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Polymer Processing Techniques Polymer Processing Techniques

Polymer Processing Techniques - PowerPoint Presentation

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Polymer Processing Techniques - PPT Presentation

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

injection mold pressure molding mold injection molding pressure forming material part process air die screw resin extrusion cavity machine

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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

MoldSlide5
Slide6

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.Slide7
Slide8

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

machineSlide12
Slide13

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

)Slide15
Slide16

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.Slide26
Slide27

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.Slide28
Slide29

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)Slide32
Slide33
Slide34
Slide35
Slide36

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 cornersSlide43
Slide44
Slide45

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.Slide46
Slide47

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.Slide48
Slide49

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