Injection Molding

What it's all about.

Injection molding is the most widely used polymeric fabrication process. It evolved from metal die casting, however, unlike molten metals, polymer melts have a high viscosity and can not simply be poured into a mold. Instead a large force must be used to inject the polymer into the hollow mold cavity. More melt must also be packed into the mold during solidification to avoid shrinkage in the mold. Identical parts are produced through a cyclic process involving the melting of a pellet or powder resin followed by the injection of the polymer melt into the hollow mold cavity under high pressure.

Injection molding can be used to form a wide variety of products. Complexity is virtually unlimited, sizes may range from very small to very large, and excellent control of tolerances is also possible. Most polymers may be injection molded, including thermoplastics, fiber reinforces thermoplastics, thermosetting plastics, and elastomers. Structural injection molding is also possible in which a core and skin may be made of different polymers. Reaction injection molding and liquid injection molding, which differ in the manner of mixing ingredients, involve the injection of liquid polyurethane systems that polymerize within the mold.


Injection Molding equipment

Injection molding machines consist of two basic parts, an injection unit and a clamping unit.

Injection Unit

The injection unit melts the polymer resin and injects the polymer melt into the mold. The unit may be ram fed or screw fed.

About the Programs

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The ram fed injection molding machine uses a hydraulically operated plunger to push the plastic through a heated region. The high viscosity melt is then spread into a thin layer by a "torpedo" to allow for better contact with the heated surfaces. The melt converges at a nozzle and is injected into the mold.

The reciprocation screw injection molding machine is the most common injection unit used. The screw rotates and axially reciprocates. Rotation is produced by a hydraulic motor and acts to melt, mix, and pump the polymer. A hydraulic system controls the axial reciprocation of the screw, allowing it to act like a plunger, moving the melt forward for injection. A valve prevents back flow of the melt from the mold cavity.

When the polymer melt is not well homogenized by the screw, the project will not appear as solid colored if there is die present.

Clamping Unit

The clamping unit holds the mold together, opens and closes it automatically, and ejects the finished part. The mechanism may be of several designs, either mechanical, hydraulic or hydromechanical.

The Mold

The polymer flows from the nozzle to the mold which is coupled to the nozzle by a sprue bushing. In a mold with multiple cavities, the melt flows to each cavity by runners and is fed to the cavity through a gate. The gate is simply a restriction in the flow path just ahead of the mold cavity and serves to direct the flow of the melt into the cavity and to limit back flow. In a cold runner mold, the melt in the runners is allowed to cool and solidify. The runners are later separated from the part and may be reground and recycled into the feed. In hot runner molds, the runners are heated, therefore the molds are much more expensive to build but regrind costs are minimized. Melting of plastic resin. Injection of melt into the mold. Cooling of the mold. Removing the part.

Injection Molding Cycle

*Melting of plastic resin.
*Injection of melt into the mold.
*Cooling of the mold.
*Removing the part

The injection molding process occurs cyclically. Typical cycle times range from 10 to 100 seconds and are controlled by the cooling time of the thermoplastic or the currying time of the thermosetting plastic. The plastic resin in the form of pellets or powder is fed from the hopper and melted. In a reciprocating screw type injection molding machine, the screw rotates forward and fills the mold with melt, holds the melt under high pressure, and adds more melt to compensate for the contraction due to cooling and solidification of the polymer. This is called the hold time. Eventually the gate freezes, isolating the mold from the injection unit, the melt cools and solidifies. Next the screw begins to rotate and more melt is generated for the next shot. In the soak time the screw is stationary and the polymer melts by heat conduction from the barrel to the polymer. The solidified part is then ejected and the mold closes for the next shot.



Advantages/Disadvantages

Advantages

*High production rates
*Design flexibility
*Repeatability within tolerances
*Can process a wide range of materials
*Relatively low labor
*Little to no finishing of parts
*Minimum scrap losses

Disadvantages

*High initial equipment investment
*High startup and running costs possible
*Part must be designed for effective molding
*Accurate cost prediction for molding job is difficult

**References:

a): Fundamentals of Polymer Processing
McGraw-Hill, NY
b): Principles of Polymer Processing
John Wiley & Sons, NY
c): Plastics: Product Design and Process Engineering
Hanser Publishers, NY
d): Compiled by jobwerx
Jobwerx makes no claims to the accuracy of the information provided and accepts no responsiblity for the use of such information.
Always consult an expert for proper reference and usability.

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