Injection Molding
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Introduction
Injection Molding Machines and Operations
Safety
Polystyrenes
Dimension Stability
Measure Residual Stress Using Photoelasticity
Weld-line Influence on Strength
Breaking Gear Teeth
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Importance of Plastics
Plastic industry ~ 100 billion/yr business, 1.4 million production jobs
World consumption of raw material by weight
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Importance of Injection Molding for Plastics
Plastic consumption by processes
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Injection Molding Machines (1000GGB)
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Basic Elements of an Injection Molding Machine
Material
hopper
Mold
Gear
train
Motor
Screw
Hydraulic
pressure
Clamping
Unit
Heaters
Spline
Limit switches
Main IM machine spec. include clamp force (tons),
shot size (ounces or pounds) and injection rate (in3/sec)
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The Basic Operation
1. Raising the temperature of the plastic to melt it. This is
achieved by heaters and the screw (mixing + friction heat).
This overall process is called plastification of the material.
2. Inject the melted plastic (through various flow channels) into
the mold cavity and allow it to solidify in the mold.
3. Open the mold and eject the product
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Typical IM Process Cycle
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Functions of IM Elements
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Safety
• The barrel temperature is usually 350-550° F (177-288°C),
and can be as high as 700° Fahrenheit (371° Celsius).
• Plastic at its molding temperatures is hot and sticky. If it
gets on skin or clothes, usually it is not easy to remove it.
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Safety (cont.)
• The electrical wires to the heater bands are high voltage
(typically 460 volts) and very high current.
• Operating personnel must be aware of all the moving parts
in the molding machine. Protective guards are placed
around potentially hazardous areas but caution from
operators is still necessary.
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Polystyrenes
Polystyrenes (Comes in two forms: foam and solid)
- Foamed polystyrene is used to make cups, bowls, plates, trays,
clamshell containers, meat trays and egg cartons as well as
protective packaging for shipping electronics and other fragile items.
- Solid polystyrene is used in products such as cutlery, yogurt and
cottage cheese containers, cups, clear salad bar containers and video
and audiocassette housings.
Recycling Symbols
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Mechanical Properties of Various Plastics
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Thermofluid Properties of Polymers
• Thermoplastic
• Glass transition temperature
• Low thermal conductivity
(good insulator)
• Extremely high viscosity (high
pumping pressure and losses)
• Viscosity is temperature
sensitive
• Non–Newtonian: Viscosity
decreases as shear rate increases
(shear thinning)
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Temperature and Shear–rate Dependence of Viscosity
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IM process is quite complicated
Molding parameters and part properties are linked through fluid flow.
We will limit ourselves to 2 variables: temperature and flow rate
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Lab 1 Process Parameters
190oC, 200oC, and 210oC at 600 bar
5–6 parts/condition
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Problems from Improper Processing
• “Short shots” — results when insufficient polymer is injected
into the mold.
• “Flash” — caused by excessive packing and/or holding pressure
prying open the mold and driving polymer through the resulting
crack
• Dimensional instability — from removing part without proper
cooling time
• “Sink marks” — caused by excessive shrinkage of the polymer
during the cooling cycle
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Importance of Proper Temperature and Pressure
CAVITY PRESSURE
Pressure: without proper holding pressure, excessive shrink
mark or flashing may result
PEAK PRESSURE
PACKING
GATE SOLIDIFICATION
MOLD FILLING
HOLDING
IN-MOLD
SHRINKAGE
TIME
START INJECTION
MOLD OPEN
Temperature:
Too high: chemical degradation, cooling time, shrink mark
Too low: high viscosity
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Dimensional Stability (Shrinkage)
At temperatures higher than T2 = Tg(shown below), the
higher energy level of molecules creates a “free volume”.
This free volume may be “frozen in” during rapid cooling.
Theoretical density at 0, 10, 100
and 210 sec during quenching (PS)
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Effect of manufacturing parameters on quality
• Gear strength is influenced by
– Shrinkage (gear size and geometry)
– Homogeneity (weld line? Residual stress due to uneven cooling?)
u _bending( p,T)
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Effect of Flow Orientation on Strength
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Effect of Weld Line on Strength
GATE 1
GATE 2
FLOW FRONTS
(a)
WELD LINES
GATE
FLOW
OBSTRUCTION
(b)
WELD LINE
MELT
FLOW
MELT
FLOW
NO DIFFUSION
PARTIAL DIFFUSION
COMPLETE DIFFUSION
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Measure Residue Stress by Photoelasticity
Photoelasticity is a nondestructive, graphic stress-analysis technique
based on the birefringence property possessed by many transparent
polymers. It is mainly used for 2-dimensional (planar) problems.
A polariscope (polarimeter) measures birefringence in specimens.
It consists of a light source, a polarizer, an optional quarter-wave
plate, a specimen, another optional quarter-wave plate, and a
second polarizer called the analyzer.
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Working Principle of Polariscopes
The theory of photoelasticity is based on light refraction that
depends on the orientation of polarized light
Refraction: When light passes through any medium other than
a vacuum, its velocity decreases
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Double Refraction
If similar light waves pass through the same thickness h of two
media having indices of refraction n1 and n2, a phase difference d
between the two waves after they emerge from the media results
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Birefringence
Photoelastic materials are birefringent, i.e., they refract light
differently depending upon the state of stress in the material.
In an unloaded state, the material exhibits an index of refraction
n0 independent of orientation. In the loaded state, however,
the magnitudes of the principal stresses, determine the index of
refraction for that light wave. The change in index of refraction is
found to be (Maxwell Equations)
n1  n0  c11  c2 (  2   3 )
n2  n0  c1 2  c2(  3  1 )

n2  n1  (c 2  c1 )( 1   2 )
n3  n0  c1 3  c2 ( 1   2 )
For birefringent material, c1 c2.
Stress-optic coefficients
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Example Birefringence Patterns
crack
tensile
bending
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Additional Safety Issues
• Sessions 1&2 involve bending fracture of the gears, so it
is very important to wear safety glasses in the lab.
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Torque Test
Single tooth: Torque is carried by one tooth‚ easier to analyze.
Normal engagement: true torque capacity of the gear.
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Model the gear tooth as a rectangular prism:
Thickness b
Force W
Length L
width h
M b y WL (h / 2) 6WL



3
2
bh
I yy
bh
12
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Predicting Gear Strength
• Given the gear geometry you can relate gear torque to the
stress at the root of a gear tooth.
• If you knew the stress that would cause the polystyrene to
fail in tension‚ you could predict the torque at which the
gear teeth would fail.
• Guess what you’ll be doing in lab next week?
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Process Conditions for Session #2
Polystyrene: 190oC, 200oC, 210oC
500 bar
5 parts each for the case with and without weld lines
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Analysis of Session 1 Results
• Make gears at given temperatures and flow rates.
Qualitatively explain which combinations of processing
conditions lead to short shot, flash and sink marks. How do
processing conditions affect final part dimensions?
• Measure pertinent geometrical quantities, including contact
point position when two gears are meshed together.
• From your optical observations, identify regions of high
residual stress in molded part. Qualitatively assess effect of
injection rate and melt temperature (processing parameters)
on residual stress field. What other processing parameters
will impact residual stress field? Qualitatively explain why
residual stress is concentrated only in certain regions.
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Analysis of Session 1 Results, Continued
• Break gear teeth and record failure torque. Does failure
torque depend on temperature or flow rate?
• Perform a strength analysis on gear and relate failure
torque to gear tooth stress. You will measure failure stress
for polystyrene in Session 2.
• Observe fill and cooling times of current injection molding
cycle. Also observe and record mold temperature for your
analysis and computer simulation.
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