PRODUCT DESIGN:
Deals with conversion of Ideas in to Reality.
BASIC CONCEPTS OF DESIGN
1. Size
2. Shape
3. Function
4. Eye-Appeal
5. Quality
6. Cost
Design by Evolution
1. In the past, designs used to evolve over long spans of time. The leisurely pace of technological change reduced the risk of making major errors. The circumstances rarely demanded analytical capabilities of the designer. This was design by evolution.
2. Development of the bicycle from its crank operated version to its present present day day chain chain and and sprocket sprocket version version over over a period period of of about about a century is a typical example of design by evolution.
The disadvantages of evolutionary design are:
• Unsuitability for mass production
• Difficulty in modification
• Inability to tap new technologies
Design by Innovation
Following a scientific discovery, a new body of technical knowledge develops rapidly, the proper use of this discovery may result in an
almost complete deviation from part practice. Every skill, which the designer or the design team can muster in analysis and synthesis, is
instrumental in a totally novel design. Examples of design by innovation are:
•Invention of laser beam which has brought about a revolution in medical and engineering fields. Laser based tools have made surgical
knife in medicine and gas cutting in engineering obsolete.
•Invention of solid state electronic resulting in miniaturization of electronic products, which has made vacuum tubes obsolete.
Essential Factors of Product Design
•Need
•Physical reliability
•Economic worthiness
•Financial feasibility
•Optimality
•Design criterion
•Morphology
•Design process
•Sub problems
•Reduction of uncertainty
•Economic worth of evidence
•Bases for decision
•Minimum commitment
•Communication
Why Plastics
Why we use plastic material in the first place instead of traditional and familiar material such as metal. In general plastic offers impressive advantages over metals. Some of it are listed below:
•They are not subjected to corrosion
•They are light in weight with good strength to weight ratio
•Very cost effective
•The speed with which they can be produced
• They give design freedom
•They provide with good electrical insulation property
•They are available in wide range of colours
•Reduced assembly time
The Case for Plastics:
•Light weight
•Toughness
•Resilience
•Vibration Damping
•Resistance to Fatigue
•Low Co-efficient of Friction
• Thermal Insulation
•Corrosion Resistance
•Colour Possibilities
•Manufacturing methods
•Integrated Design
•Price
The Apparent Limitations of Plastics:
It would be misleading not to mention that plastics have some disadvantages, as
do all materials. These disadvantages frequently turn out to be not so much
limitations as challenges for the designer to think of plastics as materials in their
own right rather than as substitutes.
1. Strength, Surface hardness and Abrasion resistance
2. Modulus
3. Temperature resistance
4. UV Resistance and Outdoor weathering
5. Flammability
6. Thermal Expansion
7. Electrostatic charges
8. Orientation
The designer should therefore bear this in mind and take appropriate steps to
overcome the same, in order to meet the specified requirements of application.
Principles & Concepts in Plastic Product Design
Successful manufacture of good plastic products required a combination of
sound judgement and experience. Design of a good plastic product requires
knowledge of plastics & their properties, various moulding methods, post
moulding procedures and information on key design areas, such as
1. Wall thickness
2. Parting line
3. Ribs and Bosses
4. Fillets, Radii and gussets
5. Taper & draft
6. Holes
7. Coring
8. Gate size & location
9. Location of Ejector pins
10. Tolerance
11. Undercuts
12. Use of metal inserts
13. Threads
14. Fasteners
15. Surface & Finish
16. Shrinkage
Nominal wall thickness:
The determination of wall thickness should be the result of an analysis of the following requirements.
Functional Requirements
1. Structure
2. Strength
3. Dimensional stability
4. Weight
5. Insulation
Manufacturing Requirements
Moulding
Flow, - Setting & - Ejection
Assembly
Strength & Precision
Proper wall thickness - Success of the product
In adequate wall thickness - Poor performance or Structural failure
Too thick section - Product unattractive, Over weight or Too expensive & Defective (Warpage, Sink mark, etc., )
Parting line :
Selection of Parting line - To assist easy ejection
Types of parting line
Ribs and Bosses :
The function of ribs
• To increase the strength and rigidity without increasing the wall thickness.
• To prevent warpage during cooling.
• Facilitate smooth flow during moulding.
Bosses:
It is a protruding studs.
Assist in the assembling of parts
Radii, fillets & gussets:
Advantages of fillets and radii are:
1.Improving flow of plastics material
2. Eliminates cracking and increase impact strength
3.Better structure with more rigidity and
better stress distribution
4.Reduction in cycle time
5.Uniform density of the molded article
6.Ensure more economical and long life of mold
7.Prevent cracking of mold parts during heat treatment.
Coring:
Heavy section should be cored to provide uniform wall thickness.
Parts having heavy cross sections are subject to longer cycles and causes laminations or sinks, blisters, warpage and increased manufacturing costs. Core out or thin down heavy sections to preclude diffic
Undercuts:
Indentation or projection on a molded part - Ejection almost impossible
There are many different types of undercuts in
molded plastic parts.
Threads:
Molded, Tapped or Part of insert.
Threads used in plastics are :
1. Square 2. Buttress 3. ACME 4. Bottle 5. Unified thread forms
Coarse threads generally preferred
Fine threads - Closer tolerance - Increase mould cost
Inserts:
1. To provide strength
2. To fulfill electrical requirements
3. To speed up assembly operation
4. To support loads
5. To metal functional requirements
6. Combination of metal and plastics
Gate size and location:
Types of gate. - Appearance
Size of gate. - Or
Location of gate. - Function of the part
A small gate is desirable so that:
1. Gate freezes soon. Prevent voids due to suck back.
2. Easy degating or Automatic degating.
3. Small witness mark remains.
4. Better control of filling of multi - impression Possible.
5. Packing in excess for compensating the shrinkage.
Location of ejector pins:
Positioning of ejector pin - Marks in the visible area Uniform lifting of the product
Tolerance
Bilateral Unilateral - As generous as possible
Wider tolerance - Easy to achieve
Closer tolerance - Difficult & expensive
to achieve
Fastening:
1. Mechanical fasteners
2. Mechanical means
3. Welding
4. Adhesives
1. Mechanical fasteners
1. Screws
1. Thread forming
2. Thread cutting
Clasps: Clasps are used on plastic containers or boxes.
Standard pin hinge
1. Require moulded holes or drilled holes.
2. Expensive or require cam.
Inexpensive pin hinge
1. Eliminate drilling or cam operation
Heat sealed hinge
1. Very strong and durable
2. Two tabs are heated and bent permanently around the two pins.
Ball grip hinge
1. Used in small boxes.
2. An accepted standard in the box industry.
3. Balls of 3.2 mm in diameter are molded.
4. Depth 0.45 mm.( approx.)
Shrinkage:
1. Value differ from material to material
2. Generally shrinkage datas in two values
Lesser value - For thin parts ( 1.8 mm or less )
Higher values - For thick parts ( 3.8 mm or more )
The choice of shrinkage for a selected material and a specific design is the responsibility of the mould designer, moulder
and product designer.
Transfer or Injection moulding - Higher shrinkage value
Compression moulding - Lower shrinkage value
Higher shrinkage due to - Imparted directional flow.
Use of small gates do not permit
the application of high pressure to
the cavityVu
The shrinkage of injection moulded thermoplastics will be affected as follows:
1. Higher cavity pressure will cause lower shrinkage
2. Thicker parts (3 mm or more) will shrink more than thinner ones.
3. Mould temperature 27°C or less will bring about lower shrinkage, whereas
temperature 40°C and above or more will produce higher shrinkage.
4. A melt temperature of the material at the lower end of the recommended range will
produce a lower shrinkage, but the upper end of the range will produce a higher shrinkage.
5. Longer cycle time, above the required solidification point, will bring about lower shrinkage.
6. Openings in a part will bring out lower and varying shrinkage than the part without opening.
7. Larger gates permit higher pressure build - up in the cavity and will cause lower shrinkage.
8. For crystalline and semi crystalline materials, the shrinkage value will be higher
in flow direction and lower in perpendicular direction. But in a symmetrical part,
when center gated, the shrinkage will average out and be reasonably uniform.
9. Glass reinforced or otherwise filled thermoplastics have considerably lower
shrinkage than the basic polymer.
Surface finish:
Decorative texture - Improve aesthetic look in moulding
Electroplating of metal on plastics - Decorative effect.
Hide defects -
Weld line, flow line, shrink marks, etc.,
Necessary radii are provided so that while plating the concentrated effected is uniform through the surface.
Raised letter - Easier & cheaper to produce
Depression letter - Costlier, because lettering is machined inside the cavity.
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