2. INJECTION MOLDING:
Definition:
The injection machine is a machine that melt plasticize the molding material inside the heating
cylinder and inject this into the mold tool to create the molded product by solidifying inside it.
The injection machine is constructed of a mold clamping device that opens and closes the mold
tool, and device that plasticize and inject the molding material.
Composition of Molding Machine:
Injection molding machine consists of two main units:
1. Injection Unit
2. Clamping Unit
Injection Unit:
This unit is located on the right side of molding machine and consists of the following
parts:
a. Hopper
b. Barrel
c. Injection cylinder
d. Charging motor
Electromotive Drive for Injection
The recent development of stronger and more accurate electric servomotors has created the
Groundwork for realizing linear electromechanical screw movement. Basically, electromotive
Linear drives are an added disadvantage to hydraulic cylinders when it comes to building up
the high forces necessary for injection (Table 4.19). However, they offer major advantages in
terms of stiffness of force transmission and accuracy, as well as ease of control. Development
work has focused on implementing the rotary motion into a linear motion. Ball screws have
become the standard component.
Clamping Unit:
This unit is located on left side of machine and consists of the following parts:
a. 3 Platens
b. Tie bars(4 holes)
c. Moving cylinder
d. Adjustment device
3. Injection Molding Process:
Overall process of injection molding can be summarized as follows steps:
1. Mold close
2. Mold clamp
3. Barrel Forward
4. Injection
5. Holding
6. Charging
7. Suck Back
8. Cooling
9. Mold de clamp
10. Mold open
11. Ejects forward
12. Ejects backward
13. Door open
Molding Parameters:
Following are the parameters that need to be fulfilled:
1. Temperature - in degree Celsius
2. Stroke - in millimeters
3. Time - in seconds
4. Speed - in percents
5. Pressure - in bars
These parameters are different and have their own specific values and quantities for specific
materials used for the molding process.
Some important points regarding molding parameters are as follow:
a. Maximum pressure is 160 bars.
b. Temperature varies in the barrel and is summarized as follows:
Barrel consists of three temperature zones:
1. Feeding zone
2. Mixing zone
3. Flight(Metering) zone
c. When a new mold is setup in the machine, working of it is concerned with a new
program to be installed in the machine. A program is first sampled, then programmed
and then finally saved so that it may be helpful in the future too.
d. Ratio of color to be added he material is 2% per one bag.
e. Die used for the production is made by machining rather than casting.
f. Size of machine depends on the size of die to be fit in it.
4. g. Hydraulic oil used in injection molding is 68 number. Number of hydraulic oil specifies
its thickness. Lubricant is applied manually.
h. Hydraulic pumps to be used in hydraulic system of injection machine depends on the
quality of oil.
i. Size of barrel depends on size of machine and size of die to be used.
Barrel Section:
Torpedo is used to straighten the plastic material resins which are composed of molecular
chains so that holes cannot appear on the piece. Moment of screw in the barrel is towards right
while the material moves to left. In the barrel, heating f material is done by different ways. 40
% of heating is done by heaters, 40 % is done by friction while the rest of 20 % is done due to the
injection of material into torpedo so the material gets enough heating.
Nozzle structure:
Open nozzle is common. The nozzle of commercially-supplied injection machine
can be open nozzle or shut-off nozzle (Figure 1-1) but in any type, it is necessary to have a
temperature control. If drooling from the nozzle is concerned, use the shut-off nozzle. However,
it might cause burn and sunspot object by resin retention at the slide part, so be careful.
Faults which can be faced during working of machine:
1. Leakage of oil
2. Sometimes material gets struck in the hopper . this happens mainly due to over addition
of material in hopper and then it gets struck there due to lack of water or over heat.
Adjustment of Mold:
Adjustment of mold in the injection machine is carried out by keeping several
aspects under consideration such as follows:
a. Platen size(3 platens: fixed, movable, adjustment)
b. Injection capacity of machine
c. Clamping force(every mold has its own clamping force)
d. Machine daylight
5. Distance between moveable and fixed platen while in closed position is called as “machine
daylight” while the same distance in open position is called as “mold stoke.”
Mold stoke should be 1.5 times than machine daylight so ejection of mold can be made easier.
Screw jacks used in this machine are mainly for the movement of mold.
2 jacks are used for injection. 1 jack is used for carriage unit. Sometimes in machines for more
accuracy, toggle is used which can be related to modified jack.
Hydraulic and pneumatic systems used in the machine:
a. Double directive system is used in hydraulic while single direction system is used in
pneumatic system.
b. In pneumatic systems, motor, pump and relief valve is not present while compressed air
source instead of hydraulic oil.
Following is the cycle for hydraulic system:
Whereas working cycle of pneumatic system is as follows:
Materials:
Plastic materials used in molding process are of two types:
1. Thermoplastic materials - which can be recycled
2. Thermosetting materials - cant be recycled
6. Separation of materials for recycling process:
Mainly separation of materials is done on the basis of identification related to their
properties. Process consists of the following steps:
1. Burning
2. Smoke
3. Odor/smell
4. Density
Density method is mostly preferred in molding works specially in industrial terms.
Some key points regarding the materials are as follows:
a. ABS is the only material which is dried.
b. Every material that observes humidity is to be dried.
For Drying, there are two parameters:
1. Time - 2 hours
2. Temperature - 82 degrees Celsius
c. Temperature drying for ABS is 200-230 degrees Celsius in feeding zone. This specified
temperature is also called as Barrel temperature.
PP – polypropylene is extremely chemically resistant and almost completely impervious
to water. Black has the best UV resistance and is increasingly used in the construction
industry.
Application: automotive bumpers, chemical tanks, cable insulation, battery boxes, bottles,
petrol cans, indoor and outdoor carpets, carpet fibers,
PUR – polyurethane materials are widely used in high resiliency flexible foam seating,
rigid foam insulation panels, microcellular foam seals and gaskets, durable elastomeric
wheels and tires, automotive suspension bushings, electrical potting compounds, hard
plastic parts (such as for electronic instruments), cushions,
PVC – poly-vinyl-chloride has good resistance to chemical and solvent attack. Its vinyl
content gives it good tensile strength and some grades are flexible. Colored or clear
material is available. Application: automobile instruments panels, sheathing of electrical
cables, pipes, doors, waterproof clothing, chemical tanks,
ABS – acrylonitrile-butadiene-styrene is a durable thermoplastic, resistant to weather and
some chemicals, popular for vacuum formed components. It is a rigid plastic with
rubberlike characteristics, which gives it good impact resistance. Application: car
dashboards, covers,
PS – polystyrene is very popular, ease to manufacture, but has poor resistance to UV
light.
Application: equipments housings, buttons, car fittings, display bases,
PE – polyethylene has good chemical resistance. Two types are used, low density
polyethylene (LDPE) and high density polyethylene (HDPE) can be manufactured in a
range 30 of densities. Application: glass reinforced for car bodies, electrical insulation,
packaging, where strength and aesthetics are important,
7. POM – polyoxymethylene (also know as polyacetal or polyformaldehyde) has big
stiffness, rigidity and excellent yield, which are stable in low temperatures as well. Very
good chemical and fuel resistance. Application: interior and exterior trims, fuel systems,
small gears,
PC – polycarbonate has good weather and UV resistance, with transparency levels almost
Good as acrylic. Applications: security screens, aircraft panels, bumpers, spectacle
lenses,
Headlamp lenses,
PMMA – acrylic is more transparent than glass, has reasonable tensile strength (shatter
proof grades are available) and good UV and weather resistance, high optical quality and
surface finish with a huge color range. Application: windows, displays, screens
