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DESIGN FOR PLATICSDESIGN FOR PLATICS
A synthetic material made from a wide range of organic polymers such as polyethylene, PVC, nylon, etc., that can be molded intoshape while soft, and then set into a rigid or slightly elastic form.
POLYMERISATION:- Polymerization 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 polymerization process are called Polymers.
WHAT IS PLASTIC?
THERMOPLASTICS: Describes a substance which becomes flexible when heated and hardens on cooling with no change in its properties. Examples:Polyethylene: packaging, electrical insulation, milk and bottles, packaging film Polypropylene: carpet fibers, automotive bumpers, microwave Polyvinyl chloride (PVC): electrical cables cover, credit cards.Polystyrene: disposable spoons, forks, Styrofoam™Acrylics (PMMA: polymethyl methacrylate): paints, fake furPolyamide (nylon): textiles and fabrics, gears, bushing and washersPET (polyethylene terephthalate): bottles for acidic foods like juicesPTFE (polytetrafluoroethylene): non-stick coating, 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, toysSilicones: surgical gloves, oxygen masks in medical applications, joint seals
Characteristics of plastic:Creep and shrink as time passesBad conductor of heatShrinkage problemTheir properties change over the temperature range of everyday lifeThermoplastics undergo a physical change when processed; the process is repeatable.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 molding and thermoforming
Thermosets undergo a chemical change; the process is irreversible
DESIGN RULES FOR PLASTIC PARTS• Allow for shrinkage after molding.• Allow draft of at least ½ or 1°.• Avoid under cut which requires cores or split cavity mold.• Locate hole part in one plane.• Locate holes at right angle to part surface. Oblique holes add to mold cost.• Design grille element parallel to the flow of plastic mold.• 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.
APPROCH TO DESIGN WITH PLASTICEvery 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.
VISCOELASTIC BEHAVIOR OF PLASTICSThe 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.
1. Wall Thickness
• 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
DESIGN FOR INJECTION MOLDING
2. Corners
• 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.
DESIGN FOR INJECTION MOLDING
3. Ribs
• 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.
DESIGN FOR INJECTION MOLDING
4. Bosses
• 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.
DESIGN FOR INJECTION MOLDING
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.
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
Injection Molding: designing injection molds
1. molding directions - number of inserts/cams required, if any2. parting lines 3. parting planes - by extending the parting line outwards4. gating design - where to locate the gate(s) ?5. multiple cavity mold - fix relative positions of the multiple parts6. runners: flow of plastic into the cavity7. 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
• 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
• 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
• 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
• 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.
• 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
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