“Blasthole” of Casting Defect Series
Sand hole defect is also a defect with relatively high frequency in investment casting production. Sand hole, as the name suggests, is sand drilled into the surface or inside of the casting, occupying a place in the metal material of the casting. It is like a metal eye socket with one or more grains of sand in the eyeball, so it becomes a trachoma. See Figure 1.
Figure 1 Trachoma defect
The characteristic of sand hole defects is that the surface or inside of the casting is filled with sand or refractory powder. It may come from outside the shell, or it may come from inside the shell. If the trachoma defect comes from the inside of the cavity, because it is a metastatic defect, there is one more trachoma here, and a small sarcoma will naturally appear in other places where the grit is lost. The root causes of trachoma defects are nothing more than external and internal.
External, it may be accidentally brought into the shell cavity. For example: the wax is not treated in place and remains in the wax material, and the injected wax enters the wax mold or runner. Of course, the probability of this situation is relatively small; the wax boiling during dewaxing will flood the free sand in the dewaxing kettle into the runner; dewaxing After cleaning the residual edges of the mold shell, the sand particles were accidentally brought into the mold shell; when the mold shell was roasted, the furnace lining was flying and fell into the mold shell due to the pressure of the combustion nozzle in the roasting furnace.
Internally, there may be gaps in the combination of wax parts. The paint penetrates into the gap and enters the cavity after dewaxing; the inner surface of the mold is not locally dried and falls off after dewaxing; the local paint is washed off for a long time when the molten metal is poured. Drop into the cavity.
Because the bladder defect is a mobile defect, its final position is generally difficult to determine, but the internal location that easily causes sand loss is easier to find. Generally, sharp corners, inner corners, insufficient or excessively dry parts, inner gates, etc., are all hidden dangers that easily cause trachoma defects.
So, based on the above analysis, let’s take a look at how to avoid the occurrence of trachoma defects.
From the external cause, we must do the following:
- 1. The wax treatment must be strictly filtered and precipitated to ensure that the wax is clean and has less impurities and ash.
- 2. Some units use sprue mandrels and the drying line on the mold shell, so the mold shell mandrel is not allowed to be cleaned (regardless of using shot blasting or sand blowing).
- 3. After dewaxing, you must check whether there is free sand in the pouring system and clean it up in time.
- 4. Be careful to clean up the remnants of the mold shell on the sprue cup to avoid sand particles from entering the mold cavity. Try to use runners with flanged sprue cups. (See Figure 2)
- 5. Try to cover the sprue cup when the shell is roasted to reduce the chance of sand entering the cavity. The roasting furnace must be cleaned at regular intervals.
- 6. For important products, clean the shell and vacuum the shell before pouring.
Figure 2 Gate rod with flange
Looking at the internal factors, we must do the following:
- 1. When the wax mold is combined welding, the welding seam must be strictly inspected to ensure that the welding seam is tight and seamless.
- 2. Strictly implement the process discipline to dry the shell, neither insufficient nor over-dried, especially in deep cavities, corners, sharp corners, and corners. Strictly control the drying conditions of the surface layer, especially low temperature and high humidity.
- 3. The design of the gating system, especially the inner gate design, avoids feeding from one place; avoids the inner gate directly impacting the core.
Source: China Forged Flange Manufacturer – Yaang Pipe Industry (www.steeljrv.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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