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EN0575 Manufacturing Technology 2013/14 Lecture Notes

Dr. Martin Birkett

Mechanical Engineering Division Faculty of Engineering and Environment Northumbria University

Recommended Further Reading Three key texts that are used throughout these notes are as follows. It is recommended that you refer to these books to extend your understanding of the topics covered during the lectures. 1.) Manufacturing Engineering and Technology, (2010), Sixth Edition in SI units, Kalpakjian, S. & Schmid, S., Pearson Education Inc. 2.) Principles of Modern Manufacturing, (2011), 4th Edition in SI units, Groover, M., John Wiley & Sons Inc. 3.) Manufacturing Technology, (2012), Youssef, H., El-Hofy, H. & Ahmed, M., CRC Press.

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1.0

Introduction to Manufacturing

1.1 What is Manufacturing The word manufacturing is derived from the Latin manu factus, meaning “make by hand”. Manufacturing technology is the largest sector of modern industry including activities such as:

Manufacturing is the art of processing materials. It involves the use of machines, tools and labour to convert the raw materials usually supplied in shapeless forms into finished products with specific shape, structure and properties designed to fulfil specific consumer needs. Manufacturing is a value adding process where the conversion of materials into products adds value to the original material. A well designed manufacturing system is achieved through minimising waste and maximising efficiency. 1.2 Manufacturing System

Manufacturing

Smelting processes Melting and pouring in moulds Changing material shape Removing some parts of the material Joining and assembling parts Alteration to the surface properties or appearance Changing the properties of the material without changing the shape

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1.3 Product Design Product design involves the creative and systematic prescription of the shape and characteristics of an artefact to achieve specified objectives while simultaneously satisfying several constraints. Design is a critical activity. It has been estimated that as much as 80% of the cost of product development and manufacture is determined by the decisions made in the initial stages of design. The Design Process - Traditionally the activities of the design process have taken place sequentially as shown below. However this can be very wasteful of resources as any changes in material and process specification will necessitate a repeat of the design stage. This can also lead to delays in getting product to market.

Traditional Design Process

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Concurrent Engineering – Also known as simultaneous engineering is a method of bringing products to market as rapidly as possible, to gain a higher market share and higher profits. Although this concept consists of the same product flow sequence as the traditional process, all the stages are now taking place simultaneously and any iterations will require less effort and result in less wasted time. It should also be apparent that communication amongst and between departments is critical to the success of this method

Concurrent Engineering

In concurrent engineering, the design and manufacture of products are integrated to optimize all elements of the life cycle. The product life cycle generally consists of the following four stages:

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1.4 Selection of Materials for Manufacture An increasingly wide variety of materials are now available, each type having its own:    

Material properties and manufacturing characteristics Advantages and limitations Material and production costs Consumer and industrial applications

Manufacturing characteristics of materials typically include their ability to be: 

Cast



Worked



Formed .

 

Ground .

 

Heat treated

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The following aspects should be carefully considered when selecting a material for manufacturing:     

Can the selected material be replaced by others that are less expensive Does the selected material have properties that unnecessarily exceed the minimum requirements of the product Does the selected material have appropriate manufacturability to be processed by the selected process Is the material to be ordered available in the required size, dimensions, tolerances and surface finish Is the material supply reliable without significant price increase, and supplied with the required quantity in the desired time without delay

1.5 Selection of Manufacturing Processes

Classification of Manufacturing Processes

Plastic forming for metallic materials

Powder metallurgy and ceramic processing

Heat treatment processes

Surface treatment processes Manufacturing Processes Assembly

Modern processes

Complementary processes

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The selection of a manufacturing process is governed by some important considerations such as: before and after processing.



considerations.

 

Accuracy and

quality.



requirements of the product.



.

 

Level of

. .

The following aspects should be considered when selecting a manufacturing process:        

All alternative manufacturing processes have been investigated. A part can often be made by several techniques The part is formed to the final dimensions without additional processes The tooling required is available in the plant, specially manufactured, or purchased as a standard item The scrap produced is minimised and recyclable The process parameters are optimised The inspection tools required are available The inspection techniques and quality control are being implemented properly All components of the product are manufactured in the plant, or some of them are available as standard items from external sources.

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1.6 Materials and Process Selection - Form, Function and Environment There is a direct link between the decision making processes in design, materials selection and manufacturing process.

Design evaluation tools may incorporate materials and process data at the design stage and also during activity such as product benchmarking.

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Form, function and operating environment all inter-relate and will impact upon the design, materials and process decision making process.

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Geometric Forms and Shapes There are various definitions and decision making trees which may be employed, for example definitions of form.

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Case Study – 2012 Exam Question The purpose of this case study is to illustrate the complex interactions between form function and environment and materials and manufacturing process selection. The following Figure shows a universal joint yoke used in the drive shaft which transmits the power to the rear wheels of a medium sized car.

a) Briefly describe the parts form, function and operating environment.

b) Identify any key geometric features and critical dimensions on the figure. c) Comment on the materials and processes that could be used to manufacture the part in high volume (>100kpcs/annum). How would your choices change if the part was made in low volume (20,000

Laminar flow Mixture of laminar and turbulent flow Severe turbulence

Values of Re in excess of 20,000 can lead to air entrapment and the formation of dross or slag on the surface of the molten metal. This is caused by the reaction of the liquid metal with air and other gases. Turbulence can be minimised by proper gating design. Sudden changes in the flow direction of the liquid metal and also in the geometry of the gating system should be avoided. Filters can also be used. The only method to completely eliminate slag is to cast under vacuum. However it can be reduced for atmospheric casting by optimising the design of the pouring basin and runner system and by skimming and using filters.

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Example – Pouring calculation A mold sprue is 20 cm long and the cross section at its base is 2.5 cm2. The sprue feeds a horizontal runner leading into a mold cavity whose volume is 1560 cm3. Determine: (a) Velocity of the molten metal at the base of the sprue. (b) Volume rate of flow (c) Time to fill the mold

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Example – Design of a sprue The desired volume flow rate of the molten metal into a mold is 0.01 m3/min. The top of the sprue has a diameter of 20 mm and its length is 200 mm. Determine: (a) What diameter should be specified at the bottom of the sprue to avoid aspiration? (b) What is the resultant velocity and Reynolds number at the bottom of the sprue if the metal being cast is aluminium and has a viscosity of 0.004 N-s/m2?

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2.2.3 Fluidity of Molten Metal Fluidity is the capability of molten metal to fill the mold cavities. It consists of two basic factors: 1.) 2.)

Molten Metal Characteristics Viscosity – Fluidity decreases with increase in viscosity and viscosity index. Surface tension – Fluidity decreases with a high surface tension on the surface of the liquid metal. Therefore oxide films on the surface can significantly reduce fluidity. E.g. an oxide film on the surface of molten aluminium triples its surface tension. Inclusions – Fluidity decreases with increasing inclusions because they are insoluble and thus increase viscosity. Solidification pattern of the alloy – Fluidity is inversely proportional to the freezing range. Therefore pure metals, which have a shorter freezing range than solid-solution alloys, have higher fluidity. Casting Parameters Mold design – Fluidity is influenced by the design and dimensions of the sprue, runners and risers. Mold material and surface finish – Fluidity decreases with increase in thermal conductivity and surface roughness of the mold material. This situation can be improved by heating the mold but this in turn can slow down the solidification time of the cast, leading to a course grained structure with reduced strength. Pouring temperature – Fluidity increases with increasing pouring temperature as solidification is delayed. Pouring rate – Fluidity decreases with decrease in pouring rate as the rate of cooling increases. Fluidity Test Fig 2.11 shows a spiral mold which is used to test fluidity. The molten metal is poured in the pouring cup and flows through the spiral channel which is at room temperature. The distance the metal flows before it solidifies and stops is a measure of its fluidity. This distance is a function of the thermal properties of the metal and the mold and also the design of the channel.

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Fig 2.11 – Spiral mold for fluidity testing

2.2.4 Heat Transfer Heat transfer during the casting cycle is an important process. The heat flow at different locations in the casting system is a complex process depending on several factors related to the mold and process parameters and the material cast. Fig 2.12 shows a typical temperature distribution at the mold/metal interface. The shape of the curve depends on the thermal properties of the molten metal and the mold. Heat from the molten metal is given of through the mold wall and to the outside air. The sudden temperature drops at the mold/metal and mold/air interfaces are caused by poor contact and boundary layers

Fig 2.12 – Temperature distribution at the mold/metal interface. EN0575 Manufacturing Technology – Student Notes – M. Birkett 2013

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Solidification time After pouring into the mold the molten metal begins to form a thin skin at the cold mold walls. The thickness of this skin increases with time until the casting is completely solid, see Fig 2.13.

Fig 2.13 – Increasing thickness of steel casting skin with time (the remaining liquid metal has been poured out after the times shown) For flat mold walls the thickness of the cast is proportional to the square root of time. Therefore if the time is doubled the skin will be 41% thicker (t=√2=1.41) Chvorinov’s rule states that the solidification time is a function of the volume and surface area of a casting:

Where

C is a constant related to the mold material, metal properties and the temperature n is usually taken as 2

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Example – Solidification times for various shapes 3 metal pieces being cast have the same volume, but different shapes: One is a sphere, one a cube, and the other a cylinder with its height equal to its diameter. Which piece will solidify the fastest, and which one the slowest? Assume that n is 2.

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Shrinkage Metals usually shrink during cooling due to their thermal expansion characteristics. The shrinkage is caused by three sequential events: 1.) 2.) 3.)

The vast majority of shrinkage occurs during stage 3 when the solid casting is cooling to room temperature. Table 2.1 shows the amount of contraction that various metals undergo during solidification.

Table 2.1

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2.3 Defects There are several defect types which can develop in castings. These can be categorised into seven standardised types as follows: A– and rough surfaces

– consisting of fins, flash or projections such as swells

B– – consisting of rounded or rough internal or exposed cavities including blowholes, pinholes and shrinkage cavities C– – Such as cracks, cold or hot tearing and cold shuts. If the solidifying metal is constrained from shrinking freely, cracking and tearing may occur. Although several factors are involved in tearing, coarse grain size and the presence of lowmelting-point segregates along the grain boundaries (intergranular) increase the tendancy for hot tearing. Cold shut is an interface in casting that lacks complete fusion because of the meeting of two streams of liquid metal from different gates D– sand layers and oxide scale

– Such as surface folds, laps, scars, adhering

E– – Such as misruns (due to premature solidification), in-sufficient volume of the metal poured, and runout (due to loss of metal from the mold after pouring). Incomplete castings can also result from the molten metal being at too low a temperature or from pouring the metal too slowly F– – Due to factors such as improper shrinkage allowance, pattern mounting error, irregular contraction, deformed pattern, or warped casting G– – Form during melting, solidification and molding and are generally non-metallic. They are harmful as they act as stress raisers, reducing the casting strength. Inclusions can form when the molten metal reacts with the environment or the crucible or mould. Slag and other foreign materials can also become entrapped in the molten metal forming inclusions. Finally spalling of the mold and core surfaces can produce inclusions.

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Fig 2.14 – Examples of hot tears in casting

Fig 2.15 – Examples of common defects in casting

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2.3.1 Porosity Porosity in castings may be caused by shrinkage, entrained and/or dissolved gases. As thin sections cool faster than thick sections, the molten metal flows into the thicker regions that have not solidified. Porous regions can develop due to contraction as the surfaces of the thicker region begin to solidify first. Microporosity can also develop between dentrites as the liquid metal solidifies. Porosity can have a detrimental effect on the ductility and surface finish of a cast. It can also make the casting permeable thus affecting its pressure tightness. There are a number of ways to reduce porosity caused by shrinkage:    

Adequate liquid metal should be provided to avoid cavities being formed during shrinkage. Internal or external chills can be applied to increase the rate of solidification in critical areas, see Fig 2.16. Steep temperature gradient by using mold materials that have high thermal conductivity to increase the rate of solidification Hot isostatic pressing of the casting

Fig 2.16 – (a) internal chills (b) external Chills

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Questions from Casting Video Questions from video for Sand Casting: Q1. What is the maximum size of casting that can be produced? A1. Q2. Is it a permanent or expendable mold process? A2. Q3. What are the upper an lower halves of the mould called? A3. Q4. What is the function of the vent holes? A4. Q5. What is the function of the riser? A5. Q6. Why are thin sections difficult to sand cast? A6.

Questions from video for Investment Casting: Q7. What is another name for investment casting and why? A7. Q8. What is the shell mold made from? A8. Q9. Why is the shell mold fired? A9. Q10. What are the main advantages of investment casting? A10. Questions from video for Evaporative Foam Casting: Q11. What is evaporative foam casting? A11. Q12. What is another name for evaporative foam casting? A12. Q12. What is the mold made from? A12.

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Chapter 2- Metal Casting - Tutorial Questions 1. Describe the stages involved in the contraction of metals during casting.

2. Explain the effects of mold materials on fluid flow and heat transfer in casting operations.

3. It is known that pouring metal at a high rate into a mold can have certain disadv...