1. Plastic Injection Molding

2. Injection Molding
3 major functional units; injection, mold, clamping

3. Plastic Injection Molding
is a manufacturing technique for making parts from thermoplastic and thermoset materials
In contrast to the extrusion (which makes continuous parts of constant cross section), injection molding make discrete parts (with complex and variable cross section)
Molten plastic is injected at high pressure into a mold, which is the inverse of the desired shape.
The mold is made from metal, usually either steel or aluminium
widely used for manufacturing a variety of parts, from the smallest component to entire body panels of cars

4. Plastic Injection Molding
The key to success in injection molding are to have;
Proper machine for good melting and injecting of the resin
The proper resin to appropriate part performance
A good mold for part definition and removal
Proper operation for efficient molding cycle (mold cycle depends on the design of the mold and manufacturing parameters)

5. The most commonly used thermoplastic materials are;
polystyrene (low-cost, lacking the strength and longevity of other materials)
ABS or acrylonitrile butadiene styrene (a co-polymer or mixture of compounds used for everything from Lego parts to electronics housings)
nylon (chemically resistant, heat-resistant, tough and flexible - used for combs)
polypropylene (tough and flexible - used for containers
PVC (more common in extrusions as used for pipes, window frames, or as the insulation on wiring where it is rendered flexible by the inclusion of a high proportion of plasticiser).

6. Injection Unit
Purpose: to liquify the plastic materials and then inject the liquid into mold
Resin is introduced through hopper
Some machines can have several hoppers (to fed filler, colorants, other additives)-Injection molding act as mixer
However, due to limited size of barrel, mixing capability is poor

7. Injection Unit From hopper – hole (feed throat)
Barrel made of heavy steel cylinder to withstand the pressure and temperature involved in melting the resin
2 types of system used in injection molding;
Reciprocating screw- similar to extruder screw but with unique reciprocating action
Ram injector

8. Injection Unit Design of screw- similar to an extrusion screw
3 sections;
Feed section- to advance the resin
Compression section- to melt the resin
Metering section- to homogenize the resin and pump it forward
The screw of injection molding machine is shorter than extruder, L/D ratios are 12:1 and 20:1
Low L/D ratios suggest the mixing is less efficient in the injection molding machine
The compression ratio (diameter of root at feed zone to the diameter of root at metering zone) often in the range of 2:1 and 5:1
Low compression ratio means less mechanical action is added during melting process

9. Injection Unit
Important measure of the size of an injection molding is weight of resin that can be injected, called shot size
Typical shot size range from 20g to 20 kg
Since shot size depends on the density of the plastic, PS has been chosed as the standard for rating the machine

10. Reciprocating Screw Injection Molding Machine
Resin is melt by mechanical shear and thermal energy from heaters
The molten resin is conveyed to a space at the end of the screw- collects in a pool
Here, the mold is closed

11. Reciprocating Screw Injection Molding Machine
The entire screw move forward and pushes the molten resin out through the end of barrel
To ensure the resin does not flow backward, a check valve or nonreturn valve is attached to the end of screw
Normally the screw will stay in the forward position, until resin began to harden in the mold

12. Reciprocating Screw Injection Molding Machine
Retraction of the screw, create space at the end of the screw
Cooling of the part in the mold, until it can be removed
While the part is cooling, the screw turns and melts additional resin

13. Reciprocating Screw Injection Molding Machine
Advantages
More uniform melting
Improved mixing or additives and dispersion throughout the resin
Lower injection pressure
Fewer stresses in the part
Faster total cycle

14. Ram Injection- Injection Molding Machine
In this type of injection molding, the resin is fed from a hopper into the barrel, and heated through thermal energy from the heaters
The molten resin is collect in a pool in a barrel celled injection chamber
The molten resin is then push forward by the action of plunger (ram or piston)
To five better mixing, the molten resin is pushed past a torpedo/spreader, impart shear to the melt

15. Molds
Designing and making mold for injection molding is more complicated than making extrusion die
Mold Parts – mold is placed in between stationary plate and the moveable plate

16. Molds
The connection from the injection unit to the mold is through the nozzle
The channel that run through the stationary plate of the mold is called the sprue channel (material that is in the channel is called the sprue)
The solid sprue is removed from the finished part assembly after the part is ejected from the mold
Resin flow from the sprue through the runner (connecting channel) to the mold cavities

17. Mold Bases Assembly of various mold parts
Mold bases can be purchased as entire units, then the cavities are cut from A & B plates

18. Runners
Distribution system for the resin from the sprue to the cavities
Flow characteristics (viscosity), temperature and other factors are important in determining the runner diameter and length
If the diameter of the runner is too small or the length is too long,the resin can freeze in the runner before the mold is completely full
If the runner system is too large, excess material would be ejected and too much regrind created

19. If the resins have a high viscosity, larger runners are needed compared to low viscosity resin
The optimum flow of the resin through the runner system depends on the shape and diameter of the channel
Round channel give the best flow characteristics but difficult to machine
Machining cost can be reduce by machining one side of the mold plates
Better shape where the depth of the channel is at least two-thirds the size of the width and the sides are tapered between 2 to 5º.

20. Runner Channel Shapes

21. Secondary Runners
Secondary runner channel are used for multicavity molds
The flow into the secondary channel should be streamlined (angle in flow direction)
The streamlined minimizes shear on the resin

22. Gates The end of runner and the entry path into the cavity
The gate shape can also affect the filling of the cavity, dimension and properties of the parts
Gate is the most restricted point in injection molding system, i.e. for reinforcement and filler + polymer systems

23. Gate Design
Small rectangular opening at the end of the runner channel, connect to the edge of cavity
Edge gate can be below the parting line if the channel and part are also below the parting line
Or it can be symetricaly about the parting line, if the runner channel and part are at both side of parting line
Edge gate

24. Submarine Gate
Starts from the edge of the runner, and goes into the cavity edge at an angle
It narrows to a point as it moves from the runner to the cavity
The advantage; separation of the parts and the runner is automatic
Disadvantage; gate cannot be used for some resins because of high shear
Submarine Gate

25. Tab Gate
By connecting the runner directly into the cavity with no reduction in runner cross-section
Used for very large parts where a reduction in flow would disturb the resin’s flow pattern and might result inadequate flow into the cavity

26. Fan Gate
Made by reducing the thickness and not the diameter of the runner channel as it goes into the cavity
Used for intermediate size, and when reinforcement in the resin cannot flow through the edge gate

27. Ring Gate Used to make hollow cylinder parts
The ring gate covers the entire top of the cylinder part so that the resin flow is downward into the wall of the part

28. Cavities Are actual molding locations
Resin enter the cavities through gate, fills the cavities, and cools to form the solid. The parts are ejected and finished
Cavities are the heart of the molding process, and must be precisely prepared
The shape of the cavities determines the shape of the part

29. Materials & Product Consideration
Almost all thermoplastic can be injection molded
Resin with low melt viscosity is required; so that the flow through runner, gate, cavity – easily done with minimum injection pressure
Resin with injection molding grades have low molecular weight and narrow molecular weight distribution

30. Shapes
Hollow parts can be created by allowing the moveable plate to protrude into the cavity of the stationary plate

31. Shapes
Threads can be placed on the inside of a part by using a core pin that is inserted into the cavity where the threads are desired

32. Shapes
A hollow part with a hole on the side is even more complicated (the core pin is used)
The core pin slide into position after the mold is closed
The core pin seals against the surface of the moveable plate, prevent flow of resin into the area