PAINTING ACRYLIC SHEET
The superior clarity and long-term weatherability of acrylic sheet allow for easy painting and printing of fabricated signs, displays, and other decorative items. The following guidelines and correct materials will help produce quality painted acrylic sheet products.
A variety of paints, specifically designed for acrylics, are commercially available. Most are supplied as concentrates and require thinning to obtain the correct viscosity.
As a rule, use mild thinner consisting of an alcohol blend, rather than one containing aromatic hydrocarbons, such as benzene and toluene. Always follow paint suppliers' recommendations about the amount and type of thinner.
Acrylic sheet's extraordinary transparency allows for backpainting (applying paint to the side of the material not exposed to weather). The unpainted outer surface of acrylic sheet provides weather protection for the paint. If you must paint the outer surface, apply a suitable protective coating over the paint.
Use any efficient atomizing spray gun system that uniformly distributes paint, and consult suppliers regarding equipment needed for a particular application.
To obtain enough atomizing air, the available line pressure should be in the range of 60-90 psig. Use a 25 ft., 5/16" minimum diameter hose to avoid pressure drops in the line.
The air compressor should be large enough to provide the volume and pressure required for all guns. Water and oil in the atomizing air will spot the paint and cause uneven distribution. To prevent this, use water and oil traps in the line. Metal screen strainers at the pressure tank and spray gun will ensure equipment cleanliness, crucial to uniform paint dispersion. Clean these items and the spray gun daily.
Install an adequately vented spray booth large enough to fit your biggest work piece. Use a light box to judge paint uniformity on transparent plastics. Be sure the light box conforms to building code regulations for a spray painting area.
Fabricating acrylic sheet using incorrect techniques prior to painting may cause stress in the material, resulting in crazing (numerous tiny cracks) after painting. You can eliminate crazing by following manufacturers' recommendations for fabricating acrylic sheet.
Always clean acrylic sheet before painting to remove dust and assure paint adherence. Use care in cleaning since acrylic sheet is sensitive to solvents like aromatic hydrocarbons, concentrated alcohols, and ketones. Clean parts with a 25 percent solution of denatured alcohol and distilled water. For stains such as oil or grease, use stronger cleaning agents like hexane, aliphatic naphtha, or kerosene. Be sure the sheet is fully dry and clean before painting.
It is important to neutralize electric or static charges that accumulate on the sheet's surface before painting. Dust on the sheet causes paint agglomeration and uneven layers. Since tearing the masking off the sheet will create a static charge, all acrylic pieces should be treated with an ionizing air gun, which safely and effectively neutralizes electric charges. Alternate techniques, such as wiping the sheet with a damp, lint-free cloth or cleaning with a dilute alcohol-water solution, are also effective. Avoid anti-static cleaners since they may leave a residue and cause paint adhesion problems.
Acrylic sheet's protective paper masking is usually used as a protective layer for spray painting flat signs. Paint manufacturers also distribute liquid maskants, which are commonly used to spray paint designs onto acrylic sheet. Maskant is supplied as a thick liquid consisting of water-soluble latex resins in solution. It is applied through the use of air or airless atomizing spray equipment. Because they are water solutions, maskants must be stored above 32° F to prevent freezing.
Clogging may occur due to dried film mixed with the solution. Prevent this with a special nozzle (available from paint equipment suppliers) which reverses the flow of product and cleans out the build-up. Spray the film on evenly to a wet film thickness of 10-12 mils, which will dry to above 4-5 mils.
Drying time is usually about two hours, although it is better to plan overnight drying to assure complete evaporation. Using forced-air heating at 110°F can accelerate drying. Going above this temperature might dry the surface and prevent evaporation of the water in the layers beneath.
To prevent dust accumulation on the plastic surface, leave the dried film in place just prior to painting. Before painting, score the design on the film using an X-Acto knife with just enough pressure to cut the film without scratching the plastic. After painting, leave the film in place until the paint is thoroughly dry or you will get smeared paint and uneven edges.
Practice on a few test pieces to be sure that paint viscosity and air pressure are correct before painting. Too high a delivery rate will result in too much paint and cause paint sag. It can also cause crazing of the sheet due to too much solvent. Too low a delivery rate will result in dry spray (a matte surface caused by too much dusting).
As a rule, use the lowest pressure possible to obtain correct results. Hold the gun 12-14 inches from the workpiece, too close or too far will cause the above listed defects. Move the gun at an even pace and in a straight line. Its movements should never start or stop directly on the sheet surface.
Vary the direction of the spray, horizontally and vertically, to assure uniform coverage. Four or five passes with several seconds between coats will provide sufficient paint. A light box behind the acrylic sheet will help in judging the uniformity and intensity of color.
Screen printing, used for volume production, is fast and economical. For beginners, it is best to purchase a screen from a local screen supply house.
After setup, apply paint with the squeegee in a uniform and even motion in one direction. The paint will pass through the open screen mesh, transferring the pattern onto the acrylic. The most important factors are the paint's viscosity and the size of the mesh openings, which will determine paint flow through the screen and the paint's appearance on the acrylic.
Since many different fabrics are used for screening, and paint viscosity depends on the application and temperature conditions, it is difficult to generalize what these conditions should be. Consult your paint manufacturer for advice on thinning paint.
If paint removal is necessary, do so immediately from acrylic sheet's surface using the paint manufacturer's recommended cleaner. Apply remover with a rag, then wipe off paint using another clean rag. Because paint removers contain organic solvents, minimize the time the remover is in contact with acrylic sheet to reduce the chance of crazing.
NOTE: Acrylic sheet is a combustible thermoplastic. Precautions should be taken to protect this material from flames and high heat sources. The information and statements contained herein are believed to be reliable, but are not to be taken as warranty or representation for which CYRO assumes legal responsibility nor as permission, inducement, or recommendation to practice any patented invention without a license. Users should undertake sufficient verification and testing to determine the suitability for their own particular purpose. Be sure to follow manufacturer's safety recommendations for equipment and materials used with acrylic sheet.
Trouble Shooting Guide
|Weak and brittle maskant||Air bubbles in film||Dilute slightly|
|Film not thoroughly dry||Wait recommended drying time|
|Maskant film too thin||Increase film thickness to 3-5 mils (10-12 mils wet)|
|Too much adhesion||Maskant film too thin||Increase film thickness to 3-5 mils (10-12 mils wet)|
|Coating exposed to UV||Do not store faces outside|
|Poor adhesion||Incorrect paint||Use paints recommended for use with acrylic|
|Dirt or residue on sheet||Clean sheet thoroughly before painting|
|Blotches of paint||Static electricity||Neutralize charges with ionizing gun, or wipe with damp cloth|
|Uneven paint application||Apply paint in more passes using less paint per pass|
|Paint not applied uniformly||Use backlighting to check paint as it is being applied|
|Poor detail||Screen mesh too coase||Neutralize charges with ionizing gun, or wipe with damp cloth|
|Paint too thin||Use less thinner|
|Worn Screen||Replace screen|
|Paint drying on screen||Hot, dry weather||Add retarder to slow paint drying|
|Too much time between screening||Flood screen between passes|
|Crazing||Stress from fabrication||Too much time between screening|
|Flame polishing||Flame polish as last step|
Written by Grant LaFontaine, Sheet Products Technical Service Manager, CYRO Industries.
For more information, contact D. Artz, CYRO Industries, 100 Enterprise Drive, Rockaway, NJ 07866, 800-631-5384, Fax: 973-442-6117, Canada: 800-268-4743, Web: www.cyro.com.