Thermoforming techniques are secondary
processing techniques. it is The combination of two words THERMO and FORMING. Thermoforming
process is used to make various plastic products from Their sheets.
Principle
Ø
First of all we heat The Thermoplastic
sheets so that’s it softens.
Ø
We force a hot and pliable material
against The surface of a mould by using eiTher mechanical, air or vacuum
pressure.
Ø
The sheet is held against The mould
Then after some time allowed to cool so that The plastic sheet gets The shape
of The mould.
Ø
The Thermoforming process is
different from oTher plastic processing because here we do not melt The raw
material but only softened The raw material.
Advantages
of Thermo forming processes:
Ø
Tooling cost is expensive.
Ø
Suitable for large part.
Ø
Low capital cost.
Ø
Low temperature required.
Ø
Fast moulding cycle, low pressure
required
Ø
They are lightweight and durable.
Ø
Thermoforming can be recycled thus
technology is environment friendly.4
Ø
Mould can readily modify and quickly
changed.
Ø
Lower machine original cost.
Disadvantages
Ø
Limited space Complexibility.
Ø
Trimming or secondary process is
required.
Ø
These are high amount scrap
generated
Ø
The cost of The sheet material is
high
Ø
internal stresses are produced
Ø
It is not suitable for large
production.
Ø
Holes and undercuts are not
produced in this process.
Limitation
Ø
The process is confirmed to The use
of sheet material only
Ø
Ribs or mounting loss cannot be made.
Ø
All The parts to be made by this
process must have uniform wall thickness
Ø
Internal stress is common.
Material characteristics for Thermoforming:-
Ø
Ability of The material to be Deep
Drawn without tearing.
Ø
Plastic memory.
Ø
Good hot melt strength.
Ø
Hot elongation.
Ø
Forming temperature range wide
range is preferred and No sharp melting point should be There.
Materials
for Thermoforming:-
Ø
Basically Thermoplastic materials (PS,
ABS, PVC, PMMA, CAB, PC, HDPE, PP) used for Thermoforming.
Ø
Such type of material when heated
will exhibit a reduction in Their modulus of elasticity There stiffness and
load bearing capacity.
Ø
The material to be Thermoformed
should have higher Thermal expansion and Thermal stability of material must be
good.
Ø
Thermal diffusivity should be ideal.
Ø
Thermal diffusivity is equal to Thermal
conductivity/ density x specific heat
Sheet
for Thermoforming:-
The sheets are manufactured form
Ø
extrusion process
Ø
calendaring
Ø
casting
Thermoforming
mould:-
Mould
materials
Plaster
of Paris:- Most commercial moulding Plaster are not strong enough to be
used in prototyping Plaster are inorganic calcious material that hydraulically
react and harden when mixed with water.
Advantages Cheap
quick and intimate production of details is possible
Disadvantages: - A maximum of only about so
forming is possible.
Wood:- Hard woods are used for prototype and short
production. The woods must be thoroughly clean dried before shaping to minimise
stress relief during fabrication.
Advantages:-
Cheap, higher impact strength longer life span than plaster mould
Disadvantages:- Limited life for approximately so forming.
Plastic moulds:-
Plastic mould is used where
mould surfaces temperature do not exceed 60°C wear bra or vacuum forming used.
Advantages Fairly cheap easily manufactured, low Thermal
conductivity, little or no finish is required, last longer
Disadvantages Some material
is sensitive to high forming temperature 2. mould surface must be cleaned one and buffed
prior to use.
Aluminium Moulds:- Aluminium is frequently The material of choice
for Thermoforming moulds. It is light weight tough metal. Typical machined
aluminium hardness is 130 Brazil has high Thermal expansion Coefficient
Advantages dimensionally stable, good surface
finish, very good abrasion resistance and indefinite lifetime.
Heating system There
is 3 methods for heating The sheet. 1. Convection
oven 2. Infrared Radiant heater. 3.
Electrically power infrared heater.
Convection
oven Convection ovens are originally The most common device used to heat
The plastic sheets for Thermoforming The heat can be supplied by gas flame or
by electric resistance unit. Forced circulation of air and buffing to equalise The
air at around 200 feet per minute are crucial to obtain temperature uniformly.
Process parameter
Ø Temperature
Ø Pressure
Ø Position
Ø mould temperature
Different forming process
Straight vacuum forming
v
This technique is most versatile
and widely used.
v
In this process a plastic sheet is
clamped in a frame and heated when The hot sheet become rubbery or elastic.
v
It is placed over a female mould
cavity. The vacuum is now applied.
v
The atmospheric pressure forces The
hot sheet against The wall and contours of The mould.
v
When The plastic sheet is cooled The
formed part is removed and final finishing and decoration may be done. This
process is limited to draw ratio of 1 and half. Draw ratio is The ratio of The draw
depth to The part width.
Pressure
forming
v
In this process air pressure is
applied on The top of The plastic to force The sheet to take The shape of The
mould.
v
The air pressure is interview on
top and forced out of The moulding oven from The download site
v
The advantage that The moulding
cycle is faster than vacuum forming
v
There is more dimensional control
in The product so that The thickness of The product is better maintained.
v
Sheet can be formed at lower
temperature because The forming process is high.
PLUG ASSIST FORMING
v
In this method a male plug is used.
v
Plastic sheet is clamped in The
female mould and after The heat softened and it is sealed across The mould
cavity The plug pushes The sheet to stretch it.
v
When The plug has completed its penetration
stroke, vacuum are compressed air is introduced to transfer The sheet from to The
cavity mould surface.
v
The plug size combined with The
rate and depth of penetration affect The amount of stretching that occurs and
is primarily responsible for The ultimate material distribution in The finished
product.
v
The plug is made up of metal, wood
or Thermoplastic.
Advantages
v
Deeper and more uniform wall
thickness is obtained
v
It is also responsible for The ultimate
material distribution in The finished product.
DRAPE FORMING
v
In this process The plastic sheet
is clamped and heated Then drawn over The mould eiTher by pulling it over The
mould or by forcing The mould into The sheet The seal is created.
v
Vacuum applied beneath The mould
that part of The sheet over The male mould.
v
By draping The sheet over The mould
that part of The seed which is touching The mould remains close to The original
thickness of The sheet. Side walls are formed from The material draped between The
top edge of The mould and bottom seal area at The base.
FREE FORMING
In this technique
v
The sheet is expanded with pressure
v
The size of bubbles is monitored by
an electronic eye
v
When The bubble reaches The desired
size The air pressure is reduced to a label that maintains The size of The
bubble while The parts cools.
Advantage
v
The product have high optical
clarity.
v
no mould is used.
v
no transfer and handling of The sheet.
v
simple and economical
v
uniform cooling
SNAP
BATCH FORMING
v
It is The modified form of The
drape farming
v
The sheet is heated to The SAG
point.
v
Then it is drawn slightly into a
vacuum box below the part.
v
This pre stretching creates
thinning effect at the centre of The part.
v
A second step is activated to give
more draw that is the main mould is pressed against The material to draw it furTher.
v
During this stage, The thickness of
The material is constant at centre and thinning occurs near The edge.
v
Finally The part is formed by
applying a vacuum through The male mould and causing The part to snap back
against outside of mould.
v
The part cool against The mould to
take its final shape .
Advantage
v
uniform wall thickness
v
Complex shape can be formed
Disadvantage
v
longer cycle time
