Platens

The mats are moved along the line to the press loader. When the loader is filled and the press opens to remove the load of freshly pressed boards, the loader pushes the new boards into the unloader and deposits the load of mats on the press platens. The press closes as quickly as possible to the desired panel thickness. More pressure, as much as 4.8—6.9 MPa (700—1000 psi) is required to press high density dry-process hardboard, because the dry fiber exhibits much more resistance to compression and densification than wet fiber. Press temperatures are also higher, in the range of 220—246°C. No screens are used in the dry-process, but the moisture in the mats requires a breathe cycle during pressing to avoid blowing the boards apart at the end of the cycle. Because no screens are used, the products are called smooth-two-sides (S-2-S), in contrast to the wet-process boards, which have a screen pattern embossed into the back side and are known as smooth-one-side (S-l-S). 

A separate mention is merited for a special molded hardboard product. These are made by a process in which either a fiber mat or hardboard panel is placed between two shaped platens and press-molded to a three-dimensional configuration. The most common resulting shape is a doorskin which resembles a wood panel door. The doorskins are bonded to wood frames to make an excellent, attractive, and relatively inexpensive door. This ftber/panel mol ding process is also used to make a wide variety of molded interior linings used in automobile manufacture. 

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. 

Because cycle time to inject, flow, set, open, eject, and close is finite, and the face area or platen size is limited, the effective mol ding area is increased by increasing the number of mold cavities so that the number of finished pieces per cycle may be multipHed many times. 

Fig. 13. Injection molding machine A, hydiaulic motoi foi turning the screw B, hydraulic cylinder and piston allowing the screw to reciprocate about three diameters C, hopper D, injection cylinder (a single-screw extmder) E, no22le F, fixed platen G, tie rods H, mold I, movable platen , hydraulic cylinder and piston used to move the movable platen and supply the force needed to keep the mold closed and K, machine base.

When the mold is opened, the part should be easily removable. Cavities are made with a slight taper to reduce frictional drag of the part on the mold. The half of the mold attached to the movable platen is equipped with ejector pins, which push the part out of the cavity while the mold is being opened. When the mold is closed, the pins are flush with the cavity surface. Release agents or lubricants facilitate ejection and shorten the mol ding cycle. Some complex parts require that the mold open in several directions in addition to the direction of the platen movement. For a threaded part, eg, a bottle cap, part of the mold must be rotated to remove the article from the mold. 

Compression tests, in which agglomerates ate cmshed between parallel platens, ate probably most universal. To obtain reproducible and accurate results, the rate of loading and method of load appHcation must be strictly controUed. A variety of commercial testers ate available to allow this needed control over the compression process. Several means of distributing the load uniformly at the point of contact ate used, including covering the platen... 

Plastic laminated sheets produced in 1913 led to the formation of the Formica Products Company and the commercial introduction, in 1931, of decorative laminates consisting of a urea—formaldehyde surface on an unrefined (kraft) paper core impregnated with phenoHc resin and compressed and heated between poHshed steel platens (8,10). The decorative surface laminates are usually about 1.6 mm thick and bonded to wood (a natural composite), plywood (another laminate), or particle board (a particulate composite). Since 1937, the surface layer of most decorative laminates has been fabricated with melamine—formaldehyde, which can be prepared with mineral fiUers, thus offering improved heat and moisture resistance and allowing a wide range of decorative effects (10,11). 

In the hydraulic system, oil under pressure is introduced behind a piston connected to the moving platen of the machine. This causes the mould to close and the clamp force can be adjusted so that there is no leakage of molten plastic from the mould. 

Foamed plastic articles may be produced with good results using normal screw-type injection moulding machines (see Fig. 4.43(a)). However, the limitations on shot size, injection speed and platen area imposed by conventional... 

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