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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Different Types of Plastics- Advantages and Disadvantages

Plastics can be classifies into two broad categories -
Thermosets and Thermoplasctics. A brief description of these two classes will be discussed here.

• Thermosets are the kind of plastics which are not recyclable. They undergo a kind of chemical change at the time of heating. As a result  they can be shaped once because of this permanent chemical change. This process is known as heat hardening. They are also known as thermosetting plastics. Examples : Epoxy Resin, Melamine formaldehyde , polyester resin , Urea formaldehyde. 
• Thermoplastics are the recyclable types. While heating they undergo a temporary chemical change. After the heat is removed they retain their original shape. The process is known as heat softening.  Examples : Polyamide (Nylon), Polymethyl methacrylate, polypropelene, polystyrene, Low density Polythene (LDPE), High density Polythene (HDPE). 

Different Manufacturing Processes for Plastics 

Different manufacturing process for plastics include 

• extrusion
• lamination 
• foaming
• thermal forming 
• molding 
• solid-phase forming
• molding, 
• spinning  
• casting   

The molding process alone have five different procedures -

• Compression
• Transfer
• Blow
• Rotational 
• Injection 

The injection molding, Blow molding, Extrusion as well as calendering uses thermoplastics for fabrication. High Pressure Lamination, Compression Molding and reaction injection method uses thermosets

Advantages and Disadvantages of using Plastics 

Advantages

• Plastic is very light in weight. 
• It has high strength to weight ratio. 
• Complex parts can be easily manufactured. 
• Plastics can be of variety of colors or can be clear. 
• It is free from corrosion. 
• It can work as a electrical as well as thermal insulator. 
• It can damp the vibration. 

Disadvantages 

• Creeping can be problem. 
• It cannot withstand extreme heat. So it is thermally unstable. 
• It can be UV light sensitive. 
• The stiffness is relatively low in comparison to other tough materials. 
• It has a low strength.
• Reworking or repairing of plastic is very tough. 
• Recycling of plastic is a hard job.  

Please Read :

• A complete guide to casting patterns 
• Sand Casting 
• Die Casting 
• Centrifugal casting 
• Basic Steps of Sand casting 
• Different types of pattern materials
• Complete guide of pattern allowances in casting process 
• Different types of casting defects  
• Properties of various natural and synthetic molding sands
• Different types of molding sand

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Diferent Parts of Sand Mold- Features

Cope and drag in Sand Mold :

Cope and Drag are the two parts of the casting flask. Cope is the upper part and drag is the lower part. Even if the casting process is flaskless , the same terms are used for the upper ans lower parts. Generally the flask is made of wood or metal. It contains molding sand. When metal is poured into the mold cavity the flask supports the mold.

Gating system:

If the molten metal is poured directly from the ladle , it will erode the bottom of the mould cavity. So molten metal is poured from the ladle to the cavity through a gating system. The gating system in casting creates a series of channels through which molten metal reaches the cavity. Gating system has -

• Pouring Basin : It receives the molten metal from the liquid metal container. 
• Sprue : Pouring cup is attached to the sprue. It is vertical in shape . On the other part of the sprue there is part called runner. 
• Sprue Base : Its the base of the sprue
• Runner : It is the horizontal part of the gating system. It connects the spues with the gates. 
•  The next part is the choke. 
• Then comes the skim bob 
• Gates and ingates : It controls the movement of the metal from the runners into the cavity. 
• Riser 

Riser:

The risers are also known as feed heads. When the metal solidifies it starts to shrink. And then risers comes into play. These feeder supply metals to the cavity when shrinking of the metal starts. 

Core:

Cores are required to create the castings with holes. It can be made of refractory materials. Most often core sand is used to make it. Metal cores are also available but less frequently used. 

Chaplets:

Chaplets are the supports for the cores. These are needed particularly when the cores are very big. Usually metal pieces are used to support the core. Without chaplets the core can be displaced and the casting can be spoiled. These chaplets are set-up between the core and mold surface. Caution should be taken while placing chaplets. Clean, oil and moisture free pieces should be used as chaplets. 

Chills:

These are huge metal pieces used to reduce the effect of shrinkage. These increases the thermal conductivity and heat capacity. It helps in speeding up the cooling process. So thick metal parts are cooled quickly. They can be used along with the risers. 

Please read : 

• A complete guide to casting patterns 
• Sand Casting 
• Die Casting 
• Centrifugal casting 
• Basic Steps of Sand casting 
• Different types of pattern materials
• Complete guide of pattern allowances in casting process 
• Different types of casting defects  
• Properties of various natural and synthetic molding sands
• Different types of molding sand
• Advantages and Disadvantages of Different Sand Casting Processes 

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Lost Wax or Investment Casting Method Steps

Investment or lost-wax castings:

The Lost-wax method, sometimes, also called simply as precision-investment casting has been used for many years by jewelers and dentists. Since world-war-II, the method has been adapted to the production of small and precise, industrial fasting. Basically, the method involves the use of expendable (heat disposable) pattern unrounded with a shell of refractory material to form the casting mould.  Example: Piston making.
Cast parts are formed by putting the molten metal in the cavity. The wax pattern is melted and it is not reusable because of that the method is named "lost - wax method".

Follow :

• A complete guide to casting patterns 
• Sand Casting 
• Die Casting 
• Centrifugal casting 

 The steps involved in this Lost Wax Method  method are explained below:

• Wax injection
• Assembly
• Shell building
• Dewax
• Conventional casting
• Knockout
• Cut-off
• Finished castings

   (a) Wax patterns are usually made by injection molding



   (b) Multiple Patterns are assembled in central wax sprue .



(c) The assembly is immersed in a liquid ceramic slurry. After that this structure is dipped into a bed of very fine sand. More than one layer of sand may be necessary.



(d) The ceramic becomes dry. The wax is then melted. To melt all the wax ceramic is fired.

(e) Simple gravity pouring is used for filling the mould. After solidification all the parts of the ceramic structure become casting. Hollow castings can be made by pouring out the extra metal before solidification.

(f) The metal cools to room temperature and after that the ceramic structure is broken off by water blasting or by applying vibration. 

(g) The parts are separated by the high speed friction saw. Minor finishing is given to the parts. 

Please Read :

• Basic Steps of Sand casting 
• Different types of pattern materials
• Complete guide of pattern allowances in casting process 

Advantages of Investment Casting Method:

1. Intricate details can be cast.
2. Undercuts and other shapes, which would not allow the withdrawal of a normal pattern, are easily obtained.
3. The surface is very smooth and there is no parting line.
4. High accuracy can be obtained so that much of complicated and costly machining can be eliminated.
5. Unmachinable alloys can be cast.
6. More than one casting can be made at a time.
7. It can be reused.
8. Preferred for casting weight less

Limitations of Lost Wax Casting :

1. It is involved and thus expensive
2. It has limitations in use of and location of holes
3. The parts are limited in size to a few kg

Application:

1. Parts for aerospace industry
2. Parts for computers
3. Parts for food and beverage
4. Costume jewellery

Please Read : 

• Different types of casting defects  
• Properties of various natural and synthetic molding sands
• Different types of molding sand
• Advantages and Disadvantages of Different Sand Casting Processes 

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Die Casting Principles

Die casting process is very different from sand casting as the mould used here is permanent. The mould is not expandable and need not to be broken after the cooling and solidification. Here the mould is called the die. 

Please Read : 

Comparison of different casting process 

A complete Guide to casting process 

Gravity die casting:

When the molten is metal is poured under a gravity head into the permanent mould it is called gravity die casting. It is also known as permanent mould casting. 

Gravity Die Casting properties:

(a)    It uses permanent mould and cavity. 

(b)   Accurately machined. 

(c)    Decent thermal conductivity 

Applications of gravity die casting 

These are used to make precision aluminum precision casting parts, machine parts, motor parts, heavy metal parts. 

Pressure die casting:

When molten metal is injected into the permanent metallic mould by the means of the external pressure it is known as die casting or pressure die casting. In this process the liquid metal is forced into the permanent die. 

Applications of pressure die casting  

Cast parts obtained from pressure die casting are used in automobiles , electrical equipment, motors, machine parts, telecommunication equipment, building materials, auto ancillary and many other sectors. Home appliances and children's toys also use pressure die cast parts . 

Please Read : 

Sand Casting Process 

Centrifugal Casting Process 

Casting Defects 

Types of Patterns used in casting 

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Sand Casting Process with Diagram

Sand casting:

In sand casting which is also known as sand molded casting, an object is produced by sand mold. The process involves pouring of the molten metal in to the mold cavity. The molten metal is then cooled to the room temperature. The metal is solidified. After cooling, the metal object is separated from the mold.  Sand casting process has its advantages and disadvantages. So care should be taken while making delicate products. 

The six  main steps in this process are - 

• Pattern making and placing it the sand mold. 
• Making a proper gating system in the mold. 
• Removing the pattern. 
• Pouring the molten metal in the mold cavity. 
• Cooling the mold to the room temperature. 
• Breaking the mold and getting the casting. 

Please read : A complete guide to steps involved in casting process 

Before making the mold the properties of the molding sand should be checked.  There are different types of molding sands. Each type of sand is used in molding according to the cast materials and application of the materials. Mainly Green or Natural and Synthetic Sands are used. 

Another important factor is the pattern. Different types of patterns are available . Before making patterns these thing should be kept in mind. 

• Pattern material. 
• Pattern Allowances 
• Cost of making pattern. 

Production steps in sand castings:

Figure: Steps involved in sand casting. This production sequence involves casting operation and pattern and mold making. 

After removing the casting engineers should look for the casting defects. If the defects are there then it should be identified. 

Please read 

• Centrifugal Casting advantages and disadvantages 
• Basic Steps of Casting 

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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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Different Types of Pattern Allowances in Casting

Pattern allowances:

There are some allowances which are responsible for the difference in the dimensions of the casting and the pattern. These allowances are considered when a pattern is designed for casting. In this article we will discuss those allowances -

1.     Shrinkage allowance:

After solidification of the metal from further cooling (room temp.) dimensions  of the patterns increases. So pattern size is bigger than that of the finished cast products. This is known as shrinkage allowance. 

It depends on:

a)      Dimensions of casting

b)      Design and intricacy of casting

c)       Resistance of mol to shrinkage

d)      Molding materials used

e)      Method of molding used

f)       Pouring temp of the molten metal

2.     Draft or taper allowance:

Pattern draft is the taper placed on the pattern surfaces that are parallel to the direction in which the pattern is withdrawn from the mould (that is perpendicular to the parting plane), to allow removal of the pattern without damaging the mould cavity.

It depends on:

a)      the method of molding

b)      the sand mixture used

c)       the design (shape and length of the vertical side of the pattern)

d)      economic restrictions imposed on the casting

e)      intricacy of the pattern

3.     Distortion allowance:

This allowance is taken into consideration when casting products of irregular shapes. When these are cooled they are distorted due to metal shrinkage.

4.     Finishing or machining allowance:

Machining allowance or finish allowance indicates how much larger the rough casting should be over the finished casting to allow sufficient material to insure that machining will "clean up" the surfaces.

This machining allowance is added to all surfaces that are to be machined. Machining allowance is larger for hand molding as compared to machine molding.

It depends on:

a)      Machining operation

b)      Characteristics of metal

c)       Methods of castings

d)      Size, shapes and volumes of castings

e)      Degree of finish required in castings

f)       configuration of the casting

5.     Shaking or rapping allowance:

To take the pattern out of the mould cavity it is slightly rapped to detach it from the mould cavity. So the cavity is increased a little.  

Please Read :

• Different types of casting defects 
• What is casting ? Main application of castings 
• different types of patterns that are used on the sand casting process. 
• different types of molding sand
• properties of various natural and synthetic molding sands
• Comparison of different casting processes. 
• Advantages and disadvantages of centrifugal castings.  
• Different Materials Used for Patterns 

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What is Brazing-Principles

Discovering Brazing (Braze Welding)


Brazing is a gas welding process in which you join metals by using heat that surpasses the 800 degree Fahrenheit mark and a nonferrous (iron-free) filler rod with a melting temperature below that of the base materials. The most important point about brazing is that you can use it to join dissimilar metals — cast iron to steel, brass to steel, or copper to steel, just to name a few examples.

Keeping a few brazing rules in mind
A successful brazing job requires that you stick to a few rules, as follows:

✓ The surfaces of the metals must be free of contaminants. Use steel wool to clean off all the metals, and use flux (a material that dissolves or removes oxides and other contaminants from the surface during brazing) for additional cleaning when the surfaces are heated.
✓ The joint to be brazed must have a tight fit. You use two basic joints for brazing: the butt joint (two pieces of material lying together on the same plane) and the lap joint (two pieces of material overlapping each other, usually in a parallel plane). If the joint has sizeable gap, brazing just doesn’t work. But if the joint is too tight, the melted filler rod can’t penetrate the entire joint, and you get a weak, ineffective weld.
✓ The base metals you’re brazing must remain stationary during the
heating and cooling process. If the base metals move while you’re welding or before everything has cooled off after welding, the joint’s integrity will likely be compromised


Giving brazing a try
If you’re trying brazing for the first time, I recommend going with an oxyacetylene torch in the flat position and using the forehand method I discuss in “Making the weld” earlier in the chapter. You can start by creating a brazed corncob, which is a piece created when you join two different metals,
following the steps in this section.

Brazing principles 

1. Acquiring a section of carbon steel pipe 1 inch in diameter and five inches long, as well as four 1⁄8-inch fluxed brazing filler rods that are 36 inches long.

2. Clean the pipe thoroughly with steel wool to remove any contaminants on the surface.

3. Lay the pipe between two fire bricks, leaving a 3⁄4-inch space between the bricks.
Fire bricks are special bricks that can withstand extremely high temperatures; find them at your welding supply or hardware store.
4. Set up and light the oxyacetylene torch, using the steps I describe in “Working through the Basics of Welding with Gas” earlier in this chapter, and adjust the torch so you have a neutral flame.
5. Preheat the pipe to burn off any grease or varnish that may be left on
the surface.
6. If you’re right-handed, start on the right end of the pipe and melt off a small portion of the filler rod onto the end of the pipe. If you’re left-handed, start on the left end of the pipe. The molten puddle should be very fluid. Be sure that when you use more of the filler rod, you let only the molten puddle (not the flame from the torch) melt the rod. If you notice white smoke coming from your molten puddle, that means you’re burning the zinc out of your filler rod, which will result in a poor
weld. Avoid that problem by welding at a lower temperature or moving more quickly with the molten puddle. When you’re practicing the brazing process, you can quench the metal between passes with the torch. Use pliers to pick up the section of pipe you’re practicing on and place it in a tank of water to cool it very quickly. Quenching isn’t good for the integrity of the brazed weld, though, so don’t quench unless you’re just practicing.

7. After you make your first pass with the torch, start a second pass by pointing the tip of the flame at the edge of the previous pass; when part of the first braze starts to melt, add some of your filler rod and proceed. This pass laps over the first braze bead 1⁄3 to 1⁄5.

8. When you’ve completed your second pass, quench the welded metal (or allow it to cool) and go ahead and start on a third pass. When you’re finished with the third pass, you should have a finished
product that resembles the brazed corncob in Figure 13-3. For some brazing projects, you may need to make even more passes. (For example, projects involving thick pieces of metal definitely require more than three passes.)

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 
• Aluminothermic or Thermit Welding Principles  
• Submrged Arc Welding 
• Flux shielded metal arc welding process principles 
• Resistance Projection Welding 
• Resistance Spot Welding 
• Resistance Seam Welding 
• TIG Aluminum Welding Advantages  made easy !
• Advanced TIG welding operations 

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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 

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Match Plate Pattern - Used in Casting (With diagram)

What is Match - Plate Pattern 

The match plate pattern is almost similar to the mounted pattern but it can have part of the casting in the cope and part in the drag like split piece pattern. These parts are generally attached to the plate / board in opposite sides in the perfect positions. When the plate is removed and mold is in close position the cavities in the cop and drag match perfectly. The mounting procedure matches the procedure of one sided mounted plate.

schematic of match plate pattern

Characteristics of Match Plate Pattern 

• It is split pattern 
• Cop and Drags on the opposite site of the metallic (generally) plate. 
• The gates are runners are on the match plate. 
• Can be used for large number of casting with very little hand work. 
• A match plate can be single pattern or a combination of many small patterns
• Example : IC engine piston rings can be produced by match plate pattern 

Important parts of Match Plate Patterns  

Cope and Drag Mounts

• Two separate pattern mounts are there.
• One is fitted to the female guides (for drag) other is fitted with pins (for cope). This must match the profile of the flask that is used.
• The half part of the pattern associated by the cope is attached to the cope mount and drag part pattern is attached to the drag mount.
• Cope and drag portions are manufactured disparately and combined together for pouring.
• Generally one molder makes copes other makes drags.
• Cope and drag mounts are also known as tubes.
• These (cope and drag mounts) are very similar when producing large castings. 

cope and drag mounts in match plate pattern 

Follow board

• It is a board with a cavity or socket.
• The cavity must conform to the shape of the pattern.
• It also defines the parting surface of the drag.
• Materials: It can be made of plaster or metal. If sand is used to manufacture follow board then it is called dry sand match.
• When the drag half of the mold is made the pattern rests on the follow board.
• It creates the correct sand parting.
• Pattern is rammed in the drag and follow board is removed.
• The follow board is replaced by the cope.
• A simple follow board may have a hole in it so that the pattern rests firmly when the drag is being rammed.   

follow board

simple follow board

Please Read :

• Different types of casting defects 
• What is casting ? Main application of castings 
• different types of patterns that are used on the sand casting process. 
• different types of molding sand
• properties of various natural and synthetic molding sands
• Comparison of different casting processes. 
• Advantages and disadvantages of centrifugal castings.  

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