ppt plastics 19oct12

Engineering plastics And basic design guidelines

What are Plastics? ●A plastic material is any of a wide range of synthetic or semi-synthetic

organic solids that are moldable.

●Plastics are typically organic polymers of high molecular mass, but they

often contain other substances.

●They are usually synthetic, most commonly derived from

petrochemicals, but many are partially natural

classification There are two types of polymers

Thermoplastics &Thermosetting polymers.

Thermoplastics

Thermoplastics are the plastics that do not undergo chemical change in their composition when heated and can be molded again and again. Examples include

• Polypropylene (PP)

• High Impact Polystyrene (HIPS)

• Expandable Polystyrene (EPS)

• Crystal Polystyrene (GPPS)

• Acrylonitrile Butadiene Styrene (ABS)

• Low Density Polyethylene (LDPE)

• Poly vinyl chloride (PVC)

• Polycarbonate (PC)

• Nylon 6 (PA6, PA66)

Thermosetting polymers

While thermosets are assumed to have infinite molecular weight. These chains are made up of many repeating molecular units, known as repeat units, derived from monomers; each polymer chain will have several thousand repeating units.

Thermosets can melt and take shape once; after they have solidified, they stay solid. In the thermosetting process, a chemical reaction occurs that is irreversible.

Applications

● Due to their relatively low cost, ease of manufacture, versatility.

● plastics are used in an enormous and expanding range of products, from paper clips to spaceships.

● They have already displaced many traditional materials, such as wood, stone, horn and bone, leather, paper, metal, glass, and ceramic, in most of their former uses.

Wide range of applications

●Automotive

●Home appliances

●Electronics

●Electrical

●Packaging

Automotive applications

Household applications

Electronic

applications

Packaging applications

Plastics Product Design

Considerations

•Wall Thickness

•Drafts

•Radii

•Ribs/Bosses/Gussets

•Undercut

•Holes

Uniform Wall Thickness

Minimizes

•Warpage

•Molded-in Stress

•Sink Marks

•Voids Improved Design

Poor Design

Uniform wall thickness promotes better flow

Efficient Wall Design

Minimum Wall Thickness Increases Productivity:

•Reduce Cycle Time •Reduce Material Consumption

Limits to Minimum Wall Thickness:

•Flow Length

•Structural Stiffness & Strength

•Flammability Rating •Uniform Thickness

Wall Transition When wall thickness transitions cannot be

avoided, the transitions should be made

gradually, on the order of 3 to 1. This

avoids stress concentration

and abrupt cooling

differences

Why Warpage? Factors Affecting Warpage

•Uneven Wall Thickness

•Unequal Cooling Rate

•Unequal Pressure

(Uneven Shrinkage Causes Warp)

Molded-in Stress

Mold-in

Assembly

Application

Low Molded-in

Stress

High Molded-in

Stress

High Performance

Low Performance

Molded-in Stress

•Over packing •Uneven Shrinkage

Causes

Overpackng

•More Material than Required in the Mold Cavity

•Results in Molded in Stresses

Shrinkage •Decreased spacing between polymer chains

at cooler temperatures.

•Must be accommodated

in tooling design.

•Differs from material to

material.

Factors Affecting Shrinkage:

•Material Properties

•Part Geometry

•Manufacturing

Shrinkage and Part Geometry (Stress Due to Uneven Shrinkage)

Non-Uniform Part Thickness Cools Unevenly

Causing High Molded-in Stress

Voids Already Cooled Surface

Will Not Yield to Shrinking Interior

Sink Marks Surface Yields to the

Still Shrinking Interior Mass

Uneven Shrinkage Defects

Material Considerations

Some materials shrink differently than others

and require different part & tool

design considerations

Crystalline Amorphous Filled Amorphous

- - - Part

Rapidly cooling skin pulls apart the slower cooling

core, resulting in a void.

Void Example

Coring should be employed where possible to eliminate material masses in the part. Coring results in more efficient designs and faster more productive cycle times. It also provides more uniform shrink and avoid sink marks.

Coring

Differential Shrinkage Leads to: •Warpage

•Voids

Gating from thick to thin sections reduces differential shrinkage. It avoids a restricted flow and reduces molded in stress. It also allows for

more uniform packing.

Gating Wall

Transitions

Parting lines are the lines on the part where the tool

halves come together. Often the parting line will be noticeable as a bump on the surface of the part which is

caused by the material that seeped into the joint.

Ejection is the process of removing the solidified part from

the mold. Common ejection methods include ejector

pins,ejector sleeves, ejector blades and stripper plate. All this leave “witness mark” on the finished part.

Parting Line and

Ejection

On the surfaces in the draw of the die, a minimum of 1/2 degrees should be specified. Typical draft is 1 degrees. More draft aids

ejection but may generate a material mass on sections contained in one side of the mold.

No Draft Angle

Difficult Ejection Easier Ejection

Draft Draft is the tapering of surfaces parallel to the line of draw (opening and

closing of the mold) for easier part removal.

The larger the draft angle,

the easier the ejection.

Typically draft angle is 10 with 1/20 on

The amount of draft angle depends on the depth

of the part in the mold, and its required end use

function

Mismatch

Specify mismatch on the parting line. Note establishes what is

acceptable to the engineer for the molder.

• For blind holes , the length

over diameter ratio should remain below 2. As the diameter of holes increases above 3/16” , the length over

diameter ratio can increase to 3. •For through holes, support of the core pin can be obtained on

both ends or the pins can meet

in the center which allows the diameter to length ratio to

increase to 4. For diameters greater than 3/16”, the length

to diameter ratio should remain below 6.

Holes and Depressions

Mold - Filling Plot

Reinforcement

Structures

R => .020”

The Larger the Better

Limited by the possibility of sinks and voids

Thermoplastics are notch sensitive!

t t = 0.6 T for T < .125” t = 0.4 T for T > .125”

1/2 to 1 1/2 degrees draft

Rib Design

H > 3T

H < 3T

Improved Design

Rib Height is Limited By the Thickness of the Base.

Rib Design Strategies Initial Design

RIB DESIGN

• Should not be placed at the corners of the part. • Instead of one heavy rib use series of Ribs. • Should be rounded to add Strength.

Careful of wall thickness at intersection

Radius all corners

Rib Intersections

Bosses are commonly found on injection molded plastic parts. They are the standing features on a part which often serve as

mounting or fastening points. They also facilitate alignment during assembly.

Bosses

Boss Design

•Should be avoided at the corners of the part. •Should be of round shape.

•If boss is of square or rectangular shape, corners should have a radii of at least 1/64”.

R = .25 T H = 2 to 5T

Gussets are often placed at points of attachment, support, or contact with other components. Gussets are effective in stiffening

structural features and in distributing stress

Gussets may be considered a subset of ribs and the guidelines

for ribs apply to gussets.

Gussets

•The thickness of the gusset at the intersection with the nominal wall should be 50% of the nominal wall.

•The height of the gusset can be 95% of the height of

the boss it attaches to. Generally the height will be less than 4 times the nominal wall thickness and the preferred height is 2 times the nominal wall.

•The length of the gusset may vary from 30 to 100% of the height of the gusset.

•The intersection of the gusset with the feature or the nominal wall should have fillet with a radius of 25% of

the nominal wall.

•The spacing between gussets should be at least twice the nominal wall thickness

Guidelines for Gussets

A fillet radius should be between 25% to 60% the nominal wall

thickness. The larger fillet radius is suggested for load carrying features. A minimum radius of 0.020 in (0.508 mm) is

suggested. Break any sharp corner with at least a 0.005 in (0.127 mm) radius.

The outside corner radius should be equal to the

inside radii plus the wall thickness (R = r + t)

Radius, Fillets and Corners

Stress Concentrator

r = 0.5 T

R = 1.5 T

Corners

Points of Consideration…for Product Designer

• Product should be molded i.e. Part can be withdrawn after Molding.

• Parting line is going to be extended up to Work piece, check for its

smoothness.

• Check for draft / undercut, specially at ribs, bosses etc.

• Avoid round at parting line.

• Prefer larger radius at vertical round.

• Check draft in direction of slide core movement.

Thank you

ppt plastics 19oct12

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