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Mechanical Engineering

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    Plastic Overview

    Plastics are a wide range of synthetic materials made primarily of polymers. Their plasticity allow them to be manipulated in a variety of manufacturing methods, including molding, extrusion, and pressing. Their adaptability and wide range of properties has led to their widespread use across the globe.

    [Add Diagram: Global plastic waste / recycling flow chart]

    What are Polymers?

    Polymers are long molecular chains formed by repeating smaller units called monomers.

    $$ \text{Monomer + Monomer + Monomer } \rightarrow \text{ Polymer Chain} $$

    Examples of monomers include ethylene, propylene, styrene, and vinyl chloride.

    Polymerization is the chemical reaction that links monomers together.

    [Add Diagram: Chain of monomers becoming polymer]

    Major Polymer Categories

    Thermoplastics

    Thermoplastics are solid at room temperature but become viscous liquids when heated. Ex: ABS, Polycarbonate (PC), Polyethylene (PE), Polypropylene (PP), Nylon, PVC, PET

    • Can be easily and economically shaped into products
    • Melting can be done repeatedly
    • Best for injection molding
    • Can be welded or recycled

    Applications: LEGO bricks, Water bottles, Electronic housings, Automotive trim

    Thermosets

    Thermosets can be molded during initial heating and mixing, but are permanently cured through cross-linking that activates in elevated temperatures. Ex: Epoxy, Phenolic, Polyester Resin, Polyimides

    • Cannot be remelted- degrades and chars when reheated
    • High temperature resistance
    • High stiffness

    Applications: Circuit boards, adhesives, handles, composites

    Elastomers

    Highly elastic polymers capable of large deformation. Ex: Rubber, Silicone, Neoprene, Urethane

    • Some elastomers can be stretched by 10x and remain elastic
    • Low modulus
    • Used for seals and tires

    [Add Diagram: Thermoplastic vs Thermoset vs Elastomer molecular structure]

    Important Polymer Temperatures

    • Glass Transition Temperature (\( T_g \)): Polymer changes from hard/glassy to rubbery.

      • Above \( T_g \), stiffness may drop by 100-1000\( \times \).

    • Melting Temperature (\( T_m \)): Crystalline regions melt into viscous liquid.

    • Processing Temperature: Temperature used during molding.

    [Add Graph: Modulus vs Temperature showing \( T_g \) and \( T_m \)]

    Mechanical Properties of Polymers

    Additives

    Used to modify plastic behavior:

    • Fillers — strengthen or lower cost
    • Plasticizers — soften material and improve flow
    • Colorants — pigments or dyes
    • Flame retardants — reduce flammability
    • Lubricants — reduce friction and improve flow
    • UV stabilizers — reduce sunlight degradation
    • Antioxidants — reduce oxidation damage
    • Cross-linking Agents — for thermosets and elastomers
    • Fibers (glass/carbon) — reinforcement

    Recycling Codes

    1. PET
    2. HDPE
    3. PVC
    4. LDPE
    5. PP
    6. PS
    7. Other

    Best commonly recycled: PET and HDPE

    [Add Diagram: Recycling triangle symbols 1--7]

    Injection Molding

    Injection molding is the most common manufacturing process for thermoplastic mass production. It is similar to high-pressure metal die casting-- pellets of thermoplastic are heated and forced into a split-die chamber.

    Similar to high-pressure die metal casting, there is a feed system to deliver the melted plastic to the cavities. The cavity is filled with many small, narrow openings called gates.

    Examples:

    • LEGO bricks
    • Bottle caps
    • Keyboard keys
    • Medical syringes
    • Consumer product housings

    Method

    Plastic pellets are fed into a heated barrel where a rotating screw:

    • conveys pellets forward
    • melts material using heat + shear
    • meters the required shot volume
    • injects melt into mold cavity

    Then the plastic cools, solidifies, and is ejected.

    [Add Diagram: Injection molding machine labeled hopper, barrel, screw, nozzle, mold]

    Cycle Steps

    1. Mold closes
    2. Screw moves forward and injects melt into cavity
    3. Packing / holding pressure applied
    4. Cooling begins
    5. Screw retracts and prepares next shot
    6. Mold opens
    7. Ejector pins remove part

    Typical cycle time: 5-60s depending on part thickness.

    [Add Diagram: 6-step cycle illustration]

    Terms

    • Sprue: Main channel from nozzle
    • Runner: Horizontal flow channel
    • Gate: Small opening into part cavity
    • Parting Line: Separation between mold halves
    • Ejector Pins: Push part out
    • Core/Cavity: Internal and external mold surfaces

    [Add Diagram: Mold feed system showing sprue, runner, gate]

    Cooling Time

    Cooling is often 50-70% of the total cycle time

    $$ t_{cool} = \frac{d^2}{\pi^2a}ln(\frac{\pi}{4}\frac{T_p-T_d}{T_e-T_d}) $$
    • Note the quadratic dependency on \( d \), meaning:
      • Thick walls cool much slower
      • Thin walls reduce cycle time
      • Uniform walls reduce warpage
    • \( a = \frac{\gamma}{\rho c} \) is thermal diffusivity \( [\frac{m^2}{s}] \)
    • \( T_p > T_e > T_d \) (melt processing, ejection, and mold wall temperatures)

    Clamp Force

    The mold must withstand large pressures without deformation, with a typical lifetime of 1 million "shots". The mold halves are pushed together with a clamping pressure of \( 70 - 200 \) MPa. Machine must hold mold shut against cavity pressure with a clamping force:

    $$ F = P \times A $$

    where:

    • \( F \) = clamp force
    • \( P \) = cavity pressure
    • \( A \) = projected area

    Too little of a clamp force causes flash formation, so molders often use more than the above force.

    Cost Drivers

    Tooling Cost:

    • highly dependent on mold complexity and environment (temperature, pressure, cycle speed)
    • Dominates at low production volume

    Processing Cost:

    • highly dependent on cycle time- longer cycles have higher labor/overhead per part
    • Thicker parts take longer to cool— \( t_{cool} \propto d^2 \)

    Material Cost:

    • Entirely variable
    • Dominates at high production volume

    Common Defects

    • Flash — excess plastic at mold parting line
    • Witness marks — contact from ejector pins
    • Sink marks — depressions over thick sections
    • Warpage — uneven shrinkage
    • Short shot — incomplete fill
    • Weld lines — two flow fronts meet
    • Burn marks — trapped gas overheats
    • Flow lines — visible flow patterns

    [Add Image Grid: examples of common defects]

    Design Guidelines

    • Use uniform wall thickness
    • Add ribs instead of thick solid walls (parallel to material flow)
    • Use multi-cavity molds for high volume
    • Include draft angle (1-3\( ^\circ \) minimum)
    • Round corners and avoid undercuts entirely, if possible
    • Place gates in thick sections
    • Bosses can be used like cylindrical ribs to prevent air entrapment and avoid thickness variations

    [Add Diagram: Good vs bad wall thickness design]

    Plastic Product Properties:

    • Strength and Stiffness — generally not as good as metals
    • Strength/Weight Ratio — competitive with metals
    • Creep — problem for thermoplastics, not for thermosets
    • Temperature Range — limited relative to metals and ceramics
    • UV protection needed to prevent sunlight degradation
    • Absorbs impacts well
    • Often soluble; resistant to most acids and bases

    Types of Molds

    • Two-plate mold
    • Three-plate mold
    • Hot-runner mold
    • Multi-cavity mold
    • Family mold

    Hot runner advantage: less scrap, faster cycles.

    [Add Diagram: Two-plate vs three-plate vs hot-runner]

    Other Plastic Processes

    • Extrusion — pipes, tubing
    • Blow molding — bottles
    • Compression molding — thermosets
    • Reaction Injection Molding (RIM)
    • Structural Foam Molding (SFM)
    • Vacuum Forming/Thermoforming
    • Rotomolding — hollow tanks