thermoforming (1).ppt

5. THERMOFORMING
CORPORATE TRAINING AND
PLANNING

INTRODUCTION
• It is the combination of two words Thermo &
Forming.
• The plastic sheet retains the moulds shape and details.
• The process involves heating a thermoplastic sheet
to its softening temp (pliable State).
• Processing or forcing the hot & flexible sheet against the
contours of mould by applying vacuum or air pressure.
•The sheet is held there for cooling and then removed.
•Thermoforming is secondary processing technique.
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PLANNING

• The sheet is heated to the point only enough to
soften it.
• Cooling step is usually short due to low wall
thickness of the part as compared to other parts.
• The essential characteristics of thermoplastic
sheet material should be such that when they
are heated to just below melting point they
should become rubbery or plastic in nature to an
extent which enables them to be stretched out
rather like a balloon.
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APPLICATION
• Refrigerator door liners
• Cheese containers
• Soft drink cups
• Signs
• Packaging of Tablets and capsules
PLANNING

• Ice cream cups
• Plastic tray
• Helmets
• Telecommunication Joints
• Luggage
• Light and instrument panels.
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ADVANTAGES
• Tooling cost is inexpensive.
• Suitable for large parts
• Thin walled components can be made by this
method only.
• Suitable for small number of parts, samples,
prototypes etc.
• Low capital cost.
• Moulds can readily modified and quickly
changed.
PLANNING

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LIMITATION
• The process is confined to the use of sheet
material only.
• All the parts to be made by this process must
have uniform well thickness.
• Ribs or mounting bosses cannot be made.
PLANNING

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MATERIALS CHARACTERISTIC
FOR THERMOFORMING
1. Ability of the materials to be deep drawn without
tearing.
2. Plastic Memory.
3. Good hot melt strength.
4. Hot Elongation.
5. Forming temperature range
a. Wide range is preferred
b. No sharp melting point should be there.
PLANNING

 Basically thermoplastic materials used for
thermoforming process.
 Such types of material when heated will exhibit a
reduction in their modulus of elasticity, their stiffness
and load bearing capacity.
TYPES OF MATERIAL USED
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 A wide softening range, i.e a broad temperature span in
which plastic is soft, pliable and elastic is desirable
since, during thermoforming process the temperature of
material drops rapidly.
 High molecular weight thermoplastics mostly preferred
for thermoforming.
 The material to be thermoformed should have higher
thermal expansion.
 The thermal stability of the material must be good.
Thermal diffusivity is ideal for establishing cooling time
for thermoformed parts.
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PLANNING

Thermal Diffusivity = Thermal Conductivity
Density X specific heat
 The water absorption capacity of the plastic material
should be low for thermoforming, because slow rate
of water absorption also causes difficulties in
thermoforming
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PLANNING

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PLASTIC MATERIALS FOR FORMING
PS, ABS, PVC, PMMA,
CAB, PC, HDPE, PP
PLANNING

 Most available sheet materials are prepared by sheet
extrusion process which employs medium to high
molecular weight polymers that are subjected to
minimal heat stress.
 Sheets with excellent optical properties are obtained
either by casting or by laminating and / or press
polishing of otherwise manufactured sheets.
MATERIAL PREPARATION
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PLANNING

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SHEET FOR THERMOFORMING
The sheets are manufactured from :
• Extrusion Process
• Calendaring Process
• Casting Process
The sheet thickness ranges from 0.25mm to 12.5mm
PLANNING

Mould Materials
(i) Plaster of Paris :
 Most commercial moulding, plasters are not strong
enough to be used in prototyping.
 Plasters are inorganic calcious materials that
hydrolytically react and harden when mixed with water.
THERMO FORMING MOULD
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PLANNING

 Soaps such as Murphy’s Oil soap, found in lather goods
stores can also be used as a surface release agent.
Vents should be designed in by placing release-agent-
coated wires perpendicular to the pattern surfaces
before coating.
 A very hard surface (Void-free) can be achieved by
“Splitting” a thin layer of relatively high water content
plaster slurry against the pattern.
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PLANNING

Advantages :
Cheap, quick, and intimate production of details is
possible.
Disadvantage :
A maximum of only about 50 forming is possible, the
surface being very soft and the mould itself is very
fragile.
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(ii) Wood :
 Hardwoods are used for prototype and short
production.
 The woods must be thoroughly Klin-dried before
shaping to minimize stress relief during fabrication.
 After thorough drying, the surface can be sealed with
temperature–resistant enamel or varnish.
 Recently, epoxy enamels and varnishes have been
developed that protect wood surface for hundreds
of cycles with out refinishing.
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PLANNING

Advantages :
Cheap, longer life span than plaster moulds, higher
impact strength.
Disadvantages :
Limited life say for approximately 500 forming. During
repeated forming, wooden mould should not be allowed
to become too hot and its dimensions should be checked
regularly.
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PLANNING

(iii) Plastic moulds:
 In particular, plastic tooling is economically preferred
for thick sheet forming.
 Plastic moulds are used where mould surface
temperature do not exceed 60oC. where drape or
vacuum forming used, epoxy and unsaturated
polyester resin (UPE) together with glass fiber are the
mould materials of choice.
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PLANNING

Advantages:
1. Fairly cheap, easily manufactured, low thermal
conductivity, little or no finish is required, lasts long.
Disadvantages:
1. Some materials are sensitive to high forming temp.
2. mould surface must be adequately cleaned, waxed
and buffed prior to use.
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PLANNING

iv) Aluminium moulds:
 Aluminium is frequently the material of choice for
thermo forming moulds.
 Because it can be easily fabricated and it has very
high thermal conductivity and so sensible heat
from plastic material can easily be removed.
 It is light weight, tough metal.
 Thermo forming tools can be made from either
machined plate or caste material.
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PLANNING

 The Aluminium mould mostly consists of 1 to 2 % Cu.,
0.5 to 1% mg, 0.5% Mn, 4 to 8% Si, 1% (max) Fe, 15 Ni and
traces of Ti and Zn.
 Typical machined aluminium hardness is 130 Brinell and
Aluminium has relatively high thermal expansion co-
efficient.
Advantage:
Dimensionally stable, good surface finish, very good
abrasion resistance and indefinite life time.
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PLANNING

(i) Convection Ovens:
 Convection ovens are originally the most common
device used to heat plastic sheets for
thermoforming.
 The heat can be supplied by gas flames or by
electric resistance units.
HEATING SYSTEMS
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PLANNING

 Forced circulation of air and baffling to equalize the
air flow at around 200 feet per minute are crucial to
obtain temperature uniformity.
 Good thermal insulation of the oven walls and the
strategical position and size of entrance and exit
doors increase energy efficiency.
 Automatic temperature regulators must be provided
to keep air temperature fluctuation as low as
possible.
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PLANNING

(ii) Infrared Radiant Heaters
 Besides dialectic heating, oil submersion heating and
contact heating, IR radiant heating is the fastest way of
heating plastic sheet or films to thermoforming
temperature.
 Although heater densities may vary with equipment,
there are also differences in regard to materials.
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PLANNING

 High temperature plastics, such as polycarbonates
and polyesters are the highest, with about 30 watt/sq.
 The cellulosic, styrene and vinyl Polymers are the
lowest 15 watts/sq. Thin films can be heated at higher
energy densities in a considerably short time.
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(iii) Electrically Powered Infrared Heaters
 Electrically powered infrared heaters are available in
a wide range of designs. In order of decreasing
radiant surface temperatures (i.e. increasing wave
length of energy emitter).
They are:
 Tungsten wire filament heaters in quartz tubes and
tungsten wire filament glass lamps.
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 Nichrome wire coil heaters in quartz glass tubes.
 Nichrome wire or band in refractory materials
embedded or surrounded and protected by
stainless steel round.
 Heat distribution over entire sheet is more uniform
in case of radiant heaters than hot air convection
ovens.
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PLANNING

A sheet of plastic initially of thickness to and surface area
Ao is stretched to provide a part having a surface area A (A
> Ao) and an average thickness ta (ta < to)
The plastic volume given by :V = toAo = tdA = taA
The stretch ratio is given by :
Ra = A/Ao or Area ratio Other wise called areal draw ratio
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STRETCH RATIO
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DIFFERENT FORMING PROCESSES
• Straight Vacuum Forming Process.
• Pressure forming
• Plug-Assist Forming
• Free forming
• Drape forming
• Snap-back forming
• Matched-die forming
• Mechanical forming
PLANNING

STRAIGHT VACUUM FORMING
• This techniques is most versatile and widely
used.
• The plastic sheet is clamped in a frame and
heated.
• The hot sheet becomes rubbery or elastic.
• Then it is placed over a female mould cavity.
• The vacuum is now applied.
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PLANNING

• The atmospheric pressure forces the hot sheet
against the walls and contours of the mould.
• It is allowed to cool there.
• The formed part is removed and final finishing and
decoration is done.
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• This technique is used when the outside of the part (the
side against the mould) must have fine details or close
tolerances .
• This process is limited to draw ratio of 1 ½ .
• Draw ratio is the ratio of the draw dept to the part width.
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FIGURE
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PRESSURE FORMING
• It is similar to straight vacuum forming process.
• Here also plastic is formed in a female mould.
• Here instead of applying vacuum a positive air
pressure on the top of the plastic is used to force
the material against the female mould.
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PROCESS
• The sheet is clamped and heated till softened.
• The softened sheet is transferred to the
moulding area and a seal is made so that the upper
chamber, above the plastic is airtight.
• The sheet is also sealed against the mould as is
done with vacuum farming.
• Air pressure is applied into the area above the
softened plastic and vacuum is created below
it.
• The air pressure and the vacuum forces the plastic
against the mould.
PLANNING

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• Moulding cycle is faster.
• The sheet can be formed at lower temp because
the forming pressure is higher.
• A greater dimensional control and part definition
can be achieved.
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ADVANTAGES
PLANNING

PLUG ASSIST FORMING
• A male plug is used.
• Plastic sheet is clamped in the female mould and
after the heat-softened sheet is sealed across the
mould cavity,the plug pushes the sheet to stretch
it.
• After completion of penetration stroke vacuum and
/or compressed air is introduced to transfer the
sheet from the plug surface to the cavity mould
surface.
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PLANNING

• Plugs are made up of metal,wood or thermoset
plastic.
• Plug is heated to a few degree less than the temp
of the plastic in order to prevent premature
cooling.
• The plug size combined with the rate and depth of
penetration affect the amount of stretching that
occurs.
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PLANNING

• Deeper and more uniform wall thickness is
obtained.
• It is also responsible for the ultimate material
distribution in the finished product.
ADVANTAGES
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PLANNING

• The sheet is expanded with pressure.
• The size of bubble is monitored by an electronic
eye.
• When the bubble reaches the desired size,the air
pressure is reduced to a level that maintains the
size of the bubble while the part cools.
FREE FORMING
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PLANNING

• The products have very high optical clarity.
• No mould is used.
• No transfer or handling of the sheet.
• Simple and Economical
• Uniform cooling.
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ADVANTAGES
PLANNING

• Complexity of shapes can’t be made.
• The control over the shape is
difficult.
DISADVANTAGES
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PLANNING

• 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.
• Vacuum is applied beneath the mould and forces the
sheet over the male mould.
• By draping the sheet over the mould, that part of the
sheet 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
edges of the mould and bottom seal area at the base.
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DRAPE FORMING
PLANNING

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PLANNING

• Greater depths of draw can be achieved without
excessive thinning compared to forming in a
female mould.
• Uniformity of thickness is much better.
ADVANTAGES
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• It is the modified form of Drape forming.
• The sheet is heated to the sag point.
• Then it is drawn slightly into a vacuum box below
the part.
• This pre-stretching creates thinning effect at the
center of the part . It is generally to 1/2 to 2/3 of
total draw.
SNAP-BACK FORMING
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• A second step is then activated to give more draw
i.e the male mould is pressed against the material
to draw it further.
• During this stage, the thickness of the material is
constant at center and thinning occurs near the
edge.
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• Finally the part is formed by applying a vacuum
through the male mould and causing the part to
snap back against the outside of mould.
• The part cools against the mould to take its final
shape.
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Advantages :
• Uniform wall thickness
• Complex shape can be formed.
Disadvantages :
• Longer cycle time.
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• Many cases & luggage sheets.
• Computer Housing.
• Acrylic cast sheets etc.
APPLICATIONS
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MATCHED DIE FORMING
• This method resembles to that of compression
moulding.
• The plastic sheet is heated to the sag point.
• It is trapped and formed between male and female
dies.
• The clearance between the male and female dies
decide the wall thickness.
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• Vent holes on both mould is provided to avoid the
entrapment of air.
• Moulds are placed until the plastic cools and
cures.
• No vacuum air pressure is applied in this
process.
• Mould materials are generally wood,plaster,
epoxy or others.
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ADVANTAGES
• Very good reproduction of mould details.
• Very good dimensional accuracy.
• Lettering and grained structure can be easily formed.
DISADVANTAGE
• Internal cooling of mould is desirable.
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MECHANICAL FORMING
• The sheet is clamped and heated.
• Mechanical pressing is done against the inside of
a forming tool such as bracket to give the desired
shape.
• The plastic sheet cut to appropriate shape and
size and then heated to the sag point.
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• The pressing can be done by a roller, a block or
any other instrument to apply a relatively uniform
pressure on the plastic sheet, when it is still hot
to create the desired shape.
• The formed plastic sheet is cooled in that place.
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PLANNING

 Inline thermoforming mainly meant for the
continuous sheet forming & production.
 If the forming process can be accomplished during
the time it takes to extrude the sheet and if long
production runs are involved, inline extruder
thermoformer could be considered.
INLINE THERMOFORMING PROCESS
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PLANNING

 The advantages is that the materials is already
available in very uniform temperature and it might not
have to be preheated.
 Any cut-outs, margins and trim materials can
continually be regrinded and re-extruded, thus
solving the problem of eliminating scrap material.
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ADVANTAGES
PLANNING

 Since the virgin regrind ratio remains constant for
each product, the disturbances found when
external regrinds are reprocessed are avoided.
 Improved extruder technology and the increased
use of gear pumps, which eliminate extrusion
surging helps too.
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PLANNING

The problem arising at any point may require the
shut down of the whole production line and that no
pre-printing of the sheet is possible.
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DISADVANTAGES
PLANNING

 Co-extruded and laminated sheets have gained
favour for thermoforming process since for many
applications.
 By the term co-extrusion is meant the formation of
sheet produced by simultaneously employing two or
more extruders.
CO-EXTRUSION AND LAMINATES
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PLANNING

 The term laminate should always be employed
when either two or more previously extruded or
calendered sheets combined or bonded.
 Usually laminates are obtained by pressing several
layers between Chrome Plated Steel Sheets.
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PLANNING

 Co-extruded products and laminates are used when :
1. Parts are required which have different colours on the
inner and outer surface, and plain coat of paint is not
adequate.
2. Parts are subjected to ultra violet radiation on the
outside, but lower cost material suffices to provide
mechanical strength.
3. low cost grind or scrap materials ( of undesirable
colour) are available but parts with high quality
appearance are demanded.
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PLANNING

Thermoforming Injection moulding
Thermoforming requires
more expensive sheet as
raw stock
Injection moulding uses
material in granule form
which is cheapest form of
raw material
Thermoforming dies are
made up of wood, plaster
of Paris, aluminium and
its cost is substantially
low.
Here the mould cost is
very high.
If the number of articles to
be moulded is less then
the choice would be
thermo forming.
Injection moulding is
suitable for large
production.
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PLANNING

Thermoforming operation
sets up quickly.
Injection moulding
operation takes more time.
Thermoforming sheet can
be printed or decorated
before forming
In Injection Moulding
decoration is not possible
before processing.
Holes and undercuts
cannot be produced by
thermoforming
Holes and undercuts be
produced by injection
moulding.
Thermoforming is
adaptable to the
production of very large
parts such as trailers roof.
Injection moulding
produces not as large part
as thermoforming.
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PLANNING

Thermoforming parts
require final trimming
operation
Injection moulded in parts
don’t require any final
trimming operation.
Pressure required is lower
than injection moulding
Pressure requirement is
very high.
Here scrap Production is
very high
Scrap production is very
low.
Lower machine original
cost
Higher original machine
cost.
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PLANNING

THERMOFORMED PRODUCTS
AND
ITS APPLICATION
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PLANNING

(i) Packaging and
related items
 Blister packs.
 Bubble packs.
 Cosmetics, Cases, Packages,
 Meat, Poultry trays
 Wide mouth jars.
 Vending machine hot cup.
 Egg Cartoons.
(ii) Vehicular  Automotive door inner liners.
 Windshields.
 Motorcycle windshields, Mud
guards.
 Recreational vehicle interior
components.
 Window Blisters.
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PLANNING

(iii) Industrial  Pallets
 Part Trays, Transport trays
 Equipment Cases.
(iv) Building
Products
 Shutters, Windows.
 Skylights, Translucent Domes.
 Exterior lighting shrouds.
 Storage modules, Bath-Tubs.
(v) Others  Exterior Signs, Luggage trays.
 Boat Hulls (with PUR foam)
 Advertising signs.
 Lighted indoor signs.
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PLANNING

FAULTS, CAUSES & REMEDIES
IN
THERMOFORMING PROCESS
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PROBLEM PROBABLE
CAUSE
REMEDIAL ACTION
BLISTERS
(a) Heating too
rapidly
 Lower heater temperature
 Use slower Heating
 Increase distance between
heaters and sheet. Blow air
across sheet surface during
Heating.
(b) Excess
Moisture
 Predry sheet
 Pre heat sheet
 Heat from both the side.
(c) Uneven
Heating
 Check heat out put power
consumption.
 Use pattern heating.
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PLANNING

INCOMPLETE
FORMING
(a) Sheet too
Cold
 Heat sheet longer.
 Raise heater temperatures.
 Use more heaters.
 Change to more efficient
heater design.
(b) Insufficient
Vacuum
 Check vacuum holes for
obstruction.
 Increase number of vacuum
holes.
 Increase diameter of vacuum
holes.
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PLANNING

(c) Vacuum not applied
rapidly enough.
 Use vacuum slots rather than
holes.
 Too many bends in vacuum
line.
 Check vacuum leaks.
(d) Applied pressure too
low.
 Increase air pressure.
 Use plug, silicone slab rubber,
or Bladder as plug assist.
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PLANNING

SCORCHED
SHEET
(a) Sheet
surface too hot.
 Shorten heat cycle.
 Use slower, soaking Heat.
 Consider convection
heating.
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PLANNING

COLOUR
INTENSITY
CHANGE
(a) Insufficient
Heating
Length heating cycle.
Raise heater temperature.
Change to more efficient
heaters.
(b) Excess
Heating
Reduce heater temperature.
Shorter heating cycle.
If localised, check heater
efficiency.
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PLANNING

(c) mould too Cold Warm the mould
(d) Sheet cools before fully
formed.
 Transfer sheet faster.
 Increase forming rate.
 Increase mould, plug
temperature.
(e) Poor mould design  Reduce draw ratio.
 Increase draft angle.
 Increase corner radius.
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PLANNING

WHITENING
(a) Stretching
below forming
temperature.
Increase sheet temperature.
Increase forming speed.
(b) Sheet Dry-
coloured
Poor extrusion.
Material unsuitable for
pigmentation.
Local blemished removed
with hot air gun.
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PLANNING

SURFACE
BLEMISHES
(a) Poor Vacuum Increase vacuum hole area.
Check plugged vacuum holes.
(b) mould Too Hot Reduce mould temperature.
(c) mould Too Cold Increase mould temp.
(d) Rough mould
surface
Polish mould.
Use Aluminium
moulds
(e) Scratched sheet Inspect handling
procedures.
Use Polish Sheet
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PLANNING

SHINY
STREAKS
(a) Local overheating
 Check heater temperature.
 Pattern Heat.
 Air cool locally.
 Reduce Heating Cycle.
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PLANNING

WARPED
PARTS
(a) Uneven part
cooling
Change coolant channel
configuration.
(b) Poor material
distribution in
part wall.
 Use Pre-stretching or plug
assist.
 Poor temperature uniformity.
(c) Poor mould Design Increase vacuum hole area.
Redesign rim area to stiffen.
Add plugged vacuum holes.
(d) mould temperature
too low
 Increase mould temperature
to just below material set
temperature.
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PLANNING

SHRINK
MARK
(a) Inadequate
Vacuum
Vacuum leak
Plugged vacuum holes.
Vacuum hole are inadequate.
(b) Surface too
smooth
 Roughen mould surface.
 Change to lower conductivity
mould material
(c) Part shrinking
during forming
Increase forming pressure.
Increase mould temperature.
Change to less elastic material.
(d) Inadequate air
pressure
Increase air flow rate.
Increase air pressure.
Increase cycle time under
pressure.
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PLANNING

PARTS
STICK
IN mould
(a) Part temperature too
high
Increase cooling time
Lower mould temperature.
Reduce heating time
(b) Inadequate Draft Rework mould for more draft.
Use female mould.
Remove part early.
(c) mould undercuts  Remove part early
 Consider more sophisticated
ejection system.
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PLANNING

(d) Sticking in one spot Uneven mould temperature.
Uneven sheet temperature prior to
forming.
Vacuum brake inadequate.
(e) Wooden mould Lubricate with dry mould release.
(f) Rough mould
surface
Polish especially corners.
Use dry mould release.
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PLANNING

SHEET
TEARS
WHILE
FORMING
(a) mould Design Increase corner Radius.
(b) Sheet is too hot Decrease sheet temperature.
Preheat sheet, then bring for
forming.
Sheet thickness may not be
uniform.
(c) Sheet too cold Increase heating time.
Preheat sheet.
(d) Improper
material
 Depth of draw excessive for
material.
 Change forming technique.
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PLANNING

CORNER
CRACKING
IN SERVICE
(a) Stress
Concentration
Increase radii.
Corner too cold during forming.
Increase mould temperature.
Increase sheet temp.
Increase forming rate.
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PLANNING

Excessive
Sag
a) Sheet too hot (a) Reduce Heater temp.
(b) Reduce Heating Cycle
b) Melt index too high (a) Use lower MI Olefin.
(b) Change Resins.
(c) Increase sheet orientation.
(c) Sheet area excessive (a) Pattern Heat to reduce
temperature of sheet centre.
(b) Add sag bands.
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PLANNING

VERY THIN
CORNERS
(a) Incorrect forming
technique
Try Plug assist
(b) Sheet too thin Increase sheet thickness
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PLANNING

(c) Sheet temperature variation Check Material allocation.
Pattern heating
Increased rate of forming
(d) Variation in mould
temperature
Change coolant line
configuration.
Check free surface cooling.
(e) Incorrect material Use stiffer resin
Use more elastic resin
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PLANNING

thermoforming (1).ppt

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