PLASTIC INJECTION MOLDING

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Plastic Injection Molding Overview

Injection Molding

Injection Molding machine

Injection molding processes

Injection molding materials

Injection molding problems and solutions

Typical applications and Products

Overview Injection molding is the most widely used polymeric fabrication

process. It is the primary process for manufacturing thermo plastic

products. 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.

The major reasons that injection is so popular is that it produces light weight parts of specific color with built in surface texturing.

Most polymers may be injection molded, including thermoplastics, fiber reinforces thermoplastics, thermosetting plastics, and elastomers.

Injection molding Injection molding is a method to obtain molded products by injecting

plastic materials molten by heat into a mold, and then cooling and solidifying them.The method is suitable for the mass production of products with complicated shapes, and takes a large part in the area of plastic processing.

Advantages of Injection Molding High production rates Wide range of materials can be used Low labor costs Minimal scrap losses

Types of Injection Molding Machines

Hand injection molding machine Piston type injection molding machine Reciprocating type injection molding

machine

Injection Molding Machine

Injection molding machine is divided into 3 units i.e. a clamping unit, an injection unit and mold.

The functions of the clamping unit are opening and closing a die, and the ejection of products. There are 2 types of clamping methods, namely the toggle type shown in the figure below and the straight-hydraulic type in which a mold is directly opened and closed with a hydraulic cylinder.The functions of the injection unit are to melt plastic by heat and then to inject molten plastic into a mold.

Machine components Injection system

The injection system consists of a hopper, a reciprocating screw and barrel assembly, and an injection nozzle, as shown in Figure 1. This system confines and transports the plastic as it progresses through the feeding, compressing, degassing, melting, injection, and packing stages.

FIGURE 1. A single screw injection molding machine for thermoplastics, showing the plasticizing screw, a barrel, band heaters to heat the barrel, a stationary platen, and a movable platen.

The hopper

The barrel

The reciprocating screw The reciprocating screw is used to compress, melt, and convey the material. The reciprocating screw consists of three zones (illustrated below) the feeding zone the compressing (or transition) zone the metering zone

FIGURE 2. A reciprocating screw, showing the feeding zone, compressing (or transition) zone, and metering zone.

The nozzle

FIGURE 3. (a) Nozzle with barrel in processing position. (b) Nozzle with barrel backed out for purging.

Mold systemThe mold system consists of tie bars, stationary and moving platens, as well as molding plates (bases) that house the cavity, sprue and runner systems, ejector pins, and cooling channels, as shown in Figure 4. The mold is essentially a heat exchanger in which the molten thermoplastic solidifies to the desired shape and dimensional details defined by the cavity.

Mold System

3 plate mold 2 plate mold

Hydraulic system Control system Clamping system The delivery system

The delivery system, which provides passage for the molten plastic from the machine nozzle to the part cavity, generally includes:

a sprue cold slug wells a main runner branch runners gates.

FIGURE 6. The molded system includes a delivery system and molded parts.

Injection molding processesThe Injection molding process basically is divided into 6 major steps1. Clamping2. Injection 3. Dwelling4. Cooling5. Mold Opening6. Ejection

Cycle time in injection molding The injection molding cycle can be divided

into several stages. They aremold closing, filling, packing, cooling, mold opening and ejection as shown here.

Cooling Time is a major fraction of the total cycle time.

Materials for injection molding Acrylonitrile butadiene styrene (ABS) Polyethylene Polypropelene Polystyrene

Typical Applications Injection molding is widely used in a wide variety of

industries: automotive, appliance, computer, communication and industrial equipment. Examples of injection molded parts in the automotive industry include sail panels, radiator end caps, door panels, lamp housings, and fuel rails.

     

Automobile parts Automotive Sail Panel Radiator End Cap  Fuel Rails with Injectors

Injection Molding Problems and Solutions Fault

Air trap

Black streaks

Flash

Short shot

Sink mark

Solution

Reduce injection speed,

Proper venting.

Lower the barrel temp.

Clean the injection unit.

Adjust the clamp force.

Reduce the injection pressure.

Increase the injection value.

Increase the cooling time.

Conclusion As the injection molding technology is now

living its most rapid development times. The most important factors behind this situation is There is no potential competitors Injection molding is especially suitable for mass

production. The profitable molding in today’s competitive

market depends on precise clamping action coupled with both high plasticizing performance and reliability integrated with complete plant operating performance.

References Injection Molding Hand Book

Dominick V Rosato Donald V Rosato

Theory of Molds R S Nagesh

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