Casting Processes

Casting Processes

  Shell Moulding

   It is a process in which, China precision machining the sand mixed with a thermosetting resin allowed to come into contact with a heated metallic pattern plate, so that a thin and strong shell of mould is formed around the pattern. Then the shell is removed from the pattern and the cope and drag are removed together and kept in a flask with the necessary back up material and molten metal is poured into the mould.

  Generally, dry and fine sand (90 to 140 GFN) which is completely free of the clay is used for preparing the shell moulding sand. The grain size to be chosen depends on the surface finish desired on the casting. Too fine a grain size requires large amount of resin which makes the mould expensive.  the synthetic resins used in shell moulding are essentially thermosetting resins, which get hardened irreversibly by heat. The resins most widely used, are the phenol formaldehyde resins. Combined with sand, they have very high strength and resistance to heat. The phenolic resins used in shell moulding usually are of the two-stage type, that is, the resin has excess phenol and acts like a thermoplastic material. China precision machining During coating with the sand the resin is combined with a catalyst such as hexa-methylene-tetramine (hexa) in a proportion of about 14 to 16% so as to develop the thermosetting characteristics.

  Additives may sometimes be added into the sand mixture to improve the surface finish and avoid thermal cracking during pouring. Some of the additives used are coal dust, pulverised slag, manganese dioxide, calcium carbonate, ammonium borofluoride and magnesium silicofluoride. Some lubricants such as calcium stearate and zinc stearate may also be added to the resin sand mixture to improve the flowability of the sand and permit easy release of the shell from the pattern. 

  The first step in preparing the shell mould is the preparation of sand mixture in such a way that each of the sand grain is thoroughly coated with resin. To achieve this, first the sand, hexa and additives which are all dry, are mixed inside a Mueller for a period 1 min. Then the liquid is added and mixing is continued for another 3min. To this cold or warm air is introduced into the Mueller and the mixing is continued till all the liquid is removed from the mixture and coating of the grains is achieved to the desired degree. 

 The metallic pattern plate is heated to a temperature of 200 to 350 ºC depending on the type of the pattern. It is very essential that the pattern plate is uniformly heated so that the temperature variation across the whole pattern is within 25 to 40 ºC depending on the size of the pattern. A silicone release agent is sprayed on the pattern and the metal plate. The heated pattern is securely fixed to a dump box, as shown in

Fig. 6.7 (a), wherein the coated sand in an amount larger than required to form the shell of necessary thickness is already filled in.   

Fig. 6.7 Shell moulding procedure 

     

 When a desired thickness of shell is achieved, the dump box is rotated backwards by 180 so that the excess sand falls back into the box, leaving the formed shell intact with the pattern as in Fig. 6.7 (d). The average shell thickness achieved depends on the temperature of the pattern and the time the coated sand remains in contact with the heated pattern. The actual shell thicknesses required depends on the pouring metal temperature and the casting complexity. This may normally be achieved by trial and error method.  

               Fig.6.8 Shell mould ready for pouring

 Since the shells are thin, they may require some outside support so that they can withstand the pressure of the molten metal. A metallic enclosure to closely fit the exterior of the shell is ideal, but is too expensive and therefore impractical.

  Alternatively, a cast iron shot is generally preferred as it occupies any contour without unduly applying any pressure on the shell. China precision machiningWith such a backup material, it is possible to reduce the shell thickness to an economical level.  

Chill

   Chill

  In a casting, metallic chills are used in order to provide progressive solidification or to avoid the shrinkage cavities. China CNC machining Chills are essentially, large heat sinks. Whenever , it is not possible to provide a riser for a part of the casting which is heavy, a chill is placed close to it as shown in Fig. 5.29, so that more heat is quickly absorbed by the chill from the larger mass making the cooling rate equal to that of the thin sections. Thus, this dose not permit the formation of a shrinkage cavity. But use of a chill means essentially providing higher cooling rate which is also likely to form a hard spot at the contract area with the chill and may therefore cause a problem if that area needs further processing way of machining.   

  Melting Practice

 The preparation of molten metal for casting is referred to simply as melting. Melting is usually done in a specifically designated area of the foundry, and the molten metal is transferred to the pouring area where the molds are filled. After moulding, melting is the major factor which controls the quality of the casting. There are a number of methods available for melting foundry alloys such as pit furnace, open hearth furnace, rotary furnace,China CNC machining cupola furnace, etc. The choice of the furnace depends on the cupola in its various forms is extensively used basically because of its lower initial cost and lower melting cost.  

  cleaning

  Cleaning refers to all operations necessary to the removal of sand, scale, and excess metal from the casting. The casting is separated from the mold and transported to the cleaning department. Burned-on sand and scale are removed to be improved the surface appearance of the casting. Excess metal, in the form of fins, wires, parting line fins, and gates,China CNC machining  is removed. Castings may be upgraded by welding or other procedures. Inspection of the casting for defects and general quality is performed 

  Other processes 

Before shipment, further processing such as heat-treatment, surface treatment, additional inspection, or machining may be performed as required by the customer's specifications.  

Sprue base well

Sprue base well

  This is a reservoir China precision machining  for metal at the bottom of the sprue to reduce the momentum of the molten metal. The molten metal as it moves down the sprue grains in velocity some of which is lost in the sprue base well by which the mould erosion is reduced.

 This molten metal changes direction and flows into the runners in a more uniform way.   

Runner

 It is generally located in the horizontal plane (parting plane) which connects the sprue to its ingates, thus letting the metal enter the mould cavity. The runners are normally made trapezoidal in cross section. It is a general practice for ferrous metals to cut the runners in the cope and the ingates in the drag. The main reason for this is to trap the slag and dross which are lighter and thus trapped in the upper portion of the runners.

 For effective trapping of the slag, runners should flow full. When the amount of

molten metal coming from the down sprue is more than the amount flowing through the ingates, the runner would always be full and thus slag trapping would take place. But when the metal flowing through the ingates is more than that flowing through the runners, then the runner would be filled only partially and the slag would then enter the mould cavity. 

   Runner extension

  The runner is extended a little further after it encounters the ingate. This

extension is provided to trap the slag in the molten metal. The metal initially comes along with the slag floating at the top of the ladle and this flows straight, going beyond the ingate and trapped in the runner extension.

   Riser  Most of the foundry alloys shrink during solidification. Table 10.1 shows the various volumetric shrinkages for typical material. As a result of this volumetric shrinkage during solidification, China precision machining  voids are likely to form in the castings unless additional molten metal is fed into these places which are termed as hot spots since they remain hot till the end. Hence a reservoir of molten metal is to be maintained from which the metal can flow readily into the casting when the need arises. These reservoirs arecalled risers. 

 As shown in Table 5.6, different materials have different shrinkages

and hence the riser requirements vary for the materials. In grey cast iron, because of graphitization during solidification, there may be an increase in volume sometimes. This if course, depends on the degree China precision machining   of graphitization in grey cast iron which is controlled by the silicon content. 

  In order to make them effective, the riser should be designed keeping the following in mind.

 1. the metal in the riser should solidify in the end. 

 2. the riser volume should be

sufficient for compensating the shrinkage in  the casting. 

      Table 5.6 volumetric liquid shrinkages

  Material            Shrinkage,(%)

Medium carbon steel     2.50 to 3.50

High carbon steel         4.00

 Nickel               6.10

Monel               6.30

Aluminum              6.60

Aluminum alloy (11-13% Si)   3.50

 Aluminum bronze         4.10

 Copper                  4.92

 70-30 brass              4.50

 Bearing bronze           7.3

 Grey cast iron             1.90 to negative

 White cast iron           4.00 to 5.75

 Magnesium              4.20

  Zinc                   6.50

Pouring basin

   Pouring basin 

  The molten metal is not directly poured into the mould cavity because it may cause mould erosion.China precision machining  Molten metal is poured into a pouring basin which acts as a reservoir from which it moves smoothly in to sprue. The pouring basin is also able to stop the slag from entering the mould cavity by means of a skimmer or skim core as shown in Fig.5.27. It hold back the slag sand dirt which floats on the top and only allow the clean metal underneath it into the sprue. The pouring basin may be cut into the cope portion directly or a separate dry sand pouring basin may be prepared and used as

shown in Fig.5.27. the molten metal in the pouring basin should be full during the pouring operation, otherwise a funnel is likely to form through which atmospheric air and slag may enter the mould cavity.

   

  (a) Green sand

(b) Dry sand Fig. 5.27 Pouring basin  

 One of the walls of the pouring basin is made inclined at about 45°to the horizontal. The molten metal is poured on this face such that metal momentum is absorbed and vortex formation is avoided. In some special cases the pouring basin may consist of partitions to allow for the trapping of the slag and maintaining constant metal height in the basin.    Sprue 

 China precision machining  Sprue is the channel through which the molten metal is brought into the parting plane where it enters the runners and gate to ultimately reach the mould cavity.

The molten metal when moving from the top of the cope to the parting plane grains in velocity and as a consequence requires a smaller area of cross section for the same amount of metal to flow at the top. If the sprue were to be straight cylindrical as shown in Fig. 5.28(a), then the metal flow not be full at the bottom, but some low pressure area would be created around the metal in the sprue. Since the sand mould is permeable, atmospheric air would be breathed into this low pressure area which would then be carried to the mould cavity. To eliminate this problem of air aspiration the sprue is tapered to gradually reduce the cross section as it moves away from the top of the cope as China precision machining  hown in Fig.5.28(b).  

 (a)Straight sprue

   (b) Tapered  

Coremaking

    Coremaking

   China precision machining Cores are the materials used for making cavities and hollow projections which cannot normally be produced by the pattern alone. 

  Any complicated contour or cavity can be made by means of cores so that really intricate shapes can be easily obtained. These are generally made of sand and are even used in permanent moulds. In general, cores are surrounded on all sides by the molten metal and are therefore subjected to much more severe thermal and mechanical conditions and as a result, China precision machining the core sand should be of higher strength than the moulding sand.

As defined earlier, gating system refer to all those elements which are connected

with the flow of molten metal from the ladle to mould cavity. The various elements that are

connected with a gating system are (Fig. 5.26): 

  

  Pouring

  basin 

  Sprue  

 Sprue base well 

  Runner  

  Runner extension 

  Riser 

 Chill  

  Fig.

Typical gating system  

In this section the functions and the design of the various elements of a gating system

will be discussed. Any gating system designed should aim at providing a defect

free casting. This can be achieved by making provision for certain requirements while designing the gating system. These are as follows: 

 1.the mould should be completely filled in the smallest time possible without having to raise metal temperature nor use higher metal heads.

 2. the metal should flow smoothly into the mould without any turbulence.

A turbulent metal flow tends to form dross in the mould

 3. unwanted material such as slag, dross and other mould material should not be  allowed enter the mould cavity

 4.the metal entry into the mould cavity should be properly controlled in such a

way that aspiration of the atmospheric air is prevented

5. a proper thermal gradient should be maintained so that the casting is cooled  without any shrinkage cavities or distortions

 6. metal flow should be maintained in such a way that no gating or mould  erosion take place.

 7. the gating system should ensure that enough molten metal reaches the mould  cavity 

8. the gating system design should be economical and easy to implement and  remove after casting solidification 

9. ultimately, the casting yield should be maximized 

  ToChina precision machining  have all these requirements together is a tall order, still a mould designer should strive to achieve as many of the above objectives as possible. Before going into the mechanics of gating design, let us describe some of the functions and types of various gating system elements

Renoho Precision Machinery Technology Co.,Ltd: Basic Design of Metal Processes

Renoho Precision Machinery Technology Co.,Ltd: Basic Design of Metal Processes: Basic Design of Metal Processes  Patternmaking   The pattern is a physical model of the casting used to make the mold.     China prec...

Basic Design of Metal Processes

Basic Design of Metal Processes

 Patternmaking

  The pattern is a physical model of the casting used to make the mold. 

  China precision machining The mold is made by packing some readily formed aggregate material, such as molding sand, around the pattern. When the pattern is withdrawn, its imprint provides the mold cavity, which is ultimately filled with metal to become the casting.  

 If the casting is to be hollow, as in the case of pipe fittings, additional patterns, referred to as cores, are used to form these cavities. As has been defined earlier,   China precision machining a pattern is a replica of object to be made by the casting process, with some modification.

 The main modifications are:     

a) the addition of pattern allowances,

(b) the provision of core prints,  

(c) the elimination of fine details which cannot be obtained by casting and hence are to be obtained by further process. 

 Core prints 

 For all those casting where coring is required, provision should be made to support the core inside the mould cavity. One of the methods that is universally followed is to provide core prints where possible. In Fig 5.22 is shown an example of the provision of core prints. The size of the core prints to be provided is to be estimated based on the specific casting.  

 Fig.5.22 Typical job, its pattern and the mould cavity 

Moulding Materials 

 A large variety of materials are used in foundries for manufacturing moulds and cores. They are:  Moulding sand,   System sand (backing sand),  Rebounded sand,  Facing sand,  Parting sand,   Core sand.   

 The properties of moulding sand

  The choice of moulding materials is based on their processing properties. The properties that are generally required in moulding materials are:  

 Refractoriness: it is the ability of the moulding material to withstand the high temperatures of the molten metal so that it dose not cause fusion.   Green strength: The moulding sand that contains moisture is termed as green sand.

 The green sand should have enough strength so that the constructed retains its shape.   Dry strength: When the moisture in the moulding sand is completely expelled, it is called dry sand. When molten metal is poured into a mould, the sand around the mould cavity is quickly converted into dry sand as the moisture in the sand immediately evaporates due to the heat in the molten metal. At this stage, it should retain the mould cavity and at the same time withstand the metallostatic forces.     Hot strength: After all the moisture is eliminated, the

sand would reach a high temperature when the metal in the mould is still in the liquid state. The strength of the sand that is required to hold the shape of the mould cavity then is called hot strength.  Permeability: During the solidification of a casting, large mounts of gases are to be expelled from the mould. The gases are those which have been absorbed by the metal in the furnace, air absorbed from the atmosphere and steam and other gases that are generated by the moulding and core sands. If the gases are not allowed to escape from the mould, they would be trapped inside the casting and cause defects. The moulding

sand should be sufficiently porous so that the gases are allowed to escape from the mould. This gas evolution capability of the moulding sand is termed as permeability. 

  Beside those specific properties, the moulding sand should also have collapsibility so that during the construction of the solidified casting, it dose not provide any resistance which may result in cracks in the casting, they should be reusable and should have good thermal   China precision machiningconductivity so that heat from the casting is quickly transferred.