Casting in shell molds at the foundry production of the concern “Nicmas”
The main advantages of special casting methods are the ability to obtain more accurate castings with a better surface. The use of these methods allows to reduce or even cancel allowances for mechanical processing, in addition, metal savings are achieved and the cost of mechanical processing of castings is reduced.
What is the advantage of shell molding?
Casting in shell molds is used mainly in conditions of mass and large-scale production of complex and responsible shaped blanks from cast iron, steel and non-ferrous alloys weighing up to 100 kg.
Casting in shell molds is one of the progressive methods of obtaining castings, the dimensions of which are close to the dimensions of the finished part. This method allows you to mechanize and automate all production processes.
Casting in shell molds compared to casting in sand molds has the following advantages:
- the consumption of liquid metal decreases by 20-30% due to the reduction of allowances for mechanical processing due to the increased accuracy of the casting dimensions and the reduction of the dimensions of the sprue system;
- the volume of mechanical processing is significantly reduced;
- the consumption of molding and core materials and the costs of preparing the mixture and its transportation are reduced by 5-6 times;
- the labor intensity of casting is reduced by 30-60%;
- shell molds and rods can be stored in the warehouse for a long time, given their non-hygroscopicity.
The process of obtaining shell removable molds and shell rods is based on the fact that a number of thermosetting resins have the property of melting and then irreversibly hardening under the influence of elevated temperatures.
What process is taking place?
A dry sand-resin mixture consisting of fine quartz sand and 6 – 8% powder of thermosetting (Bakelite) resin is applied to a model plate heated to 220 – 250° C. The layer of the mixture adjacent to the plate is heated, the thermosetting resin melts and within 12 – 20 seconds. forms a shell 6-10 mm thick around the model and plate.
After removing the unsoftened mixture, the model plate together with the sand-resin shell is placed in an oven heated to a temperature of 350 – 370° C for final hardening; at the same time, the resin goes into a solid, irreversible state. Hardening takes 1.5 – 2 minutes. The tensile strength of the shell reaches 10 – 15 kG/cm2 (100 – 450 MPa).
Sheathed, hollow rods are made according to the same principle.
The process of making a shell half-form is as follows. The molding mixture is poured into a rotary hopper and a model plate heated to a temperature of 220 – 250° C is attached to it. After turning the hopper by 180° and holding it in this position for 12 – 20 seconds, a layer of sintering mixture 6 – 10 mm thick forms on the model.
Then the hopper is returned to its original position and the model plate with the formed shell is removed from it. The model plate together with the shell is placed in an electric furnace for final hardening of the shell, which is removed from the plate using pushers.
The lower shell – a semi-form, after the rods are installed in it, is covered with an upper shell – a semi-form. Half-forms are glued or connected with staples.
The finished forms are placed in a container and covered with shot or sand to prevent their destruction during the pouring process.
The shell half-forms made in this way are assembled and firmly fastened together into a single system before pouring by gluing, filling the mold with shot and other methods.
Thanks to the professional work of the “Nicmas” Concern, we have the accuracy of castings obtained in shell molds that correspond to classes 7-9 (GOST 2689-54).