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§ 16

Silicone mould: consumption of silicone and hardener

How much silicone and hardener should I mix for the mould?

Answer: When you need to manoeuvre a rosette, bracket or piece of ornament, you mould the model in silicone in a mould box. The silicone should fill the box minus what the model itself takes up. The calculator works out the volume and converts to kg with the silicone's density and divides the amount into silicone and hardener after the manufacturer's mixing ratio by weight. You measure the model's volume most easily by lowering it into a measuring cup with water and reading how much the water rises.

Formula

Rumfang = kassens længde × bredde × højde (cm³ ÷ 1000 = liter) − modellens rumfang

Samlet masse = rumfang × densitet × (1 + spild ÷ 100)

Silikone (A) = samlet masse ÷ (1 + hærderdele ÷ 100) · Hærder (B) = samlet masse − A

Example with 100:5: 1.00 kg total gives 0.952 kg silicone and 0.048 kg hardener

Symbols and units in the formula
SymbolMeaningUnit
kasse_lForm box's internal lengthcm
kasse_bForm box's internal widthcm
kasse_hSilicone height in box (model + 1–2 cm above)cm
model_volModel volume (measured by water displacement)liter
densitetSilicone density (from datasheet)kg/l
haerder_pctHardener per 100 parts silicone (by weight)dele
spildWaste in cup and on spatula%
=Silicone and hardener in totalkg
=Of which silicone (A)kg
=Heraf haerder (B)kg
=Volume to be filledliter

Validity and source

Source
Manufacturer data (RTV-2 mould silicone, typically 100:5 by weight, density approx. 1.1 g/cm³)

Does not apply when

  • The mixing ratio is by WEIGHT, not by volume, and it differs from product to product (100:5, 100:10 and 1:1 all exist). Density typically ranges between approx. 1.05 and 1.35 kg/l. Read both numbers in the specific product's datasheet, and weigh the hardener on a scale that can show grams.

Worked example

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

Substituted

  • Form box's internal length30 cm
  • Form box's internal width30 cm
  • Silicone height in box (model + 1–2 cm above)8 cm
  • Model volume (measured by water displacement)1.20 liter
  • Silicone density (from datasheet)1.10 kg/l
  • Hardener per 100 parts silicone (by weight)5 dele
  • Waste in cup and on spatula10 %

Giver

  • Silicone and hardener in total7.26 kg
  • Of which silicone (A)6.91 kg
  • Heraf haerder (B)0.346 kg
  • Volume to be filled6.00 liter

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Sådan bruger du den

Når du skal kopiere en roset, en konsol eller et ornament, afformer du originalen i silikone. Silikone er dyr, så du skal kende mængden, før du blander. Beregneren regner formkassens rumfang minus modellen om til kg og deler det op i silikone og hærder efter vægt.

Step by step

  1. Byg formkassen, så der er 1-2 cm luft rundt om modellen, og mål den indvendigt.
  2. Mål modellens rumfang ved at sænke den i et målebæger med vand og aflæse stigningen.
  3. Skriv silikonens densitet og blandingsforhold fra databladet.
  4. Vej silikonen op, og vej hærderen på en vægt, der viser gram.

Typical errors

  • At blande efter rumfang, når producenten angiver forholdet i vægt.
  • At glemme at trække modellens rumfang fra. Så bestiller du alt for meget.
  • At lave formkassen for stor. Hver centimeter ekstra rundt om koster silikone.

Examples from everyday life

Afformning af en roset

En gammel roset på 30 cm i diameter skal kopieres. Formkassen er 34 × 34 cm, og silikonen skal stå 7 cm op. Rosetten fortrænger 1,6 liter vand. Silikonen har densitet 1,1 og skal blandes 100:5.

Form box's internal length
34 cm
Form box's internal width
34 cm
Silicone height in box (model + 1–2 cm above)
7 cm
Model volume (measured by water displacement)
1.6 litres
Silicone density (from datasheet)
1.1 kg/l
Hardener per 100 parts silicone (by weight)
5 parts
Waste in cup and on spatula
10 %

It means: Der skal fyldes 6.49 litres, og du skal blande 7.86 kg: 7.48 kg silikone og 0.374 kg hærder. Det er en stor mængde silikone. Til flade rosetter kan en hudform med støttekappe spare meget.

Form til et konsolstykke

Et konsolstykke skal i en aflang formkasse på 45 × 14 cm med 10 cm silikone. Konsollen fortrænger 2,3 liter, og den hårdere silikone har densitet 1,3 og blandes 100:10.

Form box's internal length
45 cm
Form box's internal width
14 cm
Silicone height in box (model + 1–2 cm above)
10 cm
Model volume (measured by water displacement)
2.3 litres
Silicone density (from datasheet)
1.3 kg/l
Hardener per 100 parts silicone (by weight)
10 parts
Waste in cup and on spatula
10 %

It means: Rumfanget er 4 litres, og du blander 5.72 kg i alt, heraf 5.2 kg silikone og 0.52 kg hærder. Rør langsomt, og lad gerne blandingen afgasse, før du hælder.

Rule of thumb

1 liter silikone vejer ca. 1,1 kg. Med 100:5 skal der ca. 50 g hærder pr. kg silikone.

How to check the answer

Silikone plus hærder skal give det samlede tal, og hærderen skal være den rigtige brøkdel: 0,374 ÷ 7,48 = 5 %.