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Renault improves yield and produces over 520 more parts per hour

The ESI group recently responded to a challenge from Renault to define design rules for filling and feeding systems across different product lines to enhance competitiveness and achieve quality casting production.

Traditional methods used in the past by Renault had produced not fully optimised layouts with lower yields. As the company was set to make a significant investment with the installation of a new moulding line at Fonderie de Bretagne (FDB) in Western France, their team employed ESI QuikCAST to quickly test different designs and understand the effects of these designs on the casting quality. QuikCAST was the chosen simulation route because of its ability to provide designers with an opportunity to try several preliminary designs on the computer, to arrive at an optimal design / process condition, and to reach the expected casting quality and competitiveness. Renault then put in place a standard approach and tools to achieve quality castings across various system designs.

Renault had an existing mould layout of a cast iron knuckle (Renault Traffic front knuckle – project code:X82). This was a six-cavity mould (fig.1), running in production with some noticeable shrinkage porosity problems (fig.2).

Each knuckle weighed around 5.5kg and the total casting weight, including the gating and feeding system, was about 97kg. The existing moulding line was derived from several trials, and it was a complex task to instigate improvements using this design. Renault set-up simulation of this existing layout using ESI QuikCAST, defining the right material properties and boundary conditions, allowing them to obtain the same defects observed on the shopfloor. Once a sufficient correlation was obtained (fig.3), these set of parameters formed a base for future work.

STANDARDISED METHODOLOGY

Needing to set up a new moulding line at FDB, Renault decided to go through the simulation route from the early development stages. Thus, a methodology needed to be established to create standard mould layouts. As a first step, they abandoned the existing layout and defined a new approach.

Determining the hotspots

A single casting simulation was then made. Here the casting has no filling/feeding system, and was assumed to solidify in the sand starting from a temperature around the pouring temperature. During this solidification, the natural thermal gradients of the casting were used to identify the solidifying liquid path and then identify the last solidifying regions in the casting. The thermal modulus of these last solidifying regions is shown in fig.4. These two regions needed to be fed well while designing the feeders to avoid any lasting solidifying regions in the casting, which could lead to shrinkage.

Developing a feeding system

The CTIF feeding rules [ cf : “Masselotage en moulage sable”- ETIF] were used by Renault to design the feeding system.

Using QuikCAST, they designed six different possible feeding systems suitable to feed these high thermal modulus regions (fig.5). An external feeder, internal feeder and a combination of external and internal feeders was proposed. The feeders were designed to have higher modulus than that observed on the single casting following the CTIF rules of feeding.

A thermal simulation including shrinkage calculation was performed on each of these proposed designs, followed by an analysis of the solidification cluster evolution, last solidifying regions / isolated liquid pockets which could lead to shrinkage in the part. Design five (D5) – with an internal feeder connected to the knuckle arm from both sides – provided the best solidification pattern showing a feed path towards the feeders with no signs of shrinkage in the casting (fig.6). D5 was the chosen option for the filling system design.

Gate positioning

With D5 as the chosen feeder, it was then important to identify the right gate positioning. Instead of designing a full cluster gating system, Renault opted to first identify the right ingate positions. Four different gating positions were proposed (fig.7).

Assuming the gates as inlets with average gate velocities, Renault conducted a filling coupled solidification simulation with all these four gate positions. There was no noticeable difference in the filling behaviour/fluid velocities between these four inlet positions. Apart from a slight increase (+0.4mm) in the maximum modulus, found with these inlet simulations, there was also no soundness impact on solidification (see shrinkage porosity maps shown in fig.8).

Developing the complete mould design

With CTIF rules [cf. “Le remplissage des empreintes de moules en sable”- ETIF] as the base of designing the filling system, Renault designed the pattern.

An eight-cavity moulding layout (combined with D5 feeder and the gate position-4) was the outcome from their design engineers (fig.9).

As opposed to the old design (fig.1), the new layout was designed in a more structured way, thanks to the step-by step methodology and the casting simulation software. Avoiding external feeders and opting for only internal feeders meant the possibility to have a uniform and symmetrical placement of the filling system. This helped in setting up the right process, as now each cavity had the possibility to fill almost identically and with similar filling time (<1s difference).

A full cluster validation simulation was performed on this moulding layout. The filling simulation helped to check the filling behaviour and the fluid velocities inside the mould. Slight modifications on the gate dimensions were suggested to reduce risk of sand erosions, slightly increasing the gate cross section. The solidification patterns were very healthy and yielded no shrinkage on the knuckle (fig.10).

CONCLUSION

A spheroidal grey iron Renault Traffic front knuckle was used as the first study to show the standardised methodology that Renault implemented for its new moulding line at Fonderie de Bretagne. The knuckle mould was improved from a six-cavity 97kg to a lighter, eight-cavity 82kg cluster weight. The new design also solved the shrinkage sensibility faced during previous production. As the yield improved, it saved 37 per cent metal for every part produced, providing two additional parts per mould (approximatively +520 parts per hour). The cost savings with the new design was consequently substantial. This standardised methodology is now used successfully by Renault across various product lines.

THE BENEFITS

  • Enhance competitiveness in the market.
  • Reduction in development time and cost, cutting needless trials to optimise the pattern design and process parameters.
  • Improvement of overall quality level.
  • Know-how capitalisation of filling and feeding systems designs.

“After using ESI QuikCAST, not only were we able to increase our bottom line by saving on metal and creating more castings than before, but we also implemented a completely new system for casting that we now use routinely across various product lines within the company,” says Laurent Soulat, cast iron design referent at Renault, France.

About Renault & Fonderie de Bretagne

A 100 per cent subsidiary of the RENAULT Group, created in 1965 in Lorient, FDB produces rough and machined parts in spheroidal graphite cast iron. FDB manufactures safety parts – suspension arms and rocket doors for the chassis, exhaust manifolds and elbows for the engines, and differential boxes for the gearboxes. Annual tonnage is 27,000 tonnes (2015). FDB has two moulding lines and a machine shop on 150 hectares including 40 hectares of buildings and employs 464 people (end of 2015). www.group.renault.com/en/