ENME307: Manufacturing design for polymers

Concurrent engineering

design engineer, material supplier, mould designer, converter

Uniform wall thickness decreases residual stress

Scope of thermoplastic family

commodity, engineering, high performance, ultra (keep as low as possible)

Amorphous

broad softening, good mechanical properties, dimensionally stable, transparent

Semicrystalline

sharper melting, resistance to chem environment (blocks diffusion), anisotropic shrinkage, good electrical properties, high heat capability

Selection Mechanical

load, impact, stiffness

Selection Environmental

chemicals

Selection Special needs

UC, FDA, outdoors, transparency

Selection Process considerations

blow, injection, foam, extrusion, thermoform

Selection assembly requirements

paint, plate, coat, water resistant, adhesion

Selection reverse engineer

look for similar applications

Physical Material Properties

mould shrinkage, rheology

Mechanical Material Properties

flexure, strength, modulus, fatigue, creep

Thermal Material Properties

deflection temp under load, GTT, thermal conduction, expansion

Regulatory Material Properties

flammable, high voltage

Chemical Material Properties

weather, UV, solvents, temp, moisture

Physical degradation

stress crack, microcrack, soft, swell, discolour

Glass/mineral additive

reinforce, flame retardant, UV stabaliser, helps releace from mould

Additives

increase viscosity, waer out tools, anisotropuc shrinkage

Material freeze in walls

shut off (flow length/pressure)

Wall thickness

cooling time (low good), flow length (high good as if low gating)

Die swell

expands laterally due to pressure change. Design die for swell, low flow, low stress, less swelling

Newtonian

shear rate effects stress

Land length

length of tube relative to diameter – higher ratio, less die swell

Injection mould

raw polymer, reciprocating screw, dry polymer, inaction, mould closing, packing, plasticising, cooling, mould open, ejection

Fountain flow

cold mould freezes and forms a skin with the hot plastic

Uniform part thickness

minimises moulded stress, warpage, voids, sink marks, uneven shrinkage.

Gate location

position gate so that it flows from thick to thin, as thick freezes first. If not inadequate packing occurs and there are voids

Nip line

imperfection of polymer going (microscopic air gap), aesthetics

Weld lines / knit lines

flow paths meet, make sure not near impact area

Parting / split line

outside of part surface is formed by the cavity

Draft

lets product exit mould without damage (tables for angles)

Boss design

joining 2 cases together with rib

Assembly design

minimise number of parts, minimise assembly surfaces

Shear thinning

viscosity decreases with increased shear, fills narrower parts better

Shot size

20-80% max capacity

Cost

materials, labour, machine rate, number of parts

Higher Cooling time

higher cost