Exploring injection moulding

Materials World magazine
,
1 Nov 2017

Sara Cammarano*, Senior Industrial Issues Executive at the British Plastics Federation, gives an overview of injection moulding.

Injection moulding is one of the prime processes for producing plastic products because of its reliability, flexibility and versatility, which make it ideal for either highly engineered components or multi-use single piece packaging. 

The process introduces material to the injection moulding machine in the form of plastic granules from a hopper into a heated barrel. The plastic granules are slowly moved along the barrel by a large screw-type plunger, before the plastic is forced into a heated chamber, where it begins to melt. As the plunger advances, the melted plastic is forced through a small nozzle that leads directly into a cooled mould. The plastic solidifies almost as soon as the mould is filled. It then opens and the part is ejected.

Efficiencies in output and cost are achieved through the correct mould selection. The end product must be carefully designed to facilitate injection moulding. Ideally, parts should be designed for manufacture with injection moulding in mind from the earliest stage. For instance, giving consideration to simplifying the parts geometry, minimising assembly and optimising the mould tool design and moulding process from the outset will avoid costly modifications later on in the process and prevent delays in getting the product to market. Once the component has been defined, the mould is designed to specification, and then built around the important factors of speed, quality and demand. Injection moulding can produce complex and intricate shapes without the need for extra plastic material, while multi-cavity moulds yield more parts per cycle, reducing energy consumption per part. Injection moulding is precise and quick – each cavity will produce a consistent product every time, achieving high-quality standards and minimising scrap waste. Once the mould is made, you can change the material and colour of the component with ease.

During both the injection and cooling stages there are several factors that may affect the quality of the final product and the repeatability – for example, different polymers have different melt temperatures. If it is too low for the particular polymer then not all the material will melt, resulting in unmelted pieces, which affects performance and appearance. On the other hand, setting temperatures too high could result in the plastic degrading, leading to the same results. Residue in the barrel can also lead to degradation, so the weight of the part and cycle time become crucial factors.

The mould tool provides a passageway for molten plastic to travel from the injection cylinder (barrel) to the mould cavity and allows the air that would be trapped inside when the mould closes, to escape. 

If the air was unable to escape then the moulded component would contain voids (air bubbles) and have a poor surface finish. The tool controls the temperature of the mould as it sets to ensure it cools at the correct rate to avoid distortion and stress.

Many designers work on the inside of the part because that is where ribs or bosses and other structural elements are. These areas are important because they enable the part to fulfil its function. Here is a brief list of what to consider:

  • Stiffness – when a part requires a certain degree of stiffness, operating temperatures, length of time and frequency need to be considered.
  • Strength – designing for strength can be defined as the maximum load that can be applied under certain conditions without causing failure.  
  • Precision – plastic mouldings shrink on cooling, which can present problems. A thick section will either sink in so that the surface is not flat, as designed, and/or there will be voids within the material mass.
  • Appearance – although design contributes a great deal to the appearance of a part, material also has a bearing.  
  • Mouldability – the aim for the most cost-effective solution is to design parts in their finished form. The ideal would be without the requirement for secondary operations and with no waste of materials.
  • Assembly – if the part is not used in isolation then consideration must be made to assembly with other plastic or non-plastic parts. Methods of assembly include adhesive, welding and snap-fit, and can be manual or automated. In general, very little post-production work is required because the parts are finished upon ejection. However, secondary processes are available for further customisation, including decoration, labelling and foil stamping. 
  • Recyclability – environmental legislation is increasingly focusing on the recyclability of products. This can already be seen in the automotive and consumer goods industries and is increasingly prevalent.

Talking with injection moulders, particularly their technical team as early as possible, is hugely beneficial in terms of saving time and money. The injection moulding process has helped to push the boundaries of plastic designs and is still driving innovation, at times indirectly. 

3D printing, for example, provides a huge opportunity for the industry, allowing designers and inventors to quickly explore and understand the potential of a new product. It offers the potential of producing prototypes with complex shapes and surfaces that would be challenging to reproduce by conventional prototyping, therefore becoming the first step of a high volume production that will still be done via injection moulding.

Viper 20 speeds-up

ENGEL, Austria, recently presented a new, faster version of its Viper 20 linear robot for injection moulding, which offers part removal times of less than one second. The total cycle time (time to manufacture product) of the machine is around four seconds.

The Viper can produce food containers or medical sample cups, providing the manufacturer with greater flexibility. The machine can also be integrated into the CC300 control unit to allow for simple monitoring of the process via a display. 

ENGEL claims that the Viper offers greater energy efficiency, shorter cycle times and a quick setup.

Netstal showcases new ELION

Netstal, Switzerland, has showcased an all-electric, two-component ELION injection-moulding machine at the international for plastics processing trade fair Fakuma 2017, which can deliver a clamping force of 1,200kN. The machine is being used to manufacture three-layered coffee capsules. 

Using a sandwich injection moulding process, the ELION is able to efficiently manufacture plastic capsules and other packaging products with a ethylene vinyl alcohol copolymer and polyvinyl alcohol to help keep the packaged product fresh. The cycle time is around 4.9 seconds, and an automated Beck handling system places in-mould labels into the mould and removes the capsules after the process. 

Control the mould

Micro-Epsilon UK has developed a new measurement system, moldCONTROL – an inline thermal imaging system for the detection of defects and quality fluctuation in injection moulded parts.

The system inspects large, complex, multi-layered and transparent moulded parts made from plastics, polymers, elastomers, rubber and glass. It can be retrofitted to existing injection moulding machines.

Saving energy 

ExxonMobil has developed An Energy Saving Guide for Injection Moulding Companies to, it claims, help plastics processors save energy and optimise operations. The free handbook/webinar presents tactics for saving money, such as switching hydraulic oil to advanced oil to increase hydraulic efficiency.

The guide has been developed in collaboration with Dr Robin Kent, of Tangram Technology, and Applied Market Information. 

The guide can be found at bit.ly/2yYMwKJ

Proto Labs add to injection-moulding capabilities

Additional services have been added to the Proto Labs portfolio of injection-moulding services, including liquid silicone rubber (LSR) capabilities to support the manufacture of larger parts. The company will now allow customers to order LSR moulded parts of up to 3004 x 203 x 100mm, in batches of 100,000 or more.

Lee Ball, General Manager of Proto Labs UK, commented, ‘We are in a phase of growing all of our services, as demonstrated by our largest ever European investment (in injection moulding and CNC machines) earlier this year [...] Now, in response to customer feedback and demand, we’ve expanded our LSR service offering by increasing the size of the parts we can manufacture.’

*Sara Cammarano is a materials engineer with a PhD in polymer science and a specialisation in rheology. Sara has been working for the British Plastics Federation for just over three years and currently holds the position of Senior Industrial Issues Executive, responsible for H&S, moulders and specialist processors, rotational moulders and the composites group.