Die castings often produce parts with rough surfaces. To improve these surfaces, manufacturers use several casting surface finishing options. They use machining to remove residues, grinding for better accuracy, polishing for smooth, shiny surfaces, etc.
Electroplating adds a shiny metal coat to stop rust and make them look nicer. Powder coating sprays on colored powder that bakes hard for a strong finish. Anodizing makes aluminum parts super tough and stops them from rusting. Painting adds color and protects the parts. Polishing makes the parts smooth and shiny.
In this article, learn various surface finishing options to increase the quality and appearance of castings in depth.
Surface Finishing Options
A. Mechanical Finishing Methods
Manufacturers use mechanical finishing methods. These methods help in increasing the dimensional accuracy and quality of casting parts. They remove metal residue, smooth the surface, and make the final part’s feature better.
1. Machining
The machining process includes cutting tools. These tools are used to shape a workpiece by removing material. Common machining methods are:
- Turning: The manufacturer uses cutting tools to cut the excess metal while rotating the cast part. This method works well for cylindrical parts. It helps in getting a dimensional accuracy of ±0.05 mm.
- Milling: A rotating cutter removes material when you keep the part in a stationary state. You can make very tough shapes with milling and get an accuracy of up to ±0.02 mm.
- Drilling: You can use a drill machine to make holes in parts. Generally, drill bits can create holes with diameters as small as 0.5 mm. These holes tolerance ranges from ±0.05 mm to ±0.1 mm, depending on the size.
- Boring: The boring process is suitable for expanding existing holes. It provides accuracy as fine as ±0.01 mm.
- Shaping and Planing: These techniques are used to perform straight-cutting actions and make plain surfaces. They offer an accuracy of around ±0.1 mm.
Tool Materials:
The selection of tools depends on the metal being cast and finishing standards. You can choose High-Speed Steel (HSS) for soft metals. It offers a cutting speed of up to 30-40 m/min. Carbide tools are good options for hard and tough materials. These tools provide speeds of 150-300 m/min.
Tool Wear and Its Effects:
Tool wears often due to repeated use for a long time. Defective tools cause rough surfaces and dimensional inaccuracies. They also increase the risk of defects. Therefore, proper tool maintenance is important to avoid these effects.
2. Grinding
Grinding is a finishing technique. It uses abrasive wheels that smoothen a part’s surface. Various effective grinding methods include:
- Surface Grinding:
Provide flat surfaces and can achieve tolerances of ±0.001 mm. The image shows a surface grinding process using a horizontal spindle and a reciprocating table. Here, surface grinding is categorized into two types:
- The traverse grinding: The grinding wheel moves side-to-side across the surface.
- The plunge grinding: The wheel feeds straight down into the material.
- Cylindrical Grinding: Manufacturers use this method for parts with cylindrical shapes and obtain tolerances as fine as ±0.002 mm.
- Centerless Grinding: This method is ideal for making accurate round workpieces. It achieves tight tolerances of ±0.001 mm.
- Creep Feed Grinding: Manufacturers remove residue from deep cuts in harder materials using this technique. It allows them to get a tolerance of ±0.002 mm.
Abrasives:
Abrasive is a kind of gritty material. This is actually used for cutting or rubbing during grinding and is based on the metal being ground. For instance, manufacturers use aluminum oxide for steels with a grit size of 60–120. For harder materials (ceramic), they use silicon carbide with grit sizes 120-240.
Coolants:
Coolants are important to protect parts from thermal damage. They control the temperature of parts and grinding wheels. Coolants also reduce wheel wear as they provide lubrication and make chip removal easy. Common types of coolants include water-soluble oils and synthetic coolants.
3. Polishing
Die casters improve final product surfaces using polishing methods. These methods create shiny and aesthetic appeals in parts as well as increase their lifespan. Types of polishing include:
- Buffing: This process uses buffing wheels whose speed ranges from 2000 to 4000 RPM. That is produced with canvas or felt materials. The manufacturers smooth the surfaces with these wheels. They often apply abrasive and non-abrasive methods to further finish.
- Chemical Polishing:
A rotating tool (a urethane ball) smoothens a workpiece’s surface in this method. The tool gives pressure through a spring support. The manufacturer flows slurry beneath the tool. That polish the surface through chemical and mechanical action.
- Lapping: Lapping involves using fine abrasives (diamond paste). This process provides higher smooth surfaces. It can be used for high-precision parts to achieve finishes with RA values even lower than 0.01 µm.
- Honing: Honing methods work well on internal surface finishes like cylinder bores. It uses abrasive stones. That removes residue and gives an accurate, smooth finish. Honing gives tolerances around ±0.01 mm to ±0.02 mm, depending on the material.
B. Chemical Finishing Methods
Chemical Etching
Chemical etching removes certain areas from a part using etchants. For example, manufacturers use ferric chloride for copper and nitric acid for steel. They use this technology for intricate metal designs in electronics (PCBs) and decorative pieces. Etching depths vary from 0.01 to 0.5 mm, based on the metal and exposure time.
Chemical Milling
Chemical milling removes the material’s controlled amount via chemical etching. Its material removal varies from 0.1 to 2 mm, according to what the component needs.
Additionally, there is no need for mechanical tools, as this can create complex shapes without them. You can use this method for the aerospace and automotive industries.
Pickling
Pickling removes oxides, scale, and rust to clean metal surfaces. It uses immersion in acidic solutions. For instance, hydrochloric acid or sulfuric acid.
Pickling can clean with depths ranging from 0.05 to 0.3 mm. However, it depends on the acid being used and immersion time. You can use this process for preparing metal surfaces for electroplating, painting, or welding.
C. Coating and Plating Methods
1. Painting
Painting uses several types of coating. For instance, solvent-based, water-based, and powder coatings. Before painting, proper surface preparation and cleanliness (over 95%) are important for better adhesion. That includes degreasing and phosphating.
You can paint the part using spray painting, dip coating, or electrostatic spraying. Use them according to material types and finish needs.
2. Plating
Plating refers to the process of applying coating to metal via electroplating and electroless plating. Particularly, electroplysis deposits metals like chrome, nickel or gold in electroplating. Meanwhile, electroless plating gives uniform coatings without needing external current.
Plating’s thickness ranges from 0.005-0.25 mm. It resists rust and wear and gives decorative finishes for industries like automotive and electronics.
3. Powder Coating
Powder coating uses negatively charged powder to apply on grounded workpieces. Manufacturers heat the item under 160°C to 200°C temperature. That causes the powder to form a smooth finish.
Powder coating lasts longer, prevents corrosion, and is eco-friendly. This is suitable for automotive parts, appliances, and furniture.
Advanced Surface Finishing Techniques
1. Laser Polishing
Laser polishing improves additively manufactured metal surfaces. It creates single tracks layer by layer to improve surface finish and reduce roughness.
A Gaussian laser beam scans in a certain direction. They melt and smooth the material layer. This laser spot overlaps with a hatch spacing of 0.1–0.5 mm for consistency.
Laser polishing uses high-energy laser beams to liquefy and re-solidify the surface layer. It gives an accuracy of up to ±0.1 µm. This process is preferable for tough shapes and devices like medical implants, aerospace parts, and precision optics.
Electrochemical Polishing
Electrochemical polishing (ECP) uses electricity. It smooths and refines the surface of metal parts with a corrosion resistance of 80–95%.
This process is like reverse electroplating. Because manufacturers remove a thin layer instead of adding metal. So that they achieve a highly polished finish.
ECP is ideal for medical devices, turbine blades, and aerospace parts.
Ultrasonic Finishing
Ultrasonic finishing uses high-frequency sound waves. These waves create vibrations in a slurry of abrasive particles and a liquid metal. Vibrations agitate abrasive particles to bombard the part surface. That removes the metal and causes a higher polished result.
Ultrasonic processing can get a surface finish of up to 0.1-0.3 µm. It is effective for deburring, polishing, and contaminant removal from complicated forms.
Casting Defects: Causes, Types, and Prevention
1. Porosity
Porosity looks like small holes or voids. It forms inside the metal and weakens it. The reasons for its occurrence are:
- Improper handling of molten metal
- Environmental factors
- Mold-related issues (old or damaged)
Gas Porosity:
Gas porosity occurs during solidification. Gases like hydrogen, nitrogen, or oxygen enter or mix with metals. These gases accumulate inside and form bubbles. For instance, hydrogen in aluminum exhibits a solubility of 0.69 cm³/100g at 660°C. It reduces to 0.034 cm³/100g when metal becomes a solid.
Sand Porosity:
Sand porosity happens in the sand casting process. Some sand grains or gases from molds mix with molten metal.
Prevention:
Manufacturers degas with argon or melt the metal in a vacuum to remove trapped gases.
To prevent sand porosity, it is important to use fine-grain sand with low permeability. Also, moisture should be applied to sand mold. These steps can decrease defects by 3 to 5%.
2. Shrinkage
When metal becomes solid, it shrinks unevenly, leaving voids inside, known as shrinkage porosity. This is a common issue and often occurs in aluminum die casting. This alloy can shrink up to 6.6% by volume. The various factors influence shrinkage, such as:
- Cooling rate
- Metal type
- Casting design
For instance, a higher cooling rate for aluminum (30 °C/min) causes cracks. Meantime, 5°C/min ensures uniform structure.
Prevention
You can use feeders to add extra molten metal during solidification to avoid shrinkage. Additionally, maintain constant pouring temperature. For example, 650–700°C for aluminum. Design a uniform mold. Also, apply chills for thicker sections. These chills accelerate the solidification process and minimize shrinkage.
This image shows shrinkage porosity. That is reduced during pressurized solidification. You can see the difference between the experiment and simulation sections. A color scale demonstrates the level of total shrinkage porosity. For instance, blue is zero, the highlighted green section is around 40 to 50 and the red shrinkage cavity shows 100% shrinkage.
3. Rough Surfaces
Rough surfaces refer to imperfections. This issue affects the appearance and performance of cast parts. They form because of:
- Mold roughness: Uneven mold surfaces cause mold roughness, often due to coarse sand grains. For instance, molds with AFS GFN <50 make surfaces very rough.
- Sand inclusions: When manufacturers pour molten metal into the sand mold, several loose sand grains stick to the molten metal and affect the surface finish.
- Erosion: A greater speed of molten metal than 2 m/s forms erosion.
Surface roughness:
You can measure surface roughness with tools like profilometers. The average roughness (Ra) for sand-casting parts usually falls between 6–25 µm. Meanwhile, this ranges from 0.4 to 0.8 µm for polished parts.
However, Rz (roughness with larger height variations) for sand-cast parts fluctuates between 50 µm or more.
Prevention
Improve the mold quality and use fine-grain sand (e.g., AFS GFN >60). Apply smooth refractory coatings. Additionally, control the pouring speed to 1.5–2 m/s to reduce erosion. For a final touch, you can use grinding or polishing methods.
Considerations for Choosing a Surface Finishing Option
The right surface finishing option depends on several factors, as given in the table. For example, cost, material compatibility, desired finish quality, and production volume. Choose them wisely to justify your investments.
Conclusion:
Manufactured parts often have rough surfaces that need to be fixed. Rough surfaces can also happen due to defective issues like porosity, shrinkage, or mold wear. Anyhow, you can fix them by using several surface-finish options. These methods include grinding, machining, polishing, or coatings. The selection of techniques depends on how you want to turn your rough surfaces into fine finishing or the usage of applications.