What are the manufacturing processes for plastics?

D W Plastics Ltd is a thermo-plastics extrusion product manufacturer, but there are many different processes for manufacturing plastics.

The most common methods are:

  • Plastic extrusion
  • Injection moulding
  • Rotational moulding
  • Extrusion & injection blow moulding
  • Vacuum casting
  • Thermoforming & Vacuum forming)
  • Compression moulding

What are the basic differences between plastic manufacturing processes? Here is a very brief comparison:

Extrusion

During the extrusion process, plastic powder or pellets are fed into the extrusion machine via a hopper. The polymer is heated inside a barrel at a controlled temperature and a screw pushes molten plastic through a metal die, which is then cooled to give the plastic a fixed, continuous shape while being continuously pulled and formed into the final shape. The product can be cut or trimmed to the desired length. This is one of the most common ways to manufacture plastic products. The extrusion process works well for high-volume production of a wide range of products, including pipes, construction products such as ventilation, door and window frames and seals.

Injection moulding

A hopper feeds the plastic polymer into a heated barrel and screw. The screw melts the plastic and injects the liquid polymer into a temperature-controlled split mould tool that creates the shape of the product. Injection moulding is used for high volume manufacturing and many components can be manufactured in a short space of time, from tiny parts to large components such as vehicle bumpers and bins. Unlike the extrusion process molten plastic is forced into a die to form its final shape.

Extrusion and injection blow moulding

Very similar to the extrusion and injection moulding processes, but air pressure forces the hollow plastic to expand into the mould or extrusion shape, leaving the interior of the object hollow. This process is used for the mass production of inexpensive containers such as bottles, cups and beakers.

Rotational moulding

This is the second option for manufacturing hollow plastic products. During the process, the plastic polymer is placed into the mould before heating. The closed mould enters a furnace and rotates, which allows the plastic polymer to coat the entire inside of the mould evenly. The heat melts the plastic into a single layer that conforms to the shape of the mould cavity while leaving the interior of the final product hollow. A water spray cools the mould while still rotating which solidifies the polymer. Rotation is stopped, the mould opened and the plastic part removed. Suitable for short, economical production runs and is not suited for precision forming due to finish of the part. It is ideal for making large, complex shapes with a uniform wall thickness, e.g. large storage tanks for water, chemicals and fuel, crates, cooler boxes, bins, bollards, canoes, toys and playground equipment.

Vacuum casting

Method of making small functional plastic parts, especially high-quality prototypes, and suitable for low-volume production. Vacuum casting is a highly versatile technology for elastomers which uses a vacuum to pull the liquid raw material (e.g. Polyurathane resins, cast nylon, waxes) into the mould. This process is used when air entrapment is a problem, if there are elaborate details or recessed surfaces, or if the material is reinforced with fibre or wire (e.g. glass-filled Nylon). The raw material is poured into the two-piece silicone mould and the vacuum released. The mould is removed from the chamber and the casting is cured in the oven. and mould then removed to release the casting. Mould can be reused.

Thermoforming

The process involves heating previously extruded plastic sheet at a pliable temperature which is then stretched into a specific shape over a mould, then trimmed to create the required product. The machines can make thousands of components quickly. Examples of products are disposable cups, containers, lids, trays, blisters, various products for the food, medical, and general retail industries. A simplified version of thermoforming is vacuum forming.

Compression moulding

Generally used in thermoset polymers (irreversibly becomes rigid when heated). The raw material is pre-heated and placed into an open mould cavity. The mould is closed with a top and pressure is applied which forces the material into all areas of the mould. Heat and pressure are maintained until the material has hardened. Once cured, the formed product can be removed. This method of moulding is used regularly in the manufacture of automotive parts such as hoods, fenders, spoilers, as well as smaller more complex parts. It is also widely used to produce sandwich structures such as a honeycomb or polymer foam.

How to choose the right plastic manufacturing process?

You need to consider the following aspects when choosing a plastic manufacturing process for your product:

Product characteristics:

Does your product have complex internal features or tight tolerance requirements? Manufacturing options may be limited depending on the complexity of the design of the product or they may require changes to the design to make it suitable for certain methods of manufacturing and make them economical to produce.

Material:

Consider what your product will need to stand up to? Several factors determine the ideal material for a given application. It is important to consider functional and aesthetic requirements, as well as cost. Make sure that you have considered the ideal characteristics for your specific application and compare them with the available choices in a given manufacturing process.

Volume/cost:

What is the total volume of products that you require? High volume manufacturing often has high initial costs for tooling, but the products can be inexpensive on a per-part basis. In contrast, low volume processes may have additional set-up costs on top of tooling costs and the cost per part is higher due to shorter manufacturing cycles depending on which manufacturing process is chosen.

Lead time:

How quickly do you need parts produced? Some processes can create first parts within 24 hours, while tooling and set-up for certain high-volume production processes will take several months to manufacture.