§ 15

Preload lift and core drift

How much should I load the overcase with so it doesn't lift?

Answer: When the mould is filled, liquid metal presses up against the drag with pressure ρ × g × h times the part's projected area in the parting line. Cores that lie in the metal also get buoyancy like a plug in water: core volume times the difference between metal and core sand density. If the lift is bigger than the drag's own weight, it lifts, and metal runs out in the parting line. The calculator gives lift force and the weight that should be put on or clamped, including a safety factor.

Formula

Løft fra metallet på overkassen: F = ρ × g × h × A (h = metalhøjde over delefladen, A = projiceret areal)

Kerneopdrift: F = V_kerne × (ρ_metal − ρ_kerne) × g

In kg: ρ (g/cm³) × h (cm) × A (cm²) ÷ 1000 + V_core × (ρ_metal − ρ_core) ÷ 1000

Ekstra vægt = samlet løft × sikkerhedsfaktor − overkassens egen vægt

Symbols and units in the formula
Symbol	Meaning	Unit
areal	Item's projected area in the parting line	cm²
hoejde	Metal height above the parting line (to the top of the downsprue)	cm
rho_m	Liquid metal density (cast iron and steel approx. 7.0 aluminium approx. 2.4)	g/cm³
kerne_vol	Volume of cores in the metal (0 if none)	cm³
rho_k	Core sand's density	g/cm³
overkasse	Drag weight with sand	kg
sf	Safety factor on lift	—
=	Extra weight to be placed on (or tensioned with)	kg
=	Total lift from metal	kN
=	Of which core buoyancy	N
=	Core buoyancy as weight (what the core marks or core supports must hold)	kg
=	Metal pressure in the parting line	kPa

Validity and source

Source: Hydrostatik (p = ρ × g × h) og Arkimedes

Does not apply when

Estimate. Metal shot during casting (dynamic pressure) comes on top, so the safety factor. A core that is lifted must be held by core marks or core supports (chaplets) and the core mark sand must be able to stand the pressure.

Worked example

The calculator's own default values inserted. The figures are calculated with exactly the formula the tool uses.

Substituted

Item's projected area in the parting line1,500 cm²
Metal height above the parting line (to the top of the downsprue)20 cm
Liquid metal density (cast iron and steel approx. 7.0 aluminium approx. 2.4)7 g/cm³
Volume of cores in the metal (0 if none)2,000 cm³
Core sand's density1.55 g/cm³
Drag weight with sand120 kg
Safety factor on lift1.50

Giver

Extra weight to be placed on (or tensioned with)211 kg
Total lift from metal2.17 kN
Of which core buoyancy107 N
Core buoyancy as weight (what the core marks or core supports must hold)10.9 kg
Metal pressure in the parting line13.7 kPa

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Sådan bruger du den

Når formen fyldes, presser metallet overkassen opad, og kernerne vil flyde op som en prop i vand. Er det samlede løft større end overkassens egen vægt, åbner delefladen, og metallet løber ud. Du regner det før støbningen for at vide, hvor meget der skal lægges på, eller om kassen skal spændes, og om kernemærkerne kan holde kernen nede.

Step by step

Mål emnets projicerede areal i delefladen i cm², altså emnet set ovenfra.
Mål metalhøjden fra delefladen op til toppen af nedløbet.
Skriv flydende metals densitet, kernernes rumfang og kernesandets densitet.
Skriv overkassens vægt med sand og sikkerhedsfaktoren, typisk 1,5.
Læs den ekstra vægt, der skal på, og kerneopdriften.

Typical errors

At glemme kerneopdriften. En stor kerne i jern kan løfte mere end en mand kan holde.
At bruge hele formkassens areal. Det er kun emnets (og løbesystemets) projicerede areal i delefladen, metallet trykker på.
At regne uden sikkerhedsfaktor. Stødet, når formen fyldes, giver et tryk oveni det statiske.
At lægge vægten skævt, så kassen vipper i den ene side.

Examples from everyday life

Støbejernshus med kerne

Et pumpehus i støbejern har 1.500 cm² i delefladen og 20 cm metal over den. Kernen til hulrummet fylder 2.000 cm³, og overkassen vejer 120 kg med sand.

Item's projected area in the parting line: 1,500 cm²
Metal height above the parting line (to the top of the downsprue): 20 cm
Liquid metal density (cast iron and steel approx. 7.0 aluminium approx. 2.4): 7 g/cm³
Volume of cores in the metal (0 if none): 2,000 cm³
Core sand's density: 1.55 g/cm³
Drag weight with sand: 120 kg
Safety factor on lift: 1.5

It means: Læg 211 kg ekstra på overkassen, eller spænd den med klammer. Metallet løfter i alt med 2.17 kN, og kernen alene løfter med 10.9 kg, som kernemærkerne skal holde.

Aluminiumsemne i en lille kasse

Et aluminiumshus har 800 cm² i delefladen og 15 cm metal over den. Kernen fylder 500 cm³, og overkassen vejer 40 kg.

Item's projected area in the parting line: 800 cm²
Metal height above the parting line (to the top of the downsprue): 15 cm
Liquid metal density (cast iron and steel approx. 7.0 aluminium approx. 2.4): 2.4 g/cm³
Volume of cores in the metal (0 if none): 500 cm³
Core sand's density: 1.55 g/cm³
Drag weight with sand: 40 kg
Safety factor on lift: 1.5

It means: Der skal kun 4 kg ekstra på. Aluminium løfter ca. en tredjedel af, hvad jern gør, og kerneopdriften er kun 0.4 kg.

Rule of thumb

Støbejern og stål løfter ca. 7 kg pr. dm² pr. 10 cm metalhøjde.

How to check the answer

Løftet i kg er ρ × h × A ÷ 1000 plus kernens rumfang × (ρ_metal − ρ_kerne) ÷ 1000: 7 × 20 × 1500 ÷ 1000 = 210 kg plus kernen.