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Compression Mould
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Compression Molding
A two-piece mold provides a cavity in the shape of the desired molded
article.The mold is heated and an appropriate amount of molding
material is loaded into the lower half of the mold.The two parts of the
mold are brought together under pressure.
The compound, softened by heat, is thereby welded into a continuous
mass having the shape of the cavity.The mass then must be hardened,
so that it can be removed without distortion when the mold is opened.
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Compression Mould
A technique whereby molding compound is introduced into an open
mold and formed under heat and pressure.
Types of Compression Mould
1.Flash Type mould
2.Positive-type mould
3.Landed Positive mould
4.Semi-positive mould
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Flash-type molds
A type of mold designed to permit excess molding material to escape
during the final closing.
The material flowing across the land area has no other restriction to its
movement. This characterizes the name Flash Mold.
Flat and shallow parts should be molded in a flash-type mold.
The flash is always horizontal. If the mold is closed too slowly, a heavy
flash will result.If the mold is closed too quickly, the density of the
molded part will be low and the strength impaired.
Advantages
1.Lower mold cost
2.Good for small parts
3.Ease of loading inserts
4.Experimental molds may be made for prototype parts.
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Flash Type Compression Mold
A-Plunger or force
B-Cavity
C- Plastic part
D- Guide pins
E- Land area
F-Parting line
G-Knockout pins
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Positive-type mold
The type of mold fully confines the molding material, and full mold
pressure is exerted at all times.There is sufficient clearance between the
cavity and plunger for the molding material to escape.The full pressure
is exerted on the molded part.
The travel of the force or plunger is limited only by the amount of
plastic molding powder placed in the die cavity.No external pressure
pads or stop blocks are used to limit the closed height of the mold.
The clearance space between the plunger and the cavity is called
vertical flash ring is 0.002 to 0.005 inch. per side.
It is used for molding high impact thermosetting plastic materials.
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Positive – type mold
A- Plunger or force
B – Cavity
D – Guide pins
E – Knockout pins F – Loading space
G -Vertical flash ring
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C – Plastic part
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Landed Positive Type
A modification of the positive mold is known as a landed positive
mold. It is essential that the width of the land should not exceed
3/8inch. If it does, the applied pressure will be absorbed by the land.
This will result in thick flash or fins, and the piece will not be filled
out properly.
The force or plunger is cut back or relieved about 1/16 inch., thus
producing a shoulder of this width on its lower end.
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Landed Positive mold
ISO 9001
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A – Plunger or force
B – Cavity
D – Guide pins
E – Knockout pins F – Land
G – Loading well
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C – Plastic part
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Semi positive mould
Semi positive compression molds differ from positive molds, because
the force or plunger only telescopes the body of the mold enough to
exert positive pressure at the final closing of the mold.
The effective pressure on the molding material is assured during the last
of the closing cycle, because of the short distance of vertical positive fit
between the force and cavity.
A slow uniform closing will allow the material to fuse completely
before the mold is closed and will increase the effective pressure on the
compound.
This type of mold is best suited for quality production mold.
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Semi positive mold
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A – Force or plunger
B – Cavity
D – Guide pins
E – Knockout pins F – Land
G – Loading well.
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C – Plastic part
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Types of Loading Chamber
Plastic powder fed in to the mould has a greater need for space in a loose
state than has the pressed molding. The mould cavity design must allow for
sufficient space for the loose powder.
The size of the charge, in unmolded powder form, regulates the size of the
“powder well” or space provided above the cavity impression.
Two advantages are obtained by keeping the powder well area as small as
practicable:
a)Molding pressure is increased
b)Less obstruction is put in the way of the escaping excess molding
material(flash)
For the materials having high bulk factor then pressuring such materials
often requires an extra high loading chamber and therefore when pressing
bushes or similar parts the punch being not only long but also thin-a
removal loading chamber extension is fixed to the mould.
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Extension of the Loading Chamber
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Bulk Factor
The ratio of the volume of the loose plastic powder to the volume of
the molding is called bulk factor.
The inclusion of different fillers affect the bulk factor of the material.
The bulk factor for general purpose material and most mineral fillers
varies between 2 and 3.
In the case of light fabric filler the bulk factor will increases to between
8 and15 and use of a heavy fabric or string filler can increase the bulk
factor to between 12 and 24.
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Flash Thickness Allowances
Allowances for flash thickness in compression moulds, using
thermosetting compounds are:
Rag-filled high impact compound
- 0.25mm
Cotton – flock compounds in large moulds - 0.2mm
Wood – flour compounds in small moulds
- 0.1mm
All other moulds and for all other compounds allow0.13mm
Because of the flash thickness that we are considering in the mould
design the depth of cavity become:
Depth of cavity = Minimum dimension of molding + Shrinkage of
compound
The flash thickness adds to the total thickness of the part and this
thickness must be sub-tracted from the basic cavity depth in order that
the finished piece may have the desired thickness.
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Calculation of Loading Chamber Depth
D = ( V T – VC ) / A
Where,
D – Depth of loading space from top of cavity to pinch-off land
VC – Volume of actual cavity space (cm2)
VT – Total volume of loose powder (cm2)
A – Projected area of the loading chamber (cm2)
Where,
V – Total volume of part including flash factor 10 to 20%. In the way
the depth of loading chamber can be determined.
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Loading chamber design
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Projected area
The projected area is the total area of the molding as seen when viewed in
the direction that it will be placed on the press in the plane normal to the
press opening.
The projected area can vary depending on the size of the component as
well as on the design of cavity or loading chamber and the compression
pressure also can vary based on the type of plastics materials.
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Compression pressure
The pressure needed to mould a particular article depends on the flow
characteristics of the material, the cavity depth and the projected area of
the piece part.
It is recommended that minimum molding pressure of 250kg/cm2 of
projected area be used.
But for calculation purpose the compression pressure for the particular
material is get from the material manufacturers or from the plastics hand
books or data books.
Clamping force
Clamping force(kgf) = Projected area of the molding(cm2) x
Compression pressure of the plastic material(kgf/cm2)
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Deciding number of impression by technological method
The calculation of the number of cavities or impressions by technological
method for multi-cavity molds, the following parameters should be
considered.
On the machine side – available machine, clamping force and size of the
platen.
Clamping force available in the machine
Number of impression = --------------------------------------------------------Clamping force required for an impression
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Heating of Compression Molds
Thermosetting materials which are used in compression molding are
cured by heat and pressure. Heating of molds using thermosetting
materials serves two purposes:
-Heat must soften the material sufficiently to allow it to flow under the
influence of the press pressure into any opening in the mold to the
desired shape.
- Enough heat must be applied to bring about the chemical change or
polymerize the material into its hard, infusible finished state.
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Heating systems
The molds are generally heated electrically, but steam or hot water may
be used.Electrical heating is more up-to-date than steam or hot water
heating.The mold temperature can be controlled easily and over wide
range with electrical heating than with steam or hot water heating.
The maintenance of the switching and control devices in electrical
system is easy when compared with maintenance of complicated
equipments in steam heating.Less wasteful of time than for the steam or
hot water systems.
In electrical system the heating coil should be placed at equal distance
from the molding surface, as local overheating or under heating can
occur more easily than with steam or hot water systems.
There is no great difference in costs between the two systems, the
generally higher cost per calorie of the electric current being balanced by
the wastages found in steam systems.
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Types of heaters
Frames, tapes and cartridges are used as heaters.
Heat equipment & heater capacity
Heater tapes have diameter from 50 to 500mm and are available with
different widths.Their heating power amounts up to 3watt/cm2.
The cartridge diameters are 17 and19mm, and the cartridge length is
generally 100mm.The required number of cartridges may be calculated
from the current rate.The heating power must correspond to the weight
of the mold.
Medium sized and big molds are sometimes induction heated. Insulated
copper conductors of large cross-section are embedded in the mold.
Alternating current of 50Hertz at 20volt and several hundred ampere
cause eddy currents that heat up the mold.
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Advantage of Compression Molding
1.Waste of material in the form of sprue, runners and transfer-culls is avoided,
and there is no problem of gate erosion.
2.Internal stress is the molded article is minimized by the shorter and
multidirectional flow of the material under pressure in the mold cavity.
3.A maximum number of cavities can be used in a given mold base without
regard to demands f sprue and runner system.
4.Compression molding is readily adaptable to automatic removal of molded
articles.
5.This technique is useful for thin wall parts that must not warp and must retain
dimensions.
6.For parts weighing more than 3pounds, compression molding is recommended
because transfer or screw injection equipment would be more expensive for
larger parts.
7.For high-impact, fluffy materials, compression molding normally is
recommended because of the difficulty in feeding the molding compound from a
hopper to the press or performer.
8.In general compression molds usually are less expensive to build than transfer
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or injection types.
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Disadvantages of Compression Molding
1.In the case of very intricately designed articles containing undercuts,
side draws, and small holes, the compression method may not be
practicable, because of the need for complicated molds and the
possibility of distorting or breaking mold pins during the flow of the
material under high pressure.
2.For heavy handle, compression molding would be slower than transfer
or injection because in transfer the plastic is thoroughly heated and is
precompressed almost to its final density prior to entering the mold.
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