What is a bit of confusion between sand mold and sand core?

The sand mold is made of sand to make the appearance of the casting by compacting, and the sand core is usually used to make the inner cavity shape of the casting, but the sand core can also be used to make the appearance (that is, we usually say the sand core full shell mold).

The black sand used for the sand mold is fine sand mixed with pulverized coal (easy to demold) and some other additives (starch, dextrin and water, etc.) to ensure that the surface of the casting is smooth, and the sand core is more demanding, and it is made of fine sand and resin!

The sand mold is only stirred by black sand, and the sand core is sintered by high temperature (usually 200-280 degrees Celsius), and the sand core is usually yellow or dark brown, depending on the amount of resin added.

For example: 1. Put a brick on a flat plate, buckle a drawer without a bottom, fill the drawer with sand (sand can be bonded into a whole), and after taking out this brick, the remaining sand is the sand mold, which is the sand mold of this brick.

2. If the brick has a hole in the horizontal direction, the sand mold of the brick is not easy to do, because after the hole is filled with sand, the brick cannot be taken out, and the sand core needs to be used at this time. According to the size of this hole in the brick, make a sand rod as thick as the hole and place it in the corresponding position of the sand mold, so that you can make a sand mold of bricks with holes. This sand rod is called a sand core.

3. Of course, the actual operation is not so simple.

Sand casting. Merit.

1. Compared with other casting methods, sand casting has low cost, simple production process and short production cycle.

2. Because the molding materials used in sand casting are cheap and easy to obtain, the casting mold is easy to manufacture, and it can be adapted to the single production, batch production and mass production of castings.

Shortcoming. The quality of molding sand and core sand directly affects the quality of castings, and poor quality of molding sand will cause defects such as porosity, sand holes, sticky sand, and sand inclusion in castings.

The defects of sand casting castings are: cold separation, insufficient pouring, porosity, sticky sand, sand inclusion, sand hole, sand expansion, etc. 1) Insufficient cold insulation and pouring Insufficient filling capacity of liquid metal, or poor filling conditions, before the cavity is filled, the molten metal will stop flowing, which will cause the casting to produce insufficient pouring or cold insulation defects.

When the pouring is insufficient, the casting will not be able to obtain a complete shape; Although the casting can obtain a complete shape during cold interval, the mechanical properties of the casting are seriously damaged due to the existence of incomplete fusion joints. Prevent under-pouring and cold separation: increase the pouring temperature and pouring speed.

2) Porosity The porous defects generated in the casting by the gas that does not escape in time before the molten metal crusts. The inner walls of the stomata are smooth, bright or with a slight oxidized color. When porosity is created in the casting, its effective bearing area will be reduced, and stress concentration will be caused around the porosity, which will reduce the impact resistance and fatigue resistance of the casting.

Porosity can also reduce the compactness of the casting, resulting in the scrapping of some castings that are required to withstand hydrostatic testing. In addition, porosity has a negative effect on the corrosion resistance and heat resistance of castings. Prevent the formation of stomata:

Reducing the gas content in the molten metal, increasing the air permeability of the sand mold, and adding an air riser at the highest part of the cavity. 3) Sticky sand: A layer of sand particles that are difficult to remove is adhered to the surface of the casting, which is called sticky sand. Sticky sand not only affects the appearance of the casting, but also increases the workload of casting cleaning and cutting processing, and even affects the life of the machine.

For example, if there is sand sticking on the surface of the cast teeth, it is easy to damage, and if there is sand sticking in machine parts such as pumps or engines, it will affect the flow of fluids such as fuel oil, gas, lubricating oil and cooling water, and will stain and wear the entire machine. Prevent sand sticking: add pulverized coal to the molding sand, and apply anti-sticking sand paint on the surface of the mold.

4) Sand inclusion The grooves and scar defects formed on the surface of the casting are very easy to occur when the thick and large plate castings are cast with wet molding. Most of the parts that produce sand inclusion in the casting are in contact with the upper surface of the sand mold, and the upper surface of the cavity is affected by the radiation heat of the molten metal, which is easy to arch and warp, and when the raised sand layer is continuously washed by the molten metal, it may break and break, stay in place or be brought into other parts. The larger the upper surface of the casting, the greater the volume expansion of the molding sand, and the greater the tendency to form sand inclusions.

5) Sand hole: Hole defects that are filled with molding sand inside or on the surface of the casting. 6) Sand expansion Under the pressure of molten metal during pouring, the mold wall moves, and the casting is partially expanded. In order to prevent sand expansion, the strength of the sand mold, the stiffness of the sand box, the pressure or fastening force of the box should be increased, and the pouring temperature should be appropriately reduced, so that the surface of the molten metal can be crusted early, so as to reduce the pressure of the molten metal on the casting mold.

Features of sand casting:

Chemically hardened sand molds have much higher strength than clay sand molds, and after being made into a sand mold, they are defilmed after hardening to a fairly high strength, and do not need to be modified. Therefore, the casting mold can accurately reflect the size and contour shape of the pattern, and it is not easy to deform in the subsequent process. The dimensional accuracy of the castings is high.

Because the viscosity of the binder and hardener used is not high, it is easy to mix with the sand particles, and the sand mixing equipment has a light structure, low power and high productivity, and the sand treatment work can be simplified.

The mixed molding sand has good fluidity before hardening, and the molding sand is easy to compact during molding, so there is no need for a large and complex molding machine.

When molding with chemically hardened sand, pattern materials such as wood, plastic and metal can be selected according to production requirements.

The binder content in chemically hardened sand is much lower than that of clay sand, and there is no powdered excipients in it, and the gap between the sand particles is much larger than that of clay sand. In order to avoid the metal seeping between the sand particles during casting, the surface of the sand mold or core should be coated with a good quality coating.

The cost of chemically hardened sand with water glass as a binder is low, and the working environment is odorless in use. However, the molding sand is not easy to collapse after pouring the metal in this casting mold; The used old sand cannot be used directly, it must be recycled, and the regeneration of sodium silicate sand is more difficult.

<>1. Molding sand is generally mixed by casting raw sand, molding sand binders and auxiliary additives and other modeling materials in a certain proportion. Molding sand can be divided into clay sand, sodium silicate sand, cement sand, resin sand, etc. according to the different binders used. clay sand, sodium silicate sand and resin sand are the most used;

2. Core sand is mixed by casting sand, molding sand binder and auxiliary materials and other modeling materials in a certain proportion. For cores with complex shapes, thin and thin sections, high dry strength and good collapsibility, vegetable oil core sand, grease core sand or resin core sand are used.

1) Casting process design steps:

Dimensional design: determine the size of the casting according to the size and processing requirements of the bracket.

Cavity design: design the shape and size of the cavity according to the shape of the casting, processing requirements and the nature of the casting material.

Casting body and system design: According to the size and shape of the casting, the gating system is designed, including the pouring channel, gating opening, pouring riser, liquid level rising speed, etc.

Temperature design: According to the envy properties of the casting material and the size and shape of the casting, the pouring temperature and cooling rate are designed.

Technical parameter design: According to the size, shape, material properties and pouring temperature of the casting, determine the technical parameters of casting, such as pouring rate, crystallization rate, pouring pressure, etc.

Trial casting and testing: Through trial casting and testing, the feasibility and effect of the casting process are evaluated.

2) Calculate the cross-sectional area of the choke:

The streak product of the flow blocking section refers to the physical section inside the casting that obstructs the flow of fluid. It can be calculated by the following formula:

a = frac \left(\fracight)^}

Where: $q$ is the pouring amount of the casting.