Injection Moulding Process: Mould and Process Optimisation

Plastic injection moulding is widely acknowledged as being the manufacturing technique to produce highly complex geometric plastic parts at low cost. The quality of the final product is heavily dependent upon careful selection of processing conditions and material selection which in turn is used to perform design analysis and dimensioning of the mould (Figure 1).

Figure 1

Figure 1 : Optimisation of injection moulding process

An inadequate selection of set process conditions is known to cause a decrease in product quality as a result of defective final products due to residual stresses, warpage, voids, sink marks, etc. Cavity pressure and mould temperature are of paramount importance for optimisation of the injection moulding process as these have a direct effect on the quality of the final product [1]. For example, higher mould temperatures with amorphous polymers such as ABS and PC have been shown to produce lower stress levels, leading to improved impact and stress-crack resistances and fatigue performances [2].

When moulding semi-crystalline materials, it has been reported that mould temperature should be kept higher than glass-transition temperature, allowing sufficient time for the polymer to crystallise. Overall, it has been found that lower melt temperatures in conjunction with higher mould temperatures tend to provide improved quality performance.

However, most injection machines are often operated at higher temperatures in an attempt to reduce melt viscosity.  When increasing temperature the viscosity reduces and it is this higher temperature which could lead to increased degradation, cooling time and energy consumption. It has been shown that an injection moulding machine in idle mode with no production consumes 80% of the full load power [3], whilst cooling represents approximately 10-15% of the total energy used.

The design of the mould plays an important role when examining polymer processability and its effect on energy consumption. Moulded parts with thinner walls, particularly suitable for fibre-reinforced thermoplastics (FRPs), require shorter cycles and compare more favourably in terms of energy. A reduction in the moulded part wall thickness of 25% has been shown to reduce cooling time by more than 40% [4].

It has also been found that uniform temperature distribution within the mould maximises efficiency. As a result, conformal water lines have been used by using selective laser melting technology (SLM) because of their ability to perfectly shape according to the moulding surface. This ensures a more uniform mould temperature distribution, improving quality of the final product (Figure 2) [5].

Figure 2

Figure 2 : SLM and conformal channels [5]

The application of high performance alloys for mould design provides another potential solution for the optimisation of the injection moulding process (Figure 3). This fabrication method enables a unique combination of strength and thermal conductivity that offers important benefits such as shorter cycle time and improved part quality [6].

Figure 3Figure 3 : Example of a copper alloy mould design [4]

Pulsed cooling is also a reliable approach for the optimisation of the cooling phase of the injection moulding process when compared to continuous cooling [7], leading to reduced cycle times (approximately 20% reductions) and lower energy consumptions.

By judicious selection of materials, mould design and processing conditions it is clear that injection moulding processes can be optimised without any reduction in quality performance. Therefore, an understanding of the energy consumption of the injection moulding process and its relationship to set process conditions, mould design and polymer being used may result in potential energy savings. It has been shown that simple no cost or low cost energy practices can reduce energy consumption by between 10 and 20%, which would result in product cost savings of £38 million per annum [8].

References

[1] H. Hassan, An experimental work on the effect of injection molding parameters on the cavity pressure and product weight, International Journal of Advance Manufacturing Technology, Vol. 67, pp 675- 685, 2013.

[2] M. Sepe, The importance of Melt & Mold temperature, Plastics Technology, December, 2011

[3] R. Kent, Energy Management in Plastics Processing, Plastics Information Direct, Bristol, 2008

[4] D. Godec, Processing parameters influencing energy efficient injection moulding of plastics and rubbers, Polimeri, Vol.33, pp 112-117, 2012

[5] Inglass, Mold conditioning optimization and SLM-Selective Laser Melting technology, available from: http://www.inglass.it/upload/user_files/schede/slmen.pdf

[6] J. C. J Kuli and R. Kusner, A review of Copper Alloys for Plastic Injection Molding, Plastics Technology, December, 2011

[7] A. G. Smith et al, Optimisation of Continuous and Pulsed Cooling in Injection Moulding Processes, Plastics, Rubber and Composites, 36, pp 93 -100, 2007

[8] R. Kent, Energy management in plastics processing – framework for measurement, assessment and prediction. Plastics, Rubber and Composites, Vol.37, pp 96 -104, 2008