TECHNIQUE

General information about the lost wax technique

An exact wax model is used for both the precision casting and the investment casting. Such a model is produced in an aluminum mould through injection moulding (extrusion). These models are combined to form a wax casting unit, the so called casting tree. In several steps this casting unite is coated with ceramics. After the ceramic shell has dried the wax is burned out. The shell is baked at 900°C and the liquid metal alloy is then poured into the hot ceramic shell. Trapped gas escapes through the permeable ceramic layer. The feeding via the central core prevents the forming of shrink marks and air locks (cavities) in the hardening casting.

The ceramic shells of the cooled-down casting unit are removed and the castings are separated from the casting unite. They are sandblast or even etched in order to remove remaining ceramics on the inside, e.g. in bore holes or undercuts. Then the ingate on the surface is ground off. The visual check and the dimensional check of the castings are very costly since they have to be carried out by hand. Depending on the further use of the castings the following steps can be taken: the parts can be processed mechanically (threads, fits) or be heat-treated (tempering, case hardening, nitrating). Even a surface treatment may be required (hand polishing, powder coating, cathodic dip-coating, fibration polishing etc.).

The quality assurance for initial samples and serial parts can be carried out using different procedures. It is possible to perform chemical analyses, mechanical tests, ultrasonic tests, leakage tests, dimensional checks and radiographic examinations.

Precision casting

What distinguishes the precision casting from the investment casting is the use of ceramic dip-coating compositions and waxes with coarser grain sizes. This leads to greater tolerances and a rougher surface than the investment casting would produce.

Precision casting characteristics:

• Part size: edge lengths or diameters up to 900 mm
• Weight per unit: 0.5 kg to 85 kg
• Wall thickness: 2.5 mm or more
• Dimensional tolerance: about 1.2 % of the nominal size
• Surface roughness: min. N 9 (Ra = 6.3 ym)
• Processing of all castable materials (also well weldable qualities)
• Castings can be fully heat-treated
• Undercuts are representable
• Small and large-scale production series are possible
• Low tool costs
• Tool life: almost infinite
• Delivery periods: about 8-10 weeks after ordering (stockpiling is possible)

Investment casting

• Part size: edge lengths or diameters up to 500 mm
• Weight per unit: 0.01 kg to 35 kg
• Wall thickness: 1.5 mm or more
• Dimensional tolerance: about 0.8 % of the nominal size
• Surface roughness: min. N 8 (Ra = 3.2 ym)
• All castable materials (also well weldable qualities) can be processed
• Castings can be fully heat-treated
• Undercuts are representable
• Small and large-scale production series are possible
• Low tool costs
• Tool life: almost infinite
• Delivery periods: about 8-10 weeks
  

The technical bulletin VDG P 690 provides the basis for the lost wax technique concerning dimensional tolerances, surfaces and machining allowances. This bulletin can be purchased at:

VDG-Informationszentrum Giesserei
D-40042 Düsseldorf

Casting of prototypes

Several casting methods for producing functional prototype castings can be chosen from.

Rapid prototyping investment casting (single castings)

• A master model is moulded in plaster or ceramics and then it is cast.
• Delivery period about 1 week
• All castable materials

Rapid prototyping investment casting with silicone mould (small production series up to 50 pieces)

• Wax models are produced in a silicone mould, then formed with plaster or ceramics and cast.
• Delivery period about 2 weeks
• All castable materials

Investment casting with plastic tools for small production series (100 to 1,000 pieces)

• Wax models are produced through injection moulding in a plastic mould, then formed with plaster or ceramics and cast.
• Delivery period about 4 weeks
• All castable materials, even magnesium alloys

Precision casting with plaster (small and medium-sized production series; 100 to 2,500 pieces)

• Ideally suited for producing prototypes of future pressure castings
• The plaster forms are created within a milled aluminium tool.
• No wax model necessary
• Therefore more cost-effective, faster and more accurate than investment casting
• Delivery period about 5 weeks
• Only aluminium alloys are castable

Rapid prototyping sand casting (from single castings to medium-sized production series of 1,000 pieces)

• A model is moulded directly into sand and then it is cast.
• Laser-sintered cores can be used.
• Delivery period about 1 - 4 weeks (depending on complexity and quantity)
• All castable materials, even magnesium alloys

Depending on type, size and complexity of the component, on the required number of pieces and the casting material we will choose the most economic production technique.

All prototype castings can be mechanically reworked, hardened, painted and combined to form a assemblies.

Please enclose a 3D CAD drawing (IGES) when making an inquiry.

Materials

The casting technique allows the use of a wide range of different materials. The lost wax technique basically allows the use of all castable alloys.
The following list of the most common materials is not intended to be exhaustive. We will gladly consider your special wishes. Please inform us about the materials or special alloys you would like to use. Explain to us the application or the component's intended use. Many years of experience provide us with the necessary know-how to assist you in choosing the most suitable material.

Steel castings for general engineering purposes according to DIN 1681 / steel casting

Case hardening steels according to DIN 17210 / case hardening steel casting

Nitriding steels according to DIN 17211 / nitriding casting steels
GS-31 CrMo 12
GS-35 CrMoV 10 4
GS-34 CrAlNi 7

Steels for quenching and tempering according to DIN EN 10083 part 1, part 2 / heat treatable steel casting

Heat-resisting steel castings according to DIN 17245 17465 / hot work steel casting

Stainless steel castings according to DIN 17445 / stainless steel casting
Ferritic:

Martensitic:

Austenitic:

Fully austenitic/duplex:

Nickel- and cobalt-based alloys according to SEW 471
GCoCr 28 Nb
GNiCr 28 W

Non-magnetic steel castings according to SEW 395-87
GX 25 MnCrNi 8 8 6
GX 2 CrNiMoN 18 14

Manganese hard steel castings according to SEW 395-87
GX 120 Mn 13 solution annealed at 1050 °C water quenched
GX 120 MnCr 13 2 solution annealed at 1050 °C water quenched

Aluminium alloys

AIMMC (Aluminium Metal Matrix Composites) reinforced aluminium

Magnesium alloy