Core and core making

  Introduction

1)      A core is essentially a body of materials which forms components of the mold. It possesses sufficient strength to be handled as an independent unit.

2)      Core is an obstruction which when positioned in the mold, naturally does not permit the molten metal to fill up the space occupied by the core. In this way a  core produces hollow casting

3)      Cores are required to create the recesses, undercuts and interior cavities that are often apart of castings.

4)      Cores are employed as inserts in moulds to form design features that are otherwise extremely difficult to produce by simple molding.

5)      A core may be defined as a sand shape or form which makes the contour of a casting for which no provision has been made in the pattern for molding.

6)      Cores are made up of sand, metal, plaster or ceramics.

7)      Cores are used to:

a.       Form the air-space between the cooling fins of an air cooled engine cylinder

b.      Make the water cooling chamber in internal combustion engine.

Different functions, purpose of cores:

1. For hollow casting, core provides the means of forming the main internal cavities.
2. Cores may form a part of green sand mould.
3. Cores may provide external undercut feature
4. Cores may be employed to improve the mould surface.
5. Cores may be inserted to achieve deep recesses in the castings
6. Cores may be used to strengthen the mould.
7. Some times the mould may be completed simply by assembling the core pieces or core.
8. Cores may be used to form the gating systems of large size moulds

Essential characteristics of (dry sand) cores:

Cores may possess:

1. Sufficient strength to support itself and to get handled without breaking.
2. High permeability to let the mould gases escape through the mould walls.
3. Smooth surface to ensure a smooth casting.
4. High refractoriness to withstand the action of hot molten metal(metal penetration).
5. High collapsibility in order to assess the free contractor of the solidifying metal.
6. Those ingredients which do not generate mold gases.

Core making procedures:

1. Core sand preparation
2. Making core
3. Baking core
4. Finishing core
5. Setting core

Making the core:

1. Small cores can be made manually in hand rammed core boxes.
2. Cores on mass scale are rapidly produced on a variety of core making machines, to name a few,
1. Jolt machine.
2. Core roll over machine
3. Sand slinger
4. Core extrusion machine
5. Core blower
6. Shell core machine

Core box:

1. A core box is basically a pattern for making cores
2. Core boxes are employed for ramming cores in them
3. Core boxes impart the desired shape to the core sand.
4. Core boxes range from simple wooden structures to precision metal assemblies which possess long life under extracting condition.

Types of core boxes:

a.       Half core box

b.      Slab or dump core box

c.       Split core box

d.      Left and right hand core box

e.       Strickle core box

f.       Loose piece core box.

g.      Gang core box.

            Equipments used for the baking of cores:

A.    Core ovens

a.       Batch type

                                                                          i.      Drawer type

                                                                        ii.      Rack type

b.      Continuous type

B.     Dielectric bakers

C.     Radiant bakers

            Finishing of cores:

1.      Baked cores are finished before they can be set in the mald.

2.      Core finishing consists of

a.       Cleaning

                                                                          i.      Trimming

                                                                        ii.      Brushing

                                                                      iii.      Coating

                                                                      iv.      Mudding

b.      Sizing

c.       Core assembly

Setting of cores:

1. Core setting means placing cores in the mold
2. In order to obtain correct cavities in the castings the cores should be accurately positioned in the molds.
3. Cores in the moulds should be firmly secured so that they can withstand the buoyancy effect of the being poured molten metal.
4. Small cores are set in the moulds by hand whereas big cores may required a crane for the purpose.

Types of cores

1. The state or condition of core
1. Green sand core
2. Dry sand core
2. The nature of core materials employed
1. Oil bonded cores
2. Resin bonded cores
3. Shell cores
4. Sodium silicate cores
3. The type of core hardening process employed
1. Co2 process
2. The hot-box process
3. The cold set process
4. Fluid or castable sand process
5. Nishiyama process
6. Furan no-bake system
7. Oil no-bake system
4. The shape and position of the core
1. Horizontal core
2. Vertical core
3. Hanging or cover core
4. Balanced core
5. Drop core or stop off core
6. Ram up core
7. Kiss core

1. Green sand core:
1. Green sand cores are formed by pattern itself.
2. A green sand core is a part of the mold.
3. A green sand core is made out of the same sand from which the rest of mold has been made i.e molding steel.

2. Dry Sand cores
1. Dry sand cores, unlike green sand cores are not produced as a part of the sand.
2. Dry sand cores are made separately and independent of that mold.
3. A dry sand core is made up of core sand which differs very much from the sand out of which the mold is constructed.
4. A dry sand core is made in a core box and it is baked after ramming.
5. A dry sand core is positioned in the mold on core seats formed by core print on the patten.
6. A dry sand core is inserted in the mold before closing the same.

1)      Oil bonded cores:

a)      Conventional sand cores are produced by mixing silica sand with a small percentage of linseed sand.

2)      Resin – bonded cores:

a)      Phenol resin bonded sand is rammed in a core box.

b)      The core is removed from the core box and baked in a core oven at 375 to 450 f to harden the core.

3)      Shell cores:

a)      Shell cores can be made manually or on machines.

b)      The procedure of making shell cores is as follows:

1)      The core box is heated to temperature of the order of 400 to 600 F.

2)      Sand mixed with about 2 to 5 % thermosetting resin of phenolic type is either dumped or blown into the preheated metal core box.

3)      Where sand blowing is employed, it is preferred to use resin precoated sand to avoid resin segregation.

4)      The resin is allowed to melt to the specified thickness.

5)      The resin gets cured.

6)      The excess sand is dumped and removed.

7)      The hardened core is extracted from the core box.

8)      Cores thus produced needs no further baking.

c)      Shell core posses very smooth surface(3125 micro mm root mean square) and close tolerance .(+_ 0.003 mm/mm)

d)     Shell core making process can be mechanized and several core making machines are commercially available.

e)      High permeability is achieved in shell core making .

f)       Shell cores can easily stored for future use.

g)      Shell cores are costly as compared to cores produced by other methods.

4)      Sodium silicate – CO2 cores:

a)      These cores use a core material consisting of clean, dry sand mixed with a solution of sodium silicate.

b)      The sand mixture is rammed into the core box.

c)      The rammed sand while it is in the core box is gassed for several seconds with CO2 gas. As results a silica gel forms which binds sand grains into a strong solid form.

Na2SiO3+CO2 –Na2CO3+SiO2 (silica sand)

d)     Cores thus produced usually need no baking.

e)      Cores thus formed possess more strength than the oil /resin bonded cores.

f)       Unhardened cores are not handled so that there is no chance of braking or sagging of cores.

g)       Core dryer is not required.

h)      Core formed by CO2 process are used in the production of cast iron, steel, aluminum and copper base alloy castings.

i)        The used sand mixture however can not be recovered and reused.

5)      Hot box process :

a)      It uses heated core boxes for the production of cores.

b)      The core box is made up of cast iron, steel or aluminum and possesses vents and ejectors for removing core gases and stripping cores from the core box respectively.

c)      Core box is heated from 350 to 500 F.

d)     Heated core box is employed for making shell cores from dry resin bonded mixtures.

e)      Hot core boxes can also be used with core sand mixture employing liquid resin binders and a catalyst.

6)      The cold set process :

a)      While mixing the core sand, an accelerator to the binders.

b)      The sand mixture is very flowable and is easily rammed.

c)      Curing begins immediately with the addition of accelerators and continues until the core is strong to be removed from the core box.

d)     A little heating of the cores hardens it completely.

e)      Cold set process is preferred for jobbing production.

f)        Cold set process is employed for making large cores.

7)      Castable sand process :

                                                              i.      A setting or hardening agent such as dicalcium silicate is added to sodium silicate at the time of core sand mixing.

                                                            ii.      The sand mixture possesses high flowability and after being poured in the core box, it chemically hardens after a short interval of time.

                                                          iii.      As compared to CO2 process, where it may not be possible to gas the full core uniformly and to obtain uniformly hardened cores, castable sand process produces much better and uniform results.

                                                          iv.       Castable sand process is best suited for large jobbing work.

1)      Nishiyama process :

a)      Nishiyama process uses sodium silicate bonded sand, which is mixed with 2% finely powered ferrosilicon.

b)      Hardening occurs because of exothermic reaction of silicon with Noah produced by hydrolysis in the solution of sodium silicate.

c)      Cores thus made posses short bench life.

2)       Furan no-bake system :

a)      The core sand mixture contains washed sand dried sand with clay content less than 0.5%, furan no-bake resin 2% and activator (phosphoric acid) 40%.

b)      The basic reaction between the furan resin and phosphoric acid results in an acid dehydration of the resin.

c)      The core sand mixture has high flowability and needs reduced rodding.

d)     Uniform core hardness, exact core dimension, better fitting cores, lower machining and layout costs and reduction of oven baking are some of the good characteristics of cores made by this system.

3)      Oil no-bake process :

a)      The process employs a synthetic oil binder which when mixed with basic sands activated chemically produces cores that can be cured at room temperature.

b)      The sand may consists of:

-          Washed and dried sand                500 kg

-          Oil no-bake binders and catalyst   7 kg

-          Oil no-bake cross linking agent      1.4 kg

c)      In oil no-bake process, the polymerization reaction results in a complete and uniform setting of the complete core sand mass.

d)     This process assures better depth of set, fast baking, easier core withdrawal and lower production cost as compared to furan or oil bonding process.

  

4)      Horizontal core :

5)      Vertical cores :

6)      Hanging or cover core :

7)      Drop or stop off core:

8)      Balanced core :

9)      Ram up core :

10)  Kiss core :

Core applications :

1)      Core and core forms greatly increase the versatility of molding and casting operations.

2)      Before being used for forming recesses and holes in the castings, cores are also employed:

a)      As a strainer, gates and pouring cups.

b)      As riser core

c)      For making molds

d)     As core mold in centrifugal casting process

e)      As slab core for increasing casting output from one mold.

f)       For increasing production from mold plate molding.