Archive for the ‘MANUFACTURING PROCESS’ category

Compressor Types

September 21, 2012

Contents

    • Piston Compressors
    • Screw Compressors
    • Dynamic Compressors
    • Air Treatment
    • Air Lubrication
  • Pressure Regulation

The majority of air compressors work on positive displacement, this means a fixed volume of air is delivered from each rotation of the compressor.

Piston Compressors
Piston compressors are the most common form of compressor for compressing air, some smaller compressors use a single cylinder compressing air in one direction of its stroke, this type of compression method can however cause an uneven air supply, this can be due to pressure being lost through use of the air supply while the piston is moving back drawing in more air. The double-acting construction (shown below) uses both sides of the piston and compresses on both strokes during one revolution, this gives a smoother air supply.

Double Acting Compressor

The above image shows a combined two stage compressor which is fine as it is, however when high pressures are required from a compressor set up like this then the compressed air temperature can rise to over 200°C and motor power needed to drive the compressor rises with this temperature.
So for these higher desired pressures it is far more economical to include a cooling system between each stage, this would then be known as a multistage compressor. These cooling systems are commonly known as intercoolers which work by drawing the hot compressed air over a very large surface area where the heat can dissipate quickly, this cooled air will then enter the second stage compression to be compressed even further by the last piston.

Two-stage compressor
Screw Compressor

Click for Larger Image

Screw compressors

Screw compressors are best suited when only medium pressures are required (<10 bar).
Screw compressors offer a steady flow of air without pulses and fluctuating pressure that can be associated with piston compressors, they also offer a simple system with fewer moving parts which equates to a more reliable system with less maintenance involved.

Screw compressors work on the basis of two intermeshing rotating screws with minimal clearance between each screw fin, they draw in air and squeeze it up along the screw with air getting compressed tighter the further along the screw its drawn until it reaches the discharge port, it is here that it is delivered out of the compressor.

The two intermeshed screws can be synchronised by external timing gears meaning that the screws should remain close but without touching each other. The other version is known as wet rotary screw compression, this is where the first screw is driven by a motor while the second screw isn’t, the second screw is rotated by being in physical contact with the first. This requires oil to be sprayed in with the air to relieve friction and wear; some of this lubricant will make its way into the air system so it is removed by later oil separation units.
Dynamic Compressors

Dynamic compressors are capable of delivering large volumes of air but little pressure (0.5-3bar). These compressors are usually known as blowers and work by drawing air in and throwing it out with the use of rotary fins, these fins rotate at very high speed.

There are 2 main types of dynamic compressor, they are centrifugal and axial.
Centrifugal systems use centrifugal force to hurl air out from the fins, centrifugal systems can generally obtain greater pressures than the axial type compressor.

Axial type compressor use a set of fan blades in line to generate large air flow, pressures from this method aren’t expected to reach much over 0.5bar. The axial compressors are largely used for ventilation and as part of air processing.

Axial Compressor

Axial Compressor

Air Saturation

The air we breathe from our atmosphere contains moisture in the form of water vapour; people often refer the amount of moisture in the air as levels of ‘humidity’. Humidity levels are a percentage of the saturation of air with water.
The maximum amount of water saturation in air varies greatly with temperature; hotter air can hold much more moisture than cold air. Due to the change in air pressure when compressing air its ability to hold moisture is seriously reduced, so some of the moisture content that was previously in the air will simply condense out of it when the air is compressed. This is why drainage systems and dryers are an important part of compressor systems.
If the water was allowed to travel out of the compressor into the pneumatic components that it was powering then this could cause big problems, for example ceasing valves and actuators.

Air Saturation

Relative Humidity = Mass of water vapour present           X 100
Mass of water needed to saturate it

Stages of air treatment

Filters
Compressed air requires filtration and drying before it leaves for pneumatic components, atmospheric air carries many particles of debris and moisture that unless filtered out can block valves or cause increased wear. Compressors usually filter air before its compressed, this is mainly to remove relatively large particles which could damage the compressor. A fine grade filter used after it is compressed helps filter the small particles which could clog or damage the pneumatic components that are used in pneumatic system set ups.

Air dryers
Before air can be used the excess moisture has to be removed, where well dried air isn’t a requirement all that may be needed is a basic intercooler followed by a separator unit; this is where the condensed water collects and is drained off.

Refrigeration units
The dew point can be lowered even further by cooling the air with a refrigeration unit which cools the air even further to around 0°C helping more moisture collect in the separator tank. This cold air is then routed through a separate section of the initial heat exchanger to help cool the warm air which is flowing through from the compressor.
This system of drying is suitable for most systems as air is dried out well.

Deliquescent dryer
Where completely dry air is a necessity then a chemical dryer should be used, the chemicals can remove moisture in two ways.
1) A chemical agent called a desiccant is used, this chemical agent collects water vapour and holds it while it eventually turns itself into a liquid once so much vapour has been collected and runs to the bottom of the unit where it is drained off. This chemical agent needs to be replaced quite regularly.

Absorption dryer
2) Within an absorption dryer a material called silicone dioxide or copper sulphate is used, this process works by attracting moisture to the sharp edges of these granular materials. Once these materials become saturated they are dried by passing heat through them, this process cant be carried out while it is still drying incoming air so they are usually set up with 2 of these moisture collectors which can be swapped over instantly, this allows one collector to collect the moisture while the remaining collector is left to dry itself out.
Oil is often added to the air to lubricate moving parts, the oil is released as a fine mist into well dried air, if the air is poorly dried then the oil will mix with the moisture and become tacky which will cause problems sticking valves ect.

Lubricator for compressed air systems

The oil is drawn up from the oil reservoir due to the way the air flows through the lubricator; a pressure difference between the lower and upper chambers is created as the air is channelled down past the oil mist opening. The screw valve is used to adjust this pressure difference.

Pressure Regulation

Pressure regulation in pneumatics is vital for the correct operation of circuits and for damage prevention to circuit components. As you would imagine all pneumatic components have a maximum operating pressure.

Relief valves
These are basic pressure regulators and are usually used in pneumatic circuits as a backup device should the main pressure controller fail. They work with the use of a ball valve which is under an adjustable amount of pressure to keep the valve shut, once this pressure is exceeded the ball forces the spring back allowing the excess pressure to escape. Care should be taken when employing a pressure relief valve, this is because the valve must have sufficient ability to vent the pressure and flow quickly enough should a fault occur, and should the worst happen the relief valve may have to dump the entire compressor output.
Once a fault has triggered the release mechanism and the pressure has dropped sufficiently then the valve automatically closes and re-seals itself. This type of valve is always ready to release pressure again if needed, however a safety valve must be manually reset once it has cracked to release pressure.

Non-relieving pressure regulators
Non-relieving pressure regulators work by restricting flow rather then venting it should over pressure occur. The regulator restricts flow when the pressure gets too high because the pressure acts on the diaphragm forcing it up against the spring pressure, the diaphragm has what is called a ‘poppet’ attached on the end of it which is drawn up with the diaphragm and restricts the passing air flow.

Non-Relieving Pressure Regulator (Pneumatics)

Relieving pressure regulators
Relieving pressure regulators use a similar diaphragm system to that of the non-relieving regulator. The diaphragm drops if the outlet pressure is too high closing the inlet valve and opening the outlet vent, releasing pressure until it falls back to the preset pressure.

Relieving pressure regulator

Drop Forging – Introduction

September 21, 2012

Contents

    • Introduction

What is drop forging?
Drop forging is a metal shaping process, the metal to be formed is first heated then shaped by forcing it into the contours of a die, this force can be in excess of 2000 tons. The drop forging process can be performed with the material at various temperatures;

  • Hot ForgingDuring hot forging the metals are heated to above their recrystallization temperature. The main benefit of this hot forging is that work hardening is prevented due to the recrystallization of the metal as it begins to cool.
  • Cold ForgingCold Forging is generally performed with metal at room temperature below the the recrystallization temperature. Cold forging typically work hardens the metal.

There are two types of drop forging, open die and closed die.
Open die drop forging requires the operator to position the work piece while it is impacted by the ram. The die attached to the ram is usually flat or of a simple contour, most of the shaping is achieved by the operator physically positioning the work piece before each stroke of the ram. There are also special dies which can be used to cut the metal, form holes or notches. see more

Closed die drop forging comprises of a die on the anvil which resembles a mould, the ram which falls and strikes the top of the metal billet can also be equipped with a die. The heated metal billet is placed on the lower die while the ram drives down forcing the metal to fill the contours of the die blocks. see more

 

Drop Forging Diagram
Diagram of basic drop forging set up

Investment casting

September 21, 2012

Investment casting is one of the oldest known metal forming techniques, dating back over 5’000 years ago when beeswax was used to create the pattern. Beeswax back then didn’t allow the accuracy and intricate shapes we can produce today. Investment castings are used in a huge array of items, golf clubs are investment cast, aeroplanes use investment cast parts as do cars and other motor vehicles.

Creating The Mould

To begin the process wax patterns are made by injecting hot molten wax into an aluminium die, this sets the wax pattern to the exact size and shape of the required part. Many of these wax moulds are attached to a wax sprue which forms a stem linking all of the individual moulds together.
Sprue

Once the sprues are filled with the mould attachments they are dipped into a cleaning bath to ensure the future layers of shell cling to the mould profile correctly. Before the first layer of the shell is added the sprue assembly is dipped into a bath of slurry, this will form the bonding agent to the layer of ceramic powder which is added in either a rainfall sander or a fluidised sand bed, this process gradually builds up a ceramic shell around the moulds and sprue, with progressively coarser layers of ceramic coating being added over several coats until the desired shell thickness is achieved.

After the ceramic thickness is achieved the sprue along with its moulds are oven baked to both harden the ceramic shell and to melt all of the wax from within it, this will include all of the wax moulds and the wax sprue leaving a hollow ceramic shell. The wax moulds once melted leave a perfect void of the desired mould to be cast, the sprue once melted leaves open channels for the liquid metal to flow and fill all of the hollow shell (see image below). The molten metals are poured into the now hollow ceramic shell via the filling cup and left to cool off. Once the metal has cooled sufficiently the ceramic shell is broken up by either being submitted to vibrations or a water jet.

Investment Casting Shell

Advantages

Investment casting offers a number of advantages, the first being the excellent surface finish achievable, although of course the mould itself must first have these finishes. Investment casting leaves no parting or flash lines and the high quality finishes mean that the expense of another machine process to clean up surface finish is rarely required.

High dimensional accuracy is achievable which again can save a big cost on later machining to bring parts within the required tolerances.

Almost any metal can be investment cast and there is very little material waste from the process, any defect parts can usually be melted down and re used.

Investment casting has the potential to create extremely intricate parts; the molten metal will run and fill the mould very neatly, and because the castings mould is broken up to be retrieved there is little constraint to the shape of a casting.

Disadvantages

The creation of a mould can be fairly expensive and time consuming, a lot of labour is also required throughout the rest of the casting process. Occasional defects do occur for example air pockets within the casting.

Welding Types

September 21, 2012

MIG (Metal Inert Gas)

MIG welding is one of the more simple welding methods as it does not require high skill to achieve results. The process is semi automatic because an electrode wire and gas is automatically fed through the gun at a user defined speed or pressure when the operator pulls down the trigger, the electronic arc can also be user defined and carried out automatically on operation. MIG is a quick and easy form of welding, it is used often by robotics in automated production lines. MIG however does not offer the best penetration when welding therefore is not the best choice for use on thick plates where a strong weld is required.

Mig Welder

Mig Welding Content Menu

 Above: Mig Welder

 

MMA (Manual Metal Arc)

This is a manual form of welding that requires a good level of skill to be fully utilized. The welding gun holds what are called welding rods and these come in various sizes which determine how large of a weld will be put down. These rods are a consumable electrode coated in flux, which when a current flows through will melt and vaporize the flux which acts as a gas shield for the weld as it is being laid. MMA can be used on a range of metals such as iron, steels, stainless steel, and can even be used on aluminium, and copper alloys.

Arc Welder

 Above: Arc Welder

 

TIG (Tungsten Inert Gas)

TIG is a manual welding process where by a Tungsten electrode is used to produce the weld along with a shielding gas and often a metal filler. TIG is best used for thin sections of metals such as stainless steel, copper alloys, and aluminium. TIG is also much slower than MIG and MMA, it also requires a higher skill to master it. TIG Welds are generally very high quality and allows the welder greater control of the weld.

Water Jet Cutting

September 21, 2012
Water Jet Cutter Diagram

 

What is it?

A water jet cutter is a tool capable of slicing into metal or other materials using a jet of water at a very high velocity and pressure, the water can be mixed with other abrasive particles to aid the erosion of the metal. These abrasive particles can be in the form of suspended grit or aluminium oxide. To get the pressure used into context the pressure fired out the end of the nozzle in a water jet cutter is around 30 times more than a car pressure washer.

 

What can it do?

Water jet cutting can be very versatile in its potential, e.g. it can be used to cut things such as fish sticks all the way to titanium, however some tempered glass cannot be water jet cut, some ceramics also have this problem, while diamond is just too tough.
The cut kerf (width) can be adjusted by changing the nozzle parts along with the abrasive grain size.
The typical cutting jet size when using abrasive particles is around 1mm-1.3mm, although this can be reduced to almost 0.5mm. When cutting without the use of abrasive particles then the jet size can be further reduced to around 0.08mm.

 

How fast does a water jet cut?

An abrasive jet can cut half-inch thick titanium at the rate of 7 inches per minute when a 30 HP pump is used. The abrasive jet moves in a manner very similar to a slowed-down pen plotter.

 

Advantages of water jet cutting

Water jet cutting does not apply heat to the material it is cutting, therefore there is no possibility of work hardening happening.
Water jet cutters are also capable of producing rather intricate cuts in material. With specialized software and 3-D machining heads, complex 3-D shapes can be produced.
Water jets are capable of achieving an accuracy of 0.13 mm

Water jet cutting is safer for operators and the environment – avoids vapour, dust and smoke and does not require expensive coolants
Clean finished product eliminates secondary cleaning operations

 

Disadvantages of water jet cutting

Cutting hard metals such as tool steel seriously affects the cutting speed, therefore water jet cutting is not an efficient method of machining these hardened materials.
Cutting thick materials results in a taper down the cut resulting from the water jet widening as it get further away from the nozzle. This means dimensional accuracy in thick cuts becomes a problem.

Plastic injection moulding

September 21, 2012

Plastic injection moulding is a hugely common method of plastic forming, it is used in a wide variety of objects you see and use in day to day life, things like shampoo bottles, hair brush handles, children’s toys, car dashboards, games consoles. Plastic injection moulding is more efficient when producing parts with thicknesses no larger than around 3mm, this is because thicker parts take longer to cool off before they are released from the mould. Thicker walls in moulded parts also risk having problems with material shrinkage, to combat this when wall strength is required the use of ribs or fillets are implemented.

Plastic Ribs

Plastic Injection Mould

Above shows the use of ribs to help strengthen the plastic structure.  The inside face of a plastic injection mould

Moulds

Moulds have two plates, the injection mould and the ejector mould, plastic resin is injected under pressure into the mould cavity via runners or channels, these runners are grooves machined out of the mould which allow the plastic resin to be directed to points throughout the mould where it can enter and fill the cavity. Small air vents are machined into the moulds to help air escape, however if air became trapped within the mould it would prevent plastic resin flow from filling its space causing a defect. Once the mould cavity is filled the plastic is usually helped to cool off with a coolant which runs through machined channels within the mould plate.

Advantages of injection moulding

Injection moulding is an efficient means of mass producing parts to the same high tolerances over and over. Injection moulding is not labour intensive and little waste is produced from the process as there is little in the way of off-cuts. The moulding process leaves a good finish therefore further finishing is not always required.

Disadvantages of injection moulding

The initial set up cost is high, this is because all of the required moulds must be produced first, also all parts must be designed with the ability to be moulded in mind.

Injection moulding machine

Laser cutting

September 21, 2012

Laser cutter head

Laser Cutter Head Diagram

What is it laser cutting?

Laser cutting is a fairly new technology that allows metals and some non metallic materials to be cut with extreme precision if required. The laser beam is typically 0.2 mm in diameter with a power of 1-2 kW .

Types of laser cutting

Depending on the application of the laser cutter a selection of different gases are used in conjunction with the cutting. For general boring, cutting and engraving then Co2 is typically used.
If high powered laser pulses are used then neodymium (Nd) gas is required, this set up is mainly associated with boring. For a constant high powered beam neodymium yttrium-aluminium-garnet (Nd-YAG) is used.

Gaseous laser cutting uses an electrical current pumped through the gas which gives the laser its cutting properties, however this has recently been revised and RF energy is now preferred as this method does not require the use of electrodes like the DC current does. These electrodes were susceptible to erosion

What can it do?

Laser cutting can cut through a wide range of different materials, these can range from acrylic, wood, paper and foam core to high carbon and stainless steels, Laser cutting is not best suited to metals such as aluminium and copper alloys as they have good heat conductive and light reflective properties, these materials require the use of a more powerful laser. Laser cutters are generally best suited to thin materials of <12mm.

Advantages

Contamination of materials while laser cutting is reduced as there is no real physical contact between metal and cutter.

Great accuracy as laser can be focused into very small points, and there is also no wear in a laser while it is cutting as there is with more conventional methods, such as milling.

There is also a reduced chance of warping the material when laser cutting as the laser only generates a small area of heat.

No mechanical force is applied therefore no physical damage can occur.

Disadvantages

Laser cutting has high energy consumption, and can draw a lot of power to perform its cutting. Although it uses a large amount of power it goes some way to making up for this cost with its fast and precise cutting speed. The cost and setup of a laser cutter can also be expensive when compared to other methods.

Work hardening along the edges of cuts can mean harder work if any further machining is required.