Demonstration Guide for Sand Casting of Metal

            Metal casting, generally, involves heating of a metal beyond its melting point and pouring the molten metal into a mold where it cools and solidifies.  In expendable mold metal casting techniques, the mold cavity is built around a pattern.  Therefore, it may be necessary to prepare a pattern and prepare a mold, while preparing the material for casting.  The sand casting process is an expendable mold metal casting technique.  It may employ a permanent or expendable pattern, both of which are examined in this package.  There are a variety of tools used to prepare the mold, some are common to both expendable and permanent pattern casting, while others are unique to the technique used.

            The sand casting process typically takes place in a foundry.  A foundry is a place where the tools and materials are located to produce many types of sand castings, using different and varied processes.  Casting, in our case, is the process of pouring molten metal into a mold cavity, formed of sand, to produce a near net shape metal object.  The sand used in the mold production phase is a silica sand (silicon dioxide), mined from river deposits, lakes,  and other large bodies of water.  Regardless of the pattern production method, or the binding mechanisms that may be employed to allow the cohesionless sand to maintain a shape, the five following steps are generally common to all sand casting processes:

                        1. Mold preparation

                        2. Material preparation

                        3. Introduce material into mold

                        4. Solidify material

                        5. Remove part

            Even when considering a high volume production process, mold preparation remains a time liability.  Once a large crucible of material is heated to pouring temperature, it can be maintained, whereas the molds still need to be individually produced.  This package describes methods employed to hand pack (ram) sand around the pattern to produce a mold cavity.

I.      Permanent Pattern Sand Mold Casting

A.      General Description - This process uses a permanent pattern, typically made from  wood, plastic, or aluminum, that is a duplicate of the part to be produced (although it is often somewhat oversized to allow accurate sizing when the cast metal shrinks during cooling). Sand, mixed with some form of binder, is placed around the pattern and   compacted to the point that it will hold the cavity shape when the pattern is removed  Molten metal may then be poured directly into this open cavity, or it may be poured  through passageways if the cavity is closed.  After the metal has cooled to the point that it solidifies, the sand shell is broken away to remove the part.

B.      Machine Design - The actual process of pouring (sometimes called casting) of the  metal into the  mold cavity may not use any machinery (as is the case in this demonstration).  However, machinery is typically involved during the mold preparation  phase.  Two machines are common: the mull and the riddle.  The mull is a large mixing vat that employs power driven rotating wheels inside a hollow canister.  Sand particles  placed into the mull are thoroughly impregnated with binders such as clay, oil, water, and/or catalysts through compression and mixing by the rotating wheels.  The mull typically has a hinged top for adding materials, and a chute in the bottom that is used to expel mulled sand when the gate in the chute is opened.

                                         Inside View of Mull

                                       Often, the sand expelled from the mull is clumped into large globs of                         material.  If left in  this form the sand particles may not compact tightly against the                         pattern during ramming,   leaving imperfections in the cavity when the pattern is                         removed.  To help overcome this problem, mulled sand is typically riddled before                         ramming.  The power riddle is a large  sifter, having a screen wire element that                         breaks up the clumped sand into finer particles  through agitation (shaking) by a cam                         drive attached to an electrical motor.  The  riddle consists of a ring having the screen                         attached to one open face, a yoke frame attached to the ring for air suspension, and                         the motor/cam drive attached to the frame to transmit the vibration that feeds mulled                         sand through the riddle.  Riddled sand will fall  through gravitational forces through                         the screen into a container (or a pit) and may be considered ready for ramming                         although a hand riddle (having a finer screen) is typically  used as a final filter as sand                         is being poured onto the pattern.

Power Riddle

                        C.      Tooling and Materials -  Most of the materials and tooling used in                         permanent  pattern sand mold casting are also used (or usable) in expendable                         pattern mold casting.  Unless otherwise noted the following tools and materials will                         be used in both processes.

Silica Sand:  SILICA SAND is SiO₂ (silicon dioxide). It is used in sand casting for   several reasons. It is an inexpensive, readily available, and recyclable material. Two   major types are bonded and unbonded. Washed and dried sand that is unbonded is usually core sand. A naturally bonded silica sand contains 70% or more of sand grains, with the balance being clay as the bonding agent. Molding sands need to have cohesive properties, along with permeability and refractory capabilities.

  Permanent Pattern: (Only used in permanent pattern sand mold casting). A full sized model of        the object to be cast, the pattern can be made from many different materials. It is the tool which the sand is compressed around, to form the cavity of the mold. The different types of these are single sided, double sided (match plate), loose, cope and drag, and special patterns or pattern  devices. Along with the model of the pattern, the pattern may have some other geometry that is  normally part of the feeder system. The most common attachment is the sprue well configuration. This allows the molten material entering the cavity a place to settle any foreign materials in the pour.

Pattern Board

  Flask: A frame into which the sand is compressed around a pattern. This frame can be a one piece, a one piece removable, or a two piece. When it is a two piece frame, the halves are named. What will eventually become the bottom half is called the drag half. It can be identified by the alignment pins protruding from the flanges at the ends of the frame. The top half, called the cope, can be identified by the alignment holes machined into the flanges at the ends of its frame. Other than the alignment pins, the frame halves are usually identical.

Assembled Flask and Pattern Board

Peen Rammer: A ramming tool with a flat hammer configuration at one end, and a narrow edge  forming design at the other. This tool is used to compress sand around the pattern, and around the  inside of the flask.

Hand Riddle: The riddle is a sifting tool that allows the mold-maker to be sure of a very fine sand texture directly in contact with the pattern. These can be of varying degrees of size and  mesh. All riddling of the sand is done to reduce it to its purest usable form.

Hand Riddle and Peen Rammer

Slick: A small spoon shaped tool used for contouring of the mold such as cutting the pouring basin

Molders Trowel: Used to contour and smooth large surfaces of the mold.

Strike-Off Bar: The strike-off  bar is a straight edged steel bar, used for leveling the extreme  upper and lower flask surfaces. These surfaces are flattened to the height of the flask edge. The bottom side is used for a level surface to set the mold on. The top is leveled prior to cutting the sprue and the pouring basin using the strike-off bar.                                                      

 Sprue Cutter: (Only used in permanent pattern sand mold casting).  A tapered hollow tube that       cuts through the compacted sand of the cope half of the mold. The sprue is the passageway into which the molten material is poured into the cavity created by the pattern.

Typical Hand Tools

Molders Bench: The metal workbench that is designed to hold some of the molders tools and accommodate the pattern board, flask, and area for retaining a good portion of the sand. This bench is provided with a bottom tray for catching sand from the riddling and strike off  procedures. The molders bench also has rails and ties to support the mold/flask halves.

Molders Bench

D.      Mold Preparation Procedure - With some minor exceptions the procedure for              preparing the mold in both permanent and expendable pattern sand mold casting is the same:

1.         Using the molders bench, lay the pattern board on the rails, sprue well up, and  insert the             cope side of the flask into the alignment holes that coincide with the drag pins.

 2.         Place a chalk marks on the edges of the pattern board, in alignment with the sprue  well.              There should be a mark on both the long and short axes of the pattern  board. This will              allow for cutting of the sprue at a later time.

  3.         Dust a mold release, or parting compound, made of talc, over the pattern  geometry. This              will facilitate removal of the pattern once the flask half has been  rammed.

 4.         Riddle prepared sand (mulled) over the pattern that is facing up through the cope side of the              flask. This is done to a depth of about 1 to 1-1/2 inches. The finely sifted sand helps ensure              that a smooth surface finish is attained. This fine layer of sand is hand packed as evenly as              possible over the pattern geometry.

 5.         Continue to introduce sand into the mold (no need to riddle) until the flask half is about 1/2              to 2/3 full.

  6.         Peen ram the sand around the edges of the flask and directly over the pattern  geometry.

 7.         Fill the mold half to completely overflowing with prepared sand. Ram this sand  until it is              thoroughly compacted.

 8.         Using the strike-off bar, "grade" the sand until it is level with the sides of the flask  half.

  9.         At this time the bottom (cope) half of the mold is complete. The mold half now  needs to be              turned over to facilitate the completion of  the mold.  Place a blank  moldboard on top of the              cope half of the mold.  Gripping the flask  and moldboard, turn the flask over with the              alignment pins pointing up.

 10.       Guide the "drag half" of the flask onto the "cope" alignment pins, and settle it onto the pattern               board.

  11.       Continue this half of mold preparation by following steps #3 through #8.

 12.       After completing the strike-off procedure for this half of the mold, it is time to cut the sprue.              First find the alignment chalk marks on the edge of the pattern board. Using the sprue cutter,              and lining up the marks, firmly press the cutter down  through the compacted sand, turning it              slightly from side to side, until it bottoms out on the pattern board.

  13.       With the sprue cutter still buried in the sand, cut the sprue basin with the spoon shaped tool              (slick). Do this by digging a shallow bowl around the sprue cutter.  Pack any remaining loose              sand by hand.

 14.        Cut another shallow bowl a few inches away from the sprue basin. Connect the two bowls               with a small trough that is not as deep as the two bowls. This second basin allows the               molten material to be poured first into it, and then flow up and through the trough into the               sprue basin and down the runway (sprue passage) into the mold cavity.

 15.        Remove the sprue cutter, being careful not to let any loose sand fall down the sprue. Pack               any loose sand by hand that may be on the top of the mold.

                II.    Full Mold Sand Casting

  A.      General Description - This mold making process uses a consumable pattern, typically made of an expanded foam (Styrofoam) product.  In the previous sand mold making process the mold cavity was made up in the flask halves.  In the full mold  process the mold cavity is created by vaporizing the foam pattern while simultaneously filling the cavity with molten material.  This process utilizes the production capabilities of foam molding and takes advantage of faster mold making, typically better surface finish on the part, and elimination of many core-making processes.  The latter is accomplished by filling any voids that need to be cast in the part geometry with the dry molding sand. The sand used in this process is essentially the same with the exception of needing less preparation. It will need to be riddled and mulled as before, but  does not require large amounts of additives, if any.

B.      Machine Design - The machinery required in the full mold process is essentially the same as used in the match plate process.  The riddle and mull will be used, as before, and in addition we will use the squeeze/jolt machine.  This machine assists in the settling of the sand around the foam pattern, and to fill any voids that may be needed.  This settling may also be accomplished with vibrating tables or shakers.

C.      Mold Preparation Procedure

1.         The full mold will be prepared at the molders bench, using a two piece flask.  A mold   blank              board is placed on the bench first.  The flask is assembled and set on the moldboard

2.         A thin layer of sand is rammed evenly on the bottom of the flask (moldboard).  Loose sand             is placed on top of the compressed sand, and the pattern (sprue up) is placed and settled into             the layer of loose sand.

3.         Loose sand is used to cover the entire pattern to the upper edge of the flask (filled).

 4.        The flask and moldboard are moved to the squeeze/jolt machine, and placed on its lower             table.

5.         The air valve is opened on the machine and it will begin to move up and down, jolting the             mold on the down stroke.  This action settles the sand all around the pattern.  If needed, sand             may be added to the mold to maintain a full flask.

6.         Make sure the mold is full and well settled.  At this time the sprue needs to be checked to              ensure it is sticking up though the top layer of sand.  If it is well above the flask top, it will              have to be trimmed flush with the top.  A small bowl shaped basin can be formed around the              sprue to ease pouring, if needed.

1.                  Describe “draft” and its effect on the mold-making process.

2.                  Describe the effects of draft on finished or as-cast parts.

3.                  Explain why a metal casting process demonstrated in lab is called permanent pattern, expendable mold casting?

4.                  Give three reasons for using silica sand as a mold-making medium.

5.                  How would the casting of different metals require changing of the pattern?

6.                  Why might the process of casting metal be considered economically advantageous?

7.                  Name two metal casting processes that use expendable mold casting.

8.                  Describe full mold metal casting in terms of cavity content?

9.                  In our metal casting demonstration, the metal was heated in a container inside the furnace.  What is this container called?

10.              List the five steps common to all sand casting processes.

11.              What tool is used to level the compacted sand in a full flask?

12.              What is the function of both mechanically operated and hand operated riddles?

13.              How is sand reconditioned prior to the mold making process?