Casting Processes: Differentiating Composite Molds and Solidification in Sand Casting, Study notes of Design

Download Casting Processes: Differentiating Composite Molds and Solidification in Sand Casting and more Study notes Design in PDF only on Docsity!

Material Processing

IE 3130

Module 5 – Fundamentals, Metal

Casting Processes and

Equipment

Objective of this module Upon completion of class activity, lecture, and homework, student should be able to do the following:

1. Differentiate between casting processes
2. Describe the solidification of alloys in molds.
3. Describe methods of improving mechanical properties of cast.
4. Briefly describe factors affecting the fluidity and quality of melts.
5. List process for casting shapes into expendable and reusable molds.
6. Describe methods of improving the properties of castings.
7. Given a product and production volume, choose a casting process and design part for ease of casting.

Module 5 - Metal Casting Processes and

Equipment

TABLE 11.2 General Characteristics of Casting Processes

Major Categories of Casting Processes and Equipment

1. Expendable Molds
2. Permanent Molds
3. Composite Molds

FIGURE 11.2 Outline of production steps in a typical sand-casting operation. 11.2 Expendable-mold, Permanent-pattern Casting Processes Major categories: sand, shell mold, plaster mold, ceramic mold, and vacuum casting

Types of Sand Molds

• Three basic types of sand molds: green-sand, cold-box, and no- bake molds.
• Most common is green molding sand: mixture of sand, clay and water. “Green” refers to the moist or damp sand mold while metal is poured. About 3% water content in sand.
• Green sand mold is least expensive, easily recycled.
• Cold-box process, various organic and inorganic binders are blended into the sand to bond the grains chemically for greater strength. More dimensionally accurate but more expensive.
• In the no-bake molds, a synthetic liquid resin is mixed with the sand, the mixture hardens at room temperature.
• Both cold-box and no-bake mold are called cold-setting processes because bonding of the mold takes place without heat.

Characteristics of Expendable Mold, Permanent-Pattern Casting E.g. Sand Mold Figure 11.3 Schematic illustration of a sand mold, showing various features. Major features:

1. Flask
2. Pouring basin
3. Sprue
4. Runner system
5. Risers
6. Cores
7. Vents
8. Cope
9. Drag

Risers (feeder heads) should have a high volume-to-area ratio compared to the product. Directional solidification Progressive solidification Aids to directional solidification Improper positioning of riser Chill

➢ A casting may show shrinkage cavities and microporosity (a), ➢ Feader heads or risers, removed after solidification, provide hot metal (b). ➢Microporsity may be eliminated with directional solidification by incorporating a metal chill into the mold (c). ➢Microporosity may be eliminated by tapering the thinnest section (d)

Characteristics of Expendable Mold, Permanent-Pattern Casting E.g. Sand Mold Figure 11.3 Schematic illustration of a sand mold, showing various features.

Patterns – Design Characteristics

➢All dimensions of patterns are increased to account for the contraction (solid shrinkage) of the casting from the solidus to room temperature. ➢If the casting is to be machined, an appropriate thickness (machining allowance) is added. ➢Patterns are made of wood or a metal or strong plastic for greater durability and dimensional stability. ➢For easy removability from the consolidated mold, patterns are made in two halves. ➢Surfaces parallel to the direction of withdrawal are given a draft to allow removal of the pattern without damaging the mold. ➢Cavities, undercuts, and recesses in the cast shape must be formed by the insertion of core. Nesting holes (core prints) are used for accurate location of cores. Cores are made of refractory materials. ➢The simplest pattern for producing the shape shown in Figure would be in one piece, and gates, runners, and risers would be added during molding. ➢For higher productivity, elements of the feeding system are incorporated into the pattern.

FIGURE 11.4 A typical metal match-plate pattern used in sand casting.

FIGURE 11.5 Taper on patterns for ease of removal from the sand mold.