THE WORLD OF PLASTICSEngr. Lina D. dela CruzChemical Engineering Departmnt

The synthetic or man-made polymers, play a major industrial role in our society today.

The polymer industry produces more than 10,000 different polymeric products for numerous diverse uses. Many of these products are referred to as “plastics”

Plastics have been used to replace natural materials in existing products.

Their low cost and superior performance as compared to wood, metals, and fabrics have allowed industry to cut manufacturing expenses, improve product performance, and reduce cost to the consumer.

Chemists are now learning to design products with specific chemical and physical properties for use in all new applications.

Since students are surrounded by plastic products, they should have general understanding of how these products are processed in industry to make the objects they are familiar

The objective of this module is to aid students in developing an understanding of the relationship between science and technology as well as view science as a human endeavor.

The simulation of the plastic manufacturing processes of extrusion, injection molding and blow molding will provide the framework to reach these objectives

Plastics Processing:

The plastics industry is composed of three divisions.

One is the manufacture of resins or the pellets made from starting materials such as crude oil.

For example polyethylene resins are made from "cracking" or breaking the bonds of petroleum molecules to make ethylene gas,C2H2.

Under high pressure and temperature, ethylene molecules are joined together in a chain or "polymerized" to form polyethylene.

High density polyethylene (HDPE) contains lots of straight chain molecules that are thousands of units long.

Low density polyethylene (LDPE) contains side branches of atoms to the parent chain.

Both HDPE and LDPE are long-chained molecules composed of carbon and hydrogen atoms.

A polymer of polyethylene where each carbon atom bonded to two hydrogen atoms are then bonded carbon to carbon to form the polymer.

However, creating a polymer is just the first step in making a useful product.

The second division in the polymer industry involves the manufacture of a compound with either plastic or elastomeric mixtures.

(Elastomers are rubber and rubber-like products.) Here the plastic resins or elastomers produced by the first division are mixed with other ingredients such as pigments, fillers, plasticizers, and other chemicals.

The creation of various forms of the mixed compounds allows ease of handling and these include powders, flakes, granules, liquids or in the case of elastomers, slabs or strips for processing by the third division. The processing plant then produces the plastic or elastomer objects for the consumer.

In the case of the plastic compound, it can be either thermoset or thermoplastic. If it is a thermoset type of material, crosslinking takes place while curing and it can not be reshaped.

However, thermoplastic materials can be shaped and reshaped (Recycled).

Three of the most common plastics processing methods involved in the shaping or molding of plastics are extrusion, injection molding, and blow molding.

They all involve melting the molding compounds by heat and pressure and forcing the melted or softened plastic into a mold or through an opening to shape the object.

In the case of the thermoplastic material, the object must be cooled to stop the flow of plastic and to allow the object to harden.

Extrusion:

Extrusion (similar to a meat grinder) is a process that involves forcing a melted plastic through a "die" or nozzle. Dry plastic powder or granules are subjected to heat and pressure as they go through an extruder.

As the plastic melts into a thick viscous material, it is forced through a die or hole in the shape of the finished product.

The extruded material then lands on a moving conveyer belt which carries the plastic into a stream of cool water or air.

This allows the manufacturer to continually produce a product with a desired shape. Extruded pieces can then be cut to specific lengths

Examples of extruded products are; auto trim, vinyl windows frames, house siding, pipes, tubing, fibers, films (bags, coverings and laminates), and coatings.

More examples can be identified if the viewer looks for long, continuous pieces with a consistent cross-sectional shape (not tapered).

These pieces are typically cut to specific lengths so they may also show a sharp cut edge.

Extrusion Process

Extruded Parts

Injection Molding:

Injection molding is the most important commercial method for transforming thermoplastics into useful products.

A thermoplastic polymer is one that can be reheated and reshaped many times.

Thermoset plastics set in a desired shape and cannot be reheated and remolded.

During the injection molding process, plastics are heated until they soften into a molten state. The melted plastic is then forced at high pressure through a nozzle into a cold mold.

The plastic cools in the mold and takes on the mold’s shape. Once the plastic is cooled, the mold opens and the product is released from the mold.

The resulting product often needs no further work before being put into use since details such as ribs and screw threads can be formed during this process

Examples of injection molded items are; cabinets for computers, computer screens, and computer keyboards

parts of plastic model or hobby kits, plastic bottle caps, brake and engine parts, reflectors and headlights,

circuit breakers, toaster oven panels, power tool and lighting housings, and even tableware.

Injection Molding Process

Injection Molded Toys

Blow Molding

Blow molding technology has been adapted from the glass industry for molding of hollow containers from thermoplastic materials.

The most widely known blow molded objects are bottles, jars, cans, and containers of all types for the food, beverage, cosmetic, medical, pharmaceutical, and home products industries

Among other blow molded objects are balls, toys, automobile fuel tanks, and car bumpers. These objects are hollow and thin walled.

Blow Molding

Examples of Blow Molded Toys