Female mold

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. 

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. 

Undercuts and reentrant shapes are possible in many designs. They require movable or collapsible mold members, but with small undercuts they can often be sprung from a female mold while the formed product is still warm. This type of action works best when the plastic has some flexibility, as do the TPEs, or the material is very thin. Guidelines for the maximum amounts of undercutting that can be stripped from a mold are as follows 0.04 in. (0.1 cm) for acrylics, PCs and other rigid plastics 0.060 in. (0.15 cm) for PEs, ABSs, and PAs 0.100 in. (0.25 cm) for flexible plastics such as the PVCs. 

Some draft is required in side walls to facilitate the easy removal of the product from the mold. Female molds require less draft since products tend to pull away from mold walls as they shrink during cooling. With female or male tooling, for most plastics the draft on each side wall should be at least 1 degree (Fig. 3-39). 

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. 

The process basically forms the sheet after it has been heated to the point at which it is soft and flowable, and then applying differential pressure (atmospheric pressure, air pressure, vacuum, or their combinations) to make the sheet or film conform to the shape of the male or female mold producing many different products (Table 8-23). The more precise and controlled pressure applied, the more efficient in reproducing products at the lowest cost occurs (Fig. 8-50),... 

Blow molding uses compressed air to blow and expand a hot plastic tube ( rubbery melt ) in a female mold cavity, until the plastic conforms to the walls of the mold. This has developed into a major way to produce plastic bottles, and also more specialized hollow shapes such as fuel tanks, seat backs, tricycles, surfboards, and so on. The leading material is HDPE for milk, water, and household chemicals. Second largest is PET for carbonated beverages. A number of other thermoplastics are blow molded in smaller amounts for more specialized uses. 

Slush Molding. This is the reverse of dip molding. A female mold is used to give any desired surface finish on the product. The mold is preheated, plastisol is pour into it, and gels onto the surface of the mold. The excess liquid plastisol is poured out, and the internally- coated mold is heated to gel, then cooled to solidify the rubbery product, and the product is stripped out of the mold. Typical products are arm rests, head rests, road safety cones, anatomical models, dolls, toys, and auto parts. 

Rubber Plug Molding. Liquid silicone rubber is cast into a female mold cavity and cured. The plug is removed, and layers of mat or fabric are built onto the plug. This assembly is then inserted into the mold cavity, pressed, and heated until cured. 

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. 

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. 

B Formed sheet into female mold, a - Preheated, clamped sheet, b - Female mold with vacuum holes, c - Vacuum. 

Vacuum Forming - Vacuum forming is a type of thermoforming process consisting of preheating the plastic sheet prior to forming. The sheet is formed into the female mold by application of vacuum through holes in the mold. 

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 vacuum forming (Fig. 10.3), the main forming force is air pressure, caused by normal atmospheric pressure on one side of the sheet and a near vacuum on the other side. Typically a negative, or female mold, with a concave shape is used. The heated sheet is clamped onto the mold, and vacuum applied, so that the sheet is forced into 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. 

Matched male and female molds are employed, in a vertical press, giving good surfaces to both sides of the molding and a higher degree of dimensional and quality consistency than is usually obtained from hand lay-up operations. The molds are prepared and a release agent is applied. The reinforcement (usually tailored to size and shape) is then laid in the open mold and a charge of liquid catalyzed resin is poured over the lay-up. The mold is then closed, to allow cure to take place. 

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