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How Much Waste is Involved in Injection Moulding?

Hand turning knob to select the lowest waste level - concept image

The impact of injection moulding on the manufacturing process cannot be overstated. Today, the process is integral to supporting the design of a wealth of components and small products. From automotive parts to packaging, wire spools and most plastic products, it would be difficult to envision how a broad range of products would look without injection moulding.

Beyond the versatility of injection moulding, there’s a wealth of benefits to using the process to manufacture parts and components. For starters, injection moulds can withstand extremely high pressure. Not only that, the moulding process allows for complex and intricate shapes of varying materials to be designed which otherwise would be too complicated and expensive to manufacture.

Additionally, given that plastic injection moulding is an automated process, it helps to reduce manufacturing costs. There’s no need to employ an expansive labour force, for instance. These commercial savings can then be passed on to the customer.

The benefits aside for a moment, the injection moulding process is not immune from a handful of challenges. One of the most often cited is waste. So, how much waste does injection moulding create and how can waste be reduced?

Injection Moulding and Waste

As we all strive towards a sustainable future, one that places emphasis on preserving the environment for future generations to enjoy, businesses are looking to optimise their manufacturing processes with a zero-waste goal. Whereas it’s true that when compared to CNC, injection moulding is a far less wasteful process, improvements can always be made.

Injection moulding waste from manufacturing typically occurs in four core areas: the sprue, the runners, the gate locations and any overflow material that leaks from the cavity, a condition that’s called ‘flash.’ However, the waste from injection moulding is negligible and is only bettered by 3D printing.

3D Printing

One of the most talked about technologies of recent years, 3D printing has myriad of applicational potential and has already transformed the manufacturing process. In fusing layers upon layers of materials atop one another each of which is approximately 0.1mm thick and consisting of liquid, powder and sheet materials, you can print most anything that you can think of.

Anything from firearms to figurines, medical models, even guitars can be made using 3D printers. However, despite being less wasteful, 3D printing does not have the capability to manufacture products on a large scale which is why an injection moulding machine is still preferred by manufacturers. Whether wholesale migration to 3D printing will occur in the future remains to be seen.

Low Scrap Rates

Relative to traditional manufacturing processes, like CNC machining which cut away substantial percentages of an original plastic block or sheet, injection moulding has low scrap rates.

The sprue, for example, guides molten plastic from the nozzle of the injection moulding machine to the entry point for the entire injection moulding tool and is separate from the mould itself. It produces very little waste.

The runner is a system of channels that connect with the sprue, typically within or part of the mould tool and guide the molten plastic into part cavities located within the tool. There are two principal categories of runners (hot and cold). Hot runners ensure a higher quality part and are particularly beneficial to moulding challenging part geometries which require a lower margin of error.

The advantage of hot runners is that if designed properly the plastic will flow uniformly from the nozzle to the gate locations – the point at which molten plastic enters the injection mould cavity. This is paramount as the gate location, temperature of the plastic, design of the internal mould cavities and the material properties of the plastic and mould tools themselves play a crucial role in the success or failure of the injection moulding process.

Conversely, cold runners consist of two or three plates contained within the mould. The system injects plastic into the mould through the sprue and feeds the runners which then filters into parts of the mould cavity. The simplest cold runner system consists of two plate moulds with a parting line that separates the two plates. This system has the runner and part attached. You can only gate the part of its perimeter. An ejection system separates both the runner and part from the mould.

The benefit to cold runners is that they are cheap to produce and maintain, are suitable for a wide range of polymers, allow for quick and easy colour changes and feature a faster cycle time. Both hot and cold runners are renowned for creating very little waste.

Moreover, any flash (or excessive material found where mould separates, notably at the parting surface, moveable core, vents or venting ejector pins) is minimised by using an injection moulding machine.

Providing that there is no undesirable gap between the cavity and the core sides of the mould, the plates are strong enough to avoid deformation during moulding, there are adequate venting dimensions (its recommended that venting size is 0.025mm for crystalline polymers and 0.038 mm for amorphous polymers) and that the mould surface is clean, flash is unlikely to occur.

The Takeaway

We’re continually moving towards a more sustainable and environmentally conscious society. A big part of this is minimising waste across all commercial sectors and our daily lives. Injection moulding is one of the best ways that we can minimise waste whilst creating components and products intrinsic to businesses and our lives the world over.

The minimal waste associated with the injection moulding process and superior components and products that can be created means that demand is only likely to increase in the future.