Sand casting core making operation guide

1.Pattern preparation

Wooden molds, metal molds and epoxy resin plastic molds can be used. For castings with low requirements for single pieces, disposable molds made of foam plastic can also be used. Using metal molds or metal molds in winter will affect hardening, so wooden molds should be used as much as possible. The draft angle of the mold should be appropriately increased, and the core box should be made into a disengaged or rice bucket type as much as possible. For molds with limited draft angle, they can be made into a core-pulling type. The pouring system and fillets should be made on the mold as much as possible, and the mold bolts should be installed and tightened. Before molding, check whether the mold is intact and complete. The surface of the wooden mold should be flat and smooth, without cracks or scratches, and a release agent before molding and core making. Before placing sand, carefully check whether the live blocks are complete and in place.

2.Preparation of core bones and hanging clasps

Prepare the necessary core bones and hanging clasps before core making. Due to the high strength of resin sand, the core bone can be greatly simplified. Generally, it is not necessary to make an integral cast iron core bone. It can be made by inserting iron rods loosely. However, it is also necessary to consider the rigidity and strength requirements of the mud core during demoulding, transportation, core lowering and box closing, and the buoyancy of molten iron during pouring. Sufficient core bones should be placed. For large mud cores, cast iron keels can be placed. Pay attention to inserting crossbar iron bars at the lower hook of the hanging climb.

3.Tightening of the model

Resin sand has good fluidity and good self-compacting performance. However, during the modeling, it is still necessary to pay great attention to the compacting operation of the concave parts, corners, and under the boss of the movable block, which are not easy to fill. Tightening can be done by pounding with a wooden stick or plugging with hands, and the sand-filling surface can be compacted by stepping on it or with a wooden hammer. The surface stability of the mold or local surface that is not well compacted is poor, which is easy to cause mechanical sand sticking (iron seepage), sand washing and sand holes, and the looseness of the mold after molding is difficult to repair. During the sand placement and compaction, attention should be paid to the movement and looseness of the mold movable block, the gate rod, the venting and riser rod, the gate ceramic tube, etc. The sprue rod and the vent and riser rods should be pulled out within a few minutes after molding. The head sand of the continuous sand mixer should be placed in the bucket as back sand because of the abnormal amount of resin and curing agent. Otherwise, if there is a part of the inner surface of the mold that is not hardened or the strength is insufficient, the whole box will often be knocked off. The strength of the molding sand at the sprue should be taken seriously. The sand flushing caused by the insufficient surface stability of the sprue is often an important cause of sand eye defects in castings. Similarly, for the sand filling surface of the core box in contact with the molten iron, it should be scraped flat with a trowel in time to enhance the surface stability. When molding with a large sandbox, the sand placement time may exceed the usable time of the resin sand. Therefore, attention should be paid to the advancement route of the sand placement, and the part that has begun to react should not be dug as much as possible. Therefore, all preparations must be made before sand placement, including full mental preparation, especially the placement direction and location of the hanging climbers should be carefully planned. The hanging climbers on the sand-filling surface should be dug out immediately after molding. For multi-box molding of non-pattern patterns, a layer of parting agent (usually talcum powder) should be brushed on the split surface.

4.Demolding and repairing

The demolding time of resin sand should be properly controlled. It is easy to deform if it is too early, and it isn’t easy to demold if it is too large. Therefore, generally, about half an hour after molding, use nails (3″ to 4″) to pierce the mold. If it cannot be pierced, it can generally be de-molded. Since the demolding of resin sand molds is carried out in a semi-hardened state, a large impact force is required. At this time, attention should be paid to the balance of the demolding. Do not use an iron hammer to hit the wooden mold directly. A wooden block should be placed on it, or a rubber, plastic, or wooden hammer should be used. When dismantling the core box, pay attention to the demolding order and direction of each active part, and dig out the hanging head. For the parts that are accidentally damaged during demolding or transportation, the blocks can be coated with a special core repair adhesive and returned to their original place. If necessary, nails can be used to reinforce. Other damaged parts can be filled with newly mixed resin sand and scraped flat with a trowel. The strength and surface stability of the repaired part is poor, so the most important thing is to carefully do the demolding, handling, turning and other interactions to minimize the repair operation. Pay special attention to the release of air during modeling and core making.

5.Coating

It is not easy to paint the mold and core that have just been de-molded immediately because the hardening reaction of the resin is still in the primary stage at this time, and the moisture (solvent) in the paint will affect the normal hardening. If alcohol quick-drying paint is used, it needs to be ignited and burned immediately, which will cause the unreacted resin to overburn, which will reduce the surface stability of the mold (core). Generally, the coating operation should be carried out 4-8 hours after modeling. In order to prevent excessive solvent and moisture from penetrating the core and affecting the drying degree of the coating and to ensure the thickness of the coating to improve the resistance to metal penetration, the coating must have a certain concentration. When the brushing performance (such as leveling, easy coating, etc.) is acceptable, the coating concentration should be above 30 “Be. Stir evenly before coating. The coating should be applied from top to bottom, and the coating should not be allowed to drip and accumulate. The coating should be applied evenly to reduce brush marks. Also, care should be taken not to miss any coating. The entire pouring system must be coated, and the pouring port ceramic tube should be prevented when it is inconvenient to apply the coating. In order to prevent sand particles from falling into the cavity when closing the box, the pouring riser and the air outlet on the top of the cover box should also be painted. However, do not apply the coating on the core head to avoid affecting the mold matching accuracy and hindering the exhaust. This should be paid special attention to during dipping. After the alcohol-based coating is applied, it should be ignited and dried in time. In order to improve the combustion performance of the coating, alcohol above 95% should be used as the diluting solvent. After the water-based coating is applied, it should be dried in a drying room at 150-180 ° C for 1-2 hours. The full drying of the coating is crucial to prevent pores. In the case of poor combustion, a spray gun should be used to supplement the drying, but be careful not to use too much firepower to cause the resin to burn.

6.Lower core box:

Generally the same as clay sand. Resin sand has high strength and is easy to operate when matching molds, but the following points should be noted:

• Due to the large amount of gas generated during sintering, it is necessary to leave air holes at the high points of the cavity and the end of the core head on the box to allow the gas in the cavity and the core to be discharged smoothly
• Take measures to prevent molten iron from drilling the core head to avoid gunfire and air holes
• The head of the hanging should be smoothed with newly mixed sand and painted and dried. When repairing the hanging place, nails of appropriate length should be inserted
• To prevent the box from escaping when lifting, the sandbox clips should be Tighten and tightened well, and the mold surface should be with fire mud or asbestos rope
• Ensure the molten iron pressure head and all the pouring cups, riser rings and air outlet rings must be glued to the mold surface or buried to prevent fire
• The closed box should be placed empty to facilitate smooth exhaust on the lower surface
• After the core is placed, the gaps on the upper and lower left and right sides of the core head should be filled and leveled before closing the box to prevent the core from moving or floating. The floating sand must be blown (sucked) clean. Use a spray gun to dry the entire mold once before closing the box. The cold iron, core support, etc., should be dry, clean and free of rust