injection molding - san diego state universityeon.sdsu.edu/~johnston/injection molding.pdf ·...
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Injection Molding:
The Art of Making Plastic Parts
By Helen Anderson and Jason Atkins
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History
Stemmed from die-casting
bullets in the Revolutionary War.
First modern injection mold was
for a billiard ball using the
synthetic organic resin celluloid.
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mold cavity melt
nozzle
plastic pelletshopper
screwheating
coilscold side hot side
parting line
plates
Injection Mold
Major
parts of
the
injection
molding
machine
barrel
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Process
Plastic pellets fill the hopper…
…to form melt in the nozzle…
…fall into
the barrel…
…where
heat and
pressure
are
applied……which is pushed
into the mold.
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Process
After cooling, the mold opens
and the finished part is removed.
The sprue is then removed
from the final part.
It is reground
and placed
back into the
hopper.
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The diameter of the screw inside the barrel gradually increases to generate more pressure and heat helping to melt the material.
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Shape Complexity
Advantages
Quantity
Quality
Materials Available
Cost per Unit
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Disadvantages
Complex geometry can render parts
impossible or extremely expensive.
Unless mass producing, the cost of the mold can
outweigh the benefits of injection molding.
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Materials
Two main types:
Thermosets: Thermal Insulator
High Strength and Hardness
Nonreversible
Thermoplastics: Low Cost
Manufacturability
Reversibility
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Polyethylene(UHMWPE)
20Minimum Operating Pressure (MPa)
120 -180Mold Temp. Range (°C)
~0.2Mold Shrinkage (%)
144 – 152Melting Temp. Range (°C)
-81 - 100Operating Temp. (°C)
25Strain at Yield (%)
500Elongation at Break (%)
1.06+Notched Izod (kJ/m)
0.5Flexural Modulus (GPa)
35Tensile Strength (MPa)
RR50Surface Hardness
0.945Density (g/cm3)
ValueProperty
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Plastic pellets come in all colors, shapes, and sizes.
Dyes can be included to achieve specific colors
when mixed in the barrel.
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Materials
No longer limited to plastics!
Powder Injection Molding (PIM)
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Injection molding the parts for this assembly keeps functionality up and cost down.
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Design
Rib features provide rigidity without adding
lots of weight and extra material.
Draft on surfaces perpendicular to the ‘pull’
direction help remove parts from molds.
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Assembly for Injection Molded Parts
Snap fits are common in injection molded
parts. They can be separable or not,
depending on the demand of the part.
Heat staking is a good secondary
operation for permanent assembly of parts.
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FEA
Finite Element Analysis (FEA)
helps engineers of today design
parts to be injection molded.
Although not an exact science, FEA
can help determine whether or not a
mold will work the way it is expected to.
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Injection molding in action.
Watch the real thing!
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Mold Cavity
Interior of mold cavity.
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Multiple Cavity Molds
Eight parts can be made at
once in this mold.
Each cavity must fill at the same
rate for the parts to be consistent.
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Removal of Parts
Parts can be removed manually…
…or with the help of ejector pins.
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Flow Defects
Jetting is caused by an
undersized gate.
Surface defects can be caused by
improper melt flow throughout a part.
‘Rheology’ is the study of flow under stress
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From art…
…to part.
Injection Molding