Here’s everything you need to know about foundry core production using the Shell Moulding process. Discover how this innovative technique can improve the efficiency and quality of your manufacturing production in terms of casting precision.
Shell Moulding process
The most commonly used hot moulding process for cores is known as Shell Moulding or Croning in Germany, from his creator’s name. It has been available since 1950.
This process involves coating sand with a mixture of phenolic resins and waxes, which is prepared in a dedicated plant and supplied to the foundry ready for use.
Manufacturing companies can supply customised sands to foundries based on their needs, such as grain size and resin percentages.
When the sand grain is heated in the mould, the resin film that surrounds it softens and fuses with the neighbouring grains.
During the transport of the pre-coated sand through pneumatic systems, it is important to use the lowest possible pressure to prevent the resin from separating from the sand grains, causing the formation of dust and possible problems of stickiness or excessive gas developments during casting.
In the production of cores and forms in series, this process is used to create products that are hollow, mechanically robust and with a compact and smooth surface. It is suitable for cores of medium size (from a few grams to 20 kg) and large quantities of production (thousands of pieces) thanks to its short reaction times.
Based on the foundry processing and the final product, suitable sands and resin percentages are selected for the process. For example, anti-veining sands to prevent cast iron clumps, precoated sands with a lower resin percentage to prevent blowholes, or regenerated sands for pumps that have already undergone thermal shock and will respond better to contact with cast iron.
Materials for precoated sand include:
- Round grain quartz sand for better permeability
- Synthetic resin (2-6%) that binds the siliceous grains
- A hardener (6-10%) for the binder.
For the production of cores or shells, pattern plates and metal core boxes are used that are heated to 200-300°C.
The sand and resin mixture is “shot” onto the pattern or inside the core boxes with special machines. Heat melts the resin and hardens the sand, creating a solidified shell of sufficient thickness. Excess mixture is evacuated (for hollow cores) or waiting for the complete solidification of the core (for solid cores).
In many cases, to increase the resistance or smoothness of the part in contact with the molten metal, a core coating is required.
The finished product is then ready to be used in casting.
- An irregular heating of the pattern plates can cause cracks or damage to the shape
- Excessive heat on the pattern plate can cause the combustion of the resin and the separation of the sand during casting
- Regeneration not allowed
Advantages and problems
This process has the following advantages:
- Excellent mechanical strength
- Optimal smoothness for filling the core box
- Good resistance to deformation during casting
- Ability to make hollow cores with even complex and thin shapes
- Simplicity in forming the shell shape
- Compact, smooth and low roughness surface
- Ability to obtain hollow cores, facilitating the escape of gases and preventing porosity
The disadvantages are as follows:
- Metal core boxes (expensive)
- Long production times for cores
- Possible gas formations, especially in solid cores
- High cost of sand
Foundry cores production
To produce castings of different sizes and characteristics, the system that best meets our needs in Shell Moulding production must be selected.
There are two types of machines for producing solid or hollow, complex or simple castings:
- Tiltable with oscillations, for hollow and complex castings
- Fixed shot, with a sand tank positioned above or below depending on the type of casting you want to produce: solid and complex with shot from the top, simple and empty with shot from below
Type of machinery
The Shell Moulding foundry involves the production of precise castings with minimal dimensional tolerances and smooth surfaces, using a sand shell coated with heat-hardening resins.
The shell mould is filled with molten metal, through a specific casting and feeding system for each casting to be produced. The method involves the insertion of “cores” inside the shell, to obtain a casting with geometry and dimensions similar to the finished piece.
The advantages of Shell Moulding technology over traditional sand casting include:
- The ability to produce complex-shaped castings with thin thicknesses and low dimensional tolerances, smooth surfaces with low roughness,
- Higher permeability of the shell that reduces scrap due to gas bubbles
- Absence of moisture that reduces blowholes and hard spots
- The ability to cast complex details in alloy steels
Regarding the processing at the Shell Moulding foundry, we handle a wide range of materials including:
- Gray iron
- Vermicular iron
- Spheroidal iron
- Ni-hard iron
- Ni-resist iron
- Cr-hard iron
- Stainless steels
Regarding the production at the Shell Moulding foundry, we manufacture a wide range of components for the automotive, maritime, aerospace, racing, and industrial textile and mechanical sectors. The machine, which has the shape of a book resting on a table, consists of two parts: a plate with the model and a pre-coated sand container.
To create the model, the plate is flipped over onto the sand-containing part and the container is returned to the initial position, where it remains for a sufficient amount of time to reach the desired thickness.
After the desired baking, the shell is carefully removed. It is important to pay particular attention to the correct baking of the cores and shells, to avoid problems such as erosion, gas development, low mechanical strength, and deformations. The ideal color of the cores and shells should be between a dark yellow and light brown.
The production of cores and mould models for shell moulding uses temperatures ranging from 280°C to 380°C, depending on the type of metal used in the construction of the cores. However, temperatures that are too high can cause deformations.
The models used to create shells should be equipped with extractors, positioned outside the model intended for casting.
The cores should be equipped with filters, positioned so as not to interfere with the model and to obtain a core that is as compact as possible.