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 ID: 713128
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Slide1
Polymer Processing Techniques Slide2
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.Slide3
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.Slide4
Injection Line
It consists of three major units:
Injection
Unit
Clamping
Unit + Control
MoldSlide5Slide6
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.Slide7Slide8
Process
& machine schematics
*
Schematic of thermoplastic Injection molding machineSlide9
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 poorSlide10
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 actionSlide11
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
machineSlide12Slide13
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 cycleSlide14
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
)Slide15Slide16
Mold Basics
Cavity Plate
Cavity
Moulding
Core
Core Plate
Runner
Cavity
Gate
Nozzle
Sprue
Melt DeliverySlide17
Part
Cavity
Core
Stripper plate
Injection Molding for a plastic cup
Runner
Knob
NozzleSlide18
Complete IM for a plastic cup
Runner
Part
Cavity
Nozzle
Part
Cavity
Knob
Stripper plate
Runner
Part
Cavity
NozzleSlide19
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. Slide20
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).Slide21
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
.Slide22
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 specificSlide23
Extrusion Line
Figure 10.1 Slide24
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. Slide25
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.Slide26Slide27
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.Slide28Slide29
Drag the extrudate up to the top nip
Pass it through idle rolls to the winderSlide30
Adjust the blow up ratio by introducing the air by
slowly
turning the air valve.
After the desired size is obtained, close the valve Slide31
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)Slide32Slide33Slide34Slide35Slide36
Bottle forming processSlide37
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. Slide38
1)
Placement
of Charge
5)
De-moulding
3)
Squeeze to
final dimensions
2)
Mould
Closure
4)
Resin
CureSlide39
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.Slide40
Pros & Cons
Pros
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 stressesSlide41
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 FormingSlide42
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 cornersSlide43Slide44Slide45
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.Slide46Slide47
Rotational
Molding
Process Description
Mould
Arm
Plate
Material
a) Charging the Mould
Final
Part
d) Demoulding
A large hollow mould is charged with thermoplastic powder, and rotated.
Heat
b) Heating the Mould
The rotating mould is transferred into an oven, polymer sticking to the walls.
Spray Nozzles/Fan
c) Cooling
Once the powder consolidates, the mould is cooled, and the part removed.Slide48Slide49
Rotational
Molding
Products
Wide range of complex hollow products: Slide50
TP
Pultrusion
Process Description
Prepreg is fed into a preheating section, bringing TP close to T
melt
.
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
Heated Die
Cooled Die
PreheatingSlide51
TP Pultrusion
Equipment
Experimental TP
Pultrusion
Line
Close-up of
preheater
and dies