding technique produces a molded lip seal with the flash already removed. In general, when using a vacuum chamber on the press, to ensure that all the air and/or gases are evacuated from the mold after closure, one or...

This technique overcomes the drawbacks of unidirectional pressing and produces a charge of uniform density and homogeneity. In this method, the explosive formulation in the mold is compressed into position in two modes. In the first mode, pressure acts from top by the downward movement of the ram. In the second stage of pressing, the spacer is removed and the base plate moves upwards on application of pressure. Thus the formulation experiences equal pressure throughout the length. The L/D ratio is two in this case. 

During high temperature compression molding, the polymeric powder is poured into a positive pressure mold that is heated and then cooled under pressure. The cooled mold is then opened to yield a fully sintered UHMWPE article. 

To imprint more complex structures, a Si master with positive relief structures was first created. Then the master was used to imprint the channel pattern on a PMMA plate (135°C for 5 min) (see Figure 2.22) [1011]. This method is called compression molding or embossing. In using this method, a vacuum chamber is needed to remove air bubbles trapped at small features and to remove water vapor from the polymer [7]. 

This process is a take-off from compression molding that uses solid material male and female matching mold halves. This unique process uses a precision-made, solid shaped heated cavity and a flexible plunger that is usually made of hard rubber or TS polyurethane. This two-part system can be mounted in a press, either hydraulic or air-actuated. Rather excellent product qualities are possible at fairly low production rates. The reinforcement can be positioned in the cavity and the liquid TS resin is poured on it. Also used are prepregs, BMC, and SMC. 

Example of mold types (a) positive compression mold,... 

FIGURE Compression molds, (a) A simple flash mold, (b) A positive mold. Knockout pins could extend through plunger instead of through cavity, (c) Semi-positive mold as it appears in partly closed position before it becomes positive. Material trapped in area b escapes upward, (d) Semipositive mold in closed position. 

In-mold decoration can be done with either injection molding of thermoplastics or compression molding of thermosets. Thermosetting plastics are decorated with a two-stage process. For melamine products, for example, the mold is loaded with the molding powder in the usual manner and closed. It is opened after a partial cure, and the decorative "foil or overlay is placed in position. 

An interesting application of MF resins in compression molding involves decorative foils made by impregnating a printed or decorated grade of paper with resin and then drying. The foil may be applied to a compression molding shortly before the cure is complete, and the resin in the foil may be cured in that position to produce a bonding. 

Figure 5,81 Cross sectional view of a multi-cavity semi-positive mold used for compression molding...

Figure 5.82 Example of mold types (a) positive compression mold, (b) flash compression mold, and (c) semipositive compression mold...

A carbon fiber bicycle wheel rim is fabricated by Sun Rims saving 400 g over an A1 rim and with 30% less carbon than that achieved by competitors using carbon fiber braid. A core of foam is rolled into a circle to fit the mold rim periphery and a braid sleeve drawn over the core with the ends overlapped. Extra pieces of carbon fiber prepreg are positioned at high stress points, the reinforcement is impregnated with epoxy resin and the lay-up is then compression molded. The pressures are sufficient for the overlap joint to be uniformly incorporated into the rim thickness. 

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