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Sand casting process


Deshengrui is mainly engaged in machining production and casting production. The blank parts come from precision casting, sand casting and forging. Deshengrui has rich experience in casting processing. During casting production, we must first determine the casting process plan according to the structural characteristics, technical requirements, production batches, production conditions and other factors of the casting. It includes the determination of the pouring position, parting surface, and casting process parameters (machining allowance, draft angle, casting fillet, shrinkage, core head, etc.), and then make the casting drawing with the prescribed process symbols or text Illustration. The casting process chart is a technical document to guide the casting production.

1.The determination of the pouring position

The position of the casting in the mold during pouring is called the pouring position. The casting position of the casting has a great influence on the quality, dimensional accuracy, and difficulty of the molding process. The pouring position is usually determined according to the following basic principles.

(1) The important processing surface of the casting should be facing down or on the side. Gas, slag and grit in the mold will float during pouring, which may cause defects such as pores, slag inclusions and trachoma in the upper part of the casting, while the possibility of defects in the lower part of the casting is small and the structure is denser. As shown in the figure, in the pouring position of the machine bed, the guide rail should face down to ensure the quality of this important working surface. As shown in the figure, the quality of the circumferential surface of the hoisting drum is relatively high. The vertical pouring scheme can be used to keep the circumferential surface on the side to ensure uniform and consistent quality. As shown in the pouring position of the machine bed, the guide rail should face down to ensure the quality of this important working surface.


sand casting

iron casting

The main working side of the bed is facing down

Work surface placed on side wall


(2)The large plane of the casting is poured downward or inclined. Because the hot metal liquid has strong heat radiation to the upper part of the mold during pouring, it causes the top surface sand to expand and arch and even crack, causing defects such as sand inclusion and trachoma in the large plane. The large plane faces down or the oblique casting method can avoid casting defects in the large plane. The following figure shows the pouring position of the flat casting。

Large plane facing down

(3)The thin wall of the casting is facing downwards, sideways or inclined. In order to prevent the thin-walled part of the casting from being cold-separated and unable to cast defects, the thin-walled area with a larger area should be placed on the lower part of the casting or placed on the side wall or inclined position, as shown in the figure.

Pouring position

Pouring position of thin-walled castings

(4)Most of the thickness of the casting should be placed on the top or on the side of the parting surface. The main purpose is to facilitate the placement of the riser in the thick place for replenishment, as shown in the valve body's riser replenishment and the important face of the hoisting drum shown on the side.

2. Selection of parting surface

The parting surface is the joint surface between the casting components. In order to facilitate ejection, the parting surface is generally selected at the largest cross section of the casting. The parting surface should be selected to ensure convenient mold opening, simplify the casting process, and ensure the quality of the casting. The following principles should be followed to determine the parting surface.

(1) The parting surface should be selected at the largest cross-section of the pattern to facilitate the ejection of the mold. as the picture shows.

Parting surface

Parting surface is selected at the largest diameter

(2) Minimize the parting surface. With less parting surfaces, it is easy to ensure the accuracy of the casting and simplify the molding process. For machine modeling, generally there can only be one parting surface. For the mass production of rope sheave castings, as shown in the figure below, in order to facilitate machine modeling, according to a parting scheme, a ring-shaped core is used. The parting surface is reduced to one parting surface. Of course, in the case of single-piece production, when manual molding is used, in order to reduce the manufacturing of tooling, it is reasonable to use the b plan, three box modeling, and two parting surfaces.


Parting surface

(a) (b)

Parting surface of sheave casting


(3) Make the parting plane as straight as possible. In order to make the pattern manufacturing and modeling process simple, the parting surface of the curved connecting rod as shown in the figure should not be a curved parting surface (plan b), but a straight parting surface (plan a).

Parting surface of curved link

Parting surface of curved link

(4) Try to keep all or most of the castings in the same flask. The castings are located in the same sand box, which is convenient for fitting and avoiding wrong shapes to ensure the precision of the castings. In the figure below, there are two types of typing schemes for water pipe plugs. The type a scheme in the figure is more reasonable, so that the reference plane and the machining plane are located in the same sand box, and the accuracy of the casting is easy to ensure.

Parting surface of water pipe plug

Parting surface of water pipe plug


(5) Place the core in the lower box as much as possible, and pay attention to reduce the height of the sand box. This simplifies the molding process, facilitates lowering and fitting, and facilitates ejection and repair. As shown in the figure, the parting scheme of the machine tool column is adopted. The Ⅱ scheme is more reasonable, so that most of the cavity and the core are in the lower box, which is convenient for ejecting the mold, lowering the core, and closing the mold.

Parting surface of machine column

Parting surface of machine column

3.Selection of process parameters

(1) Machining allowances and tolerances

Machining allowance refers to the thickness of the metal layer reserved on the machining surface of the casting to be removed. The machining allowance depends on the accuracy level of the casting, and is related to the casting material, casting method, production batch, casting size, pouring position and other factors.

The following table shows the tolerance grades and the corresponding machining allowance grades for single-piece and small-batch production of commonly used casting alloys for sand casting.

Shell material

CT/MA

Cast steel

grey cast iron

Ductile iron

Malleable cast iron

Dry and wet sand

13-15/J

13-15/H

13-15/H

13-15/H

Self-hardening sand

12-14/J

11-13/H

11-13/H

11-13/H

Tolerance grades of castings and their corresponding machining allowance grades in single and small batch production (taken from GB / T1350-89)

  

(2) Draft angle

In order to make the pattern (or core) easy to take out from the mold (or core box), the slope of the wall on the pattern (or core box) parallel to the direction of the mold is called the mold slope, and the angle α or width can be used. a means to promote the use of width a. There are three ways to increase the slope of the pattern, such as increasing wall thickness, increasing or decreasing wall thickness, and decreasing wall thickness, as shown in the figure. For walls that require machining, the wall thickness increase method must be used.

The value of the increase and decrease of the draft of the die can be selected according to relevant standards. The draft of the die when using clay sand can be determined according to JB / T5105-1991. The inclination α of general wood molds is 0.3 ° ~ 3 °, a = 0.6 ~ 3.0mm; the inclination α of metal molds is 0.2 ° ~ 2 °, a = 0.4 ~ 2.4mm. The higher the appearance, the smaller the slope. When the ratio of the height of the hole to the diameter of the casting is less than 1 (H / D <1), the hole can be cast by the method with its own core.



Draft Angle

(a) thickness-Wall thickness <8mm

(b) Increase or decrease casting thickness-Wall thickness: 8mm ~ 12mm

(c) Reduce casting thickness-Wall thickness> 12mm

Select the right Draft



(3) Shrinkage

In order to compensate the shrinkage of the casting during the cooling process and make the cooled casting meet the requirements of the drawing, the size of the pattern needs to be enlarged, and the amount of enlargement depends on the size of the casting and the linear shrinkage of the alloy. Generally, the linear shrinkage of small and medium gray cast iron pieces is about 1%; the casting shrinkage of non-ferrous metals is about 1.5%; the casting shrinkage of steel castings is about 2%.

(4) Casting fillet

The connection between the wall and the corner of the pattern and the corner must be made into a circular arc transition, which is called casting fillet. Casting rounded corners can reduce or avoid the sharp corner damage of the sand mold, and prevent sand sticking, shrinkage and cracks. However, the corners of the parting surface of the casting must not have rounded corners. The size of the cast fillet can be selected according to 1/3 ~ 1/5 of the average wall thickness of the adjacent two walls, and the radius of the fillet is half of the fillet.

(5) core head

The core head refers to the extended part of the sand core, which is used to locate and support the sand core. as the picture shows. There are two types of cores: vertical and horizontal. The core seat refers to the cavity in the mold for the core. Core head and core seat size mainly includes core head length L (height H), core head inclination α, core head and core seat assembly gap s, etc., and its value is related to the core length (height) and diameter.

Core structure

a) Vertical core head (b) Horizontal core head

Core structure

4.The pouring system

The gating system is a series of channels opened in the mold to fill the cavity and riser.

(1) Composition and function of the gating system It usually consists of a gating cup, a straight runner, a horizontal runner, an inner runner and a riser. Reasonably design the pouring system, which can make the metal liquid fill the mold cavity smoothly; control the flow direction and speed of the metal liquid; adjust the temperature of each part on the casting, control the cooling and solidification sequence; The riser plays a role of shrinking, venting and slag collecting.

(2) Type of the pouring system According to the position of the metal liquid introduced into the cavity, the pouring system can be divided into bottom injection, top injection, middle injection, stepped, etc. See the figure below.


iron castingductile castingiron casting<a href=http://www.desheng-precision.com/Precision-casting.html target='_blank'>investment casting</a>

  

Type of Pouring

5.casting process diagram example

The casting process drawing is a drawing showing the parting surface, pouring position, core structure and size, pouring system, process parameters, etc., which can be marked on the casting drawing according to the prescribed process symbols or characters or the process drawing can be drawn separately.

The following figure shows the bushing parts. The material is HT200. Sand casting is used. The annual output is 200 pieces.

(1) Structural analysis, determining modeling method, pouring position and parting surface. The φ 48mm hole on the part is to be cast, but the small step of the inner hole is not cast, so a simple round rod core is used. Not cast. The height of the casting is not large, and two boxes can be used for overall mold modeling and vertical pouring. The parting surface is selected at the end face of φ160mm, and the two-box integral mold is adopted.

(2) The process parameters are determined.

Machining allowances Castings have to be machined on all sides, so there should be allowances. The tolerance and associated machining allowance grade of sand casting gray iron parts are 14 / H. The machining allowance level of the top surface and the hole is reduced by one level (take the J grade). The machining allowance value can be found in GB / T11350-1989. The φ160mm and φ104mm circumferential surfaces are processed on both sides. The machining allowance is 6.0 mm, the machining allowance of the top surface is 7.0 mm, and the machining allowance of each side of the inner hole is 6.0 mm.

Draft angle At the direction perpendicular to the parting plane (parallel to the draft surface), determine the draft angle according to the thickening method. Take the width a = 1.0mm. "7/6" in Fig. 9-21b indicates the allowance of the upper and lower ends after considering the machining allowance and the draft angle.

Linear shrinkage Since it is a small batch production, the casting shrinkage of each dimension of the casting can take the same value, and the casting shrinkage is 1%.

Mandrel This mandrel is a vertical mandrel. Check the relevant manual (the book is omitted) to get the core size, as shown in the casting process drawing.

Casting fillet Casting fillet according to (1/3 ~ 1/5) wall thickness method, taking R inside as 8mm; R outside as 4mm.

(3) Draw the casting process diagram. As shown below (excluding the gating system).

6.  castings drawing

The casting drawing is a figure that reflects the actual size, shape and technical requirements of the casting, and is the main basis for acceptance and inspection of casting production. The casting drawing should be drawn on the basis of the complete casting process drawing. The following figure is the casting drawing of the bushing.


casting process

    Parts drawing Casting process drawing Casting drawing

Bushing


Deshengrui is mainly engaged in machining production and casting production. The blank parts come from precision casting, sand casting and forging. Deshengrui has rich experience in casting processing. If you have more question or idea, please visit our website for more details: www.desheng-precision.com



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