Rapid Tooling in Metal Casting

Sand Casting



Scematic of RT application in sand casting

It is one of the major processes used to manufacture cast parts. In this process, molten metal is poured into a disposable mold formed out of foundry sand containing binder. The cavity in the sand is formed by using a pattern. A pattern, which has approximately the same shape as an actual casting, is the main tooling in the process of sand molding. Patterns in sand casting are typically made out of wood, plastic, or metal (since pattern material dictates pattern life, its selection depends on the volume of production). If a cast part has internal cavities or passages, then a sand core or cores should be inserted into mold cavity. Core boxes, which are used for sand cores production, are second tooling in sand casting. Major requirements for sand casting tooling are:

Direct Tooling

Theoretically all RP technologies are capable of creating models that can be used directly as patterns in sand molding after appropriate finishing. LOM is the most common application used for pattern making for very large casting sizes.

gray iron casting and its lom-generated prototype

Gray iron casting and its
LOM-generated prototype

The only weakness of paper made LOM patterns and core boxes are their susceptibility to humidity. The latter can be fixed by applying special coatings or paintings.

Indirect Tooling

All of RP generated models can be used as indirect, or intermediate tooling for sand molding. For example as patterns in silicon rubber molds production, which in turn are used for production of durable plastic patterns. The only limitation here is the size of a model.

Paternless Sand Molding

Current progress made in SLS technology allows sintering foundry sand coated by ultraviolet curable resin. It provides the opportunity to generate sand molds and cores directly on a SLS machine. Both manufacturers of SLS systems, 3D Systems and EOS GmbH, produce systems with this capability.

A complicated sand core made by laser sintering of foundry sand (3D System) and an actual aluminum casting made utilizing the core

Laser sintered sand molds and cores (EOS System, EOS S)

Steering block

Hydraulic controller

Impeller

However, making sand cores or molds by laser sintering takes significantly more time than traditional methods utilizing patterns and core boxes. Therefore, it is advantageous to use pattern less molds production in cases, when:

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Investment Casting





Scematic of RT application in investment casting

This process utilizes disposable molds made by use of a lost pattern made from wax or some other material that can be melted or burned away. This lost pattern is dipped in refractory slurry, which coats the wax pattern and forms a skin. The shell then is dried and the process of dipping in the slurry and drying is repeated until a desirable shell thickness is achieved. After this, melting or burning removes the lost pattern. This leads to a mold that can be filled with the molten metal.
The tooling in this process is reusable mold for lost pattern pouring. These molds traditionaly are made from metal (usually aluminum) in case of high-production runs, or from epoxy, or even rubber in case of medium and low-runs productions.

Direct Tooling

Investment casting wax can be injected into a mold, which was generated by one of RP technologies. An example of investment cast tooling generated by LOM machine is shown in Figure bellow. The cavities in the mold halves are covered by special paint to increase mold durability, beside this, the mold was completely done by the RP-device.

LOM-generated mold, wax lost pattern, and final casting

Indirect Tooling

RP-generated objects can be utilized indirectly in investment casting processes, for example as a master pattern for creation of a silicon rubber mold, which can be employed for small slots of wax pattern production.

Toolless Process

Lost patterns can be produced directly by RP machines.
RP systems based on FDM principle (as well as SI machines) are capable to dealing with traditional investment casting wax as a build material. Consequently, they have found very wide applications as a lost pattern making systems. Nonetheless, other RP technologies also have been utilized for direct lost pattern production.

Polymer materials used in SLA are more difficult to burn out than traditional materials used for lost patterns producing. These materials also have a tendency to expand and crack the mold. To solve these problems, the SLA-generated pattern is built in hollow, thin sections, which tend to crumple during burn out rather than expand.

SLA-generated investment casting pattern and the actual aluminum casting

Plastics utilized in SLS processes are also capable to be used as a build material for lost patterns.

SLS-generated investment casting pattern and the actual aluminum casting

Modification of LOM technology, designed especially for lost pattern production, called "Lost paper", generated objects that do not expand and do not crack the shell during the burn cycle.

Among the materials used in 3D printing, suitable for manufacture of lost patterns are also available.

LOM-generated investment casting pattern and the actual aluminum casting

Major requirements to lost patterns made by direct RP technology are high dimension accuracy and low cost. NASA built a plenum gas-diverter component as a test case to evaluate the comparison of various RP methods. The pattern built from the RP technique is then used to develop the casting using investment casting. The produced castings and the patterns are then evaluated for their critical dimensional accuracy and costs. Their results show that high dimension accuracy of a pattern is not really critical, the accuracy of final casting has more important concern. For example, lost pattern made by SLA differs from initial CAD model not more than on 0.006'', while same parameter for SI-generated pattern is 0.013''. However, final casting differs from initial CAD drawing on 0.029'' for SLA and 0.013'' for SI. Therefore despite of variety of applicable methods, only 3D printing and SI methods are really effective in high volume production of cast parts by investment casting technology.

Direct Shell Production

This technique can be considered as next level in toolles manufacturing since even need in a lost pattern is eliminated. The technology is based on 3D Printing technology utilizing ceramic build powder. The thin wall ceramic shell made by this method has internal cavity, which repeat shape of desirable casting. This RT approach can be considered as method of investment casting shell creation with no lost patterns.

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Permanent Mold Casting





Scematic of RT application in permanent mold casting

This process utilizes reusable (permanent) molds made of a metal. Cores, used in permanent mold casting may also be made from metal (reusable cores) or sand (disposable cores). Since metallic molds and cores are reusable, they are the immediate tooling in permanent mold casting. RP technologies can be utilized in process of permanent mold making as direct or indirect implements. Besides metallic cores, sand cores made by RP methods may also be used in permanent mold casting.

Direct Tooling

Currently advanced SLS machines are capable to sinter metallic powders. It provides an ability to directly create permanent molds and cores for pouring low melting point metals such as aluminum, zinc and their alloys.

Currently two different approaches to sinter metallic powders exist. In the 3D System's version of metal sintering technology, parts are built by using polymer coated metal powder. Polymer is melted by laser beam and metallic particles are bound together. To achieve required mechanical properties, parts must go through finishing process in which part is heated, polymer burns away, and metal particles are sintered together. To eliminate porosity, the part must be infiltrated with another metal (typically copper based alloy).

In contrast to 3D System, in EOS machine parts are built by using clean metal powder with no binder or coating. Laser partially melts the powder, and particles are bound together. For removing porosity, the part also must be infiltrated with another metal, like copper.

Parts sintered by laser are comparable in durability to cast or forged parts made from same or similar material. Major limitation for both (3D System and EOS) metal sintering systems is size of created models:
There is also a possibility to create metallic mold directly from CAD data without any other additional treatment processes like infiltration or sintering. This possibility is brought by LENS process, which allows creating full dense metallic molds. Limitation of LENS process in terms of generated parts (in our case molds) dimensions is:

Indirect Tooling

Most common traditional method of permanent mold making is sand casting with or without finishing by machining. This technology can be improved by using RP generated models as patterns. Approach of usage of RP generated models as a pattern in sand casting was already described above.

SLA-generated mold prototype and final permanent mold set
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Pressure Die Casting Process

In terms of tooling, there are no significant differences between Die-casting and Permanent Mold casting, since both technologies utilize reusable metallic molds. However, pressure die casting dies will have to withstand a very high pressure and therefore are more difficult to produce compared to permanent molds. As a result there is a limitation in terms of the technologies used for this application. The SLS process with metal powders is used for direct tooling for die-casting. An example of tool inserts for die-casting produced with steel by direct metal laser sintering is shown below.

Mold for aluminum casting, made of direct sintered steel

The die proved that acceptable aluminum castings could be produced using this tool insert. It was claimed that total 500 castings were produced using the die and the die is still usable after 500 parts. Another example of tool inserts for magnesium alloy die-casting generated by direct metal sintering is shown below.

Tool inserts (Ericsson T28 mobile phone frame) made using SLS for die-casting process

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