Secondary Operations Chapter 9

1. Secondary Operations Chapter 9 Professor Joe Greene CSU, CHICO MFGT 144

2. Chapter 9 Topics • Need for Secondary Operations • Assembly Operations • Ultrasonic welding; Hot-gas welding • Induction bonding; Spin (Friction) welding; Adhesive bonding • Machining Operations • Drilling and tapping; Reaming; Turning and milling • Automatic Shape Cutting • Water jet; Laser cutting • Surface Finishes and Decorating Procedures • Surface Prep: Flame, plasma process, acid etch • Applied Finishes • Painting;Electroplating; Vacuum Metallizing; Hot stamping • Pad printing and screen printing • Molded-in-color and symbols

3. Need for Secondary Operations • Secondary operations • Any operation to a molded part that occurs after the part is made. • Painting, trimming, drilling, fasteners, assembly • Should be minimized through injection molding design • Will generally be more expensive than molded-in features. • When to consider secondary operations • When volumes are small • When tooling costs are excessive • When time to build mold jeopardizes sales schedules • When a labor is available from other sources in company

4. Assembly Operations • Ultrasonic welding (Figure 9-1) • Uses high frequency mechanical vibrations (20 to 40 kHz per second transmitted through thermoplastic parts. • Vibrations generate friction between the plastic parts which leads to melting of the plastic. • The two plastic parts melt and fuse together as bond. • Can be used for staking, surface vibration welding, spot welding, and inserting metal inserts. • Thermoplastic materials can be welded • Amorphous materials are easy to ultrasonically weld. • Crystalline materials require greater amounts of energy and are much more sensitive to joint design and fixturing

5. Ultrasonic Welding • Parameter Effects • Materials: crystalline versus amorphous • Melt temperature • Melt index and viscosity • Material stiffness • Chemical makeup of plastic: Some dissimilar amorphous plastics can be welded. • Energy Directors (Fig 9-2) • Purpose- direct energy from the horn of machine to the desired point of welding. • Focuses the ultrasonic energy to the point and causes the material to melt

6. Ultrasonic Welding • Ease of welding • Table IX-2 and IX-2 for Amorphous and Crystalline • Function of joint design, part geometry, energy requirements, amplitude, and fixturing. • Based on near field welding, welding joint within 0.25 inches of horn contact surface • Frequency is usually 20 kHz versus 40 kHz (20% of jobs) • Vibration welding is lower frequency: 250 to 300 Hz • automotive bumpers, or materials that are damaged by high Hz

7. Ultrasonic Welding • Variables that Influence Ultrasonic Welding • Polymer structure • Amorphous- molecules are random arrangement. • Efficiently transmit ultrasonic vibrations and can be welded under a wide range of force-amplitude combinations • Crystalline- molecules are are spring-like in solid state. • Internally absorb a percentage of the high-frequency mechanical vibrations of the ultrasonic generator reducing efficiency of transmitting to joint interface. • Requires a higher amplitude • Melt temperature • Higher melt temperature more energy required. • Stiffness (modulus of elasticity) • Higher stiffness the better the transmission of the ultrasonic energy to the joint interface.

8. Ultrasonic Welding • Variables that Influence Ultrasonic Welding • Moisture content • Hygroscopic materials- nylon, ABS, PC, Psulfone, PET, PBT • Higher moisture content the lower the bonding efficiency • Moisture turns into steam during welding step and creates porosity in part and degrade resin at the joint interface. • Molded parts should be dried prior to welding. • Flow rates or viscosity • Rate at which material flows when it becomes molten. • Different materials should have similar viscosities or melt index • Mold release agents • Added to increase release of part from mold. • Higher mold release the lower the bond strength. • Especially poor bonding of silicone.

9. Ultrasonic Welding • Variables that Influence Ultrasonic Welding • Plasticizers • High temperature boiling liquids or low temperature melting plastics added to increase flexibility and elongation • Higher plasticiser amount results in lower bond strength. • Plasticizers interfere with a resin’s ability to transmit vibrations. • Plasticizers swell polymer like a sponge. • Flame retardants • Inhibits ignition or modifies burning chacteristics • Generally inorganic oxides or halogenated organics • Aluminum, antimony, boron, chlorine, bromine, sulfer, nitrogen, • Typically, 1% to 2% • not weldable • Higher flame retardant amounts results in lower bond strength

10. Ultrasonic Welding • Variables that Influence Ultrasonic Welding • Regrind • Regrind is added to reduce cost of part • Regind reduces melt temperature and reduces bond strength • Colorants • Generally do not inhibit weld strength unless greater that 5% • Resin grade • Different resin grades can have different melt temperatures and molecular weights. • Different weld grades are weldable if the two resins have similar molecular weights and the melt temperatures should be within 40°F of each other.

11. Ultrasonic Welding • Variables that Influence Ultrasonic Welding • Fillers • Added to reduce the price of the polymer and increase (Slightly stiffness) and reduce CLTE • talc, calcium carbonate, kaolin, organic fillers, silica, micas, etc. • Enhance some plastics ability to transmit ultrasonic energy by imparting higher stiffness. (For up to 35% filler) • Are very abrasive and can cause excessive wear on surface. • Require use of hardened steel or carbide-coated titanium horns • Reinforcements • Added to increase strength and stiffness and reduce CLTE • glass fiber, carbon fiber, aramid fiber. • Enhance the weldability of resin • Short fibers result in better welds • Long fibers clump at gate and reduce weldability

12. Hot-gas Welding • Similar to metal welding • Welding rod composed of same material being welded is placed along a beveled joint area. • Heat is applied to the area by hot gas (air or nitrogen) • Hot plastic melts the plastic and welding rod • PVC (rigid) is most common material hot-gas welded • Figure 9-3

13. Induction (Electromatic) Bonding • Figure 9-4 (Time required = less than 10 seconds) • Process consists of activating an electrodynamic field to excite a conductive bonding agent (metal wire strands) • Heat is absorbed by the plastic components that surround the bonding agent, causing the plastic to melt. • Melted plastics fuse together and to the bonding agent. • Slight pressure is applied during welding.

14. Spin (Friction) Welding • Figure 9-5 (Time required = less than 2 seconds) • Process consists of one part spinning at speeds of 100 to 1000 RPM located near second part. • The spinning produces friction & heat when parts touch. • Slight pressure is applied during welding. • Can produce weld with drill press or lathe. • Can be used with most hard plastics. • Requires part to be cylindrical.

15. Adhesive Bonding • Figure 9-6 (Time required = 10 seconds to minutes) • Process consists of one adding a thermoset material • The thermoset material (urethane and epoxy polymers) • Ashland Chemical PLIOGRIP (Modified urethane) • Acrylics, Phenolic resins, Structural Adhesive, Welding Adhesives, Roofing Adhesives, Wood Bonding Adhesives • Structural Adhesives • Solventless PLIOGRIP®, AROWELDTM and AROGRIP® structural adhesives bond reinforced thermosetting composites, thermoplastics, metals and other substrates

16. Machining Operations • Drilling and Tapping Thermoplastics • Carbide drills are most suitable • Carbide tipped or diamond-tipped drills for mirror finish • Flutes should be highly polished and drill cutting surfaces should be chrome plated or nitrided to reduce wear • Details of drill dimensions are shown in Fig 9-7 • drill land, L, should be 1/16 in or less • Helix angle should be 30° to 40° • Point angle should be 60° to 90° • Drill feed should be approximately 0.0005 per revolution of drill bit • Drill speeds should range from 5000 rpm to 1000 rpm • Thermosets are more abrasive and require special bit

17. Machining Operations • Reaming Thermoplastics and Thermosets • Reamers should be fluted for best surface finish • Reamer feeds and speeds should approximate those of drilling operations. • Water soluble coolants should be used to reduce heat generation by friction. • Turning and Milling • Lath and mill cutters should be tugsten carbide or diamond-tipped with negative back rake and front clearance (Figure 9-8)

18. Automated Shape Cutting • Water Jet Cutting • Most popular automated cutting process in industry • Employs a force of a thin stream of water pressure (20 kpsi to 50kpsi) to create a powerful cutting point. • Pierces plastic or composite material cleanly. • Dust and chips are non-existent • Used for flat sheet stock mostly but can with the use of three and five axis machines cut complex parts

19. Automated Shape Cutting • Laser Cutting • Used when a fine polished finish on plastic edge is required, such as on the edges of an acrylic sign. • Laser cuts by focusing its concentrated beam at the exact point of the cut, which causes the plastics to melt, vaporize, and solidify, thus producing a smooth finish. • Advantages • Straight, burr-free cuts • Narrow cutting-width • Oxidation-free cuts, • Smooth profile • High cutting speeds http://www.lmclaser.com.au/

20. Surface Finishes and Decorating Procedure • Preparation of Surface • Products that require postmold painting or decorating need clean surface to ensure proper adhesion to paint or bond. • Flame treatment • Most common method of preparing polyolefins and acetals • These materials are slippery in nature and resistant to paints • Flame treatment consists of passing the molded product through a flame • Causes the surface to oxidize and making it receptive to paints • Surface is oxidized without charring surface. • Corona Discharge • Surface oxidation of plastic is achieved by passing the plastic over an insulated metal cylinder beneath a high voltage conductor. • An electric discharge strikes the surface of the plastic causing plastic to oxidize

21. Surface Finishes and Decorating Procedure • Preparation of Surface • Plasma Process • Low pressure air is directed through an electrical discharge and expand into a vacuum chamber containing the plastic. • Nitrogen and oxygen gases are partially disassociated radicals in air react with the surface • Acid Etch • Some plastics, e.g., PC and ABS, need additional surface preparation • Acid wash attacks surface of the plastic and creates microscopic craters of exposed resin. • Craters will physically capture the decorative coating and lock it to the plastic surface.

22. Surface Finishes and Decorating Procedure • Applied Finishes • Painting • Applied with brushing, spraying, rolling, or dipping • Manually, mechanically, or robotically • Most are sprayed with standard spray process (Fig 9-9) • Need proper surface prep, primer, oven Temp • Plating (Electroplating) • Requires plastic to be made conductive • Apply conductive base metal to plastic surface. • Metallic plating is used for decorative or functional • plumbing fixtures, jewelry, circuit board traces, EMI shields, corrosion resistant surfaces

23. Surface Finishes and Decorating Procedure • Applied Finishes • Vacuum Metallizing (Deposition) Figure 9-10 • Plastic is coated with lacquer base coat. • Then placed on a rack inside a vacuum chamber along with small clips of the metal to be deposited, • The metal clips are heated to the point of vaporizing • Depoited on all line-of sight surfaces due to vacuum • Gives bright metallic finish • Less expensive than electroplating

24. Surface Finishes and Decorating Procedure • Applied Finishes • Hot Stamping (Fig 9-11) • Three methods of hot stamping • Roll-on decorating: (Fig 9-12) • Ideal for applying rolls or preprinted heat transfers to part surfaces • Silicone rubber roller applies heat and pressure to release the print medium onto the plastic substrate. • Peripheral marking (Figure 9-13) • Ideal for periphery of cylindrical or slightly conical parts • Plastic product is rolled under a flat stamping die to release the print medium onto plastic substrate • Vertical Stamping (Fig 9-14) • Ideal for small areas of flat or slightly crowned products • Silicone rubber die is mounted to the heated head of a vertical machine and positioned directly over the part to be decorated • Rubber die contains raised graphics to be stamped and is heated • Rubber die is powered and pushes the foil against the plastic

25. Surface Finishes and Decorating Procedure • Applied Finishes • Pad Printing (Fig 9-15) • Done like printing paper on a press • A pad of rubber is inked with the image that is pressed onto a steel or nylon plate on which the image is etched with ink screened into that image. • Ink pad is brought to the plastic surface and pressed. • Screen Printing (Fig 9-16) • Ink or paint is forced through a mesh of a plastic or a metal screen by pulling a squeegee across a screen that is placed against the surface of the plastic.