Plastic manufacturing

Plastic is the name given to a vast collection of materials that can be molded into an ever-expanding list of shapes, colours and sizes.

Products made entirely from plastic, or containing plastic components, surround us. Examples span simple items such as paperclips all the way to complex feats of engineering such as space satellites.

Plastic gained this popularity by being relatively low cost, versatile, easy to work with, durable, moldable and water-resistant.

Plastics consist of a wide range of synthetic or semi-synthetic organic compounds, with the main element being long molecule chains called polymers. These polymers give plastic the strength and flexibility to be formed into solid objects.

Scientists and chemical engineers have spent the past 100 years developing a broad range of different plastics. These are generally grouped together by the chemical structure of their primary polymer.

Some of the most important groups include:

• Acrylics
• Polyesters
• Silicones
• Polyurethanes
• Halogenated plastics

The most frequently used plastics are:

• Acrylonitrile Butadiene Styrene (ABS) – applications include protective outer cases for electronics and electrical equipment, automotive trim components, LEGO bricks and pipes and pipe fittings. It is also widely used as a material for 3D printing (see What are the plastic materials used in 3D printing?)
• Polyamide (PA) – occurs both naturally, for example in wool and silk, or synthetically as nylons. Applications include textile fibers, toothbrush bristles, tubing, fishing line and low-strength machine parts.
• Polycarbonates (PC) – a tough material used to manufacture compact discs, eyeglasses, riot shields, security windows, traffic lights and lenses.
• Polyester (PES) – found extensively in the clothing and textile industries.
• Polyethylene (PE) – the most common plastic, mainly used in the packaging sector for bags, films and containers. Sometimes referred to as polythene for certain applications.
• High-density polyethylene (HDPE) – Often used in molded bottles, cases, plastic lumber, and piping.
• Low-density polyethylene (LDPE) – widely used in outdoor furniture, floor tiles, shower curtains and clamshell packaging.
• Polyethylene terephthalate (PET) – the most common thermoplastics polyester polymer, with applications include clothing fibers and containers for liquids and foods.
• Polypropylene (PP) – applications include automotive components, packaging, labelling, textiles, stationary, reusable containers and polymer banknotes.
• Polystyrene (PS) – used to make foam packaging peanuts, food containers and disposable tableware.
• High-impact polystyrene (HIPS) – commonly used to manufacture toys, refrigerator liners, food packaging and vending cups.
• Polyurethane (PUR and PU) – the most common plastic found the automotive industry. Other uses include cushioning foams, thermal insulation forms, surface coatings and printing rollers.
• Polyvinyl chloride (PVC) – common applications for rigid PVC (RPVC) include plumbing pipes, gutters, doors, windows and bank or membership cards. Flexible PVC (FPVC) is used for shower curtains, flooring, imitation leather, inflatable product and as a replacement for rubber.

Whether derived from petrochemicals or biological material, the process to make plastic involves five steps:

• Extraction 

Raw ingredients are removed from either petroleum (crude oil), natural gas and coal or renewable feedstock such as plant starch or sugar cane. The extracted material contains many different compounds which need to be split apart and separated from each other.

• Refining

Heating the material converts it into different monomers (molecules), making it easier to separate out the useable elements from impurities and contaminants. In the case of petroleum, these monomers include liquid fuels, lubricants, petrochemicals and naphtha, a key ingredient in the manufacture of plastic.

• Polymerization 

A combination of heat and pressure is then used to chemically bond chains of these monomers together to create polymers, the building blocks of all modern plastics.

• Compounding

Additives are blended with the polymers to create plastics with different strengths and applications. UV-stabilizers, for example, are added to protect the plastic in a similar way to how sunscreen protects skin. Other additives include glass fibre to add strength, or chemicals to enable the plastic to breakdown and biodegrade.

• Extrusion

The last step is to mechanically extrude, or squeeze out, this molten mixture, usually in long tubes or sheets. These are then broken up into sheets or pellets of different shapes, sizes and colors depending on their intended end-use, and shipped to manufacturers to be transformed into countless products.

The techniques used in plastic manufacturing vary, but almost all start with heating the plastic sheets or pellets. The most common processes include:

• Extrusion – this is a very similar process to how plastic itself is made. Plastic pellets are heated inside a temperature-controlled barrel and forced through a metal die by a mechanical screw. Cooling the plastic as it emerges fixes the shape and allows it to be cut to the desired length.   

• Injection molding – heated pellets are melted and injected via a screw into a temperature-controlled mold cavity. The mold is then clamped and cooled in order for the plastic to harden into its final shape. This process is ideal for high volume manufacturing as it can produce many items in a short space of time. It can be used to create small parts such as bottle caps or large items such as automobile bumpers and wheelie bins.

• Injection blow molding – this is similar to injection molding, but uses compressed air to force the molten plastic into the mold cavity. This technique enables the manufactured object to be hollow, rather than solid.

• Rotational molding – similar to injection blow molding in that it produces a hollow object. The difference is that the mold is spun once filled, using centrifugal pressure to force the plastic into the cavity rather than air. Objects made using this technique can lack the refinement of those made by injection blow molding.

• Thermoforming – this involves heating a plastic sheet until pliable and then stretching the softened material over a mold so it conforms to the shape. For more complex shapes, a vacuum inside the mold can be used to suck the material tightly to the form. This is known as vacuum forming. Both thermoforming and vacuum forming are capable of producing many objects very quickly.

Plastic is so prevalent because it provides several key advantages: 

1. Versatility – There are numerous types of plastics, each with its own unique set of properties.
2. Manufacturability – plastic is easy to work with and can be quickly molded into shapes of different sizes, colours and transparency.
3. Durability – Depending on how they're manufactured, plastics can be very tough, hardwearing and resistant to water and chemicals. Their strength can be further increased by mixing in additives such as glass or carbon fibers. 
4. High strength to weight ratio – substituting heavier metal or glass items with plastic can significantly reduce distribution costs, in the case of food and beverage containers, or increase the fuel economy of a vehicle.  
5. Low cost – Despite having a relatively complicated manufacturing process, plastic is easy to mass-produce, making it affordable and convenient. 
6. Recyclability – the list of plastics which can be made into new products continues to grow, although some of the processes involved aren’t yet widespread. 

Plastics do have a number of downsides that are worth noting:

1. Non-renewable sources – the vast majority of plastics manufactured today are derived from finite resources such as crude oil, natural gas or coal.  
2. Single use – much of the plastic currently in use is designed to be used once and discarded. If not properly disposed of, such as being collected for recycling, the material is likely to remain in the environment for hundreds of years.
3. Reprocessing costs – the cost of recycling some types of plastic is very high and the process can require complicated sorting and cleaning systems. Additionally, plastics can usually only be recycled a handful of times before the quality becomes too poor for use. Whereas, a material like glass or aluminium doesn’t degrade during the process, so can be recycled an infinite number of times.
4. Low melting point– many plastics have relatively low melting points which makes them unsuitable for use at high temperatures. 
5. Flammability – Plastics which contain carbon and hydrogen in their polymer molecules are often highly volatile. They burn quickly and release toxic gases which can be explosive and serve as an accelerant.