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§ 15

Thermal expansion of nameplates

How much does my sign plate expand, and how large should the holes be?

Answer: Sign boards expand in sun and contract in frost. Acrylic and polycarbonate move about three times as much as aluminium composite. If screws are too tight in too small holes, the board bulges or cracks at the holes. The calculator computes the movement over the temperature range and the minimum hole diameter when the board is fastened in the middle.

Formula

ΔL = α · L · ΔT (α i mm pr. meter pr. grad, L i meter)

If the plate is fixed in the middle, the outer screw moves α · (L/2) · ΔT in each direction from the assembly temperature.

Huldiameter = skruens diameter + 2 × den største bevægelse ved den yderste skrue, så skruen kan sidde i midten af hullet ved montering.

Dark plates in direct sun can become much warmer than the air. Use the surface temperature.

Symbols and units in the formula
SymbolMeaningUnit
laengdeSheet length (the longest side)mm
alfaMaterial (linear expansion)mm/(m·K)
t_minLowest temperature (winter)°C
t_maxHighest surface temperature (sun)°C
t_montTemperature during assembly°C
skrue_dScrew or spacer bolt diametermm
=Total movement over temperature rangemm
=Expansion from installation to warmest daymm
=Contraction from installation to coldest daymm
=Minimum hole diameter at the outermost screw (board fixed in the middle)mm

Validity and source

Source
Manufacturer data: PLEXIGLAS® (Röhm) 0.07 mm/(m·K), Makrolon® (Covestro) 0.065, Dibond® (3A Composites) 0.024 · technical annex: materials' structure in relation to thermal conditions

Does not apply when

  • The expansion coefficients are the manufacturers' typical values. Use the datasheet for the panel you have. Clamp the screws so the panel can slide, e.g. with distance bolts or washers, and position the screw in the middle of the hole. If there are gaps between multiple panels, the gap must also be able to take the movement.

Worked example

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

Substituted

  • Sheet length (the longest side)2,000 mm
  • Material (linear expansion)0.07 mm/(m·K)
  • Lowest temperature (winter)-20 °C
  • Highest surface temperature (sun)60 °C
  • Temperature during assembly15 °C
  • Screw or spacer bolt diameter5 mm

Giver

  • Total movement over temperature range11.2 mm
  • Expansion from installation to warmest day6.3 mm
  • Contraction from installation to coldest day4.9 mm
  • Minimum hole diameter at the outermost screw (board fixed in the middle)11.3 mm

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Sådan bruger du den

Når en akrylplade, en polycarbonatplade eller en kompositplade skal skrues op udendørs, skal hullerne være store nok til, at pladen kan arbejde. Ellers buler den om sommeren eller revner ved skruerne om vinteren. Beregneren viser bevægelsen og den huldiameter, du skal bore.

Step by step

  1. Mål pladens længste side.
  2. Vælg materialet. Akryl og polycarbonat arbejder ca. tre gange så meget som komposit.
  3. Indtast laveste og højeste temperatur. Brug overfladetemperaturen i sol, især på mørke plader.
  4. Indtast temperaturen den dag, du monterer.
  5. Bor hullerne i den beregnede diameter, og sæt skruen i midten af hullet.

Typical errors

  • At bore huller i skruens diameter i akryl udendørs. Pladen kan ikke bevæge sig og revner ved hullerne.
  • At bruge lufttemperaturen. En mørk plade i sol bliver langt varmere.
  • At spænde skruerne helt. Pladen skal kunne glide under skiven.
  • At fastgøre pladen stramt i begge ender. Så skal hele bevægelsen tages i den ene ende.

Examples from everyday life

Akrylplade på en facade

En 2 m lang akrylplade skal skrues op på en sydvendt facade. Du regner med −20 °C om vinteren og 60 °C i sol, og du monterer en forårsdag ved 15 °C med 5 mm afstandsbolte.

Sheet length (the longest side)
2,000 mm
Material (linear expansion)
Acrylic / PMMA (0.07)
Lowest temperature (winter)
-20 °C
Highest surface temperature (sun)
60 °C
Temperature during assembly
15 °C
Screw or spacer bolt diameter
5 mm

It means: Pladen arbejder 11.2 mm over året. Fastgjort på midten skal de yderste huller være mindst 11.3 mm. Bor dem med et trinbor, så kanten ikke revner.

Kompositplade i fuld længde

Et facadeskilt i aluminiumskomposit på 3.050 mm monteres samme dag med de samme temperaturer.

Sheet length (the longest side)
3,050 mm
Material (linear expansion)
Aluminium composite (0.024)
Lowest temperature (winter)
-20 °C
Highest surface temperature (sun)
60 °C
Temperature during assembly
15 °C
Screw or spacer bolt diameter
5 mm

It means: Selv den lange kompositplade arbejder kun 5.9 mm, og de yderste huller skal være mindst 8.3 mm.

Sort akrylskilt monteret i frost

Et sort akrylskilt på 1,5 m monteres en vinterdag ved 0 °C. I sol kan den mørke plade nå 70 °C.

Sheet length (the longest side)
1,500 mm
Material (linear expansion)
Acrylic / PMMA (0.07)
Lowest temperature (winter)
-20 °C
Highest surface temperature (sun)
70 °C
Temperature during assembly
0 °C
Screw or spacer bolt diameter
5 mm

It means: Fra monteringen til den varmeste dag udvider pladen sig 7.4 mm. Hullerne skal være mindst 12.4 mm. Sæt skruen midt i hullet, så er der plads til, at pladen vokser i sol.

Rule of thumb

Akryl arbejder ca. 5-6 mm pr. meter over et år udendørs. Komposit kun ca. 2 mm.

How to check the answer

Bevægelsen skal være præcis udvidelse plus sammentrækning. Dobbelt længde giver dobbelt bevægelse.