1. Molding Processes and Techniques
When heated to the molding temperature, transparent acrylic plastic becomes soft and flexible, allowing it to be formed into almost any shape. After cooling, it retains the formed shape. For thin sheets, cold bending into single curvature is possible if the bending radius is at least 180 times the sheet thickness. Exceeding these limits during cold bending applies excessive stress to the plastic surface, creating tiny cracks or fissures known as crazing.
2. Heating
Wear cotton gloves to avoid leaving fingerprints on the soft surface.Remove all protective paper and adhesive from the acrylic sheet before heating. If the sheet is dusty or dirty, wash it with water and rinse thoroughly. Dry gently with a soft, absorbent cloth.
Heat the acrylic to the manufacturer-recommended temperature for optimal results. Use a forced-air oven with a temperature range of 120–374°F (49–190°C). Overheating may cause surface bubbles, compromising optical quality.
For uniform heating, suspend sheets vertically using spring clamps attached to their edges and hang them on a rack (Figure 90).For smaller sheets or those without sufficient edge area, place the sheet on a rack covered with soft felt or flannel. Ensure adequate airflow around the sheet.
Figure 90: Suspending Acrylic Sheets
For small projects (e.g., lamp shades):Use a kitchen oven.Infrared heat lamps spaced 7–8 inches apart (18 inches from the sheet) can evenly heat thin sheets.Avoid direct contact with hot water or steam, as this may cloud the acrylic.
3. Molding
Acrylic plastic requires minimal pressure during molding, so simple molds made of pressed wood, plywood, or plaster suffice for basic curves. Complex or compound curves may require reinforced plastic or plaster.
Ensure the mold surface is perfectly smooth by sanding and covering it with soft fabric (e.g., outdoor flannel or billiard felt).The mold should extend beyond the part's trim lines and include provisions to hold the heated plastic firmly against the mold during cooling.Reassemble broken parts with tape, wax or oil the interior to prevent plaster adhesion, and fill with plaster. Once hardened, smooth rough areas and cover with soft fabric to create a reusable mold.
4. Molding Methods
4.1 Simple Bend Forming
Heat the sheet to the recommended temperature, remove from the heat source, and drape it over the prepared mold. Press the heated plastic gently onto the mold and clamp or hold it in place until cooled (10–30 minutes). Do not force-cool.
4.2 Compound Bend Forming
Used for complex shapes (e.g., aircraft canopies or wingtip light covers), this method requires specialized equipment. Common techniques include:
4.3 Stretch Forming
Preheated acrylic is mechanically stretched over a mold. Maintain uniform thickness, as some sections stretch more than others.
4.4 Male and Female Die Forming
Uses costly matched male/female molds. The heated sheet is placed between the dies, which close until the plastic cools.
4.5 Shaped Template Vacuum Forming
Used for aircraft canopies:A cutout template is attached to a vacuum box.The heated acrylic sheet is clamped over the template.Air is evacuated from the box, allowing external air pressure to force the plastic through the template opening. Surface tension shapes the dome.
4.6 Female Vacuum Forming
For non-dome shapes:Place a female mold below the heated sheet.Evacuate air from the mold cavity, forcing the plastic to conform to the mold.
5. Cutting and Drilling
5.1 Sheet Cutting
Straight Cuts: Use a hollow-ground circular saw with side-set teeth for smooth edges.Curved Cuts: A bandsaw is ideal. Cut within 1/16 inch of the final size, then sand to precision using a drum or belt sander.
5.2 Drilling
Unlike soft metals, acrylic plastics have poor thermal conductivity, requiring careful heat management during drilling. For deep holes, cooling is essential, and water-soluble cutting oil is an ideal coolant as it does not degrade the plastic.
Drill bits used for acrylic must be precisely sharpened with no nicks or burrs that could mar the surface finish. As illustrated in Figure 91, the drill tip should be ground with a larger included angle compared to those used for soft metals. Additionally, the rake angle must be set to zero to enable scraping (rather than cutting), minimizing chip adhesion.
Figure 91: Twist drill with 150° angle for acrylic
Patented Unibit® is designed for drilling holes in aircraft windshields and windows. As shown in Figure 92, it can cut holes ranging from 1/8 to 1/2 inch in 1⁄32-inch increments, producing smooth, clean holes with no stress cracks around the edges.
Figure 92: Unibit® drilling acrylic
6. Adhesive Bonding
Polymerizable adhesives are formulations where a catalyst is added to a viscous monomer-polymer liquid to accelerate rapid hardening. Examples include PS-30® and Weld-On 40®, which are suitable for bonding all types of Plexiglas®, acrylic cast sheets, and parts molded from Plexiglas® pellets.At room temperature, the adhesive hardens (polymerizes) in the container within 45 minutes after mixing.Higher temperatures significantly accelerate curing.Bonded joints can typically be handled within 4 hours of assembly.For optimal strength, allow 24 hours before subjecting the joint to stress.This method ensures durable, high-strength bonds for acrylic materials. Let me know if you need additional details!
7. Adhesive Applications
PS-30® and Weld-On 40® joints retain excellent appearance and color stability after outdoor exposure. These adhesives form clear, transparent gel-like bonds, making them essential for applications where the color and visual integrity of the joint are critical.
PS-30® and Weld-On 40® must be applied at temperatures no lower than 65°F (18.33°C). If curing is performed indoors below 65°F (18.33°C), the hardening time will substantially increase, and the resulting joint strength will diminish.
The adhesive must be prepared in strict accordance with the manufacturer's specified ratios, ensuring thorough mixing. Use clean glass or polyethylene mixing containers, as improper containers or tools may introduce discoloration or interfere with curing. Due to their extremely short pot life (approximately 45 minutes), mixed PS-30® and Weld-On 40® must be used immediately after preparation. Pre-assemble all components before mixing the adhesive to minimize time loss during application. For optimal workability, pour the adhesive into joints within 20 minutes of mixing. To achieve maximum seam strength, the adhesive bond must remain free of bubbles. Typically, allowing the mixed adhesive to rest for 10 minutes before application enables bubbles to rise to the surface. If bubbles form within the joint, they can be carefully lifted using a small non-copper wire or similar tool; however, minimize disturbances to the adhesive layer after pouring.
Gap-filling techniques are suitable only for clear Plexiglas®, acrylic materials, or hidden seams. For inconspicuous joints on colored Plexiglas® or acrylics, components must fit tightly using closed V-grooves, butt joints, or radiused edges.
Masking tape can be used to create bonding templates, provided the adhesive surface does not contact the tape's adhesive layer. This is easily achieved by placing a strip of cellophane tape over the masking tape's adhesive side. Careful tape selection is critical: adhesives on standard cellophane tapes may inhibit PS-30® and Weld-On 40® curing. Always test tape systems on sample joints before production to confirm compatibility.
Polymerizable adhesives shrink during hardening. To compensate, freshly poured adhesive should slightly overfill the joint. Excess material can be machined off after full curing (24 hours) if aesthetic requirements demand. Apply only contact pressure during bonding to ensure adhesive remains confined to the joint.
Bubbles tend to rise to the top of the adhesive layer in gap-filled joints. If removed during processing, these pose no issue. Avoid using copper tools, as copper ions may disrupt polymerization.
8. Repair
Whenever possible, replace rather than repair acrylic plastic that has been extensively damaged. A carefully patched section will never equal a new part, either optically or structurally. At the first sign of crack development, drill a small hole (#30 or 1⁄8-inch drill bit) at the extreme end of the crack, as shown in Figure 93. This stops crack propagation and distributes strain over a broader area. For minor cracks, drilling to block the crack may suffice until replacement or a permanent repair is feasible.
Figure 93: Crack Stop-Drilling
(All initial strain causing the crack concentrates at Point A, promoting crack expansion. Drilling a small hole (A1) at the crack's endpoint with a #30 or 1⁄8" bit redistributes strain. Every crack originating from holes or openings should be drilled similarly.)
9. Cleaning
While plastics offer advantages over glass in aircraft use, they lack glass's surface hardness. Exercise care during maintenance to avoid scratches or surface damage. Clean plastics with ample water and mild soap, using a clean, soft, grit-free cloth, sponge, or bare hands. Avoid gasoline, alcohol, benzene, acetone, carbon tetrachloride, fire extinguishers, deicing fluids, paint thinners, or window-cleaning sprays, as these soften the plastic and induce cracking.
Never rub plastics with a dry cloth, as this risks scratches and generates static charges that attract dust. After removing dirt and grease, apply a high-quality commercial wax if no significant scratches are visible. Apply wax in a thin, uniform layer and buff gently with a soft cloth for a polished finish.
10. Polishing
Do not attempt manual polishing or buffing until surfaces are clean. Use soft, open-weave cotton or flannel buffing wheels. Light scratches may be addressed by vigorously rubbing the affected area with a clean cloth dipped in a turpentine-chalk mixture or a damp cloth coated with automotive cleanser. Remove cleanser residue with a dry, soft cloth. Note that acrylic and cellulose acetate plastics are thermoplastic; prolonged buffing generates friction heat, softening the surface and causing visual distortion.
11. Windshield Installation
Replacement panels must match the material originally used by the aircraft manufacturer. Transparent plastics vary widely in properties-thermal expansion, low-temperature brittleness, UV resistance, surface integrity, etc. Refer to MIL-HDBK-17, Plastics for Aircraft, Part II: Transparent Glazing Materials (available from the Government Printing Office) for detailed specifications. Substituting materials with differing properties may lead to operational issues, as manufacturers select materials based on these critical traits.
12. Installation Procedures
When replacing panels, adhere to the aircraft manufacturer's installation method. While specifics vary by aircraft type, follow these principles:
1,Avoid Forced Fitment: Never deform plastic panels to fit frames. If a replacement panel resists installation, replace it or gently heat the entire panel for reshaping. Ideally, cut and install panels at room temperature.
2,Stress Management: Do not subject plastic to excessive compressive stress during clamping or bolting. Over-torquing nuts/bolts beyond 1,000 psi risks damage. Tighten nuts firmly, then back off by one full turn to maintain finger-adjustable tension.
3,Hardware Precautions: Use spacers, collars, shoulder washers, or locknuts to prevent over-tightening bolts. Retain original hardware quantities (bolts, washers, etc.) as specified. For riveted installations, ensure adequate spacing to avoid frame-induced stress.
4,Gasket Integration: Install plastic panels with rubber, cork, or similar gaskets to waterproof joints, dampen vibrations, and distribute compressive stress.
5,Thermal Expansion: Plastic expands/contracts significantly more than metal channels. Install windshield panels into channels with sufficient depth (minimum 1⅛ inches) and a 1⁄8-inch gap at the channel base to prevent detachment during cold contraction or load-induced deformation.
6,Hole Design: For bolt/rivet installations, drill holes 1⁄8-inch oversized and centered to prevent edge binding or cracking. Slotted holes are recommended.
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