injection moulding; an introduction

injection moulding; an introduction

injection moulding; an introduction

Injection molding is the most used process responsible for the mass production of thin-shelled plastic parts with three dimensional complex shapes. The injection molding operationis the most crucial plastic manufacturing technique because it, alone, is capable of producing large quantities of intricate parts at relatively low cost. For example, an injection molding machine can produce 48 plastic spoons, in any color, every 14 seconds. That's 205 a minute; 12,343 an hour; or almost 100, 000 in one eight hour shift. Each spoon would cost less than one cent to produce and package. The most common injection molded items include: pen bodies, bottle caps, computer keys, automotive components, telephone parts, eye glasses, plastic utensils, compact disks, video tape cases, automotive interiors and exterior parts. Recently, the most innovative injection molded parts included: ship propellers, bicycle frames, medical implants, automotive leaf springs, wristwatch parts the internal parts used in copiers and printers.

The injection molding process begins with the plastic material itself in the form of pellets, grind, or powders. The plastic is fed through a funnel shaped hopper into a high pressure and high temperature screw pump. The plastic must be changed from a solid to a melt and then injected into a mold. Depending on the material and its viscosity, melting temperatures can range from 275 to 500 o F. After the plastic is melted, it must be thoroughly mixed to insure a uniform temperature throughout the melt. When the plastic reached the right homogeneous viscosity and temperature, it is injected, under high pressure, into a steel mold. The pressures range from 10,000 to 30,000 pounds per square inch (PSI). To withstand this pressure, molds are made out of the highest quality steel and the cavity surfaces are hardened. The plastic materials are then held in the mold until it cools to a solid condition. Once cooled, the mold opens and the molded plastic part is ejected. The entire process of closing the mold, injecting the plastic, cooling, and ejecting the solid part is called the injection molding cycle.

The injection moylding cyclus consists of three phases:

1. Injection of the melted plastic which includes:

Heating and melting the plastic material.

Mixing the melt with additives.

Homogenizing the melt to a uniform temperature.

Injecting the melt into the mould cavity.

2. Holding the mold closed until the material cools.

3. Opening the mold and ejecting the part.

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