Manufacturing Consideration
Manufacturing Considerations
• Injection Molding is a high speed, automated
process that can be used to produce simple to very
complex parts
• The part designer must recognize that the design
of the part determines the ease of molding, the
tooling requirements and the cost
• Also the designer must recognize that the
properties of the part are greatly affected by the
mold design and processing conditions
Manufacturing Consideration
• Injection molding is a series of sequential
process steps, each of which has an
influence on the properties of the resultant
part
–
–
–
–
Mold filling
Packing
Cooling
Ejection
Manufacturing Consideration
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•
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•
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•
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Gating
Orientation
Pressure losses
Frozen in stress
Shrinkage and Warpage
Weld/Meld lines
Flow leaders/restrictors
Gating
• The gate is the melted plastics entry into the mold
cavity
• Usually the thinnest cross section in the system
• The gate type, number of gates and gate location
has a dramatic effect on overall part quality
– Determines the mold filling pattern
– Induces shear and shear heating
– Affects shrinkage and warpage
Gating
• Gating determines the type and cost of the
mold
– Edge or sub gated parts can be produced with a
standard cold runner two plate mold
– Top center gating or multiple top gating
required a three plate mold
Gate Design Rules
• Gate centrally to provide equal flow length
• Gate symmetrically to avoid warpage
• Gate into thicker sections for better filling
and packing
• Gate long, narrow parts from an end for
uniform flow
Gate Design Rules
• Position the gate away from load-bearing
areas
• Hide the gate scar
• Gate for proper weld-line location and
strong weld lines
• Multiple gates shorten flow lengths
• Locate gates on either side of a weak core
or insert
Orientation
• Almost all injection molded parts have some
degree of frozen-in molecular orientation
• The degree is determined by the molecular weight,
relaxation characteristics, and processing
conditions
• Orientation greatly affects the properties of the
part
– Shrinkage
– Strength
– Residual stresses
Orientation
• Mold filling related orientation can be
affected through process variables that
affect mold filling pressure requirements
– Flow direction and speed
– Channel dimensions
– Temperatures
• Residual Orientation = Orientation due to
flow - relaxation
How Molecular Orientation Occurs
• Molecular orientation develops during mold
filling as the plastic is injected through the
nozzles, runner, gate and cavity
• The polymer chains become stretched out
due to velocity gradients
• The orientation tends to be in the direction
of flow
How Molecular Orientation Occurs
• The blunted shape of most polymer melt velocity
profile causes most of the orientation to occur
toward the surface.
• The molecules at the core remain random
• Extreme in injection molding where the melt
adjacent to the cold mold will freeze first, leading
to high interfacial shear stresses and not allowing
for relaxation
• Problems are most significant for higher molecular
weight plastics and fiber reinforced plastics
How Molecular Orientation Occurs
Effects of Molecular Orientation
• Orientation creates different directional
properties
– Stronger is the flow direction
– Weaker in the transverse direction
Effects of Molecular Orientation
• Typical directional property of an injected
molded part
Orientation
• The degree of orientation caused by mold
filling is influenced by processing
conditions, material properties, mold design
and part design
– Large diameter runners, sprues, gates along
with shorter flow lengths will reduce
orientation
– Faster fill rates and higher melt temperatures
tend to promote molecular relaxation
Mold Filling Pressure Loses
• When selecting a gate location, it should be such
that the mold fills uniformly, the pressure drop is
not excessive and the shear rate does not exceed
the limit of the polymer
• The designer must obtain an estimate of the
pressure drop to evaluate the moldability of the
part with respect to a proposed gating scheme
• The pressure drop depends on the material, mold
and processing conditions
Mold Filling Pressure Loses
• Assuming isothermal, laminar, Newtonian fluid (ok for
engineering estimate) the equations for pressure drop and
shear rate are:
– Cylindrical
Rectangular


4 *Q
 
 * r3
8 * Q * * L
P 
 * r4
6*Q
W *H2
12 * Q * * L
P 
W *H3
 
r
L
W
H
L
Mold Filling Pressure Loses
•  is the shear viscosity
– Pa-sec, lb-sec/in2
•  is the apparent wall shear rate
– Sec-1
• Q is the volumetric flow rate
– M3/s, ft3/s
Apparent vs Corrected Shear Viscosity
• Most viscosity data is of the form apparent
shear viscosity at the wall as a function of
wall shear rate and temperature
• If shear viscosity is described as apparent, it
is not corrected for pseudo-plastic behavior
Apparent vs Corrected Shear Viscosity
• The corrected shear viscosity is
– Cylinder
Rectangle
4n
t  a [
]
(3n  1)
3n
t  a [
]
(2n  1)
t  true  vis cosity
a  apparent vis cosity
n  power  law  index
Estimating Pressure Drop
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Determine part volume
Determine volumetric flow rate
Determine apparent shear rate
Determine apparent shear viscosity
Determine true shear viscosity
Determine pressure drop
Estimating Pressure Drop Example
• High impact polystyrene ruler
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Sprue 0.313”diameter by 2” length
Runner 0.25”diameter by 2.25” length
Edge Gate 0.08”deep by 0.4”wide by 0.12” length
Cavity 0.1”deep by 1.5”wide by 6.03” length
• Single cavity
• 200 degree centigrade
• 1.5 seconds fill time
• n=1
Estimating Pressure Drop Example
• Determine part volume
– Cylinder
• V = *r2 *L
– Rectangle
• V = L*W*H
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•
•
•
Sprue
Runner
Edge Gate
Cavity
0.154in3
0.110in3
0.004in3
0.905in3
Estimating Pressure Drop Example
• Determine volumetric flow rate
– For single cavity mold
– QT=Qs=QR=QEG=QC
– QT=VT/tF
• VT is total volume = 1.173in3
• tF is fill time = 1.5 seconds
• QT=0.782in3/sec
Estimating Pressure Drop Example
• Determine apparent shear rate
– Cylinder
Rectangular

6*Q
 
W *H2

4 *Q
 
 * r3
–
–
–
–
Sprue
Runner
Edge Gate
Cavity
259/sec
510/sec
1830/sec
312/sec
Estimating Pressure Drop Example
• Determine apparent shear viscosity
– From figure
– Conversion factor
• Lb*sec/in2 = 6894.7 Pa*sec
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•
•
Sprue
Runner
Gate
Cavity
320 Pa*sec
270 Pa*sec
180 Pa*sec
305 Pa*sec
0.046lb*sec/in2
0.039lb*sec/in2
0.026lb*sec/in2
0.044lb*sec/in2
Estimating Pressure Drop Example
Estimating Pressure Drop Example
• Determine true shear viscosity
– Cylinder
Rectangle
4n
t  a [
]
(3n  1)
3n
t  a [
]
(2n  1)
– n=1
•
•
•
•
Sprue
Runner
Gate
Cavity
0.046lb*sec/in2
0.039lb*sec/in2
0.026lb*sec/in2
0.044lb*sec/in2
Estimating Pressure Drop Example
• Determine pressure drop
• Cylinder
Rectangular
•
•
•
•
•
8 * Q * * L
P 
 * r4
Sprue
Runner
Gate
Cavity
Total
305 psi
716 psi
149 psi
1650 psi
2820 psi
12 * Q * * L
P 
W *H3
Frozen in Stress
• Molding factors, such as uneven part
cooling, differential material shrinkage or
frozen in flow stresses cause undesirable
residual stress
• Residual stresses can adversely affect
– Chemical Resistance
– Dimensional stability
– Impact and tensile strength
Shrinkage and Warpage
• Injection molding is used to produce parts with
fairly tight dimensional tolerances
• Many plastics exhibit relatively large mold
shrinkage values
• If a plastic exhibits uneven directional shrinkage,
warpage will result
• Shrinkage is affected by the material, the mold,
the part geometry and the processing conditions
Shrinkage and Warpage
• Parts with thick and thin wall sections can
easily warp because the thick sections take
longer to pack and cool, resulting in uneven
shrinkage
– When the part is ejected the thicker hotter
sections will continue to cool and shrink
PVT Behavior of Plastics
• Plastics have a positive coefficient of
thermal expansion and are highly
compressible in the molten state
• Volume of any given mass will change with
both temperature and pressure
• Semi-crystalline plastics shrink more than
amorphous because of the ordered
crystalline regions
PVT Behavior
PVT Behavior
Linear Mold Shrinkage
• Volumetric shrinkage can be predicted
theoretically if PVT characteristics and the
processing conditions
• We need linear shrinkage for cavity design
– Linear Shrinkage = 1-(1-volumetric shrinkage)1/3
– Cavity dimension=Part dimension/(1-linear shrinkage)
– Expressed in in/in or mm/mm or %
Uneven Shrinkage and Warpage
• Uneven shrinkage is undesirable because it can
lead to not hitting dimensions, internal stresses
and warpage
• Main causes
– Differential shrinkage due to orientation
– Differential cooling due to differences in cooling rate
from cavity to core
– Cavity pressure differences due to too much pressure
drop through the cavity
Mold Shrinkage Data
Mold Shrinkage Sample Problem
• The material that a part is made from has a
volumetric shrinkage of 0.1in3/in3.
• What must be the cavity dimensions be to
make a part
– 3.02 inches wide
– 5.67 inches long
– 0.1 inches thick
Mold Shrinkage Sample Problem
S L  1  (1  SV )
1
3
S L  1  (1  0.1)
1
3
in
S L  0.0345
in
3.02in
Width 
 3.128in
in
1  0.0345
in
5.67in
Length 
 5.872in
in
1  0.0345
in
0.1in
Thickness 
 0.104in
in
1  0.0345
in
Flow Leader and Restrictors
• Ideally the melt should flow from the gate,
reaching the extremities of the cavity all at
the same time
• To achieve balanced fill, the filling pressure
drop associated with each and every flow
path must be equal
• Pressure drops can be balanced by making
local adjustments in the part wall thickness
Flow Leader and Restrictors
• Flow Leader are local increases in wall thickness
to promote flow
• Flow restrictors are local decreases in wall
thickness to reduce flow
• If flow is not balance
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Overpacking/underpacking
Variable shrinkage
Residual Stress
Tendency to warp
Flow Leaders and Restrictors
Weld and Meld Lines
• Formed during filling
when melt flow front
separates and
recombines
• Cause by
– Multiple gates
– Cores/Holes
• Looks like a crack on
the surface of the part
Weld and Meld Lines
• The strength of the weld line can be
significantly lower
• Try to eliminate completely or locate in non
critical area in terms of load and appearance
– Vary part geometry, part wall thickness and
gating scheme
Weld and Meld Lines
• Processing conditions affects the weld
strength
– Molecular diffusion and entanglement are
necessary to improve weld strength
• Increase the temperature
• Increase the pressure