Core Moulding
A core is a pre-determined shaped mass of dry sand, which is made separately from mould. It is positioned in a mould to obtain a shape in the casting which can’t be readily obtained by the mould properly. Core box pattern equipment, baking equipment and handling facilities are required for making of cores.
Core can be eliminated when of small size particularly, if the hole is to be machined. Cores should be of sufficient thickness and free of fragile or overhanging projections, which might be easily broken during the necessary handling and transportation involved in production.
The main material of the core is refractory, either a natural or a synthetic sand. The sand-mix used for core making consists of the sand together with a binder which is used to give strength after baking. A natural binder (sometimes called core-gum) is linseed oil. A synthetic binder is a synthetic resin.
A recent development is to mix sand with sodium silicate solution so that the grains are coated and obtain the necessary hardness by ‘gassing’ with carbon dioxide, causing the sodium silicate to bond the grains together and produce strong core, thus eliminating the baking process.
Core Binder:
Since pure sand used for preparing core has no natural bond, binder is used. A core binder is an agent added to sand to produce a good ‘green’ bond and also hardness after baking. A typical natural binder is linseed oil; a typical synthetic binder is a resin of the thermosetting type.
Generally, core oils (composed of 50 to 60% linseed oil, 25% resin and balance mineral oil) are used as they are very economical to produce better cores. In the case of thermoplastic binders (resin and pitch), the powdered binder is mixed with core sand, and on heating the binder liquefies and coats the sand grains.
On cooling, the dispersed liquid binds the sand grains together to form a united mass. Use of synthetic resins is limited as they are costly. Addition of resin to core oil speeds up drying of core and reduces the volume of core gas. Pitch compounded with dextrin and steam coal is used for large cores.
Phenol and urea formaldehyde thermo setting plastic core binders are becoming common due to their high strength, low gas formation, collapsibility and resistance to moisture absorption.
Molasses adds hardness to the core but lacks in strength. On the core, molasses water may also be sprayed.
Protein binders (gelatine, glue, caserim etc.) are used where collapsibility is the main criterion.
Excepting very small cores, these are internally reinforced with core wires or grids to help cores retain their shape while damp, for transportation and for strengthening purposes.