Phenolic resin coated sand casting precautions

1. Knowledge and understanding of coated sand (coated sand belongs to organic binder type, core sand)
The characteristics of coated sand: it has suitable strength properties; good fluidity, the produced sand mold and sand core have clear outline and dense structure, and can produce complex sand core; the surface quality of the sand mold (core) is good, and the surface roughness can reach Ra=6.3~12.5μm, dimensional accuracy can reach CT7~CT9 grade; good collapsibility, easy to clean the casting. Scope of application: The coated sand can be used to make both casting molds and sand cores. The molds or cores of the coated sand can be used in conjunction with each other or with other sand molds (cores); not only can be used for metal mold gravity casting or low pressure Casting, can also be used for iron sand-covered casting, and can also be used for thermal centrifugal casting; not only can be used for the production of cast iron and non-ferrous alloy castings, but also for the production of steel castings

2. Production process of coated sand
The preparation process of coated sand mainly includes cold coating, warm coating and hot coating. At present, the production of coated sand is almost all by hot coating. The thermal coating process is to first heat the raw sand to a certain temperature, then mix and stir with resin, urotropine aqueous solution and calcium stearate, and then cool, crush and sieve. Due to the difference in the formula, the mixing process is different.

3. The main process of core making with coated sand
The heating temperature is 200-300°C, the curing time is 30-150s, and the blasting pressure is 0.15-0.60MPa. The sand core with simple shape and the coated sand with good fluidity can choose a lower sand shooting pressure, and the thin sand core can choose a lower heating temperature. When the heating temperature is low, the curing time can be appropriately extended. The resin used in the coated sand is phenolic resin. The advantages of the core-making process: suitable strength properties; good fluidity; good surface quality of the sand core (Ra=6.3-12.5μm); strong anti-hygroscopicity of the sand core; good collapsibility, easy to clean the casting. Mold (mold) temperature The mold temperature is one of the main factors affecting the thickness and strength of the shell, which is controlled at 220~260°C and selected according to the following principles:
3.1 Ensure sufficient heat for softening and curing of the resin on the coated sand.

3.2 Ensure that the required shell thickness is formed and the surface of the shell (core) is not coking

3.3 Minimize the time of crusting and hardening to improve productivity

4. Sand shooting pressure and time
The sand shooting time is generally controlled at 3~10s, if the time is too short, the sand mold (core) cannot be formed. The sand shot pressure is generally about 0.6MPa; when the pressure is too low, it is easy to cause insufficient shot or loose phenomenon. Hardening time: The length of hardening time mainly depends on the thickness of the sand mold (core) and the temperature of the mold, generally around 60~120s. If the time is too short, the strength of the shell layer will be low if the shell layer is not completely cured; if the time is too long, the surface layer of the sand mold (core) will be easily scorched and affect the quality of the casting.

5. Problems and solutions in the application of coated sand
There are many types of core-making methods, which can be generally divided into two categories: thermosetting methods and cold-setting methods. Coated sand core-making belongs to the thermosetting method category. Any core-making method has its own advantages and disadvantages, which mainly depends on the comprehensive factors such as product quality requirements, complexity, production batch, production cost, product price, etc. to decide which core-making method to use. It is very effective to use coated sand cores for sand cores with high requirements on the surface quality of the inner cavity of the casting, high dimensional accuracy requirements and complex shapes.

5.1 Determination method of strength and gas generation of coated sand
Under the premise that the quality of raw sand and resin is certain, the key factor affecting the strength of coated sand is mainly taken from the amount of phenolic resin added. When the amount of phenolic resin added is large, the strength will increase, but the amount of gas generated will also increase, and the collapsibility will decrease. Therefore, in the production application, the strength of the coated sand must be controlled to reduce the amount of gas generated and improve the collapsibility, and a balance point must be found when the strength standard is formulated. This balance point is to ensure the surface quality of the sand core and the strength under the premise that no deformation or core breakage occurs during pouring. In this way, the surface quality and dimensional accuracy of the casting can be guaranteed,
It can also reduce the amount of gas production, reduce the porosity defects of castings, and improve the sand production performance of the sand core. For the storage of sand cores, station equipment and sand core trolleys can be used during the handling process, and 10mm~15mm thick sponges are laid on them, which can reduce the loss rate of sand cores.

5.2 Storage period of coated sand cores
Any sand core will absorb moisture, especially in the south where the relative humidity is high. The storage period of the sand core must be stipulated in the process documents, and the storage volume and storage period of the sand core must be reduced by using the lean production method of FIFO.

5.3 Control the supply quality of coated sand
When the coated sand enters the factory, the quality assurance data of the supplier must be attached, and the enterprise shall inspect it according to the sampling standard, and only after the inspection is qualified can it be put into storage.

5.4 Qualified coated sand is found to be fractured and deformed during core making
The fracture deformation of the sand core during core making is usually considered to be caused by the low strength of the coated sand. Fracture and deformation of sand cores are actually involved in many production processes. When an abnormal situation occurs, the real cause must be found before it can be completely solved. The specific reasons are as follows:
5.41 The temperature and mold retention time of the mold during core making are related to whether the hardening thickness of the sand core crust meets the technological requirements. The process parameters specified in the process all need to have a range, and this range needs to be adjusted by the skill of the operator. When the mold temperature is at the upper limit, the mold retention time can be taken as the lower limit, and when the mold temperature is at the lower limit, the mold retention time can be taken as the upper limit. Operators need continuous training to improve their operational skills.
5.42 Phenolic resin and sand particles will stick to the mold during core making, which must be cleaned in time and sprayed with release agent, otherwise the sand core will be broken or deformed when the mold is opened.
5.43 The spring ejector on the static mold of the hot core box mold will cause elastic failure due to long-term operation at high temperature, resulting in fracture or deformation of the sand core. The spring must be replaced in time.
5.44 The movable mold and the static mold are not parallel or not on the same center line. When the mold is closed, under the pressure of the oil cylinder or cylinder, the front end of the positioning pin has a certain slope, and the mold will still be tightly closed, but when the mold is opened, the movable mold and the static mold are closed. Still reverts to its original state to fracture or deform the core. In this case, the sand will run away when the sand is shot, and the size of the sand core will become larger. The solution is to adjust the parallelism and coaxiality of the mold in time.
5.45 When the hollow sand core is produced on the shell core machine, when the unhardened coated sand is poured out of the sand core and needs to be reused, the unused coated sand must be screened and mixed in a ratio of 3:7 before use, so that In order to ensure the surface quality and core strength of the shell core sand core.

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