In order to gain good quality lost foam castings, each manufacturing process must be strictly controlled from the purchase of raw materials, modeling, sand falling, and cleaning of the castings. A slight improper operation will cause various defects in the castings.

 

1. Causes of carbon defectsand solutions

Carbon defect is a relatively common defect in lost foam casting, which is specifically manifested as a carbon defect formed by a part of the residual combustion product remaining in the casting material after the white mold is melted. The main reasons for this carbon defect include the following:

 

(1) The negative pressure generated during the production process is too small, which is manifested as:

a: The production preparation work in the early stage of production is not in place: in order to control the sand sticking           phenomenon of machine tool bed castings, most of the enterprises design the system's theoretical negative pressure value to be relatively low.

Measures to be changed: Revise the process parameter values ​​again, and increase the actual vacuum in the flask to a certain value.

b: Unreasonable performance in the production, manufacture and use of machinery and equipment.

 

(2) The selected coating has poor air permeability

Proposed change measures: re-replace paint in multiple areas.

 

(3) The size of the pre-selected foamed plastic foam particles is too small

In order to make the surface of the white mold smooth and clean, some companies use small-particle foaming materials.

Proposed change measures: control the density of pre-expanded foam.

 

(4) The carbon content in the casting liquid is too high

Proposed change measures: appropriately reduce the carbon content.

 

machine tool castings

 

2. Causes and solutions of cold barrier defects

 

The so-called cold barrier refers to the fact that the casting material may not be poured through due to the low temperature of the casting material during the cold barrier. Cracks or fractures appear after the casting of the workpiece after a certain force, and oxidized inclusions appear in the fracture. Or there is no fusion phenomenon. There are three reasons for the low temperature of the casting material:

 

(1) Before pouring, the liquid temperature of the casting material is low. Casting under this condition will cause a lot of cold isolation.

Measures to be changed: Keep the pouring temperature of the casting material at a higher temperature level.

 

(2) The design of the gating system is defective

The performance: the unreasonable position of the inner runner is introduced, which causes the runner of the casting liquid to be relatively long, which causes the phenomenon of cold isolation in the casting.

Proposed change measures: correctly design the gating system, and add risers and other devices at appropriate positions.

 

(3) The improper operation taken by the caster in the pouring operation of the bed casting can easily cause the temperature of the molten casting material to drop too quickly, mainly in:

a: The casting speed of the casting liquid is too low: If the casting speed of the casting liquid is too low, the casting liquid poured in first loses a lot of heat, and the temperature of the casting liquid decreases too much, which will cause the liquid to flow poorly, and the subsequent casting liquid will not flow in, causing cold Separation phenomenon.

Proposed change measures: Under the premise of not affecting the casting process and the quality of the castings, the casting speed should be accelerated as much as possible.

b: The back spray phenomenon occurs during the casting process: If the back spray phenomenon occurs during the casting process, it will inevitably affect the smooth entry of the casting liquid. Causes current interruption, which may lead to cold isolation.

Proposed change measures: increase the air permeability of the paint and extend the drying time of the model. If necessary, you can change multiple areas of the paint, change the brushing process or reduce the density of the foam plastic.

c: The casting liquid cuts off during the pouring process: If the casting liquid produces discontinuous cut-offs during pouring, the cold cut phenomenon may occur at any time if the successively poured casting liquids cannot be fused together.

Proposed change measures: Resolutely avoid cut-off.