Design of Pattern, Mould and Core.

1.DESIGN OF PATTERN:

1.1. What Is Pattern?

Ø Pattern is basically mould forming tool in the hands of foundrymen.

Ø Pattern is also defined as a model or replica of the object to be cast, but it only looks like a casting object. It has different shapes than the object to be cast.

Ø These extra dimensions of the pattern than the casting product are known as ‘Pattern Allowances.

NPTEL

1.2. Steps In Designing Of Pattern:

  The steps in designing of the pattern are given as follows:                                                            

              [A]. Selection of Pattern Material

              [B]. Determining Pattern Allowances

              [C]. Determining Pattern Type to Be Used For Particular

                     Application.

          We will discuss these steps in brief.

         [A].Selection Of Pattern Material:

Ø The common materials of which the patterns are made are as follows:                                                                             Wood,Metal,Plastic,Plaster,Wax

Ø Wood patterns are commonly used because they are cheap and easily available in large quantities.

Ø But wood patterns are weak & they absorb the moisture hence they get warped.

Ø Wooden patterns are used where the number of casting to be made is less and the size of the pattern is large.

Ø Metal patterns are cast from wooden patterns.

Ø Metal patterns are used when a large number of castings has to be produced from the same pattern.

Ø Plastic patterns are used because of their low weight, high strength & less costliness.

Ø Plaster patterns are used for making small and intricate patterns.

Ø Wax patterns are exclusively used in the investment casting process.   

       

         [B].Determining Pattern Allowances:

Ø Pattern is always made larger than the final casting because it carries certain allowances due to metallurgical and mechanical reasons.

Ø Allowances provided on the pattern:

1.    Shrinkage allowance

2.    Machining allowance

3.    Draft and taper allowance

4.    Distortion allowance

5.    Rapping allowance

Ø All the metals used In casting work shrink or contract during cooling, to compensate contraction adequate allowances are provided on the pattern.

Ø Machining allowance is the amount of dimension on a casting which is made oversized to provide stock for machining.

Ø Draft allowance is given for easy removal of the pattern from sand.

Ø The casting can distort sometimes, so to eliminate the distortion problem opposite distortion is provided on the pattern is known as distortion allowance.

Ø While taking out the pattern mould is rapped and shaken to make pattern free from adjoining sand, so there is little increase in mould size. So to compensate for this problem negative allowance i.e. rapping allowance is provided.

    [C].Determining Pattern Type to Be Used For Particular Application:

      1. Single piece or solid pattern: e.g. stuffing box of steam engine

      2.    Split pattern: e.g. casting of taps & water stopcocks

      3.    Loose piece pattern: e.g. fixed in the moulding sand

      4.    Cope & drag pattern: e.g. used for producing the large casting

      5.    Gated pattern: e.g. used for producing the small casting

      6.    Match plate pattern: e.g. piston rings of IC engine

      7.    Sweep pattern: e.g. large kettles of cast iron

      8.    Skeleton pattern: e.g. turbine casting, water pipes, L-Bends

      9.    Segmental pattern: e.g. used for producing large circular castings

     10.   Follow board pattern: e.g. casting master patterns

     11. Lagged up pattern: e.g. casting of the cylindrical workpiece

MOULD DESIGN:

When the pattern is made sand is packed around it.  When the pattern is removed from this sand cavity looking like the pattern is formed in the sand. This cavity is known as mould or mould cavity. In other words, the mould is the container which when poured with molten metal produces the casting of the mould shape.

Designing of the mould depends upon selecting mould material according to the application, selection of gating system, selection of runner system, selection of the de-gating system, a venting system for hot gases producing in the system. Mostly refractory sand is used as the mould material.

CORE DESIGN:

 A core is a sand shape or form which makes the contour of casting for which no provision is not made in the pattern for moulding. Core is made up of sand, plaster, metal or ceramics. The core is used to produce hollow castings, shapes which are difficult to produce by simple moulding.

Essential parameters in core design are,

1.    Diameter of the core
2. Core aspect ratio
3. Inter-core distance

DIAMETER OF THE CORE:

In most of the castings, low diameter holes can’t be produced by using cores. The main reason is thick sections of the castings try to fuse with small core material i.e. sand, metals. And then core can’t be separated from casting easily. So the minimum recommended core depends on the metal which to be cast, the casting process, the thickness of the section in which the hole is located, and the length of the core.

CORE ASPECT RATIO:

Aspect ratio means the ratio of the length of the core to a diameter of the core. If this ratio is increased then the core is said as Narrow Core. And if the aspect ratio of the core is large, metal cores can distort & sand cores can break under pressure. So the aspect ratio of the core should be selected within the limits.

INTER-CORE DISTANCE:

This parameter covers the point of limiting the centre distance between two adjacent cores. When two cores are placed very closely then it leads to a thin section between two holes. Because of that, the poured metal can’t occupy these thin sections completely. The heat transmission in the small section will be less due to less area for heat transfer. Because of less heat transfer in between two cores leads to shrinkage porosity in this small section due to this section will be porous an weaker. So to avoid these problems distance between the cores is kept greater than the core diameter.