Classification of Coated Sand

Overview

Before molding and core making, the molding sand and core sand with a layer of solid resin film on the surface of the sand particles are called coated sand. Coated sand was originally a kind of thermosetting resin sand. The process is to mechanically mix powdered thermosetting phenolic resin with raw sand and solidify it when heated. It has now developed into a method of using thermoplastic phenolphthalein resin, latent curing agent and lubricant to prepare coated sand through a certain coating process. In addition to the general dry granular state, coated sand also has wet and viscous coated sand. The shell process is mostly used to produce precision castings.

Compared with other resin sands, coated sand has the following characteristics:

1. It has suitable strength properties, which can meet the requirements of high-strength shell core sand, medium-strength hot core box sand, and low-strength non-ferrous alloy sand.
2. It has excellent fluidity, good sand core formability and clear contours, and can manufacture the most complex sand cores, such as water jacket sand cores for cylinder heads and engine bodies.
3. The surface quality of the sand core is good, dense and not loose. Even if less or no coating is applied, good casting surface quality can be obtained. The casting size accuracy can reach CT7~CT8 level, and the surface roughness can reach Ra 6.3~12.5μm.
4. Good disintegration, which is conducive to casting cleaning and improving product performance.
5. The sand core is not easy to absorb moisture, and the strength is not easy to decrease after long-term storage, which is conducive to storage, transportation and use.
6. Coated sand can be supplied as a commodity, and the user unit has a large choice.

Types of coated sand

Ordinary coated sand

Main features: composed of silica gel, thermoplastic phenolic resin, urotropine and calcium stearate, without relevant additives: room temperature specific strength is 1.0~1.1MPa/1% (mass fraction) resin.

Application: suitable for the production of some cast iron parts with low requirements and simple structure.

High-strength low-gassing coated sand

Main features: adding relevant characteristic additives and adopting new preparation process, its resin specific strength is more than 30% higher than that of ordinary coated sand; gassing speed is 3~5s slower than that of ordinary coated sand.

Application: sand molds of water-cooled cylinder heads, valves and pumps bodies of small, complex and precise multi-cylinder engines, cast steel and cast iron parts.

High-temperature resistant coated sand

Main features:

1. High high-temperature strength, long heat resistance time, small high-temperature deformation. For example, the compressive strength of ordinary coated sand at 1000° is less than 0.2MPa, and the heat resistance time is less than 90s; while the compressive strength of high temperature resistant coated sand at this temperature is greater than 0.8MPa, and the heat resistance time is greater than 150s.
2. High temperature compressive strength is greater than 0.8MPa, heat resistance time is greater than 150s, thermal expansion rate is less than 0.6%, gas emission is less than 15mL/g, cold drawing strength is greater than 3.5MPa, and cold bending strength is greater than 7.0MPa.
3. A coated sand developed for non-ferrous alloy castings that are difficult to clean, with good strength and excellent low-temperature collapsibility.
4. It is wet at room temperature and will not dry naturally after long-term storage, and the storage period is generally greater than 1 year.
5. The density of coated sand is relatively large, the gas emission is relatively low, and the gas emission speed is slow.

Application:

1. Sand cores of complex thin-walled precision iron castings (such as automobile engine cylinder blocks, cylinder heads, etc.) and high-demand steel castings (such as container corners and train brake buffer housings, etc.).
2. Intake manifold, cylinder head water jacket sand core, turbocharger housing and other castings.
3. Wet manual coated sand is used for manual core making: wet mechanical coated sand is used to directly replace hot core box sand, and the core is made by core shooting machine, and the shooting head does not need to be modified.
4. It is suitable for hot mold centrifugal casting process, and it can be used to replace paint to produce centrifugal casting pipes, etc.