The Mold

Two-Stage Resins. The ratio of formaldehyde to phenol is low enough to prevent the thermosetting reaction from occurring during manufacture of the resin. At this point the resin is termed novolac resin. Subsequently, hexamethylenetetramine is incorporated into the material to act as a source of chemical cross-links during the molding operation (and conversion to the thermoset or cured state). 

More complex shapes can be made by cold isostatic pressing (CIP). CIP uses deformable mbber molds of the required shape to contain the powder. The appHcation of isostatic pressure to the mold suspended in a pressure transfer media, such as oil, compacts the powder. CIP is not as easily automated as uniaxial pressing, but has found wide appHcation in the preparation of more complex shapes such as spark plug insulators (26). 

The fluid plasticizer (solvent) consists of an energetic compound, eg, nitroglycerin, an inert carrier, and a stabilizer. The system is evacuated to remove volatiles, moisture, and air, and the plasticizer is then pressurized and passed slowly upward through the powder bed while the powder is held stationary by a pressure plate on the powder column. Casting solvent may also be added from the top of the mold. 

Tack. Tack causes layers to adhere when they are pressed together. This property can be reduced by employing fillers with a finer psd or by dusting the stock with a laminar filler such as mica. In a related value, fillers such as mica can inhibit adhesion to the mold during processing. 

To improve homogeneity, the preformed article is heated to 370—390°C. The time required for heating and sintering depends on the mold dimensions cooling, which affects the crystallinity and product properties, should be slow. 

Eree-sintered articles do not have the same dimensions as the mold cavity because they shrink at right angles to the direction of the preform pressure and grow in the direction of the appHed pressure. 

The cycle can usually be estimated on the basis of about 30 s/3 mm of thickness most of it is devoted to ram-in-motion time (except for very thin sections). The mold temperature used with PFA 340 is often the highest temperature that allows the part to be ejected undamaged from the mold and retain its shape while cooling. 

Sufficient time must be allowed to cool the molded part under pressure to 90°C to prevent vacuum voids and distortion. 

In the low pressure process, a short shot of a resia containing a blowiag ageat is forced iato the mold where the expandable material is aUowed to expand to fiU the mold under pressures of 690—4140 kPa (100—600 psi). This process produces stmctural foam products with a characteristic surface swid pattern produced by the coUapse of ceUs on the surface of molded articles. 

In the high pressure process, a resia melt containing a chemical blowiag ageat is iajected iato an expandable mold under high pressure. Foaming begias as the mold cavity expands. This process produces stmctural foam products with very smooth surfaces siace the skin is formed before expansion takes place. 

For passenger car seating about 90% is made by the molded foam process. The transportation market has expedenced a decline since 1979 due to decreased automotive production and also because U.S. cars have been downsized, resulting in the use of less polyurethane foam per car. 

The pressure of the extmder forces uniform plastic distribution throughout the mold. Cooling the mold solidifies the plastic with slight shrinkage. The mold is maintained closed by mechanical or hydraulic pressure while the thermoplastic is injected and solidified. 

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. 

Injection-molded articles can be decorated by in-mold labeling or by post-mold decoration. In the former method, printed film is inserted into the mold cavity before injection. The plastic forms an intimate contact with the graphic material. Post-mold decoration includes hot stamping, dry offset printing, and decal printing. 

Thermoforming. Thermoforming is the most common method of fabricating sheet into three-dimensional packaging. In conventional thermoforming, the sheet is heated to its softening point or just below the melting temperature. The softened plastic is forced by differential air pressure into an open-top mold to assume the shape of the female mold. The mold is chilled and the plastic sheet solidifies and is then removed from the mold. 

The mold is usually prepared with orifices to permit air trapped between the sheet and the mold to escape and ensure uniform, close contact of the plastic with the mold surface. By clamping the sheet beyond the perimeter of the piece, plastic may be drawn from the peripheral areas into the mold, ensuring uniformity. Both pressure and vacuum are employed to force the softened plastic sheet into the mold. 

The heated web is conveyed to the forming section where pressure or vacuum force the softened web into the mold. The mold opens and the web is conveyed to a die-cutting station. 

In the cuspation—dilation thermoforming process developed in AustraHa, sheet formation is promoted by expanding blades extending into aU areas and distributing the material uniformly throughout the mold. This process is claimed to deHver uniform distribution of high barrier components of sheet coextmsions and laminations. The process also permits almost vertical side waUs to cups (2). 

Conventional extmsion or coextrusion may be performed on vertical or horizontal rotary or shuttle mold configurations. In shuttie blow mol ding the extmder and die are in fixed horizontal and vertical position two or more molds shuttie into and out of position beneath the die. By reciprocating in two planes, the mold may remove a parison and permit the extmder to function continuously. 

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