The document discusses injection molding. It begins with an overview of injection molding and the types of products made through the process. It then covers the fundamentals of polymers used in injection molding, including their properties and how they behave under heat and pressure. The document outlines the key components of injection molding machines and mold tooling, and reviews the injection molding process parameters and how they impact the filling and cooling cycles. It also discusses common defects and guidelines for part design. Advanced topics covered include side-action molding, insert molding, and overmolding.
11. 2.008x
Kalpakjian and Schmid, Manufacturing Engineering and Technology
Groover, Fundamentals of Modern Manufacturing
Poly (many) + mer (structural unit)
-[C2H4]n- = poly[ethylene]
12. 2.008x
The ‘families’ of materials: modulus vs. density
Ashby, Materials Selection in Mechanical Design.
20. 2.008x
Viscosity: resistance to shear
*at typical injection shear rate and melt temperature
Material Dynamic viscosity
Water (room temp) 1×10-3 kg/m-s [Pa-s]
Honey 10
Liquid thermoplastic* 102-103
Molten aluminum (600 C) 3×10-3
y
U
¶
¶
= µt
Ux(H) = Ux
Ux(y)
Ux(0) = 0
h
Ux
x
y
21. 2.008x
Viscosity of polypropylene versus shear rate
and temperature
From Solidworks Plastics
µ = k !γ(n−1)
100 C
250 C
Ux(H) = Ux
Ux(y)
Ux(0) = 0
h
Ux
x
y
27. 2.008x
An injection molded cap
§ What features do you notice?
§ Compare quality to LEGO bricks; what is different?
§ What do the molds look like (draw the molds)?
36. 2.008x
How would you choose
an IM machine?
(important specs?)
§ Clamping force: force available
to hold plates together.
§ Injection pressure: maximum
pressure that can be developed
to force the plastic into the mold
cavity.
§ Shot size: amount of material
that can be transferred to the
mold (i.e., the part volume plus
runners, gates, etc).
43. 2.008x
Simple scaling of injection parameters for a
2D rectangular channel
÷
÷
ø
ö
ç
ç
è
æ
µ
÷
ø
ö
ç
è
æ
=D
=D
2
3
2
3
12
12
h
wL
F
h
L
P
wh
QL
P
fill
clamp
fill
t
µ
t
µ
µ
44. 2.008x
Simulating injection using Solidworks Plastics
A simple plate:
§ L = W = 100 mm
§ h (thickness) = 2 mm
§ Polypropylene (PP)
§ Tmelt = 250C
§ Tmold = 70C
à Above, we predicted injection
pressure DP = 3 MPa
46. 2.008x
Viscosity of polypropylene versus shear rate
and temperature
From Solidworks Plastics
µ = k !γ(n−1)
100 C
250 C
Ux(H) = Ux
Ux(y)
Ux(0) = 0
h
Ux
x
y
51. 2.008x
How do we model cooling of the part?
2
2
2
2
y
T
y
T
c
k
t
T
p ¶
¶
=
¶
¶
=
¶
¶
a
r
Mold
Mold
Part
x
y
L
h
h/2
h/2
52. 2.008x
Exact solution for a plate
Tm = melt temperature
Tw = wall temperature
Te = ejection temperature
Drawing from Leinhard, A Heat Transfer Textbook
Also see BASF ‘estimating cooling time’ http://www2.basf.us/PLASTICSWEB/displayanyfile?id=0901a5e1801499d3
a4
2
h
tcool =
MoldMold Part
a = thermal diffusivity = k / rcp
~0.1 mm2/s for thermoplastics
÷
÷
ø
ö
ç
ç
è
æ
-
-
=
we
wm
cool
TT
TTh
t
pap
4
ln2
2
à We define the cooling time
as the time until the
temperature at the centerline of
the part reaches the specified
ejection temperature
‘Rule of thumb’
if (Tm-Tw) ≈ 10(Te-Tw)
53. 2.008x
Cooling time scaling for plate geometry
Tm = 200 ºC = 473 K
Tw = 77 ºC = 350 K
tcool =
h2
4α
tcool =
h2
π2α
ln
4
π
Tm − Tw
Te − Tw
54. 2.008x
How do process parameters vary with
part size?
ΔP =
12µ
τ fill
L
h
"
#
$
%
&
'
2
Fclamp ∝
µ
τ fill
wL3
h2
"
#
$
%
&
'
tcool ∝
h2
4α
58. 2.008x
Polymers change volume with pressure and
temperature
à imagine a sponge that tries to shrink but is glued to
the inside walls of a rigid container: residual stress!
60. 2.008x
In other words…
§ Polymers shrink during cooling; that’s a fact.
§ If the shrinkage is constrained by the mold, residual
stresses are ‘trapped’ because the part cannot relax as
the polymer shrinks.
§ During injection molding, the variation in shrinkage
both globally and through the cross section of a part
creates internal stresses or residual stresses that act
on a part with effects similar to externally applied
stresses.
§ These residual stress can cause the part to will warp
upon ejection from the mold or crack when loaded
during use.
73. 2.008x
Corners, fillets and hinges (‘living hinges’)
R = 0.2 mm
2 mm
R = 1 mm
Fillets
Corner radius
2 mm
0.25 mm
Hinges
Note
blistered
edges
74. 2.008x
Draft angles
2 mm
Fins on Protomold cube
à Draft angles enable
easier part ejection.
à The required draft
angle depends on
thickness, and surface
texture.
LEGO brick
100 µm
76. 2.008x
The process window always applies, but the
conditions are different everywhere in your
part!
à Therefore it’s not good enough to be in the process window!
à Beware of common defects, design for maximum uniformity, and
reduce risk by following DFM guidelines (see supplements)
82. 2.008x
Metal injection molding (MIM)
à Perform injection molding using a metal powder mixed
with polymer binder; then anneal the part to achieve
higher density (with significant shrinkage)
83. 2.008x
8. Conclusion: the big four
Injection Molding Machining
Rate High Low-Medium
Quality Good As good or better!
Cost Low (at high volume) Almost always greater
Flexibility Low (tooling cost high) High (within machine
constraints)