Optimization solutions to decrease ejection friction

On reviewing polymer-based microfabrication technologies Becker and Gartner identified some important features of replication tools (Becker and Gärtner 2008): (a) the geometrical replication depends upon the geometrical

accuracy of the master, (b) for successful demoulding no undercuts in the structure itself can be allowed, (c) the surface roughness of the master should be as low as possible for replicating structures and (d) a suitable interface chemistry between master and substrate has to be selected.

To ensure a good solution for the demoulding issues the principle that rules the better solutions assumes that the tool and the part designs can be optimized to maximise the likelihood of successfully demoulding. Well-known examples for injection moulded products are to add draft angles on all tool cores, to have a constant wall thickness throughout the part and to gate the part on the thickest region. The part deformation problems can be approached by increasing the structural rigidity of the part for successful demoulding in terms of design such as adding bosses/ribs where possible and the selection of optimum materials and processing parameters (Delaney, Bissacco et al. 2012).

Other less known solutions to part design which may be more applicable to micro-structured parts include sacrificial barriers. These are non-critical structures deliberately included in the part geometry to resist overall shrinkage in the vicinity of the microstructures. In the microhot-embossing context Worgull et al. used a frame to limit the in-process flow front (to reduce warpage and shrinkage) and create sacrificial features to take up the high contact stress during demoulding (Worgull, Heckele et al. 2005). A similar auxiliary structure as a thermal stress barrier in the form of an additional circular structure around the field of microstructure has been proposed by Guo et al. (Guo, Liu et al. 2007). The simulation results by finite element modelling predicted a significant reduction in the stress experienced by microstructures. One disadvantage of this approach is the additional space on the component to locate the sacrificial stress barrier.

Wang and co-workers studied an optimum ejector pins layout that distributed the overall ejection force among a series of ejector pins In these works different layouts, location, dimension, quantity and distribution of the ejector

2. STATE OF THE ART 21

Correia, M.S. Modelling the ejection friction in injection moulding

pins were considered. The objective was to identify the balanced layout causing minimum stress and deformation to the product and developed a strategy of numerical optimization of the demoulding stage. The studies dealt with conventional demoulding concept of ejector pins to physically push off the component from the mould core. To predict the distribution of the ejection force among ejector pins a finite element thermoviscoelastic solidification analysis was performed. An assumption of uniformly friction distribution cannot be generalized and the balanced ejection is not simply balancing the ejector pins layout according the interface areas. The primary premise, according to Wang et al. (Wang, Kabanemi et al. 2000), is that the corners of the moulding will limit the shrinkage and thus minimise the contribution of warping to demoulding force. On the other hand the local stiffness of the part must be considered, so in reality the local contact pressure will be influenced by both the shrinkage and stiffness of the part.

Bataineh and Klamecki (Bataineh and Klamecki 2005)studied improvements to the ejector pins layout to predict local mould-part force. Experiments were made using ring and box-shaped parts to provide input of the coefficient of friction, material properties and total and local ejection forces, to the simulation process. Michaeli and Gartner proposed and trialled non-destructive methods to do the demoulding without ejector pins or plates (Michaeli and Gartner 2006). The method used was demoulding with ultrasonics. It was expected that with the utilization of ultrasonics the oscillation between the mould and the part would reduce the wall adherence this resulting in the reduction of the demoulding force, but the experimental results did not report this assumption.

Despite the improvement of the ejection system, the surface topography has been used as an indicator of the most dominant friction mechanisms. The principle of solution is that the replication tool surface has a topography which will minimise the overall demoulding force. In the context of minimizing the

overall time required to finish rapid tools Majewski and Hopkinson summarized the effects of tool surface roughness on part quality and demoulding force for parts injection moulded using laser sintered tools (Majewski and Hopkinson 2003). In this work it is suggested that the ejection force can be minimised through the use of very low surface roughness.

However, Ferreira et al. (Ferreira, Neves et al. 2001) mentioned that very good polished surfaces (mirror-like) may facilitate the formation of a seal which prevents air entering the gap between the core and the part resulting in the local formation of vacuum forces that can make difficult to separate the part from the core. Finishing the core in the ejection direction air can enter the gap allowing atmospheric pressure to exist between the plastic and the steel core, eliminating the vacuum force. The existence of an optimum core surface roughness was reported by Sasaki et al. (Sasaki, Koga et al. 2000) with similar results observed by Pontes et al. (Pontes, Ferreira et al. 2004) and noted by Pouzada et al. (Pouzada, Ferreira et al. 2006). As the previous authors Kyuichiro (Kyuichiro 1995) verified in several pin-on-disk tests the same behaviour.