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Secondary Operations Chapter 9. Professor Joe Greene CSU, CHICO. MFGT 144. Chapter 9 Topics. Need for Secondary Operations Assembly Operations Ultrasonic welding; Hot-gas welding Induction bonding; Spin (Friction) welding; Adhesive bonding Machining Operations

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Secondary operations chapter 9 l.jpg
Secondary Operations Chapter 9

Professor Joe Greene


MFGT 144

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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

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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

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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

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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

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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

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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.

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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.

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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

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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.

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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

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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

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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.

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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.

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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

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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

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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)

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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

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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

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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

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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.

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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

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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

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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

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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.