Processes

Question 0

Overview of casting:

Answer 0

Permanent mould - continuous casting, gravity die casting, pressure die casting, centrifugal casting, reaction injection moulding, injection moulding, rotational moulding

Permanent pattern - sand casting or shell moulding

Expendable mould and pattern - investment casting, ceramic moulding, full evaporative pattern casting

Question 1

Advantages and disadvantages of machining:

Answer 1

Advantages: versatile, simple and easy to implement

Disadvantages: high amount of waste

Question 2

Overview of Injection moulding:

Answer 2

Discontinuos process capable of generating 3d parts. Unlike extrusion, this has idle times and feed all material into mould, not sizing die.

Small (10^-6) to big (10^2) parts can be made. Mould is filled via gates and runners. Runners are cut via robots in high production, otherwise hand cut. Screw acts as piston.

Parts are light, can be made in a variety of colours, and have good dimensional accuracy. Low stiffness unless reinforced through design features such as ribs.

Advantages: high productivity

Disadvantages: expensive tooling and machines

Question 3

What are the steps involved in injection moulding?

Answer 3

1. The mould is closed while the polymer is melted by mechanical shearing of the screw and thermal energy of the heaters. The melt polymer is pushed in front of the screw.
2. The mould is filled with polymer through the axial movement of the screw. Any back flow is stopped by a check valve.
3. The mould remains under pressure to ensure that the polymer completely fills the mould.
4. The injection unit then retracts.
5. The screw recharges with another shot of plastic while the whole unit is retracting. Coolant cools and solidifies the plastic in the mould.
6. The mould opens and the part is ejected via ejector pins on an ejector bar.

Question 4

What is the process capability of Injection moulding?

Answer 4

Mass production - over 10,000 parts

High initial costs

High production rates - 1-50 parts/min

Low scrap rates

Can all be automated

Limited flexibility once invested in mould. But flexibility can be enhanced by using inserted cavities

Low labour costs

High capability to recycle materials/scraps

If constant temperature and pressure are achieved, and the mould is cooled properly, injection moulding won’t go wrong.

Question 5

Overview of cutting:

Answer 5

Electromachining - electrochemical machining or electrical discharge machining

Mechanical machining - single point cutting, multiple point cutting, grinding

Question 6

What considerations need to be taken into account when cutting sheet metal?

Answer 6

The punch needs to put enough stress on the metal to change it from elastic to plastic properties.

When the blank is the required part, the diameter of the hole is S and the size of the punch is S-2(delta), where delta is the clearance on either side of the punch. When the blank is scrap, the size of the hole is S+2(delta) and the punch is S.

When cutting is not balanced, a hold down plate is needed.

Question 7

What is cutoff cutting?

Answer 7

Cutting that is done along a line. The blanks have to fit and nest into the original strip of metal. Uses a hold down plate, as only one edge is cut.

Question 8

What is parting?

Answer 8

A method of cutting blanks that do not nest perfectly in the metal strip. Repetitive scrap forms are generated as scrap is what is punched. This is less effective at material utilisation than cutoff method. These cuts are done via open contours and a punch, therefore no hold down plate is required.

Question 9

What is blanking?

Answer 9

Similar to parting, except the blank is punched out, and the remaining strip is the scrap. This results in more scrap metal. The blank is a complete or enclosed shape punched out by the punch. Has a stripper plate to hold the material down so it is not pulled up by punch, which acts as an ejector.

Question 9

How to bend sheet metal?

Answer 9

Opposed forces are placed apart at a bigger distance than in shearing (cutting). Here the clearance will be equal to the thickness of the metal plus the radii of the punch and the die to allow for stretching, rather than just being a fraction of it.

This allows plastic deformation without damage.

Bending involves stressing in tension the outer region of material and stressing in compression the inner part of the material. The neutral axis is where the stresses change from tension to compression. The material is only stressed locally.

The neutral axis is at a distance (approx. 0.4xthickness of material, depending on radius length) from the inside surface at the bend, otherwise is at 0.5xthickness. Neutral axis gets closer to inner surface when bending radius increases while sheet thickness does not, sheet thickness increases while bending radius does not, or bending degree increases while sheet thickness and bending radius stay the same.

Question 10

How do you calculate the length of a bend?

Answer 10

Lb = (bending angle/360) x 2pi x (radius of inner bend x neutral axis distance from inner surface)

The error will be tiny the closer the neutral axis is to 0.5t. In thicker sheet metals, the error may become an issue.

Question 10

What are spring backs, and how can they be avoided?

Answer 10

Spring backs happen when the metal has only been elastically deformed and not plasticity deformed. Ie. Not sufficiently deformed.

The higher the steel strength, the higher the elastic limit, and therefore the higher the spring back.

Sharp/small bending radii have a higher plastic deformation, and therefore have lower spring back.

The higher the bending angle, the higher the plastic deformation, and therefore the lower the spring back.

Thicker material have higher plastic deformations, and thus lower spring back.

There are two ways to reduce the spring back effect:

1. By bottoming - using a bead at the end of the V-die punch to hit the radius area. The material will be plasticity deformed at high volume in the radius area.
2. Overbending - a V-die can be made at a present angle (eg. 88 degrees), so that the part springs back to 90 degrees when released

Question 11

What is flanging?

Answer 11

Flanging differs from bending as the dimension of the flange is much smaller than the overall dimensions of the part. This is used for increasing part stiffness, rounding sharp edges, and easy assemblies.

Looks like a lip, can be wavy, straight, curved, hole flange, stepped, etc.

Question 12

What is hemming?

Answer 12

A hem is a fold at the edge of a sheet of metal. It is used to improve the edge quality, to stiffen the edges, and to hide burs.

Three types of hemming exist: flattened hem, open hem, and teardrop hem. These can be used in three ways: as a simple edge hem, as a single hem assembly, where a bent part is connected under the open hem of another part, and lastly a double hem assembly or lockseam, where two parts are assembled, both with open hems.

Question 13

What is curling and wiring?

Answer 13

Wiring is when an edge of metal is curled over the length of a wore. Curling and wiring strengthen the edge of the part.

False wiring or curling is when the the edge is not completely curled over.

Curled edges of different parts can be assembled via double curl assembly.

Question 13

What is corrugated?

Answer 13

Waves or ridges are put into a sheet of metal just like cardboard. It is applied mainly for improving the stiffness of the part.

Question 14

Overview of forming:

Answer 14

Sheet - sheet metal forming, vacuum forming, blow moulding, superplastic forming

Bulk - forging, rolling, extrusion, drawing

Powder processing - pressing and sinter one or isostatic pressing

Question 14

What three types of sheet metal bending are there?

Answer 14

V-die which symmetrically bends the sheet metal at required angle.

U-die which symmetrically bends the sheet metal down around a die to form a u cup shape.

Wiping die which assymetrically bends sheet metal at one end.

Question 18

What is extrusion moulding?

Answer 18

Continuous 2D shaping process (the extrusion is done in the third axis). Used for wires, films, pipes, etc. Constant cross section.

Plastic goes into hopper, then into feed and compression zone, screw rotated by mother moves plastic into platicizing zone where it is heated. Plastic is pushed into metering zone and then into die.

Question 19

What is extrusion blow moulding?

Answer 19

Used for small or medium sized enclosed symmetrical (bottles) and non-symmetrical (car fuel tanks) containers.

Discontinuos but low idle times.

Question 20

Overview of Flow forming:

Answer 20

The thicker part of a perform is plasticity deformed using a roller.

Little waste is produced and a medium precision can be achieved.

Question 21

What is the difference between extrusion blow moulding and injection blow moulding?

Answer 21

In injection blow moulding, the polymer is injected rather than extruded.

Question 23

Overview of Deep drawing:

Answer 23

A blank of metal is held by a retainer and pushed by a punch into a form. The wall thickness is approximately the same as that of the blank.

The sheet metal material flows, like that of a viscous fluid, over the edge of the die and into the cavity. The material has to have the right mechanical properties like strength and ductility, but also needs an equal surface quality, to accommodate the stretch.

Parts can be light and have good dimensional accuracy. Parts will not be stiff unless reinforced through design features such as corrugates and flanges.

Advantages: low skill required, faster production, complex tools can be made, medium production rates, automated, medium material utilisation, low labour costs, complex components can be produced by combining other operations with deep drawing

Disadvantage: expensive tooling and machinery (but medium cost can be achieved if use commercially available standardised tooling), complex parts require trimming which means more scrap

Question 24

What are the steps of the deep drawing process?

Answer 24

1. A flat circular blank, with the correct radius to avoid cutting, is placed on the die steel, under the punch. The punch gets in contact with the blank.
2. As the punch moves down, the metal is bent to the same radius as that of the punch. The metal on the die steel flat surface is pushed down and begins to bend over die radius.
3. The punch continues to move down. More metal is bent over the radius of the die. The material that was previously bent is straightened to form the cylinder of the future part.
4. The punch moves down the final time, the force of the punch is higher than the friction force between the metal and the flat surface of the die, otherwise metal will simply tear. When the material is compressed as forced into the die, it has a tenancy to wrinkle. To avoid this, a blank holder presses the blank to be drawn.

Question 25

What are potential consequences of Deep drawing?

Answer 25

There is tension between the internal wall of the die and the material, can cause up to 25% thinning, which can lead to tearing.

If the blank holder exerts too much force (Q) on the blank it can cause cracking. If the blank holder does not exert enough force it can result in wrinkling.

Wall wrinkling is when the sheet thickness cannot support the large dimension of the part, the material then looses stability and buckles.

Parts without flanges may suffer earing. This is caused by sheet anisotropy where the mechanical stress along the x axis is different to that in the y axis.

An orange peel effect is when the surface of the part appears opaque and rough due to the high levels of tension coupled with course metallurgical grain of the material.

Burnishing marks can appear if the material is heavily worked because the clearances between the punch and the die are too small or not well aligned.

Scoring happens when the lubricant is penetrated and the drawn material comes in contact with the die elements. This results in material welding that afterwards scores the part.

Lastly, miss-strike happens when the blank is not perfectly aligned with the punch and die.

Question 26

What is the difference between a deep drawing simple-action die and a deep drawing simple-action inverted die?

Answer 26

A simple-action die is for drawing flanged cavities. The die steel is composed of the die and a spring based striper which is lubricated for ejection. The blank holder is actuated by one ram movement of the punch and is called a spring pressure pad. As the pad is pressured by strings, it makes it suitable for light drawing. The Disadvantage is that different settings are needed for different forces for different material thicknesses. Because of the use of springs, the force of the punch is not constant.

The single-action inverted die is the opposite setup. The die is mounted on the ram, while the punch and pad are fixed (press bed). Here, the draw ring inserts can be replaced under the ram. There is no use of springs, meaning that the pressure can be controlled. There is an air vent that runs through the punch to let out leftover air.

Question 27

What is a double-action inverted die?

Answer 27

I don’t know why it is called inverted…

The press has two rams. The outer ram is connected to the blank holder and the inner ram is connected to the punch. The outer ram descends first, so that the blank holder presses the sheet. Then the punch moves down and the drawing process begins.

Also has replaceable die rings, but in the bottom half. Also has air vent. And has a delayed action kicked, topped by a kicker pad for ejection.

Question 28

How to avoid or reduce wrinkling when deep drawing?

Answer 28

Employing blank holders
Using thicker sheets of metal
Redesign the blanks to reduce their flange dimensions (a)
Use successive drawings (redrawings): first draw larger cup diameter which induces less compressive stress

Question 29

What is the role of lubrication in drawing?

Answer 29

The lubricants are thin layer films that enable separation between the die and the metal surfaces. They have to withstand high pressures (resistance) while maintaining low coefficients of friction.

Question 30

What kind of metals are used in deep drawing?

Answer 30

Metals with high strength and ductility (to allow material flow). They have to comfort to thickness tolerances as specified by standards, the surface quality needs to have uniform roughness with no scoring, and needs to specify the isotropy (uniform of crystalline/grain orientation).

These include low carbon steel, titanium, copper, aluminium alloys, magnesium, and stainless steel (austenistic).

Question 31

What optional post-processes can a deep drawn component go through?

Answer 31

Machining, joining, and assembly to enable functionality

Or polishing and coating to enhance appearance or resistance

Question 32

Overview of powder metallurgy:

Answer 32

Metal powders are compacted and then heat treated (for diffusion of material) to create required shape. Lot of force required to compact powders.

Can mix powders to create new alloys/composites. Powders can be sourced from solid-state reduction (ore) or atomization (molten metal) which pass through protective gas as can’t mix with air.

Powders are then mixed with lubricants such as graphite etc. to lower friction for when powder is compacted. This lubricant can usually be removed during sintering. Powders are mixed in v type or double cone mixers.

Filters (pourous) and fragile graphite parts. High strength parts such as cutting tools, small parts like surgery equipment, self-lubrication bearings, complex parts like cams and gears, magnets, electrical contacts, abrasive elements like grinding wheels

Advantages: produce part to or close to final dimension, uses around 97% of initial raw material, generate many alloys, good surface finishes, complex geometries, control porosity for self-lubrication or filtration, moderate to high volume production and cost effective

Disadvantages: raw powder material needs careful prep and so increases cost, expensive tooling and equipment, threads only possible in isostatic compaction, grooves holes and undercuts can not be produced perpendicular to pressing direction

Question 33

Flowchart of power metallurgy processes:

Answer 33

1a) metal powders
1b) additives (lubricants)

…|…

2) mixing

…|…

3a) hot compaction (the best compaction): isostatic, extrusion, die compacting, pressure less sintering
3b) warm compaction: die compacting, injection moulding
3c) cold compaction: die compacting, isostatic, rolling, injection moulding

…|…

4) only for 3b and 3c: sintering (like an oven): atmosphere, vacuum - this is when the materials are diffused together

…|…

5) for 3a and 4: optional processing: repressing/sizing, impregnation (improve strength)

Question 34

What is the sintering phase in powder metallurgy?

Answer 34

After compaction, the parts are heat treated to chemically bond the compressed particles.

Sintering temperature is usually below melting point and targets fraction of liquid over whole component mass. The more liquid phase, the higher the density of final component.

If temp is too high, the part will loose it’s shape. If temp is too low, not enough bonding takes place.

The smiller the particle, the more contact surface it has with other particles, resulting in higher diffusion. The less spherical in shape the particle is, the more contact surface it has with other particles, again resulting in higher diffusion.

Discontinuous sintering happens on a batch furnace, eg. Bell furnace, where all sintering happens in the same chamber.

Continuous sintering happens in a continuous furnace eg. Belt furnace, where the parts are continuously loaded and unloaded onto the belt and the sintering cycle happens in different parts of the oven.

During sintering, protective atmospheres are used to avoid further oxidation or reduce the oxides on the powder grains. Hydrogen is used to eliminate oxides as oxygen and Hydrogen become water which evaporates. But Hydrogen is very flammable, so a small amount is used. A neutral (inert) gas can make up the rest of the atmosphere such as argon or nitrogen which only protect the surfaces from further oxidation. This is all done in a medium vacuum which retains the chemistry of the parts during sintering.

Question 35

How does cold die powder compaction work?

Answer 35

Die compaction can be single or double action pressing. Densities of up to 90% of full density (mass of powder x new volume) can be achieved, depending on compaction technique, tooling, metal and lubricant powder properties.

In a single action press, there is not a constant pressure due to friction, in a double action press there is more compaction, due to even pressure.

Cold compaction results in high porosity and low strength.

Cold isostatic compaction is when the die is made of flexible materials (rubber/plastic) that enable pressure to be transmitted through fluids (oil/water). Advantages are high, uniform densities, complex shapes, reduced tooling and post-manufacture costs, composite structures.

The principle is that the powder is placed into a flexible mould and then pressurised. The dry bag technique is when the bag is fixed in the pressure vessel and the powder is loaded without removing the bag. While the wet bag technique is when the bag is submerged in the pressure vessel that contains pressurising fluid. Might need secondary process for surface.

Cold compaction can also be done through rolling where the powder (with irregular shapes) is fed between the gap of pairs of rolling mills. This results in a continuous compacted strip with densities between 80-85% of full density. The strip is the sintered into continuos furnaces. This method has a high productivity rate, however parts have low porosity and strength.

Question 36

Cold filling powder metallurgy (without compaction):

Answer 36

This process is used to control the porosity of the final component that usually targets fluid/gas filtration applications.

Loose powder is filled into a die made of heat resistant material so that it can be transferred into an oven for consolidation.

The part does not have a high strength and so cannot be machined.

Cheap.

Question 37

What is hot compaction power metallurgy?

Answer 37

Simultaneously pressing and sintering. Method often used for materials that are difficult to shape through PM.

The powder or precompacted parts are loaded into the mould. The mould is heated to plastic the powder. A force to compact the powder is applied as the temperature slowly increases. Temperature and pressure are maintained. Then the system is cooled down.

In the hot consolidation unit, silica is used above the punch to prevent heat loss. Around the die there are heaters and around the heaters there is a quartz insulator. A thermocouple is in contact with the part and the air in the consolidation unit, this is to measure how long it takes to get uniform heat on the outside and inside of the part.

Advantages: high density at lower compaction forces, consolidate powders with low sintering properties, less sensitive to powder particle shapes and sizes, higher strength

Disadvantages: more expensive tooling

Hot isostatic pressing can lead to 100% compaction and is used to produce parts made out of cemented carbides or ceramics. It can be done in three ways:

Press cold and then heat
Press and Heat
Heat first and then press

First flexible dies are filled with powder. They are loaded into the furnace which is inside a pressure vessel and is filled with argon. The flexible dies are then unloaded as the parts are consolidated. The flexible dies are made from a carbon steel can which is ductile and cheap. Often a problem arises where the material sticks to the part and the part can’t be removed from the can. When this happens, mechanical material removal is used, eg. Water jet.

Question 38

What is powder injection moulding?

Answer 38

Similar to plastic injection moulding, used for medium to large production volumes of small parts with accurate and complex geometry.

The powders are spherical and between 0.5-20 nanometers. The powders are mixed with a thermoplastic binder, at its meeting temperature, which coats the particles and allow a uniform compound. The mixture is then formed into pellets for transportation. The pellets are heated up and pressed into a mould like in injection moulding. The binder is removed before sintering, usually by thermal debinding. The part is then sintered.

Advantages: tiny complex parts with high densities at high dimensional tolerances can be produced

Disadvantages: difficulty debinding without causing cracks or distortions, not enough pressure could lead to poor compaction which means bad mechanical properties for the unsintered part which could be damaged during mould extraction or manipulation, insufficient consolidation can arise when the sintering temperature or time is incorrect, the part could have poor consolidation and the natural colour of the powder could change if extensive oxidation happens at sintering temperatures, cracks can appear during compaction and or consolidation due to mechanical or thermal induced stresses

Question 39

What are the part classes of PM?

Answer 39

Class 1 is for thin single level parts with a maximum thickness of 7mm, obtained by single action pressing.

Class 2 is for thick single level parts with any contours and thicknesses, obtained by double action pressing.

Class 3 is for two level parts with any contours and thicknesses, obtained by double action pressing.

Class 4 is for multilevel parts with any contours and thicknesses, obtained through multiple forces applied from two directions.

Question 40

What post-processes are common after PM?

Answer 40

Re-pressing and resintering can be done to improve the density and strength of the parts. The part is first compressed at an intermediate density and then presintered. The part is then pressed in a die with slightly different dimensions and then followed by a final sintering.

Sizing and coining can be done to calibrate the dimensions of the part using supplementary pressing after sintering. Coining engraved and embossed small features.

Heat treatment is used on ferrous alloys that need to improve their properties.

Tumbling in a rotating barrel or vibratory deburring aims to remove the burs produced during pressing. This can also be done by grinding.

Impregnation is used to improve the lubrication properties, machinability, or reduce the porosity of the part. This can be done by impregnating the porosities of the part with lubrication oils or plastics.

Machining is employed to generate features or surface properties that cannot be achieved through powder metallurgy.

Question 41

Overview of fabrication:

Answer 41

Fusion welding - electric arc welding, gas welding, laser welding, electron beam

Solid state welding - forge welding or friction welding

Gluing - adhesive bonding, brazing, soldering

Mechanical joining - threaded fasteners, riveting, crimping