Stress Limits - Plexiglass Acrylic sheet offers good tensile, impact, and flexural strength properties as shown in the table below; however, stress considerably below the test values shown will produce light surface checking or fractures known as crazing. To avoid crazing, loads should not exceed 1,500 psi.

Tensile Strength 10,500 psi (Tensile strength is a measure of the force required to pull a specimen apart.)

Flexural Strength 16,000 psi (Flexural strength is a measure of the force required to break a specimen by bending it.)

Compressive Strength 18,000 psi (Compressive strength is a measure of the compressive load required to cause failure.)

Modulus of Elasticity 450,000 psi (Modulus of elasticity is a measure of the force required to produce a given change in dimension. A high modulus is associated with rigidity and a material's ability to hold its shape under momentary loading.)

Thermal Expansion and Contraction - All materials expand and contract to a greater or lesser degree due to changes in temperature and humidity. Allowances must be made for these changes in the construction and fabrication of products; for example, the expansion joints in cement sidewalks and on steel bridges. Plexiglass Acrylic sheet is subject to greater dimensional change, due to thermal expansion and contraction, than other materials with which it is used in construction.

Comparison of Co-Efficient of Thermal Expansion

Plexiglass Acrylic Sheet vs. Other Materials

Inches/Inch/F

Acrylic Sheet .0000410

Aluminum .0000129

Plate Glass .0000050

For indoor applications where temperature normally remain the same (+/- 20 degrees F), acrylic sheet does not generally require special considerations for expansion and contraction other than providing for a snug rather than tight fit since its movement is approximately 1/32 of an inch per foot length for each 20 degrees of temperature change. In outdoor use where temperature varies widely, acrylic sheet should be handled in accordance with the following recommendations:

1. Sheets or panels should be installed in a channel frame engaging the edges of the material so that it is free to expand and contract without restraint.

2. The channel frame depth should be sufficient to allow for thermal contraction of the sheet without withdrawal of the edges of the frame.

3. Through-bolting or other inflexible fastenings which do not provide for expansion and contraction may cause failure of the installation.

4. Before installation in the channel frame, the sheet or panel should be cut sufficiently shorter than the channel frame dimensions to allow for thermal expansion.

5. Sealant compounds and tapes should be types which are sufficiently extensible to accommodate thermal expansion and contraction of the material and which adhere to both sheet and frame. A practical rule to follow in outdoor installations is that the frame depth should be 1/4" plus 1/8" per running foot of acrylic sheet in length and width to provide for contraction. To provide expansion clearance, the flat sheet should be cut 1/16" per running foot shorter than the frame depth in both dimensions.

Rigidity - Plexiglass Acrylic sheet is not as rigid as many other materials such as metal, wood, and glass. This can cause the material to deflect under load and foreshorten as a result of the deflection. Observation of the following recommendations will contribute to the good performance of the finished part.

1. Channels engaging the edges of the sheet must be sufficiently deep to allow for foreshortening of the sheet due to deflection under load, as well as thermal expansion and contraction.

2. Thermoforming the Plexiglass sheet into dome-shaped or corrugated sheets increases its rigidity. Wherever practical, formed panels should be used in large unsupported area usage, where wind loads or snow loads are involved.

3. If it is not practical to form the sheet, increasing the thickness of a flat sheet will impart greater rigidity to the panel.

4. Another possibility is to insert a curved wedge under a Plexiglas skylight to give it a convex shape.

5. As a general rule, any horizontal outdoor glazing for skylights, etc., should be 1/4" thickness for minimum deflection with maximum unsupported span of 36".

Notch Sensitivity - Like glass and many other materials, Plexiglass acrylic plastic is notch sensitive. Notches and chipping along the edges of the sheet, at drilled holes, and inside corner cuts are potential points of cracking or breaking. Scraping, sanding or joining the rough cut edges will eliminate these notches and insure the maximum impact resistance of the part. Inside corner cuts should be avoided whenever possible or at least be designed to have a slight radius.

Surface Hardness - The surface hardness of acrylic sheet is approximately equal to that of copper or brass. Abrasives will scratch the surface of acrylic. This is not ordinarily a handicap in the use of colored or patterned sheet but may be one in the use of clear sheet where scratch-free appearance is desired. The exercise of reasonable care in cleaning colorless acrylic sheet will minimize scratching.

Combustibility - Acrylic plastic must be used with an appreciation for the fact that it is a combustible material. The fire precautions observed to protect wood and other ordinary combustibles should be used to protect acrylic plastic from ignition sources and involvement in fire. Applications subjected to high heat sources of flame such as high wattage incandescent light bulbs, radiators, heaters, fire places or candles, etc. should be avoided. Items normally made from non-combustible materials should be considered carefully before making them in acrylic as an unknowing user might assume they are safe when they might, in fact, present a hazard.

Service Temperature - The maximum temperature at which acrylic plastic can be subjected to for any period of time is 180 degrees F to 200 degrees F. The material softens when heated to temperatures above 260 degrees F which is 40 degrees below the temperature used in thermoforming, and 300 degrees below its ignition temperature. Applications subjected to high heat sources should be avoided or temperatures checked to allow for an adequate safety factor. An alternative clear plastic to use would be Tuffak Polycarbonate with a service temperature of 270 degrees F.