Mold flatness

Certain basic facts must be observed in the manufacture of molds. Flat surfaces should be avoided if possible, because slight domes or dish effects will allow the sheet to stretch over the entire surface. The curved surface prevents the slight bumps that usually may appear in flat sections. Maximum allowable vent hole diameters will vary with materials and sheet thickness. Air evacuation holes should be as small as possible and to minimize restriction of plastic flow through vent holes, the openings should be back drilled mold surface, as reviewed above. 

Fig. 6 The molded flat array of air-breathing DMFCs, shown held between the fingers on the right, generates power in the range 0.5-1 W in a concept prototype of a DMFC system for portable consumer electronics applications. (MTI Micro Fuel Cells).

The second polymer (sample E) was an ethylene-hexene copolymer which Is slightly branched, and had an My of 211,000 and a nominal density of 0.955 g/cm. Specimens approximately one millimeter thick were prepared from both polymers by molding flat sheets from which dumbbell-shaped bars were cut with a die. The sheets were molded by heating the polymer for ten minutes at 440K In a press at which time the heat was turned off and pressure applied. The polymer was then cooled In the press over... 

Sheet Molding Flat sheet One centimeter to 1 meter in width and thickness of 3-75 millimeters A few hundred grams to a few tens of kilograms... 

Back drilling of large holes (to 0.125 in.) on the underside can be used when smaller holes are required on the part side. Male molds can be mounted on a vacuum plate with thin washers or shims, and large vacuum holes can be drilled under the mold. Narrow slots also can be used, and they offer much less resistance than holes when air is evacuated through the mold. Flat areas, segmented sections, or male portions of a mold can be joined with shims, providing long slots. 

Surfacing mats are relatively strong and stiff and are applied when molding flat areas or parts of simple contour. Overlay mats are more drapable and harder to handle but can be used on parts of more complex shape. 

Figure 1.8 Illustration of typical molded flat sheet fatigue-testing specimens, (a) Flat sheet fatigue specimen with rectangular cross-section (b) flat sheet fatigue specimen with circular cross-section.

Uses and Treatments of Hardwood Plywood. Most early appHcations of hardwood plywood were those where the hardwood plywood was better adapted to the use than soHd wood. One of the most important early uses was in curved or formed parts, an appHcation particularly suited to the use of veneers which could be molded into intricate shapes during the pressing and bonding process. Then, as furniture manufacturers realized the inherently superior stabiHty of plywood compared to soHd wood, lumber-core or plywood panels began to be used for most flat-panel constmctions in furniture. 

A small amount of particleboard is made with a fire-retardant treatment for use in locations where codes require this material, as in some offices and elevators. Particleboards receive overlay and finishing treatments with ease. Wood veneers, melamine overlays, printed paper overlays, vinyl overlays, foils, and direct grain printing can all be done quite simply. A small amount of particleboard is also made in the form of shaped, molded articles such as furniture parts, paper roU plugs, bmsh bases, and even toilet seats. There is another small increment of particleboard made by the extmsion process. These products are made in small captive operations owned by furniture manufacturers which consume all of this production in their furniture. The extmsion process differs from conventional flat-pressed particleboard in that the wood furnish is forced between two stationary heated surfaces. The mats are formed from one edge and this edge is alternately formed and pushed between the heated platens, which are maintained at a distance equal to the thickness of board produced. This is an old, slow, small-scale process, but is stiU in use in at least one location. 

Insert injection mol ding is used to manufacture snap closures for yogurt and ice cream cups and tubs and for breakfast cereal cans. In insert injection mol ding, a die-cut printed paperboard or other flat material is placed in the mold. The plastic is extmded around the insert to form a precision skeletal stmcture. 

Newer resins include polysulfone, polyethersulfone, polyetherimide, and polyetherketone. Some of these newer materials are high temperature thermoplastic, not thermoset, resins. They are being promoted for the design of injection-molded printed circuit boards in three-dimensional shapes for functional appHcations as an alternative to standard flat printed circuit boards. Only semiadditive or fully additive processing can be used with these devices. 

Ultem PEI resins are amber and amorphous, with heat-distortion temperatures similar to polyethersulfone resins. Ultem resins exhibit high modulus and ate stiff yet ductile. Light transmission is low. In spite of the high use temperature, they are processible by injection mol ding, stmctural foam mol ding, or extmsion techniques at moderate pressures between 340 and 425°C. They are inherently flame retardant and generate Httie smoke dimensional stabiUties are excellent. Large flat parts such as circuit boards or hard disks for computers can be injection-molded to maintain critical dimensions. 

Veneers laid up on a flat table. Complete assembly is positioned in a mold at one time used primarily on curved surfaces to be molded by the flexible bag process. 

The variation in temperature across the top and bottom halves of the mold is a function of the press platen size, the flatness of the mold and platen surfaces at the two mold/platen interfaces, the mold construction, and the platen heating system. The larger the platen size, the more difficult it is to maintain a constant temperature across the platen. Therefore, one solution to the problem is to use a large number of molding presses with a relatively small platen size, for example, four presses with platens 355 x 355 mm will have approximately the same production capacity as one press with a 710 x 710 mm platen. In reality, for high-volume production, large presses are more economical from the cost point of view and the floor space required. 

The pressure distribution across the mold is a function of both the mold surface and the platen flatness. The larger the mold size and the platen, the more difficult it is to maintain the high degree of flatness required. The mold and platen flatness is also critical in the area of heat transfer from the platen to the mold itself. 

As discussed in the previous section on injection molding, a sink mark almost always occurs in extrusion on a flat surface that is... 

Products in this process can have deep sections and relatively sharp comers. However RM flat, particularly large relatively uniform wall thickness surfaces are difficult if not impossible to produce. This process can be used to mold complex products that may require three or four split molds. Also, different finished surfaces are obtained. For example, the products surface finish is dictated by the inside surface of the mold. This makes it easy to obtain smooth as well as textured surfaces on the product. Raised or depressed letters, fluting, and other decorative inscriptions may also be molded. 

By contrast with ICM, a compression mold design is used where male plug fits into a female cavity rather than the usual flat surface parting line mold halves for IM (Fig. 8-27). The melt is injected into the cavity as a short shot thereby not filling the cavity. The melt in the cavity is literally stress-free it is literally poured into the cavity. Prior to receiving melt, the mold is slightly opened so that a closed cavity exists the male and female parts... 

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