Types of Shell core shooter machines
In the Shell Moulding process, it is important to choose the right equipment based on the characteristics and dimensions of the cores to be produced.
There are two types of core-making machines:
- Tiltable machines with oscillations, suitable for complex and hollow cores
- Fixed shot machines, which can have the sand tank positioned with top shot for solid or complex cores, or bottom shot for simple and hollow cores
Folding down machine for hollow cores
The folding down machine for hollow cores is composed of equipment that is connected to a hydraulic control unit, which controls the tilting and closing of the plates.
The core box, to which sand is fixed, is heated by burners or electric heaters.
Once the core box reaches the desired temperature, the production cycle can begin.
The core box is closed and starts to rotate, with the sand tank in a high position.
Subsequently, the sand is introduced into the core box by air pressure, which is located at the bottom with the sand tank in a vertical high position for a set time, called the investment time, for the formation of the core thickness.
After the cooking time, the core box opens and the core can be extracted, while the used sand is reintegrated into the tank.
Fixed shooting machine
The fixed shooting machine is a variant of the folding down machine, but without rotation.
It is mainly used to produce solid cores, but can also be used for small or medium-sized hollow cores.
The sand tank can be positioned under the core box to produce hollow cores, while for solid cores or problematic core boxes, the tank is positioned at the top of the machine; this system allows for better filling of the core box.
Core shooting machines can be divided into three types based on the shooting system used:
- Shooting from the top
- Shooting from the bottom
- Shooting from the top with core ejection through a tilting mechanism
Shooting from the top
The top injection method is very versatile and can be used to create cores with complex shapes and thin sections.
The sand is pushed downwards by compressed air, aided by gravity.
This method does not have any particular constraints regarding the characteristics of the resin in cold processes, but in hot processes, it is important to position the sand tank to avoid variations in the resin softening point due to the heat emitted by the core box.
Shooting from the bottom
The bottom injection method involves launching sand into the core box cavity using compressed air, and allowing it to remain suspended until the resin-bonded sand core has hardened and polymerized.
When the air pressure is stopped, the sand in the central parts of the cavity, which has not received sufficient heat to harden, falls into the machine tank, partially emptying the core.
This method is particularly suitable for the formation of hollow cores and requires the use of pre-coated sands with a high softening point and relatively long flow to avoid peeling.
The wide section flow connected to the most voluminous zones allows the injection of cores with relatively simple or medium complexity shapes.
The process of forming cores involves using compressed air flows to drag the pre-coated sand into the core box cavity. Both the top and bottom injection methods rely on the high flowability of the pre-coated sand to achieve complete and uniform cores with relatively low air pressures. However, to prevent sand bridging and ensure core quality, the core box must be filled as quickly as possible, preferably within 2 seconds. In addition, the piping connecting the blowing machine to the compressed air distribution network must be large enough and the valve must be opened rapidly. Finally, the sand introduction holes in the core box should be as large as possible and oriented towards the deepest part of the cavity.
In both injection methods, both top and bottom, sand is moved into the core box cavity through a high-speed compressed air flow.
Thanks to the flowability of pre-coated sands at the time of shooting, which are dry and covered with a hard and lubricating resin film, relatively low air pressures (from 0.5 to 5 atm.) are sufficient to obtain complete and uniform density cores.
However, higher air pressures are required to create thicker cores or when injecting sand from the bottom.
To obtain quality cores, the core box must be filled in a very short time (maximum 2″), so the pipes connecting the blowing machine to the compressed air distribution network must be sufficiently sized and the valve must be opened quickly.
The sand inlet hole or holes into the core box must be as large as possible and oriented towards the deepest part of the cavity.
A short blowing time also avoids the negative effect of “channeling” that occurs when compressed air is supplied with insufficient flow rate, where the air follows a preferential path through the sand in the container, causing a reduction in the amount of sand carried per unit volume of air.
Shooting from the top with emptying
The top injection method with the rotation of the shooting head combines the positive characteristics of the previous methods.
During the shooting phase, the head is positioned high or in rotation, in order to obtain hollow cores thanks to the emptying of the not yet polymerized sand.