CASTING DESIGN AND METHODS

DIPLOMA IN FOUNDRY TECHNOLOGY – CASTING DESIGN AND METHODS

SESSION 1 : CASTING PROCESS

1.1 Introduction

Aluminium alloy castings are being produced industrially by five principal methods.
These are:

(1) sand casting
(2) gravity die casting [permanent or semi-permanent mould casting]
(3) high pressure die-casting
(4) low pressure die-casting
(5) investment casting

In the sand casting process molten metal is poured into a sand mould and allowed to solidify. The mould is then shaken out and the casting removed. A new mould is necessary for each casting.

The permanent mould casting process [gravity die-casting] makes use of metal moulds and cores which may be used many times over. The moulds and cores must be carefully designed so that they can be separated from the casting without damage to the product, mould or cores. (Split line). Sand cores may have to be incorporated in the process due to design limitations, thus making it a semi-permanent process.

The die casting process – both high-pressure and low pressure – also makes use of metal moulds but unlike the permanent mould process, the molten metal is forced into the mould cavity under pressure: either high or low pressure.

The investment casting process consists of pouring metal into a mould produced by surrounding or “investing” an expendable pattern with a refractory slurry that sets at room temperature. After which the wax or plastic pattern is removed through the use of heat – prior to filling the mould with liquid metal. This process is also known as “precision casting” or the “lost-wax process”.

The choice of an appropriate process when considering the production of a new casting is rather difficult as many features of the design will influence the method of casting and vice versa. Each casting must be considered as a separate “challenge” and given careful study from both the technical and economical viewpoints to try to achieve a balance between the two.

Technical considerations:
Here we will need to consider a number of factors including:

– the size and shape of the casting
– the characterization of the alloy in terms of mechanical and physical properties
– the minimum and maximum section thicknesses
– the intricacy of the design
– the tolerance and type of surface finish

Economic considerations:
Here our concerns are:

– the number of pieces required
– possibility of repeat orders
– the relative machining and finishing costs of the castings produced by the various
processes

Careful estimating – based on experience – is the best guide in choosing a casting process.

1.2 Sand Casting
1.3 Gravity Die Casting
1.4 Pressure Die Casting
1.5 An Approach to the Design of Aluminium Castings
1.6 Casting Design
1.7 Designing for Minimum Casting Stresses
1.8 Model Making as an Aid to Design
1.9 Some Design Considerations
1.10 General Design Suggestions
1.11 Homework

SESSION 2 : PERMANENT MOULD CASTING [GRAVITY DIE]

2.1 Introduction

Permanent mould casting, or gravity die casting, is a widely used method of producing castings by pouring liquid metal into a die cavity; gravitational forces are then applied to assist the feeding process during solidification. In designs where sand cores are required the process is often called “semi-permanent” mould casting.

The choice between the various methods of producing castings depends largely upon both the technical requirements and the economic considerations.

Sand castings would be chosen for the production of up to a total of one thousand components. Above this figure the lower price for permanent mould casting begins to offset the initial tooling cost and future requirements are best met using this method.

When quantities amounting to ten thousand or more are to be produced, pressure die casting becomes a more economical proposition, due to the lower unit cost.

In some cases of larger quantities, permanent mould castings are still used when “undercuts” in the casting shape require either expendable sand cores or multi-piece steel cores – or where a particular alloy which is not easily pressure-die-cast is required. An example here would be an alloy casting made from a particular alloy that would later require heat-treatment.

In general, greater foundry skills are required in permanent mould casting than in pressure die casting. Permanent die casting is however considerably slower in production, although it is far more versatile in the type of castings that it can produce.

2.2 Casting Size
2.3 Die Design
2.4 Die and Core Materials
2.5 Casting Tolerances
2.6 Manually Operated Dies
2.7 Homework

SESSION 3 : LOW PRESSURE DIE CASTING

3.1 Principles of the Process

Molten aluminium alloy is contained in a crucible, housed in an electrical resistance furnace. A riser tube is housed in the cover and reaches down to near the bottom of the crucible.

The die, which is mechanically or hydraulically operated, is filtered by means of a controlled amount of air pressure that forces the level of molten metal downwards in the crucible so that a suitable volume of metal rises up the stalk and into the die. “1.5 to 2.5 Bar, depending on fill time required”

Any air in the die escapes by means of “venting”. The molten metal is held under pressure until it has solidified in the die, after which the pressure is removed, allowing the surplus metal to fall in the riser tube. The optimum pressure required for any design of casting is calculated making use of the appropriate tables and formulae. The flow rate is carefully controlled by a mano-metre tube or pressure gauge and timers.

3.2 Low Pressure Die Construction
3.3 Furnace Pressurisation
3.4 Riser Tube Material
3.5 Economics of Low-Pressure Die Casting
3.6 Homework

SESSION 4 : CASTING DEFECTS

4.1 Introduction

By its very nature, the casting process can give rise to a wide variety of defects in the final component. Nevertheless, with care and application, such faults can be held at a level which not only ensures profitability, but enhances customer confidence in our foundry’s ability to provide quality castings. Some casting defects may have no effect on the function or the service life of the cast components but will give an unsatisfactory appearance or make further processing, such as machining, more costly.

In any foundry that is endeavouring to improve the general quality of its castings or is attempting to correct existing defective castings, the first consideration must be the
correct identification of the defect so that the proper measures can be taken to correct it. Often the incorrect identification of defects has resulted in effort being wasted on corrective measures which are not related to the defect.

The design of a die has an important bearing upon the incidence of defects in castings. In some cases defects due to poor design can be counteracted by the application of special foundry techniques. But if the design is basically bad this invariably results in a greater scrap incidence than is acceptable. This in turn increases production costs and lowers productivity.

The most common defects due to bad design are:

– shrinkage cavities’
– shrinkage porosity
– hot tears
– cracks
– blow-holes

Design can also influence the incidence of such defects as:

– misruns
– cold laps
– warped castings

Castings should be designed to allow progressive solidification to take place. Abrupt sectional changes should be avoided and where sections must be increased, as in the case of flanges, etc., the change in section should be made gradual and machining allowances kept to a minimum. Constraining members, T-sections, and unnecessary bosses, should be avoided and suitable radii included at all corners.
Castings should be designed to allow for an adequate taper.

Whilst the remarks on design are applicable to all castings, it should be remembered that different metals and alloys vary in pouring temperature and in their contraction and solidification characteristics. As a result of this these factors will also need to be taken into account during the design phase.

Although the emphasis here is towards aluminium, it must be appreciated that factors which constitute a hazard risk with one metal may give little difficulty with another. The casting design should always be considered in relation to the characteristics of the particular metal involved.

The objective here is to illustrate a number of common defects experienced, together with their identification, and to suggest a number of possibilities for their elimination in subsequent production runs.

4.2 Inclusions
4.3 Oxidation and Dross
4.4 Gas Defects
4.5 Causes and Remedies
4.6 Check List for Pressure Die Casting Defects – “Pressure”
4.7 Check List of Pressure Die Casting Defects – “Gravity”
4.8 Check List for Pressure Die Casting Defects – “Sand Casting”
4.9 Homework

SESSION 5 : FILTRATION

5.1 Introduction

The filtration process consists of passing the molten metal through a porous device [a filter] in which the “inclusions” contained in the flowing metal are trapped or captured by one or more filtration mechanisms.

The filter material itself must have sufficient integrity – that is:

– strength
– refractoriness
– thermal shock resistance
– corrosion resistance

so that it is not destroyed by the molten metal before its task is accomplished. The best filter media to achieve this are the ceramic materials which are available in a variety of configurations.

5.2 Filter Sizes
5.3 Benefits of Filtration
5.4 Ceramic Filters in Gating Design
5.5 Filter Locations and Positions
5.6 Filter Types
5.7 Use of Ceramic Filters

SESSION 6 : PRESSURE DIE CASTING

6.1 Designing for Die Casting

Design conversions from alternative methods of manufacture must be determined by the function of the part and not by the previous history of its production. To attain maximum advantage from the process, each new or existing item must be designed specifically for die-casting. Although almost any shape can be die-cast, the simpler and cleaner the shape the more economical the product. The following will help to produce an efficient design with minimum production problems, and one that will result in a number of advantages, including:

– Minimum tooling cost and manufacturing time
– Short die-casting cycle times – therefore higher productivity
– High quality standards
– Minimum trimming costs
– Precision casting
– High Production
– Loss Height
– Lower Cost Components

6.2 Die Design
6.3 Bow-Shaped Castings
6.4 High-Pressure Die-Casting Machines
6.5 Process Summary
6.6 Homework

SESSION 7 : HIGH PRESSURE DIE CASTING

7.1 Purpose

To introduce the learner to the field.

7.2 Outcomes
7.3 Load Chart
7.4 Homework

SESSION 8 : GUIDE TO THE SELECTION OF ALUMINIUM CASTING ALLOYS

8.1 Introduction

In practice, the majority of the aluminium alloy castings produced are made from one of the following general purpose alloys: LM2, LM4, LM6, LM24, LM25 and LM27, and quite a large proportion of the remainder are pistons.

The alloys LM2, LM4, LM6, LM24, LM25 and LM27 (see Table 8-1) provide the essential basis of this guide and the user is reminded of the advantage to be gained from using one of these alloys wherever this is practicable.

They are generally cheap and readily available alloys and are suitable for a wide range of applications. They also have the advantage that more information is available from experience of the properties and production methods than for less commonly used alloys. Furthermore, the use of the minimum number of alloys in a foundry or in an engineering shop undoubtedly results in considerable simplification and economy.

This session is arranged to facilitate the selection of alloys according to:

(1) design of casting
(2) casting characteristics
(3) mechanical, physical and other properties of the alloys
(4) suitability for finishing processes

The most suitable alloys for each requirement are named under the appropriate heading, and for ease of comparison the data for the general purpose alloys are assembled in Table 8-1.

8.2 Design of Casting
8.3 Casting Characteristics
8.4 Mechanical Properties
8.5 Other Properties
8.6 Suitability for Finishing Treatments
8.7 Uses and General Remarks
8.8 Composition and Properties of Aluminium Alloys
8.9 Homework

SESSION 9 : ALLOYS FOR PRESSURE

9.1 Introduction

One of the most significant trends in the United Kingdom’s light alloy foundry industry has been the persistent increase in the production of pressure die-castings which have exceeded 22 410 tons – nearly four times the quantity produced ten years ago.

9.2 Effect of Composition
9.3 Mechanical Properties
9.4 Microstructure
9.5 Casting and Other Characteristics
9.6 Finishing

SESSION 10 : PERMANENT MOULD COATINGS

10.1 Introduction

When liquid metal is cast into a sand mould or against a core, there may be both a chemical reaction and a physical effect at the interface between the metal and sand. Either of these two conditions is likely to result in surface defects on the finished casting.
A coating on the mould or core can reduce or eliminate metal penetration and sand burn-on. For many years foundries made their own coatings but now, with modern technology advancing at the rate it is, coatings have now become highly developed and some very effective coatings are available from a variety of sources.

10.2 Uses and Application
10.3 Spray Equipment
10.4 Method of Application
10.5 Selecting the Correct Coating
10.6 Economics of a Coating
10.7 Homework

SESSION 11 : CASTING ECONOMICS

11.1 Introduction

One of the first questions we should perhaps ask ourselves is why go into the foundry business in the first place? Why not put our money in the bank to earn interest and let the bank have the worry?

There are basically three reasons why foundries form and these can briefly be described as follows:

– To provide a product or service to customers:

We may generally categorize businesses as either producing a product (manufacturing or process industries) or providing a service (tertiary industries). Both have a series of inputs (resources) which they intelligently use to provide outputs, the goods they produce or the services they provide.
– To make a profit:

One could argue that the underlying reason for being in business is so that we can make a profit. If this is the case then we need to realize that profit is not necessarily having a lot of money. Profit is having a bigger business at the end of the year than we had at the beginning of the year.
– Because we have a social responsibility:

Foundries are expected to use some of their surplus funds to assist the community where they are situated in developing its people and infrastructure. Foundries also provide employment for the local inhabitants of the area.
11.2 Profit and Productivity
11.3 Accounting
11.4 The Accounting Equation
11.5 The Business Cycle
11.6 Financial Accounting
11.7 Cost Accounting
11.8 Information Gained from Financial Accounts
11.9 Assets and Liabilities
11.10 Costing a Casting
11.11 Marginal Costing
11.12 Saving Costs in the Foundry
11.13 Homework