Manufacturing Flashcards
1
Q
A numerically controlled machine is
A
a machine whose movements are controlled by numerical data
2
Q
Numeric data can be fed into an NC machine by medium of:
A
- paper tape
- magnetic tape
- computer file
- being directly keyed into the machine
3
Q
How does a CNC machine operate?
A
After a program has been entered, the computer processes this program line by line and for each line produces discrete positional commands for the feed drive and velocity commands for the spindle drive, these commands are converted to voltage signals by the digital to analogue converter,these voltage signals are amplified by the servoamplifiers and sent to the motors
4
Q
A tachometer is
A
a velocity feedback sensor, it works by producing a voltage proportional to the velocity of the servomotor
5
Q
An encoder is
A
a device that measures the position of the tool. It consists of alternating dark and transparent sections which the tool crosses and light is shone through. The device counts the number of transparent sections which are crossed and hence the position of the tool
6
Q
The mechanical components of a feed drive are:
A
- the servomotor
- a torque reduction gear set
- the leadscrew
- a nut which moves along the leadscrew and is rigidly attached to the table
- the table with the workpiece on it
7
Q
An open loop system is
A
a system where the location of the tool is not measured and hence there is no guarantee that the tool will arrive in the required position
8
Q
A open loop system is suitable for operations such as
A
drilling
9
Q
The advantages of an open loop system are
A
- it is considerably less expensive than a closed loop system
- maintenance is less complicated than a closed loop system
10
Q
A closed loop system is
A
a system where the actual system achieved by the machine is constantly compared to the required position and the error is fed back to the drive unit
11
Q
In the indirect feedback method,
A
the output of the servomotor (the velocity) is constantly monitored, it is popular but less accurate than the direct feedback method because it cannot measure backlash
12
Q
Backlash is
A
lost motion in a mechanism due to gaps between parts
13
Q
In the direct feedback method, also called a positional feedback system,
A
the position of the machine slide is constantly monitored and fed back to the drive unit, it is more accurate but more expensive to implement
14
Q
In a point-to-point system,
A
both motors operate at their maximum speed until one of the coordinates is correct, at which point it moves in the single axis it needs to
15
Q
If both motors operate at an equal full speed in a point-to-point system
A
the tool will travel at an angle of 45 degrees
16
Q
Point-to-point systems are used:
A
- in between machining operations
- when machining is only performed at discrete positions
17
Q
In contouring or continuous path systems
A
motion is controlled simultaneously in more than one axis
18
Q
In a two and a half axis system
A
contouring can be done in the x-y plane with only linear motion in the z-plane. These motions cannot be done simultaneously
19
Q
Advantages of NC machines:
A
- Minor design changes to a part can be easily handled by making corresponding changes to the part program
- Better accuracy and repeatability than conventional machines
- Different operations can be performed on one NC machine. This results in less fixturing and less floor-to-floor times
- Components with complex geometries can be manufactured
- Facilities can be incorporated into an NC machine, such as a robotic arm or an automatic tool change, to reduce non-productive time
- The output of one NC machine can match that of three conventional machines
20
Q
Disadvantages of NC machines:
A
- NC machines are expensive, about three times more expensive than conventional machines
- They require trained personnel to program and operate the machines
- The electronic parts have a limited life, around 10-15 years
21
Q
Disadvantages of manual part programming:
A
- Geometrical calculations involved
- Error prone
- Time consuming
22
Q
Alternatives to manual part programming:
A
- Computer-assisted part programming
- Manual data input
- CAD/CAM approach
23
Q
When labeling the axes of machines, these assumptions must be made:
A
- the tool moves relative to the coordinate system and the component is stationary
- the positive direction of an axis must increase the clearance between the cool and the component
- a right-hand co-ordinate system is used
24
Q
Labeling the z-axis:
A
- if there is a spindle which imparts cutting power, the z-axis is parallel to it
- if not, the z-axis is parallel to the work-holding surface
25
Labeling the x-axis:
- on machines with cutting tools, if the spindle is horizontal, the positive x-axis is to the right when looking from the spindle towards the workpiece, if the spindle is vertical, positive x movement is to the right when looking from the spindle towards the column
- in the case of lathes, the positive x is radially outwards
26
Each line of a part program is referred to as
a block
27
The first character of a word in part programming is referred to as
a word address
28
( and ) are used as
control-out and control-in characters, all coded information between them is ignored by the machine
29
N words
are sequence numbers used for program command identification
30
F words
specify the linear velocity the drives should move during cutting in mm/min or mm/rev (no decimals)
31
S words
specify the spindle speed in rpm
32
The rotational axes around X, Y and Z are
A, B and C respectively
33
Secondary and tertiary axes to X, Y and Z are
U, V, W and P, Q, R
34
Linear interpolation
causes the tool to move along a straight line to the required position
35
Circular motion can be specified in two ways:
- specifying the radius (ambiguous, and the machine will often chosen the wrong one for angles greater than 180 degrees)
- specifying the centre through incremental coordinates
36
The machine zero is
the datum point of the machine, a preset point within the working volume
37
If the machine crosses the software travel limits
the controller will generate an emergency stop
38
The program zero is
the datum from which the coordinates in the part program are specified, it is defined by the user
39
The distances in x, y and z between the machine zero and the program zero are known as
absolute offsets
40
A canned cycle is
a short-hand method of coding a pre-determined sequence of movements and spindle rotations to perform specific functions such as drilling, tapping and boring
41
In the case of counter-bored holes
it is essential that the tool pauses with the spindle still running to ensure a flat surface is completely machined
42
P words
specify the dwell time in seconds or revolutions depending on whether the feed rate is specified in mm/min or mm/rev
43
If the hole depth/diameter ratio is greater than three
the hole must be drilled in multiple pecks to clear the chips at several intervals
44
Q words
specify the height of each peck in a deep hole before the tool returns to R height
45
If it is required to tap a hole
a G84 tapping cycle is used
46
To end a program
used M02, or M30 for rewind
47
The direction of cutter radius compensations refers to
the direction of the tool relative to the workpiece when looking from behind the path of the tool
48
Spindle speed (rev/min) =
(cutting speed (m/min) * 1000) / (tool diameter (mm) * pi)
49
Feed velocity (mm/min) =
spindle speed (rev/min) * feed (mm/rev)
50
Metrology is so important today because
we require interchangeable parts
51
It is important not to measure things as accurate as possible, but rather
as accurate as necessary
52
The concept of traceability allows
for the comparison of accuracy of measurements worldwide, it means that the result of a measurement can be related to a national or international measurement standard through an unbroken chain of comparison
53
Accuracy is
the closeness of agreement between a quantity value obtained by one measurement and the true value
54
Trueness is
the closeness of agreement between the arithmetic mean of a number of measurements and the true value
55
Precision is
the closeness of agreement between quantity values obtained by replicate measurements of a quantity
56
The resolution of a measuring device is
the smallest change it can detect in the quantity it measures
57
Errors can be classified as
systematic errors and random errors
58
When a measurement is repeated many times
any difference between the mean and the true value is likely to only be systematic error
59
Sources of systematic error include:
- imperfect calibration of measuring devices
- faulty components on malfunctioning instruments
- fluctuations in environmental conditions
60
Uncertainty is
an estimate of the inaccuracy of a measurement that includes both the random and systematic components. It is the quantification of doubt about the measurement result.
61
The level of confidence explains
how confident you are that the taken
| measurement lies within the uncertainty region
62
Sources that can be used to estimate uncertainty:
- analysis of repeated measurement
- data provided in calibration
- uncertainties assigned to reference data taken from handbooks
- manufacturer's specifications
- general knowledge of behaviour and property of relevant materials and instruments
63
If a caliper has a 1/10th nonius,
The nonius is 1/10th smaller than the unit of the main scale, so the resolution is 1/10th of the main scale
64
Thimbles in micrometers typically have a fine pitched thread as
less distance is covered with each turn, so the resolution of the reading is high as a smaller distance can be split into the same number of parts
65
Slip gauges are
ground and polished blocks of steel or ceramic that represent one distance
66
Odd distances can be measured using slip gauges by
stacking two or more blocks together
67
The main purpose of slip gauges is
to check if the size of a hole/slot is larger or smaller than a given value
68
Go/no-go gauges are
two slip gauges, one slightly smaller than a given size, one slightly larger, that indicates if a given dimension fits within the range of the two sizes
69
Disadvantages of slip gauges:
- they are not flexible
| - they are expensive
70
CMMs function by
moving in a touch probe in the x, y and z directions until it touches a surface. The coordinates of the touch probe at that point are considered to be part of the component's surface
71
The most common configuration of CMM is
a moving bridge CMM, consisting of a stationary table and a moving bridge
72
Advantage of moving bridge CMM:
fairly lightweight, thus no foundations are needed
73
Disadvantage of moving bridge CMM:
the columns supporting the bridge can move at different paces, causing it to twist (yaw)
74
In a fixed bridge CMM
the bridge is fixed and the table provides one axis of motions
75
Advantages of a fixed bridge CMM:
- yawing is eliminated
| - reduced Abbe offset error
76
Disadvantages of a fixed bridge CMM:
- reduced operation speed as the table must move, and hence reduced throughput
- limited part weight
- heavy
77
Throughput can be increased when using horizontal arm CMMs by
using two horizontal arms at the same time
78
CMM stands for
co-ordinate measuring machine
79
Advantages of horizontal arm CMMs:
- ideal for measuring large parts (measuring range of up to 25m in the long axis)
- excellent accessibility to all of the side of the part
- high measurement speed
- since table is stationary, can measure heavy parts without affecting accuracy
80
Disadvantages of horizontal arm CMMs:
- relative large Abbe offset errors
| - limited accuracy (can be partially compensated using counterweights)
81
To avoid yawing in gantry CMMs
dual drive systems are used in a master and slave configuration
82
Advantages of gantry CMMs:
- free access for the operator to all parts in the machine volume
- reasonable accuracy even for large measurement volumes
83
Disadvantage of gantry CMMS:
very expensive if high accuracy is required as special isolated foundations are required
84
It is known when the touch probe touches the component as
the touch probe is attached to three rods that complete a circuit with each resting on two balls to create six contact points, with springs and gravity holding them down, but when the touch probe touches the component, one of the these balls is lifted, cutting the circuit
85
Pretravel refers to
the length the machine travels after the probe touches the machine and before a reading is made
86
Pretravel is necessary as
the vibrations caused when the machine moves would otherwise be sufficient to record false readings
87
It is known when a point of contact is detected as:
1. A current flows through the contacts between the rod and balls
2. The resistance of this electrical circuit is constantly being measured
3. On contact between stylus ball and workpiece, contact patches reduce in size as stylus forces build up
4. Probe triggers once the resistance breaches the threshold
5. Before rod and balls actually separate, the current is being cut to avoid arcing between rod and balls
88
The typical material of a stylus ball is ____ because ____
ruby ____ it has high wear resistance and low mass
89
Measurement procedure for CMMs:
1. Acquire points on component's surface
2. Define the component's geometric features
3. Extract information about the component's geometric features
4. Inspection report
90
To define a perfect circle from a set of points the CMM
employs the least squares fitting algorithm
91
Number of points needed to measure a circle:
3
92
Number of points needed to measure a sphere:
4
93
Number of points needed to measure a cylinder:
5
94
Number of points needed to measure a plane:
3
95
When measuring the dimensions of a circle
choose points around the circumference of the circle and not near each other
96
Advantage of point-to-point mode on CMMs:
accuracy of co-ordinates measure more accurate due to lack of vibrations
97
Advantage of scanning mode on CMMs:
more points are fed back in a given time which can give useful data regarding shape
98
To divine the datum of a coordinate system using a CMM:
take three points on one surface, two on another, and one on the other
99
Portable CMMs record co-ordinates
by using a polar coordinate system with their many axes of rotation, a button is manually pressed when it comes into contact with the surface
100
Advantages of conventional CMMs over portable CMMs:
- high accuracy
- accuracy is stable over all positions
- high repeatability
- can be automated through part programming
- possible to operate 24/7
101
Advantages of portables CMMs over conventional CMMs:
- unlimited work volume
- can be brought to machines, so parts can be machined and inspected at the same time, increasing accuracy and decreasing time
102
Disadvantages of contact measuring probes:
- thin walled sections can give way when the stylus makes contact
- components made of soft materials can be difficult to measure
103
Non contact probes measure based on the principle of
laser triangulation
104
Triangulation determines distances by
projecting a light beam onto an object, part of which is deflected to a detector by the object, the distance can be measured depending on the position of the deflected beam
105
Non contact probes can measure an area by
using a resonating mirror to measure over a line and moving the laser perpendicular to this line
106
Processing of non contact probe data:
- scanning
- point-cloud handling (filtering outliers, merging scans, meshing points)
- alignment to CAD model
- compare to CAD
- generation of report
107
Laser trackers measure points by
firing a laser at a retro reflector, the displacement is found using interferometry, and the position is found using the head tilt angle
108
Interferometry is
a technique where the superposition of waves are used to extract information such as distance
109
Difference between laser trackers and laser scanners:
laser trackers follow a predefined path whereas laser scanners scan their entire environment
110
Laser scanners measure points by
measuring the head position and the time it takes for the beam to travel to find the distance
111
When using laser scanners, to ease the identification of features
a camera can be mounted on the scanner and the photos taken can be superimposed on the scan
112
Surface metrology is
the measuring of small-scale features in or shortly below surfaces
113
Why is surface metrology important?
- it is important to understand how it was influenced by its history (the way it was manufactured, whether it was subject to any kind of wear or fracture)
- it is important to understand how it will behave in future (in its environment or with respect to adhesion or friction)
114
Disadvantages of sample surfaces:
- optical or fingernail scratch comparison rather crude and qualitative
- sample surfaces get damaged over time
115
Surface roughness testers measure
the deflection of a needle (stylus) being dragged across the surface
116
The raw profile of a surface can be deformed into
the form profile (due to misalignment between the tester and the workpiece) and the primary profile, which can further be decomposed to the roughness profile and the waviness profile
117
In describing roughness, Rt is
the maximum peak to valley height, the maximum distance between highest peak and lowest valley within the sample length
118
In describing roughness, Rz is
the mean peak to valley height, the average height difference between the highest peak and the lowest valley across a number of sections within the sample length
119
In describing roughness, Ra is
the arithmetic mean roughness, the arithmetic mean of the departures of the profiles from the mean line (this is done by squaring the departures as the sum of the departures from the mean line will be zero)
120
If a profile has a negative skew
the modal distribution is towards the top
121
A single scratch on the surface affects the roughness reading the most when described using
maximum peak to valley height
122
It is best to record surface roughness at a spot
without any visible irregularities
123
Measuring the true surface is impossible because
of the finite roundness of the stylus tip
124
An atomic force microscope works by
dragging a small probe tip attached to a cantilever over a surface, a laser is shone at the back of the cantilever and is reflected onto a four quadrant photo detector, when the cantilever bends the laser's path of deflection changes and can be measured
125
The wider the tip of a probe,
the narrower holes appear and the wider protrusions appear
126
Advantage of atomic force microscope over surface roughness tester:
probe tip is much smaller so part's surface can be followed much more accurately
127
What are the forces that cause the cantilever to deflect?
- Short-range atomic forces
- Van der Waals forces
- Electrostatic forces
- Capillary forces (caused by water vapour condensation, by far the largest)
128
The two operating modes of atomic force microscopes are
contact mode and tapping mode
129
In tapping mode
intermittent contact with the surface causes the tapping to decrease in amplitude, and this reduction is used to measure the features
130
Disadvantage of contact mode:
excessive tracking forces are created by dragging the probe across the sample
131
If the tip of the probe has multiple apexes
features with considerably smaller size than the tip radius can be mapped twice
132
Advantages of electron microscopes over optical microscopes:
- Higher magnification
| - Larger depth of field
133
How does a scanning electron microscope work?
- Cathode emits beam of electrons which is focused by lens system towards object
- A scan unit deflects the electrons so they scan a surface
- As electrons hit the target, they cause the target material to emit electrons which are detected by a detector close to the object
134
The types of electrons used as signals for SEMs are:
- secondary electrons (emitted 5-50nm into the material)
| - back-scattered electrons (scattered back 50-300nm into the material)
135
The amount of electrons emitted/reflected by a surface depends on
the angle of inclination towards the beam direction, the more it is inclined, the more electrons are reflected/emitted, and hence the brighter it appears
136
The hardness of a material is a function of
the force applied to a material and the shape change under that force
137
HB stands for
Hardness Brinell
138
Brinell hardness is determined by
the diameter of indentation left after a tungsten carbide ball is pressed into it for a duration
139
HV stands for
Hardness Vickers
140
Vickers hardness is determined by
the length of the diagonals of the indentation left after a diamond pyramid with 132 degree cone angle is pressed into it for a duration
141
HRC stands for
Hardness Rockwell C scale
142
Rockwell hardness is determined by
pressing a diamond cone with 120 degree cone angle into a surface, first using a minor force of 98.1 newtons, before a force of 1373 is added before being removed, the depth is then used
143
HRB is used
for very soft materials, a ball is used in place of a cone
144
Vickers hardness numbers are reported as
x HV y where x is the hardness value and y is the load, if the duration wasn't 15 seconds, / z is added where z is the duration
145
Rockwell hardness numbers are reported as
x HRC where x is the hardness value
146
Advantage of Rockwell hardness:
Dial is included so extra dimensional measurement not required
147
Disadvantage of Rockwell hardness:
Only can be applied to a narrow range of hardnesses
148
Limitation of conventional hardness tests:
Cannot measure hardness of very thin layers
149
The width of a layer to have its hardness measured must be
at least ten times of the size of the indentation
150
What does EDX stand for?
Energy dispersive x-ray spectroscopy
151
EDX is used for
chemically characterising surfaces and the near surface region
152
How does EDX work?
1. Bombarding a substrate with x-rays knocks electrons out of the inner atom shell, creating unstable conditions
2. As a result, an electron from the outer shell moves into the empty space left behind
3. As it moves into the empty space, it emits a photo electron quantum with a particular energy, which is characteristic for the atom and, thus, for the chemical element
4. The number and energy of the photo electrons emitted are detected by a spectrometer, and is an indication for the concentration of chemical elements in the substrate being analysed
153
CMM tables are made from granite because
they are hard, can be polished to a very smooth surface finish, exhibits low thermal expansion and they dampen vibrations
154
How many points need to be taken to find the intersection between 2 planes?
6
155
Extenders permit
a large volume of plastic to be produced with very little polymer
156
Lubricants
reduce the viscousity of molten plastic and improve forming characteristics
157
Additives to be included in plastics include
fillers, flame retardants, lubricants, pigments, stabilisers, plasticisers and anti static agents
158
The crystalline degree is dependent on
thermal history
159
In thermoplastic materials
The polymers are held together by weak Van Der Waals forces, when the material is heated these forces are weakened and the material becomes flexible, before solidifying again when cooled
160
Properties of amorphous polymers:
- transparent due to looser structure
- low shrinkage
- low chemical resistance
- poor fatigue and wear
161
Properties of crystalline polymers:
- sharp melting point
- usually opaque
- high shrinkage
- good fatigue and wear resistance
162
Advantages of thermoplastic polymers:
- can be easily and economically shaped into products
| - can be subjected to heated and cooling cycles repeatedly without significant degregation
163
Properties of polymers:
- can be formed into intricate shapes, usually without further processing
- cost and energy competitive with metals
- can be transparent
- low density
- good strength to weight ratio
- high corrosion resistance
- low electrical and thermal conductivity
164
Branches make polymers:
- stronger in the solid state
| - more viscous at a given temperature
165
The curing reaction is
a polymerisation process characterised by chemical cross-linking reactions that create an infusible, insoluble and highly cross-linked three-dimensional network
166
What kind if bonds are thermosetting characterised by?
Carbon-carbon double bonds
167
How are thermoplastics supplied?
As pellets, powders or granules
168
Thermoset resins are supplied as
Liquid chemicals that undergo a chemical reaction
169
How is curing initiated?
Through exposure to light, heat, or an electric potential
170
Two main events occur during the cure reaction:
Gelation and vitrification
171
Polymers are
large molecules made my joining thousands of small molecules called monomers
172
Polymers that contain only a single type of repeat unit are known as
homopolymers
173
Polymers containing a mixture of repeated units are known as
copolymers
174
Gelation:
- the resin dramatically decreases in viscousity
- a gel phase and sol phase are created, the gel phase is insoluble and cross linked whereas the sol phase remains soluble
175
How does increasing temperature affect the viscousity of thermoplastics?
The viscousity decreases
176
How does increasing temperature affect the viscousity of thermosets?
The viscousity starts to decrease but when curing begins the viscousity starts to increase
177
Vitrification occurs when
The glass transition temperature of the curing resin increases to the current curing temperature
178
Is the resin 100% cured at vitrification?
No
179
In the injection moulding process
A plastic is melted and then forced into the cavity of a closed mould which gives shape to the plastic, after sufficient time for the part to solidify it is removed
180
The equipment for injection moulding is divided into three units:
- injection unit
- mould
- clamping system
181
Clamping force=
Injection pressure x total projected cavity area
182
Stages of the injection cycle:
- plastification
- injection
- packing/cooling
- demould/ejection
183
Conformal cooling channels
Make use of cooling lines in an injection mould that curve and closely follow the geometry of the part, they can be produced by additive manufacturing
184
The melting temperature is generally below the melting point of the plastic as
The velocity the polymer travels at increases in temperature due to shearing
185
In terms of strain rate, polymers exhibit
psuedoplastic behaviour, where strain rate increases at a quicker rate than stress
186
When choosing the temperature of the mould, what factors must be considered?
- shot size
- injection rate
- size of runner system
- part thickness
187
Shrinkage is minimised during injection moulding as
more plastic is added during shrinkage by controlling the pressure
188
Part with only thin sections are filled at high injection rates to
prevent freezing before filling
189
The dwell time is
The time that pressure is applied to the cavity after it is filled
190
The mould time consists of
fill time, packing time, cooling time and ejection time
191
Coinjection moulding or ___ is ___
Sandwich moulding ___ a process that allows multiple polymers to be injected into one mould to make a single part
192
How coinjection moulding works:
- the skin material is injected first immediately followed by a core material
- the core material displaces the skin material, pushing it ahead
- the skin material freezes on the walls with the core material filling the cavity
193
Three stages if coinjection moulding:
- short shot of skin material
- injection of core material
- more skin material
194
Advantages of coinjection moulding:
- reduces cost by using a cheaper fill material as the non-visible core of a product
- combining desired properties of polymers
195
Disadvantages of coinjection moulding:
-machines are more expensive and difficult to maintain than standard moulding machines
196
Biinjection moulding is
a process in which two different polymers are simultaneously injected at different locations in the same mould
197
Biinjection moulding is only used in
simple, low-tolerance parts
198
In some designs of the gas assisted injection moulding process, has may be allowed to escape from the cavity so that
the gas can be recovered for reuse
199
Benefits of the gas assist process:
- reduced cycle time
- capable of producing hollow, light-weight and rigid parts
- reduced pressure and clamping forces
- part consolidation with both thick and thin sections
200
In gas assisted injection moulding, how does melt viscousity affect wall thickness?
Higher viscousity results in thicker walls
201
In gas assisted injection moulding, how does melt viscousity affect length and area of gas channels?
Lower melt viscousities produce longer and larger cross-sectional area gas channels
202
In gas assisted injection moulding, how does melt viscousity affect wall uniformity and reproducability?
Lower melt viscousities produce less uniform wall thickness and hence less reproducibility
203
In gas assisted injection moulding, how does melt viscousity affect strength?
Lower melt viscousity produces lower strength so that the gas can break through the melt front more readily
204
Sink marks occur largely due to
The difference in cooling between thick and thin sections of the part
205
Weld lines form
Whenever two molten flow fronts meet
206
Burn marks or discolouration are generally caused by
Trapped gasses in the cavity or thermal degradation of the polymer
207
Main causes of sink marks:
- insufficient polymer to fill the mould
- polymer flowing back out of the mould into the runner system and barrel
- polymer temperature too high
- ejected parts too hot
208
Main causes of weld lines:
- slow injection speed
- under packing of the part
- polymer too cold
- multiple gates
209
To rectify incomplete filling of the cavity (short shots):
- increase melt and mould temperatures
- increase pressure
- increase injection speed
210
Corrective action for insufficient polymer in the mould:
- increase packing pressure
- increase injection hold time
- increase injection speed
- increase gate size
211
Corrective action for polymer flowing back out of the mould:
- increase injection hold time
- decrease mould temperature
- increase cooling time
212
Corrective action for polymer temperature too high:
- reduce back pressure
- reduce barrel temperature
- improve mould temperature controls
213
Corrective action for ejected parts too hot
Increase cooling time
214
Corrective action for slow injection speed
Increase injection speed
215
Corrective action for underpacking of the part:
Increase hold pressure
216
Corrective action for polymer too cold:
- increase melt temperature
- increase mould temperature
- increase injection speed
217
Corrective action for multiple gates in weld lines:
- decrease number of gates
- increase size of gates
- change gate locations
218
Problems with weld lines
Can affect appearance and mechanical properties
219
Corrective action for trapped gasses in the cavity:
- clean vents
- provide additional vents
- decrease injection speed
220
Corrective action for thermodynamics degradation:
- decrease injection speed
| - decrease melt temperature
221
Applications of ceramics:
- Semi conductors in computers
- Heat resistant ceramic tiles
- Catalytic converters
222
Properties of metals:
- High Young's modulus
- Strong by alloying and heat treatment
- Ductile
- Susceptible to fatigue failure
- Least resistant to corrosion
- Tough
223
Properties of ceramics:
- High Young's modulus
- Hard and stiff
- Abrasion resistant
- Corrosion resistant
- Retains strength at high temperatures
- Brittle
- Much stronger in compression than in tension
224
Properties of polymers:
- Can be as strong as metals
- High strength per unit weight
- Easy to shape
- Easy and cheap assembly of parts
- No finishing operations required
- Corrosion resistant
- Low friction coefficients
- Low Youngs modulus
225
Ceramic products are produced by
compressing fine ceramic powder under certain temperature and pressure conditions
226
Important stages in modulus changes with temperature for polymers:
Glassy plateau, glass transition, rubbery plateau, viscous flow
227
Uses of composites:
Maximise the attractive properties of two components
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The two components of a composite are
the matrix material and the reinforcing material
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The stress-strain behaviour of a composite can be found from
the weighted average (in terms of volume fraction) of that of the matrix and the reinforcing material
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Fibres are woven because
it makes them stronger in a given direction and in multiple directions
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A bundle of fibres are called
tows
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All weaves have a corresponding
unit cell
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In an 8-harness satin weave
tows come up to cross one other tow before going below to cross sevens tows
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Why can property data vary so much between different polymers?
- all polymers contain a spectrum of molecular lengths, a slight change in processing changes this spectrum
- polymerisation changes the molecular branching and degree of crystallinity in the the final product
- properties also change due to mechanical processing which can align the molecules in varying degrees
- variation in additives
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The compressive stress of foams
increases with strain as the cell walls buckle
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Foams are
gaseous composites
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Foams are made
by stirring molten materials in a gaseous environment
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In resin transfer moudling
- foam is saturated with resin
- fibremats that have been stamped into certain shapes are introduced
- resin is forced out of the foam and into the sheets
- the resin is cured and a part with a light core and a strong skin is produced
- produces a near net shape product quickly
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Requirements of reinforcement material:
- strong interface bond between the fibre and matrix
| - chemical compatibility
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Types of fibre reinforcement:
- continuous
- chopped
- woven
- hybrid
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Flake composite shapes are used when
high bending stiffness is required
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To produce isotropic properties in an element
laminates can be layered at different directions
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Manufacturing of glass fibres:
- glass is fed into a furnace and drawn into filaments
| - the fibres are collected together to form a strand
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Stages of manufacture of carbon fibres:
- stabilisation (200-300 C)
- carbonisation (1000-1500 C)
- graphitisation (2500 C)
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In extrusion
- polymer granules in a hopper are fed into a heated barrel with a screw thread in
- the screw turns and pushes the melting polymer forwards
- the polymer is pushed through a die to create a prism of a particular cross-section
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In injection moulding
- the same process occurs for extrusion, but the polymer is fed into a mould which can have a complex shape
- pressure can be applied to the mould gives good surface finish
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In compression moulding
- the polymer is injected into a die of two parts which compress it under heat
- it can be used with prepregs
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A laminar is
a sheet reinforced by unidirectional fibres
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A prepreg is
a sheet of fibres with semi-cured resin
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Essential steps in manufacturing fibre-reinforced polymeric composites:
- fibre alignment
- fibre moulding
- polymer infiltration
- cross linking
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In hot-melt prepregging
- fibre reinforcement are pulled in between resin sheets
- it is put between heated rollers to semi-cure and apply pressure
- a slitting device takes film off the top and bottom to get the correct volume fraction
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Sheet moulding compound is produced by
- resin filler paste is applied to plastic carrier film under a doctor blade
- a chopper chops continuous strand rovings onto the film
- rollers apply pressure to combine the paste and the chopped fibres between films layers
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Fibres can negatively affect fluidity if:
- they are too long
| - they make up too much of the volume fraction of the composite
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What is the difference between thermoset and thermoplastic prepregs?
Thermosets must be cooled when stored whereas thermoplastics can be stored at room temperature
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In open mould hand lay-up
- a wax coat is added to prevent the resin and the mould interacting
- resin and fibre mats are added by hand to the mould
- pressure is applied by hand with rollers and it is left to cure
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Applications of open mould hand lap up:
- prototype fabrications
- large components in small quantities such as boat hulls or aircraft exterior parts
- development work
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Materials for open moulds:
- plaster (used for large moulds, unheated and hence curing takes a long time)
- metal (for high production, can be heated for curing)
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What can be used in compression moulding?
Prepreg tape or sheet-moulding compounds
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In open mould spray up
resin and fibres are fed into a chopper-spray gun which chops the fibres and combines them and sprays them on a mould
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What is the advantage of spray up over hand lay-up?
It is faster
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What is the advantage of hand lay-up over spray up?
Fibres can be layed more precisely and need not be chopped
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The open mould hand lay-up process can be improved by
automated tape-laying
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In autoclave
- a prepreg is placed on a mould under a vacuum bag in a heated pressure chamber
- a vacuum pump removes the air from between the vacuum bag and the prepreg so there are no air traps in the component
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In filament winding
- fibre rovings are bathed in resin and wound round a mandrel
- a tension adjust controls the volume fraction of resin
- the carriage moves transversely across the mandrel to change the position and angle the rovings go on the mandrel at
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Types of windings possible in filament winding:
- hoop windings (when hoop stresses are priority)
- helical windings
- polar windings (when axial stresses are priority)
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How can the mandrel be removed?
- if it is made of plaster, it can be broken and taken out
- washing it away with water if it is made from soluble plaster
- alkaline solutions can be used to dissolve it if it is made from a thin sheet of metal
- they can be made collapsible or deflatable
- sometimes they can be left in
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In pultrusion
fibre rovings are bathed in resin and pulled through a heated die
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In reinforced reaction injection moulding
the resin is mixed with a chemical hardener to speed up curing
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In pulforming process
the products of pultrusion are put into a die to curve them
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Compared to open mould, close mould:
- costs more
- has higher production rates
- gives higher surface finish
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Whether a thermoplastic or thermosetting polymer affects injection moulding as
the mould must be cold and the barrel must be hot for thermoplastics and viceversa
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Applications of high performance ceramics:
- Cutting tools
- Drills
- Engine parts
- Wear resistant parts
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What determines a high performance ceramic?
- Fracture toughness
| - Distribution of microcracks
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Sintering process:
- Pack the powder
- Heat to a temperature at which diffusion becomes rapid (around 2/3 melting temperature)
- This leads to bonding of particles, reducing surface area and causing the powder to densify
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Rate of sintering:
- finer particles sinter faster
| - higher temperatures cause faster sintering
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In production of ceramics
- press the poweder into an initial shape in a die
- mix it with a binder to produce a compact with enough strength to be handled
- fire the compact
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Problems in production of ceramics:
- shrinkage
| - grain growth (smaller grains give stronger products)
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In hot-isostatic pressing
- powder is squeezed in a pressure vessel with high has pressure
- powder is contained in a thin steel perform which is heated at compressed by high pressure argon
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In liquid phase sintering
- one percent volume of low melting glass is added
| - this means diffusional transport through the melt is high meaning it occurs quicker, but the strength is reduced
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In chemical vapour deposition
-a chemical reaction causes a layer to be deposited on the substrate
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Grain growth occurs when
a lattice is exposed to a temperature for a long period of time
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Processing that improve on manufacture of ceramics:
- die pressing
| - hot isostatic pressing
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In the slurry infiltration process
- fibres are fed through a tank of glass slurry to create glass impregnated fibre tape
- tape if stacked in the required order
- binder that is used in the slurry in burned out at 500 C
- pressure is applied at 800-925 C in hot pressing to produce fibre glass composite
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In diffusion bonding
- layers of foil and fibre mats are consolidated and stacked
- heat and pressure is applied
- the product is cleaned and trimmed
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Components made using filament winding must be
exosymmetric
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The number of monomers in a polymer chain is known as
the degree of polymerisation
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The rate of reaction slows down from vitrification as
it becomes controlled by diffusion as the mobility of the molecular chains decreases
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How does crosslinking affect the glass transition temperature?
It causes the viscousity to increase and hence the glass transition temperature to increase
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The time when the temperature of a thermosetting polymer is increase before the viscousity begins to increase is known as
induction time
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Co-injection molding utilizes what property of liquid polymers to keep the core material inside the skin material?
"fountain flow"
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How does melt viscousity of the polymers affect products of co-injection moulding?
The most uniform thickness distribution can be obtained by injecting a core material with a slightly higher viscosity than the skin material. Higher melt viscosity of the core material produces thinner walls.
