Advantages of Shell Mold Casting Process

Lost foam casting (also known as solid casting) is to bond and combine paraffin or foam models similar in size and shape to castings into model clusters. After brushing refractory paint and drying, they are buried in dry quartz sand and vibrated for modeling. The new casting method is a new casting method in which the model is gasified by pouring down, and the liquid metal occupies the position of the model, and solidifies and cools to form a casting.

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 Shell mold casting process is a kind of base sand with high-quality selected natural sand, through special performance resin coating system and the most ideal process technology. The most perfect combination of fluidity, surface roughness of castings, etc., are widely used in automobile engines, diesel engines, hydraulic parts and other industries.  

 Compared with lost foam casting, coated sand casting has the following advantages:

  1. The pollution is small, and the worksite environment is obviously improved. Since the clay sand is added with a large proportion of coal powder, which is a combustion-supporting material, it is mainly used to increase the appearance quality and air permeability of the workpiece. The coal powder added to the sand cannot be reused, resulting in a waste of resources. Ash residues after combustion have a great impact on the quality of our working environment. At the same time, the reducing gas and PM2.5 produced during combustion cause serious pollution to the atmosphere. Although the molding method based on coated sand also has a certain impact on the environment, after all, it uses a small amount of sand, and at the same time, all the waste sand after use is recycled. The worksite is clean and free of dust pollution. 

  2. The sand coating layer is thin, the dosage is small, the production cost is low, the waste is small, and the used molding sand can be recycled and reused. Due to the thin sand coating layer of this molding method, 5-10mm coating sand layer is usually designed according to the wall thickness of the live piece, while the minimum sand coating amount for clay sand molding is usually 30-50&#;, and the amount of sand used is 10-20 mm of coated sand. times. Although clay sand can also be recycled for the second time, the high content of coal powder, bentonite and other additional raw materials have been basically consumed, and the quality of the sand has been greatly reduced, which affects the appearance quality of the product. After the molding sand of coated sand is roasted at high temperature, the expansion is greatly reduced, the process steps of pickling are reduced in the secondary use, and the addition amount of solid resin and curing agent is also greatly reduced, which reduces the production cost.

  3. Change the appearance quality of the blank, reduce the cutting allowance, the process yield is high, and the quality is stable. Due to the low hardness of the clay-sand molding sand and the large concessionality, the casting defects such as box expansion, sand expansion, sand loss, porosity, shrinkage and other casting defects are serious, which not only cause poor product appearance but also have many internal casting defects, which are serious and serious. The service life of the product is affected, and the scrap rate is high. The aggregate in coated sand molding is pickled or calcined at high temperature, which reduces the expansion of the sand mold after combustion, and has many advantages such as high strength and small gas generation after curing. Appearance quality, inherent casting defects, and product size accuracy are far superior to those produced by clay sand molding.

  4. Improve the internal quality of products. Due to the fast cooling rate of the coated sand during casting, the graphite morphology of the casting is greatly improved, the mechanical properties of the product are improved, and the service life of the product is increased. The products produced by coated sand molding, especially ductile iron, are far superior to those produced by clay sand molding regardless of spheroidization level, matrix structure, hardness, tensile strength, yield strength, etc. Invisibly improve the inherent quality of the product. It increases the service life of the product and indirectly provides a strong guarantee for meeting customer needs. 

  5. Coated sand molding adopts automatic production line operation, which improves production efficiency, reduces the labor intensity of workers, and shortens labor time. The traditional clay sand molding adopts manual molding or molding machine molding. The labor intensity of workers is high and the work efficiency is low, which is 1/3 to 1/4 of the molding efficiency of the production line.

Sand casting is an economical way to produce parts in a variety of metals. Shell mold casting can be an alternative process to consider given specific part design needs. This blog post introduces the shell mold casting process and compares it with sand casting so readers may understand their respective strengths and limitations.

Shell Mold Casting Basics

All casting processes need a pattern for the shape of the part to be cast, and shell molding is no exception. As with sand casting, investment casting and other casting processes, the pattern is used to create the cavity into which liquid metal is poured.

In shell molding a metal pattern tool is coated with a layer of sand. This contains a resin that melts when heated and cools to form a thin shell, (typically 1/4 &#; 3/8&#; thick,) around the pattern. Melting is achieved by heating the pattern before coating in sand, then heating the shell to fully cure the resin.

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The shell is made in two halves so it can be released from the pattern. After each half shell, (sometimes called a &#;biscuit&#;), is lifted away from the pattern the two shells are bonded together, forming the cavity in the space between them. The assembled shell is then placed into a flask where it&#;s supported with granular material like sand or shot.

The shell is filled with liquid metal which is allowed to cool. Once solid, the shell is taken out of the flask and broken apart to release the cast metal part. A limitation is that while it&#;s not impossible to add cores to form hollow regions, it is difficult to do.

Two Types of Sand

Shell mold casting uses a different sand to that used in green sand casting, and this is what gives the process its advantages. In sand casting the &#;green&#; sand, (green meaning not dried and not the color,) is held together by a binder of clay, usually Bentonite.

The silica sand used for shell molding is finer than that used for sand casting. The American Foundry Society uses a &#;fineness number&#; to characterize average grain size where a higher number indicates smaller grains. Green sand is typically 60 &#; 80 on this scale while shell molding sand is 100 or higher. In addition, rather than using a clay binder, the sand is given a thin coating of phenolic resin which is what melts to form the shell.

Consequences of Fine-Grained Sand

The fine grained sand packs more tightly than green sand, which reduces gas permeability. Sand casting requires a degree of permeability to let moisture, in the form of water vapor, out. This isn&#;t a problem in shell molding though because the shell is dried and heated before metal pouring.

The major benefits of finer sand are that it conforms more closely to find surface features on the pattern and provides a smoother surface for the metal.

For even better replication of surface detail, and a smoother finish, a foundry may use zircon sand. This can have a fineness number as high as 150. It also has a lower coefficient of thermal expansion which allows application of tighter tolerances.

How Shell Molding Benefits Cast Parts

Superior surface finish and reproduction of fine detail are two of the major strengths of this process. In addition, shell molding requires minimal draft angles to release the pattern from the shell.

Whereas sand casting typically needs 2° &#; 3° of draft, the shell molding process gets away with 1°, and on short vertical walls it may be possible to have zero. This allows application of smaller machining allowances, which saves time and money in finishing operations.

Sand Casting and Shell Mold Casting Compared

Sand casting:

• Works for ferrous and non-ferrous metals
• Can produce parts weighing from ounces to hundreds of pounds
• Can&#;t reproduce fine surface detail from the pattern
• Has low geometric precision
• Leaves a rough surface finish
• Requires large machining allowances
• Is an economical process

In comparison, shell molding:

• Works for ferrous and non-ferrous metals
• Is limited to parts weighing less than 30 lbs
• Is unsuited for parts that need cores
• Reproduces fine surface detail
• Can maintain tight tolerances
• Leaves a smooth surface finish

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In terms of capabilities and costs, shell mold casting falls between sand and investment casting. It&#;s more complex than sand casting and can produce more complex parts, but without the sophistication and complexity of a wax pattern process.

Impro makes aluminum aerospace parts by shell mold casting when it makes sense. If you&#;d like to discover whether the process is right for your parts, contact us. Our casting specialists will be happy to help.

Contact us to discuss your requirements of shell mold casting products. Our experienced sales team can help you identify the options that best suit your needs.