The Definitive Principles FOR Reliable Castings

Immediately before casting, the melt shall be prepared, checked, and treated, if necessary, to bring it into conformance with an acceptable minimum standard. Prepare and use so far as possible only near-defect-free melt.

This is the requirement that the liquid metal front (the meniscus) should not go to fast. Maximum meniscus velocity is approximately 0.5 m/s for most liquid metals. This requirement also implies that the liquid metal must not be allowed to fall more than the critical height corresponding to the height of a sessile drop of the liquid metal. The maximum velocity may be raised to 1.0 m/s or even higher, and the critical fall height might be correspondingly raised to approximately 50 mm, in sufficiently constrained running systems or this section castings.

This is the requirement that no part of the liquid metal front should come to a stop before the complete filling of the mould cavity. The advancing liquid metal meniscus must be kept "alive" (i.e. moving) and therefore free from thickened surface film that may be incorporated into the casting. This is achieved by the liquid front being designed to expand continuously. In practice, this means progress only uphill in a continuous uninterrupted upward advance (i.e. in the case of gravity-poured casting processes, from the base of the sprue onwards). This implies

• Only bottom-gating is permissible.
• No falling or sliding downhill of liquid metal is allowed.
• No horizontal flow of significant extent.
• No stopping of the advance of the front due to arrest of pouring or waterfall effects, etc.

No bubbles of air entrained by the filling system should pass through the liquid metal in the mould cavity. This may be achieved by:

• Properly designed off-set step pouring basin; fast back-fill of properly designed sprue; preferred use of stopper; avoidance of the use of wells and all other volume-increasing features of filling systems (such as expanding channels sometimes known as 'diffusers'); small volume runner and/or use of ceramic filter close to sprue/runner junction; possible use of bubble traps. A naturally pressurized filling system fulfills most of these criteria.
• No interruptions to pouring.

• No bubbles from the outgassing of cores or moulds should pass through the liquid metal in the mould cavity. Cores to be demonstrated to be of sufficiently low gas content and/or adequately vented to prevent bubbles from core blows.
• No use of impermeable clay-based core or mould repair paste.

• No feeding uphill in larger section thickness castings. Feeding against gravity in unreliable because of (1) adverse pressure gradient and (2) complications introduced by convection.
• Demonstrate good feeling design by following all seven Feeding Rules, by an approved computer solidification model, and by test castings.
• Once good feeding is attained, fix the temperature regime by controlling (1) the level of flash at mould and core joints, (2) mould coat thickness (if any), and (3) temperatures of metal and mould.

Assess the freezing time in relation to the time for convection to cause damage. Thin- and thick-section castings automatically avoid convection problems. For intermediate sections, either (1) reduce the problem by avoiding convective loops in the geometry of the casting and rigging, (2) avoid feeding uphill, or (3) eliminate convection by rollover after filling.

Predict segregation to be within limits of the specification, or agree out-of-specification compositional regions with customer. Avoid channel segregation formation if possible.

No quenching into water (cold or hot) following solution treatment of light alloys. (Polymer quenchant or forced air quench may be acceptable if casting stress can be shown to be acceptable.)

All castings to be provided with location points for pickup for dimensional checking and machining. Proposals are to be agreed on with quality auditor, machinist, etc.