Male mold

Cast molding is an increasingly used manufacturing process for both rigid gas-permeable and hydrogel contact lenses. In this process, two molds, made from a variety of plastics, are used. A female mold forms the lens front surface (convex) and a male mold forms the lens back surface (concave). The plastic molds are made from metal tools or dies that are usually stainless steel, precision lathed, and polished to the specified lens design. A variety of mold materials are used. The polymerized, hardened lens is released from the mated molds and is processed in much the same way as the spin-cast lenses described above. 

The dimensional tolerances with the more conventional single-mold system are generally 0.6% ( 0.35% for close tolerances). With female molds 0.5% ( 0.3% close) with male molds under 3 ft., 0.8% ( 0.4% close) and with male molds over 3 ft., 30% ( 10% close) for wall thicknesses. 

Positive mold A projecting mold over which the product is formed, usually referred to as a male mold. 

In drape forming, the thermoplastic sheet is clamped and heated and the assembly then sealed over a male mold. The mold may be forced into the sheet or the sheet may be pulled into the mold by introduction of a vacuum between the sealed sheet and mold. By draping the sheet over the mold, the part of the sheet touching the mold remains close to the original thickness. Foamed PS and polyolefins are generally used in this procedure. 

In pressure-bubble vacuum snapback, the heated sheet is clamped and sealed against a pressure box. Air is forced through the female pressure box forcing the sheet to push outward from the pressure box. A male mold is then pressed against the bubble and as it pushes into the pressure box, excess air is forced from the pressure box forcing the heated sheet to take the shape of the male mold. The major difference between the vacuum snapback and pressure-bubble vacuum snapback is that in the vacuum snapback process vacuum from a female... 

In matched-mold forming, a heated sheet is placed between a matched female and male mold parts. As the two mold halves close, they distort the sheet to their shape. The air between the mold halves is removed. The article walls are more uniform than for many of the thermoforming techniques. This technique is used for the production of foamed PS and foamed polyolefins food containers. 

Dip Molding. Instead of a heated product, a heated male mold is dip-coated in the same way. In this case, the final flexible vinyl product is stripped off of the mold. A typical product is medical gloves. 

Male molds provide for tighter tolerance controls. What causes female molds to have more difficulty in controlling tolerances is due to plastic shrinking away from the mold during cooling. Prestretching or extended localized heating can be used with either type mold to provide a more uniform wall thickness. 

The female molded product has the greatest wall thickness with the thinnest bottoms. The reverse occurs when using a male mold. During the forming, the part of the hot sheet that touches any part of the mold will start to cool resulting in a thicker wall with possible frozen stresses. With multiple cavities the female mold permits the cavities to be spaced closer together. Costwise the lower cost is a male mold. 

Drape forming process is the simplest technique for use with a male mold. The heated clamped sheet is lowered over the mold until it seals with the mold base. This action allows the heated sheet to conform by gravity or pressure. The mold form acts as a crude sheet pre-stretch plug. 

It combines drape forming with vacuum action. The sheet is clamped into a movable frame, heated, and draped over high points of a male mold. Vacuum is pulled to complete the forming operation. In this technique a male or female mold is closed into the hot sheet. 

Vacuum (or pressure) is used both to billow pre-stretch and to form the sheet. The heated sheet is first clamped across a vacuum box or chamber, which is then partially evacuated, causing atmospheric pressure to billow and stretch the sheet. A male mold is then advanced into the billowed sheet, and forming is completed by drawing a vacuum (or pressure) on the mold while venting the vacuum chamber to the atmosphere. The process is a takeoff to drape forming but with the advantage of sheet pre-stretch, which produces a much more uniform distribution of wall thickness. 

Plug functioning as a male mold is forced into a heated plastic sheet held in place by a clamping ring. This is the simplest type of mechanical forming that involves more than a fold into two planes. 

Like the snap back process, billow forming is a sheet pre-stretch process used with a male mold. The sheet is clamped across a pressure chamber and is billow pre-stretched by applying a low magnitude positive air pressure to the chamber. The male mold is then moved into the stretched sheet until the clamp seals on the mold periphery. Contact between the mold and controlling the rate at which the pressure chamber is vented to compensate for the advancing mold can regulate the sheet. 

This version of the billow forming method is used with grained or polished stock on a male mold to preserve the surface finish. The... 

The air slip process uses a positive pressure billow sheet pre-stretch that is generated by the movement of a male mold towards the heated sheet that is clamped across a pressure chamber. The mold platen acts as a piston in the chamber and drives a volume of air ahead of it that serves to billow the sheet. At the full extent of its forward travel, the mold periphery seals against the sheet frame, and forming is completed by drawing a vacuum through ports in the mold. 

B Sheet formed by simultaneous motion of two mold halves, a - Male mold half, b - Preheated, clamped sheet, c - Female mold half d - Applied force. 

In this case, a hot sheet is stretched over a mold or matched molds without the use of air or pressure. For example, in matched mold forming, the heated sheet is clamped over a female mold or draped over the mold force (male mold) (Figure 11.13). The two molds are then closed. The resulting part has excellent dimensional accuracy and good reproduction of the mold detail, including any lettering and grained surfaces. 

In drape forming (Fig. 10.2), the main forming force is the influence of gravity on the hot plastic sheet. Typically, a positive, or male mold of the part, with a convex shape, is used. The forming is assisted by the application of vacuum to pull the material down around the shape. 

As the sheet is stretched into the shape of the part, it thins and cools. As soon as the sheet touches the mold surface, dramatic cooling takes place and the change in dimensions of that portion of the part is nearly stopped. Drawing of the sheet involves three-dimensional deformation, because as the planar dimensions increase, the thickness decreases, as shown in Fig. 10.5. Thus, the choice between use of male and female molds has implications for the characteristics of the final part. Imagine a bowl shape as the final package. If this shape is produced with a male mold, the first contact between the mold and the sheet will occur at the base of the bowl, so this will be the first to stop deforming, and consequently the thickest section. If that same part is produced with a female mold, the first contact is at the rim of the bowl, and the last at the base. Consequently, the thickest section of the bowl will be at the rim. 

Various mechanical techniques have been developed for thermoforming that use neither air pressure nor vacuum. Typical of these is matched mold forming (Figure 2.41). A male mold is mounted on... 

Present day developments include Lotus for production of car bodies, the female mold half is first coated with a gelcoat and dry reinforcement (mainly continuous filament mat and some woven roving) and PUR foam formers are placed on it. The foam inserts have previously been wrapped in continuous filament mat to create a stiff torsion box on each side. The inner male mold, with an airtight peripheral seal, is then placed over the loaded mold and bolted in place and resin is then drawn evenly into the mold by vacuum. 

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