Metal casting mould core: bonded sand cores for internal cavities

01

Trade core strength against knock-out after casting

Obzina and co-authors at VŠB – Technical University of Ostrava cast AlSi7Mg0.3 at 720 °C around four core systems and counted the knocks needed to remove each core. Transverse (three-point bending) strength ranged from 0.92 MPa for 3D-printed furan cores to 7.36 MPa for geopolymer (GEOPOL) cores, and knock-out went the same way: about 6 knocks for the printed furan cores and about 46 for the geopolymer cores. [1]

Conventional furan cores reached 3.45 and 2.67 MPa with two sand grades and showed excellent collapsibility; resin-coated sand (RCS) cores reached 4.47 MPa, gave the smoothest cast surfaces and sat between furan and geopolymer for knock-out. The authors report a correlation: as transverse strength rises, so does the number of knocks needed to remove the core. [1] The printed furan cores, at the lowest strength, gave the roughest cast surfaces, which the authors attribute to erosion by the liquid alloy. [1]

Budavári and co-authors at the University of Miskolc found the same tension in warm-box cores: more acid hardener gave better thermal stability early in heating but lower residual bending strength after 15 minutes of thermal loading at 400 °C, which helps knock-out. [2] Editorial note: specify the strength the core needs for handling and metal pressure, then ask the core maker for a knock-out or residual-strength result on your casting, not the strength alone.

Give the core maker the casting alloy and pouring temperature, core drawing or 3D model, core weight and wall thickness of the casting around it, required handling strength, surface finish of the cored passage, venting route and core-print design, permitted binder systems (organic or inorganic), coating, quantity and delivery packaging.

02

Vent the gas the core releases through its core print

A 2023 study in Materials poured grey iron at 1360 °C around 3D-printed furan sand cores whose only exit was the core print. Gas generation was highest in the first 60 s, with a peak pressure of 1.09 kPa at the print, and gas release lasted 10 minutes. [3]

A core with a blind core print peaked at an exhaust speed of 1.10 m/s and held about 1 m/s for 500 s, while a hollow core peaked at 1.89 m/s and then slowed. The authors warn that gas pores can form early in casting from this eruption of gas, and that burnt core sand cut off from air produced 30.7% more gas than sand burnt with air, so open-air gas evolution tests understate what a core releases inside a casting. [3] The same paper cites earlier tube-furnace measurements of 10–15 mL of gas per gram of resin sand at 700–850 °C. [3]

Editorial note: ask how the core is vented through its print, whether hollow sections or vent channels are included, and what gas evolution test the core maker runs and in what atmosphere.

03

Compare binder processes by published core-making settings

These are settings reported for laboratory and test cores, not recommendations for a production core.

04

Sand grain, binder, core box or printer, and the core print that vents gas

05

Transverse strength, knock-out, pour temperature and permeability for a sand core

The reviewed papers measure strength in bending and collapsibility in knocks, not the tensile or hardness values a generic spec sheet asks for; permeability, density and thermal expansion are not stated and need the core maker's own data.

06

Send the casting, core and process data together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Casting: [alloy; pouring temperature; wall thickness around core]
Core: [drawing or 3D model; weight; tolerances]
Process: [binder system allowed; organic or inorganic; printed or shot]
Performance: [handling strength; knock-out requirement]
Gas: [core print; venting; gas evolution test]
Supply: [quantity; coating; packaging; storage time]

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