Spotlight: How to... reduce plastic packaging wall thickness using injection compression moulding

Materials World magazine
,
29 Oct 2018

Sumitomo (SHI) Demag’s UK packaging expert Kevin Heap talks through the commercial and ecological benefits of injection compression moulding and how the technique compares with traditional moulding of ultra-thin plastic packaging.

What is injection compression moulding?

The method of pressing material in a mould is quite a common industrial process, typically using rubber and resin and a combination of heat and pressure to mould products. It has generally been used to produce thicker sectioned parts on slower cycle times, for example, optical lens or thermoset applications. However, the technique hasn’t been applied to the mass-production of polypropylene packaging applications, until recently.

Why is it beneficial to packaging?

Global forecasts for rigid plastics, which is the category thin walled containers sits in, is predicted to rise at an annual rate of 3.7% from 52.9 million tonnes in 2017 to 63.4 million tonnes in 2022, according to research by Smithers Pira. The value of materials used to produce rigid plastic packaging is also forecast to grow across the same period at an annual rate of 3.9% to reach US$202bln. Polyethylene is projected to be the largest-growing segment during this forecast period. Lightweighting and consumer convenience are important factors in this expansion, with the combination of environmental pressures and high polymer prices placing increasing pressures on packaging manufacturers.

Most companies operating in the competitive arena produce millions of packaging containers every year. Volume, raw material waste and precision are fundamental to each company’s financial viability, with many facilities operating 20+ machines to meet supplier demand.

By applying injection compression techniques to stack moulds, packaging moulders can now reduce the wall thickness of containers and lids from 0.45-0.35mm. This saves around 25% in raw polypropylene materials compared with the standard injection moulding process, while maintaining comparable mechanical properties.

Given the dominant role that materials play in low-cost, mass-produced disposable food packaging containers and with packaging converters striving to enhance their green credentials, being able to reduce the wall thickness of packaging by up to 25% is a significant advantage.

How is injection compression moulding different to conventional injection moulding?

It’s all linked to the closure mechanism and the consequent clamping pressure applied within the machine, which helps to reduce shrinkage and ensure even distribution of the plasticised material in the cavity.

In conventional thin wall injection moulding, faster filling and higher pressures are required to drive molten plastic material into thinner cavities to prevent it freezing off between shots. It would be extremely challenging to repeatedly accomplish a 0.35mm wall thickness using this method.

During injection compression moulding, plasticised material is introduced into the cavity prior to complete locking of the mould. The machine closing movement aids distribution of plasticised material in the cavity. This injection compression reduces the injection pressure and the required clamping force, and this consequently removes the internal stresses from the product, resulting in less warping due to shrinkage.

Fill time for injection compression moulding when operating at maximum speed is 0.2 seconds, with a total cycle time of around 3.3 seconds, depending on the application.

Are there other environmental benefits?

Yes, switching to injection compression moulding can deliver savings on energy consumption as a result of the significantly lower clamp force pressure. Additionally, because less clamp force is required, thin wall packaging manufacturers can opt for a smaller machine with reduced tonnage and a smaller footprint.

Being a hybrid machine means that speed and precision are not compromised on the Sumitomo El-Exis SP range. This is all due to the servo-driven hydraulic clamp unit, engineered to execute precise movements. For high-volume thin wall packaging manufacturers, these enhancements aid company profitability with capacity and flexibility to meet future growth demands.

Due to speed of production, most systems integrate robotics. These usually perform a dual purpose, automating the placement of labels into the mould and taking packaging components out of the mould and stacking them.