INJECTION MOLDING Learning Objectives After attending to this

INJECTION MOLDING

Learning Objectives After attending to this presentation you will be able to § Describe the Injection molding machine. § Classify the plastic material used in Injection molding. § Explain with sketch components of a molding machine. § Describe with sketch reciprocating screw zone. § Describe with sketch injection molding operation sequence.

INTRODUCTION § Injection Molding is one of the most popular and mostly used processes in Plastics molding. § The largest variety of parts are made with Injection Molding. § Injection molding is generally used to produce thermoplastic polymers.

. . . INTRODUCTION § It consists of heating of thermo plastic materials until it melts, and then injecting into the steel mold, where it cools and solidifies to take its final shape. § It is a high-speed process that can make quite large to very small, solid parts with simple or highly intricate 3 D geometries.

. . . INTRODUCTION § Process can hold extremely high tolerances. § Tooling costs are high, so the process is best suited for high-volume applications. § It Can produce high-gloss parts with high depth of image and a wide variety of special effects in the final product.

Types of Material Use in Molding Commonly used thermoplastic materials are § Polyethylene § Polypropylene § Polystyrene § Polyvinyl chloride (vinyl or PVC) § Nylon § Acrylonitrile butadiene styrene (ABS) § Acrylic

Machine Components Visual Courtesy: http: //nptel. ac. in/courses/107103012/module 2/lec 4. pdf

Molding Process § The plastic materials are usually received in the granular form.

Molding Process § Plastic granules get placed in the hopper of the molding machine from which it is fed to a heated cylinder.

Molding Process § Granules are heated in the cylinder to melt or plasticize by the external heater provided in to the barrel and also by the friction with internal body of barrel and reciprocating screw. Heater Reciprocating Screw Barrel

Molding Process § The melting temperature varies according the plastic material. § The mold is usually made up of steel or aluminum and it is water cooled.

Molding Process § A reciprocating screw or plunger forces the molten plastics from the cylinder into the mold.

Molding Process § The mold get cool by cool water or cool air so that the molted plastic material get solidifies. § The mold is opened and the molded part as well as the attached runner is removed.

Reciprocating Screw Zones The reciprocating screw is used to compress, melt, and convey the material.

. . Reciprocating Screw Zones § The reciprocating screw consists of three zones 1) Feeding zone 2) Compressing zone / Transition zone 3) Metering zone Visual Courtesy: http: //old. bgk. uni-obuda. hu/ggyt/targyak/seged/bagim 1 ennb/imw. pdf

Reciprocating Screw Zones § Feeding zone – During this stage, the screw rotates and pumps the polymer material in front of the screw. This polymer cannot flow into the mold, so it starts to collect in front of the screw and feed to the next zone.

. . . Reciprocating Screw Zones § Compressing zone / Transition zone – The pressure that builds up, because of the material being forced in front of the screw and get compress, homogenize.

. . Reciprocating Screw Zones § Metering zone – At the preset temperature of plastic, it is get transfer to the mold. When the mold closes, the screw is pushed forward. This fills the mold to 98% full. This filling is done at high speed.

Molding operation sequence 1 - The mold closes and the screw begins moving forward for injection. 2 - The cavity fills as the reciprocating screw moves forward, as a plunger.

. . Molding operation sequence 3 - The cavity is packed as the screw continuously moves forward. 4 - The cavity cools as the gate freezes off and the screw begins to retract to plasticize material for the next shot.

. . . Molding operation sequence 5 - The mold opens for part ejection 6 - The mold closes and the next cycle

Application § Computer parts and its accessories § Bumper for Autos § Automobile parts § Electrical Connectors § Medical Instruments § Toys § Housings Appliance like TV, Refrigerator § Furniture

. . Advantages § Parts can be produced at high production rates. § Large volume production is possible. § Relatively low labour cost per unit is obtainable. § Process is highly susceptible to automation. § Parts require little or no finishing. § Many different surfaces, colours, and finishes are available.

. . . Advantages § Good decoration is possible. § For many shapes this process is the most economical way to fabricate. § Process permits the manufacture of very small parts which are almost impossible to fabricate in quantities by other methods. § Minimal scrap loss result as runners, gates, and rejects can be reground and reused.

Limitations § Intense industry competition often results in low profit margins. § mold costs are high. § Molding machinery and auxiliary equipment costs are high. § Lack of knowledge about the fundamentals of the process causes problems. § Lack of knowledge about the long term properties of the materials may result in long-term failures.

Summary § Injection molding is mostly use process. § Most of the plastic parts except large parts, hollow parts can be assembled from injection molding. § Injection molding can make equivalent parts at higher tolerances, assuming volumes justify higher tooling and extra assembly costs, and aesthetic standards will allow a welded area around the part.

Thank You Presented By Anup P Pardey Lecturer Government Polytechnic Nagpur