INJECTION MOLDING

Introduction

Injection molding is an extensive global manufacturing process for making simple to intricate plastic, ceramic, and metal parts. Injection molding converts wax, thermoplastics, thermosets as well as powdered metals, and magnesium into thousands of products (1,2). Commercial processes and equipment have been developed  for a wide variety of materials, though the bulk of what is injection molded are thermoplastics. 

Applications appear limitless, from compact discs to jet plane canopies, to medical implants. Nearly all consumer products have some injection-molded components. In the year 2000 injection molding consumed approximately 32 wt% of all plastics sold worldwide, roughly 4.95 × 107 metric tons (3). This is second

to extrusion which consumed 36 wt% of all plastics sold worldwide. The United States consumes about 33% of the plastic sold worldwide. In the United States, the plastics industry ranks as the fourth largest after motor vehicles, petroleum refining, and automotive parts. Plastic processing and products continue to grow at a faster pace than the national average. This is in spite of the trend of the United States going toward an information and service industry and away from manufacturing. In the United States there are about 6000 molding shops, down from a high of about 8000 in the early 1990s. Consolidation continues as monitored

by the trade journals, for which web sites are listed (4–8). There are an estimated 80,000 injection presses in operation within the United States.

The injection-molding process provides low cost fabrication of large and small parts to precision tolerances. Many of these parts are nearly impossible to make in production volumes by other techniques. Injection molding can form parts as small as a cubic millimeter (micromolding), and parts as large as garbage dumpsters

and 1.2 × 2.4 m (4 × 8 ft) filter panels are in production. A wide range of injection-molding processing capability characterizes the industry. Quality and tolerances can be high if scientific principles are followed. Tolerances near 0.025 mm (0.001 in.) are difficult to achieve but are possible if concurrent engineering

is applied to certain parts. One of the strengths of injection molding over other plastic forming processes is that parts can have fine three-dimensional (3-D) details in their shape and surface finish. Typical nominal-wall thickness ranges from 0.5 to 6.4mm(0.020–0.250 in.); however, thickness of 0.08–50mm(0.003–2.0 in.) are also possible. Injection molding is the plastic processing method of choice for large quantities of identical parts. Quantities in the millions are possible with one or more molds that can form several parts at one time. The efficiency of this process has changed the quality of life for most Americans, especially low income households.

Properties of injection-molded parts can be unique. Given a metal or plastic’s typical properties, injection molding imparts some of its own. Because of flow patterns, molecular orientation, and other factors, molded part properties are often different than those of the base material. An example of the benefits of orientation is the “living hinge”, the flexible plastic hinge found on spice, shampoo, and dental floss containers. Properties of injection-molded parts can be flexible or stiff, soft or hard, tough or brittle, clear or opaque, specific chemically resistant and flame retardant. Parts can be tinted, colored, plated with various metals, and recycled.

Encyclopedia of Polymer Science and Technology.

Copyright John Wiley & Sons, Inc.

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