1. Mechanical Engineering
Plastics (Polymers)
The word plastics is from the Greek word Plastikos, meaning “able to be shaped and molded”
Ken Youssefi
Mechanical Engineering

2. Why Design with Plastics?
Density
Light weight, high weight to strength ratio, particularly when reinforced
Cost
Relatively low cost compared to metals and composites
Ken Youssefi
Mechanical Engineering

3. Why Design with Plastics?
Corrosion resistance
Low electrical and thermal conductivity, insulator
Easily formed into complex shapes, can be formed, casted and joined.
Wide choice of appearance, colors and transparencies
Ken Youssefi
Mechanical Engineering

4. Disadvantages of using Plastics
Low strength
Low useful temperature range (up to 600 oF)
Less dimensional stability over period of time (creep effect)
Aging effect, hardens and become brittle over time
Sensitive to environment, moisture and chemicals
Poor machinability
Ken Youssefi
Mechanical Engineering

5. Mechanical Engineering
Ken Youssefi
Mechanical Engineering

6. Mechanical Properties of Various Plastics
Steel: 350 to 1900 MPa
Brass: 200 to 850 MPa
Aluminum: 100 to 550 MPa
Ken Youssefi
Mechanical Engineering

7. Mechanical Engineering
Polymers
The earliest synthetic polymer was developed in 1906, called Bakelite.
The development of modern plastics started in 1920s using raw material extracted from coal and petroleum products (Ethylene). Ethylene is called a building block.
Polymers are long-chain molecules and are formed by polymerization process, linking and cross linking a particular building block (monomer, a unit cell).
The term polymer means many units repeated many times in a chainlike structure.
Most monomers are organic materials, atoms are joined in covalent bonds (electron-sharing) with other atoms such as oxygen, nitrogen, hydrogen, sulfur, chlorine,….
Ken Youssefi
Mechanical Engineering

8. The structure of polymers
Ken Youssefi
Mechanical Engineering

9. Classification of polymers
There are two major classifications of polymers
Thermoplastics
As the temperature is raised above the melting point, the secondary bonds weaken, making it easier to form the plastic into any desired shape. When polymer is cooled, it returns to its original strength and hardness. The process is reversible. Polymers that show this behavior are known as thermoplastics.
Thermosetting Plastics (thermosets)
Thermosetting plastics are cured into permanent shape. Cannot be re-melted to the flowable state that existed before curing, continued heating for a long time leads to degradation or decomposition. This curing (cross-linked) reaction is irreversible. Thermosets generally have better mechanical, thermal and chemical properties. They also have better electrical resistance and dimensional stability than do thermoplastics.
Ken Youssefi
Mechanical Engineering

10. Mechanical Engineering
Polymer’s Structures
Bonding – monomers are linked together by covalent bonds, forming a polymer chain (primary bonds). The polymer chains are held together by secondary bonds. The strength of polymers comes in part from the length of polymer chains. The longer the chain, the stronger the polymer. More energy is needed to overcome the secondary bonds.
Linear polymers
A sequential structure resulting in thermoplastics like nylon, acrylic, polyethylene. A linear polymer may contain some branched and cross-linked chains resulting in change in properties.
Branched polymers
Side branch chains are attached to the main chain which interferes with the relative movement of the molecular chains. This results in an increase in strength, deformation resistance and stress cracking resistance. Lower density than linear chain polymers.
Ken Youssefi
Mechanical Engineering

11. Mechanical Engineering
Polymer’s Structures
Cross-linked polymers
Three dimensional structure, adjacent chains are linked by covalent bonds. Polymers with cross-linked chains are called thermosetting plastics (thermosets), epoxy and Silicones.
Cross-linking is responsible for providing hardness, strength, brittleness and better dimensional stability.
A three dimensional network of three or more covalent bonds. Thermoplastic polymers that have been already formed could be cross-linked to obtain higher strength. Polymers are exposed to high-energy radiation.
Network polymers
Ken Youssefi
Mechanical Engineering

12. Mechanical Engineering
Additives in Plastics
Additives are added to polymers in order to obtain or improve certain properties such as strength, stiffness, color, resistance to weather and flammability.
Plasticizers are added to obtain flexibility and softness, most common use of plasticizers are in PVC.
Ultraviolet radiation (sunlight) and oxygen cause polymers to become stiff and brittle, they weaken and break the primary bonds. A typical treatment is to add carbon black (soot) to the polymer, it absorbs radiation. Antioxidants are also added to protect against degradation.
Fillers such as fine saw dust, silica flour, calcium carbide are added to reduce the cost and to increase harness, strength, toughness, dimensional stability,…..
Ken Youssefi
Mechanical Engineering

13. Mechanical Engineering
Additives in Plastics
Colorants are added to obtain a variety of colors. Colorants are either organic (dye) or inorganic (pigments). Pigments provide greater resistance to temperature and sunlight.
Flame retardants such as chlorine, phosphorus and bromine, are added to reduce polymer flammability. Teflon does not burn and nylon and vinyl chloride are self-extinguishing.
Lubricants such as mineral oil and waxes are added to reduce friction.
Ken Youssefi
Mechanical Engineering

14. Applications of Thermoplastics
Design requirement: strength
Applications: Valves, gears, cams, pistons, fan blades, …
Plastics: nylon, acetal (delrin), polycarbonate, phenolic
Design requirement: wear resistance
Applications: bearings, gears, bushings, wheels, ….
Plastics: nylon, acetal (delrin), polyurethane, phenolic, polymide
Ken Youssefi
Mechanical Engineering

15. Applications of Thermoplastics
Design requirement: functional and decorative
Applications: knobs, handles, cases, moldings, pipe fittings, …
Plastics: ABS, acrylic, polyethylene, phenolic, polypropylene, polystyrene
Design requirement: functional and transparent
Applications: lens, goggles, signs, food processing equipment, …
Plastics: acrylic, polycarbonate, polystyrene, polysulfone
Design requirement: hollow shapes and housings
Applications: pumps, helmets, power tools, cases, …
Plastics: ABS, polyethylene, phenolic, polypropylene, polystyrene, polycarbonate
Ken Youssefi
Mechanical Engineering

16. Mechanical Engineering
Popular Plastics
Polyethylene (LDPE (low density) and HDPE (high density)
Properties: good chemical and electrical properties, strength depends on composition
Applications: bottles, garbage cans, housewares, bumpers, toys, luggage
Acetal (Delrin)
Properties: good strength, good stiffness, good resistance to heat, moisture, abrasion and chemicals
Applications: mechanical components; gears, bearings, valves, rollers, bushings, housings
ABS
Properties: dimensionally stable, good strength, impact and toughness properties, good resistance to abrasion and chemicals
Applications: automotive components, helmets, tool handles, appliances, boat hulls, luggage, decorative panels
Ken Youssefi
Mechanical Engineering

17. Mechanical Engineering
Popular Plastics
Polycarbonates
Properties: very versatile and has dimensional stability, good mechanical and electrical properties, high resistance to impact and chemicals
Applications: optical lenses, food processing equipments, electrical components and insulators, medical equipments, windshields, signs, machine components
Nylons
Properties: good mechanical and abrasion resistance property, self-lubricating, resistant to most chemicals but it absorbs water, increase in dimension is undesirable
Applications: mechanical components; gears, bearings, rollers, bushings, fasteners, guides, zippers, surgical equipments,
Ken Youssefi
Mechanical Engineering

18. Applications of Thermosetting Plastics
Epoxies
Properties: good dimensional stability, excellent mechanical and electrical properties, good resistance to heat and chemicals
Applications: electrical components requiring strength, tools and dies, fiber reinforced epoxies are used in structural components, tanks, pressure vessels, rocket motor casing
Phenolics
Properties: good dimensional stability, rigid, high resistance to heat, water, electricity, and chemicals
Applications: laminated panels, handles, knobs, electrical components; connectors, insulators
Ken Youssefi
Mechanical Engineering

19. Applications of Thermosetting Plastics
Polyesters (thermosetting, reinforced with glass fibers)
Properties: good mechanical, electrical, and chemical properties, good resistance to heat and chemicals
Applications: boats, luggage, swimming pools, automotive bodies, chairs
Silicones
Properties: excellent electrical properties over a wide rang of temperature and humidity, good heat and chemical properties
Applications: electrical components requiring strength at high temp., waterproof materials, heat seals
Ken Youssefi
Mechanical Engineering

20. Mechanical Engineering
Website:
Plastics
Stress vs. Strain curve
Ken Youssefi
Mechanical Engineering

21. Mechanical Engineering
Structural and mechanical Appl.
Light duty mechanical & decorative
Gears, cams, pistons, rollers, fan blades, rotors, pump impellers, washing machine agitators
Handles, knobs, steering wheel, tool handles, pipe fittings, camera cases, eyeglass frames X
ABS
Acetal (Delrin)
Acrylic
Cellulosics
Fluoroplastics
Nylon
Phenylene Oxide
Polycarbonate
Polyester
Polyethylene
Polyimide
Polyenylene sulfide
Polypropylene
Polystyrene
Polysulfone
Polyurethane
Polyvinyl chloride
Phenolic X X X X
Thermoplastics X X X X X X X
Thermosets
Ken Youssefi
Mechanical Engineering

22. Mechanical Engineering
Parts for wear applications
Optical and transparent parts
Gears, bearings, bushings, tracks, wheels, ware strips
Lenses, safety glasses, signs, refrigerator shelves, windshields
ABS
Acetal (Delrin)
Acrylic
Cellulosics
Fluoroplastics
Nylon
Phenylene Oxide
Polycarbonate
Polyester
Polyethylene
Polyimide
Polyenylene sulfide
Polypropylene
Polystyrene
Polysulfone
Polyurethane
Polyvinyl chloride
Phenolic X X X X X
Thermoplastics X X X X X X X
Thermosets X X
Ken Youssefi
Mechanical Engineering

23. Mechanical Engineering
Small housing & hollow shapes
Large housing & hollow shapes
Phone and flashlight cases, helmets, housings for power tools, pumps, small appliances
Boat hulls, large appliance housings, tanks, tubs, ducts, refrigerator liners X X
ABS
Acetal (Delrin)
Acrylic
Cellulosics
Fluoroplastics
Nylon
Phenylene Oxide
Polycarbonate
Polyester
Polyethylene
Polyimide
Polyenylene sulfide
Polypropylene
Polystyrene
Polysulfone
Polyurethane
Polyvinyl chloride
Phenolic X X X
Thermoplastics X X X X X X X X X X X
Thermosets X X X
Ken Youssefi
Mechanical Engineering

24. Mechanical Engineering
Structural & Mechanical
Light duty mech & deco
Small housing & hollow shapes
Large housing & hollow shapes
Parts for wear applications
Optical and transparent parts
Plastic X X X
ABS
Acetal (Delrin)
Acrylic
Cellulosics
Fluoroplastics
Nylon
Phenylene Oxide
Polycarbonate
Polyester
Polyethylene
Polyimide
Polyenylene sulfide
Polypropylene
Polystyrene
Polysulfone
Polyurethane
Polyvinyl chloride
Phenolic X X X X X X X X X X X X
Thermoplastics X X X X X X X X X X X X X X X X X X X X X X X X X
Thermosets X X X X X
Ken Youssefi
Mechanical Engineering

25. Manufacturing Processes for Plastics
Fabrication of Plastics
Injection Molding
Ejector pin
Molded part
Heaters
Granular plastic
Plunger
Torpedo
Ken Youssefi
Mechanical Engineering

26. Mechanical Engineering
Ken Youssefi
Mechanical Engineering

27. DFM Design Guidelines Injection Molding
Provide adequate draft angle for easier mold removal.
Minimize section thickness, cooling time is proportional to the square of the thickness, reduce cost by reducing the cooling time.
Ken Youssefi
Mechanical Engineering

28. DFM Design Guidelines Injection Molding
Avoid sharp corners, they produce high stress and obstruct material flow.
Keep rib thickness less than 60% of the part thickness in order to prevent voids and sinks.
Ken Youssefi
Mechanical Engineering

29. DFM Design Guidelines Injection Molding
Keep section thickness uniform around bosses.
Provide smooth transition, avoid changes in thickness when possible.
Ken Youssefi
Mechanical Engineering

30. DFM Design Guidelines Injection Molding
Use standard general tolerances, do not tolerance;
Dimension Tolerance Dimension Tolerance
0 ≤ d ≤ ± 0.5 mm 0 ≤ d ≤ 1.0 ± 0.02 inch
25 ≤ d ≤ ± 0.8 mm 1 ≤ d ≤ 5.0 ± 0.03 inch
125 ≤ d ≤ ± 1.0 mm 5 ≤ d ≤ ± 0.04 inch
± 1.5 mm ± 0.05 inch
Minimum thickness recommended;
.025 inch or .65 mm, up to .125 for large parts.
Standard thickness variation.
Round interior and exterior corners to in radius (min.), prevents an edge from chipping.
Ken Youssefi
Mechanical Engineering

31. Mechanical Engineering
Rotational Molding
Rotational molding process consists of six steps
A predetermined amount of plastic, powder or liquid form, is deposited in one half of a mold.
The mold is closed.
The mold is rotated biaxially inside an oven.
The plastics melts and forms a coating over the inside surface of the mold.
The mold is removed from the oven and cooled.
The part is removed from the mold.
Ken Youssefi
Mechanical Engineering

32. Rotational Molding Machines
Turret machine
Vertical wheel machine
Shuttle machine
Rock and roll machine
Ken Youssefi
Mechanical Engineering

33. Mechanical Engineering
Rotational Molding
Advantages
Molds are relatively inexpensive.
Rotational molding machines are much less expensive than other type of plastic processing equipment.
Different parts can be molded at the same time.
Very large hollow parts can be made.
Parts are stress free.
Very little scrap is produced
Ken Youssefi
Mechanical Engineering

34. Mechanical Engineering
Rotational Molding
Limitations
Can not make parts with tight tolerance.
Large flat surfaces are difficult to achieve.
Molding cycles are long (10-20 min.)
Materials
Polyethylene (most common), Polycarbonate (high heat resistance and good impact strength), Nylon (good wear and abrasion resistance, good chemical resistance, good toughness and stiffness).
Ken Youssefi
Mechanical Engineering

35. Mechanical Engineering
Rotational Molding
Nominal wall thickness
Polycarbonate wall thickness is typically between .06 to .375 inches, .125 inch being an ideal thickness.
Polyethylene wall thickness is in the range of .125 to .25 inch, up to 1 inch thick wall is possible.
Nylon wall thickness is in the range of .06 to .75 inch.
Ken Youssefi
Mechanical Engineering

36. Rotational Molding Examples
Ken Youssefi
Mechanical Engineering

37. Rotational Molding Examples
Ken Youssefi
Mechanical Engineering

38. Mechanical Engineering
Blow Molding
Blow molding is generally the same process as glass blowing adapted to polymers.
In extrusion blow molding a tube is extruded and clamped in a split mold. Air under pressure ( psi) is injected into the tube blowing the plastic outward to fill the mold cavity.
Ken Youssefi
Mechanical Engineering

39. Mechanical Engineering
Blow Molding
Blow molding is used for medium size, hollow thin-walled shapes; containers, tool cases, hollow structures, ….
Blow molding is limited to thermoplastics such as polyethylene, polycarbonate, ABS.
Wall thickness between
Maximum tolerance
Ken Youssefi
Mechanical Engineering