Die casting Figure M2.3.1

Transcription

1 Die casting Die casting is a moulding process in which the molten metal is injected under high pressure and velocity into a split mould die. It is also called pressure die casting. The split mould used under this type of casting is reusable. Die casting is categorized two types namely- hot chamber and cold chamber as shown in Figure M Metals like Zinc, tin and lead alloys are casted in hot chamber die casting having melting point below C whereas aluminum alloys are casted in cold chamber die casting machine. Aluminum dissolves ferrous parts in the die chamber and hence preferred to be used in cold chamber die casting. Continuous contact of molten metal is avoided by using a ladle for introducing molten metal directly to the machine.. Figure M2.3.1: Die casting process

2 Advantages of the die casting process 1. High production rate. 2. High accuracy in part dimensions. 3. Smooth surface finish for minimum mechanical finishing. 4. Ability to make many intricate parts such as hole opening slot trademark number etc. 5. Much thinner wall sections can be produced which can t be produced by other casting methods. 6. Varieties of alloys can be used as per design requirements. For example zinc can be used for intricate forms and plasticity, aluminum for higher structural strength, rigidity and light weight. 7. Ability to cast inserts such as pins studs shafts, fasteners etc. Disadvantages of the die casting process 1. Microporosityinthe die casting products is a common problem because of faster solidification, trapped air and vaporized die lubricants. 2. Undercuts cannot be found in simple two piece dies. 3. Hollow shapes are not readily casted because of the high metal pressure. 4. Limited sizes of the products can be produced based on the availability of the equipment 5. High melting temperature alloys are practically not die casted 6. Flash is present except for very small zinc die casting.

3 Applications Die casting process is preferred for nonferrous metal parts of intricate shapes. Examples of products are automobiles appliances, hand tools, computer peripherals, toys, optical and photographic equipment etc. Suitable material consideration Since dies are made from tool steel, for die casting products lower melting point nonferrous materials are used. Popular among them are aluminum and zinc alloys. In recent days, ferrous metal die casting is carried out on an experimental basis. Also on limited production basis, dies with refractory material are used for materials having high melting temperature of about C. Different material properties of various types of alloys are shown in Table M Table M2.3.1: Various types of alloys recommended for die casting (Source: Design for Manufacturability Handbook by James G Bralla, 2nd Ed) Material Aluminum Zinc Commercial alloy name Melting point, C Ultimate tensile strength, kpa Castability Remarks ,000 Excellent Most popular aluminum alloy; best combination of properties and ease of use ,000 Good Used when better corrosion resistance and ductility are required AG40A (Zamak No. 3) ,000 Excellent Used for the majority of commercial applications for thin walls and good platability Magnesium AZ91B ,000 Excellent Used for the majority of commercial applications for lightness with strength Brass ,000 Fair Used when high strength, elongation, and corrosion resistance are required

4 General design consideration The following design guidelines need to be followed for die casting: 1. Identify the possibility of incorporating several functions in to one die casting. Full advantages must be taken for the reduction of machining that die casting can afford. 2. Before the design and construction of die begin the designer should finalize the design of die casted product and its producibility. Dies after machined undergo heat treatment for hardening. 3. The designer must consult die cater about the location of the ejector pin as early as possible in the product design stage itself. If the impression left by the pins are not tolerable or can t be cored out, as an alternative solution rings or sleeve ejection can be opted. 4. Abrupt section changes, sharp corners and wall at an acute angle to one another need to be avoided. These features disturb the continuity of metal flow and lead to form a porous structure and surface irregularities. Therefore, it is recommended to provide radii as generous as possible with differing sections blending into one another. 5. Blind recesses are needed to be provided in the die to form bosses. Due to trapped air subsurface porosity is developed and this causes the drills to wander and taps to break in secondary machining and hence recommended to be avoided. Specific design recommendation Wall thickness: Sharp changes in sectional area and heavy sections over 6 mm thickness should be avoided if possible. Uniform wall thickness need to be maintained so as to achieve a minimum porous die casted product. The injected liquid metal cools rapidly on contact with the die casting surface resulting in a fine grained dense structure (the surface is termed as skin) generally devoid of porosity and is considered to be the strongest region of the die casted product. Depending upon the casting size, the recommended skin measures between 0.38 and 0.63 as mentioned in the Table M2.3.2.

5 Table M2.3.2: Recommended wall thickness (mm) (Source: Design for Manufacturability Handbook by James G Bralla, 2nd Ed) Surface area(cm 2 ) Zinc alloys Aluminum and magnesium alloys Copper alloys Up to Figure M2.3.2: To avoid surface shrinks, connect the boss to the wall with a short rib. During cooling process, due to shrinkage in the metal inside the boss even after the cooling of the adjacent walls, bosses behind the surface can cause visible sinks on decorative parts having flat, expansive areas. Sink effects are magnified in thinner walls. To avoid such surface shrinks the bosses are relocated and connected to the wall with a short rib as shown in Figure M2.3.2.

6 Ribs and fillets: Ribs are provided for structural reinforcing. It must be perpendicular to the parting line so as to allow the removal of casting from the die. In order to avoid sink, it is recommended that the width of the rib should not exceed the wall thickness of the casting. The minimum distance between the two adjacent ribs must be the sum of their heights. Sufficient (at least 2 0 per side) drafts should be provided to help in ejection. Ribs and fillets are also used to improve the rigidity and strength of the standing bosses. Ribs can be designed on both the halves of the die instead of providing on one side of the wall, thereby strengthening it avoiding local thickening effect. In case, ribs are designed to cross, they should do so at right angles. Acute angle intersections are avoided because it causes the die to overheat in the area between the ribs. (Figure M2.3.3 to M2.3.6) Undesirable Desirable Figure M2.3.3: Incorporating internal ribs for Box-shaped components and provide radius for corners Undesirable Desirable Figure M2.3.4: Introduction of further coring for heavy section

7 Undesirable Desirable Figure M2.3.5: Avoid external-wall undercuts Figure M2.3.6: Kissing cores in opposite die halves. Draft: The side wall of the die casting and other features perpendicular to the parting line must be tapered or drafted as much as possible to easy removal from the die. Draft angles depend upon the type of alloy is used and draft requirements have been provided in Table M For the case of outside wall, the recommended draft angel should be the half of the draft angle used in the inside wall.

8 Table M2.3.3: The recommended draft angel for inside wall (in degree) Draft angle for inside wall(in degree) Depth of wall (mm ) Copper alloys Aluminum alloys Magnesium alloys Zinc alloys Radii: Sharp internal corners in die casting are to be avoided. The various reasons for avoiding sharp corners are: stress concentration at the corner, subsurface porosity due to abrupt change in metal flow direction during injection, and premature erosion of the die material because of heat concentration at sharp corners. It is recommended that radii and fillets should be 1.5 times the wall thickness for both inside and outside radii. (See Figure M2.3.7)

9 Figure M2.3.7: Allow generous radii at internal and external corners. Holes: The die-casting process uses cores to produce holes or openings. However, the cores used to create hole are to be placed at right angles to the parting line and there are core length limits that should not be exceeded depending upon the diameter. Sufficient draft should be provided to the cores to ensure longevity (Refer Table M2.3.4 & Table M2.3.5). (Figure M2.3.8 to M2.3.9) Table M2.3.4: Draft provided for different material a/c to diameter of hole. Diameter of hole (mm) Alloy Maximum depth(mm) Zinc Aluminum Magnesium Copper

10 Undesirable Desirable Figure M2.3.8: Countersink on both sides to avoid a deburring operation. Undesirable Desirable Figure M2.3.9: Core slides can be avoided by using this hole design. Table M2.3.5: Draft Requirements for Cored Holes in Die Castings Draft, Depth of hole, mm Copper alloys Aluminum alloys Magnesium alloys Zinc alloys

11 Core slides: Avoid designing for core slides that must fit accurately through both die halves. Drill holes in internal ribs. (Figure M2.3.10) Undesirable Desirable Figure M2.3.10: Avoid drill holes in internal ribs. Threads: External screw threads can be formed on die casting but not for a precision fit. Production of threads by machining is recommended for precision type of fit. The most practical way to cut the threads are between the die halves and eject the part in normal manner. Due to accuracy reasons, pitches finer than 24 threads per inch for Aluminum and Magnesium and 32 threads per inch for Zinc are not recommended. In fact, it is possible to die cast internal threads either by unscrewing the casting from the threaded core or rotating the core out of the casting during ejection. Zinc is recommended for this type of design since other alloys shrink more tightly onto the core. Inserts: Inserts can be incorporated into die casted parts where necessary. The most common type of insert is the threaded stud used for assembly operation. It must be designed in such a manner that material will shrink on to its shank with sufficient force thereby preventing the movement in use. (Figure M2.3.11) Figure M2.3.11: Insert designs to prevent rotation and pullout

12 Machining allowance: When die casting operation requires machining operation the machining allowance (the added material need to be removed) should not exceed 0.5 mm. At the same time, the allowance should not be less than 0.25 mm to avoid excessive tool wear. If an area is to be machined covers ejector pin location their impressions should be left standing to 0.4 mm to make sure that they are removed during machining. (Figure M2.3.12) Undesirable Desirable Figure M2.3.12: Limit machining allowance to 0.25 to 0.50mm. Flash and gate removal: A well-built trimming die generally removes flash almost to the most extremely drafted point of the casting wall. In order to avoid the wear of the cutting edges certain allowances must be provided. Commercial die castings are considered to be adequately trimmed if flash and gates are removed to within 0.38mm of the casting wall. In case of very heavy gates having size more than 2.2mm thick, gates are removed within 0.75mm. Angled junctions of an external wall are avoided with parting line. It is preferable to add a minimum draft shoulder at the parting line. (Figure M2.3.13) Figure M2.3.13: Provide1.5-mm minimum height shoulder at parting line.

13 Lettering: The easy way to specify the character is that, by raising it in the casting. This can be achieved by engraving of the die. If the character is to be depressed into the casting all the background steel on that face of the die must be painstakingly removed around the character. Some design rules for lettering are: Minimum character width: 0.25mm Character height: 0.25 to 0.5 mm Draft for clean ejection: at least 10 Surface design: Large plain area is generally not advisable because, any slight casting imperfection will be readily visible. Recommended option to resolve this issue is to mask irregularities by designing in ribs, serrations or mould texturing. Dimension and tolerances The main cause of dimensional variation in die casting are due to the thermal expansion of both the die and the casting. For designing die, a shrink factor of 0.6% is provided for both the contraction and expansion mentioned above. Recommended tolerances for die casting by cavity dimensions in either half of the die is shown in Table M2.3.6.

14 Table M2.3.6: Recommended tolerances for die casting by cavity dimensions in either half of the die. Dimensions to 25 mm Die casting alloys Zinc Aluminum Magnesium Copper For critical dimensions Each additional 25 mm over 25 to 300 mm Each additional 25 mm over 300 mm For noncritical dimensions Dimensions to 25 mm Each additional 25 mm over 25 to 300 mm Each additional 25 mm over 300 mm