Keuntungan Pengecoran dengan Metode Lost Foam Casting

Ada beberapa keuntungan pengecoran dengan metode Lost Foam Casting (ini menurut dosen kita tercinta lho, Pak Dr. Suyitno, S.T. M.Sc.) antara lain sebagai berikut :

1. Fleksibel dalam pembuatan pola, karena pola dibuat dari styrofoam sehingga untuk merubah pola dapat dilakukan dengan mudah dan cepat
2. Ketelitian dimensinya tinggi, karena pola telah dibentuk sesuai dengan ukuran benda yang akan dihasilkan
3. Adanya pengurangan core, metode ini memungkinkan bentuk model yang lebih kompleks
4. Tidak dibutuhkan adanya cupe dan drag seperti digunakan pada pengecoran pasir cetak tradisional
5. Ramah lingkungan, pasir cetak dapat digunakan berulang-ulang.

Nah tunggu apalagi catat sekarang siapa tahu besok keluar di ujian. Toh yang penting kalau dalam pengecoran logam tuh prakteknya, otomatis teori kudu di luar kepala.

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Sand Casting and Processing of Sand Casting

A sand casting or a sand molded casting is a cast part produced by forming a mold from a sand mixture and then pouring molten liquid metal into the cavity in the mold. The mold is then cooled until the metal has solidified. In the last stage, the casting is separated from the mold. There are six steps in this process:

1. Place a pattern in sand to create a mold.
2. Incorporate a gating system.
3. Remove the pattern.
4. Fill the mold cavity with molten metal.
5. Allow the metal to cool.
6. Break away the sand mold and remove the casting.

There are two main types of sand used for molding. Green sand is a mixture of silica sand, clay, moisture and other additives. The air set method uses dry sand bonded to materials other than clay, using a fast curing adhesive. The latter may also be referred to as no bake mold casting. When these are used, they are collectively called "air set" sand castings to distinguish these from "green sand" castings. Two types of molding sand are natural bonded (bank sand) and synthetic (lake sand), which is generally preferred due to its more consistent composition.

With both methods, the sand mixture is packed around a master pattern forming a mold cavity. If necessary, a temporary plug is placed to form a channel for pouring the fluid to be cast. Air-set molds often form a two-part mold having a top and bottom, termed cope and drag. The sand mixture is tamped down as it is added, and the final mold assembly is sometimes vibrated to compact the sand and fill any unwanted voids in the mold. Then the pattern is removed with the channel plug, leaving the mold cavity. The casting liquid (typically molten metal) is then poured into the mold cavity. After the metal has solidified and cooled, the casting is separated from the sand mold. There is typically no mold release agent, and the mold is generally destroyed in the removal process.

The accuracy of the casting is limited by the type of sand and the molding process. Sand castings made from coarse green sand impart a rough texture on the surface of the casting, and this makes them easy to identify. Air-set molds can produce castings with much smoother surfaces. Surfaces can also be ground and polished, for example when making a large bell. After molding, the casting is covered in a residue of oxides, silicates and other compounds. This residue can be removed by various means, such as grinding, or shot blasting.

During casting, some of the components of the sand mixture are lost in the thermal casting process. Green sand can be reused after adjusting its composition to replenish the lost moisture and additives. The pattern itself can be reused indefinitely to produce new sand molds. The sand molding process has been used for many centuries to produce castings manually. Since 1950, partially-automated casting processes have been developed for production lines.

source : http://wikipedia.org

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Advantages and disadvantages of Lost Foam Casting

This casting process is advantageous for very complex castings that would regularly require cores. It is also dimensionally accurate, maintains an excellent surface finish, requires no draft, and has no parting lines so no flash is formed. As compared to investment casting, it is cheaper because it is a simpler process and the foam is cheaper than the wax. Risers are not usually required due to the nature of the process; because the molten metal vaporizes the foam the first metal into the mold cools more quickly than the rest, which results in natural directional solidification.Foam is easy to manipulate, carve and glue, due to its unique properties. The flexibility of LFC often allows for consolidating the parts into one integral component; other forming processes would require the production of one or more parts to be assembled.

The two main disadvantages are that pattern costs can be high for low volume applications and the patterns are easily damaged or distorted due to their low strength. If a die is used to create the patterns there is a large initial cost.

source : http://wikipedia.org

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Processing of Lost Foam Casting

Lost-foam casting (LFC) is a type of evaporative-pattern casting process that is similar to investment casting except foam is used for the pattern instead of wax. This process takes advantage of the low boiling point of foam to simplify the investment casting process by removing the need to melt the wax out of the mold.

Processing of Lost Foam Casting

First, a pattern is made from polystyrene foam, which can be done many different ways. For small volume runs the pattern can be hand cut or machined from a solid block of foam; if the geometry is simple enough it can even be cut using a hot-wire foam cutter. If the volume is large, then the pattern can be mass-produced by a process similar to injection molding. Pre-expanded beads of polystyrene are injected into a preheated aluminum mold at low pressure. Steam is then applied to the polystyrene which causes it to expand more to fill the die. The final pattern is approximately 97.5% air and 2.5% polystyrene. Once the pattern is made pre-made pouring basins, runners, and risers can be hot glued to form the final pattern.

Next, the foam cluster is coated with ceramic investment, also known as the refractory coating, via dipping, brushing, spraying or flow coating. This coating creates a barrier between the smooth foam surface and the coarse sand surface. Secondly it controls permeability, which allows the gas created by the vaporized foam pattern to escape through the coating and into the sand. Controlling permeability is a crucial step to avoid sand erosion. Finally, it forms a barrier so that molten metal does not penetrate or cause sand erosion during pouring. After the coating dries, the cluster is placed into a flask and backed up with un-bonded sand. The sand is then compacted using a vibration table. Once compacted, the mold is ready to be poured. Automatic pouring is commonly used in LFC, as the pouring process is significantly more critical than in conventional foundry practice.

sumber : http://wikipedia.org

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Pasir Cetak

Bingun juga nih mo cari materi buat ujian besok pagi, akhirnya aku search di google, yang penting cocok dengan materi kuliah siapa tahu besok keluar. Ujian juga tinggal satu doank, Nah ini beberapa materi yang udah aku dapat :

Syarat Umum Pasir Pengecoran :

1. REFRACTORINESS (Tahan suhu tinggi)
2. COHESIVENESS (Kemudahan dibentuk)
3. PERMEABILITY (Mampu dilewati udara/gas)
4. COLLAPSIBILITY (Memberi tempat utk perubahan
5. bentuk)dalam kasus distorsi

Syarat Pasir Pengecoran

1. Kuat
2. Permeabilitas yang baik
3. Flowabilitas yang baik
4. Mempunyai distribusi pasir yang cocok
5. Sifat adhesive yang baik
6. Sifat kohesive yang baik
7. Sifat kollapsibility
8. Koefesien muai rendah
9. Bench life

B. Material Pasir

• Pasir Silika
• Pasir Chromit
• Pasir Zircon

1. Pasir Silika
Deskripsi :

• Merupakan pasir sintetis atau pasir buatan
• Kadar SiO2 95%
• Pasir berkualitas tinggi dengan sedikit impuritis
• Pasir silika Merupakan pasir yang paling banyak digunakan karena jumlahnya yang sangat banyak dan juga harga yang murah

2. Pasir Chromit

• Formula: Fe2O4·Cr2O4
• Pasir berkualitas tinggi dengan sedikit impuritis
• Ekspansi termal rendah
• Konduktivitas termal tinggi
• Refraktori bagus

3. Pasir Zirkon

• Formula: ZrO2.SiO2
• Pasir zirkon ada yang tidak berwarna dan ada yang berwarna. Warna yang dimiliki zirkon biasanya cokelat atau merah kekuning-kuningan.
• Sumber dari Zirkonium murni menunjukan penyerapan neutron yang rendah, kekuatan tinggi pada temperatur yang tinggi, dan tahan terhadap korosi kimia, selain untuk
• cetakan pasir dapat juga digunakan untuk reaktor nuklir dan radiometric dating.

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Definition of Heat Transfer

Do you know about definition of Heat Transfer ? Heat transfer is the transition of thermal energy from a hotter mass to a cooler mass. When an object is at a different temperature than its surroundings or another object, transfer of thermal energy, also known as heat transfer, or heat exchange, occurs in such a way that the body and the surroundings reach thermal equilibrium; this means that they are at the same temperature. Heat transfer always occurs from a higher-temperature object to a cooler-temperature one as described by the second law of thermodynamics or the Clausius statement. Where there is a temperature difference between objects in proximity, heat transfer between them can never be stopped; it can only be slowed. Source : www.wikipedia.org

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Heat Transfer Mecanism

Heat transfer mechanisms can be grouped into 3 broad categories:

Conduction:

Regions with greater molecular kinetic energy will pass their thermal energy to regions with less molecular energy through direct molecular collisions, a process known as conduction. In metals, a significant portion of the transported thermal energy is also carried by conduction-band electrons.

Convection:

When heat conducts into a static fluid it leads to a local volumetric expansion. As a result of gravity-induced pressure gradients, the expanded fluid parcel becomes buoyant and displaces, thereby transporting heat by fluid motion (i.e. convection) in addition to conduction. Such heat-induced fluid motion in initially static fluids is known as free convection.

Radiation:

All materials radiate thermal energy in amounts determined by their temperature, where the energy is carried by photons of light in the infrared and visible portions of the electromagnetic spectrum. When temperatures are uniform, the radiative flux between objects is in equilibrium and no net thermal energy is exchanged. The balance is upset when temperatures are not uniform, and thermal energy is transported from surfaces of higher to surfaces of lower temperature.

source : www.efunda.com

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Basics of Heat Transfer

Do you know about Heat Transfer ? Heat Transfer in food engineering proses is very important. In the simplest of terms, the discipline of heat transfer is concerned with only two things: temperature, and the flow of heat. Temperature represents the amount of thermal energy available, whereas heat flow represents the movement of thermal energy from place to place.

On a microscopic scale, thermal energy is related to the kinetic energy of molecules. The greater a material's temperature, the greater the thermal agitation of its constituent molecules (manifested both in linear motion and vibrational modes). It is natural for regions containing greater molecular kinetic energy to pass this energy to regions with less kinetic energy.

Several material properties serve to modulate the heat tranfered between two regions at differing temperatures. Examples include thermal conductivities, specific heats, material densities, fluid velocities, fluid viscosities, surface emissivities, and more. Taken together, these properties serve to make the solution of many heat transfer problems an involved process. source : www.efunda.com

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