thermoforming · introduction • it is the combination of two words thermo & forming. • the...

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.

2 CORPORATE TRAINING AND

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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5

APPLICATION

• Refrigerator door liners

• Cheese containers

• Soft drink cups

• Signs

• Packaging of Tablets and capsules


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• Ice cream cups

• Plastic tray

• Helmets

• Telecommunication Joints

• Luggage

• Light and instrument panels.


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11

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.


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12

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.


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13

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.


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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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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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17

PLASTIC MATERIALS FOR FORMING

PS, ABS, PVC, PMMA,

CAB, PC, HDPE, PP


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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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SHEET FOR THERMOFORMING

The sheets are manufactured from :

• Extrusion Process

• Calendaring Process

• Casting Process

The sheet thickness ranges from 0.25mm to 12.5mm


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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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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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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. 23


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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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(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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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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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. 27 CORPORATE TRAINING AND

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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(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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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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(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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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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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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36

DIFFERENT FORMING PROCESSES

• Straight Vacuum Forming Process.

• Pressure forming

• Plug-Assist Forming

• Free forming

• Drape forming

• Snap-back forming

• Matched-die forming

• Mechanical forming


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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. 37 CORPORATE TRAINING AND

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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40

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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43

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.


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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


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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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• 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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• 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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• 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


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


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• Complexity of shapes can’t be made.

• The control over the shape is difficult.

DISADVANTAGES


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


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• 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


PLANNING

• 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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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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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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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


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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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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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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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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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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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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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THERMOFORMED PRODUCTS

AND

ITS APPLICATION


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(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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(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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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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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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(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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SCORCHED

SHEET

(a) Sheet

surface too hot.

Shorten heat cycle.

Use slower, soaking Heat.

Consider convection

heating.


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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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(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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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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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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SHINY

STREAKS

(a) Local overheating

Check heater temperature.

Pattern Heat.

Air cool locally.

Reduce Heating Cycle.


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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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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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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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(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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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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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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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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VERY THIN

CORNERS

(a) Incorrect forming

technique

Try Plug assist

(b) Sheet too thin Increase sheet thickness


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(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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thermoforming · introduction • it is the combination of two words thermo & forming. • the...

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