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Year 2 Semester 1 > Manufacturing > Flashcards

Manufacturing Flashcards

1
Q

A numerically controlled machine is

A

a machine whose movements are controlled by numerical data

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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
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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

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4
Q

A tachometer is

A

a velocity feedback sensor, it works by producing a voltage proportional to the velocity of the servomotor

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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

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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
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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

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8
Q

A open loop system is suitable for operations such as

A

drilling

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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
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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

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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

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12
Q

Backlash is

A

lost motion in a mechanism due to gaps between parts

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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

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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

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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

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16
Q

Point-to-point systems are used:

A
  • in between machining operations

- when machining is only performed at discrete positions

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17
Q

In contouring or continuous path systems

A

motion is controlled simultaneously in more than one axis

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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

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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
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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
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21
Q

Disadvantages of manual part programming:

A
  • Geometrical calculations involved
  • Error prone
  • Time consuming
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22
Q

Alternatives to manual part programming:

A
  • Computer-assisted part programming
  • Manual data input
  • CAD/CAM approach
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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
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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
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25
Q

Labeling the x-axis:

A
  • 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
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26
Q

Each line of a part program is referred to as

A

a block

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27
Q

The first character of a word in part programming is referred to as

A

a word address

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28
Q

( and ) are used as

A

control-out and control-in characters, all coded information between them is ignored by the machine

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29
Q

N words

A

are sequence numbers used for program command identification

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30
Q

F words

A

specify the linear velocity the drives should move during cutting in mm/min or mm/rev (no decimals)

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31
Q

S words

A

specify the spindle speed in rpm

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32
Q

The rotational axes around X, Y and Z are

A

A, B and C respectively

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33
Q

Secondary and tertiary axes to X, Y and Z are

A

U, V, W and P, Q, R

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34
Q

Linear interpolation

A

causes the tool to move along a straight line to the required position

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35
Q

Circular motion can be specified in two ways:

A
  • 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
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36
Q

The machine zero is

A

the datum point of the machine, a preset point within the working volume

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37
Q

If the machine crosses the software travel limits

A

the controller will generate an emergency stop

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38
Q

The program zero is

A

the datum from which the coordinates in the part program are specified, it is defined by the user

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39
Q

The distances in x, y and z between the machine zero and the program zero are known as

A

absolute offsets

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40
Q

A canned cycle is

A

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

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41
Q

In the case of counter-bored holes

A

it is essential that the tool pauses with the spindle still running to ensure a flat surface is completely machined

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42
Q

P words

A

specify the dwell time in seconds or revolutions depending on whether the feed rate is specified in mm/min or mm/rev

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43
Q

If the hole depth/diameter ratio is greater than three

A

the hole must be drilled in multiple pecks to clear the chips at several intervals

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44
Q

Q words

A

specify the height of each peck in a deep hole before the tool returns to R height

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45
Q

If it is required to tap a hole

A

a G84 tapping cycle is used

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46
Q

To end a program

A

used M02, or M30 for rewind

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47
Q

The direction of cutter radius compensations refers to

A

the direction of the tool relative to the workpiece when looking from behind the path of the tool

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48
Q

Spindle speed (rev/min) =

A

(cutting speed (m/min) * 1000) / (tool diameter (mm) * pi)

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49
Q

Feed velocity (mm/min) =

A

spindle speed (rev/min) * feed (mm/rev)

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50
Q

Metrology is so important today because

A

we require interchangeable parts

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51
Q

It is important not to measure things as accurate as possible, but rather

A

as accurate as necessary

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52
Q

The concept of traceability allows

A

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

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53
Q

Accuracy is

A

the closeness of agreement between a quantity value obtained by one measurement and the true value

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54
Q

Trueness is

A

the closeness of agreement between the arithmetic mean of a number of measurements and the true value

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55
Q

Precision is

A

the closeness of agreement between quantity values obtained by replicate measurements of a quantity

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56
Q

The resolution of a measuring device is

A

the smallest change it can detect in the quantity it measures

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57
Q

Errors can be classified as

A

systematic errors and random errors

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58
Q

When a measurement is repeated many times

A

any difference between the mean and the true value is likely to only be systematic error

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59
Q

Sources of systematic error include:

A
  • imperfect calibration of measuring devices
  • faulty components on malfunctioning instruments
  • fluctuations in environmental conditions
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60
Q

Uncertainty is

A

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.

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61
Q

The level of confidence explains

A

how confident you are that the taken

measurement lies within the uncertainty region

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62
Q

Sources that can be used to estimate uncertainty:

A
  • 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
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63
Q

If a caliper has a 1/10th nonius,

A

The nonius is 1/10th smaller than the unit of the main scale, so the resolution is 1/10th of the main scale

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64
Q

Thimbles in micrometers typically have a fine pitched thread as

A

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

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65
Q

Slip gauges are

A

ground and polished blocks of steel or ceramic that represent one distance

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66
Q

Odd distances can be measured using slip gauges by

A

stacking two or more blocks together

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67
Q

The main purpose of slip gauges is

A

to check if the size of a hole/slot is larger or smaller than a given value

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68
Q

Go/no-go gauges are

A

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

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69
Q

Disadvantages of slip gauges:

A
  • they are not flexible

- they are expensive

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70
Q

CMMs function by

A

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

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71
Q

The most common configuration of CMM is

A

a moving bridge CMM, consisting of a stationary table and a moving bridge

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72
Q

Advantage of moving bridge CMM:

A

fairly lightweight, thus no foundations are needed

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73
Q

Disadvantage of moving bridge CMM:

A

the columns supporting the bridge can move at different paces, causing it to twist (yaw)

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74
Q

In a fixed bridge CMM

A

the bridge is fixed and the table provides one axis of motions

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75
Q

Advantages of a fixed bridge CMM:

A
  • yawing is eliminated

- reduced Abbe offset error

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76
Q

Disadvantages of a fixed bridge CMM:

A
  • reduced operation speed as the table must move, and hence reduced throughput
  • limited part weight
  • heavy
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77
Q

Throughput can be increased when using horizontal arm CMMs by

A

using two horizontal arms at the same time

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78
Q

CMM stands for

A

co-ordinate measuring machine

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79
Q

Advantages of horizontal arm CMMs:

A
  • 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
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80
Q

Disadvantages of horizontal arm CMMs:

A
  • relative large Abbe offset errors

- limited accuracy (can be partially compensated using counterweights)

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81
Q

To avoid yawing in gantry CMMs

A

dual drive systems are used in a master and slave configuration

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82
Q

Advantages of gantry CMMs:

A
  • free access for the operator to all parts in the machine volume
  • reasonable accuracy even for large measurement volumes
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83
Q

Disadvantage of gantry CMMS:

A

very expensive if high accuracy is required as special isolated foundations are required

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84
Q

It is known when the touch probe touches the component as

A

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

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85
Q

Pretravel refers to

A

the length the machine travels after the probe touches the machine and before a reading is made

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86
Q

Pretravel is necessary as

A

the vibrations caused when the machine moves would otherwise be sufficient to record false readings

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87
Q

It is known when a point of contact is detected as:

A
  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
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88
Q

The typical material of a stylus ball is ____ because ____

A

ruby ____ it has high wear resistance and low mass

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89
Q

Measurement procedure for CMMs:

A
  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
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90
Q

To define a perfect circle from a set of points the CMM

A

employs the least squares fitting algorithm

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91
Q

Number of points needed to measure a circle:

A

3

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92
Q

Number of points needed to measure a sphere:

A

4

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93
Q

Number of points needed to measure a cylinder:

A

5

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94
Q

Number of points needed to measure a plane:

A

3

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95
Q

When measuring the dimensions of a circle

A

choose points around the circumference of the circle and not near each other

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96
Q

Advantage of point-to-point mode on CMMs:

A

accuracy of co-ordinates measure more accurate due to lack of vibrations

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97
Q

Advantage of scanning mode on CMMs:

A

more points are fed back in a given time which can give useful data regarding shape

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98
Q

To divine the datum of a coordinate system using a CMM:

A

take three points on one surface, two on another, and one on the other

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99
Q

Portable CMMs record co-ordinates

A

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

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100
Q

Advantages of conventional CMMs over portable CMMs:

A
  • high accuracy
  • accuracy is stable over all positions
  • high repeatability
  • can be automated through part programming
  • possible to operate 24/7
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101
Q

Advantages of portables CMMs over conventional CMMs:

A
  • unlimited work volume
  • can be brought to machines, so parts can be machined and inspected at the same time, increasing accuracy and decreasing time
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102
Q

Disadvantages of contact measuring probes:

A
  • thin walled sections can give way when the stylus makes contact
  • components made of soft materials can be difficult to measure
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103
Q

Non contact probes measure based on the principle of

A

laser triangulation

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104
Q

Triangulation determines distances by

A

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

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105
Q

Non contact probes can measure an area by

A

using a resonating mirror to measure over a line and moving the laser perpendicular to this line

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106
Q

Processing of non contact probe data:

A
  • scanning
  • point-cloud handling (filtering outliers, merging scans, meshing points)
  • alignment to CAD model
  • compare to CAD
  • generation of report
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107
Q

Laser trackers measure points by

A

firing a laser at a retro reflector, the displacement is found using interferometry, and the position is found using the head tilt angle

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108
Q

Interferometry is

A

a technique where the superposition of waves are used to extract information such as distance

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109
Q

Difference between laser trackers and laser scanners:

A

laser trackers follow a predefined path whereas laser scanners scan their entire environment

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110
Q

Laser scanners measure points by

A

measuring the head position and the time it takes for the beam to travel to find the distance

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111
Q

When using laser scanners, to ease the identification of features

A

a camera can be mounted on the scanner and the photos taken can be superimposed on the scan

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112
Q

Surface metrology is

A

the measuring of small-scale features in or shortly below surfaces

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113
Q

Why is surface metrology important?

A
  • 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)
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114
Q

Disadvantages of sample surfaces:

A
  • optical or fingernail scratch comparison rather crude and qualitative
  • sample surfaces get damaged over time
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115
Q

Surface roughness testers measure

A

the deflection of a needle (stylus) being dragged across the surface

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116
Q

The raw profile of a surface can be deformed into

A

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

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117
Q

In describing roughness, Rt is

A

the maximum peak to valley height, the maximum distance between highest peak and lowest valley within the sample length

118
Q

In describing roughness, Rz is

A

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
Q

In describing roughness, Ra is

A

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
Q

If a profile has a negative skew

A

the modal distribution is towards the top

121
Q

A single scratch on the surface affects the roughness reading the most when described using

A

maximum peak to valley height

122
Q

It is best to record surface roughness at a spot

A

without any visible irregularities

123
Q

Measuring the true surface is impossible because

A

of the finite roundness of the stylus tip

124
Q

An atomic force microscope works by

A

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
Q

The wider the tip of a probe,

A

the narrower holes appear and the wider protrusions appear

126
Q

Advantage of atomic force microscope over surface roughness tester:

A

probe tip is much smaller so part’s surface can be followed much more accurately

127
Q

What are the forces that cause the cantilever to deflect?

A
  • Short-range atomic forces
  • Van der Waals forces
  • Electrostatic forces
  • Capillary forces (caused by water vapour condensation, by far the largest)
128
Q

The two operating modes of atomic force microscopes are

A

contact mode and tapping mode

129
Q

In tapping mode

A

intermittent contact with the surface causes the tapping to decrease in amplitude, and this reduction is used to measure the features

130
Q

Disadvantage of contact mode:

A

excessive tracking forces are created by dragging the probe across the sample

131
Q

If the tip of the probe has multiple apexes

A

features with considerably smaller size than the tip radius can be mapped twice

132
Q

Advantages of electron microscopes over optical microscopes:

A
  • Higher magnification

- Larger depth of field

133
Q

How does a scanning electron microscope work?

A
  • 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
Q

The types of electrons used as signals for SEMs are:

A
  • secondary electrons (emitted 5-50nm into the material)

- back-scattered electrons (scattered back 50-300nm into the material)

135
Q

The amount of electrons emitted/reflected by a surface depends on

A

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
Q

The hardness of a material is a function of

A

the force applied to a material and the shape change under that force

137
Q

HB stands for

A

Hardness Brinell

138
Q

Brinell hardness is determined by

A

the diameter of indentation left after a tungsten carbide ball is pressed into it for a duration

139
Q

HV stands for

A

Hardness Vickers

140
Q

Vickers hardness is determined by

A

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
Q

HRC stands for

A

Hardness Rockwell C scale

142
Q

Rockwell hardness is determined by

A

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
Q

HRB is used

A

for very soft materials, a ball is used in place of a cone

144
Q

Vickers hardness numbers are reported as

A

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
Q

Rockwell hardness numbers are reported as

A

x HRC where x is the hardness value

146
Q

Advantage of Rockwell hardness:

A

Dial is included so extra dimensional measurement not required

147
Q

Disadvantage of Rockwell hardness:

A

Only can be applied to a narrow range of hardnesses

148
Q

Limitation of conventional hardness tests:

A

Cannot measure hardness of very thin layers

149
Q

The width of a layer to have its hardness measured must be

A

at least ten times of the size of the indentation

150
Q

What does EDX stand for?

A

Energy dispersive x-ray spectroscopy

151
Q

EDX is used for

A

chemically characterising surfaces and the near surface region

152
Q

How does EDX work?

A
  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
Q

CMM tables are made from granite because

A

they are hard, can be polished to a very smooth surface finish, exhibits low thermal expansion and they dampen vibrations

154
Q

How many points need to be taken to find the intersection between 2 planes?

A

6

155
Q

Extenders permit

A

a large volume of plastic to be produced with very little polymer

156
Q

Lubricants

A

reduce the viscousity of molten plastic and improve forming characteristics

157
Q

Additives to be included in plastics include

A

fillers, flame retardants, lubricants, pigments, stabilisers, plasticisers and anti static agents

158
Q

The crystalline degree is dependent on

A

thermal history

159
Q

In thermoplastic materials

A

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
Q

Properties of amorphous polymers:

A
  • transparent due to looser structure
  • low shrinkage
  • low chemical resistance
  • poor fatigue and wear
161
Q

Properties of crystalline polymers:

A
  • sharp melting point
  • usually opaque
  • high shrinkage
  • good fatigue and wear resistance
162
Q

Advantages of thermoplastic polymers:

A
  • can be easily and economically shaped into products

- can be subjected to heated and cooling cycles repeatedly without significant degregation

163
Q

Properties of polymers:

A
  • 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
Q

Branches make polymers:

A
  • stronger in the solid state

- more viscous at a given temperature

165
Q

The curing reaction is

A

a polymerisation process characterised by chemical cross-linking reactions that create an infusible, insoluble and highly cross-linked three-dimensional network

166
Q

What kind if bonds are thermosetting characterised by?

A

Carbon-carbon double bonds

167
Q

How are thermoplastics supplied?

A

As pellets, powders or granules

168
Q

Thermoset resins are supplied as

A

Liquid chemicals that undergo a chemical reaction

169
Q

How is curing initiated?

A

Through exposure to light, heat, or an electric potential

170
Q

Two main events occur during the cure reaction:

A

Gelation and vitrification

171
Q

Polymers are

A

large molecules made my joining thousands of small molecules called monomers

172
Q

Polymers that contain only a single type of repeat unit are known as

A

homopolymers

173
Q

Polymers containing a mixture of repeated units are known as

A

copolymers

174
Q

Gelation:

A
  • 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
Q

How does increasing temperature affect the viscousity of thermoplastics?

A

The viscousity decreases

176
Q

How does increasing temperature affect the viscousity of thermosets?

A

The viscousity starts to decrease but when curing begins the viscousity starts to increase

177
Q

Vitrification occurs when

A

The glass transition temperature of the curing resin increases to the current curing temperature

178
Q

Is the resin 100% cured at vitrification?

A

No

179
Q

In the injection moulding process

A

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
Q

The equipment for injection moulding is divided into three units:

A
  • injection unit
  • mould
  • clamping system
181
Q

Clamping force=

A

Injection pressure x total projected cavity area

182
Q

Stages of the injection cycle:

A
  • plastification
  • injection
  • packing/cooling
  • demould/ejection
183
Q

Conformal cooling channels

A

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
Q

The melting temperature is generally below the melting point of the plastic as

A

The velocity the polymer travels at increases in temperature due to shearing

185
Q

In terms of strain rate, polymers exhibit

A

psuedoplastic behaviour, where strain rate increases at a quicker rate than stress

186
Q

When choosing the temperature of the mould, what factors must be considered?

A
  • shot size
  • injection rate
  • size of runner system
  • part thickness
187
Q

Shrinkage is minimised during injection moulding as

A

more plastic is added during shrinkage by controlling the pressure

188
Q

Part with only thin sections are filled at high injection rates to

A

prevent freezing before filling

189
Q

The dwell time is

A

The time that pressure is applied to the cavity after it is filled

190
Q

The mould time consists of

A

fill time, packing time, cooling time and ejection time

191
Q

Coinjection moulding or ___ is ___

A

Sandwich moulding ___ a process that allows multiple polymers to be injected into one mould to make a single part

192
Q

How coinjection moulding works:

A
  • 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
Q

Three stages if coinjection moulding:

A
  • short shot of skin material
  • injection of core material
  • more skin material
194
Q

Advantages of coinjection moulding:

A
  • reduces cost by using a cheaper fill material as the non-visible core of a product
  • combining desired properties of polymers
195
Q

Disadvantages of coinjection moulding:

A

-machines are more expensive and difficult to maintain than standard moulding machines

196
Q

Biinjection moulding is

A

a process in which two different polymers are simultaneously injected at different locations in the same mould

197
Q

Biinjection moulding is only used in

A

simple, low-tolerance parts

198
Q

In some designs of the gas assisted injection moulding process, has may be allowed to escape from the cavity so that

A

the gas can be recovered for reuse

199
Q

Benefits of the gas assist process:

A
  • 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
Q

In gas assisted injection moulding, how does melt viscousity affect wall thickness?

A

Higher viscousity results in thicker walls

201
Q

In gas assisted injection moulding, how does melt viscousity affect length and area of gas channels?

A

Lower melt viscousities produce longer and larger cross-sectional area gas channels

202
Q

In gas assisted injection moulding, how does melt viscousity affect wall uniformity and reproducability?

A

Lower melt viscousities produce less uniform wall thickness and hence less reproducibility

203
Q

In gas assisted injection moulding, how does melt viscousity affect strength?

A

Lower melt viscousity produces lower strength so that the gas can break through the melt front more readily

204
Q

Sink marks occur largely due to

A

The difference in cooling between thick and thin sections of the part

205
Q

Weld lines form

A

Whenever two molten flow fronts meet

206
Q

Burn marks or discolouration are generally caused by

A

Trapped gasses in the cavity or thermal degradation of the polymer

207
Q

Main causes of sink marks:

A
  • 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
Q

Main causes of weld lines:

A
  • slow injection speed
  • under packing of the part
  • polymer too cold
  • multiple gates
209
Q

To rectify incomplete filling of the cavity (short shots):

A
  • increase melt and mould temperatures
  • increase pressure
  • increase injection speed
210
Q

Corrective action for insufficient polymer in the mould:

A
  • increase packing pressure
  • increase injection hold time
  • increase injection speed
  • increase gate size
211
Q

Corrective action for polymer flowing back out of the mould:

A
  • increase injection hold time
  • decrease mould temperature
  • increase cooling time
212
Q

Corrective action for polymer temperature too high:

A
  • reduce back pressure
  • reduce barrel temperature
  • improve mould temperature controls
213
Q

Corrective action for ejected parts too hot

A

Increase cooling time

214
Q

Corrective action for slow injection speed

A

Increase injection speed

215
Q

Corrective action for underpacking of the part:

A

Increase hold pressure

216
Q

Corrective action for polymer too cold:

A
  • increase melt temperature
  • increase mould temperature
  • increase injection speed
217
Q

Corrective action for multiple gates in weld lines:

A
  • decrease number of gates
  • increase size of gates
  • change gate locations
218
Q

Problems with weld lines

A

Can affect appearance and mechanical properties

219
Q

Corrective action for trapped gasses in the cavity:

A
  • clean vents
  • provide additional vents
  • decrease injection speed
220
Q

Corrective action for thermodynamics degradation:

A
  • decrease injection speed

- decrease melt temperature

221
Q

Applications of ceramics:

A
  • Semi conductors in computers
  • Heat resistant ceramic tiles
  • Catalytic converters
222
Q

Properties of metals:

A
  • High Young’s modulus
  • Strong by alloying and heat treatment
  • Ductile
  • Susceptible to fatigue failure
  • Least resistant to corrosion
  • Tough
223
Q

Properties of ceramics:

A
  • 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
Q

Properties of polymers:

A
  • 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
Q

Ceramic products are produced by

A

compressing fine ceramic powder under certain temperature and pressure conditions

226
Q

Important stages in modulus changes with temperature for polymers:

A

Glassy plateau, glass transition, rubbery plateau, viscous flow

227
Q

Uses of composites:

A

Maximise the attractive properties of two components

228
Q

The two components of a composite are

A

the matrix material and the reinforcing material

229
Q

The stress-strain behaviour of a composite can be found from

A

the weighted average (in terms of volume fraction) of that of the matrix and the reinforcing material

230
Q

Fibres are woven because

A

it makes them stronger in a given direction and in multiple directions

231
Q

A bundle of fibres are called

A

tows

232
Q

All weaves have a corresponding

A

unit cell

233
Q

In an 8-harness satin weave

A

tows come up to cross one other tow before going below to cross sevens tows

234
Q

Why can property data vary so much between different polymers?

A
  • 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
235
Q

The compressive stress of foams

A

increases with strain as the cell walls buckle

236
Q

Foams are

A

gaseous composites

237
Q

Foams are made

A

by stirring molten materials in a gaseous environment

238
Q

In resin transfer moudling

A
  • 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
239
Q

Requirements of reinforcement material:

A
  • strong interface bond between the fibre and matrix

- chemical compatibility

240
Q

Types of fibre reinforcement:

A
  • continuous
  • chopped
  • woven
  • hybrid
241
Q

Flake composite shapes are used when

A

high bending stiffness is required

242
Q

To produce isotropic properties in an element

A

laminates can be layered at different directions

243
Q

Manufacturing of glass fibres:

A
  • glass is fed into a furnace and drawn into filaments

- the fibres are collected together to form a strand

244
Q

Stages of manufacture of carbon fibres:

A
  • stabilisation (200-300 C)
  • carbonisation (1000-1500 C)
  • graphitisation (2500 C)
245
Q

In extrusion

A
  • 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
246
Q

In injection moulding

A
  • 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
247
Q

In compression moulding

A
  • the polymer is injected into a die of two parts which compress it under heat
  • it can be used with prepregs
248
Q

A laminar is

A

a sheet reinforced by unidirectional fibres

249
Q

A prepreg is

A

a sheet of fibres with semi-cured resin

250
Q

Essential steps in manufacturing fibre-reinforced polymeric composites:

A
  • fibre alignment
  • fibre moulding
  • polymer infiltration
  • cross linking
251
Q

In hot-melt prepregging

A
  • 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
252
Q

Sheet moulding compound is produced by

A
  • 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
253
Q

Fibres can negatively affect fluidity if:

A
  • they are too long

- they make up too much of the volume fraction of the composite

254
Q

What is the difference between thermoset and thermoplastic prepregs?

A

Thermosets must be cooled when stored whereas thermoplastics can be stored at room temperature

255
Q

In open mould hand lay-up

A
  • 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
256
Q

Applications of open mould hand lap up:

A
  • prototype fabrications
  • large components in small quantities such as boat hulls or aircraft exterior parts
  • development work
257
Q

Materials for open moulds:

A
  • plaster (used for large moulds, unheated and hence curing takes a long time)
  • metal (for high production, can be heated for curing)
258
Q

What can be used in compression moulding?

A

Prepreg tape or sheet-moulding compounds

259
Q

In open mould spray up

A

resin and fibres are fed into a chopper-spray gun which chops the fibres and combines them and sprays them on a mould

260
Q

What is the advantage of spray up over hand lay-up?

A

It is faster

261
Q

What is the advantage of hand lay-up over spray up?

A

Fibres can be layed more precisely and need not be chopped

262
Q

The open mould hand lay-up process can be improved by

A

automated tape-laying

263
Q

In autoclave

A
  • 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
264
Q

In filament winding

A
  • 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
265
Q

Types of windings possible in filament winding:

A
  • hoop windings (when hoop stresses are priority)
  • helical windings
  • polar windings (when axial stresses are priority)
266
Q

How can the mandrel be removed?

A
  • 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
267
Q

In pultrusion

A

fibre rovings are bathed in resin and pulled through a heated die

268
Q

In reinforced reaction injection moulding

A

the resin is mixed with a chemical hardener to speed up curing

269
Q

In pulforming process

A

the products of pultrusion are put into a die to curve them

270
Q

Compared to open mould, close mould:

A
  • costs more
  • has higher production rates
  • gives higher surface finish
271
Q

Whether a thermoplastic or thermosetting polymer affects injection moulding as

A

the mould must be cold and the barrel must be hot for thermoplastics and viceversa

272
Q

Applications of high performance ceramics:

A
  • Cutting tools
  • Drills
  • Engine parts
  • Wear resistant parts
273
Q

What determines a high performance ceramic?

A
  • Fracture toughness

- Distribution of microcracks

274
Q

Sintering process:

A
  • 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
275
Q

Rate of sintering:

A
  • finer particles sinter faster

- higher temperatures cause faster sintering

276
Q

In production of ceramics

A
  • 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
277
Q

Problems in production of ceramics:

A
  • shrinkage

- grain growth (smaller grains give stronger products)

278
Q

In hot-isostatic pressing

A
  • 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
279
Q

In liquid phase sintering

A
  • 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

280
Q

In chemical vapour deposition

A

-a chemical reaction causes a layer to be deposited on the substrate

281
Q

Grain growth occurs when

A

a lattice is exposed to a temperature for a long period of time

282
Q

Processing that improve on manufacture of ceramics:

A
  • die pressing

- hot isostatic pressing

283
Q

In the slurry infiltration process

A
  • 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
284
Q

In diffusion bonding

A
  • layers of foil and fibre mats are consolidated and stacked
  • heat and pressure is applied
  • the product is cleaned and trimmed
285
Q

Components made using filament winding must be

A

exosymmetric

286
Q

The number of monomers in a polymer chain is known as

A

the degree of polymerisation

287
Q

The rate of reaction slows down from vitrification as

A

it becomes controlled by diffusion as the mobility of the molecular chains decreases

288
Q

How does crosslinking affect the glass transition temperature?

A

It causes the viscousity to increase and hence the glass transition temperature to increase

289
Q

The time when the temperature of a thermosetting polymer is increase before the viscousity begins to increase is known as

A

induction time

290
Q

Co-injection molding utilizes what property of liquid polymers to keep the core material inside the skin material?

A

“fountain flow”

291
Q

How does melt viscousity of the polymers affect products of co-injection moulding?

A

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.

Year 2 Semester 1 (2 decks)

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