The procedure has eight steps grouped into five stages. Figure 1 shows the stages and the evidence that should be in hand before moving from one to the next. The intention is that a foundry can stop at any stage with a clear reason and without having committed capital prematurely.
Stages of the proposed planning procedure and the evidence needed before each is passed
The starting point is the foundry's own work. Every active casting is listed with its annual quantity, the number of cores, the state of its pattern equipment and its recent scrap history. Jobs then fall into three groups: repeat work in quantity on sound patterns, which should stay in green sand; prototypes, one-off spares and jobs whose patterns are worn out or lost, which are candidates for printing; and heavily cored castings such as pump volutes, impellers and manifolds, where printed cores might cut assembly labour and core-related scrap. A map of products by volume and complexity, of the kind proposed for additive manufacturing generally (Conner et al., 2014), is a convenient way to display the result.
Three models are available. The foundry can buy printed moulds and cores from a service bureau, own a printer, or adopt a hybrid in which printed cores are set in conventional green sand moulds. The guide recommends that a small foundry begin with bought-in cores for a few jobs. This needs no capital, tests customer interest and exposes the practical problems. Printed sand parts are fragile and bulky, so transport distance, packaging and breakage in transit must be weighed, as must the lead time from order to delivery. Ownership becomes worth examining only when the volume of printed sand bought each month would keep a machine reasonably busy. Shared ownership through a cluster facility is a further option where an industry association exists to host it.
A printed mould should not be a copy of the green sand one. Freedom from draft and parting lines allows runners that fill from the bottom with less turbulence, feeders placed where solidification requires them, and cores consolidated into single pieces. The principles of quiescent filling and directional feeding set out by Campbell (2015) apply with fewer compromises, and published demonstrations show complex gating and cellular geometries made this way (Snelling et al., 2015). Two cautions follow from the binder content: vents must be generous, and the mould should be designed as a shell of modest wall thickness, supported by loose backing sand if necessary, to limit both gas and the volume of costly printed sand. Solidification simulation, described for foundry engineers by Ravi (2005), is well suited to checking the new layout before anything is printed.
Printers need dry, rounded sand within a narrow size range, and the foundry must establish whether a regional silica sand meets the supplier's specification or whether sand must be bought through the machine vendor. Test pieces printed in the candidate sand are checked by the standard foundry tests for bending or tensile strength, permeability, loss on ignition and gas evolution (ASM International, 2008). Climate deserves a specific test. Organic binders are sensitive to humidity, and much of South India has a long monsoon season with humid air. Strength should therefore be measured after storage for periods and conditions that match the foundry's own stores, and packaging or a dry room specified if it falls.
Acceptance criteria are agreed before the first trial is poured: dimensional tolerance, surface roughness, soundness by the customer's inspection standard, and freedom from the defects to which resin bonded moulds are prone, namely gas porosity, veining and metal penetration. Each trial casting is made alongside a green sand casting of the same part in the same melt where this is possible, so that differences can be attributed to the mould. The records kept are the defect type and location, the dimensions and the fettling time. Published comparisons of printed and conventional moulds indicate what to look for (Hawaldar and Zhang, 2018; Chhabra and Singh, 2011), but they do not substitute for the foundry's own alloy and section sizes.
The model compares cost per good casting by each route. For green sand it includes the pattern and core boxes spread over the expected order quantity, moulding and core-making labour, sand preparation and scrap. For printing it includes the price of printed sand by volume, transport, any redesign time, and the same melting and finishing costs adjusted for the trial results. Plotting both against quantity gives a break-even point for each job. Where an owned printer is under study, depreciation, maintenance contracts, imported consumables priced in foreign currency and the cost of an uninterrupted power supply are added, and the machine utilisation assumed should be stated openly, since idle time dominates the cost of additive equipment (Ruffo et al., 2006).
Furan-bonded sand cannot simply be returned to the clay-bonded system in quantity, because binder residues and acid alter the properties of the green sand. A small proportion of core sand is diluted in any foundry's system, but a planned increase needs monitoring of the system sand and, beyond some point, a separate route: mechanical or thermal reclamation, or disposal through a channel acceptable to the state pollution control board. Pouring and shake-out of resin-bonded moulds release decomposition products, so local extraction at those stations should be reviewed, and the handling and storage rules for the resin and activator obtained from the supplier's safety data sheets.
The final step is a timetable. A first phase limited to bought-in cores for selected jobs is followed by a review against the cost model and the scrap record. Only if the review is favourable does the foundry move to complete printed moulds for prototypes, and only after that to any discussion of ownership. Training runs in parallel. At least one person needs to be competent in three-dimensional modelling and mould design, and moulders need to learn the handling, cleaning and assembly of printed parts. Green sand moulding continues throughout for the work it suits.