Injection molding

Injection molding is a processing technique which converts thermoplastic and thermosetting materials into a variety of products. Approximately 25% of the 13 billion pounds of thermoplastics sold in the United States in 1971 were injection molded (Rubin 1).

Injection molding equipment may be configured in a wide variety of options. Selection of the specific type of equipment is determined by the requirements of the molding job and by the degree of versatility desired. Each type of injection unit and its associated clamping unit has its own specific advantages and limitations. The current trend is toward reciprocating screw and two-stage screw pre-plasticizer equipment (Grandilli 34). This is largely because of increased capacity and versatility when compared to conventional plunger machines. The initial cost of screw-type equipment is however, usually 20 to 30% higher.

The major advantage of a toggle clamp unit over a hydraulic clamping unit is that it permits faster machine cycling and low energy consumption. Toggle clamping is however limited to 400 tons maximum clamping force and is therefore used in smaller molding applications. Fully hydraulic systems are generally believed to offer improved mold protection and have a higher accuracy with respect to clamping force and platen parallelism (D. Lamb). For the purposes of this discussion, treatment will be limited to hydraulic systems and specifically the system classification referred to as reciprocat

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by delivering high-pressure oil to the clamp cylinder to compress the oil column. The volume of oil required to reach full tonnage is a function of the compressibility of the oil and the volume of oil in the clamp cylinder. Since oil volume can be very high, especially at minimum shut height, clamping time may be long, especially for large machines. High oil requirements tend to make these machines less energy efficient than the toggle type which are able to use the large mechanical advantage built into the linkage to minimize oil requirements. The stiffness of the oil column in a hydraulic machine is much lower than the mechanical linkage in a toggle. Thus, hydraulic machines are more prone to creating flashing during the injection molding operation (Ibid.). Disadvantages of Hydraulic Systems in Injection Molding Disadvantages inherent in using hydraulic systems in Injection Molding are as follows (Rubin 463-464): 1. The hydraulic system must be totally confined and sealed. 2. Leakage must be prevented. This can prove difficult especially when working in the pressure ranges developed using modern machine technology. 3. The mechanical (size) requirements of the system are large. 4. Heat an

Category: Science - I