Plastic Extrusion Process Principles

Extrusion Process Principles

Extrusion process is very common in the plastic industry. It is used for high volume plastic production. In extrusion process the color pigments along with other performance enhancing additives are combined with resin which is then pushed through the rotating screws. The heat and pressure generated within the barrels of screw is dispersed and melts the plastic elements. Thus a homogeneous mixture is produced. A cool die is usually placed at the end of the mixture. After passing through the die the mixture is ready to experience the finishing operation which may include pelletizing, calendaring or other processes. 

The arrangements shows a motor which runs the gear box. The speed is used by this gear reducer box. Which then moves the screw. The metal ingredients is poured through the hopper. The feeding of metal is done in the feeding zone. Then with the increased pressure and temperature of the screw melts the elements in the melting zone. The next part is the melt pumping zone which increases the pressure of the liquid. This pressurized liquid is then pushed through the filter. Then the molten metal flows through the die and cooled. The thermocouple is set at different points to measure the temperature. The figure of the whole process is shown below. Click on it to have a better view . 

Advantages of Plastic Extrusion Process

One of the main advantages of the extrusion process is it can produce pipes of any length. It is continuous. High production volumes with very low cost per pound. The melting is very efficient. The mixing of the ingredients is very good. Different types of raw materials can be used. It uses thermoplastics which can be reused.

Disadvantages of Plastic Extrusion Process

Highly complicated parts are very not suitable for production in extrusion process. It can only produce the parts which have uniform cross sections. 

Please Read : 

• Different Types of Plastics and Their Advantages and Disadvantages
• A complete Guide to Casting Process 

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A Complete Guide to Steps Involved in Casting Process

We know that casting is the process of pouring molten metal in a desired shaped cavity and allowing it to solidify. After the solidification we get the desired shaped object.

Please read :

• Comparison of different casting processes. 
• What is casting ? Main application of castings 
• Basic Steps of Sand casting 

Basic Operation of the casting process

• First we have to make a pattern 
• Then the we have to melt the metal by applying heat 
• pouring the metal into the mold and confirming that the metal gets the desired shape. 

What is Pattern in Casting Process  

Pattern is the object used to make cavity in the mold. The molten metal is poured into this cavity.

Why a good Pattern is required in casting ? 

• To obtain the desired shape and size. 
• It is easy to design and manufacture. 
• making a pattern is not expensive. 
• It is of high strength. 

Pattern Materials 

• Wood 
• Cast Iron / Brass 
• Aluminum or White metal 
• Plastic gypsum 

For a complete knowledge of pattern materials go to this link

• Different types of pattern materials

Pattern Allowances 

• Shrinkage allowance (due to cooling)
• Machining allowance (due to machining)
• Shaking allowances (due to rapping)
• Draft allowance (follow the link)
• Distortion allowance (due to irregular shape) 

For a complete knowledge about the allowances in patterns follow this link

• Complete guide of pattern allowances in casting process 

Different Types of Patterns used in Casting 

• Split piece pattern 
• Match plate pattern 
• cope and drag patterns 
• sweep patterns 
• draw backs 
• Loose piece pattern 
• Gated pattern 

Follow : A complete guide to casting patterns 

Preparing Mould for the Casting 

While preparing mould one should be very careful . Different types of sands and binders are used in mould preparation. These are 

Green sand mould: Natural sands , clay and water is used in making green sand mould. It is very cheap with acceptable errors.  

Dry Sand Mould: It is natural sand mould with cereal and pitch used binders. 

Other materials for mould preparation includes silica sand and binder with additives. 

Please Read : 

• Different types of molding sand
• Advantages and Disadvantages of Different Sand Casting Processes 

Different Properties of the moulding sand 

These are the desired properties of the moulding sand 

• Refractoriness 
• Cohesiveness 
• Permeability 
• Co-efficient of expansion 
• Adhesiveness 
• Collapsibility 
• Plasticity 

For details of each of the properties follow this link : 

• Properties of various natural and synthetic molding sands

Tests for the different moulding sands 

1. Moisture Content Test 
2. Fineness Test 
3. Testing Amount of Clay 
4. Permeability Test 
5. Mould hardness test 
6. Strength or cohesiveness test 

Core Making 

What is Core ? 

It is a structure made of refractory material . It is prepared before pouring the molten metal on to the cavity. There is another term known as core print which is the projection on a pattern used to make spaces in the mould. It is also known as core seat. 

Criteria for selecting a furnace for casting 

1. Capacity needed to hold the molten metal 
2. Melting rate 
3. Quality of the melt 
4. Temperature needed 
5. Method of pouring 

Operation in Cupola Furnace 

Cupola is the most popular furnace for casting non ferrous and ferrous materials . It is known as a shaft furnace. The shell of cupola furnace is made of steel of 8-10 mm thickness. The shell is supported by columns or legs. The inside of the furnace is lined with refractory material to save the furnace from over heating.  

Different types of casting processes 

1. Sand Casting 
2. Shell Mold Casting 
3. Expandable Mold Casting 
4. Plaster Mold 
5. Ceramic Mold 
6. Investment Mold or Lost Wax Casting  
7. Permanent Mold 
8. Die Casting 
9. Centrifugal casting 

Casting Defects 

And lastly comes the casting defects. These defects can occur for different reasons like air trapping in the sand mold, low quality sands, improper ramming , defecting sand mold etc. Go through this link for detailed knowledge of casting defects.

• Different types of casting defects  

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Electrical Discharge Machining (EDM) Principles

EDM is a non-conventional machining technique uniquely used for cutting metals which are not possible to cut with traditional methods. EDM only works with materials which are electrically conductive. Delicate cavities and intricate contours which are difficult to produce with a grinder or other machines can be done with Electrical Discharge Machining or EDM. The cutting tool for EDM may be made of hardened too steel, titanium carbide or inconel or kovar.  

EDM is also known as "Spark Machining" . Such name has been given for the fact that it removes the metal by applying a rapid series of repetitive electrical discharges. An electrode and the work piece is used for the conducting path of these electrical discharges. A continuously flowing fluid is always flowing to flush away the little amount of material that are removed. Repetitive discharge gives the workpiece a desired shape.

Please Read :

• Abrasive Jet Machining Process 
• Water Jet Machining Process .
• Ultrasonic Machining Principles 
• Laser Beam Machining

Two primary EDM methods are

Ram EDM and
Wire EDM.

Between this two methods the main difference is found in the electrodes. In ram EDM graphite electrodes are used. These electrodes are machined in traditional tools and a special shape is given which is connected to the power source. The electrodes are also connected to a ram. When all the arrangements are ready the electrodes are fed into the workpiece. The entire process is performed under a submerged fluid bath. In Wire EDM the electrode it self is a thin wire. Specially processed brass wires are used for feeding into the material. Electrical discharges cut the w/p according to desired shape. Wire EDM is generally done while submerged in a bath of water.

Ram Electro-Discharge Machining (EDM) Process: 

Rapid recurrence of spark produced between the two electrodes (tool & w/p) controls the metal erosion. It is to be noted that the workpiece must be an electrically conducting metal. An appropriate gap is necessary which is usually approximately 0.025 to 0.075 mm known as spark gap. The gap must be maintained between the w/p and the tool by a servo motor which runs by the difference between a reference and gap breakdown voltage. The tool moves upwards and downwards by this operating motor.
The rate of metal removal depends on the spark gap. If both the electrodes are made of same materials then the highest erosion is found on the positive electrode or the anode. So logically to get a high metal removal rate and a greater tool life the tool is made as cathode and workpiece works as anode, The two electrodes are kept apart and are separated by a dielectric fluid. A transient electric discharge is found across the gap between the two electrodes in the form of spark. When the potential difference between the electrodes is sufficient, the dielectric fluid is ionized and break down which in terms produces an conductive spark channel. The current is discharged across the channel by the condensers as spark. If the potential difference is less than or equal to 12 volts the dielectric fluid is deionized. The process is repeat itself as the condensers start to recharge. The spark interval is generally 10 to 30 microseconds and the current density ranges from 15-500 am/mm2. The energy is released from the sparks in the form of local heat and eventually local temperature found is in the order of 12000°C. Such high temperature and pressure melts and erodes some metals some of which is vaporized and other fine material particles are carried by the fluid circulating around the electrodes which creates a crater on the w/p. As the time interval of the sparks is very low the heat doesn't get time to conducted between the tool & workpiece. Fig below shows a schematic diagram of the whole process.

How to choose the Tool Material
Many factors are needed to be taken into consideration while selecting the tool material.

• Low erosion rate and decent work to tool wear ratio
• good electrical conductivity 
• good machinability 
• low electrical resistance 
• high melting point . 
• high rate of electron emission  

EDM has one major drawback and it is the wear ratio of the tool. Different material has different wear ratio. For Brass it is 1:1 . For metallic electrodes it is found 3:1 or 4:1 . For high melting point electrode graphite it ranges from 5:1 to 50 :1 . 

Tool Wear

While applying EDM the tool or cathode also erodes which is not desirable. It is unavoidable but remains in tolerable limit as the wear of the cathode is much less than the anode. This occurs because -

• Positive ions from the dielectric fluid hit the cathode but electrons strikes the anode. Though electrons are much lighter than the positive ions it possesses more energy as it moves it greater velocity. So anode gets more eroded. 
• At the time of spark a compressive force is created at the cathode which reduces the cathode erosion. 
• Fluid medium is generally hydrocarbon. Due to pyrolysis gases are produced which produces carbon particle and these particles create a thin layer of protection on the cathode. Thus the cathode is much safer than anode. 

Purpose of Dielectric Fluid 

• Effective Coolant for the workpiece and the tool . 
• It works as an insulating material during the charging of the condenser as a result perfect condition for efficacious spark discharge and its conduction when ionized is obtained. 
• The eroded materials are carried away by this medium. 
• It is a coolant in quenching the spark and prevents the arcing. 

Essential Requirements for dielectric fluid 

• Optimum Viscosity is necessary . If the viscosity is low then the fluid will not be able to carry the metal particles. On the other hand if the viscosity is high then it will restrict the flow of the liquid. 
• It should be non-reactive with the work piece, container or the tool material. 
• Non-expensive, easily available and inflammable. 
• It should not produce toxic gases or vapors during the operation. 
• It should be a hydrocarbon compound. 

Different dielectric fluids are : transformer oil, spirit (white), oil and kerosene etc. If some conducting power like aluminum or light graphite is added to the fluid then the metal removal rate increases. 

Advantages of EDM

• Metal having any hardness or brittleness and toughness can be machined. 
• Harder materials such as steel alloys or tungsten carbides which are used for molding and other non-conventional machining like forging and press tools can be reproduced. 
• Dies can be machined at hardened condition. 
• Complicated shapes can be reproduced. 
• Very fine holes can be done very accurately . 
• The accuracy is very high. Tolerance of 0.005 mm can be achieved. 
• Wear resistance surface can be made because workpieces produced with EDM have micro-craters which can contain lubricants effectively. 
• The physical contact between the tool and w/p is avoided. No cutting force other than blasting pressure is exerted. So fragile jobs and cylinders can be machined without causing any damage. 
• Harder metals can be machined very quickly in comparison to the conventional machining process. 

Disadvantages Electrical Discharge Machining 

• The power required for machining is much higher compared to the conventional machining. (120J/mm2)
• There are chances of surface cracking when the materials become brittle at room temperature. 

• A thin layer usually ranging from 0.01 mm to 0,10 mm containing 4 % carbon may be deposited on the workpieces made of steel 
• The Material Removal Rate  (MRR) is comparatively low (75 mm3/sec)
• Reproducing sharp corners is difficult in EDM. 
• Sometimes the micro-structures are distorted and subsequently etching occurs. 

Applications of EDM 

Generally EDM is hugely used for machining burr free intricate shapes as well as narrow slots and blind cavities. Sinking of dies , plastic molding, die casting compacting, cold heading, extrusion, press tools, wire drawings are some of the examples of its application. Negative tool geometry can also be generated on a w/p if suitable tool can be made. EDM is very useful for machining small holes. It is also used to cut slot in diesel fuel injection nozzles. It is also used in air crfat engines and brake valves etc.

Wire Electrical Discharge Machining

A very thin wire of diameter ranging from 0.02 to 0.3 mm is used as an electrode in wire cut EDM. It cuts the workpiece with electrical discharge just like a band saw. In this process either workpiece or the wire is moved. The spark discharge phenomenon is used for eroding the metal which is same as the conventional EDM. In wire cut EDM the wire acts as an electrode as a result complicated shapes can be cut easily without forming electrode. Basically the wire-cut EDM consists of a machine which has a workpiece contour movement control unit ( NC tension : a power supply which supplies electrical energy to the wire and has a unit ) . It also has workpiece mounting table and a wire driver section. The wire driver section is use for moving the wire accurately at a constant tension. Another important part is the dielectric fluid (distilled water) supplier having constant specific resistance. Wire EDM has the following features -   

•  No forming electrode is necessary. 
• electrode wear is very negligible. 
• Smooth machined surface. 
• Tight geometrical and dimensional tolerances . 
• Extremely high tolerances between punch and die. Extended die life. 
• Straight holes are possible to produce. 
• Machine can be operated without any regular supervision for long time at high operating rates. 
• No skill is needed to run the machine. 

Advantages

• Because of the absense of the split lines in the die, savings of the stages in the sequential tools occurs. It permits more punch opening per stage. 
• There will no flashes on the molded parts because the molds with draught  can be arranged without vertical divisions. 
• To necessity for tool manufacturing and storing. 
•  Workpieces are hardened before cutting . So no heat treatment distortion is not present. 
• Whole work is done in one machine . So die manufacturing cycle time is short. 
• Lesser inspection time because of single piece construction of dies with high accuracy. 
• Time is utilized perfectly as the wire cut EDM can cut throughout the day. 
• Very economical even for small batch production. 
• low thermally affected zone. High surface finish. 
• Number of rejected workpieces are very small. 

Please Read : 

• Different types of welding defects.
• How to easily understand TIG welding Basics 
• Advantages of TIG welding 
• TIG welding Principles in Easy language 
• Welding Electrodes Explained 
• MIG welding principles 

Read full post »

Electrochemical Machining (ECM) Principles

Electrochemical Machining (ECM) is based upon Faraday's law of electrolysis. Faraday's law states that the the mass of a metal altered by the electrode is proportional to the quantity of electrical charges transferred to that electrode.

• In ECM the removal of metal is controlled by the anodic dissolution in the electrolyte. 

In ECM -

•  The workpiece acts as the anode
• The tool act as cathode.
• The electrodes should be placed closely with a gap of about 0.5 mm .
• The anodes and cathodes should be immersed into electrolyte. (Here Sodium Chloride)

Please Read : 

• Abrasive Jet Machining Process 
• Water Jet Machining Process .
• Ultrasonic Machining Principles 
• Laser Beam Machining
• Electrical Discharge Machining (EDM) Principles 

Schematic illustration of the electrochemical-machining (ECM) process

The main principles of ECM are as follows - 

• A potential difference is maintained between the electrodes as a result ions existing in the electrolytes migrate towards the electrodes. 
• Conventionally positively charged ions are attracted towards the cathode and negative ions are attached towards to the anode. And thus flow of current is initiated in the electrolyte. 
• The set-up is kept stationary and tool is fed linearly. 
• The desired amount of metal is removed because of ion migration towards the tool. 
• For keeping the tool safe from damage a continuous supply of electrolyte is ensured by pumping it at high pressure (15kg/cm2). 
• In this process the temperature generated is very low and no spark is produced and thus there isn't any scope of metallurgical changes in the job. 
• When electricity is supplied to the metallic ions of the w/p is pulled out. The positive ions of the metal reacts with the negative ions present on the electrolytic solution and hydroxides of metal and other components. This hydroxides are precipitated and washed away by the electrolytic solutions.  
• In Electrochemical Machining the tool and workpiece doesn't come in direct contact with each other so negligible wear and tear is observed. 
• Metal removal rate is high and voltage supplied is very low. 
• The metallic workpiece is not damaged due to thermal stresses. 
• Dimensions up to 0.05 mm can easily machined . 

Examples of parts made by ECM 

Different parts made by electrochemical machining.  (a) Turbine blade material :  nickel alloy (b) Thin slots on a 4340-steel roller-bearing cage.  (c) Integral airfoils on a compressor disk.

 Electrochemical Grinding  (ECG) Principles  

(a) Schematic illustration of the electrochemical-grinding process.  (b) Thin slot produced on a round
nickel-alloy tube by this process.

        Functions of Electrolyte

•           The current is carried between the tool and the w/p through electrolyte.
•           The produced heat is dissipated by the liquid electrolytic solution.
•           The product of machining is removed by the solution.               
•            It keeps the reactions continuous by supplying the elements necessary for the reaction.  

What Should be The Criteria of Selecting Electrolyte in Electrochemical Machining  (ECM) and Grinding (ECG)

The selection of the electrolyte should be done by considering the following matters - 

·         Required Machining rate 

·         Required Dimensional Accuracy 

·         Surface Texture and Integrity  

            

      The properties of Electrolytes 

•            High Electrical Conductivity 
•            High Current Efficiency for machining 
•            Good Surface finish and integrity is necessary
•            Composition of the electrolyte and structure of the material controls the final surface texture. 

 Flow Arrangement of the Electrolyte in  ECM / ECG 

       Perfect electrolyte flow across the machining tool is mandatory for proper machining.

      cavitation is likely to be occurred in the tool so proper care is necessary to keep the 

      tool in shape. Tool design must ensure the uniform flow of electrolytic solution in all

      the machining areas. Optimum flow of the electrolyte is desired because excessive

      flow can cause erosion of the tool. 
      
     Mainly two types of flows are used –

     1.     Divergent flow

     2.     Convergent flow

     Convergent flow provides a smoother flow of electrolytes. At first the electrolyte has to pass a

     chamber known as ‘dam’. The dam is used to pressurize the are outside  the working tool. 

   Advantages of convergent flow system.  

  

•         Improved Surface finish 
•         Improved Uniform and predictable side over cut as well as front machining gap. 
•         Less prone to arcing. 
•         Clean operating environment 
•         Stray currents make it possible to eliminated unwanted machining.  

But it is also to be mentioned that machining in convergent flow is much expensive than divergent flow. 

Advantages of Electrochemical Machining (ECM)

1. Accurate Machining 
2. No direct contact between tool and job. 
3. Negligible wear and tear of the tool. 
4. Environment friendly 
5. no thermal or mechanical stress is induced on the tool.
6. There is no contact between worpiece and the tool so its is possible to machine non-rigid and open w/p. 
7. Jobs with complex geometric shapes can be machined with ease accurately and repeatedly. 
8. ECM is a time saver when compared to conventional machining. 
9. During drilling several holes can be done at once.  
10. Deburring can be done in hard to access areas. 
11. Fragile and brittle materials which are prone to damage can be machined easily in ECM without cracking or breaking. 
12. Surface finish up to 25 μ in can be achieved. 

Disadvantages of Electrochemical Machining (ECM)

1. Sometimes this process is costly because the equipments are expensive. 
2. Continuous supply of electrolytic solution is mandatory. 
3. Steady voltage or potential difference should be maintained. 
4. Rigid fixturing is required to withstood the high flow rate of electrolytes. 
5. Designing the tool is arduous because it must be insulated to maintain the perfect conducting paths towards the workpiece. 
6. Corrosion free material is needed for the structure and the electrolyte handling unit. 
7. If hydrogen is liberated at the tool surface then it is possible to suffer from hydrogen-embitterment of the surface. 
8. There is possibility of damages because of sparks. 
9. Conventional machining techniques produce more improved fatigue properties than ECM.  

Applications of Electrochemical Machining ECM 

• ECM is mainly used in the areas where conventional machinig techniques are not feasible . One of the main applications of ECM is found in the aerospace industries where accuracy is very important when complex shaped difficult to machine materials are needed to be machined. 
• Different Industrial techniques have been developed on the basis of Electrochemical Machining Such as 

                a. Electrochemical Cutting 

                b. Electrochemica ECM 

                c. Electrochemica broaching 

                d. Electrochemica drilling 

                e. Electrochemica deburring 

Please Read : 

• Different types of welding defects.
• How to easily understand TIG welding Basics 
• Advantages of TIG welding 
• TIG welding Principles in Easy language 
• Welding Electrodes Explained 
• MIG welding principles 

Read full post »

Ultrasonic Machining (USM) Working Principles

In this process the material of the workpiece is removed by the repetitive impact actions of abrasive particles. The erosion takes place by the abrasive particles which are carried by a liquid medium in the from of a slurry . A shaped vibrating tool is used to produce the impact. The term shaped is used to explain that the process is capable enough to create 3D profiles in correspondence to the tool shape which is not possible in AJM. The tool gets the vibrating motion from the vibrating mechanical horn.  Here is a schematic diagram of the basic system.   


Please Read : 

• Abrasive Jet Machining Process 
• Water Jet Machining Process .
• Laser Beam Machining
• Electrical Discharge Machining (EDM) Principles 

Ultrasonic Machining (USM) Principles

Working principle of Ultrasonic Machining  or Ultrasonic Impact Grinding is described with the help of a schematic diagram. The shaped tool under the actions of mechanical vibration causes the abrasive particles dipped in slurry to be hammered on the stationary workpiece. This causes micro-indentation fracture on th material. 

Small abraded particles are removed along the surface which is perpendicular to the direction of the tool vibration. When the material is removed a cavity of the same profile of the tool face is formed. The abrasive particles gradually erodes as the machining process continues. As a result fresh abrasive particles are needed to be supplied in the machining zone. Abrasive particles associated with the liquid is fed to the m/c zone and it ensures the removal of the worn out grains and material. 

Machining Time 

The machining time of the ultrasonic grinding depends on the frequency of the vibration, material properties and grain size.  The amplitude of the vibration may vary from 5 to 75 µm and frequency may vary from 19~25 kHz. Ample static force is also required to hold the job against the machining tool . A continues flow of abrasives suspension is also mandatory.

  

Advantages of USM:

1. It can be used to drill circular or non-circular holes on very hard materials like stones, carbides, ceramics and other brittle materials.
2. Non-conducting materials like glass, ceramics and semi precious stones can also be machined. 

Disadvantages of Ultrasonic Machining  :

1. It can be proved slower than the conventional machining processes. 

2. Creating deep holes is difficult because of the restricted movement of the suspension. 

3. It is arduous to select the perfect tool geometry for creating hole of certain dimension. The holes created may be of larger sizes because of side cutting.  

4. High tool wear because of continues flow of abrasive slurry. 

Applications:

1. Hard and brittle materials can be machined like tungsten carbide, diamond and glass. These are difficult to machine in conventional m/c-ing process.
2. Wire drawing dies of tungsten carbide can be drilled by this process.  

3. Circular as well as non-circular holes can  be done with straight or curved axes.
4. It has been proved successful in machining geranium, silicon quartz and synthetic ruby etc. 

Please Read : 

• Different types of welding defects.
• How to easily understand TIG welding Basics 
• Advantages of TIG welding 
• TIG welding Principles in Easy language 
• Welding Electrodes Explained 
• MIG welding principles 

Read full post »

Water Jet Machining (WJM) Advantages and Disadvantages

        In Water jet Machining no abrasive is used. Water jet alone is used for cutting. In WJM, materials like concrete, asbestos, wood, rocks, coal, textiles and leather can be cut. The material is removed by means of erosion. These days hydraulic coal mining as well as tunneling, descaling and cleaning is also done by this process. 

         Water Jet Machining (WJM) Principles 

As the name suggests the water jet machining process involves the use of high velocity  and high pressure thin jets of water to cut the job. Water Jet is the stream of high velocity water coming out from the nozzle. When high pressure water jet comes out of the nozzle it gains a large kinetic energy. After striking the work piece this kinetic energy is converted to pressure energy inducing high stress on the material. When this induced stress surpasses the ultimate stress of the material, removal starts. 

Please Read :  

•         Abrasive Jet Machining Process 
•          Laser Beam Machining
•          Electrical Discharge Machining (EDM) Principles 

         Schematic Diagram of WJM 

        The schematic diagram of  WJM process is very similar to that for AJM. To raise the pressure of the water a pump or intensifier is used. The system pressure ranges from 1600 to 4000 N/mm2. The accumulator used in the system act as a pulsation remover and an energy reservoir. Water passes through the accumulator and then nozzle through a high pressure thick tube. The tube material may be stainless steel jacketed with carbon steel. Sintered diamond , sapphire or tungsten carbide may be used as the nozzle material. The exit dia. of the nozzle may vary from 0.05 to 0.35 mm. 

a. Schematic of Water Jet Machining b. Example of workpiece machined by WJM

Advantages of Water Jet Machining Process 

1. Here water is used for cutting which is cheap, non-toxic and readily available. 
2. The water jet keeps the job clean and dust free. 
3. The only moving part used is the pump, therefore the operating and
maintenance expenses are low. 
4. The process is very safe to use. 
5. Very complicated designs and detailed work can be done. 
6. There is no damage of the workpiece due to thermal stress. Very little heat is
generated. 
7. Soft rubber like materials can be cut through this process where saw teeth
gets clogged. 

          

Disadvantages of WJM.

1. Hard Materials cannot be cut. 

2. The initial cost is high. 

Applications:

1. Cutting 

2. Milling 

3. 3D Shaping

4. Turning

5. Piercing

6. Drilling

7. Polishing 

Please read :

•          Different types of welding defects.
•          How to easily understand TIG welding Basics 
•          Advantages of TIG welding 
•          TIG welding Principles in Easy language 
•          Welding Electrodes Explained 
•           MIG welding principles 

Read full post »

ADVANTAGES AND DISADVANTAGES OF WELDING

Like all joining processes (in fact, like all processes!), welding offers several advantages but has some disadvantages as well. The most significant advantage of welding is undoubtedly that
it provides exceptional structural integrity, producing joints with very high efficiencies. The strength of joints that are welded continuously (i.e.,full length, without intentional skipped areas) can
easily approach or exceed the strength of the base material(s). The latter situation is made possible by selecting a joint design that provides greater cross-sectional area than the adjoining joint elements and/or filler that is of higher strength than the base material(s). Another advantage of welding is the
wide range of processes and approaches that can be selected and the correspondingly wide variety of materials that can thus be welded.

Advantage of Welding 

Almost all metals and alloys, many (thermoplastic) polymers, most if not all glasses, and some ceramics can be welded, with or without auxiliary filler. Still other advantages of welding are that -

(1) there are processes that can be  performed manually, semi-automatically, or completely automatically; (2) some processes can be made portable for implementation in the field for erection of large structures on site or for maintenance and repair of such structures and
equipment; (3) continuous welds provide fluid tightness (so welding is the process of choice for fabricating pressure vessels);(4) welding (better than most other joining processes) can be performed remotely in hazardous environments (e.g., underwater, in areas of radiation, in outer space) using robots; and (5) for most applications, costs can be reasonable. The exceptions to the last statement
are where welds are highly critical, with stringent quality requirements or involving specialized applications (e.g., very thick section welding).

Disadvantage of Welding 

The single greatest disadvantage of welding is that it precludes disassembly. While often chosen just because it produces permanent joints, consideration of  ultimate disposal of a product (or structure) at the end of its useful life is causing modern designers to rethink how they will accomplish joining.
A prime example is the need for the regulatory authorities in former West Germany to  dismantle the nuclear reactors in former East Germany that have designs similar to the reactor that failed in Chernobyl in the former USSR.

A second major disadvantage of many welding processes is that the requirement for heat in producing many welds can disrupt the base material microstructure and degrade properties. Unbalanced heat input can also lead to distortion or the introduction of residual stresses that can be problematic from
several standpoints.

A third serious consideration, but not necessarily a  disadvantage, is that welding requires considerable operator skill, or, in lieu of skilled operators, sophisticated automated welding systems. Both of these, along with the aforementioned specialized applications, can lead to high cost.

This table summarizes the major advantages and disadvantages or limitations of welding as a means of joining materials or parts into parts or assemblies or structures.

advantages  and disadvantages of welding

Advantages and Disadvantages of Welding as a Joining Process

Advantages	Disadvantages
I. Joints of exceptional structural  integrity and efficiency, will not accidently loosen or disassemble  2. Wide variety of process embodiments  3. Applicable to many materials within  a class  4. Manual or automated operation  5. Can be portable for indoor or  outdoor use  6. Leak-tight joints with continuous welds 7. Cost is usually reasonable	1. Impossible to disassemble joints without  destroying detail parts  2. Heat of welding degrades base properties  3. Unbalanced heat input leads to  distortion or residual stresses  4. Requires considerable operator skill  5. Can be expensive (e.g., thick sections)  6. Capital equipment can be expensive (e.g.,  welds electron-beam guns and vacuum chambers)

Please Read : 

• TIG welding Basics 
• Advantages of TIG welding 
• TIG Aluminum Welding 
• MIG Welding Principles 
• Submrged Arc Welding 
• Flux shielded metal arc welding process principles 
• Resistance Projection Welding 
• Resistance Spot Welding 
• Resistance Seam Welding 
• Different types of welding defects
• Aluminothemic Welding or Thermit Welding Principles 
• LaserBeam Machining/Welding

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Abrasive Jet Machining (AJM) Process Advantages and Disadvantages

Abrasive Jet Machining (AJM) Principles 

In abrasive jet machining process, a focused steam of abrasive particles (of size 10 to 40 microns) carried by high pressure gas or air at a velocity of about 150 to 300 m/sec is made to impinge on the work surface through a nozzle, and the work material is removed by erosion by the high velocity abrasive particles. The inside diameter (ID) of the nozzle through which abrasive particles flow is about 0.18 to 0.80 mm and the stand-off distance (i.e. distance between nozzle tip and workpiece) is kept about 0.3 to 20.0 mm. The process can be easily controlled to vary the metal removal rate which depends on flow rate and size of abrasive particles. This process is best suited for machining super alloys and refractory type of materials, and also machining thin sections of hard materials and making intricate hard holes. The cutting action is cool because the carrier gas serves as coolant. 

When an abrasive particle (like Al2O3 or SiC) having sharp edges hits a brittle and fragile material with a high speed, it makes dent into the material and lodges a small particle from it by a tiny brittle fracture. The lodged out or wear particle is carried away by the air or gas. The operating elements in AJM are abrasive, carrier gas and the nozzle as schematically shown in the following Figure

Abrasive Jet Machining (AJM) Principles 

The distance between the nozzle tip and the work surface has great influence on the diameter of cut, its shape and size and also rate of material removal. The following Figure shows the variation in the diameter of cut with change in the stand off distance (SOD). It is evident that the SOD changes the abrasive particles spreads (i.e. covers wider area) on the work surface and consequently increases the diameter of the cut.

The basic Units of AJM

The basic unit is schematically shown in following Figure. It consists of gas supply system (compressor), filter, pressure regulator, mixing chamber, nozzle assembly and the work holding device. In the mixing chamber, the abrasive is allowed to flow into the gas stream. The mixing ratio is generally controlled by a vibrator. The particle and gas mixture comes out of the nozzle inside the machining chamber of the machine tool unit. The feed motion can be given either to the work holding device or to the nozzle. 

AJM setup

Please read :

• Water Jet Machining Process .
• Laser Beam Machining 
• Electrical Discharge Machining (EDM) Principles 
• Electrochemical machining process 

Abrasive Jet Machining (AJM) Advantages:

• This process is quite suitable for machining brittle, heat resistant and fragile materials like, glass, ceramic, germanium, mica etc. 
• It can be utilized for cutting, drilling, polishing, deburring, cleaning etc. of the materials. 
• The depth of damage to the surface is very little. 
• Holes of intricate shapes could be produced efficiently.
• The surface machined can have good finish (by controlling the grain size mainly). 

AJM Disadvantages: 

The materials removal rate is low. For example, for glass, it is 0.0164 cm3/min. 

• The tapering of hole especially, when the depth of the hole is more, becomes almost inevitable. 
• A dust collecting chamber is a basic requirement to prevent atmospheric pollution to cause health hazards.
• The abrasive particles may remain embedded in the work surface. 
• Abrasive particles cannot be reused.

AJM Applications:

• Abrasive jet machining is best suited for machining brittle and heat sensitive materials like glass, quartz, sapphire, ceramics etc,
• It is used for drilling holes, cutting slots, cleaning hard surfaces, deburring, polishing etc. 

Please Read :

• Different types of welding defects.
• How to easily understand TIG welding Basics 
• Advantages of TIG welding 
• TIG welding Principles in Easy language 
• Welding Electrodes Explained 
• MIG welding principles 

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