My Science School

Plastics are not always moulded into shapes — we often need sheets of plastic. Perspex is a thermosetting plastic, ideal for making windows and roof lights. Perspex is as clear as glass and does not break easily. It is made by pouring liquid plastic between two sheets of sealed glass. When the glass sheets are clamped together and passed through a hot oven, the plastic sheet hardens to form perspex.

Table tops and other surfaces are sometimes protected with sheets called ‘laminates’. Melamine is a laminate. This thermosetting plastic forms a very strong, tough surface. Melamine is resistant to heat and does not stain easily.

Laminate sheets are made by bonding together two or more layers of materials. Laminate sheets from plastics are glued to wood to make the surface aesthetic. These sheets are manufactured by laminating different types of papers with formaldehyde. The core sheet consists of Kraff paper with phenol formaldehyde (PF) and below it, a barrier paper is provided. Above the Kraff paper, a tissue paper is impregnated with FF melamine formaldehyde (MF) resin is provided which gives protection and also enhances abrasion resistance. Then, these laminates are bonded to wooden surfaces with suitable glue and pressing for some time finishes the process.

Moulding is the process of manufacturing by shaping liquid or pliable material using a rigid frame called a mold or matrix.

When moulding plastics, a powder or liquid polymer such as polyethylene or polypropylene is placed into a hollow mold so the polymer can take its shape. Depending on the type of process used, various ranges of heat and pressure are used to create an end product.

Many different objects are made from plastic — thin sheets for wrapping food, long strips for curtain rails, hollow bottles and complicated toys. With such a huge range of items made of plastic it is not surprising that there are many different ways of moulding plastic.

The Types of Plastic Moulding

       1. Rotational Moulding

Rotational Moulding, also called rotomoulding, is a manufacturing process for producing large hollow parts and products by placing a powder or liquid resin into a metal mould and rotating it in an oven until the resin coats the inside of the mould. The constant rotation of the mould creates centrifugal force forming even-walled products. Once the mould cools, the hardened plastic is removed from the mould.

Very little material is wasted during the process, and excess material is often re-used, making it economical and environmentally friendly.

Common Uses for Rotational Moulding

Rotational moulding is commonly used to make large hollow plastic products like bulk containers, storage tanks, car parts, marine buoys, pet houses, recycling bins, road cones, kayak hulls, and playground slides.

        2. Injection Moulding

Injection moulding is the process of making custom plastic parts by injecting molten plastic material at high pressure into a metal mould. Just like other forms of plastic moulding, after the molten plastic is injected into the mold, the mould is cooled and opened to reveal a solid plastic part.

Commonuses for Injection Moulding

Injection moulding is commonly used for making very high volume custom plastic parts. Large injection moulding machines can mold car parts. Smaller machines can produce very precise plastic parts for surgical applications.

         3. Blow Moulding

Blow moulding is a method of making hollow, thin-walled, custom plastic parts. It is primarily used for making products with a uniform wall thickness and where the shape is important. The process is based upon the same principle as glass blowing.

Blow moulding machines heat up plastic and inject air blowing up the hot plastic like a balloon. The plastic is blown into a mould and as it expands, it presses against the walls of the mould taking its shape. After the plastic “balloon” fills the mould, it is cooled and hardened, and the part is ejected. The whole process takes less than two minutes so an average 12 hour day can produce around 1440 pieces.

Common Uses for Blow Moulding

Blow moulding processes generate, in most cases, bottles, plastic drums, and fuel tanks.

        4. Compression Moulding

Compression moulding is done exactly like the name suggests. A heated plastic material is placed into a heated mold and then pressed into a specific shape. Usually, the plastic comes in sheets, but can also be in bulk. Once the plastic is compressed into the right shape, the heating process ensures that the plastic retains maximum strength. The final steps in this process involve cooling, trimming, and then removing the plastic part from the mould.

Common Uses of Compression Moulding

The best use of compression moulding is the replacement of metal parts with plastic parts. It is mostly used for small parts and products in very high volume. The automotive industry uses compression moulding heavily because the final products are very strong and durable.

Picture Credit : Google

There are many different types of plastic; they can all be divided into two main groups. These are called ‘thermoplastics’ and ‘thermosetting plastics’. Thermoplastics include polypropylene, polystyrene, polyester, acrylics and nylons. These types of plastic melt when heated and become solid again as they cool. Thermosetting plastics do not behave in this way. Bakelite and melamine are both thermosetting plastics.

• When plastics are made they are either in the form of thick liquid or solid granules. This material is called ‘raw plastic’.

• The granules are tipped into a container and heated until they melt. They may contain a colouring dye.

• The liquid plastic is then poured into a mould. The shape inside the mould is exactly the same shape as the finished article.

• Until this point the thermoplastics and the thermosetting plastics are treated in the same way.

Thermoplastics are often used for clothing and packaging material, both of which can be recycled. Thermosetting plastics are resistant to high temperatures. For this reason they are often used to make saucepan handles and ashtrays.

The strong polymer links of the thermosetting plastic mean that it holds its shape even when heated. Plastics of this kind cannot be melted down and used again easily.

THERMOSETTING PLASTICS

•  When the thermosetting plastic is heated, links form between the polymers in the plastic. They become joined together into a permanent structure.

THERMOPLASTICS

• The thermoplastic, however, has different types of links and can be melted down and used again.

Two main processes are used to produce plastics - polymerization and polycondensation - and they both require specific catalysts. In a polymerization reactor, monomers such as ethylene and propylene are linked together to form long polymer chains. Each polymer has its own properties, structure and size depending on the various types of basic monomers used.

In a polymerization reactor, pressure and heat cause ethylene molecules, mixed with other chemicals, to link together. In ethylene the molecules form a colourless inflammable gas. When the same molecules are linked together in big chains called polymers, the plastic polyethylene or polythene is made — one of the world's most widely used plastics. Other chemical combinations can produce polymers of different lengths and structures.

Once the polymers have been formed, the hot liquid plastic is taken from the reactor, cooled, and cut into chips or pellets.

• Each molecule of ethylene is made up of two atoms of carbon and four of hydrogen.


• If another chemical, known as a ‘catalyst’, is added these molecules form a long chain. This is called ‘polymerization’.


• Polythene gets its name from ‘poly’ meaning ‘many’.

An oil refinery is an industrial plant that refines crude oil into petroleum products such as diesel, gasoline and heating oils. Oil refineries essentially serve as the second stage in the production process following the actual extraction of crude oil by rigs. The first step in the refining process is distillation, where crude oil is heated at extreme temperatures to separate the different hydrocarbons.

Oil refineries serve an important role in the production of transportation and other fuels. The crude oil components, once separated, can be sold to different industries for a broad range of purposes. Lubricants can be sold to industrial plants immediately after distillation, but other products require more refining before reaching the final user. Major refineries have the capacity to process hundreds of thousand barrels of crude oil daily.

In the industry, the refining process is commonly called the "downstream" sector, while raw crude oil production is known as the "upstream" sector. The term downstream is associated with the concept that oil is sent down the product value chain to an oil refinery to be processed into fuel. The downstream stage also includes the actual sale of petroleum products to other businesses, governments or private individuals.

• Crude oil is piped in and heated. As the different chemicals (or ‘fractions’) in the oil get hotter they start to boil and turn into gases (or ‘evaporate’).


• Each fraction boils at a different temperature. The ones with the lowest boiling points stay as gases for longest. They rise to the top of the column as they cool. The liquids with higher boiling points do not stay as gases for long, so they do not rise far.


• As each gas cools it turns back into a liquid (or ‘condenses’). The condensing liquids are collected at different levels in the column.


• Oil is full of useful chemicals. Did you know we get fuels, soaps, tar and some of the ingredients for drugs from the chemicals in oil?


• Certain fractions are mixed to give plastic making chemicals.

Most plastics come from chemicals in crude oil. However as the world’s a reserve of crude oil begin to run out; coal and gas are now being used more frequently. At a refinery, crude oil is separated into different fractions or chemicals. Most of these fractions are used for fuels.

In a further process at the refinery some of the remaining fractions are cracked or separated into various parts, including the gas ethylene, one of the main chemicals from which plastics can be made.

Plastic is a word that originally meant “pliable and easily shaped.” It only recently became a name for a category of materials called polymers. The word polymer means “of many parts,” and polymers are made of long chains of molecules. Polymers abound in nature. Cellulose, the material that makes up the cell walls of plants, is a very common natural polymer.

Over the last century and a half humans have learned how to make synthetic polymers, sometimes using natural substances like cellulose, but more often using the plentiful carbon atoms provided by petroleum and other fossil fuels. Synthetic polymers are made up of long chains of atoms, arranged in repeating units, often much longer than those found in nature. It is the length of these chains, and the patterns in which they are arrayed, that make polymers strong, lightweight, and flexible. In other words, it’s what makes them so plastic.

These properties make synthetic polymers exceptionally useful, and since we learned how to create and manipulate them, polymers have become an essential part of our lives. Plastics have saturated our world and changed the way that we live.

Plastics are everywhere! You are sitting on a plastic chair, leaning on a plastic-coated table or wearing plastic shoes. There are many different types of plastics. So what makes something a plastic? The first plastics were made more than 100 years ago from cellulose which is naturally found in plants.

Today plastics are made mostly from crude oil, a raw material that is fast running out. In the future, new raw materials must be found to make plastics, and there must be greater recycling of plastic waste.

Plastics is the term commonly used to describe a wide range of synthetic or semi synthetic materials that are used in a huge and growing range of applications. Everywhere you look, you will find plastics. We use plastic products to help make our lives cleaner, easier, safer and more enjoyable. We find plastics in the clothes we wear, the houses we live in, and the cars we travel in. The toys we play with, the televisions we watch, the computers we use and the DVDs we watch all contain plastics.

Plastics are organic materials, just like wood, paper or wool. The raw materials used to produce plastics are natural products such as cellulose, coal, natural gas, and salt. Plastics have become the modern material of choice because they make it possible to balance today’s needs with environmental concerns.

• The word plastic comes from the Greek Plastikos - meaning able to be shaped.


• They can be shaped into almost anything.


• Plastics are light and relatively cheap.


• They can be produced in different colours.


• Heat and electricity do not travel through plastics easily; they are good ‘insulators’.


• Unlike metals and wood, they do not rust or rot.

But plastics do have some disadvantages too.

• They are made from resources which will eventually run out, and they are difficult to recycle.


• Because they do not naturally rot (biodegrade) like wood, they are an eyesore and a hazard in the environment.


• They are not as strong as many metals and they melt at high temperatures, sometimes giving off poisonous fumes.