Casting

Question 1

2-Part-Mould

Answer 1

The mould is made in 2 halves, the upper section called the cope, and lower section called the drag.

Question 2

Mould Cavity

Answer 2

The section of the mould in which the useful part forms.

Question 3

Cores:

Answer 3

Solid inserts in the mould cavity to stop molten metal solidifying there. E.g. If you were casting a pipe, you would use a core to ensure the inside is hollow.

Question 4

Flask

Answer 4

A container to hold the mould in place.

Question 5

Risers

Answer 5

Act as reservoirs of extra material, that feeds back into the cavity as the metal shrinks when it cools. This helps avoid overall shrinkage of the part.

Question 6

Vents

Answer 6

Are used to allow air to escape the mould, this avoids pressure build up.

Question 7

Patterns

Answer 7

These are replicas of the part made of a different (cheaper) material, used for mould making. They may sometimes be destroyed when the mould/part is made.

Question 8

Pouring Basin

Answer 8

This is where the metal is initially poured into the mould.

Question 9

Sprue

Answer 9

A vertical column that channels the metal down to the right height.

Question 10

Gate

Answer 10

Controls the flow of material into the runners, usually one at the bottom of the sprue but can be multiple. Tends to be the narrowest part of the sprue.

Question 11

Well

Answer 11

Lies at the bottom of the sprue to catch and trap any unwanted impurities, stopping them from entering the cavity.

Question 12

Runners

Answer 12

Transports the molten metal from the sprue to the cavity, ensuring a lamina flow.

Question 13

Gating System

Answer 13

Includes all parts of the mould used to get metal into the cavity, that are not needed for the final part and must be removed. Includes; pouring basin, sprue, gate, runners, well.

Question 14

Fluidity

Answer 14

The ability of a metal to flow into a mould. It affects the; minimum thickness, maximum length of a part/runner, how fine the detail can be, the ability of the mould to fill extremities and take complex shapes.

Question 15

Short Run

Answer 15

When molten metal doesn’t fill the mould properly, resulting in a bad part. Ensuring the correct temperature is used is the best way to reduce the chances of this.

Question 16

Superheat

Answer 16

In casting refers to the difference between the pouring temperature, and the solidification temperature. This needs to be high enough to ensure a high fluidity, so the cast forms properly. On the other hand, it mustn’t be too high as extreme fluidity can mean a metal picks up too much texture from the mould, e.g. the porous surface of a sand casting.

Question 17

Shrinkage

Answer 17

Occurs in nearly all metals because the solid phase has a higher density than the liquid phase. It must be accounted for by making the mould (and patterns) a certain amount larger (shrinkage allowance), or by using risers.

Question 18

Chills

Answer 18

Act as heatsinks can be used to ensure even cooling. Internal chills are small solid metal parts, placed inside the cavity, around which the metal cools around first. External chills are metal inserts in the walls of the mould that can remove heat more rapidly than the surrounding sand. External chills must be machined off the part afterwards, whereas internal ones remain within the part. Because of this the metal used for internal chills is usually the same or very similar to the metal being casted.

Question 19

Top risers

Answer 19

(open air risers) are useful because they can be visually assessed. Heat is lost greater this way and you don’t want the riser to cool and solidify before the rest of the cast, as it wouldn’t do its intended job.

Question 20

Side risers

Answer 20

(blind risers) are closed off from the air and can’t be visually assessed, so you have no idea whether its working or not. However, they lose less heat and not as much material is required to fill them.

Question 21

Live risers

Answer 21

(hot risers) are attached to the gating system and receive the hottest metal (last incoming) metal. This helps them to cool after the useful casting.

Question 22

Dead risers

Answer 22

(cold risers) are positioned on a non-critical surface of the part, where it can easily be ground off afterwards. Dead risers are usually cheaper and mean the overall mould size is smaller.

Question 23

Green Method

Answer 23

Sand bonded with clay, water and additives is packed tightly around a (wooden) pattern. The ‘green’ part implies the mould hasn’t been cured.

Question 24

Chemical Method

Answer 24

Sand is mixed with organic or inorganic resins and curing agents. This gives the mould a greater strength and dimensional accuracy (less likely to deform). Reduced pattern cost as patterns can be made of cheaper materials. This is due to the sand being compacted using vibrations rather than compression, so the pattern material doesn’t need to be as strong. It has excellent shake-out characteristics when it comes to removing the metal from the mould.

Question 25

Shell Moulding

Answer 25

Sand coated with thermosetting resin is dumped onto a heated metal pattern then hardens to its shape. This is advantageous because you can work on (smooth) the shell without it easily falling apart. Chaplets are used to hold any cores in place, so they don’t move. They are small pieces of metal that support the core in the cavity. When the molten metal is poured in they melt and become part of the casting. A bonding adhesive is used to hold the cope and drag shells together (a bit like a gasket).

Question 26

Investment Casting:

Answer 26

Also known as the lost wax method. Several wax patterns are made and placed on a pattern tree. The tree is the repetitively submerged in a refractory slurry, which hardens to form a ceramic mould around the wax. The wax is then melted out of the mould (melt-out), and the mould used for casting. The mould is removed from the part by harsh vibrations that break the ceramic apart (shakeout). The pattern can be injection moulded, but recent methods use 3D printing to create a plastic pattern layer by layer. The interior structure is printed in a honeycomb fashion allowing it to collapse in on itself as it melts, rather than expanding and breaking the mould. The polymer used must have a low melting point and create as little smoke and ash as possible.

Question 27

Evaporative Foam

Answer 27

The whole pattern including the gating system is made of polystyrene, which is vaporised by molten metal being poured on it. Before this though, it is coated in refractory slurry and left to harden, so the metal has something to take the shape of. This time the mould is silicone based instead of ceramic.

Question 28

Permanent Mould Casting

Answer 28

Molten metal is poured into a mould which has been machined in to die blocks. The mould is preheated to minimise thermal fatigue due to the multiple heat cycles it goes through. If the temperature difference is smaller the fatigue per cycle will be less meaning the mould will last longer. This is important as permeant moulds are expensive and need to be reused. Cores must be retractable or collapsible to ensure the part can be removed easily.

Question 29

Low Pressure Casting

Answer 29

Requires the mould to be under vacuum. The mould is turned upside-down and put on top of an airtight crucible, ALMOST full of molten metal. A shaft from the mould to the bottom of the crucible allows metal to travel up into the cavity (a bit like a straw). A low pressure INERT gas (0.1MPa) is introduced into the top of the sealed chamber, pushing metal into the mould. There is no need for traditional risers as this method compensated for shrinkage by maintaining the pressure during the liquid phase. There is also no need for vents as the air he been removed prior to casting. The metal is very clean as it is taken from the centre of the crucible i.e. it has never seen the atmosphere so hasn’t had a chance to oxidise. > There is a high yield (85%) as there is no need for a gating system, so most of the metal melted goes into the useful part.

Question 30

Vacuum Casting

Answer 30

is a similar method where a vacuum is placed on the die from above, drawing the metal into the cavity. There is no need for a low-pressure gas. The maximum mass of the part can only be about 5Kg though. Graphite may also be used for moulds as well as metal.

Question 31

Cold chamber die casting

Answer 31

Requires additional machinery such as a furnace and way of moving the molten metal. Preferred method for metals with high melting points, such as copper, brass and aluminium alloys. These materials are usually stronger and more versatile.

Question 32

Hot chamber die casting:

Answer 32

Contains the alloy melting pot as part of the machine itself. It uses a gooseneck and plunger to inject metal into the die. As the plunger passes the hole in the gooseneck the metal inside it gets trapped and is forced into the cavity, once solidified the plunger retracts and the gooseneck fills up again.

Question 33

Group A: Metal Projections.

Answer 33

• Fins are created where extra metal has solidified at the parting line of the mould, looks like a small ridge. They can also form around cores, and if cores move slightly during solidification.
• Swells are where excessive amounts of metal form around gates or at the bottom of the sprue, as these are areas where erosion of the mould wall can occur (a bit like a waterfall). This only really occurs in sand casting (non-permanent), where the sand has been poorly compacted.

Question 34

Group B: Cavities.

Answer 34

• Can be described as depression in the surface or an internal void in the casting. Typically caused by shrinkage during solidification due to poor riser design.
• Blowholes and pinholes are holes inside the casting caused by trapped gas in the molten metal. Pinholes are smaller and there tends to be more of them together than blowholes. If the part has lots of pinholes it can become porous, caused by micro shrinkage.
• Shrinkage cavities have a rougher shape and can be quite deep. They are caused by non-progressive solidification.

Question 35

Group C: Discontinuities.

Answer 35

• A lack of fusion between 2 streams of liquid metal, where they meet but fail to join properly i.e. around a core or if using multiple sprues.
• Happens because of premature freezing of the metal, mainly due to the pouring speed/temperature being too low. Can also be down to the fluidity of the metal (if it isn’t runny enough), and if the cross-sectional area it’s trying to fill is too narrow.
• Surface shrinkage can make matters worse.
• A hot tear is a fracture formed during final stages of solidification, or early stages of solid cooling, due to hindered contraction. In other words, the casting is restrained from contracting because the rigid mould is in the way. Having lots of 90o corners is particularly bad for this.
• A hot crack is formed during solid cooling because of internal stresses developing in the casting. If the stresses are uneven they can create weak points.

Question 36

Group D: Defective Surface.

Answer 36

• Sand adherence is where sand from the mould walls sticks to the surface of the part, because it’s not been compacted properly or has dried out due to the heat. Sand can also get trapped just below the surface, being hard to remove.
• Depression scan occur if chills are not used / not used properly.
• Other surface defects such as; surface blows, scaring, blistering, surface folds/laps and wrinkles.

Question 37

Group E: Misruns.

Answer 37

• Castings solidify before completely filling the mould cavity.
• Typical causes are the same as for group C; fluidity, pouring temperature, pouring speed and thin cross-sectional area.

Question 38

Group F: Incorrect Dimensions of Shape.

Answer 38

• Pattern moulding error such as a shifted core or shifted mould halves.
• Irregular contraction causing a warped casting.
• Incorrectly/not compensating for shrinkage.

Question 39

Group G: Inclusions.

Answer 39

• Can be metal splatters (ball bearing size) that solidify quickly and become entrapped in the casting. These are known as cold shots.
• Also, can be sand from the mould wall or slag / metal oxides that get trapped inside the part.
• Caused by badly designed gating system or incorrect pouring procedures.