MECHENG236 - Plastics Flashcards
Adv of plastics
Resistant to a range of chemicals. Corrosion resistant. Good surface finish. Can be colouredright through. Flexible, good impact strength, light. Vast array of compositions and characteristics.
Types of polymers
- Thermoplastics
o In thermoplastics, the polymer chains are only weakly bonded. The chains are free to slide past one another when sufficient thermal energy is supplied, making the plastic formable, reformableand recyclable. e.gpolypropylene, polycarbonate, PVC, ABS and nylon. - Thermosets
o In thermosets, adjacent polymer chains form strong cross links. When heated, these cross links prevent the polymer chains from slipping past one another. As such, thermosets cannot be reflowed once they are cured (i.e., once the cross-links form). Instead, thermosets can suffer chemical degradation (denaturing) if reheated excessively.
Thermoplastics adv and disadv
Advantages *Highly recyclable. * High-impact resistance. *Higher toughness, or ductility. * Remoulding/reshaping capabilities. * Hard crystalline or rubbery surface options. *Faster processing times.
Disadvantages *Generally, more expensive than thermosets. * Can loose shape and mechanical integrity if overheated. *Not as stiff or strong as Thermosets. *Tend to creep over long time periods. *High viscosity of melts; requires high processing temperatures and pressures.
Thermosets adv and disadv
Advantages *More resistant to high temperatures than thermoplastics, in use. *Highly flexible design. *Thick to thin wall capabilities. *High levels of dimensional and thermal stability. *Higher tensile strength and stiffness. *Heat, chemical and solvent resistance. *Good creep resistance. *Excellent fatigue properties. *Low viscosities, simplifying physical processing.
Disadvantages *Cannot be recycled. *More difficult to surface finish. *Cannot be remouldedor reshaped. *Brittle at room temperature.
Designing with plastics – things to consider
Walls should be near constant thickness and avoid being excessively thick – to avoid sink marks as plastic cools
Design out unnecessary parts
Use fillets to reduce stress and allow easier demoulding
Use webs and ribs for strength due to the often thin walls in plastic parts
Tapers are necessary for easier demoulding
Polymer manufacturing methods
Extrusion
Injection Moulding
Rotational moulding
Blow moulding
Compression moulding
Thermo-forming
Polymer extrusion adv and disadv
Advantages–Low cost per part–Complexity of cross-section–Post-extrusion alterations Disadvantages–Size variances–Product limitations
Injection moulding adv and disadv
Advantages * High production rates. * High tolerances are repeatable. * Wide range of materials can be used. * Mainly thermoplastics; however, thermosets can also be injection-moulded. * Low labourcosts. * Minimal scrap losses. * Little need to finish parts after moulding.
Disadvantages * Expensive equipment investment. * Running costs may be high. * Lead times for production of die sets may be long. * Modifications to product, and therefore mould, can be costly. * Difficult to accurately meter shot size, due to density changes. * Variability in mouldfilling, due to variation in nozzle pressure.
Rotational moulding adv and disadv
Advantages * Low pressure therefore moulds are relatively simple and inexpensive(e.g., aluminium) and, therefore, lead times can be short. * Stress-free mouldings. * Can contain resin reinforcement and cast-in features such as thread inserts. * Can produce very large and thick-walled articles (12 mm). * Intricate shapes and undercuts are possible. * Very little wastepolymer. * Most thermoplastics and some thermosets can be used.
Disadvantages * A relatively slowprocess. * Loading and unloading is labour intensive. * Bosses and ribs are difficult to produce. *Can have non-uniform wall thickness.
Blow moulding adv / disadv
Adv: High rate of production, very fast cycle times
Disadv: Machinery is expensive
Thermo-forming adv and disadv
Advantages *Low pressures mean low-cost tooling –Wood, plaster. *Also means a short manufacturing time for tooling. *Medium to large parts can be made economically in small quantities
Disadvantages *Relatively slow process. *Plastic sheet feedstock must be prepared. *There is a certain amount of waste from brim offcut (can sometimes be reground and reused).
