1. Insufficient pressure in the dewaxing kettle or slow pressure increase
If your mold shells experience widespread or recurring cracking, it's important to inspect the dewaxing kettle's operating condition. Current dewaxing kettles are generally required to reach a pressure of at least 6 kg within 10 seconds, or 7 kg within 14 seconds, and ideally maintain a pressure of at least 6.5 kg throughout the entire process. If your dewaxing kettle pressure consistently remains below 6.5 kg, it's time to inspect your equipment. During the initial dewaxing process, the wax rapidly expands due to heat transferred from the mold shell. Engineer Zhang mentioned during his livestream that this expansion pressure shouldn't be underestimated; ice can burst pipes, bottles, and other structures. If sufficient pressure isn't maintained within the dewaxing kettle at this point, the expansion force can cause cracks in weak areas of the mold shell. Once one or two areas crack, the pressure is released, preventing further cracks. Therefore, this defect typically doesn't spread across the entire surface, unlike cracks in the mold shell's surface layer. This issue relates to equipment performance.
2. Poor dewaxing
The general reason for poor dewaxing is due to process design. If dewaxing is not fully considered during process design, it may lead to excessive residual wax and other defects; secondly, it may cause the shell to crack. Some companies often have die heads, especially three or four vertical die heads, that break at the bottom during dewaxing. This is basically caused by poor dewaxing. This situation may be divided into the following two reasons:
(1) Too much loading. The shell needs a lot of heat to heat up, but the equipment's heating capacity is insufficient, resulting in a delay in temperature rise. Temperature and pressure are directly related. For steam, it can only rise to a certain temperature when it reaches a certain pressure.
(2) Sometimes we say that the die head is too thin and causes breakage, but this situation often occurs on certain products, and other products have no problems using this die head. In this case, it is usually because these products have too many components or there are parts that are difficult to dewax. If a pressure relief port is added to the die head, this problem can basically be solved. As for the special situations in other operating processes, we cannot analyze them one by one.
3. Large Ambient Temperature Fluctuations
We've already analyzed the impact of environmental factors in the previous Basics section (Series 9), so we won't go into detail here. However, it's crucial to pay attention to the temperature stability of the backing layer. In some foundries, the temperature in the same drying room can fluctuate by over 5°C in a single day. This fluctuation, not only impacts the mold dimensions, but also challenges the mold shell's strength. Another point to keep in mind during dewaxing is the time it takes to remove the mold from the drying room, enter the dewaxing kettle, and start aeration. This is especially true in the summer, when the outside temperature is much higher than inside the drying room. Some companies have excessive distances between the drying room and the dewaxing kettle, or workers may be careless and wait by the dewaxing kettle. There have even been cases where the mold shells are placed in the dewaxing kettle without being aerated. Please be mindful of this.
4. Insufficient Wet Strength of the Mold Shell
This problem occurs on large curved surfaces or corners because stress concentration in these areas often leads to cracking. Cracks are more likely to occur in areas of the mold shell that are weaker than others. Some inner corners may not be fully dry, resulting in reduced wet strength. Strength is also affected by factors such as personnel, materials, equipment, and the environment. We have previously analyzed this in detail and will not elaborate on it here.
In fact, the causes of this defect are not limited to the ones listed above; they are just more likely.
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