Tool Design Theory (DCD)

TOOL DESIGN THEORY

(DCD)

(5

TH

SEMESTER)

DTM

MSME TOOL ROOM

INDO GERMAN TOOL ROOM

SUBJECTS

&

COURSE CONTENT

In

DIPLOMA IN TOOL & DIE MAKING

(DTM)

A Introduction A1 A.1.1.1 Die Casting Background A.1.1.1(a) What is Die Casting A.1.1.3(a) Ciassification Of Castings

A.1.1.3 Principle Of Die Design And Process A.1.1.3(a) Advantages Of Die Casting Techniques A.1.1.3(b) Technique of filling die cavity

A.1.1.4 Comparision of die casting with other products.

B Die casting operations B1 Gravity die casting B.1.1.1 Introduction

B.1.1.2 Gravity die casting process

B.1.1.3 Limitation of permanent mould castings B.1.1.4 Principle Of permanent mould casting B.1.1.4(a) Progressive solidification

B.1.1.4(b) Minimum turbulance B.1.1.4(c) Air and gas clearance

B.1.1.5 Suitable casting metals for GDC B.1.1.6 Selection of mould materials B.1.1.7 Permanent mould casting machines B.1.1.8 Gravity die casting mould

B.1.1.9 Principle of mould design B.1.1.10 Solid graphite permanent mould B2

Pressure Die casting (cold

chamber) B.2.1.1 Introduction

B.2.1.2 Pressure die casting machines B.2.1.3 Classification of die casting machines B.2.1.4 Cold chamber machines

B.2.1.5 Cold chamber machines and parts

B.2.1.6 Classification of cold chamber die casting machines Over view of mass

production of Casting Parts Produced by various Casting Process UNIT CHAPTER NO. PR AC T IC E T O PI C S & SU B T O PI C THEORY

TOPICS & SUB TOPICS

CHAPTER PAR

B.3.1.4 Hot chamber machines and parts

B.3.1.5 Classification of hot chamber die casting machines B4 Defects and remedies of die casting componentsB.4.1.1

Introduction to various defects of die casting components

B.4.1.2 Identification of defects B.4.1.3 Classification of defects B.4.1.4

Description for different types of defects and their causes and remedies

C

Elements of die casting and their

function C1 Feeding system C.1.1.1

Introduction of feeding system of die casting components

C.1.1.2 Definition of elements and fuction of feed system C.1.1.3 Feed system for gravity die casting dies

C.1.1.4 Feed system for hot chamber castings dies C.1.1.5 Feed system for core chamber casting dies C.1.1.6 Classification of gate systems

C.1.1.7 Principle of feed system C.1.1.8 Balancing of feed system

C2 Cooling system C.2.1.1 Introduction to cooling of die casting dies

C.2.1.2 Definition of elements and fuction of cooling system C.2.1.3 Principle of cooling of die casting dies

C.2.1.4 Classification of cooling systems C3 Ejection system or techniquesC.3.1.1 Introduction

C.3.1.2 Various elements of ejection techniques and fuctions C.3.1.3 Various ejection techniques

C.3.1.4 Principle of ejection system

D Material handling D1 Pre-casting D.1.1.1 Introduction

D.1.1.2 Principle of metal casting technique D.1.1.3 Classification of metal casting technique D.1.1.4 Pre-casting techinque

D.1.1.5 Pre-casting techinque related equipments D2 Post-casting D.2.1.1 Introduction of post-casting technique

E Maintenance, safety and storage E1 Maintenance, safety and storage with respect to die casting die and machineE.1.1.1 and machines E.1.1.2 Concept of safety

E.1.1.3 Concept of maintenance E.1.1.4 Concept of storage E.1.1.5 Safety of die casting die

E.1.1.6 Safety of die casting machines and their equipments E.1.1.7 Safety of personnel

E.1.1.8 Check list for maintenance of die and machines E.1.1.9 Storage of die casting die

F Specification F1 Specification of die, material and machinesF.1.1.1

Introduction to use and application of the specifications pertaining to die casting dies, materials and machines for tool design data

F.1.1.2 Die casting die specification F.1.1.3 Die casting metal specification F.1.1.4 Machine specification

F.1.1.5 Specification for processing G Computer aided information analysis G1 Introduction of simulation and analysis packagesG.1.1.1

Introduction to use and application of simulation package

G.1.1.2 Concept of process parameters G.1.1.3 Classification of simulation packages G.1.1.4

Principle of selection of process parameters using software packages

D D2 TD 1.2,5.3d

D TD 2.2g parameters

D TD 2.3 Work sheet for ijection mold

D TD 2.4 Work/Data sheet for mold Compression

D TD 2.5 Work/ Data sheet for parameters

D E1 TD 2.6 Work/Data sheet for Blow mold

E Conceptual Design Sketching Conceptual TD 1.1 Introduction

E TD 1.1a Application & atternative Conceptual Design

E TD 1.2 Definition & Concept &

E TD 1.2a Conceptual Design

E TD 1.2b Evaluation

E TD 1.3 Procedure

E TD1.3a

Develop alternative Conceptual design Using design Parameters

E F1 TD1.3c Select the optimal Design

F Design & Moulds

Draw the assembly and

Details diagrams & mold TF1.1a Flow Chart for Development & Design

F TF1.1b Preparation Design Data Sheet

F TF1.1c Preparation Concept Drawing

F TF1.1e Assembly Drawing method (TA 1.1.c)

F G1 TF1.1f Details Drawing method

G Mould Data Bill & Materials TG1.1(a) Introduction to Bill & Material

G TG1.1(b) Elements & Bill & Material

G TG1.1 ( c )

Preparation & Bill & Material by Appropriation Selection & Material, Material size

G G2 TG.1.1 (d) Representation & Standard parts in Bill & Material

G Mould Data TG. 2.1 (a) Introduction to mould Data

G TG.2 .1 (b) different Element & mold Data

G H 1 TG.2.1 ( c) Preparation & mold data

H CAD/CAE TH 1.1 (a) Introduction to CAD/CAE SoftWare

H TH 1.1(b) mould Design Soft wares

H TH 1.1 ( c ) Application & Softwares in plentic Processing

H H2 TH 1.1 ( d)

Different Types & Application SoftWarer for the plastic processing

DIE CASTING TG-2 THEORY

UNIT-1

CHAPTER – A1

OVERVIEW OF MASS PRODUCTION OF

CASTING ARTS PRODUCED BY VARIOUS

CHAPTER OUTLINE

A1.1.1 Die casting Background A1.1.1 (a) what is Die Casting

A1.1.2 Classification of Castings

A1.1.3 Principle of Die Design and Process A1.1.3 (a) Advantages of Die Casting Techniques A1.1.3 (b) Technique of filling Die Cavity

A1.1.1

DIE CASTING BACKGROUND

Die casting are among the highest volume, mass - produced items

manufactured by the metalworking industry. Die casting are important components in thousands of consumer, commercial and industrial products such as

automobiles, household appliances, recreation, hobby and leisure -time products,

farm and garden equipment, elec trical equipment and ordnance, general

hardware, power tools, computers and other business equipment, instruments, toys, novelties and a great many other too numerous to mention. In fact, die

casting have greater utility and are used in more appl ications than components produced by almost any other metal forming process.

Die –casting is a process involving the injection of molten metal at high pressure (as opposed to casting by gravity pressure). It is beloved to have begun

sometime durin g the middle of the 19 th century. According to records, in 1849,

Sturges patented the first manually operated machine for casting printing type.

Another 20 years passed before the process was extended to casting other shapes. The casting of printer’ s type led to patents which eventually resulted in development of the Linotype machine by Ottoman Megenthaler.

The earliest commercial applications for die castings occurred in 1892 when

parts were produced for phonographs and cash registers. Mass product ion was

further encouraged when the H.H. Franklin Company began die casting Babbitt alloy bearings for automobile connecting rods shortly after the turn of the century.

Various compositions of tin and lead were the first die casting alloys. Their importance and use declined, however, with the development of zinc alloys just prior to World War I. Aluminum alloys for die casting made their commercial debut about 1914. Magnesium and copper followed shortly thereafter.

During the 1930s, many of the alloys we know today had become available. Modern science and technology, metallurgical controls and research are making possible still further refinements resulting in new alloys with increased strength and stability.

Through the years, many significant technol ogical improvements have

been made to the basic die casting process, to die steels and to die

construction, as well as in casting machine design. Improvements have not only

extended the capability and production capacity of the process, they have been

tremendously effective in expanding die casting applications into almost every know market.

A1.1.1 (a) WHAT IS DIE CASTING

Die casting is a manufacturing process for producing accurately

accomplished by forcing molten metal under high pressure into reusable metal dies. The process is often described as the shortest distance between raw material

and finished product. The term, “die casting” is also used to describe the finishe d

part.

The term “gravity die casting” refers to castings made in metal molds

under gravity head. It is known as permanent mold casting in the U.S.A and

Canada. What we call “die casting” here is know as “pressure die casting” in Europe.

Ever since m an discovered that metals could be melted, he had tried to form these metals into shapes useful to him by pouring the liquid metals whose shape they retain during and after solidification. The casting of molten in moulds is one of the oldest methods developed by man to shape metal objects.

These are called molding or casting and are classified depending upon the molding method, mould material or casting process employed.

A1.1.2

CLASSIFICATION OF CASTING

1. Sand Castings:

a. Green sand molding b. Dry sand molding c. Shell molding

d. High pressure molding (using high pressure molding machine) e. Floor and pit molding

2. Metal mould castings:

a. Permanent mould (gravity die casting)

b. Semi- permanent mould casting (using metallic molding machine) 3. Plaster mould casting.

4. Investment casting.

In metal mould castings, the mould consists of two or more parts, is used repeatedly for the production of many casting of t he same form. Where as in the other molding processes the mould is destroyed for each casting produced. A great deal of these type of casting processes are still employed for the production of castings.

The summary of each of these molding and cas ting processes is furnished for comparative study at the end of this chapter. Each method has its own

advantages and disadvantage like finish, dimensional accuracy etc.

In order to produce cast articles more efficiently the permanent steel mould was developed. The molding of non - ferrous metals and their alloys with relatively

low melting temperatures in permanent steel mould under pressure is called Die

the extent that very little or no subsequent machining is necessary after removal of the gate and flash . Th e die castings made with hot or cold chamber machines are called Pressure Die Casting.

A1.1.3

PRINCIPLE OF DIE DESIGN AND PROCESS

First, a steel mold capable of producing tens of thousands of castings in rapid succession must be made at least two secti ons to permit removal of castings. These sections are mounted securely in a machine and are arranged so that one is stationary ( fixed die half) while the other is moveable ( injector die half ) To begin the casting cycle, the two die halves are clamp ed tightly together by the die casting machine. Molten metal is injected into the die cavity where it solidifies quickly. The die halves are drawn apart and the casting is ejected. Die casting dies

can be simple or complex , having moveable slides, co res, or other sections

depending on the complexity of the casting.

The complete cycle of the die casting process is by far the fastest known for producing precise non -ferrous metal parts. This is in marked contrast to sand casting which requires a new sand mold for each casting . while the permanent mold process uses iron or steel molds instead of sand it is considerably slower, and not as precise as die casting.

A1.1. 3 (a) ADVANTAGES OF DIE CASTING TECHNIQUES

Gravity Die

Casting:-The die is built up of parts or elements made of metal ( generally cast iron or steel ) . The design is adopted to the shape of the article required to be produced, so as to enable easy assembly, pouring and extraction These

operations constitute a cycle of operations which when repeated in a certain rhythm, determine the output rate of the equipment The various operations of assembly and disassembly of the die may to some extent be mechanized. In this process liquid flows into the die entirely under its own weight. It is form this features that the term “gravity Die Casting” was coined.

Pressure Die

Casting:-This technique is a development over the gravity die casting and has the following characteristics.

a. The die is mounted between the two plates called “PLATENS’ of press, generally of horizontal type, by means of which it is closed and opened.

b. The movement of the die follows that of the machine, and this determines the general directions of assembly.

c. The liquid metal is generally injected by the action of a piston, which forces it through the die from a compression chamber This is referred to as a hot chamber, if it is situated inside the molten metal which is heated by a furnace forming part of the assembly a cold chamber if it is fed with metal which has been melted in a furnace separate from the machine.

Advantages of Pressure Die Casting Process:

-Die casting components parts, decorative trim, and /or finished prod ucts

offer many features, advantages and benefits to those who specify this manufacturing process.

1. Die casting provides complex shapes within closer tolerance than many other mass production processes.

2. Die castings are produced a t high rates of production. Little or no

machining is required.

3. Die castings can be produced with thinner walls than those obtainable by other casting methods …. much stronger than plastic injection moldings with the same dimensions

4. Die casting provide parts which are durable, dimensionally stable, and have the feel and appearance of quality.

5. Die casting dies can produce thousand of identical castings within specified tolerances before additional tooling may be required.

6. Zinc castings can be easily plated or finished with a minimum of surface preparation.

7. Die castings can be produced with surfaces simulating a wide variety of textures.

8. Die cast, surfaces, as cast, are smoother than most other forms of casting.

9. Holes in die castings can be cored, and made to tap drill sizes.

10. External threads on parts can be readily die cast.

11. Die castings provide integral fastening elements, such as bosses and studs, which can result in assembly economics.

12. Inserts of other metals and some non-metals can be die cast in place. 13. Corrosion resistance of die casting alloys rates from goods to high. 14. Die castings are monolithic. They combine many functions in one,

complex shaped part. Because die castings do not consist of separ ate

parts, welded or fastened together the strength is that of the material, not that of threads or welds, etc.

16. Since the dies are fil led by pressure, castings with thinner walls, greater length to thickness ratio and greater dimensional accuracy can be

produced.

17. Production rates are higher in pressure die casting, especially when multiple cavity dies are used.

18. The castings are p roduced as almost completely finished parts, the investment in inventory and factory floor space reduced to a minimum. 19. Dies for pressure die casting can produce many thousands of castings

without significant change in casting dimensions.

20. Metal cost is often lower than in other casting process, because pressure die casting permits components of thinner sections.

21. Many die casting can be plated (finished) with minimum surface preparation.

22. Some Aluminum alloy pressure die castings can be develop ing higher

strength than compared sand castings.

The Principle Limitations of the Pressure Die Casting Process:

-1. Casting size is limited. The casting weight seldom exceeds 50 1b and normally is less than 15 lbs.

2. Depending on the casting contours and gating, difficulty may be encountered with air trapped in the in the die. Trapped air is principle cause of porosity.

3. The die casting facilities, consisting of the machine, the auxiliary

equipment and the dies are r elatively expensive. Because the die

castings are small, large quantities of castings are required for the process to be economical.

4. Commercial use of the process is limited to metals having melting

temperatures not higher than these of copper - base all oys, with few

exceptions.

Dies can be produced for simple and complex parts. Parts having external undercuts or projections on side walls often require slides which increase costs. In many cases however, resultant savings of metal or other advantages such as uniform wall sections, offset the extra cost or effect a net economy in overall costs. This is especially true when large quantities are involved.

A1.1.3 (b) TECHNIQUE OF FILLING DIE CAVITY

As the name “ Pressure Die Casting” implies, injecti on of the molten metal into the mould or die cavity is done under pressure. The thin walls as well as the various bends around the corners and the edges of complicated die castings offer considerable resistance to complete filling of the mould or die. Therefore it is necessary that the metal moves through the die with high velocity before it

settles in the mould cavity. The air present in the cavity has to be displaced by the entering metal. The air could be displaced by providing air vents in the die or by connecting the cavity to vacuum before the metal is injected into the die.

Vacuum die castings are used only for small parts made of low or high melting light alloys.

Pressure die casting necessitates a particularly care fu lly study of the design and shape of the articles to be produced.

The choice of technique involved for the production of a given article is governed by many factors, the most important of which are as follows :

1. Mechanical properties 2. Dimensional accuracy 3. Complexity of shape 4. Surface condition

5. Number of casting to be produced 6. Production time

7. Production cost

Certain considerations of strength, precision or surface condition indicate a particular technique. It is always important that the production cost of the article desired should be estimated. The production cost must take into account.

a. Cost of mould

b. Cost of the injection machine

c. Cost of subsidiary operations (melting. trimming and machining) d. Actual weight of the metal.

The careful study of the production cost of an article based on the above factors and related to the number of articles to b e produced determines the limit of viability of a particular technique to make the process economical.

The advent of mass production has made possible the study and practical application of mechanized means of pressure die casting. As a result of

considerable capital investment it is possible to mass produce articles at extremely competitive prices. For example, an electric coffee mill may be sold at a price lower than that of most of its component parts, if these were to be produced individually other than the die casting process.

A1.1.4 COMPARISONS OF DIE CASTING WITH OTHER

PRODUCTS

Plastics Injection Molding:

-Compared with plastic injection moldings, die castings are stronger , stiffer, more stable dimensionally, more heat resistant, and ar e far superior to plastics on a properties /cost basis They help prevent radio frequency and

cracking in the presence of various regents. Manufacturing cycles for producing die castings are much faster than for plastic injection moldings, Plastics, however,

may be cheaper on a unit volume basis, have color inherent pro perties which

tend to eliminate finishing , are temperature sensitive, and are good electrical insulators.

Sand

Castings:-Compared with sand castings die casting require much less machining; can be made with thinner walls; can have all or nearly all holes cored to size; can be held within much closer dimensional limits; are produced more rapidly in dies which make thousands of die castings without replacement; do not require new

cores for each casting; are easily provided with inserts die cast in place; have

smother surfaces and involve much less labor cost per casting Sand castings, on

the other hand, can be made from ferrous metals and from many non -ferrous

alloys not suitable for die casting. Shapes not producible by die castin g are

available in sand castings; maximum size can be greater; tooling cost is often less and small quantities can be produced more economically.

Permanent Mold

Castings:-Compared with permanent mold castings, die cast ings can be made to closer dimensional limits and with thinner sections; holes can be cored; are produced at higher rates with less manual labor; have smoother surface and usually cost less per die casting. Permanent mold casting involves somewhat lower tooling costs; can be made with sand cores yielding shapes not available in die casting.

Forgings:-Compared with forgings die castings can be made more complex in shape

and have shapes not forgeable; can have thinner sections; be held to close r

dimensions and have coring not feasible in forgings. Forgings, however, are denser and stronger than die castings; have properties of wrought alloys; can be produced in ferrous and other metals and in sizes not suitable for die castings.

Stampings:-Compared with stampings, one die casting can often replace several parts. Die casting frequently require fewer assembly operations; can be held within closer dimensional limits; can have almost any desired variation in section thickness; involve less waste in scrap; are producible in more complex shapes and can be made in shapes not producible in stamped forms. Stampings, on the

other hand, have properties of wrought metals; can be made in steel and in

alloys not suitable for die casti ng; in their simpler forms, are produced more

rapidly; and may weigh less than die castings.

Screw Machine

Products:-Compared with screw machine products, die castings are often produced more rapidly; involve much less waste in scrap; can be ma de in shapes difficult or impossible to produce from bar or tubular stock; and may require fewer

operations. On the other hand, screw machine products can be made from and

alloys which cannot be die cast; they have the properties of wrought metals;

and they require less tooling expense.

There are some comparison tables for Die Casting Process with other process with respect to Process, Die/Mold, Cost, Design and application etc., continued in the next page.

Table : SUMMARY OF MOULDING AND CASTING PROCESSES

Process *Choice of materials Complexity of part Number of castings relative to tool life

Casting size or weight Sand 1, 2 ,3, 4, 5, 6, 7, 8, 9, 10,

11,

Considerable limited by pattern drawing. No limit with cores.

Wide range, type of pattern depends upon total/casting.

28 gms. To Shell Mould 1, 2, 3, 4, 5, 6, 9 Considerable, limited by

removal of mold from pattern. Less limited with cores.

High metal patterns have a long life.

28 grms – 45 gms and 387 cm2

Permanent Mould 1, 3, 4, 5, 6, 7, 8, 10, 11 Limited, restricted by the rigid molds. Ability to eject casting limits shape

Moderate to high , casting metal affects life of mold.

Several grams - 23 kgs.

Die Casting 4, 5, 7, 8, 10, 11 Moderate, limited by design of movable cores

High, mold life affe cted by casting metal.

Several grams – 33 kgs. In aluminium 90 kgs. In zinc usually under 7 kgs.

Plaster moulding 4, 5 Considerable, possible to make

mold of several pieces, expendable mold

Moderate, depends on pattern material

28 grms to several kgs. In most of material.

Investment Casting 3, 4, 5, 6, 9 Considerable, very complex patterns can be assembled from pieces.

Moderate, type of pattern mold depends upon number of castings.

Under 28 gms to 45 kgs. Best for parts under 0.8 kg. Centrifugal Casting 1, 3, 4, 5, 6, 9 Casting of circular periphery Low to moderate Upto several kgs.

Process Min. section mm Min. dia cored hole mm Surface finish Microns Precision and tolerances

Sand 3 – 6 depending upon metal

4, 5 – 6 6 , 25 - 25 1, 5 – 4, 2 mm depending upon metal & casting size. Tolerance of ± 0.25 mm possible on some parts. Shell Mould 1, 5 for most materials. 3 – 6 Some what better than

sand

± 0, 003 mm/mm 0,075 total possible on some dimensions.

Permanent Mould 2.38 most materials 4, 5 – 6 2, 5 – 6, 25 ± 0,015 mm/mm for firs t 25 mm 0.025 mm to 0.05 mm for each additional 25 mm

Die Casting 0.625 0.76 – 4.5 depending upon metal

1 – 2, 5 ±0,025 – 0,125 mm depending upon material. Plaster moulding 0, 750 12.5 0,75 – 1.25 ±0.005 – 0.010 mm/mm or

less

Investment Casting 0.750 0.50 – 0.750 0.25 – 2.12 ±0.005 mm/mm Centrifugal Casting 0.750 4.3 - 6 2.25 – 6.25 or as in sand Same as permanent

Process Tool costs Direct Labour costs Finishing costs Field of application

Sand Low Wide range, much hand labour

required.

Wide range, high to low, depends upon cleaning, snagging and machining required.

Singular and batch production of medium & large components of cast iron, cast steel, not precision.

Shell Mould Low to moderate Moderate Low, often only a minimum

required.

Batch & mass production of cast iron, steel components. To reduce the cost of machining.

Permanent Mould Medium Moderate Low to Moderate Batch Production.

Die Casting High Low to medium Low, little more than

trimming necessary.

Mass production of small components of Auminium, zinc, magnesium, copper alloys. To reduce the cost of machining.

Plaster moulding Medium High, skilled operators

necessary

Low, little machining necessary.

Batch Production.

Investment Casting High High, many hand operators

required

Low, machining usually not necessary.

Steel, alloyed steel, small batch, small size to reduce the expensive

machining.

Centrifugal Casting Medium Moderate Low to moderate Singular batch production bearing

DIE CASTING COMPARISON WITH OTHER PRODUCTION PROCESSES Process Process defined Materials Rate of

production

Size & weight of parts Strength of parts Wall thickness Complex ity Other characteristics Die casting Castings made by forcing molten metal under external pressure into a metal die or mold. Lead, tin, zinc magnesium , aluminium and copper alloys. Very high, upto 500 shots/hr possible with some parts. No real size limitation. Size depends upon casting equipment available. Present max. sizes run : 15 lb for aluminum, 10 lb for magnesium and 30 lb for zinc.

High unit strength.

Very thin; upto 1 in or more max. From simple to very complex. Inserts of almo st any metal can be embedded in castings. Permanen t mold casting Castings produced by pouring molten metal under a gravity head into metallic molds.

Iron, manesium, aluminum & copper alloys. Relatively low. Not a high production process. Usually medium or large parts. Between die, castings and sand castings.

High Not so thin as die casting but much heavier sections possible. Usually not so complex as die castings. Inserts can be used. Sand Casting Castings made by pouring molten metal under a gravity head into molds prepared by packing molding sand

Principally iron, magnesium , aluminum & copper Low. Not a high production process. Medium to very large. Less than die castings or permanent mold Must be heavier than die castings & permanent mold castings can be Housings & hubs represent average degree of Inserts seldom practical.

Plaster mold casting

Castings made by pouring metal under a gravity head into molds made of gypsum with strengthening & setting agents added. Any nonferrous material having a melting point of less than 2000 F, except magnesium in large sizes. Low. Not a high production process.

Relatively small. Equal to sand castings.

Not as thin as lead, tin or zinc die castings, but sometimes equal to die castings of aluminum, magnesium & brass. Usually not so complex as die castings or permanent mold castings. Precision investment casting Castings made by pouring molten metal into refractory or ceramic molds formed around wax patterns. Patterns are removed by melting in the process of firing of the refractory. Iron, zinc, magnesium, copper alloys & especially hi gh alloy steels. Usually lowest of all processes

Small parts only. Max weight of part about 101b or up to 201b by special techniques. Section size usually limited to 7 in. or less. Equal to or better than permanent mold castings. 0.040 in. min. 1/16 in. prescribed min. tolerance on walls no less than 0.005 in. min. Intricate shapes not readily made by machining, forging or sand casting can be produced. Min. thickness of trailing edge equal to 0.015 in. min. & preferably 0.025 in. inserts not practical.

COMPARISON OF DIE CASTING WITH OTHER PRODUCTION PROCESS (Contd…)

Process Appearance & finish Cost Applications

Die casting Excellent. Can be finished with variety of mechanical, plated, chemical or organic finishes.

High equipment cost, high tool cost, & low labor cost. Low part cost on high activity items. Machining, grinding & other operations usually not

necessary.

Structural parts, machine elements & decorative members & parts for automotive, business, machine electrical appliance, & all other high production industries making both industrial & consumer products.

Permanent mold casting

Usually machined or ground but left with base metal surface.

Medium equipment cost, high tool cost, high labor cost. Fairly high part cost.

For parts similar to sand castings but which must have superior surface finish, closer tolerances, & better strength in as cast condition.

Sand casting Inferior to die or permanent mold castings. Usually machined or ground but left with base metal surface.

Low tool cost, high equipment cost, high labor cost part cost between those of die castings & precision castings.

Gears, framing members, housings motor blocks & structural members when cast structure having relatively low strength & resistance to impact satisfactory. Usually limited to cast iron & cast steel for industrial equipment.

Plaster mold casting

Excellent Low tool cost, low equipment cost,

high labor cost, fairly high part cost.

Various engineering parts, mostly of brass alloys.

Precision

investment casting

Equal to die castings, but usually left with base metal surface.

Low equipment cost, low tooling cost, high labor cost, high part cost.

Small intricate parts made in limited quantities, usually from high alloy metals such as stainless steel. Inconel Hastelloy

UNIT-2

CHAPTER – B1

CHAPTER OUTLINE

B1.1.1 Introduction

B1.1.2 Gravity Die Casting (GDC)

B1.1.3 Limitations of Permanent Mold Casting B1.1.4 Principle of Permanent Mold Casting B1.1.4 (a) Progressive Solidification

B1.1.4 (b) Minimum Turbulence B1.1.4 (c) Air and Gas Clearance

B1.1.5 Suitable Casting Materials for GDC B1.1.6 Permanent Mold Casting Machines B1.1.7 Selection of Mold Material

B1.1.8 Gravity Die Casting Mold Life B1.1.9 Principle of Mold Design

B1.1.1

INTRODUCTION

This is a casting process in which the mould is permanent and same can be

repeatedly used for making thousands of identical components.

B1.1.2

PERMANENT MOULD/GRAVITY DIE CASTING

In permanent mold casting, a metal mold consisting of two or more parts is repeatedly used for production of many castings of the same from The liquid metal enters the mold by gravity Simple removable cores are usually made of metal, but more

complex cores are made of sand or plaster when sand or plaster cores are used , the

process is called semi permanent mold casting.

Permanent mold casting is particularly suitable for the high -volume production of

castings with fairly uniform wall thickness and limited under -cuts or intricate internal

coring. The process can also be used to produce complex castings, but production quantities should be high enough to justify the cost of the molds. Compared to sand casting,

permanent mold casting permits the production of more uniform castings with closer

dimensional tolerances, superior surface finish, and improved mechanical properties.

B1.1.3

LIMITATIONS OF PERMANENT MOULD CASTING

Permanent mold casting has the following limitations:

-§ Not all alloys are suitable for permanent mold casting

§ Because of relatively high tooling costs, the process can be prohibitively expensive for low production quantities

§ Some shapes cannot be made using permanent mold casting, because of parting line location undercuts, or difficulties in removing the casting from the mold

§ Coatings are required to protect the mold from attack by the molten metal

Metals that can be cast in permanent molds include the aluminum, magnesium, zinc and copper alloys and hypereutectic gray iron.

B1.1.4

PRINCIPLE OF PERMANENT MOULD CASTING

Compared with permanent mould casting, Die Casting can be made to closely dimensional limits and with thinner sections, holes can be cored, are produced, higher rates with less manual labor, have smoother surfaces and usually cost less per die casting. Permanent mould casting involves some what lower tooling cost, can be made with sand cores yielding shapes not available in die casting.

In permanent mould casting, a metal mold consisting of two or more parts is used

repeatedly for producing many castings of the same form. The liqui d metal enters the

mould by gravity. (The process does not, however, include pouring of ingots in metal moulds). Simple cores area made of metal, but more complex cores are made of sand or

plaster. When sand or plaster cores are used, the process is called Semi permanent mold

casting.

Removal of Casting from Molds:

-After a casting has solidified, the mold is opened and the casting is removed. To facilitate release of the casting from the mold, a lubricant is often added to the mold coating. The use of as much drafts as permissible on all portions of the casting makes ejection easier. For many castings, ejector pins or pry bars must be used. Core pins and cores should be designed so as not to interfere with removal of castings from the mold.

B1.1.4 (a)

PROGRESSIVE SOLIDIFICATION

Casting requires a feeding system whic h consists of gates, runners, risen etc. Risers must be connected to heavy section of the casting. The process of solidification must be in such manner that the casting freezes from the further most point progressively towards the

risers. If this principle is not satisfied, shrinkage, porosity, cavitations or surface

depressions can occur which may causes the rejection of the component.

B1.1.4 (b)

MINIMUM TURBULENCE

This principle must be observed during the filling of die cavity with molten metal. Turbulent filling of die will leave air bubbles and oxide films entrapped within the casting. The turbulence can be controlled by suitably designing the runner system. This principle must be satisfied for setting a sound casting.

B1.1.4 (c)

AIR AND GAS CLEARANCE

For gravity die casting the care must be given to air and gas clearance otherwise entrapped air or gas will cause defects in the components (blow holes, surface depression etc)

B1.1.5

SUITABLE CASTING METALS FOR GDC

Metals that can be cast in perma nent moulds include aluminum, magnesium, zinc and copper alloys, and hypereutectic gray iron.

Aluminum alloys : -

Aluminum alloys have low density, which combined with their oxide-film-forming characteristics, make them flow some what sluggishly. The shrinkage of aluminum alloys during solidification is relatively large, and provision must be made for ample metal feed during solidifi cation. After solidification, aluminum alloys are soft at elevated temperature, and castings may distort during removal from the mold.

Magnesium alloys : -

Magnesium alloys are less castable than aluminum alloys, and have relatively poor feeding characteri stics in thin -wall castings. Also, the castings are more sensitive to hot shortness (brittleness at elevated temperature) than are aluminum alloys castings. Generous fillers are required when the casting contains large bosses or when one section of the cas ting is much larger than another. Sharp casting detail cannot be obtained with magnesium alloys, and shapes that shrink on to mold sections are susceptible to cracking and should be avoided.

Copper alloys : -

Copper alloys solidify at high temperatures, an d some have narrow solidifications ranges. They shrink on to cores and other mold elements, and shrink on to cores and other mold elements, and must be ejected from molds as soon as possible.

Zinc alloys: -

Zinc alloys can be cast in permanent molds, but because the castings are usually made in large quantities, they are more often die cast.

Gray iron: -

Gray iron is used successfully in high -volume production of small (28 g to 13.5 kg, or 1 oz to 30 lb), simple castings. However, more complex gray iron c astings, with internal coring and marked changes in section, have also been successfully made by the permanent mold process.

Maximum Size of Casting:

-Practical sizes of permanent mold castings are limited by cost. The maximum sizes that have been cast differ among the casting alloys.

Aluminum alloys: -

In high production, permanent mold castings weighing up to 13.5

kg (30 lb) are made from aluminum alloys in casting machines. However, much larger castings can be produced.

Magnesium alloys: -

It despite their comparatively low cast ability; have been cast in permanent or semi -permanent molds to produce relatively large and complex casting. For instance, 8 kg (17.7 lb) housing for an emergency power unit was poured from alloy

AZ91C in a semi -permanent mold. The mold utilized vertical parting and an oil -sand core to develop the vanes and internal surfaces of the casting. Surface finish of the casting varied from 6.4 to 12.7mm (250 to 500min.).

Copper alloy: -

These permanent molds casting weighing over 9 kg (20 lb) rarely can be justified.

B1.1.6

PERMANENT MOLD CASTING MACHINES

Permanent mold casting machines can be customized to automatically or manually operating. They are basically simple in construction.

Manually operated machine:

-Manually operated permanent mold casting machines may consists of a simple

“book” mold arrangement, such as that shown in fig.1 or for castings with high ribs or walls that require mold retraction without rotation, the machine shown in fig. 2 can be used. With

Either type of machine, after the casting solidified the mo ld halves are separated by

manually releasing the eccentric mold clamps.

Automatic machines:

-For high-volume production, the manual drives are replaced by two -way hydraulic mechanisms. These can be programmed to open and close in a preset cycle. Thus, except for pouring of the metal and removal of castings, the operation is automatic.

A method of permanent mold casting has been developed in which the metal is not ladled by hand. This is called the Wessel Process . The equipment for which is shown in fig. 3. In this method, the permanent mold is mounted on rails against the end face of the tilting reverberate furnace. As the furnace is tiled about an axis bear its center of gravity, metals flows through a pouring hole in ht wall of the furnace in the mold. The assembly

remains in its tilted position for a predetermined interval, then returns to the s tarting

position. Tilting is done by means of a hydraulic cylinder.

Molds are parted vertically, parallel to the direction of metal flow. One mold half slides on the mounting rails; the other, which is hinged, swings away from the mounting rails, pulling the casting and sprue with if to leave the pouring hole clear for the next cycle. Core manipulation and casting ejection are the same as in conventional practice.

B1.1.7

SELECTION OF MOULD MATERIALS

Four principle factors affect the selection of material s for permanent molds and

cores:

-§ The pouring temperature of the metal to be cast § The size of the casting

§ The number of castings per mold § Cost of the mold material

Mould Materials: -

As indicated in Table A, gray iron is the most commonly used

mold material. Aluminum or graphite molds are sometimes used for the small -quantity

production of aluminum and magnesium castings, and graphite or carbon linear on steel are sometimes used for molds for casting copper alloys ( see also the section “Solid Graphite Molds” in this article).

With aluminum or magnesium casting alloys, it is not unusual to obtain 100,000 castings, or more, per mold; however, molds for copper or gray iron casting alloys have a shorter life because of the higher pouring temperatures required. Pouring temperatures for specific metals are as follows:

Gray iron molds without tool steel inserts are satisfactory for long production runs of aluminum and magnesium castings that will be magnesium casting that be machined extensively and for which s urface finish is not a major consideration. In the casting of zinc, well over 00.000 pours are possible in a gray iron mold (die casting is usually selected to produce zinc castings such large quantities).

Mold Inserts: -

Full or partial mold cavities ins erts of the same material as the mold , or of a different material, are sometimes used to obtain longer mold life, or to simplify

machining handling or replacement. Inserts can also be used for venting, cooling thin walls, and heating portions of the mold o r the full cavity area . Inserts made of cast -to-shape gray iron are used for casting complex alu minum and magnesium parts that r ange in surface

area from 320 to 2900 cm 2 _{(50 to 50 in.}2_{). Tolerance on these parts range from 0.76 to ±}

1.5 mm (± 1.5 mm ± 0. 030 to ± 0.060 in.). Inserts last for 5000 to 20.000 pours, depending on casting complexity.

Core Materials: -

Core materia ls are recommended in Tables B and C on the basis of performance over a wide range of coring requirement for small and large cores. An

expendable core is used when the location or shape of the core does not permit its removal from the casting or when an in tricate design can be obtained at less cost with materials for such cores. These materials are listed below in order of increasing preference:

§ Sand (oil – bonded or resin –bonded, shell, car bon dioxide-silicate) § Plaster

§ Graphite and carbon

Table A recommended permanent mold Materials Casting alloy No. of Pours

1000 10,000 100,000

for small Castings (25 mm or 1inch Maximum dimension)

Zinc Gray Iron: 1020steel Gray Iron: 1020steel Gray Iron: 1020steel Aluminum Mg. Gray Iron: 1020steel Gray Iron: 1020steel Gray Iron with AISI-H14

inserts: 1020steel Copper Gray Iron Gray Iron Alloy Cast Iron

Gray Iron Gray Iron (a)Gray Iron (a) Quantity not Poured

for medium and large castings (upto 915mm or 36Inch maximum dimension)

Zinc Gray Iron:AISI-H11 (b)Gray Iron: AISI-H11 (b) Gray Iron: AISI-H11 (b) Aluminum Mg. Gray Iron Gray Iron Gray Iron with AIS-H11/H1(c)

inserts: 1020steel Copper Alloy Cast Iron Alloy Cast Iron Alloy Cast Iron (d)

Gray Iron Gray Iron (a) Gray Iron (a) Quantity not Poured

METAL/ALLOY TEMPERATURE, °C (°F) Zinc Aluminum Magnesium Copper Gray Iron 465-620 (870-1050) 675-790 (1250-1450) 705-790 (1300-1450) 980-1230 (1800-2250) 1275-1355 (2325-2475)

Table B Recommended Materials for small cores (<75mm or 3In in Diameter & 255mm or 10In. long) for Permanent moulds

Casting alloy Recommended core materials (a)

Zinc Sand Plaster, Gray Iron 1020steel

Aluminum Mg. 1010 or 1020 steel Sand Plaster H11 Die steel or Equivalent (b) carbon (c) Copper Sand 1020steel Gray Iron Plaster (d) Graphite9(c)

Gray Iron Sand Graphite Carbon and Gray Iron (a) Materials are listed descending order of preference (b) Hardened to 40-45HRC (c) for use with relatively few Pours (d) for Castings of Aluminum Bronze.

Table B Recommended Materials for Large cores (<75mm or 3In in Diameter & 255mm or 10In. long) for Permanent moulds

Casting alloy No. of Pours

1000 10,000 100,000

Zinc Gray Iron: 1020steel Gray Iron: 1020steel Gray Iron: 1020steel Aluminum Mg. Gray Iron: Gray Iron with Gray Iron: Gray Iron with Gray Iron: Gray Iron with

1020steel insert (b) 1020steel or H11 Insert (b) H11 Insert (b), Sand Plaster (b) Gray Iron: 1020steel H11 Die steel,

Copper Sand Sand Quantity not Poured

Gray Iron Sand, Graphite, Sand, Graphite,Quantity not Poured

Carbon Gray Iron Carbon Gray Iron

(a)Material listed in descending order of preference, (b) except for openings with complex shape, which required expandable sand cores.

B1. 1.8

GRAVITY DIE CASTING MOULD LIFE

Mold life can vary from as few as 100 pours as many as 250,000 pours (or even more), depending on the variables discussed later in the section. A mold for an aluminum piston for example, can be accepted to produce 250,000 casting before requiring repair. After the production of 250,000 more castings, the repaired mold will require a major overhaul.

Mold life is likely to be longer in the casting of magnesium alloys than in the casting of aluminum alloys of similar size and shape; this is because molte n magnesium does not attack ferrous metal molds however the difference the difference in mold life for magnesium alloys depends to a great Extent on the effectiveness of the mold coating used

in the casting of gray iron, mold coating used in the casting of gray iron, mold life is

Molds are often fabricated from cast iron is cause casting the mold close to the finished shape can decrease machining costs. In ad dition cast iron is much more resistant to attack by molten aluminum than steel; however is weld able and easier to repair than cast iron. Therefore steel molds are often used for high-production castings.

Major variables that affect the life of Permanent molds are:

-§

Pouring temperature:

The hotter the casting metal is poured, the hotter the mold is operated which leads to rapid weakening of the mold metal.

§

Weight of casting:

Mold life decreases as casting weight increases. The decrease in mold life with increasing weight of casting is shown in Fig. 2 for 25 mm (1 in.) thick gray iron mold used for casting of gray iron.

§

Casting shape :

Mold walls are required to dissipat e more heat from casting having thick sections than from those having thin sections When there is a significant variation in the section thickness of a casting , a temperature

differential is set up among different portion of the mold. As t he temperature

differential increases mold life decreases.

B1.1.9

PRINCIPLE OF MOLD DESIGN

A mold design has a marked effect on mold life. Variation in mold -wall thickness

causes excessive stress to develop during heating and cooling which in turn causes

premature mold failure from cracking Abrupt changes in thickness without generous fillets also cause premature mold failure . Small fillets and radii lead to reduced mold life because checking and cracking as well as ultimate failure , often start at these points.

Usually less draft is required on external mold surface than on internal mold surface than on internal mold surfaces because of the shrinkage in the casting A 5° draft is desirable but 2 ° on external and 3 ° on internal mold surfaces can be used Lower draft angles, however decrease the number of castings that can be made between mold repairs The effect of draft angle on the life of cores and molds used for producing aluminum alloy castings is shown in Fig 3

Projections in the mold cavities contrib ute greatly to reduce mold life These projections become extremely hot. Which increases the possibility of extrusion , deformation and mutilation when the casting is removed I t is sometimes possible to extend mold life by using inserts to replace worn or broken projections.

Undercuts

on the outside of a casting complicate mold design and increase casting cost, because additional mold parts or expendable cores are needed. Complicated and undercut internal sections are usually made more easily with expendable cores than with metal cores, although collapsible steel cores or loose metal pieces can sometimes be used instead of expandable cores.

Numbers of castings per mold

is a major consideration in designing the mold; the objective is to have the optimum number of cavities per mold that will yield acceptable castings at the lower cost. Except for vary small and thin castings, as the weight of the metal being cast per mold increases, the cycle time of the machine also increases. These increases, however, are not directly proportional. A mold with the maximum number

of cavities often will produce more castings per unit of time than a mold with a smal ler

number of cavities that was designed to operate on shorter cycle.

For relatively simple castings, cavities may be placed one above the other as shown in the below fig. the metal then flows through the lower cavities to till to those above this permits maximum utilization of the machine platen area available.

Mold Design and Dimensional

Variations:-The dimensional accuracy of permanent mold castings is affected by short -term and long -term variables Short -term and long –term variables. Short –term variables are those that prevail regardless of the length of run:

-§ Cycle- to – cycle variat ion in mold closure or in the position of other moving elements of the mold

§ Variations in mold closure caused by foreign material on mold faces or by distribution of the mold elements

§ Variations in thickness of the mold coating § Variations in temperature distribution in the mold § Variations in casting-removal temperature

Long –term variables that occur over the life of the mold are caused by:

-§ Gradual and progressive mold distortion resulting from stress relief, growth and creep

§ Progressive wear of mold surface primarily due to cleaning

Dimensional variations can be minimized by keeping heating and cooling rates constant, by operating on a fixed cycle, and by maintaining clean parting faces. It is

particularly important to select mold cleaning procedures that remove a minimum of mold material.

The mold thickness and the design of the supporting ribs both affect the degree of mould warp age at operating temperatures. Supporting ribs on the back of a thin mold will

warp the mold fa ce into a concave form. This mold –design error can alter casting

dimensions across the parting line by as much as 1.6 mm (1/16 in). Adequate mold lockup will contribute to the control of otherwise severe warp age problems.

Mould erosion resulting from me tal impingement and cavitations due to improper gating design both contribute to rapid weakening of the mould metal and to heat checking. These mould design errors contribute to rapid dimensional variation during a long run mechanical abrasion due to insuf ficient draft or to improperly designed ejection systems also contributes to the raped variation of casting dimensions.

The dimensions of many mold and core componen ts change at a relatively uniform rate; there fore, it is possible to estimate when rework or replacement will be required

To maintain castings within tolerances. It is sometimes necessary to select mold

-component materials on the basis of their wear resistance.

Cooling methods: -

Water cooling is more effective than air cooling but it substantially decreases mold life.

Heating cycles: -

Generally, a continuous run. in which the mold is maintained at a uniform temperature provides maximum mold life. Repeated heating and cooling over a wide temperature range will shorten mold life.

Preheating the mold: -

This is done to operating temperature with a gas flame or electric heaters, and it greatly increases mold life. Thermal shock i s one of the principle causes of mold failure.

Mold coating: -

This protects the mold from erosion and soldering by preventing the metal from contacting mold surfaces, thus increasing mold life ( see discussion below).

Mold materials: -

See Table 1.

Storage: -

Improper storage can lead to excessive rusting and pitting of mold surfaces which will reduce mold life.

Cleaning: -

The common practices for cleaning mold are abrasive blasting , dipping in caustic solution and wire brushing Dipping in caustic can be hazardous to the operator. Wire brushing and abrasive blasting can cause excessive mold wear if not carefully

controlled. Glass beads are the safest abrasive blast material; their use minimizes dimensional changes due to erosion from the abrasive blast.

Gating: -

A poor gating system can greatly reduce mold life by causing excessive turbulence and washout at the gate areas.

Method of mold operation: -

Although the same materials are used to make molds and cores for both automatically operated equipment and hand –operated equipment the life of the tool materials on hand –operated equipment is shorter because of the abuse

the tooling must withstand Tools for automatic equipment may last up to twice as long

as for hand-operated equipment.

End use of casting : -

If the structural function of a casting is more important than its appearance, a mold can be used for more pouring before being discarded.

Surface Finish: -

Surface finish on castings is determined primarily by the roughness of the mold coatings and is essentially independent of the surface achieved in machining the mold. A specified surface finish for the part has little bearing on the selection of the mold

material unless heat checking and pitting ( in high –volume production ) cause mold

surfaces to become rougher than the mould coating A tool steel is also selected where

it is desirable to maintain a high polish on the mold as for casting zi nc parts to be

Supplementary Heating or Cooling: -

Supplementary heating or cooling of all or selected portions of a mold, to minimize extremes of temperature during the operating

cycle , will usually increase mold life and result in the most efficient operation of the

mold regardless of the metal being cast Supplementary heating is used to :

§ Bring the mold to operating temperature especially where the quantity of heat to be removed is small

§ Equalize the temperature throug hout the mold so as to avoid thermally induced stresses

§ Avoid chilling of metal being cast

Supplementary cooling is used to:

-§ Hold mold temperature at its required level especially where the quantity of heat to be removed is large

§ Permit casting to be done at a faster rate, thus increasing production

§ Equalize the temperature throughout the mold thus avoiding thermal stresses

Erosion: -

Erosion of mold materials varies with the metal being cast and the mount of metal flowing into the mold

Scaling: -

Scaling of molds is most likely to occur when the molds are not in actual operation- for example if they are overheated when being prepared for production; this can be prevented by providing preheating torches with a reducing

Pitting and Corrosion: -

Pitting and corrosion usually occur in storage where moist and corrosive conditions cause chemical attack. This is especially true when molds are

stored without prior removal of the mold coating However, pitting may occur during

service if the mold cavity is not well protected with a mold coating.

Mold Coating: -

A mold coating is applied to mold and code surface to serve as a barrier between the molten metal and the surface of the mold while a skin of solidi fied metal is formed Mold coatings are used for five purpose:

-§ To prevent premature freezing of the molten metal

§ To control the rate and direction of solidification of the casting and therefore its soundness and structure

§ To minimize thermal shock to the mold material

§ To prevent soldering of molten metal to the mold material § To vent air trapped in the mold cavity

Types: -

Mold coating are of two general types insulating and lubricating Some coatings perform both functions A good insulating coating can be made from (by weight) one part sodium silicate to two parts colloidal kaolin in sufficient water to permit spraying. The lubricating coatings usually include graphite in a suitable carrier Typical compo sitions of 15 mold coatings are called in Table D Coatings are available as proprietary materials.

The various requirements of a mold coating are not always obtained with one

coating formulation. These requirements are often met by applying different coat ings to

various locations in the mold cavity.

TABLE -D) Typical compositions of coatings for permanent molds

Composition, % by weight (remainder, water)

Insulators Lubricants

Coating Requirements: -

To prolong mold life a coating must be non corrosive. It must adhere well to the mold and yet be easy to remove. It must also kee p the molten metal from direct contact with the mold surfaces.

Coating Procedure: -

The mold surface must be clean and free of oil and grease. The portions to be coated should be lightly sand blasted. If the coating is being applied with a spray, the mold should be sufficiently hot (205 °C or 400 °F) to evaporate the water immediately.

B1. 1.10

SOLID GRAPHITE PERMANENT MOULDS

Permanent molds can be machined from solid blocks of graphite instead of cast iron or steel The low coefficient of thermal expansion and superior resistance to

distortion of graphite make it attractive for the reproduce idle production of successive casting made in the same mold. Because graphite oxidizes at temperatures above 400 °C

Casting

No. Sodium Silicate Whiting FireClay MetalOxide Diatom-aceous earth

Soap

Stone(a) Talk(a) Mica(a) Graphite BoricAcid

1 2 4 1 2 8 4 3 7 7 4(b) 12 9 5 5 11 2 5 6 9 4 14 7 11 17 8 4 23 5 9 7 1 23 20 2 10 23 10 11 30 5 12 18 41 13 8 16 62 14 7 15 20 53

the molds and to extend their service lives they are usually coated with a wash, which is normally made of ethyl silicate or colloidal by forming minute cracks in their surface .

Graphite pe rmanent molds are used for a variety of products ( notably bronze bushings and sleeves) and graphite chills are often inserted in molds to promote

progressive or directional solidification The use of graphite as a permanent mold material is perhaps best demonstrated in the casting of chilled iron railroad car wheels ( the Griffin wheel casting process) as shown in Fig 5 Graphite is a particularly suitable mold material for this process It produces castings with closer tolerance than can be achieved with sand molding and the high thermal conductivity of graphite chills the metal next to the mold face very efficiently giving it a wear – resistant white iron structure.

However because graphite erodes easily pouring the me tal into molds from the top under the influence of gravity causes unacceptable mold wear. The technique has been used to make ferrous casting weighing up to 410 kg (900lb).

UNIT-2

CHAPTER – B2

CHAPTER OUTLINE

B2.1.1 Introduction

B2.1.2 (a) Pressure die casting machine

B2.1.2 (b) Classification of die casting machine B2.2.1 (a) Cold chamber machine

B2.2.1 (b) Cold chamber machine & parts

B2.2.2 Classification of cold chamber die casting m/c B2.2.3 Horizontal cold chamber machine

B2.2.4 Vertical cold chamber machine

B2.2.5 Comparisons of hot and cold chamber process B2.2.6 Process parameter & control

B2.2.7 Cold chamber die casting processing metals and alloys B2.2.8 Cold chamber die casting die

B2. 1.1

INTRODUCTION

Pressure die casting technique is a development over the gravity die casting Die casting is a process in which molten metal is injected into a precisely dimensioned steel mould with in which pressure is maintained until solidification is completed The die casting according reproduces with high fidelity the finest detail of the impression within which it was formed.

Die casting offers the users a means of obtaining dimensional accuracy and good surface finish in several alloys with wide range of mechanical and physical properties In the pressure die casting

1. The die is mounted between the two “ platen” of a press, generally of horizontal type by means of which it is closed and opened .

2. The movement of die follows that of ma chine and this determines the general

directional of assembly.

3. The liquid molten is generally injected by the action of piston which forces it through the die from a compression chamber. This is referred to as HOT

CHAMBER if it is situated inside the molten metal which is heated by furnace forming port of the assembly a COLD CHAMBER if it is fed with metal which has been melted in furnace separates from the machine.

B2.1.2 (a) PRESSURE DIE CASTING MACHINES

There is a wide choice ranging from small hot chamber machines intended for the production of hardware to massive cold chamber machines intended for the production of hardware to massive cold chamber machines with sophisticated equipment suitable for the automatic production of large and complex casting of high quality.

The function of pressure die casting Machines:

-1. To Hold the two Halves of the die together 2. Inject molten metal under pressure into the die

3. Close and open die halves to permit removal of finished castings 4. Eject the casting

The die costing machines essentially consists of a frame on which the die and the actuating equipment are mounted The actuating device opens and closes the die and the actuating equ ipment which in all modern machines are hydraulic cylinder and piston or hydraulic cylinder and toggle arrangement The injection system of the machine forces the molten material into the die under pressure The modern machines are equipped to give slow plunger movement a variable speed filling stroke and an intensified squaring pressure before solidification is completed The ejection system consist of knock out rod or plates hydraulic cylinder and rack and pinion The power cylinders are actu ated either hydraulically

B2.1.2 (b) CLASSIFICATION OF DIE CASTING MACHINES

There are generally two types of the die casting machines as identified by the placement of the injection chamber or metal pumping systems.

1. HOT CHAMBER (GOOSE NECK) MACHINE 2. COLD CHAMBER MACHING

i) HORIZONTAL

ii) VERTICAL

B2.2.1 (a) COLD CHAMBER MACHINES

For the die casting trade however the introduction of cold chamber machines was a considerable step forward No t only could Aluminum and magnesium alloys be die casted successfully on such equipment but large and complex zinc alloys components could be produced.

The metal is heated to pouring temperature in a separate holding furnace and transferred to the shot cylinder by ladling The metal injection chamber is separate from the

melting temperature of the alloys These machines are called Cold Chamber Casting

Machines and also know as Plunger Casting Machines.

These cold chamber machines are subdivide d into two types on the position of the injection chamber as:

-1. Horizontal cold chamber machines. 2. Vertical cold chamber machines.

The molten is in contact with shot cylinder and plunger for only a short period of time in cold chamber machines and is relatively cooler.

Cold chamber injection system can be used for metals that can be die cast. They are usually used for alumin um, magnesium and copper based alloys. Molten aluminum has a tendency to react with iron when it comes into contact with any steel at all times, resulting in contamination of the alloy and causing production of inferior castings.

This injection system is relatively free from attack of molten metal since, it is not submerged in the metal bath. This helps the use of high injection pressures and they range

form 8000- 30000 psi (560 – 2100 kg/cm2) Optimum plunger speed varies with:

a) The alloy being cast.

b) The size and shape of casting. c) The design of runner and gate.

B2.2.1 (b) COLD CHAMBER M/C AND PARTS

Horizontal Cold Chamber Die Casting M/C:

-Following are the most important characteristic of a horizontal cold chamber machine: -1) Injection System. 2) Accumulator. 3) Intensifier. 4) Ejection. 5) Core pull. 6) Pump. 7) Clamping.

Plunger speeds range from 150 to 900 ft. per minute Shot chamber and plunger of standard sizes could be fitted to the machine depending on the volume of the metal required for the casting (shot capacity).

Horizontal Cold Chamber Machines:

-The shot chamber is mounted horizontal with pouring hole in the top of the chamber wall.