School of Science and EngineeringENGR 339 (3209-81) LaboratoryMondays labFoundry Processes: Sand Casting Lab Report 1

Presented to: Dr. Mohamed Emara

Presented by:Seif Elsaie 900112577

Abstract:The main process used in this experiment is Sand casting. We created the cast using the desired mold with the specific cavity needed for the molten metal that we retained to be poured into the mold. The liquid metal solidifies and the cast is cleaned. The procedure ensures an environment friendly casting process because the mold can then be used a couple of times. In this report, we will be summaries the steps into making a proper cast and focusing on all the defaults we experienced in the lab and we will conclude with the basic techniques needed to avoid these defects.

Table of Contents:

List of figures .4

Introduction ........6

Objectives .......7

Procedure ..........10

Results....... 11

Discussion......17

Conclusion & Recommendations......18

References.19

List of figures:Figure 1....5Figure 2............8Figure 3 ...........8Figure 4....9Figure 5............9Figure 6 ...........10Figure 7....10Figure 8............11Figure 9 ...........11Figure 10..12Figure 11..........12Figure 12 .........13Figure 13..13Figure 14..........14Figure 15 .........14Figure 16..19Figure 17..........19Figure 18..........20Introduction:The main steps in to making a sand cast is to determine and to know the basic features of a closed mold. It must consist all the basic features as seen in the diagram. Two things must be taken separately in this experiment. We will start with the cope which is the top half of the pattern which contains the riser, sprue and pouring cup and the drag is the bottom half of the pattern. The molding material is prepared in each of the cope and drag separately. The molding material must ensure when the molten metal is poured through the pouring cup, which is mainly to minimize splash and turbulence as the metal flows. The molt falls through the gating system into the sprue, which is the vertical part of the gating system (and it is recommended to be designed at an angle). The molt then makes its way through the runners, which is the horizontal portion of the gating system, and the mold must ensure controlled entrances throughout the molt, which are the gates. This entire system is the gating system that defines the network of connected channels that deliver the molt.The mold fills the mold cavity and the riser and its left to cool and solidify and the riser will compensate for the shrinkage during solidification. Before the pouring of the molt in the mold we must determine whether the final design required will contain any internal features, after that it will give us a clear image of what the core material will look like. We must ensure the use of a core print that is probably higher in melting point to act as a support to the core during the process. There are two main design features that must be included to provide the best possible shape for the cast without any defaults, which is the vent, and draft design. The vents will ensure the escape of all gas entrapment and the draft design for the pattern to be extracted without the damaging of the mold.

Figure 1: Anatomy of mold used for a sand casting

Objectives: The main aim of this experiment is to: Create a sand cast in a step-by-step process (follow the procedure), thus producing a final product, the mold.

We are also aiming at identifying the main defects that might occur in the mold during the casting process.

The causes of these defects will be discussed and ways to eliminate or avoid such defects will also be mentioned as well as analyzed thoroughly.

Calculate the total heat that is necessary in order to elevate the temperature of the metal to the most suitable pouring temperature

Calculate the mold filling time

Calculate the solidification shrinkage

Procedures:Mold-Making Process:1- Position the pattern on the molding board. 2- Place the drag (lower part of the mold) on the board and pin down in order to be fixed in place. 3- Add a layer of sand around the pattern.4- Ram the layer of sand. 5- Following the ramming of the sand layer process, remove the excess sand on the rammed layer using a strike rod and ensure that the layer of sand is leveled. Drill vent holes within a few millimeters of the pattern allowing escape of the gases that are formed when the molten metal is poured using a thin rod to ensure that the vent hole is not to big. 6- Repeat the process of layering and ramming the sand until the drag is completely full. 7- The drag is turned upside down and the molding board is removed, exposing the pattern that was originally at the bottom of the drag.8- The surface of the sand is flattened with a trowel and is then covered with a fine coating of parting sand. 9- Place the cope on the drag and fix the two halves together using the pins on both sides, holding them tightly in position.10- The sprue, gate and riser are placed in their proper locations.11- Layer and ram the cope with more sand as previously done with the drag. 12- After the sprue tube is surrounded by sand and rammed till the surface is smooth, remove the sprue tube carefully creating a hollow entrance for the molt to be added.13- Remove the cope half of the flask carefully and keep it aside.14- The pattern edges are then moistened with swab to be able to remove and withdraw the pattern easily (tap the pattern gently on each side to remove).15- The core is now prepared; place a metal rod vertically through the core box and ensure that its located and the middle of the core so that the sand surrounds the part compoletely. This is called a core print and it acts as support for the core and to ensure that it doesnt melt it must be higher than the melt in terms of melting point.16- Open the core box, remove the core tube and place it carefully in the cavity of the pattern present in the drag box.17- Use a torch to apply heat onto the surface of the mold (temperature 900 degrees Celsius), drying the water surface before adding liquid metal.18- Assemble the two parts of the flask in preparation for casting process. Preparing and melting the metal:1- The amount of metal needed to create the casting is calculated (considering allowances and shrinkage).2- Metal is super heated in a gas-fired furnace.3- Degassing agents are added into the furnace with the metal.Pouring the metal into the molds:Pour the molten metal through the sprue gently at a steady rate to avoid any splashes and turbulences that might be a future problem regarding the cast.Cleaning the casting:1- Following the solidification process, break the mold block obtaining the required shape with the sprue and riser still attached. 2- The sprue and riser are then cut off and the final shape can be also be machined to reduce defects.

Figure 3: Filling the cope with sand after placing it on top of the dragFigure 2: Pattern exposed after drag is turned upside down

Figure 4: Ramming the final sand layer in the cope using a hammer

Figure 5: The drag and cope parts already filled with sand and rammed, just before removing each half of the pattern

Figure 6: Preparing the core of the mold

Figure 7: Using torch to apply heat on the surface of the mold

Figure 8: Pouring the molten metal from the furnace into the pouring cup

Figure 9: Pouring the molten metal into the sprue

Figure 10: The final casting produced from the sand casting process

Figure 11: The initial pattern used to form the casting

Figure 12: core inserted in the mold

Figure 13: tools used in the casting process

Results:

Dimensions:

Figure 14: Initial dimensions of the wooden pattern (left) and final dimensions of the final metal casting (right). All dimensions are in mm.

Diameter = 3 cm

Figure 15: Final dimension of the core in the final casting

Total heat required to raise the temperature of the metal to the pouring temperature:H = V {Cs (Tm T0) + Hf + Cl (Tp Tm)}

Where:

H = total heat required to raise the temperature of the metal to the pouring temperature, J = density g/cm^3 Cs = specific heat for the solid metal J/g C (the amount of heat required to raise the temperature of 1 g of material by 1C) Tm = melting temperature of the metal C To = starting temperature, usually ambient CHf = heat of fusion J/g Cl = specific heat for the liquid metal J/g C Tp = pouring temperature C V = volume of metal being heated cm^3

For Aluminum (metal used in the casting process): = 2.7 g/cm^3 Cs = 0.9 J/g C Tm = 660 C To = 25 CHf = 398 J/g Cl = 1.086 J/g C Tp = 900 C

V = {(6.3/2)^2 * (1.9) + (4.5/2)^2 * (5.34) + (6.1/2)^2 * (1.65)} (3/2)^2 * (8.89) = 129.5cm^3

Therefore:

H = (2.7)(129.5){(0.9)(660 25) + 398 + (1.086)(900 660)} = 200548.775 J

Mold filling time:v = 2gh Where:v = flow velocity cm/sg = gravitational acceleration constant (981 cm/s^2) h = height of the sprue cm

Assuming the height of the sprue to be 12 cm:g = 981 cm/s^2h = 12 cmTherefore: v = 2(981)(12) = 153.44 cm/s

Q = vAWhere:Q = volumetric flow rate cm^3/sv = flow velocity cm/sA = cross sectional area of the liquid cm^2

Assuming the diameter of the sprue to be uniform throughout and equal to 4 cm:Q = (153.44)(*(4/2)^2) = 1928.18 cm^3/s

MFT = V/QWhere:MFT = mold filling time sV = volume of mold cavity cm^3Q = volume flow rate cm^3/s

MFT = 129.5/1928.18 = 0.67sDiscussion:During the sand-casting, several defects occur towards the final shape that we casted. Examples of these defects are: open hole (blowhole), pinhole porosity, entrapped air or other gases, shrinkage cavities, hot tear, misrun (cold shut), inclusions, cut (wash), core shift, mold component shift, sand fusion on the casting, metal penetration of mold sand.In our experiment though, only some of these defects occurred. These experimental defects included open blow (blowholes), pinhole porosity, inclusions, hot tear, entrapped air or other gases, misrun or cold shut and also shrinkage cavities. Blowholes are one of the many defects that we encounter and it comes from three main reasons. The first reason why the defect will appear on the final casting is from the excess moisture in the molding sand, on chills, or metal. Secondly, the core is not sufficiently baked which will finally cause blowholes in the inner features of the casting. The final reason is the permeability/ porosity of sand or if core too low. This allow for gas and moisture entrapment, which will finally defect the final cast with an indentation or a certain irregularity.Pinhole porosity is the second defect encountered and it will come through the molt if it over heated above 900 degrees Celsius approximately and if gas dissolved in the alloy that is not properly degased.Inclusions occur mainly due to human errors such as having faulty gating or improper pouring of the molten metal into the downsprue. If the molten metal is poured too quickly or at an unsteady rate will lead to defects. Inclusions can be observed in the final casting as parts of slag inside the metal casting. This defect is quite large and is easy to detect.A hot tear occurs due to a faulty design creating stressed areas in the final cast and also from insufficient collapsibility of core or mold. Hot tears are also characterized as irregular internal or external cracks that usually occur instantly following the solidification process. This defect was also observed in the final casting of our experiment in the form of external cracks.Entrapped air is a major defect that was observed on the cast, and either having a porous mold or building vents avoids it. This can also be counted on human errors for example whilst pouring the molt into the mold. As the molt solidifies the entrapped air cause a big gap or void in the casting. We notice how all defects are similar and how each one follows another.A misrun or cold shut defect creates a type of incomplete or unfinished casting and the edge of the defect is usually smooth and round. This is caused by several reasons such as having a faulty design that results in having a very thin section, making it more prone to defect. Misruns can also occur if the liquid metal is too viscous, if the pouring temperature of the molten metal is too low, or if you have a faulty gating system. The final type of defect that was seen in the final casting of our experiment was shrinkage cavities. Shrinkage cavities can occur due too many things for example a faulty gating system or misplacing the risers location and the improper use of chills. The faulty design of the system may cause alterations to the castings shape calling for isolated heavy sections that the feed couldnt reach. Shrinkage cavities might be seen when the shrinkage property of the casting metal is improper with the properties of the mold.There are many more defects that we havent witnessed including: cut (wash) which could take place due to faulty gating, weak mold, insufficient good facing material on mold cavity surface. Metal penetration of mold sand could take place due to using large grain size sand, mold sand not sufficiently rammed, weak mold or core, pouring temperature of molt too high, insufficient good facing material on mold cavity. Sand fusion on the casting is due to faulty gating, mold sand not sufficiently refractory, weak mold or core sand, pouring temperature of molt too high, insufficient good facing material on mold cavity. Core shift is due to improper support or location, weak mold or core sand, faulty patterns or design. Mold component shift is due to improper placement of guide pins, faulty pattern, faulty core boxes, and faulty flask. These entire defects act towards the final casting shape and it shows how each defect is connected with one another and the slightest error will result with catastrophic changes towards the end.

InclusionMisrun

Figure 16: Inclusion and Misrun defects in our final casting

Pinhole Porosity

Blowhole (small)

Figure 17: Blowhole and Pinhole Porosity defects in the cast

Large blowholeBlowholes

Shrinkage Cavity

Hot tear

Figure 18: Blowhole, hot tear, and shrinkage cavity defects in the final casting

Conclusion & Recommendations:Finally all of these defects listed must be avoided to get the best possible shape with minimum machining and near enough dimensions too the pattern. Therefore, I have found that human error played the biggest role in this entire experiment.In order to avoid the formation of blowholes in the final casting product, the content of moisture in the sand or metal should be controlled or monitored carefully. Providing a ventilation system for the removal of any unwanted gas. Moreover, during each layer of sand the sand should be rammed enough to ensure that layers are tightly packed.In order to avoid the pinhole porosity defect, several precautions have to be taken into consideration when carrying out a sand casting process. Aluminum alloys have a common defect against pinhole porosity which was the metal used in this experiment. Inclusions are mainly due to the type of mold/ sand used in this experiment and thats why the grade of sand must be chosen carefully. Hot tears can be reduced by proper guidance throughout the experiment and following the instruction precisely because it is mainly due to human error applying a proper means of directional solidification and a steady rate of cooling of the mold, making sure the molten metal is poured at the correct temperature and last but not least controlling the hardness of the mold, are all ways in which hot tears could be avoided completely or drastically reduced.Entrapped air and other gases could be reduced or avoided by correct pouring of the molt will eventually reduce turbulence when the liquid metal flows through the gates and building small vents with a specific location according to the cavity in the mold and the specifications of the vent and vent holes depend on the size of the material. Finally shrinkage cavities is a very common defect and is usually mainly due to human errors in carrying out the experiment. Shrinkage cavities can be avoided by once more using a proper system for gating, pouring the liquid metal at a suitable temperature, and carefully monitoring directional solidification during the process.

References:

http://www.iron-foundry.com/casting-defects-pictures.html - Sand casting defects and causes of each defect.

http://www.mechanicalengineeringblog.com/tag/sand-casting-defects/ - Sand casting defects, causes of these defects and ways to reduce such defects.

http://www.substech.com/dokuwiki/lib/exe/fetch.php?w=&h=&cache=cache&media=sand_casting.png - Figure 1

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