Injection Moulding of Plastics: An Overview

Injection moulding is the primary method for creating plastic parts, components and products. Plastics are by far the most common materials that are produced using injection moulding. The process is simple in principle; a plastic material (usually called a resin) is melted and forced into a mould via injection. The mould, which usually comprises two or more pieces depending on its complexity, is cooled and disassembled leaving behind the solid plastic part.

Injection moulding is big business, with a scale of more than a third of a trillion dollars every year in product value worldwide. Despite the volatility of the prices of raw materials and the environmental concerns about permanent disposal of plastic waste, the reliance of the manufacturing sector on plastic means this market share is not expected to reduce in the near future. In the United States, and arguably in most manufacturing countries, it is a top-five industry consuming about 15 billion pounds (approximately 6.8 billion kilograms) of polymeric materials every year [1].

In this article, you will learn about:

  • The injection moulding process
  • How injection moulding stands in comparison to other plastic moulding processes
  • How prototype production and mass production differ
  • Injection moulding machinery
  • Injection moulding techniques
  • Future trends of injection moulding

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THE INJECTION MOULDING PROCESS

Injection moulding is a repeatable process that involves three main steps:

  1. Plasticization
  2. Mould filling
  3. Cooling/solidification

The raw material comes usually in the form of grains or powder, which is melted in an injection unit (plasticization) and then forcefully filled at high pressure into a mould (mould filling). This mould is then cooled, which in turn cools the plastic part inside before it is finally ejected as a solid part (cooling/solidification). Each of these processes has parameters that must be carefully controlled and monitored to ensure quality, especially over repeated cycles.

One of the major advantages of injection moulding is that it is well suited and economical for mass production without a drop in quality over time. An automated process can be created if the mould is installed with a vertical parting line, which means once the mould opens, the plastic part falls off due to gravity, and the entire process can be carried out again quickly.

Despite the popularity of injection moulding, there are several other processes that may achieve similar results. A comparison of injection moulding with these processing methods are shown in the table below. Some plastic parts may have more than one process for their manufacture. The final choice of the process will depend on factors such as speed, accuracy, and cost.

Table 1. Comparison of various plastic moulding processes [2]

Process

Part complexity

Examples of parts

Forming action

Mould

Injection moulding

Complex (solid)

TV housing, car handles

Injection

Closed

Blow moulding

Complex (hollow)

Bottles

Inflation

Closed

Extrusion

Fairly simple

Window profiles, tubes

None

None

Film blowing

Very simple

Carrier bags, packing

Inflation

None

Compression moulding

Simple

Structural parts

Compression

Closed

Transfer moulding

Simple

Thermoset mouldings

Compression

Closed

Intrusion moulding

Simple

Low strength profiles

Compression

Open

Injection-compression moulding

Complex

Thin walled mouldings

Compression

Closed

PROTOTYPE PRODUCTION vs. MASS PRODUCTION

Producing plastic parts sometimes requires prototype production to first determine the feasibility of production, optimise the production process, and sometimes, optimise the product itself. The injection moulding process may be different for prototype production and mass production due to factors such as economics, accuracy and reproducibility.

Prototype production is important because, at this stage, there is a lot of final decisions yet to be made, such as mould design or additional machining that may be required post-moulding. Usually, the prototype has already been designed or sometimes created by other processes, such as 3D printing. Injection moulding for prototype development is usually small scale with a lot of emphasis placed on dimensional accuracy. The moulds are often made of aluminium or ceramics so that they can be made quickly to incorporate new adjustments.

Mass production is the next step after prototype production, where the mould and the plastic material to be used would all be finalized by now. A lot of information gathered through tweaks and adjustments during the prototype production are incorporated at this stage. But due to the intricacies of some designs, trade-offs may be required. The mould is now a permanent type, usually made of steel because of its higher durability and sturdiness against temperature changes, and is expected to churn out hundreds of thousand parts in its lifetime. At this point, there cannot be any errors in the mould’s design.

TYPES OF INJECTION MOULDING

It is difficult to classify injection moulding processes into broad categories because the general principle is the same across the board. However, there are specific requirements that call for certain aspects as different mould designs and heating systems, and this is where deviations occur in injection moulding. These types of injection moulding are briefly discussed below [3]; however, they do not make a comprehensive list as new methods are being developed to meet the demands of new requirements.

Thermoplastic Injection Moulding

This type uses thermoplastic polymer, a plastic that changes state reversibly from solid to liquid when heated and to solid when cooled. Thermoset plastics, unlike thermoplastic polymers, cool into a permanent solid and cannot be re-melted for moulding.

Over-moulding

This process of moulding covers another material, usually metal, with a rubber-like compound (called thermoplastic elastomer) to create a product with improved properties.

Insert Moulding

This process combines multiple components to create a single product. It can be said to be a moulding process designed to fasten multiple parts together with the added advantages of reduced weight and the elimination of fastening mechanisms.

Cold Runner Injection Moulding

This uses a cold runner to deliver the molten plastic into the mould. This means that when the part is cooled, excess plastic that is also now solid, has to be ejected along with the main part, which can lead to wastage. This wastage can be minimised by reincorporating the runner back into the plastic feed, but this also leads to time wastage and may cause variation in the physical properties of the finished product.

Hot Runner Injection Moulding

This method keeps the runner heated either internally or externally so that there can be a continuous flow of plastic into the mould. Cooling the mould for the part to be ejected does not affect the temperature of the runner. This saves a lot of time and material when compared to cold runner injection moulding.

Materials processed by injection moulding

Here is a list of some polymeric materials that are processed by injection moulding to form desired parts and components, with their relative application areas.

Material

Trade names

Application areas

Epoxy resin

Epoxy NU 505 L

High-precision parts for automotive engineering

Epoxy NU 5680 V

Encapsulation of electrical devices

Bakelite®

Bobbins, car electronics, spark plug connectors

Polycarbonate (PC)

PC L-1225Y

General purpose

SABIC® PC Resin PC2203R

General purpose

LEXAN™ Resin LUX2010T

LED lighting

Polyester

EPIMIX PBT GFR 15 NC Q1B501 (PBT)

General purpose

Polyethylene Terephthalate (PET) Granule (ES306312)

General purpose

PETG K 2012

Sheet, cosmetic, construction, food, drink, decorating film

Polyethylene (PE)

SABIC® LDPE HP7022

Caps and closures, houseware and appliances

SABIC® HDPE CC453

Bottles, multi-purpose

SABIC® LLDPE M300024

Thin wall packaging, houseware and appliances

For more materials, you can go to Matmatch search and find the right material for you.

FUTURE TRENDS

Already injection moulding has begun to branch into other complex areas such as micromoulding, which as the name implies is injection moulding at a very tiny scale, a scale where accuracy is of utmost importance. The materials used for injection moulding is another aspect that may soon change with the advent of metals, ceramics and hybrids of these with plastic. The machinery used for injection moulding may also witness drastic changes and improvement which will affect the speed and accuracy with which components are being produced. Many of the current applications of injection moulding were difficult to imagine decades ago. It stands to reason that future trends will only be limited by our imagination. If there is a requirement to be met, it’s just a matter of time before a suitable injection moulding technology is developed.

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Sources

[1] "Injection Molded Plastics Market Size, Global Industry Report, 2019-2025", Grand View Research [Online].

[2] Vanessa Goodship, Arburg Practical Guide to Injection Moulding, Rapra Technology Limited and ARBURG Limited, 2004.

[3] "Types of Plastic Blow and Injection Molding", Microdyne Plastics [Online].