Compression mold flash

Sandwich construction Compression mold Flash mold... 

Excess flash—Indicates the compound flow was excessive during the mold closing process (shelf loaded compression molding) [3]. 

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...

Cast polyurethanes cured with water would need the bulk of the carbon dioxide to be flashed off and the remainder held in place by compression molding. 

When compared with compression molding, transfer molding provides better product consistency, shorter cycle times, and better bonding of rubber to metal [55]. However, considerable amount of material is lost as scrap in the transfer pad, sprues, and flash. The basic three-plate multiple cavity mold is more complex and expensive than a comparable compression mold but is suited better for intricate parts or securing inserts [56]. 

Another difference between the processes is the flash formation and its removal. As it is reinforced with fibers, it becomes difficult to remove the flash formed. As compression molding uses semipositive molds, there would be some material loss in the form of flash. 

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. 

The CDs are manufactured either by injection compression molding or injection molding. Injection compression molding minimizes molded-in stress but requires longer cycle times than injection molding and produces flash. For both processes very clean room conditions, up to 1,000 times cleaner than medical molding, are essential. 

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

To form fine structures and to support the filling stage of high viscosity materials, the use of a mold with a vertical flash face for injection compression molding is useful. Figure 13 shows a mold with a molding with two surface structured cavities for the production of bipolar plates over a gap filling the mold. 

However, attempts have been made to identify how injection compression molding could improve microfeature replication on optical devices, such as DVD pickup lens or mobile flash phone optics [12, 13]. 

Cut-off (flash groove, pinch-off) n. In compression molding, the line where the two halves of a mold come together, often along a sharp mating ridge and groove. 

Flash groove (spew groove) n. A groove in a mold force that allows the escape of excess material during a compression-molding operation. 

Flash specification. Flash consists of small amounts of plastic which adhere to a molded part beyond its desired contour, usually where wear occurs like the fitments around side actions, ejector pins, and at the parting line. In small amounts, it can be removed with a variety of cutting and scraping devices ranging from clippers to a tile knife. For large amounts, typical of compression-molded parts, a router is used. 

The necessary pressure and temperature vary considerably depending on the thermal and rheological properties of the polymer. For a typical compression molding thermoset material, they may be between 120 and 200 C and 1000 and 3000 psi. A slight excess of material is nsnaUy placed in the mold to ensure its being completely filled. The rest of the polymer is squeezed out between the mating surfaces of the mold in a thin, easily removed film known as flash. 

The same type of resin-treated material as described above can be shaped into a variety of products by modification of a compression molding technique. Such molded laminates have excellent properties similar to high-pressure laminates. Usually a semipositive type of mold is favored with ample clearance for flash around the mold area. Molding pressures range from 250 to 2000 psi with temperatures of about 150-200°C. The cycle depends on the nature of the formulation and its thickness. 

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