Transcript Processing Techniques - Department of Mechanical

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.
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.
Injection Line
It consists of three major units:
 Injection Unit
 Clamping Unit + Control
 Mold
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.
Process & machine schematics
*
Schematic of thermoplastic Injection molding machine
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
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
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
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
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)
Mold Basics
Sprue
Nozzle
Core Plate
Cavity
Gate
Cavity Plate
Runner
Melt Delivery
Cavity
Moulding
Core
Injection Molding for a plastic cup
Nozzle
Knob
Runner
Cavity
Part
Stripper plate
Core
Complete IM for a plastic cup
Nozzle
Nozzle
Knob
Runner
Cavity
Runner
Part
Cavity
Cavity
Part
Part
Stripper
plate
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.
Extrusion
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).
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.
Advantages and Disadvantages
 Uniform products
 No narrow and complex
 Continuous Process
part
 Low extrusion pressures
 Uniform products only
 Screw is almost material
specific
 High production
 Low processing cost
 Flexibility of Raw
materials
 Can act as compounder.
Extrusion Line
Figure 10.1
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.
Blown Film Extrusion
A web under 0.254 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.
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.
 Drag the extrudate up to the top
nip
 Pass it through idle rolls to the
winder
 Adjust the blow up ratio by introducing
the air by slowly turning the air valve.
 After the desired size is obtained, close
the valve
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.
1. Continuous EBM (Rising Mold System, Parison
Transfer System, Rotational Molding)
2. Intermittent EBM (Reciprocating Screw,
Accumulator System)
Bottle forming process
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.
1) Placement
of Charge
5) De-moulding
3) Squeeze to
final dimensions
2) Mould
Closure
4) Resin
Cure
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.
Pros & Cons
Pros
Cons
• Low machine cost
• High raw material cost
• Low temp. requirements
• High scap
• Low mold cost
• Limited part shape
• Low pressure requirements
• Poor part finish
• Larger parts can easily be
• Inherent wall thickness
formed
• Fast mold cycles
variation
• Common internal stresses
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
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
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.
Process Description
Rotational Molding
Material
Mould
Heat
Arm
Plate
a) Charging the Mould
Spray Nozzles/Fan
c) Cooling
b) Heating the Mould
Final
Part
d) Demoulding
• A large hollow mould is charged with thermoplastic powder, and rotated.
• The rotating mould is transferred into an oven, polymer sticking to the walls.
• Once the powder consolidates, the mould is cooled, and the part removed.
Rotational Molding
Products
Wide range of
complex hollow
products:
Process Description
TP Pultrusion
• 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.
Prepreg
Tape or
Sheets
Preheatin
g
Heated
Die
Cooled
Die
TP Pultrusion
Equipment
Experimental TP Pultrusion Line
Close-up of
preheater and dies