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Plastic Injection Molding

3 Major Componintes Injection, Mold, Clamping

Plastic Injection Molding is a manufacturing technique for making parts from thermoplastic and thermoset materials.

In contrast to the extrusion (which makes continuous parts of constant cross section), injection molding make discrete parts (with complex and variable cross section).

Molten plastic is injected at high pressure into a mold, which is the inverse of the desired shape.

The mold is made from metal, usually either steel or aluminium.

Widely used for manufacturing a variety of parts, from the smallest component to entire body panels of cars.

Purpose: to liquefy the plastic materials and then inject the liquid into mold.

• Resin is introduced through the hopper.

• Some machines can have several hoppers (to fed filler, colorants, other additives)-Injection molding act as mixer.

• However, due to limited size of barrel, mixing capability is poor.

Injection Unit

• From hopper – hole (feed throat).

• Barrel is a heavy steel cylinder made to withstand the pressure and temperature involved in melting the resin.

• 2 types of system used in injection molding;

• Reciprocating screw- similar to extruder screw but with unique reciprocating action.

• Ram injector.

Injection Unit

• Design of screw- similar to an extrusion screw.

• 3 sections: • Feed section- to advance the resin. • Compression section- to melt the resin. • Metering section- to homogenize the resin and pump it forward.

• The screw of injection molding machine is shorter than extruder, L/D ratios are 12:1 and 20:1.

• Low L/D ratios suggest the mixing is less efficient in the injection molding machine.

• The compression ratio (diameter of root at feed zone to the diameter of root at metering zone) often in the range of 2:1 and 5:1.

• Low compression ratio means less mechanical action is added during melting process.

Injection Unit

• Proper measurement of the size of an injection molding is weight of resin that can be injected, called shot size.

• Typical shot size range from 20g to 20 kg.

• Since shot size depends on the density of the plastic, PS has been chosen as the standard for rating the machine.

Injection Unit

• Resin is melt by mechanical shear and thermal energy from heaters.

• The molten resin is conveyed to a space at the end of the screw and collects in a pool.

• Here, the mold is closed.

Reciprocating Screw Injection Molding Machine

• The entire screw moves forward and pushes the molten resin out through the end of barrel.

• To ensure the resin does not flow backward, a check valve or nonreturn valve is attached to the end of screw.

• Normally the screw will stay in the forward position, until resin begins to harden in the mold.

Reciprocating Screw Injection Molding Machine

• Retraction of the screw, creates space at the end of the screw.

• Cooling the part in the mold, until it can be removed.

• While the part is cooling, the screw turns and melts additional resin.

Reciprocating Screw Injection Molding Machine

• Design of screw- similar to an extrusion screw.

• 3 sections: • Feed section- to advance the resin. • Compression section- to melt the resin. • Metering section- to homogenize the resin and pump it forward.

• The screw of injection molding machine is shorter than extruder, L/D ratios are 12:1 and 20:1.

• Low L/D ratios suggest the mixing is less efficient in the injection molding machine.

• The compression ratio (diameter of root at feed zone to the diameter of root at metering zone) often in the range of 2:1 and 5:1.

• Low compression ratio means less mechanical action is added during melting process.

Advantages:

• More uniform melting.

• Improved mixing or additives and dispersion throughout the resin.

• Lower injection pressure.

• Fewer stresses in the part.

• Faster total cycle.

Reciprocating Screw Injection Molding Machine

In this type of injection molding, the resin is fed from a hopper into the barrel, and heated through thermal energy from the heaters.

The molten resin is collected in a pool in a barrel celled injection chamber.

The molten resin is then pushed forward by the action of plunger (ram or piston).

To create better mixing, the molten resin is pushed past a torpedo/spreader, impart shear to the melt.

Ram Injection - Injection Molding Machine

Prior to the injection of the molten plastic into the mold, the two halves of the mold must first be securely closed by the clamping unit. When the mold is attached to the injection molding machine, each half is fixed to a large plate, called a platen. The front half of the mold, called the mold cavity, is mounted to a stationary platen and aligns with the nozzle of the injection unit. The rear half of the mold, called the mold core, is mounted to a movable platen, which slides along the tie bars. The hydraulically powered clamping motor actuates clamping bars that push the moveable platen towards the stationary platen and exert sufficient force to keep the mold securely closed while the material is injected and subsequently cools. After the required cooling time, the mold is then opened by the clamping motor. An ejection system, which is attached to the rear half of the mold, is actuated by the ejector bar and pushes the solidified part out of the open cavity.

Clamping Unit

Obviously, the mold must be cooled in order for the production to take place. Because of the heat capacity, inexpensiveness, and availability of water, water is used as the primary cooling agent. To cool the mold, water can be channeled through the mold to account for quick cooling times. Usually a colder mold is more efficient because this allows for faster cycle times. However, this is not always true because crystalline materials require the opposite: a warmer mold and lengthier cycle time.

Lubrication and Cooling

• A proper machine for good melting and injecting of the resin. • The proper resin to appropriate part performance.

• A good mold for part definition and removal.

• Proper operation for efficient molding cycle (mold cycle depends on the design of the mold and manufacturing parameters).

The key to success in injection molding is to have:

Polystyrene (low-cost, lacking the strength and longevity of other materials).

ABS or acrylonitrile butadiene styrene (a co-polymer or mixture of compounds used for everything from Lego parts to electronics housings).

Nylon (chemically resistant, heat-resistant, tough and flexible - used for combs).

Polypropylene (tough and flexible - used for containers).

PVC (more common in extrusions as used for pipes, window frames, or as the insulation on wiring where it is rendered flexible by the inclusion of a high proportion of plasticiser).

The Most Commonly Used Thermoplastic Materials

•Aerospace components •Automotive components •Avionics components •Cable assemblies •Computer electronics •Electronics components •Encapsulations •New product design &development •R&D labs Test specimens

•Engineering prototypes •Geophysics •Instrumentation •Marketing samples •Material quality testing •Medical & dental products •Medical laboratories •Model shops, toys, hobby

General Plastic Injection Molding Applications

Injection molding is one of the most important processes for plastics and ithas a very wide list of kinds of products it can produce, which makes it very versatile.

Conclusion

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