Semiautomatic molds

Compression molds can be divided into hand molds, semiautomatic molds, and automatic molds. The design of any of these molds must allow venting to provide for escape of steam, gas, or air produced during the operation. After the initial application of pressure the usual practice is to open the mold slightly to release the gases. This procedure is known as breathing. 

Hand molds are used primarily for experimental runs, for small production items, or for molding articles which, because of complexity of shape, require dismantling of mold sections to release them. Semiautomatic molds consist of units mounted firmly on the top and bottom platens of the press. The operation of the press closes and opens the mold and actuates the ejector system for removal of the molded article. However, an operator must load the molding material, actuate press controls for the molding sequence, and remove the ejected piece from the mold. This method is widely used. 

There are three types of compression molds hand molds, semiautomatic, and fully automatic. Hand molds are limited to prototypes, small size parts, and short runs. Semiautomatic molds are self-contained and securely installed in a conventional mold press. The... 

Automatic presses are self-contained and the cycles are computer controlled. Typically these presses are vertical and they are available from 22.7 to 273 tons. Other features such as the material loading mechanism and unloading trays are added to fully automate the operation of these presses. The tool cost is higher for automatic molds than manual or semiautomatic molds. 

A semiautomatic method for the quantitative representation of mold morphology was described (Metz, 1981) in which a variety of morphology indexes were shown to be useful, for example, the effective hyphal length, L the total hyphal length, L and the hyphal growth unit, L (cf. Equ. 5.109). 

The original process used a single ram, both to close the mold and to transfer the resin from the pot to the mold cavity this was a manual operation. The preferred process at present uses one ram to close the mold and another auxiliary ram to transfer the resin from the pot to the mold cavity this is semiautomatic. A third process, which is used occasionally, uses a screw preplasticator to warm the resin and feed it to the transfer pot this is completely automatic. 

A typical molding cycle will not start until a number of interlock conditions are satisfied. While the interlock conditions vary with the machine, the machine will not enter semiautomatic or automatic mode in the absence of these interlock conditions. Generally, the gate (operator guard door) must be closed and the gate safety (interlock) switches tripped. In hydraulic machines, the front gate has three switches (electrical, mechanical, and hydraulic) whereas the rear gate usually has one or more electrical limit switches. The electrical interlocks are usually electrical limit switches. When tripped, the hydrauHc interlock pre-... 

Compression molding presses are generally classified according to method of operation (i.e., hand, manual, semiautomatic, and fully automatic). They may be classified also according to their source of power (i.e., mechanical, hydraulic, and... 

The process is generally used for high-volume production because the cost of a modem semiautomatic press of modest capacity (50 to 70-ton clamping force) may be as much as 100,000 and a moderately sophisticated self contained multicavity mold may cost 50,000. Typical applications include melamine dinnerware toaster legs and pot handles and electrical outlets, wall plates, and switches. 

A number of different types of presses are commonplace for compression molding. They include manual, semiautomatic, automatic, automatic-rotary presses. 

The pressure forces the preheated material through runners into the closed two-plate mold cavity. This process is mostly conducted in semiautomatic closed presses. 

There are three popular designs of transfer molds loose-plate, integral, and auxiliary molds. The loose-type molds can be classified into manual and semiautomatic types based on the manner of mounting and mold operation. 

Figure 10.17 illustrates a semiautomatic floating-plate (loose-plate) mold. The floating plate is a permanent component of the press. It has an opening at its center that will accommodate different size pots and plungers that are needed to fabricate different parts. The floating plate moves either by hydraulic or pneumatic force or by bolts/latches that are built into the mold. An important advantage of these molds is their low cost. 

An integral mold comes equipped with its own pot and plunger (Fig. 10.18) and can be designed for manual or semiautomatic operation. The self-contain-ment feature increases the efficiency of the mold because the design of the material transfer pot can be optimized with respect to specific cavity. The pot could be located above or below the mold. The molten plastic may flow through a sprue-runner-gate to enter the mold or the sprue can be placed inside the cavity, as is the case in the mold shown in Fig. 10.18. 

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