Solutions to common sand and slag inclusion defects in lost foam

In lost foam products, common defects mainly include sand inclusion and slag inclusion defects. To solve sand and slag inclusion defects, we must first systematically and comprehensively analyze the causes of the defects and find the causes to better formulate corresponding measures.

Defect analysis

In the production of lost foam, the preparation of coatings is the most important process in the production of lost foam. It is the most core process of each manufacturer, and the most restrictive factor for product quality is coatings. In addition, the bonding process of lost foam white molds also has an important impact on the appearance quality of castings. To study the source of sand and slag inclusion defects, it is necessary to conduct a comprehensive analysis of the defects in castings.

Analysis of sand inclusion defects

Analyze the sand inclusion defect and find out the following reasons that may cause sand inclusion:

(1) There is loose sand on the surface of the pouring cup, which is not compacted during tamping and rushes into the cavity during pouring;

(2) The pouring cup shape is unsuitable, causing excessive scouring force on the sprue during pouring. This washes away the sprue’s coating layer. It also flushes modeling floating sand into the cavity, leading to sand inclusion.

(3) The coating layer of the sprue itself is thin and has poor resistance to molten steel scouring. The sprue needs to be painted again. material to ensure the thickness of the coating layer;

(4) During bonding, after the casting and the runner are bonded with hot melt adhesive, the paper tape that is attached has very low high temperature resistance. Once the molten steel is poured here, it will be burned immediately, and the molding sand in the sand box will rush into the cavity from the gap, resulting in sand inclusion;

(5) After the sprue and the casting are painted, the hanging method on the paint trolley is not appropriate, resulting in too many places that need to be repaired during packaging. If the repair is incomplete or the repair layer is too thin, sand inclusion will be directly generated.

Slag inclusion defect analysis

(1) The slag in the ladle is not completely removed before pouring;

(2) During the pouring process, slag continues to be generated in the ladle, and the slag remover fails to remove the slag in time. The slag flows into the cavity with the molten steel, resulting in slag inclusion;

(3) Some refractory materials in the raw materials of the coating are added too much or inappropriately, reacting with the molten steel to produce a certain compound, which is initially judged to be a silicate compound.

Solution

Adjustment of lost foam coating formula

First, we must understand the composition of the coating in order to better solve the coating problem. The lost foam coating supports and protects the foam model. It prevents liquid metal from penetrating the sand. It absorbs decomposition products and allows gases to pass through. The coating maintains the cavity’s integrity after foam evaporation. It keeps the heat of liquid metal from dissipating. The coating should have high refractoriness and mechanical strength. It must offer suitable thermal insulation and conductivity. It needs good air permeability and absorption of liquid decomposition products. The surface should be smooth, with good brushing performance. The coating should not react chemically with the foam model.

Lost foam coating is mainly composed of refractory aggregate, suspending agent, binder, dispersing medium and additives to improve certain properties of the coating.

(1) Refractory aggregate. Refractory aggregate is also called the main material of refractory coating, mainly including zircon powder, corundum powder, quartz powder, bauxite powder, magnesia powder, forster olive powder, graphite powder, talcum powder and other refractory aggregates.

(2) Suspension stabilizer. The suspending agents of water-based paints are mainly sodium bentonite and activated bentonite, followed by polymer compounds such as sodium carboxymethyl cellulose (CMC), polyvinyl alcohol, syrup, etc. Polymer compounds can be used simultaneously with bentonite. The suspending agents of alcohol-based paints are mainly polyvinyl butyral (PVB), organic modified bentonite, sodium bentonite and lithium bentonite.

(3) Binder. The suspending agent in the paint is also a binder, but the amount added is limited. If the paint is not sticky enough, additional binders are required, such as syrup, pulp residue, latex and other organic binders.

(4) Dispersing medium. The dispersing medium of water-based paint is water, and the dispersing medium of alcohol-based paint is mainly ethanol, methanol, n-butanol, etc.

(5) Additives. In order to prevent the paint from deteriorating, pentachlorophenol, sodium benzoate, industrial formaldehyde solution, etc. are often added as preservatives.

The coating mixing process is as follows:

1) Add 100 kg of water into the mixer, start the motor at the same time, and add the composite binder Guilin No. 5;

2) The composite binder should not be added too quickly to prevent agglomeration;

3) At the same time, add the mixed bentonite and mix for 30 minutes;

4) Add the required refractory materials into the mixer in batches and mix for 2-3 hours;

5) The thickness (viscosity) of the coating must be checked before use. If it is too thick, add appropriate water before use.

Adjusting the white mold bonding process

The production workshop strengthens the brushing process and adjusts the bonding process without adjusting the formula.

(1) First, the paint should be brushed evenly, and there should be no areas that are too thick or cannot be brushed;

(2) After the white mold paint is dry, brush and repair the peeling and cracking areas of the paint, and then dry it before assembling the box.

(3) When assembling and bonding the box, the white mold should be handled with care and the paint layer should not fall off. The coating layer should be completely scraped off in the reserved area for bonding the pouring channel.

(4) Improve the bonding process, no longer use paper tape, use alcohol paint instead.

(5) The thickness of the coating layer of the casting is controlled at about 1mm, and it is brushed twice. The straight pouring channel is brushed three times, and the thickness is about 1.5mm.

(6) Before pouring, each package must be scraped with a slag agent.