THE AMERICAN UNIVERSITY IN CAIRO
MENG 3209/339
Lab Report 1: Expendable Casting Processes
Submitted by: Cherif Youssef Chokeir
SID: 900140712
Date of Submission: 22/2/2017
Dr. AbdAllah Wifi
Abstract:
The purpose of this report is to study two expendable mold casting processes: sand casting and lost foam casting and to understand the basic differences between these two processes. It is also important to compare these two processes in terms of advantages and disadvantages.
Furthermore the study of several casting defects and ways of eliminating them is a major goal in this study.
Table of Contents:
List of figures………4
Introduction And Background……….5
Objective………9
Experiment Description and Analysis………6
List of figures
Fig. 1 Features of sand mold………7
Fig.2 Types of patterns……….7
Fig 3 Lost Foam process. ……….8
Figure 5: Basic Casting System (Groover)………10
Fig. 6 Patterns used in casting. ………12
Fig. 7 Drag Section placed on the board with its pin down………..12
Fig. 8 Packing sand around the pattern………..12
Fig 9 Removing Excess Sand ………13
Fig 10. The drag is flipped and the pattern is exposed ………13
Fig 11. Covering the sand with a fine coating of parting sand ………..13
Fig. 12 Placing the cope section……….14 Fig. 13 Placing the sprue, gate and riser ………14
Fig 14. Ramming and filling cope section………14
Fig.15 Opening at top to pour molten metal………..15
Fig 16 Cope part removed……….15
Fig. 17 Making Core using core box ……….16
Fig. 18 Core box opened to remove core……….16
Fig. 19 Core placed using core prints………16
Fig 20- Using Torch to dry mold……….17
Fig 21- Furnace Used to superheat aluminum to 977°C………..17
Fig 22 Pouring Molten metal into mold………18
Fig. 23 Final Casting Obtained……….18
Fig 24 Defects in the casting-external………..19
Fig 25-Blowholes………..19
Fig 25 Internal
Defects………20
Fig 26 Misrun(a) and Coldshuts(b)
……….20
Fig. 26-Lost Foam Process……… 22
Fig 27- Placing the foam pattern inside the box and packing sand around it………22
Fig 28- Pouring molten metal inside box through down sprue section of the pattern………..23
Fig 29-Casting obtained through lost
Introduction and Background:
Metal Casting is a manufacturing process in which the use of a mold made of sand, ceramic or other material help to obtain a final product in the desired shape by pouring molten metal into a mold and wait until its solidification. The main steps are 1. Melt the metal 2. Pour it into mold 3. Let it freeze. In this process the mold, which plays a crucial role must be oversized in order to account for solidification shrinkage. It could be made of sand, ceramic, plaster and metal. The main advantages of the casting process is that one can creat complex part geometries, both external and internal surfaces, can produce parts in different sizes and suited to mass production. Some of its disadvantages are the poor dimensional accuracy and surface finish in some
processes, the safety to foundry men and on the environment and the limitations on the
mechanical properties. Parts produced by casting involve compressor frames, wheels, bells etc. There are two types of metal casting processes:
1. Expendable Mold processes: in which the workers, named foundry men scarifies the mold in order to obtain the desired casting. Mold materials are sand, plaster etc. The advantages of this process is that it is capable of producing complex shape geometries. Its disadvantages are that the production rate are too low since the time is usually taken to prepare mold for each new casting.
2. Permanent Mold processes, in which the mold could be used and reused to produce castings. The advantages and disadvantages of the process are opposite to those of the expendable mold process. In this process the mold material is usually metal.
In this experiment, the purpose is to study two expendable mold processes: sand casting and lost foam casting.
Sand casting: is the most widely used metal casting process that accounts for a majority of total tonnage cast. All alloys can be sand casted, and it is capable of producing quantities from one to millions
The steps in sand casting are as follows: 1. “Pour the molten metal into sand mold 2. Allow time for metal to solidify
3. Break up the mold to remove casting 4. Clean and inspect casting
5. Separate gating and riser system
6 . Heat treatment of casting is sometimes required to improve metallurgical properties” (Groover)
In making the sand mold, the cavity is formed by packing sand around a pattern the separating the mold into two halves and removing the pattern. The mold must contain the gating and riser system. One needs to include a core to obtain internal surfaces. This is inserted into mold cavity
before pouring and as molten metal flows into mold and between the cavity and core, it solidifies giving the internal and external shapes. The pattern is a full sized model of the product that must be oversized in order to account for solidification shrinkage. A new mold is required each time.
Fig. 1 Features of sand mold
There are many types of patterns: solid pattern(a), split pattern (b), match-plate(c), cope and drag (d)
Fig.2 Types of patterns
There are some desirable properties to the mold : • Strength ‐ to maintain shape and resist erosion
• Permeability ‐ to allow hot air and gases to pass through voids in sand • Thermal stability ‐ to resist cracking on contact with molten metal • Collapsibility ‐ ability to give way and allow casting to shrink without cracking the casting
• Reusability ‐ can sand from broken mold be reused to make other molds? (Groover)
There are green sand molds and dry sand mold. The first one contains excess moisture at pouring
Lost Foam process: In this process the foundry men use a polystyrene or foam pattern and sand is packed around it in a box so that the foam vaporizes when the molten metal is poured inside the mold. Its main advantages are reduction in process timing due to the absence of need to separate the mold into halves. Also there is good dimensional accuracy and good surface finish. So high production ratios, but there are limitations in size. Steps are shown in the figures.
Fig 3 Lost Foam process.
1. Spray Molten Metal with refractory compound 2. The pattern is placed in the box
Objectives:
- Understand and study the process of sand casting
- Understand and study the process of lost foam casting
- Perform the casting operation
- Understand and study different casting defects, their reasons and means of
eliminating them
- Compare between different casting processes in terms of advantages and
disadvantages.
- Calculate total heat required to raise metal to pouring temperature
- Calculate solidification shrinkage.
-Experiment Description and Analysis
The process of sand casting, also known as foundry process that is widely used in producing castings of a variety of shapes, sizes and materials mainly includes the preparation of the sand mold, melting the metal to be used, pouring molten metal into mold, cleaning the casting and collecting the casting for future reuse.
This section explains the different steps involved in sand casting and sketches these steps. At first, it is important to explain the main casting system in order to clarify some terminology that is widely used:
Figure 5: Basic Casting System (Groover)
Figure 5 shows the basic casting system: it consists mainly of two halves :cope and drag in which a cavity is made by packing sand around a pattern that takes the shape of the part to be casted but that must be oversized in order to allow for solidification shrinkage.
The system consists of a sprue, a vertical channel into which the molten metal flows. The runners typically carry molten metal from the sprue section to the cavity. The gate could be defined as the portion of the runner from which the molten metal itself enters the mold cavity. The importance of the risers is that they could be regarded as reservoirs that help to supply molten metal in order to prevent shrinkage of the casting part during solidification.
Mold Making Process
It is this step that usually takes most of the time in sand casting and thus minimizes the production rate since the mold is sacrificed to obtain the casting part. It is thus important to understand it steps carefully.
Fig. 6 Patterns used in casting.
2. The drag section is placed on the casting board with its pin down
Fig. 7 Drag Section placed on the board with its pin down
3. The sand is packed around the pattern inside the drag section. It is flattened by using a hammer and tapping over a wooden surface.
Fig. 8 Packing sand around the pattern
4. When ramming is complete, remove off excess sand with a strike rod. To account for gases escaping during pouring of the molten metal, small vent holes are made within few mm of the pattern.
Fig 9 Removing Excess Sand
5. Flip the drag over and removed the molding board which exposes the pattern.
Fig 10. The drag is flipped and the pattern is exposed
6. Smooth the surface of the sand with a trowel and cover it with a fine coating of parting sand.
Fig 11. Covering the sand with a fine coating of parting sand
7. Place the cope on the drag, after putting a paper that will help one to separate them later. The second half of the pattern will be used like the first half and pins help to hold both sections tightly.
Fig. 12 Placing the cope section.
8. The sprue, gate and riser are all placed in their proper position.
Fig. 13 Placing the sprue, gate and riser
9. Repeat the same process of filling, ramming, and venting the cope as with the
drag section.
Fig 14. Ramming and filling cope section Rise
11. Withdraw the sprue pin scooping out at the top to produce a fairly large-funnel shaped opening in which to pour the molten metal.
Fig.15 Opening at top to pour molten metal
1. Carefully remove the cope half of the flask and set it to one side.
Fig 16 Cope part removed
2. Before removing the pattern, moisten the sand around its edges with a swab so that they hold firmly together when the pattern is withdrawn.
3. Tap the pattern gently to be able to withdraw the pattern.
4. In order to make a core, place a metal rod inside the core box to strengthen the core, then place sand inside the box and ram it.
Fig. 17 Making Core using core box
5. Open the core box to remove the core and place the core in position in the drag by means of the core prints.
. Fig. 19 Core placed using core prints
Fig. 18 Core box opened to remove core
C-clamp Core Box
6. Use a torch to dry the mold.
Fig 20- Using Torch to dry mold
7. Assemble the two halves of the mold in preparation for casting.
-Preparing & Melting the metal
1. Calculate the amount of metal needed to produce the casting, taking into consideration the allowances and shrinkage.
2. Superheat the metal in a gas-fired furnace
Fig 21- Furnace Used to superheat aluminum to 977°C
-Pouring the metal into the molds
Carefully pour the molten metal into the mold through the sprue at a steady rate.
Fig 22 Pouring Molten metal into mold
-Cleaning the Casting
1. After the molten metal has solidified, break the mold leaving the required shape with riser and sprue attached.
Fig. 23 Final Casting Obtained
Defects:
Fig 23 which showed the final casting obtained showed that the casting had numerous defects that it is important to identify and discuss.
On the external surfaces it is possible to observe many blowholes
Fig 24 Defects in the casting-external
Fig 25-Blowholes
Blowholes are balloon shaped gas cavity that are caused by escape of mold gases during pouring.
These could result of the presence of excess moisture in the molding sand, on chills or metal.
They could also result of cores not being sufficiently baked. Another reason could be a low sand or core permeability.
To be able to prevent blowholes, one could possibly improve permeability of mold by using better permeable sand, and allow venting in the mold for gases to escape by creating venting channels.
When it comes to internal defects observation of the final casting showed the presence of misrun and cold shuts.
Blowhole s
Fig 25 Internal Defects
Fig 26 Misrun(a) and Coldshuts(b)
These internal defects both occur due to premature freezing of molten metal before completely filling the mold.
The misrun occurs as the casting solidifies before the mold fills completely the casting.
The coldshut is two metal portions flowing together without fusion due to premature freezing.
They are both due to a fault in designing the section thickness which resulted in too small one.
This could also result due to low fluidity of the molten metal or a fault in the gating system, but also due to pouring at a very low temperature. To overcome these defects, it is important to redesign the gating system such that it does not allow rapid solidification of the molten metal prior to filling the cavity, but also adjust pouring temperature.
Total Heat required to raise temperature of metal to pouring temperature:
Misrun and Cold Shuts
Using the formula ρaluminum = 2.7 g/cm^3 V= 1000cm^3 Cs = 0.9 J/g-C Cl = 1.18 J/g-C Tm = 660.3°C Tp= 997°C T0 =25°C Hf = 398J/g Calculations gives H= 3500 kJ Measured mould filling time: t = 15s
Solidification shrinkage:
Pattern volume = 214.80259cm^3 Cast volume = 140.668cm^3 Shrinkage = 34.5%
Expendable Polystyrene Process (Lost Foam)
In the expendable polystyrene casting process, the foundry-men use a mold made of sand packed around a Polystyrene foam pattern such that it
vaporizes when the molten metal is poured through the mold. The process that could also be called as lost foam process, lost pattern process and so on, has the advantage that the pattern itself has the sprue, the risers and the gating system itself and could contain core if there is a need for internal surfaces. This gives the advantage that the pattern is not removed from the mold which reduces the process timing. Normally, the foam is coated with a refractory material in order to have smoother surface on the pattern which also help in obtaining high temperature resistance to molten metal.
Fig. 26-Lost Foam Process
Expanded polystyrene casting process: [Fundmentals of Manufactruing Processes, Mikell P.Groover]
The steps for this expendable casting process are as follows
(1) The Polystyrene pattern is coated with a refractory compound (not done during this experiment);
(2) The foam pattern is placed into a box (mold) in which sand is added and compacted around pattern.
(3) Molten metal is then poured into portion of pattern that represents the cup and
sprue
Fig 28- Pouring molten metal inside box through down sprue section of the pattern
(4) As the metal is poured into the cavity, it vaporizes the foam which allows it to fill the cavity easily.
Fig 29-Casting obtained through lost foam
From figure 29 it is possible to discover the presence of cavities (defects) in the casting. These defects could be explained by a decrease in gas solubility during solidification and also due to the absence of refractory compound that was not added in this experiment. To prevent these defect use refractory compound and control permeability of mold and core.
Conclusion and Recommendations:
To sum up, in this experiment, the purpose was to study two expendable mold casting processes: sand casting and lost foam casting. Each of the two casting processes has its advantages and disadvantages but the purpose remains the same: obtaining a casting product that is usually complex in shape. The main advantage of the lost foam process lies in the fact that the pattern itself contains the core, the sprue and the riser and thus need not to be removed from the mold which reduces the time of the process.
In studying these processes, the Engineer encountered several defects which were further understood and discussed: the blowholes, misrun and cold shuts that appeared in the sand casting process and the ways to prevent them were also further discussed. The same applies to the lost foam process where the gas cavities occurred due to a lack of ventilation in the mold itself.
It is further recommended that the lab responsible would possibly in the future offer other types of casting processes to compare different processes and their relative advantages and disadvantages.
