Manufacturing process Flashcards
Manufacturing process
5 Ms and classification of manufacturing process
involves science and technology aims to convert raw materials to finished product. the finished product needs the following:
- desired shape and size.
- desired mechanical properties
- Desired surface finish
5Ms
Material
Machine to convert in to product
Method
Man Power to operate machine
Money
Primary: – Casting, Forming such as rolling, extrusion, forging etc
Machining processes – The machine operations are performed on castings, rollings,forgings etc. in order to obtain the desired accuracy and shape. Ex – Turning, Drilling, Milling, Planing etc.
3) Surface finishing processes – to provide a good surface finish
to the metal surface of the product. Ex – Buffing, Lapping, Honing, Anodising, Electroplating etc.
4) Joining processes – These processes are used for joining two or more pieces of metal parts. Ex –
Welding, Soldering, Brazing etc.
5) Processes affecting change in properties- These processes are used to impart certain specific
properties to the metal part for specific conditions of use. Ex – Heat treatment, shot peening etc.
Metal casting aka founding//Sand Casting
pouring molten metal into a refractory(difficult to melt) mould with a cavity of the shape to be made
and allowing it to solidify. When solidified, the metal object is taken out from the refractory mould either by breaking the mould or taking the mould apart.
solid object = casting.
examples:
Shell Mould Casting
Precision Investment
Plaster Mould Casting
Permanent Mould
Die Casting
Centrifugal Casting
CASTING TERMS
- Flask: Bottom board - made of wood and is
used at the start of the mould making. The pattern is first kept on the
bottom board, sand is sprinkled on it and then the ramming is done in the
drag. - Facing sand - The small amount of carbonaceous material sprinkled on
the inner surface of the moulding cavity to give better surface finish to
the castings. - Moulding sand - It is the freshly prepared refractory material for mould cavity, mixture of silica, clay, moisture, amount might vary to get the results and it surrounds the pattern while making the mould.
- Backing sand : most of the refractory material
in the mould. made of used and burnt sand - Core : used for making hollow cavities in castings.
- Pouring basin - A small funnel shaped cavity on top of the mould into which the molten metal is poured on top of spruce.
- Sprue - metal from the pouring basin reaches runner, controls the flow of metal into the mould .
- Runner - The passage in before the gate through which molten metal flow is regulated
- Gate : The actual entry point through which molten metal
enters mould cavity.
Riser - It is a reservoir of molten metal in the casting so that hot metal can flow back into the mould cavity when there is a reduction in volume of metal due to
solidification.
what is a pattern
A pattern is a replica of the object to be made by the casting process, with some modifications:
- shrinkage allowance : metals shrink when cooling, thus needs to account for that
- machining allowance:
extra material is added in casting for sanding and smoother finish - Distortion allowance:
A metal when has just solidified is very weak and therefore
is likely to be distortion prone. particularly so for
weaker sections such as long flat portions,
make extra material
provision for reducing the distortion. also the
shape of pattern itself should be given a distortion of equal
amount in the opposite direction of the likely distortion
direction. - Draft allowance:
withdrawing the pattern from the sand mould, the
vertical faces of the pattern are in continual contact with the
sand which may damage the mould cavity the vertical faces of the pattern are always
tapered from the parting line with some degree tilt(making V shaped).
allowance - Shake allowance:
account for the dimensional changes that occur when a pattern is removed from the mold. It compensates for the slight expansion of the mold cavity due to the manual or mechanical shaking required to free the pattern from the mold, thus original dimensions should be reduced
types of patterns
i) solid or single piece pattern:
single piece, where its simple patterns and with no withdrawal problems, for small scale production
ii) split pattern:
the pattern is split into two parts so that one part is in the drag and the other in the cope. should be aligned properly by making
use of the dowel pins which are fitted to the cope half. These dowel
pins match with the precisely made holes in the drag half of the pattern and thus align the two halves properly.
can lead to alignment issues or
iii) cope and drag pattern
similar to split pattern
the cope and drag halves of the pattern along
with the gating and
riser systems are both attached separately to the metal or wooden plates
along with the alignment pins. castings which are heavy and
inconvenient for handling as also for continuous production.
iv) match plate pattern
similar to cope and drag
but are mounted on a single matching metal / wooden plate once plate is removed casting can became a whole
used for small castings with higher dimensional accuracy and large production
v) loose piece pattern
used when the contour(shape) of the part cannot be removed from the mould. Hence the obstructing part is held loose by a wire. and deattached, and loose parts are removed,
but expensive and high skill needed to remove loose parts
vi) sweep pattern:
a wooden board with a spindle is present, which rotates/sweeps to provide a symmetrical cross section, of mainly circular, contains a base, placed in the sand mass, creates shape in very less time , and hard to manufacture circular shapes without sweep pattern
properties of moulding materials
1) Refractoriness - ability of the moulding material to withstand the high temperatures of the molten metal and doesntcause fusion.
2) Green Strength - The moulding sand that contains moisture.
should have enough strength so that the constructed mould retains its shape.
3) Dry Strength - When no moisture in the moulding,
When molten metal is poured, the sand around the mould cavity is quickly converted into dry sand as the moisture evaporates .now, it should retain the mould cavity and at the same time withstand the metallostatic forces.
4) Permeability - During the solidification of a casting, large amounts of gases are to be expelled from the mould. If these gases are cant escape, they would be trapped inside the casting causing defects. The molding sand should be porous enough so that the gases are allowed to escape AKA gas evolution capability
5) Cohesiveness -
strength of the moulding sand. a property that enables the sand particles to stick together.
FACTORS:
* Shape and size of the grain
* Bonding material and its distribution
* Moisture
6)Adhesiveness - a property of moulding sand, which enables the sand
particles to stick with other objects such as moulding box. plays an important role in keeping the sand mass together in the moulding box and does not allow it to fall, also be ensured that the sand should not stick to the casting and strip off easily, leaving a clean surface
7) Flowability - a property of moulding sand to properly pack the moulding box all around
the pattern. Good flowability = moulding sand to flow all over the pattern when the mould is
rammed.
8) Collapsibility - The molten material in the mould needs enough time to get solidified. Once it is solidified, the mould must be collapsible enough so that free contraction of casting can occur. also a property will permit easy break down of the sand mass and its subsequent use after the casting
casting process
Melting and Pouring
transferred from the furnace to a ladle and cleaned and held until it reaches the desired pouring temperature. The molten metal is then
poured
Cooling and Shakeout:
after mould is cooled. Castings may be
removed manually or using vibratory tables that shake the refractory
material away
Fettling, Cleaning, Finishing:
Gating system is removed, using band saws, abrasive cut-off wheels
,electrical cut-off devices. A ‘parting line flash’ is removed by grinding
or with chipping hammers
Casting Defects GAS DEFECT, blow holes ISA-2
BLOW HOLES
spherical, flattened or elongated cavities present inside the casting
=> on surface =open holes
=> inside = blow holes
these are caused by moisture left in the mould
the heat in the molten metal, the moisture is converted into steam, part
of which when entrapped in the casting ends up as blow hole or open
blow hole
caused by low permeability of the sand mould or use of too fine sand grains, higher amount of binder
or over ramming of the mould, can also be caused by insufficient venting
practice.
GAS DEFECT
PINHOLE HOLE PORISITY
caused by hydrogen in the molten metal, picked up in the furnace or by the dissociation of water inside the mould cavity
As the molten metal gets solidified, it loses the temperature which decreases the solubility of gases and there by expelling the dissolved gases. The hydrogen while leaving the solidifying metal would cause very small diameter and long pin holes showing the path of escape.
main reason for this is the high pouring temperature which increases the gas
pick up.
rest of casting defects
casting defects: Any irregularity in the moulding process causes defects in castings
which can be fixed using as welding and metallisation
SHRINKAGE CAVITIES:
caused by the liquid shrinkage occurring during the solidification of the casting.
proper feeding of liquid metal is required as also proper casting design
METAL PENETRATION:
When the molten metal enters the gaps between the sand grains, the
result would be a rough casting surface.
either the grain
size of the sand is too coarse or no mould wash has been applied to the mould cavity
SWELL - enlargement of mould cavity because of molten metal pressure/metallostatic forces, which results in localized or overall enlargement for casting
DROP= The dropping of loose moulding sand or lumps normally from the cope surface into the mould cavity is responsible for this defect. This is essentially due to improper ramming of the cope flask
Mis runs:
caused when the metal is unable to fill the mould
cavity completely and thus leaving unfilled cavities.
COLDSHUNT:
A cold shut is caused when two metal streams, while meeting in the mould cavity, dont fuse together properly thus causing a discontinuity or weak spot in the casting
caused by the lower fluidity of metal or the section thickness of the casting is too small. can be rectified by proper casting
design.
increase the fluidity of the metal
HOT TEARS:
Since metal has low strength at higher temperatures, any unwanted cooling
stress may cause the rupture of the casting. main cause is the poor casting design.
advantages and disadvantages of sand casting
-possible to cast practically any material, be it ferrous or non–ferrous.
- necessary tools for casting moulds are very simple and inexpensive. As a result,for trial production or production of a small lot, it’s ideal.
- possible in casting process to place the amount of material where exactly required. As a result, weight reduction in design can be achieved.
- generally cooled uniformly from all sides and thus are expected to have no directional properties.
-Casting of any size and weight, even up to 200 tons,
DISADVANTAGES:
The dimensional accuracy and surface finish achieved by normal sand casting process would not be
adequate for final application in many cases.
The sand casting process is labour intensive to some extent and therefore many improvements are
aimed at it like machine moulding and foundry mechanisation.
With some materials it is often difficult to remove defects arising out of the moisture present in sand
castings.
Special casting process
Precision investment casting
Sand casting is not suitable and economical in many applications. thus special casting is used
Precision Investment Casting:
- mould is prepared via mould injection using wax to make a pattern
- Then the cluster of wax patterns is attached to the gating system by applying heat, (tree pattern)
- the pattern is then coated with thin layer of refractory material (made of ceramic), the full mould is formed by covering the coated tree with sufficient refractory material
- mould is held in inverted position to remove excess material
- remove the pattern from the mould by heating the mould to melt the pattern
- The moulds are then pre heated to remove any impurities. molten metal is poured into the mould
-molten metal is cooled and solidified. Then the moulds are subjected to shakeout using vibratory tables to remove the casting
Special casting process
Centrifugal Casting
- mould is rotated rapidly about its central axis as the metal is poured into it.
- a continuous pressure will be acting on the metal as it solidifies. slag, oxides and other inclusions b gets separated from the metal
and segregates toward the centre.
TRUE CENTRIFUGAL CASTING
normally used for the making of hollow pipes, tubes, hollow bushes, etc., which are axial symmetric(symmetric around a single axis.) with a concentric hole.
first metal is poured, after that the mould is rotated until its solidified
no core needs to be used for making the concentric hole. The axis of rotation can be horizontal, vertical or any angle in between.
the moulding flask(both upper and lower outlines used in sand casting) used to make
the outer contour of the pipe to be made.
the flask is dynamically balanced so as to reduce undesirable vibrations,
finished flask is mounted in between the rollers and the mould is rotated slowly.
Advantages of Hot Working:
Allows any amount of deformation without strain-hardening, as the material is above the recrystallization temperature.
Limitations of Hot Working:
Poor surface finish due to scaling of the surface.
Difficult to achieve dimensional accuracy due to thermal expansion and temperature control challenges.
Handling and maintaining hot metal is difficult.
Advantages of Cold Working:
Increases strength and hardness due to strain hardening.
Limitations of Cold Working:
Limited by the higher yield strength at lower temperatures, restricting the amount of deformation that can be applied with available equipment.
Metal forming hot working, cold working, recrystallisation temp states:
Metal forming: is a process where materials are subjected to plastic deformation with pressure, with or without the application of heat
used for large-scale production rates, generally economical and in many cases improve the mechanical properties too.
Under the action of heat and force, when atoms reach a certain higher energy level, the new
crystals start forming = recrystallisation
Recrystallisation temperature as defined by
American Society of Metals is:
“the approximate minimum temperature at which complete
recrystallisation of a coldworked metal occurs within a specified time
1/3 to 1/2 the melting point
depends on the amount of cold work a material has already received. Higher the cold work, lower the recrystallisation temperature.
Rolling
a process of reducing the thickness or changing the cross section of a long work or piece by compressive forces using 2 ‘high rollers’ (terms for 2 rollers) or
‘4 high rollers’ terms for 2 working rollers with 2 backup rollers => when metal width is high it might bend
most widely used metal-working because of its higher productivity and low cost.
angle of bite (Extent in reduction of thickness) => a
cold rolling=4,5 degree
hot rolling = 24-32 degree
X=> metal speed<roller speed
Y=> metal speed>rollers speed
A=> Neutral point => Metal Speed = Rollers Speed
=> Radial load acts on metal
Δh= ho-hf
(Δh)max=μ^2R
μ=tana
R=> Radius of rolelr
tan a=√Δh/R
sin a = LD/R = √RDH/R
Lp=>projected lenght of R
Lp = √RΔh
forging
where the metal is heated and then a force is applied to manipulate the metal in such a way to get the required final
shape
mainly hot working, rarely cold working
TWO TYPES
DRAWING OUT:
This is the operation in which the metal gets elongated with a
reduction in the cross-sectional area. force is to
be applied in a direction, perpendicular
UPSETTING:
applied to increase the cross-sectional area of the
force is applied in a
direction parallel to the length axis.
FORGING TYPES:
Smith Forging - traditional forging operation done openly in open dies
manual hammering/power hammers.
Drop Forging - in closed impression dies (like molds )by drop hammers. force for shaping the component is applied in a series of blows.
Press Forging - Similar to drop forging, the press forging is also done in closed-impression dies
but force is a continuous squeezing type applied by the hydraulic presses.
Machine Forging - the material is only upset to get the desired shape.
EXTRUSION
confining the metal in a closed cavity and then allowing it to flow from only one opening so that the metal will
take the shape of the opening.
equipment consists of a cylinder or container into which the heated metal billet is loaded.
On one end of the container, the die plate with openings fixed. the other end, a plunger or ram compresses the metal
billet against the container walls and the die plate, forcing it to flow through the die opening, acquiring the shape of the opening
extruded metal is then carried by the metal handling system as it
comes out of the die.
forward extrusion: flow of metal is formed in the same direction of that of the ram
backward extrusion: opposite to the direction of the flow
