DESIGN FOR PLATICS

WHAT IS PLASTICS?

A synthetic material made from a wide range of organic polymers such as polyethylene, PVC, nylon, etc., that can be moulded into shape while soft, and then set into a rigid or slightly elastic form.

POLYMERISATION:- Polymerisation is the process of joining together a large number of small molecules to make a smaller number of very large molecules. The reactants (i.e. the small molecules from which the polymer is constructed) are called Monomers and products of the polymerisation process are called Polymers.

Ethylene (monomeric )

Ethylene(polymeric)

• THERMOPLASTICS:- Describes a substance which becomes flexible when heated and hardens on cooling with no change in its properties.

Example:-

Polyethylene: packaging, electrical insulation, milk and bottles, packaging film

Polypropylene: carpet fibers, automotive bumpers, microwave containers,

Polyvinyl chloride (PVC): electrical cables cover, credit cards, car panels.

Polystyrene: disposable spoons, forks, Styrofoam™

Acrylics (PMMA: polymethyl methacrylate): paints, fake fur, Flexiglass

Polyamide (nylon): textiles and fabrics, gears, bushing and washers, bearings

PET (polyethylene terephthalate): bottles for acidic foods like juices, food trays

PTFE (polytetrafluoroethylene): non-stick coating, Gore-Tex™ (raincoats),

dental floss

TYPES OF THERMOPLASTICS

• THERMOSETTING:-Thermosetting plastics are polymer materials which are liquid or malleable at low temperatures, but which change irreversibly to become hard at high temperatures.

Examples:

Polyurethanes: mattress, cushion, insulation, toys

Silicones: surgical gloves, oxygen masks in medical applicationsjoint seals

Characteristics of plastic:-

• Creep and shrink as time passes• Bad conductor of heat• Shrinkage problem• Their properties change over the temperature range of everyday life• Thermoplastics undergo a physical change when processed; the process

is repeatable.1. Random tangled molecules are called amorphous -Amorphous materials can be fully transparent. Those with a degree of molecular arrangement and ordering are called semi crystalline. - More crystalline a material is, the less likely it is to have a wide 'rubbery' processing region, so making it less suitable for stretching processes like blow moulding and thermoforming

• Thermosets undergo a chemical change; the process is irreversible

DESIGN RULES FOR PLASTIC PARTS

• Allow for shrinkage after moulding

• Allow draft of at least ½ or 1°.

• Avoid under cut which requires cores or split cavity mould.

• Locate hole part in one plane

• Locate holes at right angle to part surface. Oblique holes add to mould cost.

• Design grille element parallel to the flow of plastic mould

• UTS range from 2000 to15000 psi, has to be monitored.

• Locate holes at right angle to part surface

• Arrange ejector pin so that marks will occur on concealed surfaces.

• Design grille elements parallel to the flow of plastic in mold.

• Every plastic material deforms under external load.

• Metals especially steels are good spring materials because they have high strength and

resilience.

R = (1/2Sy Ey) = (1/2 Sy2/E)

Where Sy and Ey are stress and strain respectively.

• Plastics are unsuitable as spring material because they reach the yield point at low value

of stress and behave viscoelastically.

• A short duration of load is common design criteria for plastic spring due to ‘Creep’

tendency of plastics.

APPROCH TO DESIGN WITH PLASTIC

VISCOELASTIC BEHAVIOR OF PLASTICS

• The modulus of elasticity is not constant and depends on the rate of loading.

• At constant stress, the strain will increase with time. This flow effect called creep.

• A plastic material with a history of locked-up stresses tends to unlock and reaches

lower value of stress.

DESIGN FOR INJECTION MOLDING

1) Wall Thickness

• Designer’s Notebook– Keep wall thickness as uniform as possible.– Use gradual transitions between thick and

thin sections.– Wall thickness must suit both function and

process.– Wall thickness guide range is:

0.75 mm to 3 mm for reinforced materials

0.5 mm to 5 mm for unreinforced materials

2) CORNERS

• Designer’s Notebook– Avoid sharp internal corners.– Internal radii should be at least 0.5 and

preferably 0.6 to 0.75 times the wall thickness.

– Keep corner wall thickness as close as possible to the nominal wall thickness. Ideally, external radii should be equal to the internal radii plus the wall thickness.

3) RIBS

• Designer’s Notebook– Rib thickness should be 50 - 75% of

the wall thickness.– Fillet radius should be 40 - 60% of

the rib thickness.– Rib root thickness should not be more

than 25% greater than the wall thickness.

– Rib depth should not be more than 5 times the rib thickness.

– Taper ribs for mold release.

4) BOSSES

• Designer’s Notebook– Before designing a boss,

consider its function and the forces acting on it during assembly and service.

– If the forces are not great, it may be possible to dispense with support ribs.

Plastics Processing: Thermoforming

Sheet of plastic Heated (soft) Molded using a shaped die

MOLDING OF PLASTICS

Compression Molding:-The molding material, generally preheated, is first placed in an open, heated mold cavity. The mold is closed with a top force or plug member, pressure is applied to force the material into contact with all mold areas, while heat and pressure are maintained until the molding material has cured.

Transfer molding:-Transfer molding is a manufacturing process where casting material is forced into a mold.

• The material most commonly used for transfer molding is a thermoset polymer.

Plastics Processing: Blow molding

heated glass

3-piece mold

(a) The hollow piece of heated glass (parison)is first created by a blow mold(see text-book Fig 17.25)

(b) The mold is put together

(c) Plunger and hot air push theglass up

(d) Hot air blows the glass out towardsthe mold surface

(e) Mold comes apart, bottle is removed

heated glass

3-piece mold

(a) The hollow piece of heated glass (parison)is first created by a blow mold(see text-book Fig 17.25)

(b) The mold is put together

(c) Plunger and hot air push theglass up

(d) Hot air blows the glass out towardsthe mold surface

(e) Mold comes apart, bottle is removed

- similar to glass blow-molding -

Injection Molding:-

Example:- Mobile phone , Plug , Rubber pen.

Injection Molding: designing injection molds

1. molding directions number of inserts/cams required, if any

2. parting lines

3. parting planes by extending the parting line outwards

4. gating design where to locate the gate(s) ?

5. multiple cavity mold fix relative positions of the multiple parts

6. runners: flow of plastic into the cavity

7. sprue located:

8. functional parts of the mold

- ejection system: to eject the molded part

- systems to eject the solidified runners

- alignment rods: to keep all mold components aligned

LIVING HINGES

• Designer’s Notebook– Gate position is all important.– Flow must take place across the hinge.– Beware of hesitation effects, weld lines, and

over packing.– Provide a separate hinge cooling circuit.– Flex the hinge immediately after ejection.

BEARINGS

• Designer’s Notebook– For metal shafts, the harder and

smoother the better.– Keep within the PV limit.– Use specific grade data for K-

factor and PV limit.– Except for slow-running and

lightly loaded bearings, verify the design by testing prototypes.

GEARS

• Designer’s Notebook– Consider conditions of service before

selecting the material.– Design for symmetry and avoid

excessive variations in thickness.– Make the center web symmetrical

and avoid ribs, spokes and holes.

DESIGN FOR RECYCLING

• Designer’s Notebook– Thermoplastics are better for recycling

than cross-linked thermosets.– Prefer versatile materials that have a

wide range of applications.– Use compatible materials together to

minimize dismantling and sorting.

DESIGN FOR RECYCLING

• Thermoplastics are better for recycling than cross-link thermosets.• Prepare versatile material that have wide range of applications.• Use compatible material together to minimise • The material of manufacture should be marked on all plastic parts, using standard symbols and abbreviations• Eliminate the use of non-plastic Parts• Welded joints are good for recycling but difficult to dismantle• Design for recycling, but not at the expense of function or service life

SPI RESIN IDENTIFICATION CODE

References

• Design Guides for Plastics Clive Maier, Econology Ltd

• www.injectionmoldingva.com/Plastic-Part-Design.php

• https://www.irjet.net/archives/V2/i2/Irjet-v2i234.pdf

• http://ec.europa.eu/environment/integration/research/newsalert/pdf/IR1_en.pdf

• http://www.technicaljournalsonline.com/ijaers/VOL%20III/IJAERS%20VOL%20III%20ISSUE%20I%20%20OCTBER%20DECEMBER%202013/384.pdf

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