Die casting

Konstruktion för X MMKN11

Design For Manufacturing – Die Casting

Andreas Lövberg Cedric Delorme Elisabeth Hansson Charlotta Engstrand Yasser Faraj Chrissi Jarl 16/9/2013

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Contents 1. Introduction ......................................................................................................................................... 2

2. Assembly methods .............................................................................................................................. 3

2.1 Self cutting/Forming fasteners ...................................................................................................... 3

2.2 Die cast external threads ............................................................................................................... 3

2.3 Interference fits ............................................................................................................................. 3

2.4 Threaded fasteners........................................................................................................................ 4

3. Determining the number of cavities ................................................................................................... 5

3.1 Determining the most economical number of cavities ................................................................. 5

3.1.1 The die casting processing costs, Cdc and Cdn .......................................................................... 5

3.1.2 Trimming and multi aperture trimming costs, Ctr and Ctn ........................................................ 7

3.1.3 Compilation and derivation ........................................................................................................ 7

3.1.4 Flow diagram for determining number of cavities ..................................................................... 8

4. Determinate the appropriate machine size. ..................................................................................... 10

4.1 Needed clamping force ............................................................................................................... 10

4.2 Required shot volume ................................................................................................................. 10

4.3 Dimensional Machine Constrains ................................................................................................ 11

4.4 Example to calculate the machine size ....................................................................................... 12

5. Cost estimation .................................................................................................................................. 14

5.1 Die set costs ................................................................................................................................. 14

5.2 Cavity and core costs ................................................................................................................... 14

5.3 Trim die costs .............................................................................................................................. 15

6. Design principles................................................................................................................................ 16

Bibliography ........................................................................................................................................... 17

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1. Introduction Die Casting is a metal casting process. It is a manufacturing process for producing sharply, defined, smooth or textured-surface metal parts. Molded metal is forced and injected under high pressure into a mold cavity which then is held under pressure during solidification. In principle, the process is very similar to the injection molding with another class of materials. Most die casting parts are made of non-ferrous metals such as zinc, copper, aluminum, magnesium and depending on the type of metal that is being cast, a hot- or a cold-chamber is used.

The die casting process allows products to be made with high degree of accuracy and also produce fine details such as textured surfaces or names without requiring further processing. The die casting process is a suitable choice for mass produced products because of its ability of producing highly detailed parts. Almost every product or a part of a product one uses in daily life is produced using this process.

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2. Assembly methods The most common way of making a die casted assembly is to die cast smaller parts individually and then put them together to make a final assembly. Die castings can be assembled with not only castings of the same material but also with ceramics, alloys, plastics and woods. A lot of methods that are used to assemble metal parts, such as welding, studs or bolt and screw, can also be used to assemble die castings. This report cannot possibly mention all the different assembly techniques but a few common ones are described below . The following techniques are based on using “softer” metals/alloys like aluminum, magnesium, zinc and ZA-alloys. (Dynacast, u.d.)

2.1 Self cutting/Forming fasteners This principle is about using a harder material than the die cast itself to force a thread out of a cutting process. The tool that is used is most commonly made of hard steel. Depending on the dimensions and the strength where the thread should be one uses different sizes and force of the tool that cuts.

2.2 Die cast external threads Sometimes, the part that is going to be assembled with another part of a screw has to be stronger in that area to fulfill certain strength-requirements. Since the die casted part usually is made of a weaker material, a nut made of a harder material can be used to put inside the die casted thread area to make it stronger, see figure 1.

Figure 1. Die cast external threads.

2.3 Interference fits Another technique that can be used to assembly two parts is to squeeze the one in to the other. Interference fit is a technique that could be exerted both at room temperature and by cooling/warming up the different part to assemble. It all depends on the grade of interference. If the interference is light (typically 0.001 mm/mm or less) the assembly can be performed at room temperature. That type of interference could be achieved by a larger force/pressure. For interference that is heavy, the parts need to be heated respectively cooled to make the two parts fit together and then make a solid part together after going back to room temperature again, see figure 2.

Figure 2. Interference between shaft and hole.

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2.4 Threaded fasteners Casting a part in which it is wished to have a thread inside there are some things to think about to make it strong enough. It is always important to find a place where the part is able to deal with the forces it will be exposed to. At the concerned location, if circumstances permit it, a boss is created and threaded. Since the die cast it not as strong as the steel bolts or

screws it is important to construct the design so that the bolt fails rather than the casting. A rule of thumb is to make the boss diameter twice as big as the bolt diameter. See figure 3 for an illustration.

If two parts are to be assembled and the walls are thin it is not necessary to cast a threaded boss inside the part. In those cases it is enough to pass a boss through the hole and secure it with nuts.

Figure 3. Illustration of bolt and boss.

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3. Determining the number of cavities The number of cavities is restricted by several factors. These include clamping force, flow rate, number of side-pulls and machine size. According to (Kumar, et al., 2012), the number of cavities must be economically acceptable, technically permissible and geometrically feasible – while also fulfilling given time constraints. In order to find the optimum number of cavities, we first find the most economical number of cavities - which will then be subject to physical constraints in order to ensure practical use.

3.1 Determining the most economical number of cavities According to (Boothroyd, et al., 2002), the most economical number of cavities for die casting can be calculated in the same way as for injection molding. In this method, an expression for the total cost is set up

𝐶𝑡 = 𝐶𝑑𝑐 + 𝐶𝑡𝑟 + 𝐶𝑑𝑛 + 𝐶𝑡𝑛 + 𝐶𝑡𝑎 $

Where

𝐶𝑡 = 𝑡𝑜𝑡𝑎𝑙 𝑐𝑜𝑠𝑡

𝐶𝑑𝑐 = 𝑑𝑖𝑒 𝑐𝑎𝑠𝑡𝑖𝑛𝑔 𝑝𝑟𝑜𝑐𝑒𝑠𝑠𝑖𝑛𝑔 𝑐𝑜𝑠𝑡

𝐶𝑡𝑟 = 𝑡𝑟𝑖𝑚𝑚𝑖𝑛𝑔 𝑐𝑜𝑠𝑡

𝐶𝑑𝑛 = 𝑚𝑢𝑙𝑡𝑖𝑐𝑎𝑣𝑖𝑡𝑦 𝑐𝑜𝑠𝑡

𝐶𝑡𝑛 = 𝑚𝑢𝑙𝑡𝑖𝑎𝑝𝑒𝑟𝑡𝑢𝑟𝑒 𝑡𝑟𝑖𝑚𝑚𝑖𝑛𝑔 𝑐𝑜𝑠𝑡

𝐶𝑡𝑎 = 𝑡𝑜𝑡𝑎𝑙 𝑎𝑙𝑙𝑜𝑦 𝑐𝑜𝑠𝑡

3.1.1 The die casting processing costs, Cdc and Cdn

𝐶𝑑𝑐 = 𝑁𝑡𝑛∗ 𝐶𝑟𝑑 ∗ 𝑡𝑑

Where

𝐶𝑑𝑐 = 𝑑𝑖𝑒 𝑐𝑎𝑠𝑡𝑖𝑛𝑔 𝑝𝑟𝑜𝑐𝑒𝑠𝑠𝑖𝑛𝑔 𝑐𝑜𝑠𝑡

𝑁𝑡 = 𝑡𝑜𝑡𝑎𝑙 𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑐𝑜𝑚𝑝𝑜𝑛𝑒𝑛𝑡𝑠 𝑡𝑜 𝑏𝑒 𝑐𝑎𝑠𝑡

𝑛 = 𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑐𝑎𝑣𝑖𝑡𝑖𝑒𝑠

𝐶𝑟𝑑 = 𝑑𝑖𝑒 𝑐𝑎𝑠𝑡𝑖𝑛𝑔 𝑚𝑎𝑐ℎ𝑖𝑛𝑒 𝑎𝑛𝑑 𝑜𝑝𝑒𝑟𝑎𝑡𝑜𝑟 𝑟𝑎𝑡𝑒 �$ℎ�

𝑡𝑑 = 𝑑𝑖𝑒 𝑐𝑎𝑠𝑡𝑖𝑛𝑔 𝑚𝑎𝑐ℎ𝑖𝑛𝑒 𝑐𝑦𝑐𝑙𝑒 𝑡𝑖𝑚𝑒,ℎ

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Approximation of the hourly operating rate of a die casting machine, including operator rate:

𝐶𝑟𝑑 = 𝑘1 + 𝑚1 ∗ 𝐹 �$ℎ�

Where

𝐹 = 𝑑𝑖𝑒 𝑐𝑎𝑠𝑡𝑖𝑛𝑔 𝑚𝑎𝑐ℎ𝑖𝑛𝑒 𝑐𝑙𝑎𝑚𝑝 𝑓𝑜𝑟𝑐𝑒 [𝑘𝑁]

𝑘1,𝑚1 = 𝑚𝑎𝑐ℎ𝑖𝑛𝑒 𝑟𝑎𝑡𝑒 𝑐𝑜𝑒𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑡𝑠

Figure 4. Chart over machine cost and machine clamp force.

Linear regression analysis of specifications for hot- and cold-chamber machines, see figure 4, yields

Hot-chamber: 𝑘1 = 55.4, 𝑚1 = 0.0036

Cold-chamber: 𝑘1 = 62.0, 𝑚1 = 0.0052

This shows a linear relationship between clamp force and machine costs for machines up to 15 MN. Machines in the range of 15-30 MN are associated with greatly increased cost.

Cost of a multi cavity die casting die 𝐶𝑑𝑛 , relative to the cost of a single cavity die 𝐶𝑑1 : (based on data from Reinbacker)

𝐶𝑑𝑛 = 𝐶𝑑1𝑛𝑚 [$]

Where

𝑛 = 𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑐𝑎𝑣𝑖𝑡𝑖𝑒𝑠

𝑚 = 𝑚𝑢𝑙𝑡𝑖𝑐𝑎𝑣𝑖𝑡𝑦 𝑑𝑖𝑒 𝑐𝑜𝑠𝑡 𝑒𝑥𝑝𝑜𝑛𝑒𝑛𝑡

The exponent m is chosen in the same way as for injection molding, which suggests a value of 0.7 is reasonable (this was tested for a wide range of molds). This exponent value suggests a 62% increase in cost when doubling the number of cavities.

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3.1.2 Trimming and multi aperture trimming costs, Ctr and Ctn The cost of trimming is expressed as

𝐶𝑡𝑟 =𝑁𝑡𝑛∗ 𝐶𝑟𝑡∗𝑡𝑝 [$]

Where

𝐶𝑟𝑡 = 𝑡𝑟𝑖𝑚 𝑝𝑟𝑒𝑠𝑠 𝑎𝑛𝑑 𝑜𝑝𝑒𝑟𝑎𝑡𝑜𝑟 𝑟𝑎𝑡𝑒 [$ℎ

]

𝑡𝑝 = 𝑡𝑟𝑖𝑚𝑚𝑖𝑛𝑔 𝑐𝑦𝑐𝑙𝑒 𝑡𝑖𝑚𝑒 [ℎ]

Trim rate is approximated by a constant value for trim presses of all sizes because the cost is dominated by the wage of the operator, as small forces are required for the trimming process.

Trimming cycle time:

𝑡𝑝 = 𝑡𝑝0 + 𝑛 ∗ 𝛥𝑡𝑝

𝑡𝑝0 = 𝑡𝑟𝑖𝑚𝑚𝑖𝑛𝑔 𝑐𝑦𝑐𝑙𝑒 𝑡𝑖𝑚𝑒 𝑓𝑜𝑟 𝑠𝑖𝑛𝑔𝑙𝑒 − 𝑎𝑝𝑒𝑟𝑡𝑢𝑟𝑒 𝑡𝑟𝑖𝑚𝑚𝑖𝑛𝑔 𝑜𝑝𝑒𝑟𝑎𝑡𝑖𝑜𝑛 𝑓𝑜𝑟 𝑎 𝑠𝑖𝑛𝑔𝑙𝑒 𝑝𝑎𝑟𝑡 [ℎ]

𝛥𝑡𝑝 = 𝑎𝑑𝑑𝑖𝑡𝑖𝑜𝑛𝑎𝑙 𝑡𝑟𝑖𝑚𝑚𝑖𝑛𝑔 𝑐𝑦𝑐𝑙𝑒 𝑡𝑖𝑚𝑒 𝑓𝑜𝑟 𝑒𝑎𝑐ℎ 𝑎𝑝𝑒𝑟𝑡𝑢𝑟𝑒 𝑖𝑛 𝑎 𝑚𝑢𝑙𝑡𝑖𝑎𝑝𝑒𝑟𝑡𝑢𝑟𝑒 𝑡𝑟𝑖𝑚𝑚𝑖𝑛𝑔 𝑑𝑖𝑒

Similar to that of multi cavity die cost, multi aperture trim die cost is expressed as

𝐶𝑡𝑛 = 𝐶𝑡1𝑛𝑚 [$]

Where

𝐶𝑡1 = 𝑐𝑜𝑠𝑡 𝑜𝑓 𝑎 𝑠𝑖𝑛𝑔𝑙𝑒 𝑎𝑝𝑒𝑟𝑡𝑢𝑟𝑒 𝑡𝑟𝑖𝑚 𝑑𝑖𝑒, [$]

𝑚 = 𝑚𝑢𝑙𝑡𝑖𝑎𝑝𝑒𝑟𝑡𝑢𝑟𝑒 𝑡𝑟𝑖𝑚 𝑑𝑖𝑒 𝑐𝑜𝑠𝑡 𝑒𝑥𝑝𝑜𝑛𝑒𝑛𝑡

It is assumed that the cost exponent for multi aperture trim tools is the same as that for multi cavity die casting dies.

3.1.3 Compilation and derivation Compiling the previous equations results in

𝐶𝑡 =𝑁𝑡𝑛

(𝑘1 + 𝑚1𝐹)𝑡𝑑 +𝑁1𝑛 �𝑡𝑝0 + 𝑛𝛥𝑡𝑝�𝐶𝑟𝑡 + (𝐶𝑑 + 𝐶𝑡)𝑛𝑚 + 𝑁𝑡𝐶𝑎

Assuming full die casting machine clamp force utilization:

𝐹 = 𝑛𝑓 ⟷ 𝑛 =𝐹𝑓

Where

𝐹 = 𝑑𝑖𝑒 𝑐𝑎𝑠𝑡𝑖𝑛𝑔 𝑚𝑎𝑐ℎ𝑖𝑛𝑒 𝑐𝑙𝑎𝑚𝑝 𝑓𝑜𝑟𝑐𝑒, [𝑘𝑁]

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𝑓 = 𝑠𝑒𝑝𝑎𝑟𝑎𝑡𝑖𝑛𝑔 𝑓𝑜𝑟𝑐𝑒 𝑜𝑛 𝑜𝑛𝑒 𝑐𝑎𝑣𝑖𝑡𝑦, [𝑘𝑁]

Inserted into the previous compilation gives

𝐶𝑡 = 𝑁𝑡 �𝐾1𝑓𝐹

+ 𝑚1𝑓� 𝑡𝑑 + 𝑁𝑡𝐶𝑟𝑡𝑡𝑝0𝑓𝐹

+ 𝑁𝑡𝐶𝑟𝑡𝛥𝑡𝑝 + (𝐶𝑑+𝐶𝑡) �𝐹𝑓�𝑚

+ 𝑁𝑡𝐶𝑎

Optimization of the number of cavities for a die cast operation is arrived at by using the derivative of the total cost for all the components, set to equal zero:

𝑑𝐶𝑡𝑑𝐹

=−𝑁𝑡𝑓�𝑘1𝑡𝑑 + 𝐶𝑟𝑡𝑡𝑝0�

𝐹2+𝑚𝐹(𝑚−1)(𝐶𝑑 + 𝐶𝑡)

𝑓𝑚= 0

This, with rearrangement, yields the optimum number of die cavities for any given die casting task

𝑛(𝑚+1) =𝑁𝑡�𝑘1𝑡𝑑+𝐶𝑟𝑡𝑡𝑝0�𝑚(𝐶𝑑 + 𝐶𝑡)

3.1.4 Flow diagram for determining number of cavities (Kumar, et al., 2012) have developed a flow diagram for determining the optimum number of cavities using a CAD-file as input. Necessary information - such as area, wall thickness etc. – is extracted while other information such as delivery date and material are taken interactively from the user. The die-casting machine is selected from a machine database (which contains operating rates, clamping force, geometry etc.) and the alloy-properties are taken from a material database (which contains cost, temperatures, cooling factor, cavity pressure etc.). The flow diagram is shown in figure 5 below

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Figure 5. Flow diagram for determining the number of cavities for a multicavity die

Where 𝑁𝑚𝑎𝑐 is the minimum of the machine parameters (clamping force, flow rate or machine size), 𝑁𝑑𝑒𝑙 is delivery date (order must be fulfilled within time period, cycle times gives by Boothroyd), 𝑁𝑐𝑜𝑠𝑡 is part manufacturing cost and 𝑁𝑔𝑒𝑜 is part geometric features.

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4. Determinate the appropriate machine size. When choosing the appropriate machine size with which to cast a particular die cast component, there are several factors that must be considered. The most important factors are 1. the machine performance 2. the dimensional constrains imposed by the machine. The most important machine performance capability is the machine clamping force and the dimensional factors that must be considered are the available shot volume capacity, the die opening stroke length and the platen area.

4.1 Needed clamping force It is the machine clamping force F that the die casting machines are primarily specified on. The clamp force F needs to be larger than the separating force f of the molten metal on the die during injection, this to prevent separation of the die halves. Therefor the following requirement is needed F > f.

The separating force f for a given die casting task can be represented as:

𝑓 =𝑝𝑚𝐴𝑝𝑡

10 (1)

where

𝑓 = 𝑓𝑜𝑟𝑐𝑒 𝑜𝑓 𝑚𝑜𝑙𝑡𝑒𝑛 𝑚𝑒𝑡𝑎𝑙 𝑜𝑛 𝑑𝑖𝑒 [𝑘𝑁] 𝑝𝑚 = 𝑚𝑜𝑙𝑡𝑒𝑛 𝑚𝑒𝑡𝑎𝑙 𝑝𝑟𝑒𝑠𝑠𝑢𝑟𝑒 𝑖𝑛 𝑡ℎ𝑒 𝑑𝑖𝑒 [𝑀𝑃𝑎] 𝐴𝑝𝑡 = 𝑡𝑜𝑡𝑎𝑙 𝑝𝑟𝑜𝑗𝑒𝑐𝑡𝑒𝑑 𝑎𝑟𝑒𝑎 𝑜𝑓 𝑚𝑜𝑙𝑡𝑒𝑛 𝑚𝑒𝑡𝑎𝑙 𝑤𝑖𝑡ℎ𝑖𝑛 𝑡ℎ𝑒 𝑑𝑖𝑒 [𝑐𝑚2]

The total projected area can be calculated using the following equation:

𝐴𝑝𝑡 = 𝐴𝑝𝑐 + 𝐴𝑝𝑜 + 𝐴𝑝𝑓 𝑐𝑚3

where

𝐴𝑝𝑐 = 𝑝𝑟𝑜𝑗𝑒𝑐𝑡𝑒𝑑 𝑎𝑟𝑒𝑎 𝑜𝑓 𝑐𝑎𝑣𝑖𝑡𝑖𝑒𝑠 𝐴𝑝𝑜 = 𝑝𝑟𝑜𝑗𝑒𝑐𝑡𝑒𝑑 𝑎𝑟𝑒𝑎 𝑜𝑓 𝑜𝑣𝑒𝑟𝑓𝑙𝑜𝑤 𝑤𝑒𝑙𝑙𝑠 𝐴𝑝𝑓 = 𝑝𝑟𝑜𝑗𝑒𝑐𝑡𝑒𝑑 𝑎𝑟𝑒𝑎 𝑜𝑓 𝑓𝑒𝑒𝑑 𝑠𝑦𝑠𝑡𝑒𝑚

The size of overflow wells is a matter of individual die marker judgment coupled with trial and error modifications during die tryout. Studies state that projected area of cavities together with the projected area of overflow wells appears to be 50%-100% of the projected area of cavities. The following equation can therefor instead be used to approximately calculate the total projected area:

𝐴𝑝𝑡 ≈ 1.75 𝐴𝑝𝑐 (2)

4.2 Required shot volume The shot volume required for a particular casting cycle can be represented as:

𝑉𝑠 = 𝑉𝑐 + 𝑉𝑜 + 𝑉𝑓 𝑐𝑚3

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where

𝑉𝑠 = 𝑡𝑜𝑡𝑎𝑙 𝑠ℎ𝑜𝑡 𝑣𝑜𝑙𝑢𝑚𝑒 𝑉𝑐 = 𝑣𝑜𝑙𝑢𝑚𝑒 𝑜𝑓 𝑐𝑎𝑣𝑖𝑡𝑖𝑒𝑠 𝑉𝑜 = 𝑣𝑜𝑙𝑢𝑚𝑒 𝑜𝑓 𝑜𝑣𝑒𝑟𝑓𝑙𝑜𝑤 𝑤𝑒𝑙𝑙𝑠 𝑉𝑓 = 𝑣𝑜𝑙𝑢𝑚𝑒 𝑜𝑓 𝑓𝑒𝑒𝑑 𝑠𝑦𝑠𝑡𝑒𝑚

The following relationships can be used to represent the volumes of overflow and feed systems:

𝑉𝑜 = 0.8 𝑉𝑐ℎ1.25 cm3

𝑉𝑓 = 𝑉𝑐ℎ

cm3

where

ℎ = 𝑡ℎ𝑒 𝑎𝑣𝑒𝑟𝑎𝑔𝑒 𝑤𝑎𝑙𝑙 𝑡ℎ𝑖𝑐𝑘𝑛𝑒𝑠𝑠 𝑜𝑓 𝑡ℎ𝑒 𝑝𝑎𝑟𝑡 [𝑚𝑚]

In the early-design assessment, the volume of total shot size can be reduced to:

𝑉𝑠 = 𝑉𝑐 �1 +2ℎ

� (3)

4.3 Dimensional Machine Constrains As said, for a part to be die cast on a particular machine it has to have sufficient clamp force and enough shot volume. In addition to this, there are two more requirements that must be satisfied. A first requirement is that the maximum die opening must be wide enough so that the part can be extracted without interference. The size of the opening can be calculated with the following equation:

𝐿𝑠 = 2𝐷 + 12 𝑐𝑚 (4)

where

D=depth

The second requirement is that the area between the corner tie bars on the clamp unit (platen area) must be sufficient to accommodate the required die. The clearance between adjacent cavities or between cavities and plate edge should be a minimum of 7.5 cm with an increase of 0.5 cm for each 100 cm2 of

cavity area, see figure 6

𝐶𝑙𝑒𝑎𝑟𝑎𝑛𝑐𝑒 = 7.5 +0.5𝐴𝑝𝑐100 𝑛

𝑐𝑚 (5)

where

𝑛 = 𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑐𝑎𝑣𝑖𝑡𝑖𝑒𝑠

Figure 6. Arrow showing the length of "clearance".

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Reasonable estimates of the mandatory plate size are given by allowing 20% increase of part width for overflow wells and 12.5 cm of additional plate width for the biscuit.

4.4 Example to calculate the machine size A 20 cm long by 15 cm wide by 10 cm deep box-shaped die casting is to be made from A360 aluminum alloy. The mean wall thickness of the part is 5 mm and the part volume is 500 cm3.

See figure 7 for visualization of the measurements.

Determine the appropriate machine size if a two-cavity die is to be used.

The projected area of cavities is given by:

Apc = 2 × (20 × 15) = 600 cm2

Therefor the estimated shot area is by equation (2):

Apt = 1.75 × 600 = 1050 cm2

Given that the molten metal pressure in the die for A360 aluminum alloy is

pm = 48 MN/m2, the die separating force can be calculated from equation (1):

f = 48 × 1050/10 = 5040 kN

The shot size is given by equation (3):

Vs = 2 × 500(1 + 2/5) = 1400 cm3

The clamp stroke Ls can be calculated with equation (4):

Ls = 2 × 10 + 12 = 32 cm

The clearance between the cavities and with the plate edge can be calculated with equation (5):

Clearance = 7.5 + 0.5 x 600/(100 × 2) = 9.0 cm

The spacing between the adjacent cavities and around the edges may be arranged to 9 cm. An additional 20% width is added to allow for the overflow wells, as well as 12.5 cm for the biscuit. This results in a final plate size of 67 × 42.5 cm. Although, in table 1 you can see that the appropriate machine would be the one with 6000 kN clamping force. This is because of that the separation force is 5040 kN and the clamping force need to be larger. The appropriate machine can accommodate plate sizes up to 100 × 120 cm.

Figure 7. The layout within the plate.

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Table 1. Cold-chamber Die Castings Machines.

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5. Cost estimation When constructing a die cast one have to consider the tooling costs to create the die cast. Compared to similar molding techniques they are slightly higher and this is due to higher thermal shocks that the die casting is exposed to and that finer steel must be used for the die set. Also the overflow wells and sprues take up more plate area compared to the one in for example injection molding. Thus larger die sets will be needed. A trimming tool is also necessary to remove overflow wells, feed system etc. causing the costs to increase even more.

5.1 Die set costs Die sets and mold bases come in different types of steel and often it is recommended to use steel of better quality when die casting compared to injection molding. For the same plate size and thickness the cost is 25% more expensive than for the same mold base in injection molding. Therefore on can use the equation to calculate the die set cost for injection molding and multiply it by 1.25 which leads to:

𝐶𝑑 = 1250 + 0.56𝐴𝑐ℎ𝑝0.4 $

Where

𝐴𝑐= area of die set cavity plate, 𝑐𝑚2

ℎ𝑝 = combined thickness of cavity and core plates in die set, cm.

5.2 Cavity and core costs Usually the costs for customizing the die set (creating holes, fitting electrical and cooling systems etc.) is double the purchase price. What determines the cost i.e. the manufacturing hours is the amount of ejector pins needed and the relation between these and the projected part area has been found to be:

𝑁𝑒 = 𝐴𝑝0.5

Where

𝑁𝑒 = number of ejector pins required

𝐴𝑝 = projected part area, 𝑐𝑚2

It has been shown that an approximate value of manufacturing hours is 3.125 for each ejector pin (25% more time consuming than for injection molding). Using the relation above and this factor it is possible to estimate the total amount of manufacturing hours as

𝑀𝑒 = 3.125 + 1.25𝐴𝑝0.5 ℎ

Three different types of surface finish are usually used in die casting, minimum, medium and high quality finish. The costs increase with lower tolerance i.e. better quality and the additional percentage is 10% for minimum, 18% for medium and 27% for high quality.

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5.3 Trim die costs The trim die cost depends on the complexity of the profile and is defined as

𝑋𝑝 =𝑃2

𝐿𝑊

Where

P= outer perimeter of on cast part, cm

L, W = length and width of smallest rectangle that surrounds outer perimeter of one cast part, cm

Compared to blanking dies the cost is estimated to be 50% lower if no additional punches are required. This leads to the equation for total manufacturing points.

𝑀𝑡0 = 15 + 0.125𝑋𝑝0.75ℎ

The total estimated hours for a trim tool when using standard punches is

𝑀𝑡 = 𝑓𝑙𝑤𝑀𝑡0 + 2𝑁ℎ

Where

𝑓𝑙𝑤 = 1 + 0.04(𝐿𝑊)0.7 Using the average of the curves from figure 8 for area correction of blanking dies

𝑀𝑡= tool manufacturing time, h

𝑁ℎ= number of holes to be trimmed

Figure 8. Area correction factor

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6. Design principles. Accepted guidelines for die casting are listed below.

1. Die casting should be thin-walled structures. The wall should be uniform because it will ensure smooth metal flow during filling and minimize distortion from cooling and shrinkage. Zinc die casting should have a wall thickness between 1 to 1.5 mm. Aluminum or magnesium should be 30 to 50% thicker than zinc and copper die casting are usually 2 to 3 mm thick. With these thickness ranges the components will have a fine-grained structure with a minimum amount of porosity and good mechanical properties. Thicker sections in a casting will have an outer skin of fine metal, with a center section that has a rougher grain structure, some amount of porosity and poorer mechanical properties. Therefore it is important to know that mechanical strength does not increase in proportion to wall thickness.

2. Interior undercuts, see figure 9, should be avoided in casting design because moving interior core mechanics are almost impossible to operate with die casting. Those features must constantly be produced by subsequent machining. Nevertheless, the power of die casting lies in its ability to produce complex components and parts with good surface finish. Having made the decision to design for die casting, however, bearing in mind that getting as much from the process as is economically possible is important. This way, the structure of the assembly will be simplified.

3. If there are features projecting from the main wall of a die casting, they should not add significantly to the bulk of the wall at the connection point otherwise as with the injection molding, this would produce delayed cooling of the thickened section of the main wall.

4. Features projecting from the side walls should not lie behind one another when viewed from the die opening direction. By not having the features behind one another die casting depressions between the features will be avoided.

Figure 9. Explanation of undercuts.

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Bibliography Boothroyd, G., Dewhurst, P. & Knight, W. A., 2002. Product Design for Manufacture and Assembly. 2nd red. u.o.:CRC Press.

Kumar, V., Madan, J. & Gupta, P., 2012. System for computer-aided cavity layout design for die-casting dies. International Journal of Production Research.