Sec Op Lexan Guide

LEXAN* Secondary Operations Guide /resins/include/mainmenu_xml.inc /resins/include/techsolution_xml.inc /resins/include/rate_xml.inc Technical Research Design Guides LEXAN Secondary Operations Guide

Used in applications ranging from cost-saving frameless design to high-style appearance parts, LEXAN® polycarbonate resin is one of the toughest, most versatile of all engineering plastics. In many applications, additional manufacturing steps will be required after molding LEXAN resin parts. This guide reviews a range of functional and decorative secondary operations available for parts made of LEXAN resin.

Contents Introduction Introduction

Assembly Bonding and Cementing Welding Mechanical Fastening Staking/Swaging Specialty Assembly

Finshing Appliqués Painting Metallizing Printing

Machining Drilling, Tapping, Threading and Turning

Troubleshooting Guide Troubleshooting Guide

Introduction

Used in applications ranging from cost-saving frameless design to high-style appearance parts, LEXAN® polycarbonate resin is one of the toughest, most versatile of all engineering plastics. In many applications, additional manufacturing steps will be required after molding LEXAN resin parts. These may include machining and/or assembly, using bonding, welding or mechanical fastening techniques; decorating, painting or other finishing steps. Even though these secondary operations occur in the final phases of a product’s development, they must be carefully considered early in the planning stages of a program. This guide reviews a range of functional and decorative secondary operations available for parts made of LEXAN resin. For more detailed information concerning the commercial application of these techniques, we suggest consulting the Secondary Operations Supplies and Suppliers guide in the Technical Guide Library, or calling GE Plastics Technical Sales Service at (800) 845-0600, or clicking on the GEP Live button.

Bonding and Cementing

Adhesive Bonding Parts molded of LEXAN resin can be bonded to other plastics, glass, metal, wood and other materials using a variety of adhesives. Generally, better results are obtained with solventless (100 percent reactive) materials such as epoxies, urethanes and high-performance adhesives. However, each application has unique requirements for flexibility, temperature resistance, ease of application and appearance, requiring careful adhesive selection and testing. In general, flexible adhesives should be used if there are high impact requirements or when fabric or leather is involved. Five common families of adhesives used for bonding GE resins are shown in Table 1.

Adhesive Bonding Guidelines

Following these adhesive bonding guidelines will typically aid in achieving expected bonding performance: 1. Avoid adhesives containing constituents or curing agents which are incompatible with LEXAN resin.* 2. Laboratory test for desired end-use performance (bond strength, flexibility) under expected end-use operating conditions. 3. Design the joint (size and shape) for the particular stress level and environment. 4. Use curing temperatures below 250°F (121°C) and high (low) temperature adhesives for high (low) temperature applications.

Joint Design Joint Design is critical to the performance of a bond. Factors to be considered in choosing a joint design include the following: Joint orientation will define the type of loading applied to the bond – shear, peel, tensile, etc. The area of the bond should match the adhesive strength and the expected loadings. Aesthetics in the bond area may restrict the choice of joint design. Moldability and mold design can be compromised by some joint designs. Ability to maintain tolerances on the mating parts is critical to some joint designs, such as the tongue and groove. Part design and mold design must anticipate the tolerances required at the joint interface. Gap-filling ability of the adhesive must be considered in the dimensions of the joint. Set time for the adhesive and the need for handling of the parts after assembly may make some joints more advantageous.

Surface Preparation Parts should be thoroughly cleaned before adhesive bonding. All oil, grease, paint, mold releases, rust and oxides must be removed by washing with compatible solvents such as isopropyl alcohol, petroleum ether, heptane with a boiling point of 149°F (65°C), VM&P naphtha, white kerosene, or a mild solution of soap. Bond strength may be improved by mild surface abrasion (sanding), as is customary with other materials.

Solvent Cementing The joint area should be designed so that the two parts fit precisely, without pressure, when they leave the mold. Locating pins (or tongue-and-groove shapes) may be used to align the bonding area and promote rapid part matching. These devices should be kept shallow to avoid solvent entrapment in the mating surfaces. Parts to be solvent welded must be well molded dimensionally, and possess low residual stresses. 1. Methylene chloride,** which has a low boiling point of 104°F (40°C) and an extremely fast evaporation rate. 2. When perfectly mated bonding areas are impossible to obtain, a one- to five-percent solution of LEXAN resin in methylene chloride can be used to promote smooth, completely filled joints. When solvent cementing parts molded of LEXAN resin, use the minimum amount of solvent necessary for good adhesion. Solvent should be applied to only one of the surfaces to be bonded. Figures 1 and 2 illustrate two solvent cementing procedures which usually result in minimum solvent usage.

Procedure Saturate a piece of absorbent material with solvent. Stamp the bonding surface into the saturated material and clamp to the mating part with an even pressure.

Figure 1. Solvent Cementing Procedure 1.

Procedure Make fitting counterparts from wood or metal. Dip the counterparts into the solvent and press against the LEXAN resin part surfaces, applying a thin film of solvent. Allow a few seconds for drying and then assemble the parts for welding, applying pressure evenly in a jig or fixture. Pressure should be applied as soon as the two parts have been put together. Pressure between 200 and 600 psi (1.38 and 4.14 MPa) is typically used. Holding time in the fixture should be approximately one to five minutes, depending on the size of the bonding area.

Figure 2. Solvent Cementing Procedure 2. The cemented joint should be clear and bubble-free. The presence of bubbles indicates either excess solvent and/or insufficient pressure, or too short a pressure hold. Squeeze-out is a sign of excessive pressure. Crystallization at the bond area indicates excessive solvent and prolonged time between solvent application and mating of the parts in the pressure fixture. The cemented parts should be dried for 24 to 48 hours at room temperature. If the solvent-cemented part is to be used at high temperature, a schedule similar to that in Table 2 is suggested. Generally, the solvent removal schedule should ultimately reach a temperature which is equal to or slightly lower than the maximum service temperature. This can be done by raising the temperature 5 to 10°F (-15 to -12°C) per hour, depending on the bond size, until the desired temperature is reached.

Table 2. Typical Curing Schedule.

*Contact GE Plastics Technical Sales Service for compatibility information: (800) 845-0600 or click onto the GEP Live button. **NIOSH, OSHA and EPA standards must be considered when using methylene chloride. Assembly

Table1.

Welding

Welding Methods The use of applied heat to join plastic parts is popularly called welding, although it may also be referred to as fusion welding. The following are descriptions of several of the most common welding techniques for parts molded of LEXAN resin.

Table 3. Welding Procedure Comparisons.

Induction Welding The electromagnetic welding process is based on the principles of induction heating. It is typically used when an application requires structural, hermetic, leak-proof or pressure-tight seals, and is especially common in applications where weld reliability is critical. Induction bonding can be used to weld similar as well as dissimilar materials, with little or no constraint on part size or geometry (Figure 3). In fact, the assembly technique can often accommodate poor tolerances and warp. In addition, the process is tolerant of filler loadings; glass-, talc-, and mineral-filled materials can be welded.

Figure 3. Induction Bonding Joint Design. The key to the process is the preform that is placed in the joint interface. This is a thermoplastic compound containing finely dispersed ferro-magnetic particles that can be supplied as an injection molded gasket, an extruded strand, slip tape, sheet or resin. The second component of the system is the work coil. Powered by a generator, the work coil creates a high-frequency oscillating magnetic field that causes the preform to reach fusion temperature, thereby causing the parts to be welded together. In most cases, the joint can be taken apart and reassembled by reactivating the bond line.

The process is fast, clean and reliable and can be designed for a broad range of manual, semi-automatic or automatic operations.

Hot Platen Welding Hot platen or hot plate welding is a very common process for joining thermoplastic components. It is a simple and inexpensive method that involves direct heating of the plastic to be welded. The mating surfaces of two parts are slightly melted and pressed together. There are two methods of hot platen welding: contact, where parts are brought into direct contact with a heated platen, and non-contact, where a slight clearance is maintained between the parts and the platen. This contact process consists of heating two parts to be joined with a heated insert. The heated platen, at the correct temperature, is placed at the interface of the parts to be joined. The parts are then brought into contact, or in some cases very close to, the hot surface. Stops on the insert and the fixtures which hold the parts determine the depth of the initial melt. The hold time determines the depth of the secondary softening. For many applications, this initial contact produces a good mating surface, as both are made level by the insert. At this point the insert is removed and the parts are pressed together. The stops on the fixtures now determine the amount of material displaced during mating. The entire cycle is usually completed in 15 to 20 seconds. Joint design is important in hot platen welding for mainly cosmetic reasons. The material displaced during the joining process will form a bead at the weld area. It may be necessary to design the joint to hide such a bead, or the bead can be removed after the welding process is completed. Because of the method of heat application, the flat platens, only butt joints and variations on butt joints are usually used. As with other welding techniques, hot platen has advantages and disadvantages that make it a good candidate for some applications and not others.

Non-contact Welding In non-contact welding, a distance of 0.040 to 0.060" (1.02 to 1.52 mm) is maintained between the parts and the platen to compensate for localized swelling of the molten polycarbonate, any irregularities in the joint interface, and to promote uniform heating. Table 4 gives general guidelines for hot platen welding parts molded from LEXAN resin.

Table 4. Process Conditions.

Contact Welding Contact welding LEXAN resin parts results in the formation of a residue on the platen surfaces, which should be periodically cleaned to avoid unsightly deposits on subsequent parts. Stringing or “hairing,” which may occur on part surfaces as the platen is withdrawn, can be removed by trimming or dropping the parts through a flame ring. In cases where this is functionally or aesthetically unacceptable, non-contact welding should be considered. Typical joint designs for hot platen welding are shown in Figure 4. A flanged joint is often preferred for its greater strength, and the weld bead may be enclosed, recessed or covered, depending on the functional and aesthetic requirements.

Figure 4. Hot Platen Welding Joint Design.

Ultrasonic Bonding Ultrasonic bonding is a welding technique used when high bond strength is required. If properly performed, ultrasonic bonding can result in a joint with extremely high tensile strength. This technique can be utilized when LEXAN resin is to be welded to itself and to some grades of ABS/polycarbonate alloys. The necessary equipment consists of a power supply, converter, booster, horn and nest. Ultrasonic bonding operations can be easily automated. Only one of the mating parts comes in contact with the horn. The part transmits the ultrasonic energy to the bonding area, usually by means of an energy director, resulting in a fast, consistent weld. Both mating halves remain cool, except at the joint, where the energy is quickly dissipated into heat and the plastic melts. The most important element in designing LEXAN resin parts for ultra-sonic bonding is the joint design. There are usually three major requirements for the joint area: 1. A small initial contact area energy director (a point or a line). 2. Uniform contact. 3. A means of alignment. The following suggestions should also be observed: 1. Allow for the flow of molten material. Use flash traps where necessary. 2. Locate the joint as close as possible to the point where the ultrasonic horn can be applied. 3. Provide a suitable horn contact area. 4. Provide proper proportioning of joint elements. Avoid tight joints and large mating surfaces. 5. Allow for a slight dimensional change around the circumference of the weld. 6. Provide rigid fixturing or nesting of the bottom part to ensure a strong weld. The energy director designed into an ultrasonic joint serves as a concentrator: it focuses most of the ultrasonic energy to the joint area and causes the flow of material to complete a seal. Therefore, an energy director for LEXAN resin components should be triangular in shape with a sharp point on the energy bead. This will allow for easier flow during welding. Weldability is dependent upon the concentration of the vibrational energy per unit area. Since LEXAN resin has a higher melting point than many other thermoplastics, more energy is required to cause the material at the joint to flow. Figure 5 shows a simple energy director. The dimensions given in Figure 5 are only a general guide and not the absolute dimensions suitable for every application.

Figure 5. Ultrasonic Joint Design. The interference or shear joint is often used when high bond strength and a watertight seal are required. The strength of this bond is directly proportional to the area of the weld. A flash trap (See Figure 8) should be designed into the joint when flash cannot be tolerated. Nesting is extremely important for the shear weld: the outer wall must be well supported to prevent flexing or distortion.

Linear Vibration Welding. Vibration welding is an innovative assembly technique for joining large plastic assemblies. In this process, one of two flanged components is rubbed against the other in a reciprocating motion (Figure 6) at a frequency of 120 or 240 Hz.

Figure 6. Vibration Welding – Linear Mode. Frictional heat is generated by applying pressure of 200 to 250 psi (1.38 to 1.72 MPa) to the vibrating parts and fusion takes place. A weld time of 2 to 3 seconds is typical, with a linear displacement amplitude of 0.120 to 0.200" (3.05 to 5.08 mm) in the plane of the joint at 120 Hz and 0.035 to 0.070" (0.89 to 1.78 mm) at 240 Hz.

Dissimilar materials may also be joined to LEXAN resin components by vibrational welding and a special dovetail/tongue-in-groove joint design (Figure 7). Using this method, a mechanical bond is formed by melting the material with the lower viscosity into the material with the higher viscosity.

Figure 7. Joint Design for Vibration Welding Dissimilar Materials to LEXAN Resin. Parts to be welded must be held securely by a holding fixture. Aluminum plates or epoxy castings can be used for support.

Figure 8. Vibration Welding Joint Design.

Mechanical Fastening

Thread-Cutting Screws Thread-cutting (TC) screws are often used in parts molded from LEXAN resin, with Type-25 or -23 being most common because they cut a clean thread with negligible material deformation. Thread-forming (TF) screws should be avoided because of the high induced stress developed by the forming operation of the thread. If TF screws are to be used, contact your local GEP Technical Service Representative, or click onto the GEP Live button. Generally, the designer should follow these recommendations in planning for TC screws: The receiving hole diameter should be equal to the screw pitch diameter. The boss outside diameter (O.D.) should be strong enough to with-stand possible hoop stresses developed by screw insertion. Usually, a boss O.D. equal to twice the screw major diameter is sufficient. A length at least twice the screw major diameter should be provided for thread engagement. Repeated assembly operations should be avoided. The minimum torque possible should be used to keep screw assembly stress within the design limits of LEXAN resin. A countersink or counterbore is suggested to prevent the fastener from contacting the top surface of the boss. Either of these features leaves a cylinder of material above the initial thread contact point, providing some degree of reinforcement at a sensitive area of the boss. Thread-cutting screws should not be used when repeated disassembly may occur. Unless extreme care is taken, a second insertion can result in the cutting of a second set of threads. This greatly weakens the structure, and will reduce torque retention. If repeated removal and insertion is unavoidable, a Type-23 screw should be used initially, and then replaced with a standard machine screw. An insert is preferred when repeated assembly is anticipated. Table 5 gives design and performance parameters for thread-cutting screws.

Table 5. Design and Performance of Thread-Cutting Screws (1/4" Thread Length).

Machine Screws Machine screws are commonly used to assemble components made of LEXAN resin. Suggested clearance hole diameters are provided in Figure 9. When machine screws are engaging LEXAN resin threads, torque specification is a function of: 1. Bearing force of the screw head or washer. 2. Thread shear stress.

3. Boss tensile stress.

Figure 9. Suggested Clearance Holes for Machine Screws.

Ultrasonic Inserts Ultrasonic insertion can be a fast, economical method of embedding metal inserts onto parts molded from LEXAN resin. In many applications, this technique offers a high degree of mechanical reliability with excellent pull-out strength and torque retention, combined with savings resulting from rapid production cycles. If the assembly is properly designed, ultrasonic insertion can often result in a lower residual stress compared to molded-in or pressed-in techniques. This is because a thin film of homogeneous melt is produced around the insert. The ultrasonic vibrations cause a thin section of melt to flow into the undercuts or flutes in the outer wall of the insert and lock it in place. Removal of the energy source causes the molten resin to rapidly solidify around the insert. The more undercuts provided, the more pull-out strength for the inserted metal piece. For most applications, the ultrasonic energy should be transferred through the metal insert. Due to the wear on the horn as metal touches metal during the insertion process, steel horns are usually recommended. Since steel horns cannot be driven at high amplitudes, a low amplitude is usually used for insertion. A part must be rigidly supported during the insertion operation. There are two basic insert designs: straight and tapered. Figure 10 and Table 6 provide design and performance data on the straight type of insert. Figure 11 and Table 7 provide this data for tapered fasteners.

Figure 10. Straight Ultrasonic Insert Design.

Table 6. Performance Data for Straight Ultrasonic Insert.

Figure 11. Tapered Ultrasonic Insert Hole Design.

Table 7. Performance Data for Tapered Ultrasonic Insert.

Thermal Inserts Thermal insertion and thermal staking (or heat staking) are alternatives to ultrasonic insertion and staking. In these methods, the material is melted directly, using a hot mandrel. Insert design, part design and strength are typically similar to those for ultrasonic methods. Thermal insertion requires less expensive equipment than ultrasonics, but at the cost of longer cycle times.

Staking/Swaging

Ultrasonic Staking Ultrasonic staking (Figure 12) is the process of melting and reforming a boss or stud of LEXAN resin to mechanically capture another component of an assembly, usually metal. A hole in the metal part receives a LEXAN resin stud molded into the plastic part. The surface of this stud is vibrated out-of-phase with a horn using high amplitude and a relatively small contact area. The vibrations cause the progressive melting of the plastic under light, continuous pressures. The stud reforms in the configuration of the horn tip. Tip configuration of the horn will depend upon the application, the grade of resin and the stud configuration. Rigid support of the LEXAN resin part is necessary during the staking operation.

Figure 12. Typical Ultrasonic Staking Operations. Many users find that the standard and low-profile staked head-forms (Figure 13) satisfy the requirements of their applications. The standard head-form produces a head having twice the diameter of the original stud. The low-profile head-form has a head diameter that is 1.5 times the stud diameter. The low-profile head-form has an appearance advantage over the standard head-form. However, the standard head-form has approximately three times the strength of the low-profile head-form.

Figure 13. Standard and Low Profile Staked Head-Forms.

Specialty Assembly

Rivets When riveting parts of LEXAN resin, care should be taken to avoid the high stresses inherent in most riveting techniques. This can usually be accomplished by following the procedures indicated in Figure 14. Using a shouldered rivet (Figure 15) limits the amount of stress imposed on the part. Aluminum rivets also limit the force that can be applied, since the aluminum will deform under high stress. In general, the rivet head should be 2.5 to 3 times the shank diameter, and the flared end of the rivet should have a washer to avoid high, localized stresses. Clearance around the shaft should allow easy insertion, but should not be so great as to allow slippage of the joined parts.

Figure 14. Standard Rivet.

Figure 15. Shouldered Rivet.

Boss Cap A boss cap is a cup-shaped metal ring which is pressed onto the boss by hand, with an air cylinder, or with a light-duty press (Figure 16). It is designed to reinforce the boss against the expansion force exerted by screws.

Figure 16. Boss Cap.

Double-Sided Tape Double-sided pressure-sensitive tapes are available to address a variety of environmental and load-bearing requirements. Advantages of double-sided tape include distribution of stress, compensation for variations in coefficient of thermal expansion, ease of assembly, vibration damping and economics. Double-sided tapes are often used for automotive emblems and body side moldings. Pressure-sensitive tapes, like all adhesives, require surface cleaning before assembly for optimum strength.

Hook-and-Loop-Type Fasteners Hook-and-loop-type fasteners, typified by Velcro* fasteners, are available from a number of suppliers in a wide variety of fastening configurations and load-bearing capabilities. In addition for ease of assembly, these types of fasteners provide the advantage of multiple disassemblies.

*Registered Trademark of Velcro USA, Inc.

Appliqués

Hot Stamping Hot stamping is a dry system which imprints a permanent high-quality image on the surface of a molded part using a heated die and pressure. Generally no masking is needed for this process, and the part can be handled immediately after stamping. However, the pressure necessitates the use of jigs or fixtures to support the surface of the molded part. When using this process for adhesion, it is important to use a compatible foil. Hot stamping foils (Figures 17 and 18) or roll leaf are available in a wide variety of color tones and patterns specifically tailored for LEXAN resin. The basic types are pigmented, wood grain and metallic foil. They may be used for inlaid lettering, wide surface coatings and metallic borders.

Figure 17. Hot Stamping Procedure.

Figure 18. Stamping Foil Layers.

Sizing The mold should be polished in the stamping area to provide a high-quality surface finish, and care should be taken to design part walls thick enough to withstand stamping pressure. Recommended pressure on the die surface (not run pressure) ranges from 250 to 900 psi (1.72 to 6.21 MPa), depending on the type of decoration. Stamping conditions could vary temperature from 300 to 450°F (149 to 232°C), and dwell time from 0.5 to 1.5 seconds. Additional considerations include: 1. Check the bond to be certain there are no gaps between the aluminum plate and the silicone pad. 2. Avoid mold release or other contaminants on the surface to be decorated. 3. Design the stamping die to avoid entrapping air between the part and the foil.

Decals A decal is pigmented or printed decoration that has been pre-cut or pre-printed on a plastic stock backed with pressure-sensitive adhesive and release sheet. Printed on a piece of clear plastic film, decals are unlimited by color or quality of print, and are often used on LEXAN resin parts. In high-impact applications, it is important that the chemical compatibility of the decal not compromise the impact resistance of the LEXAN resin. Two important components that affect chemical compatibility are the pressure-sensitive adhesive and the plastic film. Polyester films, such as MYLAR*, have demonstrated good compatibility. Plastic films made of PVC should be avoided because the plasticizers are generally not compatible with LEXAN resin. As with all adhesives, decals must always be tested under operating conditions.

Hot Transfer Hot transfers have a design printed on the transfer film rather than a pigmented coating or metallic layer. The design is complete before it goes on the part, thereby facilitating the application of intricate patterns in one operation.

Hot transfer is a relatively expensive method compared with silk screen-ing or hot stamping, but it eliminates the problem of color registration by printing on the surface transfer sheet as a continuous web. A wide range of colors and textures may be applied to LEXAN resin parts by using appropriate paints and conventional spray painting techniques. Special clear coatings may also be applied to: 1. Improve weatherability. 2. Improve chemical and abrasion resistance. 3. Impart electrical conductivity. 4. Change the surface finish – reduce gloss. Mold temperature for glass-reinforced grades of LEXAN resin must be maintained between 200 and 240°F (93 to 116°C) to promote surface smoothness.

*Registered Trademark of E.I. du Pont de Nemours & Co., Inc.

Painting

Paint Selection The primary consideration in selecting a paint or coating for a LEXAN resin part is the compatibility of the solvent system with polycarbonate. The solvent should evaporate easily and leave the part completely. In general, water-base and solvent paints based on aliphatic hydrocarbons (mineral spirits, heptane, hexane and alcohols) have good compatibility. Chlorinated and aromatic solvents, as well as ketones, should generally be avoided, although they may sometimes be used in other solvent systems as adhesion promoters to etch the surface. Paints should be tested for chemical compatibility before use to determine if they will craze, crack or weaken the substrate. Since molded-in stress varies greatly depending on part design and molding conditions, screening tests on actual parts must be conducted prior to final selection of a paint system. The effect of the system on the LEXAN resin part may then be evaluated by simple physical tests such as drop-ball impact. However, only actual end-use testing will confirm the acceptability of a system. In most clear applications, better results are obtained by second-surface painting. Since LEXAN resin is transparent, the first surface of a part can be masked and the underside, or second surface, painted. This process results in a clear, tough layer of LEXAN resin protecting the painted surface. In addition to many attractive colors, metallic and metal flake paints may also be applied second-surface to LEXAN resin parts. Again, these paints can affect performance. Paint selection for all applications, and especially where impact is a consideration, is dependent upon both the desired surface appearance and the effect it has on the end-use performance of the substrate. Successful results with this process will be dependent upon proper surface preparation. It is also important to obtain paint recommendations specifically for LEXAN resins from the equipment or ink supplier. It is important to follow the procedures listed below prior to painting LEXAN resin part: 1. Check that spray equipment air lines are free of water or oil. 2. Check for dust, since an accumulation of particles can make it difficult to obtain a smooth, blemish-free finish. If necessary, dust particles can usually be blown off with an ionizing gun. (Antistatic agents applied immediately after molding have also been successfully used where needed.) 3. Do not use mold release. If the LEXAN resin part is clean, special surface preparation is generally needed before painting. If the surface has become contaminated, either wipe the part with alcohol or clean ultrasonically.

Curing

Paints used on LEXAN resin can be cured using either air-dry or bake techniques. Circulating warm air ovens are the most economical method of drying. Curing conditions will vary depending on the coating and part design. Bake temperatures of 250°F (121°C) are acceptable for most standard grades of LEXAN resin. Any assembly operation which induces stress on a part should be performed after painting.

Chemical- and Scratch-Resistant Coatings Several chemical- and scratch-resistant coatings have been developed for transparent plastics. These coatings have glass-like optical qualities and offer a significant improvement in chemical and scratch resistance over uncoated resin. Licensed coating technology is available for use on parts made from LEXAN resin. Royalties may have to be paid to the appropriate company when their technology is used; therefore, each supplier should be contacted prior to selection. In addition, several clear scratch-resistant coatings are commercially available that have been used successfully on LEXAN resin parts. These differ in chemical and mar resistance as well as ease of application. Surface hardness determines the scratch resistance of coatings. If the coating is applied in too thick a section, it can reduce impact resistance by initiating a notch in the surface coating which can propagate into the LEXAN resin part. Chemical- and scratch-resistant coatings must also be evaluated for compatibility, effect of environmental exposure, adhesion, effect upon impact resistance of the molded part and, ultimately, effectiveness under actual operating conditions.

Vapor Polishing Most minor surface scratches can be removed from LEXAN resin parts by vapor polishing as follows: 1. Clean the part with a compatible solvent, finishing with a mild soap-and-water washing (1 percent liquid Joy* in water). 2. Dry the part thoroughly. 3. Remove all dust from the surface – preferably with an ionizing air gun. 4. Boil a container of methylene chloride in a hood or well-ventilated area.** 5. Suspend the LEXAN resin part in the methylene chloride fumes for less than three seconds. (Do not allow the part to contact the boiling liquid.) Exposure time is critical and must be controlled closely. 6. Remove the part to dry – allowing time for the methylene chloride to evaporate before handling. Keep in a clean area relatively free from dust. After initial evaporation, drying should be gradual (5 to 10°F per hour), accelerated by the use of an air circulating oven. A final drying cycle of one hour at 250°F (121°C) is required to reduce surface stresses as well as entrapped methylene chloride. 7. Careful handling during the polishing operation is necessary to avoid transfer of fingerprints to the vapor-softened surface. CAUTION: All methylene chloride should be removed if the part is to be painted or recoated.

*Registered Trademark of Proctor & Gamble. **NIOSH, OSHA and EPA standards must be considered when using methylene chloride.

Metallizing

Vacuum Metallizing Vacuum metallizing coats a plastic part with a thin layer of metal, usually aluminum. The part may be first sprayed with a clear basecoat to promote adhesion of the metal to the plastic substrate. The metal is then vaporized in a vacuum chamber, and the condensed metal is deposited on the surface of the part. After the part is removed from

the vacuum chamber, it is usually sprayed with a top coat to protect the metallized layer from moisture and marring. The top coat may also be tinted to vary the appearance of the metallized surface. Since LEXAN resin is naturally transparent, vacuum metallizing can be either first or second surface. Second surface is usually preferred because a tough layer of LEXAN resin protects the vacuum metallized layer. In some second-surface applications no base coat is needed, and the metal layer can be vacuum deposited directly onto the LEXAN resin part. However, glass-reinforced grades of LEXAN resin require a base coat to promote surface smoothness. The mold for a vacuum metallized part should have a highly polished surface. Gating should also be considered to avoid gate blush, because the vacuum-deposited layer will accentuate minute defects in the part. It is also important that the parts be kept clean and bagged after molding. If the surface should become contaminated, clean it with isopropyl alcohol. If the surface is scratched in machining operations, vapor polish before vacuum metallizing. The base and top coats for vacuum metallizing are generally clear paints with solvent systems compatible with LEXAN resin.

Cathode Sputtering Cathode sputtering is a technology for the deposition of metal on plastic. A gas plasma discharge is set up between a cathode, made of the material to be sputtered, and an anode which serves as both an electrode and the support for the plastic substrate. Positively charged gas ions are accelerated into the cathode, causing atoms of the metal to be knocked off and condensed on the substrate, forming a metal film. Although cathode sputtering is more expensive than vacuum metallizing, it typically promotes better adhesion of the metal. Preliminary steps are similar for both processes.

Flame Spray/Arc Spray Flame spray is an alternative to vacuum metallizing that offers greater flexibility in metallizing portions of a molded part. This process utilizes a metal powder that is metered into a special spray gun, melted by contact with super-heated inert gas, and sprayed directly onto the LEXAN resin part. The metal fuses onto the surface of the part and forms a hard, dense metal coating. The arc spray process, another variation of metallizing, is similar to flame spray. This process consists of feeding two metal wires through an electric arc which melts the ends, causing a layer of metal to be sprayed directly onto the part through a hand-held pistol. Unlike the vacuum metallizing, both flame and arc spray produce a rough sandpaper-like finish. Regular metal-finishing techniques can follow if a polished surface is desired.

Electroless Plating Electroless plating normally consists of surface modification, activation, and metallization. Surface modification typically involves the use of a solvent pre-etch to clean, soften, and disorient the outer skin of the plastic part followed by an acidic etch to roughen the surface. The modification stage is critical to promote adhesion of the metallic coating to the plastic substrate. During activation a palladium-based catalyst is absorbed onto the modified surface. The catalyst is used to initiate the electroless plating process. Finally, metal deposition occurs when the catalyzed plastic part is placed into the electroless bath containing metal ions and a reducing agent. In the presence of the catalyst, a chemical reduction occurs depositing metal atoms on the surface of the plastic part. The deposited metal itself continues to catalyze the reduction reaction so that the deposition becomes self-sustaining. Since intimate contact occurs between the electroless metal solution and the plastic part to be plated irrespective of design complexity, metal deposition takes place at equal rates on all exposed surfaces. This provides a coated plastic part with a consistent metal thickness independent of geometry. Electroless metals are typically matte in appearance and well-adhered to the plastic substrate. Copper and nickel are by far the most common metals deposited onto LEXAN resin parts by electroless deposition. In many cases, nickel over copper plating is used for EMI/RFI shielding applications. For shielding plastic parts, electroless copper acts as the primary shielding medium while electroless nickel provides enhanced durability and corrosion protection.

Printing

Today’s advanced printing processes and inks provide an economical means of applying decorative designs or lettering to the surface of plastic parts. Two of the primary methods that can be used on molded LEXAN resin parts are silk screen and pad printing.

Pad Printing Pad printing is a simple, low-pressure process for single-color printing, although additional colors can be sequentially overprinted with special, rapid-cure inks. (See Figure 19.)

Figure 19. Pad Transfer Printing. In the pad-transfer process, a metal or “cliche” plate containing an etched pattern is covered with ink. A “doctor blade” then removes the ink from all but the etched pattern. A soft silicone rubber pad contacts, presses on and picks up the ink from the etched recesses in the plate. This pad, now carrying the inked pattern in reverse, is pressed against the part to be printed, transferring the pattern to its surface. The shape and flexibility of the rubber pad permit printing onto fairly irregular surfaces (such as those of golf balls), although sharp corners are to be avoided as they can cut the pad.

Screen Printing For many applications, silk screening offers an inexpensive, quality print, good physical properties, and exerts only

slight pressure on the part during decoration. However, silk screening is a wet system and must be air- or forced-air dried after each color pass. (See Figure 20.)

Figure 20. Screen Printing. It is accomplished by means of a fine-mesh synthetic material held in a frame. A pattern is produced by selectively sealing some holes in the mesh, leaving those of the design area open. Ink is placed in the screen frame and a fixture positions the screen in contact with the plastic part. A squeegee is moved across the screen, forcing ink through the open holes of the pattern and onto the part surface. The screen is then lifted, and the part is removed and dried. The screen printing process is limited to parts having a planar surface, and to only one color per step. For multi-color designs, a separate screen is required for each color, with time allowed for drying between each step. The screen printing equipment and ink suppliers are the recommended resources for products compatible with LEXAN resins.

Ink Selection Selection of a printing technique determines what type of ink is used. A wide variety of inks from different manufacturers have been found to be compatible with LEXAN resins. A separate Secondary Operations Supplies and Suppliers Guide can be found in the Technical Guide Library.

In-Mold Decorating With In-Mold Decoration (IMD) technology, a printed film is inserted into the cavity of a molding tool and plastic is injected behind the film. Design that incorporates IMD with second-surface printing allows the graphics to be sealed behind the hard film, so that they can retain a crisp, colorful and new appearance through years of heavy use. Another key benefit of IMD is that it eliminates post-molding steps such as applying adhesive labels or pad printing onto bezels. It also allows for snaps and extensions behind the graphics. For more information on In-Mold Decoration and LEXAN film manufactured by GE Structured Products, call GE Structured Products Technical Support at (800) 451-3147, or Customer Service at (800) 323-3783, or click onto the GEP Live button.

Drilling, Tapping, Threading and Turning

Drilling, Tapping and Threading Drilling is the most commonly used machining operation because small-diameter holes are more easily drilled than incorporated into the mold design. Standard high-speed twist drills often perform satisfactorily. Drill life with LEXAN polycarbonate resin is five to six times greater than with low carbon steel. Spindle speeds for drilling will vary with the surface finish desired and the degree of induced stress acceptable. Surface speeds of 200 to 300 in/min (5080 to 7620 mm/min) are usually used for drills less than 1/4 to 1/2" in diameter, when machined dry. A cooling medium should be used with speeds of 500 to 700 in/min (12,700 to 17,780 mm/min) for drills under 1/4" in diameter, and 1500 to 1600 in/min (38,100 to 40,640 mm/min) for drills 1/4 to 1/2" in diameter. A feed rate of 0.001 to 0.0015" (0.0254 to 0.0381 mm) per revolution will typically give the best results. Use of a coolant is suggested to lower induced stress and tool temperature when drilling. Commonly used cooling media are air or air-water spray mist. Standard cutting oils are not compatible with LEXAN resin. Conventional steel-working taps are recommended for tapping LEXAN resin. Taps that produce threads with slightly rounded root diameters are preferred. Conditions for use of various tap drill sizes are shown in Table 8. Unlike drilling, a lubricant is recommended during tapping to overcome frictional resistance and reduce tap wear. Contact GE Plastics Technical Sales Service, (800) 845-0600, or click onto the GEP Live button, for information on the compatibility of specific lubricants.

Table 8. Conditions for use of Various Tap Drill Sizes.

Turning Parts made from LEXAN resin can be turned with ease and precision on most conventional metal-turning lathes equipped with variable speed control. LEXAN resin parts can be cut without coolant at turning speeds of 1500 to 2500 in/min (38,100 to 63,500 mm/min). A coolant, preferably water, should be used with higher speeds to minimize strains and reduce surface galling. The finest surfaces are obtained using a round-tip cutter, a high turning speed, a shallow cut and a low cross-feed rate (Table 9). Radii of 15 to 30 mils are suggested for round cutters.

Table 9. Suggested Cross-Feed Rates. An indication of proper turning procedure is scrap in the form of a continuous ribbon, or ribbon-like chips. This is evidence that minimal stresses are being induced in the part. Under optimum conditions, the ribbon chip will exhibit ductile elongation. Segmental chips are an indication of intermittent tool slippage and a warning that stress which may cause surface imperfections is being induced into the part.

Milling/Routing Milling can be used for either roughing or for achieving extremely high-quality surface finishes. Although virtually any standard milling cutter may be used, best results are produced with high-speed steel end mills of the four-flute type with a 15° rake angle. The use of high rotating speeds or low feed rates is advisable for end-milling LEXAN resin. Higher feed rates can be used, but only when used with increased milling speeds. Suggested milling conditions are provided in Table 10. Routing should not be attempted without using a liquid lubricant-coolant. The use of a lubricant, such as a compatible light machine oil, reduces gumming and improves surface appearance.

Table 10. Milling Suggestions for LEXAN Resins.

Cleaning It is important that LEXAN resin parts be thoroughly cleaned prior to decoration.

Sanding Sandpaper or silicon carbide (emery) abrasive-coated belts may be used to remove sprue projections, toolmarks, or for flat surfacing sheet stock. Coarse abrasives will often produce scratches difficult to remove, while too-fine grades will cause the belts to fill. For a highly polished surface, a grit size not over No. 180 is suggested. Hence, it may be desirable to use a coarse-grit sandpaper first and follow with a finer one. Belt speeds of approximately 3000 surface-feet-per-minute (sfm) are usually adequate, but somewhat faster speeds (4000 sfm) can be used so long as the pressure applied is not excessive. Hard sanding must be done with light pressure in order to avoid clogging, especially with extremely fine grades. For

example, 100 grit carborundum paper generally works well with light pressure, but clogs with heavy pressure. A 240 grit paper exhibits slight clogging even with very light pressure, but continues to produce a fine satin finish. Crocus cloth also shows a tendency to clog, but the use of a kerosene lubricant can prevent clogging and produce a fine satin finish.

Filing A single-hatched file is much better for filing LEXAN polycarbonate resin parts than a cross-hatched file, since the latter has a tendency to clog under heavy pressure. A No. 2 pillar file has no tendency to clog when used with light pressure. A sharp file is recommended.

Cutting and Buffing Polishing can be classified into two phases: surface preparation and final polishing or buffing. Secondary cutting can be accomplished on a standard high-speed buffing machine. A sewed buff muslin cotton wheel turning at 5000 to 6000 surface-feet-per-minute (sfm) used with a cutting compound and moderate applied pressure has been found to give good results. Caution should be exercised on sharp edges to avoid burring and gumming. For polishing, a loose cotton buff wheel used with Moco 862* color compound gives good gloss. Avoid excessive pressure and speed which can cause drag. Good results are also possible using a ventilated buff wheel running at 4700 sfm.

*Available from The Mosher Co., Inc.

Troubleshooting Guide

DISCLAIMER: THE MATERIALS AND PRODUCTS OF THE BUSINESSES MAKING UP THE GE PLASTICS UNIT OF GENERAL ELECTRIC COMPANY*, USA, ITS SUBSIDIARIES AND AFFILIATES ("GEP"), ARE SOLD SUBJECT TO GEP'S STANDARD CONDITIONS OF SALE, WHICH ARE INCLUDED IN THE APPLICABLE DISTRIBUTOR OR OTHER SALES AGREEMENT, PRINTED ON THE BACK OF ORDER ACKNOWLEDGMENTS AND INVOICES, AND AVAILABLE UPON REQUEST. ALTHOUGH ANY INFORMATION, RECOMMENDATIONS, OR ADVICE CONTAINED HEREIN IS GIVEN IN GOOD FAITH, GEP MAKES NO WARRANTY OR GUARANTEE, EXPRESS OR IMPLIED, (I) THAT THE RESULTS DESCRIBED HEREIN WILL BE OBTAINED UNDER END-USE CONDITIONS, OR (II) AS TO THE EFFECTIVENESS OR SAFETY OF ANY DESIGN INCORPORATING GEP MATERIALS, PRODUCTS, RECOMMENDATIONS OR ADVICE. EXCEPT AS PROVIDED IN GEP'S STANDARD CONDITIONS OF SALE, GEP AND ITS REPRESENTATIVES SHALL IN NO EVENT BE RESPONSIBLE FOR ANY LOSS RESULTING FROM ANY USE OF ITS MATERIALS OR PRODUCTS DESCRIBED HEREIN. Each user bears full responsibility for making its own determination as to the suitability of GEP's materials, products, recommendations, or advice for its own particular use. Each user must identify and perform all tests and analyses necessary to assure that its finished parts incorporating GEP materials or products will be safe and suitable for use under end-use conditions. Nothing in this or any other document, nor any oral recommendation or advice, shall be deemed to alter, vary, supersede, or waive any provision of GEP's Standard Conditions of Sale or this Disclaimer, unless any such modification is specifically agreed to in a writing signed by GEP. No statement contained herein concerning a possible or suggested use of any material, product or design is intended, or should be construed, to grant any license under any patent or other intellectual property right of General Electric Company or any of its subsidiaries or affiliates covering such use or design, or as a recommendation for the use of such material, product or design in the infringement of any patent or other intellectual property right. * Company not connected with the English company of a similar name. Lexan*, Noryl*, Noryl EF*, Noryl GTX*, Noryl* Xtra, Valox*, Ultem*, Xenoy*, Cycolac*, Cycoloy*, Enduran*, Gelon* and Geloy* are Registered Trademarks of General Electric Co., USA.