Visual Inspection uses the human eye to identify surface defects, cracks, scales, air holes, pin holes, sand holes, shrinkage porosity, slag or sand inclusions, misrun defects, cold shuts, etc. Following the investment casting industry standards and specific requirements from our customers, visual inspection is required to ensure good casting quality from start to finish. Visual inspection must be 100% carried out and need to make sure there will be no defects on the surface of investment castings. Any castings with such defects must be picked out and cannot be delivered to our customers. And all the inspection result must be documented and stored to record all the products made from our foundry.
Common Casting Defects and Discontinuities:
1.Surface defects are visible to an inspector. These include very rough or uneven surfaces, veins caused by cracking of the mold at high temperatures, burned sand and stripping defects. Stripping defects are flakes of metal on the surface caused by gas being trapped in the mold.
2.Underpour or misrun defects happen when not enough molten metal has been introduced to fill the mold. Sometimes, misruns are caused by early freezing during the pour. Some part of the casting is incomplete, usually with a rounded edge where the metal froze before it reached the mold’s wall.
3.Cold shots are caused by the premature freezing of some of the metal. They look like little iron balls or teardrops that are held, suspended, in the material around them.
4.Pinholes can be caused by gas in the mold pushing through the molten metal leaving voids or bubbles as it cools. Trapped gas can be caused by the conditions of the mold, if it’s not porous enough to allow gas escape. Rusty, hydrogen rich charges of scrap metal are more likely to cause pinhole defects as they bring more hydrogen with them.
5.Shrinkage porosity is caused by insufficient volume of metal in the mold. As the metal cools and shrinks, small holes are left behind throughout the casting. These holes are usually jagged, in comparison to the smooth floating pinholes caused by gas.
6.Hot tears or cracks can also arise in too rapid cooling. These are akin to shrinkage cavities but are more ribbonlike voids in the metal caused by rapid shrinkage. Castings with hot tears or cracks will underperform mechanically.
7.Cooling deformation is a common defect in very long, thin castings, but can happen in other shapes too. In this defect, the metal casting warps away from its intended shape during cooling, causing it to be outside of specified tolerances.
In every production run, we will choose a few samples and submit them to destructive testing. The casting will be section cut, and the properties of the metal inspected closely. The tester will look for inclusions, porosity, and shrinkage.
Dimensional Inspection is undertaken to ensure a part meets dimensional requirements/tolerances. Investment casting process can obtain precision products with ideal sizes and dimensions for its high accuracy. And even if some tolerances are not available from casting directly, we can also reach the result by further machining.
Although sometimes, the finished product looks perfect after visual inspection, it does not mean they can put into use without doubt. In this case, dimensional inspection is vital to guarantee. There are 3 common dimensional inspection methods used in Ningbo Investment Casting Co., Ltd.:
1.The dimensions can be checked manually by using measuring tools for investment castings such as vernier caliper, depth indicator, dial indicator, ect. It is the most directly and simple way to test the dimensions and tolerances by hand. To ensure qualified products, we need to test the product by measuring tools one by one according to the drawing.
2.We will also make gauges according to the sizes which will probably affect the assembly or use. We design gauge like GO-NO GO, R gauge, gauge drawings with full dimensional report and submit to the customer to get approval before using. For threaded holes, we can buy go and not go gauge according to the thread specification directly from the market. Testing gauge is mostly used when the dimensions cannot be tested by measuring tools directly. All critical dimensions like holes location and dimensions shall be checked 100% by gauges. Usually, pictures of gauging process will be submitted with QC documents.
3.The most accurate way to test the dimensions of products is using a coordinate measuring machine (CMM) that the probes to get very precise measurements. Usually, we will measure some critical and very tight tolerance by CMM.
We offer a wide variety of in-house investment casting quality verification and product testing services to ensure the castings which are produced by Ningbo Investment Casting Co., Ltd.
No matter where the product is applied, each precision investment castings is inspected and approved before delivery. If you have any investment casting part request for quotation, please do not hesitate to contact us.