Casting Lab #1

Casting LabJake TeSelle

Nathan McdonaldJarek Jensen

Michael Lundstrom

Submitted to: Dr. JohnETME 216 Section 3Aluminum Mold Casting2/12/2014

Introduction:

Purpose

The purpose of this lab was to learn introductory mold making and rudimentary casting

techniques in order to cast from molten aluminium a simple rectangular block in a silica based

sand mold. Also, by means of calculations, to determine theoretical values and compare them

to experimentally determined values. The mold is to be constructed from a simple silica sand

contained within an aluminum frame. Learning proper safety techniques and practices when

handling molten aluminium are also objectives of this lab. Finally, observing visual defects in

cast parts, how to observe them, as well as how to compensate for them to create high quality

parts. Intentionally part poor mold design has been utilized to observe defects on a macroscopic

scale. These defects and their causes, as well as ways to potentially prevent them are

discussed in greater detail below.

Scope

The scope of the experiment was limited to the study of aluminium and its material

properties. No other metal was used. The casting techniques used were sand casting. All

other casting techniques were not applicable to this lab; such as die or permanent mold casting.

In industry, other materials are used in the manufacture of molds such as styrofoam, however,

in this lab only a red silica based sand was used to construct the mold. Observing the factors of

mold erosion, turbulent flow, porosity, part shrinkage, and various defects are all purposes of

this lab. The study was limited to the part and its properties. Material characteristics, mold

material, cooling rate, and hardness were not studied in the lab.

Problem

The problem this lab is designed to demonstrate is the inherent nature of defects in the

casting process. These problems are understood further through visual examination of the

finished part. The process used to produce the part is one inherently rife with problems. Such a

process would never be used industrially. Turbulent flow, which causes mold erosion, and

porosity, which causes weak spots in a part, are both common issues and are unavoidable in

production, however, they can be minimized. Other defects include mold fill lines which is

caused by unsmooth filling of the mold. This reveals horizontal lines where the flow velocity may

have slowed, forming the line. You can also have depressions in the surface of the part cause

by un-uniform shrinkage For this lab, all defects were maximized to give an accurate

representation of their nature. Getting to see these defects maximized let us see the potential

problems they all can cause. After Seeing these defects we will be able to account for these

defects in future casting.

Test and Evaluation:

Apparatus-● Cope● Drag● Aluminum Pattern (permanent)● Sand ● Wire Brush● Dustpan● Broom● 2 12” Scales● Dial Calipers ● Aluminium Pipe● Steel Rod (testing aluminum temperature)● Molten Aluminium● Pot Furnace● Pouring Ladle● Metal Sieve● Wood sand compactor● Safety Glasses● Metal Scoops● Leather Welding Gloves● Metal Straight Edge Scraper● Talc Powder● Metal Tongs● Metal Spiked Hammer● Water Quench Tank● Phone Camera● Sharpie marker

Procedure:

The floor was first swept clean to provide a clean work surface. A three section

aluminum mold was then disassembled from it original structure so we could scrub it with wire

brushes to remove any stray particles or hardened sand that would cause defects in the mold.

Careful attention was given to the surface of the mold that would be creating the cavity. Then

the mold was assembled by putting the top piece, with pegs up, upside down. The center piece

was slid onto this such that the part creating the cavity was facing down to allow for easier

removal. The third section was then lowered onto the pegs. This was done so that when

removing the center section of the mold from the sand, the part creating the cavity was coming

up for easier removal. Next, the the height, length, and width of the mold dimensions were

measured from the bottom of the draft angle to determine the cavity dimensions, by using a dial

caliper and were recorded for use in later calculation (see below). Furthermore, the location of

the square on the side of the mold was marked by putting the rulers vertically straight towards

one of the side of the square box then had placed another ruler horizontally touching the other

ruler and marked the spot with a sharpie marker. This happened for each side of the mold so

that the locations for both vent and fill holes could be determined. Next, a screen was placed on

the top piece of the mold so that sand could be sifted of any and all particulates. The Sand was

now sifted through a screen to create a layer of sand at the base that was ultra fine and with no

air pockets (see Picture 6 in Attachments). Then the sand was compressed with hands to get

precisely on the mold of the block to eliminate air pockets. After sufficient sand had been filtered

and hand compacted, larger quantities of sand were added. This was then compacted by use

of a large wooden tool (see picture 7 in Attachments). Sand was then added in order to

completely fill the cavity and beaten together to create a very stiff sand mold. A metal tool was

then used to scrap the extra sand off the top surface of the mold and when completed, then the

mold was flipped and the process repeated on the bottom piece of the mold. The red sand was

again sifted through a screen mesh, compacted by hand onto the mold, and then filled and

compacted using the wooden tool. Using the sharpie marks and a flat metal sheet, it was

possible to use the the ruler to draw a straight line in the sand. This was done a total of four

times. The measurements of the mold were now used to locate the mold cavity within the sand

so that a sprue and vent hole could be created (see picture 8 in Attachments). A hollow

aluminium pipe was pressed into the sand by hand until it hit the mold, this was used to create a

sprue and vent for the mold cavity by gently rotating it and removing a column of sand (see

picture 11 in Attachments). Using a ruler, the sprue hole edges were chamfered to allow for

much easier pouring of the molten aluminium and its dimensions were recorded for use in a

later calculation for mold fill time.

Aluminium pellets and scraps were then superheated far beyond their melting point (see

picture 12 in Attachments). This process took approximately 20 minutes. A steel rod was used

to dip into the molten of aluminum to check the temperature. Once the aluminum stuck to the

steel rod it was a glowing red, the metal was determined to be superheated enough to be

poured into the cavity of the sand mold. Three group members wearing safety goggles and

welding gloves now positioned the mold such that it would be closest to the superheated metal.

The Molten Aluminum was then poured into the pouring ladle held by a group member (see

picture 9 in Attachments). It was then poured into the mold with filling time being recorded by a

cellphone stopwatch application(see picture 5 in Attachments). Once the molten Aluminum filled

the mold by popping out of the vent hole, the pouring and stopwatch were stopped and the mold

was immediately put outside to cool for approximately 10 minutes. After cooling the cast part

was dug and pulled from the sand mold with metal tongs and a metal spike hammer (see picture

2 in Attachments). The metal part was then put into a water bath quench tank to fully cool.

After cooling, part dimensions were recorded to be compared to the initial cavity dimensions.

Sand was then beaten out of the aluminum mold so that it could be used again for the incoming

class. A spiked hammer was used to remove the sand. Large quantities of the sand were

blackened and charred from the molten metal.

Findings:

Actual Mold Fill Time: 4.8 s

Molded Part Dimensions: L=3.906”, W=2.901”, H=1.481”

Mold Cavity Dimension: L=3.965”, W=2.97”, H=1.56”

Diameter of Sprue: .7”

Diameter of Riser: .7”

Height of Sprue: 2.75”

Flow Velocity was found using formula V=2ghWhere- g is the gravitational constant 386 in/sec^2 h is the height of the spruePredicted V= 46.0760 in/sec

Volumetric Flow Rate was found using the formula Q=vAWhere- v is the flow velocity A is the cross sectional area of the liquid Q= 17.7321 in^3/sec

Mold Fill Time was found using the formula MFT=V/QWhere- V is the mold volume Q is the volumetric flow rate.MFT= 1.0360 sec

% error MFT = 463%

Part Shrinkage was found using the formula Sv=[(1-SS)(1-STC)]^⅓Where- SS is the Solidification Shrinkage % which for aluminum is given as 7.0% STC is the Solid Thermal Contraction % which for aluminum is 5.6% Sv= 0.9575

Calculated Part Shrinkage Dimensions L = 3.7965”

W= 2.8437”

H= 1.4937”

Visual Findings:

Visually, the part had several major defects. The base of the part where the fill hole was

located had minor surface defects created by initial turbulent flow and subsequent mold erosion.

The fill location also was the site for major porous regions on the part. This porosity was

caused by gas bubbles trapped due to the turbulent flow. Viewing the part from the sides,

laminar flow lines are visible, making it possible to see how the mold filled. Looking at the side

closest to the fill hole, however, reveals an amalgamation of lines, indicating heavy turbulence.

The top of the part also contained a large dip into the metal. This sagging area was caused by

the gas trapped between the vent and fill holes as the part cooled. A similar, though much

smaller dip, was present on the underside of the part.

Interpretation and Results:

After casting and looking at the casted block, there were significant problems with the

cast part. The Part came out smaller than the dimensions we calculated. This was from non

perfect cooling and casting of the part. We also had a lot of defects. One of the defects that was

most noticeable was the surface depression on top of the mold caused by shrinkage (see

picture 1 in Attachments). Surface depressions can be fixed by molding parts with uniform wall

thickness. Another visible defect we had was porosity. Porosity is found in all metal casting, and

cannot be avoided. However, it’s location can be controlled by venting the mold where you want

the porosity in your part. For this lab, porosity was found in the corner of the block (see picture

4 in Attachments). Other defects include mold filling lines that are found on the side of the mold

and during filling (see picture 3 in Attachments). This can be reduced with faster filling time and

more consistent pouring of the Aluminum. The last defect we found was mold erosion caused by

the molten metal pouring into the mold and eroding the bottom of mold away (see picture 10 in

Attachments). This defect can be fixed by changing where the molten liquid enters the mold. In

the industry, the sprue is located to the side of the mold where a nice laminar flow can occur,

and the mold can fill without mold erosion and fill more evenly. The defects we produced were

excellent examples to show the problems that the casting process can have. This can now be

applied to make better parts and get rid of almost all defects. Porosity, however, cannot be

eliminated. It can be minimized and localized with correct mold design.

Conclusions and Recommendations:

The purpose of this experiment was to demonstrate the process of sand casting. While

being the oldest known form of casting, it is still a process that is very fundamental to

manufacturing and machining today. Sand casting is still a good way to mold complex and

large parts but is less suited for mass production. The instructions were to complete the casting

in a way that would create defects and imperfections in the final part to create better learning

objectives.

The mold was first measured with dial calipers. Then the mold walls were marked using

two rulers and a permanent marker to ensure proper placement of the sprue and riser. A metal

sieve was placed over the mold, the sand was hand ground through the sieve to cover the mold

with a fine even layer of sand. This ensured less air gaps trapped between the mold and sand.

Once the mold was covered evenly, unsieved sand was dumped and pack tightly around the

mold. The sand was beat with a wooden compactor to ensure a good compression of sand

around the mold. A straight edge piece of metal was then used to scrape the sand level with

the mold frame. The same process was then repeated on the other half of the mold. A sprue

and riser were cut into the mold using a hollow steel rod. The ruler was then used to cut a

funnel shape around the opening to help with pouring. Then the top half of the mold was pulled

off, the middle section was then pulled off carefully and evenly to not damage the mold cavity.

Aluminum was superheated in a crucible, the ladle was preheated over the top of the crucible

before pouring the aluminum into it. The ladle was then used to pour the molten aluminum into

the mold at a rapid rate to prevent defects. The pour time started as soon as the aluminum hit

the funnel of the sprue and was stopped when aluminum reached the top of the riser. It was

recorded using a digital stopwatch. After a ten minute cool down the mold was taken apart and

the part was broken from the sand with a mallet. The cast part was held with tongs and

quenched in a water bath due to its high temperature. The part was then analyzed for defects

and measured for final dimensions upon cooling.

Should the experiment be repeated, several recommendations have been made by both

the students and the instructor to improve molded part quality. The major issue present was the

high turbulence and mold erosion caused by the location of the fill hole. The poor choice of

location was also a major cause of porosity. If the fill hole were moved off to the side and

connected to the cavity at a 90* angle, this would allow all erosion and turbulence to occur off to

the side of the part and allow laminar filling of the mold.

The part had four major defects which were detectable with the naked eye. Mold

erosion, turbulence while filling, localized shrinkage forming sinkholes, and large amounts of

porosity throughout the part. The mold erosion could have been minimized by changing the

location of the sprue. The direct sprue into the mold caused too much turbulence at the bottom

of the mold. Cutting the sprue in away from the cavity would allow a nice laminar flow of the

molten aluminum into the mold cavity. The sprue being farther from the riser would have

created a more even fill rate which would have allowed gases and air to escape from the riser;

lowering the amount of porosity in the part and chances of depressions. The other defects,

such as extraneous error in the mold fill time calculation, were caused by human error during

the pouring process.

Overall the experiment was a success. The proper process of the lab was followed

correctly and the result of the aluminum part contained many defects. These defects were a

result of improper mold construction that was intentional to aid in learning the importance of

mold construction, and pouring techniques.

Attachments:Calculations,data sheets, figures, pictures, etc

(1) Shrinkage causes Depression on Surface (2) Finished Mold

(3) Mold filling lines (4) Porosity in Corner of Mold

(5) Pouring of Aluminum into Mold (6) Sifting of sand to get air gaps out

(7) Packing Sand into Mold (8) Positioning of Internal Block

(9) Molten Aluminum Transfer (10) Mold Erosion

(11) Half of mold with fill hole (12) Aluminum Pellet used to make mold

Calculations:

Flow RateV= sqrt(2*2.75 in*386 in/sec^2)= 46.0760 in/sec

Volumetric Flow Rate

Q= pi((.35)^2)(46.0760)= 17.7321 in^3/sec

Mold Fill TimeMFT= (3.965)(2.97)(1.56)/17.7321= 1.0360 sec

% error of MFT%error= 1.036/4.8= 4.63= 463%

Shrinkage ValueSv=[(1-0.07)(1-0.056)]^⅓ =0.9575

Calculated Part Shrinkage Values

L = (.9575)(3.965)= 3.7965”

W= (.9575)(2.97)= 2.8437”

H= (.9575)(1.56)= 1.4937”