60. Injection Molding Principles

61. Plastics Advantages Disadvantages Light Weight
Complex Parts - Net Shape
Variety of Colors (or Clear)
Corrosion Resistant
Electrical Insulation
Thermal Insulation
High Damping Coefficient
Creep
Thermally Unstable- Can’t
withstand Extreme Heat
U-V Light Sensitive
Difficult to Repair/Rework
Difficult to Sort/Recycle
Plastics and plastic processes are widely used in manufacturing and becoming even more so.
They have a number of excellent features, including...
Light Weight-- Have exceptional strength to weight ratios in some cases (e.g. Kevlar-- although the strongest often have limited formability).
However, despite the light weight they are all subject to creep
Plastics are generally very good insulators-- electrical, thermal, and vibrational
Plastics do not corrode as metals do,
However they do degrade under UV light
Moreover, they generally can not be reworked. If broken plastic can’t be welded and if misshapen, parts can’t be corrected, but must instead be melted and redone.
Plastics are not magnetic, thus it is difficult to separate and recycle.
Today there is an emphasis on design for manufacturability (Boothroyd and Dewhurst) which generally leads to a reduction in total number of parts. In general, if fewer parts are intended to to the same job, the parts must be more complex. Plastics (in particular processes such as injection molding) are well suited to the added part complexity.

62. Variety of Plastics & Plastics Processes
Two basic types of plastics
Thermoset- Heat hardening/ Undergoes chemical change
Thermoplastic- Heat softening/ Undergoes physical change
A wide variety of plastic manufacturing processes exist
Extrusion Expansion
Thermal Forming Lamination (Calendering)
Foaming Spinning
Casting Solid-Phase Forming
Molding
Two types of plastics Thermoset- NOT recyclable, permanent change w/ heat
Thermoplastic- is recyclable, temporary change returns to
its original state with removal of heat
A wide variety of manufacturing processes exist-
Extrusion- melt pellets, push through die
Expansion (insul/packing mat’l) expandable polystyrene bead fill mold & bond (steam)
Lamination- Stack layers onto one another
Thermal Form. heat and pressure (vacuum & pressure forming)
Foaming- liquid chemicals- combine&cure (isocyanate polyal)
Molding- heat and pressure- 5 types Compression heat/pressurize large billets to fill mold (no inserts) Transfer- billet in separate tank, pushed into mold (inserts) Blow Mold- Extrude paracem- blow to fill mold Rotational Mold- Heat and spin to fill mold- start w/ powder (good fill) Injection (IM)- Force (plunger/plunger&screw) into mold
NOTE: ITALICIZED TERMS are NOT covered in the VIDEO
Reaction Injection Molding (RIM) (in same general category are Liquid Reaction Molding- LRM or Liquid Injection Molding- LIM)- involves thoroughly mixing two reactive liquids, then injecting into mold. Initial injection pressures typically much lower than traditional IM.
Also notice the process known as calendering- basically sheets of plastics are laminated onto one another by rolling through heated rollers.
Plastic Casting Processes work about the same as any other casting- Plastic is placed in the mold and then hardened into a rigid article.
Spinning produces plastic fibers and is quite similar to extrusion.
Solid phase forming involves forming the plastic below the glass temperature and bears many similarities to stamping, forging, etc.
Thermoplastics- Injection Molding, Extrusion, Blow Molding, Calendering
Thermosets- Compression Molding, High Pressure Lamination, Reaction Inj Molding
Transfer Molding
Blow Molding
Injection Molding
Compression Molding
Rotational Molding
Reaction Injection Molding

63. Injection Molding Basic Process--
Heat plastic to beyond Glass Transition Point
Close mold, creating a fully enclosed cavity
Pressurize plastic melt and inject into mold cavity
Allow plastic melt to solidify, while keeping mold filled
Open mold and eject part
Key issues for design of part and mold
1) Filling Mold & Holding Pressure
Mold layout designed to enhance fill (use good fluid flow principles)
After part is filled, packing pressure is maintained, so part will not shrink
away from walls as it solidifies
2) Ejecting Part
Part should hang on moving side as it retracts (pulling free of fixed side)
Ejector pins then push part out of moving side of mold
Taper or draft required to ensure ejection
Injection molding began as an industrial process with John and Isaiah Hyatt in They won a prize for $10,000 for the development of a material (cellulose nitrate) which could replace ivory in billiard balls.
It is the most common of the plastic forming processes today, accounting for approximately 30% of all plastics produced.
Basic working of injection molding is as follows- plastic material is heat to glass point (or beyond) and pressurized. Mold is closed. Pressurized plastic melt is forced into mold. Mold remains closed while part hardens. Mold opens, part is removed and process repeats.
The key points to this are getting the plastic to fill the mold and keeping it there, and then ejecting part after it has solidified.
To help ensure good fill, you must employ good fluid flow principles. This means we fill parts from thick to thin. In other words put the plastic in the thickest section and flow to the thinner sections. Don’t start in a thin section and move to the thick sections. Also try to avoid sharp corners in the runner system; use nice rounded curves. To allow for ejection one must include some taper or draft in the part. Typically between 1/2o to 3o is required for this.

64. Injection Mold Layout
The plastic melt flows from the injection nozzles and enters the mold at the sprue. From the sprue the plastic can flow into the runners and ultimately through the gates into the part. Gate and runner design is an important part of the mold design.
To help ensure that the mold fills you want to balance the mold so that all cavities fill at the same time. When the cavities are the same, a symmetric layout is used. If the cavities are all markedly different, often the gates and runners must be sized/shaped differently in order to allow all cavities to fill in the same amount of time.
Plastic flows from injection nozzle into sprue then into the
runners and finally through the gate into the part
Want to balance the mold so that all cavities fill at same time

65. Plunger Type Injection Molding Press
Hopper
Shooting
Pot
Torpedo
Plunger
Nozzle
Band Heaters
The first injection molding press was used to make blanks for dental plates. The original presses were plunger type presses. These presses are simple and relatively inexpensive In the RDPL we have a 20 ton Morgan Press which is a plunger type injection molding press. The Morgan Press costs about $15,000.
Here the plastic is fed into the mold when a cylinder plunger extends and forces the plastic into the mold. After the plunger retracts more material can be fed from the hopper to the shooting pot. (Thus the stroke of the plunger determines the additional material fed in each time.) Of course the shooting pot is long enough to hold several shots, so the plastics stays in the pot for a while, giving the band heaters time to heat and melt the plastic. Notice the torpedo, which is basically an obstruction to the plastic flow in the shooting pot. As the plastic moves around the torpedo, it is better mixed.
Material is stored (in pellet form) in the hopper
Band heaters heat material as it moves through shooting pot
Stroke of the plunger meters the shot size

66. Screw Type Injection Presses
Hopper
Screw meters plastic,
plunger provides
pressure
Nozzle
Band Heaters
Reciprocating
Screw
Shooting
Pot
Virtually ALL industrial presses are screw type presses
Added benefits of screw
1) Larger throughput
2) Obtain a more homogeneous melt (better mix)
3) More consistent from shot to shot
4) Added heat to melt- from action of screw
The original plunger type has had one important modification. A reciprocating screw now forces material into the mold. This screw action ensures that the same amount of material is always metered in, and it is packed equally dense along the length of the screw. Additionally the material will be much better mixed by the screw action, all of which helps to maintain much more consistency from shot to shot. Since the screw action generally helps to pack the material in better, a given plunger travel will push more material into the cavity. Finally the action of the screw as it rotates and mixes, adds energy to the melt. (Band heaters are still needed to fully heat melt, however.)
All of this results in a much better and more consistent part. This is why the screw press is essentially the only press found in industry. (Although small plunger presses are still made for prototype/lab purposes, such as the RDPL.)

67. Press Parameters 3 parameters commonly used to describe press capacity
1) Clamping force- Force available to hold platens together (tons)
Can be from Hydraulic/Pneumatic Cylinder
Mechanical Toggle Clamp
2) Shot size- Amount of material that can be transferred to mold in a
shot (given in either cm3 or ozs)
3) Injection Pressure- Maximum pressure that can be developed at the
sprue to force the plastic into the mold cavity
Generally, when one hears the about the capacity of an injection molding (IM) press, one hears the maximum clamping force. Thus presses are talked about as being 20 ton, 50 ton, etc. Additionally, we are also concerned with the shot size and the injection pressure that the press can generate.
Shot sizes are usually speced in cubic centimeters or ozs. (Not entirely sure why one set of units specs a volume and the other specs a mass, but English spec for shot size is typically given in ozs, where ozs represent 1/16 of a pound, not 1/16 of a pint).
Finally one “specs” a press in terms of the maximum pressure it can generate at the sprue in order to force (inject) plastic into the mold.

68. Injection Molding Defects
Short Shot- Insufficient material put in cavity/
material solidifies too soon
Ejector Pin Marks- Ejector pins not flush/ undersized
or plastic too soft at ejection
Weld Lines- Occur wherever flow paths come together
Sink Marks- Material shrinks away from walls of mold cavity
Residual Stresses- Generated by constraining part while cooling
Parting Lines- Plastic seeps through seam between mold sections
Jetting- Melt moving rapidly, cools unevenly and traps flow lines
Flashing- Material overflows cavity
(too much material, not enough clamp force)
These are some of the more common Injection Molding Defects.
When designing a part and the associated mold you should keep these defects in mind as well as your fundamental objectives (fill mold & hold while solidifying, then remove part) in mind. (Note that many of the defects arise from not fulfilling the objectives.)
Discuss in more detail short shot-- no extra slide but bring several examples
Short shot occurs when there is insufficient material to fill the mold cavity. It has several causes, including insufficient injection pressure, or insufficient time allowed during the injection process. Sometimes the material will freeze in a given section before it can reach the edges of the mold.
Also discuss flashing in more detail-- no extra slides but bring some examples.
Flashing occurs when there is too much material and it pushes its way out of the die. This can be caused by too much injection pressure, too much injection time, or insufficient clamping force. It also can be caused by a poorly machined die that does not properly seal off the cavity.
Final note that the cause of sink marks, warping, and residual stresses are related. As the part cools it tries to shrink away form mold walls, other features, etc. (It shrinks as it cools.) This can result in sink marks as the material is able to shrink away from other material or from walls of mold cavity, etc. It can also result in warping or in residual stresses as material is held by geometry and not able to freely alter its shape and size with the changes in temperature and state.

69. Short Shot
Short shot occurs when there is insufficient material to fill the mold cavity. It has several causes, including insufficient injection pressure, or insufficient time allowed during the injection process. Sometimes the material will freeze in a given section before it can reach the edges of the mold.

70. Flash Part w/ Moderate-Heavy Flash Flash
Flashing occurs when there is too much material and it pushes its way out of the die. This can be caused by too much injection pressure, too much injection time, or insufficient clamping force. It also can be caused by a poorly machined die that does not properly seal off the cavity.
Flash

71. Weld Lines
Weld lines are created when two flow fronts come together in
the mold.
Can result from poor gate placement
Unavoidable w/ Solid Cores
Gate
Weld
Line
Gate
Gate
Weld lines are caused when flow fronts meet. If you have a part with a solid core weld lines are unavoidable.
In some cases, however weld lines are avoidable. Generally they should be avoided if possible. Thus the solid square pictured above should only be gated in one place.
In addition to being aesthetically unappealing, weld lines are also weaker than the rest of the part. The cooler the fronts are when the meet, the less the plastic will be able to “KNIT” together, and so the weaker the plastic will be (weld lines will be more pronounced). Amount the plastic cools is directly related to how far it must travel. For this reason multiple gates are sometimes used on parts with solid cores. There will be more weld lines, but each one will be stronger.
Note that while weld lines are unavoidable when filling parts with solid cores, things can be done to maximize the knitting and thus minimize the weld line defect. (Keep sections hotter when they meet.)
Weld Line
Weld lines cause a decrease in the strength of the part
The cooler the fronts are when they meet, the more pronounced
the weld lines will be

72. Jetting
Jetting occurs at high fill rates when there is a large open space
between the gate and the opposite wall
Material stream shoots to the opposite wall and freezes. Stream
of fresh material then folds over this and freezes, trapping flow
lines of material. Air can often be trapped between folds.
Jetting
To reduce the chance of jetting, gates should be located so that
entering material flows into wall, not into open section.
Alternate gating
eliminates
jetting

73. Ejector Pin Marks
Marks are often left on the part in the area where the ejector pins
were. These marks have three possible causes
1) Pin above
flush
2) Pin below
flush
3) Clearance
around pin
To some degree, these marks are unavoidable, thus one should
try to place the ejector pins on hidden areas of the part.
Another defect occurs when one tries to eject the part before it
solidifies and pins push through the part

74. Sink Marks
Sink marks occur at excessively thick wall sections, or where
there are abrupt changes in thickness- thick sections solidify
too late and shrink away from the wall
Proper design eliminates unneeded material (ribs, core out sections)
Thick sections cause
sink marks
Bad
Design
Improved
Design
When unavoidable, sink marks can be masked by surface texture

75. Runner Design 1) Keep runners as short as possible
2) Use a cold well at the end of each branch- collect solid chunks
3) Use cross sections that minimize perimeter for a given area
Circles are best followed by rounded trapezoids and then
trapezoids
4) Use good flow principles (round corners, etc)
5) Size the (effective) diameters of runners based on length
Determine total flow length from sprue to gate
Use table to determine needed diameter for this length- this value
is minimum diameter at END of runner
Increase the diameter of the upstream runner by 20% at each 90o turn
When n multiple runners branch off a main feed runner, the diameter
of the main runner should be increased by n*20%

76. Runner Sizing
Table 2 “Secrets of gate, runner, and vent design,” Douglas M. Bryce. Plastics Design Forum. Sept 94

77. Gate Design 1) Gate so that material flow hits a nearby wall
2) Gate into the thick sections- so material flows thick to thin
3) Gate into an understressed region of the part
(Gates introduce residual stresses as they freeze before other sections)
4) “Hide” gates whenever possible
5) Size gates based on (maximum) wall thickness of part
Also depends on how well the material flows (viscosity)
Determine relative material viscosities by comparing the minimum
allowable wall thicknesses for different materials

78. Gate Sizing w d l
“Secrets of gate, runner, and vent design,” Douglas M. Bryce. Plastics Design Forum. Sept 94

79. Venting Vents allow air to escape ahead of the plastic melt
d=depth
w=width
l=land
Vents allow air to escape ahead of the plastic melt
Vents help reduce-- Injection pressure and clamping force
Cycle time
Warping & shrinking
Residual stresses

80. Vent Sizing and Frequency
Note that runners should
be vented if possible
Vent 30% of runner length
on both sides.
Vent 20% of cavity perimeter
if runners are vented. By vent 20%
of the cavity means that if one sums the
width of each vent and divide the result
by the total perimeter of the cavity, the
ratio should be around 20%.
Vent 30% of cavity otherwise
“Secrets of gate, runner, and vent design,” Douglas M. Bryce. Plastics Design Forum. Sept 94

81. Mold Design Example The part shown on the right is to be
injection molded. 16 pieces should
be made in each shot. (Material is
polycarbonate). Design the mold.
1) Layout
The configuration shown
on the right keeps the
runners small, with few
bends, and shoots the
melt into the mold wall.
One weld line will be
present on each part, but
this is unavoidable.

82. Example- Runner Design
2) Runner Size
Total Runner Length= 1.2”+7.5”/2+3.8”=8.75”
From table EFFECTIVE diameter is ”
For a flat trapezoid, with an angle of 15o, and height and
width chosen to minimize the perimeter for a given area...
h=0.871*(0.203”) = 0.176”
w=1.135*(0.203”) = 0.230”

83. Example- Upstream Runners
Recall that these dimensions (effective
diameter = 0.203”) are for the runners
nearest the gate
Four of these cluster runners (n=4)
branch off of each feeder.
Effective diameter=(1+n*0.2)*dbranch
dfeed = (1+4*0.2)*(0.203”) = 0.366”
h=0.871*(0.366”) = 0.319”
w=1.135*(0.366”) = 0.415”
Two of these feeder runners (n=2)
branch off of the main runners.
dmain = (1+2*0.2)*(0.366”) = 0.512”
h=0.871*(0.512”) = 0.446”
w=1.135*(0.512”) = 0.581”

84. Example- Gates Wall thickness is given as 0.100”- t=0.100.”
From tables, the minimum wall thickness when injection molding
Polycarbonate is 0.040,” which is comparatively high. This
indicates that Polycarbonate does not fill or flow as well as
most plastics.
Select d=0.7*t=0.7*(0.100”) = 0.070”
Select w=2.75*d=2.75*(0.070”) = 0.193”
Select l=0.030”

85. Example- Vents Mold cavity vent depth dcv=0.002”
Runner vent depth drv=0.004”
Choose wv=0.125.”
Choose lv=0.045.”
Spacing between vents on runners
is 0.42”
Spacing between vents in cavity
is 0.500”

86. Estimating Clamping Force
Clamping force is needed to overcome the force exerted by
the pressure of the plastic melt against the mold cavity.
A reasonable estimate of clamping force may be obtained by multiplying the
pressure by the area of the mold in the parting plane.
Assume that 1/2 of the pressure drops uniformly through the runner system.
Assume that the remaining pressure holds constant in the mold cavity.
(Note that these assumptions are very conservative.)
Typical Injection Pressures (kpsi)
Polycarbonate (Lexan)
Phenylene Oxides (Noryl) 14
Polyester (Valox)
ABS (Cycolac)
(From GE Product Guides)

87. Example- Clamping Force
Force for previous mold
1) Mold Area
Treat parts as perfect squares
A=2”*2”-1.8”*1.8”
A=0.76 in2
A = 16*0.76 in2 = 12.2 in2
2) Runner Area
Main: (7.5”)*(0.581”)= 4.4 in2
Feed: 4*(3.8”)*(0.415”)= 6.3 in2
Branch: 16*(1.2”)*(0.230”)= 4.4 in2
A = ( )in2 = 15.1 in2
3) Typical Polycarbonate Injection Pressure: psi
4) Clamping force
F=0.75*(16000 psi)*(15.1 in2 )+0.5*(16000psi)*(12.2 in2 )
F= lbs = 140 tons

88. Project 3 To injection mold a Drexel keychain in Hess Lab.
Select plastic material for the keychain (read Chapter 10-5 to 10-8)
Select color for the keychain (red, blue and yellow)
Make 50 good quality key chains by adjust pressure, temperature, time, etc. on the machine
Logo design for the keychain
Prepare 10 min. power Point presentation and detail report for this project

89. Available Plastic Materials in Hess Lab
1. Nylon Resin (Zytel) 2. Polyethylene (PE) 3. Acetal Resin (Delrin) 4. Polycarbonate Resin 5. Polyvinyl chloride, PVC 6. Polypropylene 7. Polyetherimide 8. Elastomers