1. Polymer Processing Techniques

2. Injection Molding
Injection molding is one of the most important techniques for the forming of thermoplastic materials
A number of various items can be manufactured with great ease.

3. Important Terminologies
Molding: To shape something into a stable (preform) structure Injection Line: The sequencing of an injection molding machine with all other particulars to produce a product. Shot size: The weight of material that can be processed in one cycle Shot Cycle: The time required to process one shot size. Mold Cycle: The total time required to produce a stable product. Sprue Channel: The channel between barrel nozzle and the mold. Clamping unit: The mechanical assembly that is used to hold the moveable mold. Reciprocating Action: To and Fro motion Ram Action: Piston action Mold definition: The internal smoothness and surface finish of the mold Multicavity Mold: A mold in which more than two cavities are present. Family Mold: A mold containing multiple cavities of different types.

4. Injection Line It consists of three major units: Injection Unit
Clamping Unit + Control
Mold

6. Injection Molding Cycle
Injection molding involves two basic steps:
Melt generation by a rotating screw
Forward movement of the screw to fill the mold with melt and to maintain the injected melt under high pressure
Injection molding is a “cyclic” process:
Injection: The polymer is injected into the mold cavity.
Hold on time: Once the cavity is filled, a holding pressure is maintained to compensate for material shrinkage.
Cooling: The molding cools and solidifies.
Screw-back: At the same time, the screw retracts and turns, feeding the next shot in towards the front
Mold opening: Once the part is sufficiently cool, the mold opens and the part is ejected
The mold closes and clamps in preparation for another cycle.

8. Process & machine schematics*
Schematic of thermoplastic Injection molding machine

9. Injection Unit
Purpose: to liquefy 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)
However, due to limited size of barrel, mixing capability is poor

10. Injection Unit
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: plunger action

11. Injection Unit 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

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)

16. Mold Basics Melt Delivery Nozzle Sprue Core Plate Cavity Plate Cavity
Runner
Gate
Melt Delivery
Moulding
Core
Cavity

17. Injection Molding for a plastic cup
Knob
Nozzle
Runner
Cavity
Core
Part
Stripper plate

18. Complete IM for a plastic cup
Runner
Part
Cavity
Nozzle
Part
Cavity
Knob
Stripper plate
Runner
Part
Cavity
Nozzle

19. Reaction Injection Molding
Liquid monomers are placed in the mold avoiding the need to use temperature to melt the polymer or pressure to inject it. The monomers polymerize in the mold forming the part.

20. Extrusion Objective Extrusion a Greek word: To push out
An Extruder is a pump that supplies a continuous stream of material to a shaping tool or post shaping process.
Objective
One of the most famous and easiest method of plastic processing.
Use is not restricted to Plastics, some metals are also extruded (e,g. Al window frames).

21. Extruder : Mechanical assembly used to melt and push the material
Extrusion Line: A typical set up that shows the complete arrangement and sequencing of all the important auxiliaries involved along with extruder to produce any uniform product.
Barrel: The upper portion of extruder that covers the screw.
Pipes or tubings: The parts which are in hollow cylindrical shape.
Profiles: Parts that are hollow but not cylindrical.
Die: The shaping tool installed usually at the end of a typical processing equipment.

22. Advantages and Disadvantages
Uniform products
Continuous Process
High production
Low processing cost
Flexibility of Raw materials
Can act as compounder.
No narrow and complex part
Low extrusion pressures
Uniform products only
Screw is almost material specific

23. Extrusion Line
Figure 10.1

24. Extrusion
Pellets of the polymer are mixed with coloring and additives.
The material is heated to its proper plasticity.
The material is forced through a die. under pressure resulting in an endless product of constant cross section
The material is cooled.

25. Blown Film Extrusion
A web under mm thickness is called a film (Packaging industry uses 0.10mm)
Materials over these dimensions are called sheets.
Thermoplastics are usually blown into films.
Die is fed from side.
Tubular die is used after extruder
The inflated film is air cooled.
The size is controlled through die gap and size rings around the blown tube, by the internal air pressure and speed of take-off rolls.
Several guide rolls are used to align the blown tube.
A collapsing device is used for to make passage of tube through nip rolls.

27. The trapped air that forms the continuous tube is directed through mandrel via die.
Once the bubble is formed, the controlled air pressure required to make tube stable is kept constant.
To speed up lines and improve output performances, internal bubble cooling (IBC) systems are extensively used. They direct cool air at low velocity to enter and exit the inside of the bubble.

29. Drag the extrudate up to the top nip
Pass it through idle rolls to the winder

30. Adjust the blow up ratio by introducing the air by slowly turning the air valve.
After the desired size is obtained, close the valve

31. Extrusion Blow Molding
Resembles film blowing in some aspects
Cylindrical extrudate is required so a mandrel is to be used.
Air may be inserted through a pin in the mandrel or through the bottom of the mold into the parison.
Extrusion is a continuous process where as blow molding is a discrete process.
Continuous EBM (Rising Mold System, Parison Transfer System, Rotational Molding)
Intermittent EBM (Reciprocating Screw, Accumulator System)

36. Bottle forming process

37. Compression Molding Pre-formed blanks, powders
or pellets are placed in the
bottom section of a heated mold
or die.
The other half of the mold is
lowered and pressure is applied.
The material softens under heat and pressure, flowing to fill the mold. Excess is squeezed from the mold. If the polymer is a thermoset, cross-linking occurs in the mold.
The mold is opened and the part is removed.

38. 3) Squeeze to
final dimensions
5) De-moulding
1) Placement
of Charge
4) Resin
Cure
2) Mould
Closure

39. Thermoforming
Shaping of materials (sheets and films) by heating until the material softens and then forcing the material to adopt the shape of mold.
The forcing medium includes vacuum, air pressure, or mechanical actions.

40. Pros & Cons Pros Cons Low machine cost Low temp. requirements
Low mold cost
Low pressure requirements
Larger parts can easily be formed
Fast mold cycles
Cons
High raw material cost
High scap
Limited part shape
Poor part finish
Inherent wall thickness variation
Common internal stresses

41. Types of Thermoforming process
Straight vacuum forming
Pressure Forming
Plug-Assist Forming
Reverse Draw Forming
Free Forming
Drape Forming
Snap-back Forming
Matched-Die Forming
Mechanical Forming

42. Straight Vacuum Forming
Simplest from all others
Straight sheet of thermoplastic is clamped above the mold
Mold has vents connected to vacuum pump
Out side pressure of air forces the sheet to press due to vacuum.
Max. Pressure therefore will be 1 atm.
Very poor control over wall thickness and corners

45. Pressure Forming
Positive air pressure is used to force the material in heated form.
Same mechanism as that was of straight vacuum forming except the vacuum.
The process can be done at low temperatures and faster cycles.
Good finish than vacuum forming is obtained.

47. Rotational Molding Process Description a) Charging the Mould
Arm
Plate
Material
Heat
b) Heating the Mould
The rotating mould is transferred into an oven, polymer sticking to the walls.
a) Charging the Mould
Spray Nozzles/Fan
c) Cooling
Once the powder consolidates, the mould is cooled, and the part removed.
Final
Part
d) Demoulding
A large hollow mould is charged with thermoplastic powder, and rotated.

49. Products
Rotational Molding
Wide range of complex hollow products:

50. TP Pultrusion Process Description Prepreg Tape or Sheets Heated Die
Cooled Die
Preheatin g
Prepreg is fed into a preheating section, bringing TP close to Tmelt.
The heated die has a slight taper, gradually shaping to the final cross section.
The product is cooled, taking on the required solid cross section.

51. TP Pultrusion Equipment Experimental TP Pultrusion Line
Close-up of preheater and dies
Experimental TP Pultrusion Line