Pattern:
Pattern
is the solid form that is used to make the mould. Even though a pattern
generally resembles the casting being produced, it differs from the
casting in terms of certain allowances that are required for producing
the correct shape and dimensions and in eliminating some minor details
which are too complicated or small to be produced by casting and are
left to be produced later by machining.
A
pattern has to be designed to facilitate moulding and ensure good
quality castings. The quality of the castings produced depends on the
design of the pattern, its material and construction. A poor casting may
result from a good pattern due to various other factors but a poor
pattern would never make a good casting.
The
design of the pattern depends upon the shape and complexity of the
casting, the material of the costing and of the pattern, the type of
moulding adopted and the number of castings to be produced.
Types of patterns:
1. Loose pattern
2. Gated pattern
3. Match plate patterns
4. Cope and drag patterns
5. Pattern devices
6. Shell patterns
Loose pattern:
Loose
pattern are patterns which are not connected to other patterns or
mounted on a plate. Loose patterns may be further classified into:
1. Solid or Single piece pattern
2. Self core pattern
3. Split patterns
4. Loose piece pattern
Solid patterns:
Solid
or single piece patterns are the patterns with shapes having straight
draft. They are moulded with the entire cavity in one box. The simplest
type of solid patterns is the one with a flat base. Such a pattern can
be moulded in one box only with flat surface forming the parting line
between the two flasks. When using solid patterns the moulder has to cut
his own runners, feeding gates and risers which take time. Such
patterns are not useful except for limited production. Generally made of
wood, these patterns are most inexpensive of all patterns.
Self core patterns:
Self
core type solid patterns are patterns having a hollow portion with
straight draft which is used for producing its own core during moulding
process itself. This type of pattern eliminates the need for core box.
Split patterns:
Many
patterns cannot be made in a single piece because they have back draft
which does not permit their withdrawal. Such patterns are made split
into two parts. The pattern is split in such a way that the two parts by
themselves have no back draft. One part of the split pattern is molded
in the drag and the other in the cope. The pattern parts are held in
their relative position by dowel pins so as to ensure proper location of
the parts during moulding.
Some
castings that have complicated shape may require that the pattern is
made in three or more parts instead of two. Such split patterns are
called multi-piece patterns and require multiple flasks-one flasks for
each part of the pattern for moulding.
Patterns with loose pieces:
Some
castings require patterns which have parts with back draft in such a
way that they cannot be made into split patterns. In these patterns the
parts with back draft are made into loose pieces so that the loose
pieces can be removed separately from the mould after the main body of
the pattern has been removed.
The loose pieces are
initially held in a place during moulding with the help of pins or
slides but when the main body is withdrawn vertically the loose pieces
are left inside the mould. The loose pieces are then withdrawn sideways
and pulled up through cavity made up by the main piece. Loose piece
patterns have the advantage of a low pattern cost but normal take longer
in moulding.
Gated patterns:
Gated
patterns are number of loose patterns connected with a gating system.
They are moulded like a single pattern and withdrawn from the mould in
one piece thus reducing moulding time.
Gated
patterns are useful when a number of small castings are to be produced.
Of course these patterns do add an extra operation of separating
individual castings after they have solidified along with the gating
system but the overall time for casting per piece is much lower than if
castings were produced individual moulds. Both loose piece patterns and
gated patterns are more suited for hand moulding.
Match plate patterns:
These
have half mounted on two sides of a pattern plate and aligned with
reference to aligning pins on the plate. They are designed to work with
match plate moulding machines for high production rates. The two halves
of the mould are made with the pattern halves on the two sides of the
pattern plate and assembled the two mould halves match.
Match
plate patterns are expensive but their cost can be justified if the
quantity of production is large. Because the moulding is done on
machines, match plate patterns produce castings which are more accurate
than those produced by hand moulding.
Cope and drag patterns:
As
the size of the castings increases, it becomes more and more difficult
to handle match plates. For medium and large size castings separate
pattern plates are made for the cope and drag halves. The pattern halves
are geometrically located with reference to aligning pins or holes in
the pattern plates for proper matching.
The cope
and drag are separately made, may be on different moulding machines by
different workers and assembled with proper matching for pouring.
Pattern devices:
When
the size of the casting is too large and the number required is only
one or two, full patterns generally prove uneconomical. Pattern devices
like sweeps, segments, skeletons and follow boards are used in such
cases to save on pattern cost.
Shell patterns:
Shell
patterns are used mostly for piping work or for producing drainage
fittings. This pattern consists of a thin cylindrical or curved metal
piece parted along the center line. The two halves of the pattern are
held in alignment by dowels. The outside surface of the pattern is used
to make the mould for the fitting required while the inside can serve as
a core box.
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