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The main factors affecting the dimensional accuracy of castings

Casting is a very important metal processing technology. For products with complex features, we can quickly and economically obtain the required structure through casting. However, the accuracy of castings is often an important factor in choosing the process. If the accuracy requirements are different, we must choose an appropriate casting process. Obtaining the external structure of the product through casting and then ensuring the accuracy of important dimensions through subsequent machining is a relatively common processing technology.

In fact, dimensional accuracy is not highly valued by most factories because our products generally do not have such high precision requirements.

But you will find that once customers have products with high dimensional accuracy requirements, we try our best but cannot meet the requirements because we have not paid attention to the impact of many factors on dimensional accuracy in the production process, or in other words, we are powerless and cannot control the equipment, raw materials, and other aspects.

We usually start from two aspects: adjusting the mold or increasing the machining allowance. However, in fact, even if the mold is adjusted well, there will still be significant fluctuations in the size. At this time, we need to start from various details to control it.

We also listed several main factors today.

1. Mold

1. 1 Mold temperature

Many companies may not have paid too much attention to this point. Let's take a simple example. In summer, a new mold arrives at the factory and is taken from the wax workshop outdoors for verification. The mold temperature and room temperature difference are over ten degrees, and the verification size is one state. When the mold returns to room temperature, it is in another state, and the same applies in winter; Even if we don't talk about such extreme situations, is there a temperature difference between your mold warehouse and wax workshop? So in general, the mold we need to use should be kept at the same room temperature as the wax mold, and the mold should be placed in the wax mold workshop for temperature adjustment in advance.

1.2. Mold structure

The structure of the mold determines the difficulty level of mold removal. Everyone may have experienced some situations where it is difficult to remove the mold. When removing the mold, the wax part has already undergone deformation. So, how can we ensure the dimensional accuracy of the wax part. For example, for products with a core pulling structure, the mold taking time will directly affect the shrinkage of the wax parts, which in turn affects the size of the wax parts.

2. Wax material

2.1. Brand new wax material

Wax materials produced by different manufacturers have different raw materials, addition ratios, and shrinkage rates. Therefore, the size of wax parts produced by the same mold naturally varies.

2.2. Recycling wax

The vast majority of our company's wax materials are recycled, so in the process of recycling, the addition ratio of new wax is different, the shrinkage is different, the wax treatment process is unstable, the moisture content of each batch of wax materials is different, the shrinkage is different, the ash content is different, and the shrinkage is also different. These factors can directly lead to the size instability of wax parts. Zhang also gave a good suggestion, which is to use a dedicated wax injection machine to produce castings with high dimensional accuracy using new wax, in order to ensure the consistency of wax parts as much as possible. Actually, up to now, some companies are still using cylinder changing wax injection machines to produce products with high dimensional accuracy, in order to control the temperature consistency of the wax material as much as possible

3. Waxing machine

The performance of the wax injection machine directly affects the dimensional stability of the wax parts. The author once used infrared to detect the temperature of the wax material injected from the wax injection port. When the set temperature was 53 ℃, the temperature of the wax material measured varied between 52-56 ℃ (with some measurement error). This is due to the insufficient precision of the wax spraying machine in controlling the temperature of the wax material. Of course, there may also be some human factors here, such as some workers' habit of dumping waste wax directly into the wax storage tank of the wax injection machine, which will directly affect the temperature uniformity of the wax material. The frequency of wax replenishment in the wax storage tank can also affect the temperature uniformity of the wax material, and so on.

4. Wax injection parameters

This should be the easiest point for everyone to understand, as the wax injection pressure and mold taking time will directly affect the size of the wax parts. However, when the wax injection parameters are consistent, there may also be fluctuations in the size of the wax parts. This brings us back to the performance aspect of the wax injection machine. The author once tested the size condition of 40 wax molds produced continuously by the same wax injection machine. For the same flange, the size is 95mm, and the maximum and minimum sizes of the 40 wax molds produced continuously differ by 0.3mm. Two of these were due to the extended mold taking time caused by workers going to the bathroom during operation. When the author extended the mold taking time to 3 minutes, the sizes of all wax parts were almost the same. However, this will delay production efficiency, and many product structures are not suitable for extending mold taking time. The author is only conducting verification.

5. Cooling of wax components

It is not recommended to cool all high-precision wax parts with water, as even if the wax parts are covered with water, there may be deformation caused by uneven cooling speed. It is almost impossible to see the operation of using water for wax mold cooling abroad. Some products that are prone to deformation require specialized shaping fixtures for wax parts (it is not recommended to use metals such as aluminum). When producing certain aviation products, the author also has cooling fixtures for each wax component.

6. Refractory materials, shell strength

Different refractory materials have different coefficients of expansion (as mentioned in my previous articles), so the size of castings produced from different refractory materials is also different. Even if the same refractory material is used, the impact on size varies due to differences in slurry, operating techniques, and shell thickness. Some factors such as shell swelling caused by product structure or shell swelling caused by insufficient interlayer density during shell making operations can directly affect product dimensions.

7. The size of the mold shell during pouring

The casting size is also different when pouring at a shell mold temperature of 1000 ℃ and when pouring at a shell mold temperature of 600 ℃.

8. Process plan factors

The different degrees of deformation caused by different gate positions are typical cases in both the investment mold manual and the book by Shanwu Poplar. Zhang also listed several examples, which I will not elaborate on further. There are also factors such as uneven shrinkage caused by different casting structures. The factors that affect the dimensional accuracy of castings include but are not limited to the above. Many people may think of more issues, such as post-processing. However, if you want to make products with higher dimensional accuracy, the above factors must be strictly controlled.