Lab 7: Injection Molding
1. Purpose
To learn processing techniques associated with injection molding.
2. Theory
In injection molding, a plastic is heated to a molten state and then forcefully
pushed through a nozzle into a heated mold, typically made of stainless steel (though the
ones used in this lab are aluminum).2 The plastic enters the mold through the sprue, then
through runners which distribute the molten plastic evenly to all cavities in the mold.
The mold cavity is connected to the runners via a gate (Figure 8.1).1 These extra pieces
may be later removed, chopped up and recycled back into the machine (called regrind).
The mold is used to create the desired resulting part shape and may consist of two or even
three pieces.2
The two types of injection molding typically used are screw plunger and plunger.
In either case, the plastic, usually in the form of pellets,
is fed in through a hopper in the top of the barrel and
melted in the barrel.
It is next forced into mold
through the heated barrel.2
In the screw plunger method, the faster of the two
part
methods (Figure 7.2a), a screw is used to evenly
distribute the heat to the plastic pellets, causing them
showing
to melt more quickly. The screw can also be used to mix
positions of part, gate, runner,
in any additives that could cause the polymer to change
Figure
7.1:
and sprue.
1
Mold
color.2
In plunger injection molding, a ram is used to force the molten plastic into a mold
through the nozzle (Figure 7.2b). At the end of either process, the plastic part is removed
Fried, J. R., “Polymer Science and Technology”, Prentice Hall PTR, Upper Saddle
River, NJ, 1995
2
IST Amatrol, “Manufacturing Processes 3. LAP1: Introduction to Injection Molding and
Operations.” Amatrol, Inc., 1999
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from the mold (Figure 7.3).2 The molds must be able to withstand high temperatures and
pressures “and produce parts with close tolerances after numerous rapid operations.”1
b
a
Figure 7.2: Steps in injection molding: a) screw-plunger method b) plunger
method.2
Based on their thermal processing behavior, all polymers can be
divided into two groups: thermoplastics and thermosets.
Thermoplastics polymers are commonly used for injection
Figure 8.3: Mold and
cooled molded part.2
molding because they can be heat-softened to obtain a desired
form. Table 7.1 below contains commonly used thermoplastics
that can be used in injection molding. 3 In thermoset polymers,
the individual chains are covalently linked and cannot be reformed upon heating. Unlike
thermoplastics polymers, thermosets are usually not used in injection molding because
after they are formed their cross-linked networks resist heat softening and reforming.1
Thus, it is easier and cheaper to use thermoplastics over thermosets in processing
applications.
3
BFP, “Injection Moulding” http://www.bpf.co.uk/bpfindustry/process_plastics_injection_moulding.cfm
2
Table 7.1: Commonly used Thermoplastics used in Injection Molding3
Name
Acrylonitrile-Butadiene-Styrene
Nylon
Polycarbonate
Polypropylene
Polystyrene
Abbreviation
ABS
PA
PC
PP
GPPS
Typical products that can be made using injection molding include: power-tool
housings, telephone handsets, safety helmets, television cabinets, washing-up bowls and
rubber ducks (Figure 7.4).3
a
b
Figure 7.4: Various items that can be made using Injection molding: a) plastic containers3; b)
rubber duck4.
Injection molding is used for mass consumables because it is a cheap and quick
way to mass produce products. However, there are some typical engineering problems
associated with injection molding which will now be described. Table 7.2 shows common
problems and some solutions associated with injection molding.
Table 7.2: Causes and solutions of Injection Molding2
Problem
Possible Causes
Solutions
No polymer enters
the mold
Only sprue forms or
the mold is not
filling completely
Mold is not properly
aligned
Not all purging
compound has been
removed from the
barrel
Not enough material
available in barrel
Mold is not warm
enough
Air pressure is too
low
Re-align mold
If the sprue is white while it is still in the
melt state, there is purging compound in the
barrel. Remove the mold and then eject all
of the purging compound from the barrel
add more material but do not overfill barrel
Heat mold longer
Increase air pressure
Excel Step Limited (China), “PLASTIC INJECTION: PI-001”
http://www.label-pin.com/plastic-injection/plastic-injection02.htm
4
3
Nozzle temperature Increase nozzle temperature but only by 5oC
is too low
at a time
Not enough polymer
is entering the mold
Mold vents may be
clogged
There is a loud Polymer may not be
popping sound when melted
the
polymer
is
ejected from the
barrel
Not all purging
compound has been
removed from the
barrel
injection pressure is
Part has flash on it
too high
clamp pressure is too
low
Increase the amount of polymer to the mold
Unclog any polymer-clogged vents and use
less mold release
Examine the polymer coming out of the
nozzle to make sure that there are no unmelted polymer beads; allow more time to
melt the polymer
If the sprue is white while it is still in the
melt state, there is purging compound in the
barrel. Remove the mold and then eject all
of the purging compound from the barrel
This causes the mold to separate; decrease
injection pressure but make sure mold still
fills fully
This causes mold to separate; increase the
clamp pressure slightly until problem is
solved
Reduce nozzle and barrel temperature
slightly (only 5oC at a time)
increase the barrel temperature by 5oC
increments only
Heat mold longer
Temperature is too
high
Appearance of Weld Barrel temperature is
marks around a too low
manufactured hole in Mold is not warm
sample
enough
incorrect flow rate
Increase the pressure used or increase the
barrel temperature by 5oC increments only
3. Experimental Procedure
WARNING: In this laboratory experiment you MUST wear safety glasses and heatresistant gloves.
3.1 Making the Part
(1). Put on safety glasses and heat-resistant gloves.
(2). Observe the main parts of the injections molding machine before starting the lab
(3). Switch on the main power followed my starting the cooling system and hydraulic
system.
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Barrel and
screw zone
Main switch
Hopper for
feeding polymer
Injection
molding
chamber with
sliding door
Switch for hydraulic
system
Main control unit
Figure 7.5: Injection molding machine
(3). Turn the temperatures at different zones in the barrel, nozzle and mold as listed
below
Rear
160 oC
Middle
180 oC
Front
200 oC
Nozzle
200 oC
Mold
32 oC
(4). Set the back pressure and screw speed as 0.6 MPa and 50 rpm respectively.
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WARNING: You now must use EXTREME caution around the barrel and the
nozzle as they will become hot enough to leave severe burns.
(6). Do not proceed until the nozzle and the barrel have reached the set temperatures.
(7). Feed the polymer in to the hopper. Each dog bone sample requires approximately
10 grams of polymer.
(8). Start the process with manual operation mode.
(9). Apply mold release to both sides of the mold. This can be used like cooking spray
as not too much is needed and it needs to be reapplied only every third or forth time.
(10).Follow the different steps in manual operation to inject the polymer in to the
mold.
a) Filling the barrel with the resin
b) Melting it through the barrel
c) Closing the mold and injecting polymer melt in to the mold
d) Open the mold and eject the injected sample out
(11). Close the injection molding chamber door while injecting the melt in to the
mold. The machine has a safety switch that will not allow it to be run unless the door
is closed
(12). Operate the machine in semi-automatic and automatic modes by changing the
mode of operation from the main control panel.
(13). Change the polymer weight for injection to see its influence on the quality of the
product.
(28). Typical results obtained from the experiment can be seen below (Figure 7.6)
with the description of the experimental parameters below it.
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1
2
3
Figure 7.6: Dog bones obtained in injection molding procedure: 1) not enough polymer is
injected into the mold; 2) too much polymer is injected into the mold; 3) just enough polymer
is injected into the mold.
3.2 Purging the Machine
(1). Feed the purging polymer pellets in to the hopper.
(2). Set the temperature limits at different zones on the battle from the main control
unit.
(3). Wait for the temperatures at the different zones to stabilize.
(4). Move the nozzle back from the mold by switching to manual mode of operation.
(5). Desired amount of the purging polymer will be pushed out of the barrel to clean
the barrel and nozzle
(6). Clear the polymer chips struck to the injection mold and gates
(7). Turn the barrel temperature to zero.
(8). Turn the nozzle temperature to zero.
(9). Turn off the cooling system and hydraulic systems
(10). Turn the key to the off position (counter clockwise 90o).
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