Finishing the Job

Decorating: put your best foot forward

Application of a decorative treatment to a resin part can take many forms, but at its simplest, decorating is about putting a pigment or coating either directly on the part surface, or onto a substrate, for example, a label. Labeling has the advantage of standing the image off the part, dividing the process between the supplier and the firm applying the label. Costs and results vary with label material, volume, and the need for color or special effects. Durability requirements and base resin properties often dictate the label material, while part geometry usually controls placement. Direct thermal, thermal transfer, laser, offset litho and screen printed products are some of the available print options, and the process can be as automated as a processor desires. Winnipeg-based Western Label (122), for example, offers a variety of print processes, materials and adhesives for specific resin applications. Automatic labeling equipment is commonly microprocessor controlled and adaptable to a range of label sizes. An example is Flex-O-Mark’s (Mississauga, Ont., 120) FL100 automatic labeler, which uses an industrial PLC. The FL 100 can apply labels at up to 150 per minute, and is compatible with many types of product sensors such as photocells and proximity switches.

Direct marking of resin parts and assemblies generally breaks out into three categories: pad printing, hot stamping, and screen printing. Hot stamping has the advantage of providing a high-quality image from a “dry” ink plate process, but can only be done on a flat or constant radius surface. Primary advantages of the system are speed, simplicity, and the ability to produce special effects such as metallic foils. United Silicone’s (121) Model US25, two and one-half ton hot stamping machine is an example of modern equipment with automatic set up and troubleshooting with an LCD readout. The unit can apply foil or conventional heat transfers to flat or slightly curved surfaces. Besides multiple design and special effects options, hot stamping can be used where conventional labeling is difficult to automate cost effectively. An example of a more difficult high throughput application is the marking of large parts such as lawn furniture and rotomolded products. Custom or flexible machinery is widely available for unusual part shapes or sizes. Cassco Machines (Toronto, 123) ‘S’ Series machines are an example, and can be customized with modular accessories such as turntables and automatic feeders.

Screen printing uses ink forced through a photo-etched screen in a traditional “wet” process. Large surfaces can be marked, and screens can be very cost-effective, but automation of the process my be costly, particularly when tooling for part shapes without flat surfaces or constant radius curves. An example is the oval blow-molded bottle. Custom tooling can be reduced with modular equipment, such as the MOSS MS 1050 from Dependable Machine (124). The MS 1050 uses stepper motors and internal memory to recall set-up parameters, and can screen multiple colors.

Pad printing, like hot stamping, uses a plate, but picks up the image on a silicone pad and impresses it onto a surface. Controlled solvent evaporation leaves a high-quality image with good rub-resistance. Pad printing’s popularity is based on low cost and the ability of the process to mark irregular surfaces. Tooling costs are moderate, and machinery can range from manual lever-operated equipment to automatic units driven by stepper motors. Pad printing equipment is available with both “open” and “closed” ink systems. Closed systems use a hermetic ink reservoir to minimize exposure to the air, allowing better ink consistency due to minimal evaporation, and faster start-up at the next shift of printing. Pad life on smooth resin parts can approach a million impressions. Steel and photopolymer plates are available with durabilities of up to a million impressions or more. Sterling Marking Products’ (London, Ont. 125) Tosh pad printer is an example of a high volume floor mounted unit using a closed ink system and digital control with a LCD readout. Most vendors offer services which include plate manufacturing, typesetting and often design as well. Robert Pope & Co., (Toronto, 126) for example, distributes Kent pad printing equipment, and also offers complete support services, including color-matched inks.

Welding is a low-cost option

Bonding resin components can be accomplished by adhesives or welding, but for high volume work with commodity polymers, welding has notable cost advantages.

Hand-held extrusion welders are popular with the fabrication community, where custom work with sheet and profiles preclude cost-effective automation. As their name implies, hand-held units are extruders in miniature, feeding molten resin from welding rod either manually or by automatic feeder. Drader Injectiweld (127) offers the “smallest hand-held extrusion welder in the world”, used primarily by processors. Rotational molders and corrugated pipe manufacturers are primary markets, although custom fabricators use the equipment for a broad range of general purpose welding. Interchangeable tips allow different weld types such as spot, butt, and lap welds. A key advantage to extrusion welding over hot air systems is the elimination of resin oxidation where oxygen in the hot air stream meets molten polymer. Extrusion welds can yield higher strengths than even hot nitrogen gas systems, where residual oxygen or entrained gas can be a problem.

Another way to generate the energy required for welding is by very high frequency sound waves. Ultrasonic welding is useful where welds must be precise and repeatable. The process is also well suited to automatic or semi-automatic assembly. The directed energy of ultrasonics can be harnessed for multiple applications. Dukane’s (Plastisonics Equipment , Brampton, Ont. 128) SRT-300 Rotary Parts Handling System for example, can be integrated with the firm’s assembly systems to allow welding, staking, swaging, degating, and spot welding.

Medical devices are a popular application for ultrasonic welding. Another supplier of ultrasonic equipment is Forward Technology Industries, (Mineapolis, Minn 129) whose Omega III welders offer three models of increasing sophistication, with the top-end MCX model allowing welding modes with constant time, constant height, constant travel, and constant energy control.

Lasers have penetrated the precision metals welding market, and plastics applications are not far behind. Laser welding allows contour welding in difficult conditions, such as electronic components. A key advantage is the absence of a contact tip or shoe allowing joints close to sensitive areas. The heat affected zone is also very small due to the highly directed energy inherent in a laser. As a result, heat and mechanical stresses are minimized, and welds have excellent visual properties. The Leister Modulas series of laser welders (Stan Mech Agencies, Mississauga, Ont. 130) incorporates the benefits of the laser power source with non-contact weld temperature monitoring and on-line process control. Welded seams can be produced with no visible bead. CPL