A guide to plastic moulding

injection moulding material


While many people automatically think of injection moulding when they see the term “plastic moulding”, there are several ways in which you can mould plastic to the desired shape. We have put together a guide to plastic moulding, which takes in all of the main processes, their pros, cons and the particular products they are used to create.

Before we look at the different plastic moulding processes in detail, it is essential to note the difference between the material used, thermoplastics and thermoset plastics. Thermoplastics can be melted down and reused, while thermoset plastics cannot. A thermoset plastic is created using an irreversible curing process that turns a soft solid/liquid into a hardened material.


Injection moulding

The process of injection moulding is relatively simple. It involves the melting of plastic pellets, which are then injected into a premade mould at high pressure. This ensures that every cavity of the mould is filled, and the material will solidify to create the final product. While the moulds can be made of different materials, those which transfer heat more quickly are preferable. For example, many companies use aluminium moulds instead of steel because aluminium transfers heat much quicker and therefore speeds up the cooling process.

While the initial setup process, involving the design and creation of the mould, can be pretty costly, there are long-term benefits. The main advantages of injection moulding include:-

  • Swift and efficient production process
  • Relatively low labour costs after initial setup
  • Extreme detail and complex designs are possible
  • High output production
  • Wide range of plastic materials used
  • Meagre scrappage rates
  • Product consistency
  • Enhanced natural strength

As you’d expect, there are some disadvantages when using injection moulding, but they are few and far between. However, disadvantages such as the following need to be taken into consideration:-

  • Relatively high tooling/setup costs
  • Design undercuts and sharp edges difficult to incorporate
  • Varied wall thickness will result in defects such as sink marks
  • More relevant for long high quantity runs

It is safe to say that the advantages of injection moulding far outweigh the potential disadvantages. As a consequence, the following products are produced using injection moulding:-

  • Electrical switches
  • Crates
  • TV cabinets
  • Automotive dashboards
  • Wheelie bins
  • Syringes
  • Bottle lids
  • Telephone handsets
  • Automotive bumpers

Even a relatively quick glimpse at the above list will give you an idea of the variation in products created using injection moulding.

Casting moulding

While casting moulds is one of the simplest ways to create identical plastic components, there are several potential issues. Other processes such as injection moulding see plastic materials injected into a mould at speed, while casting relies more on atmospheric pressure to fill the mould. Consequently, the use of casting moulds requires liquid plastic polymers, which can dramatically cut the number of options available. Many plastic polymers will soften under intense heat, but they may still retain a bread dough feel.

While plastic polymers are certainly an option when using a casting mould, you’ll also find that many processes take in powder or solvent solutions. The liquid is converted into a solid using any of the following methods:-

  • Chemical reaction
  • Cooling
  • Evaporation
  • External heat

Once the material has solidified, it is simply a case of opening the mould and taking the component out. There are many advantages to the casting process, which include:-

  • Cost of equipment, tooling and moulds is relatively low
  • The process is straightforward
  • Little or no internal stress for the products created

There is a delicate balancing act between the advantages and disadvantages of such a simple process. The main disadvantages are:-

  • Relatively long cycle period
  • Relatively slow output rate
  • Dimensional tolerances are relatively low
  • Managing moisture/air bubbles can be challenging

The plastic casting process is used to create a whole range of different products such as:-

  • Designer toys
  • Scale models
  • Small run plastic figures
  • Ball-jointed dolls

As we touched on above, the material used needs to be liquid when heated to be poured into the mould before setting. By using this process, it is possible to create highly detailed figures.

Blow moulding

The blow moulding process uses the same principles as glassblowing to create hollow objects made of plastic. As with any other moulding process, it involves heat, liquid material (in this instance, plastic) and a mould to create a hollow shape. The process is straightforward; heated plastic is placed on the top of the mould; then air is blown onto the plastic, which then forces the material down into the mould, stretched across the interior walls of the mould cavity. This is the process used to create plastic bottles and other similar products.

There are numerous advantages and disadvantages when using the blow moulding process. The main advantages take in:-

  • Quick manufacturing
  • High-volume output
  • Wide variation of products
  • Relatively low cost
  • Benefits of automation

While there are very few drawbacks, it is worth noting the following issues which have arisen over the years:-

  • Heavy reliance on petroleum for oil-based materials
  • Significant impact on the environment
  • There are limitations such as wall thickness
  • Can involve excess wastage

As we touched on above, any hollow product can be created using the blow moulding process, which takes in an array of different bottles, including:-

  • Water bottles
  • Shampoo bottles
  • Milk bottles
  • Plastic drums
  • Plastic containers
  • Storage tanks
  • Soap bottles

Even though blow moulding has been around for some time, it is still an essential element when processing hollow products such as bottles. This is one process that has aged well!

Compression moulding

Compression moulding is a relatively labour-intensive method of creating components using molten plastic. The practicalities are very simple. There are two moulds; one at the bottom into which the molten plastic is placed with the second mould compressed from above. When the two parts of the mould come together, they put intense pressure on the molten plastic. This squeezes the material into the desired shape with overflow grooves on either side to catch excess molten plastic.

Depending on the thickness of the required component, the pressure placed on the molten plastic can be anywhere between 800 psi and 2000 psi. Once the pressure is released, the part is removed together with the excess flash round the edges.

There are numerous advantages when using a compression moulding technique, including:-

  • Relatively simple process
  • Relatively low tooling costs
  • Perfect for creating more oversized items with thick walls
  • Cost-effective for relatively short runs
  • Can create robust components

As we touched on above, there are some potential disadvantages when using the compression moulding process such as:-

  • Additional work may be required post mould release
  • Excess waste
  • Relatively slow processing times
  • Unsuitable for complex designs

There are many different components made using the compression moulding process, such as:-

  • Toilet seats
  • Melamine kitchenware
  • Trays
  • Cheeseboards
  • Rail seatbacks
  • Wellington boots

This gives you an idea of the type of products that benefit from this particular moulding process. They all have one thing in common; they require a high degree of strength, perfect for compression moulding.

Rotational moulding

In principle, the idea of rotational moulding is elementary, but in practice, it can be relatively complex, opening up an array of different variables. This process involves plastic material being placed inside a hollow mould. The mould usually consists of two parts, which are secured firmly together. The mould is then rotated on two axes while being heated, which slowly pushes the melting plastic material to the inside wall of the mould. Gradually a layer of plastic will build on the mould walls creating hollow products/components.

Once the plastic has melted, it is moved to a cooling station to solidify, using air and water mist. During the cooling process, the mould is still rotated on the two-axis. This ensures the molten plastic remains on the inside wall of the mould. Once the cooling process has finished, the mould can be opened and the hollow component removed.

There are many pros and cons to consider when looking at the rotational moulding process. The main advantages are as follows:-

  • Relatively low-cost tooling
  • Wall thickness is consistent
  • Option of double-wall construction
  • One part products result in high durability
  • High stability
  • Extremely strong

As with any moulding process, there are pros and cons, and the disadvantages of rotational moulding include:-

  • Relatively long cycle times (often counted in hours per product)
  • Limited options for material
  • Regular replacement of tooling
  • Relatively high labour costs

When you consider the number of hollow components we come across every day, it is no surprise to learn that rotational moulding is extremely popular. Some of the more common products made using this process include:-

  • Bulk plastic containers
  • Shipping containers
  • Drums
  • Barrels
  • Septic tanks
  • Water tanks
  • Plastic balls
  • Children’s toys
  • Rain barrels
  • Outdoor furniture
  • Medical stretchers
  • Plastic lockers

We could go on and on, but if there is a product that has a hollow interior, this may be an ideal opportunity to use the rotational moulding process.

Different types of plastic moulding

As you can see above, there are different types of plastic moulding processes used today. Each method has particular pros and cons and is more appropriate for specific products. For example, rotational moulding is perfect for hollow products, while injection moulding creates an enhanced natural strength due to the pressurised release of the liquefied plastic. You will also notice that some moulding processes are more appropriate for short runs while others are perfect for longer runs. It is a case of balancing the characteristics required for individual components against the pros and cons of each moulding process and the cost.