Surface Finishing for Die Casting Parts: All You Need to Know

Surface finishing is an integral part of the precision die casting industry. If you are looking for straightforward information concerning surface finishing for die casting parts, this is the right resource for you. 

Read through this article for a thorough understanding of surface finishing, its types and importance. 

Why is surface finishing required for die-casted products?

Surface finishing is important for die-casted surfaces because it makes the final products look finer and lasts longer. Other benefits obtained from the finishing process include;

Enhanced aesthetics

Finishing a die-casted surface helps remove the faults incurred in the manufacturing process leading to the surface looking clean. Some materials used for finishing have beautiful designs and colours, which add to the surfaces’ aesthetic value.

Elimination of surface defects

Various surface finishing processes include rounding sharp corners and scraping off excess residues from the surface. Abrasives are commonly used and are most effective for scraping off all faults incurred during manufacturing. 

Helping with the adhesion of paint and other coatings

Finishing surfaces reduces roughness and leaves them smooth. This increases the surface’s ability to absorb paints and coats to make more rigid layers.

Increased Corrosion Resistance

Some metals are highly reactive to moisture and can corrode when exposed to the atmosphere. Applying non-corrosive coats during finishings helps to prevent the metals from corroding, hence increases their durability.

Stainless steel surfaces have high corrosion resistance. 

Different surface finishes also yield different surface roughness. A rougher surface is more susceptible to localized corrosion forms such as pitting and crevice corrosion than a smooth surface. 

Increased resistance to chemicals

Surface furnishing involves covering surfaces with tough and highly resistant material, which gives them the strength to resist chemicals.

Improved conductivity

Electroplating helps improve the conductivity of a raw part by enhancing its electrical properties. It can impact solderability, durability, and contact resistance. When designing components for applications where conductivity is essential, you should choose the surface finish so that the conductivity value is appropriate for the application.

Types of Surface Finishes

While there are many surface finishes available, here are the most popularly used ones.

1. E-Coat

E-coat, also referred to as paint deposition, is a process that uses electricity to attract the paint product to a metal surface. It is often used on its own since it gives excellent coverage but can also be useful as an undercoat for other coatings like a powder coat. 

Traditionally it has been used for functional purposes like protection and not really for decorative purposes.

E-coats such as the clear clad processes are used for coating a wide range of consumer goods, including jewellery, hardware, giftware, eyeglass frames, among others.

2. Black Oxide

Black oxide is a conversion coating produced through a chemical reaction when dipping parts in a basic heated salt solution. It provides some corrosion resistance, acts as an absorbent oil and wax, and is resistant to peeling and chipping. 

It also acts as a protective barrier, keeping moisture from corroding the surface of the alloy. 

Black oxide is used to finish surgical instruments, gears, fasteners, and hardware used in assemblies whereby corrosion resistance is required. Components used in military applications, and hardware used in architectural and furniture assemblies also use black oxide as a finish.

3. Powder Coat

Powder coating is a dry coating process used as a metal finish, mostly on industrial equipment. It is the most popular finish in terms of toughness and resistance to ding and scratch. 

It occurs in different gloss levels, textures, and colours and can be useful on different surfaces, including plastic, steel, and concrete. It is one of the most cost-effective finish options, suitable for both indoors and outdoors.

Powder coating is used in the automotive industry to coat various components as well as a basecoat. This includes hubcaps and wheel trims, under-hood components such as shock absorbers and radiators, vehicle frames, and many public transport items, including bars and handles.

4. Anodizing

Anodizing is a cost-friendly, corrosion-resistant, durable, and non-conductive protective coating that seals the part. It exists in various colours, including red, blue, and black, and is one of the most environmentally friendly coating industry processes. Since it is not a metal plating process, it produces no hazardous waste.

In the aviation industry, aluminum anodizing is used for the chemical treatment of challenging aircraft parts and frame components and for high-grade aircraft parts, aerostructures, and other aluminum components.

5. Chromate

Chromate is a corrosion-resistant process classified as a conversion finish rather than an additive finish. It comes in many options: chrome plating, chromate with zinc and without zinc, etc. It is a cost-effective bulk process conversion coating that occurs in different colours.

The application of the chromate finish is often observed on hardware and power tools.

6. Chrome Plated

It is one of the most expensive finishes due to the labour involved in chrome plating. There are two types of chrome plating – bright and satin. 

Bright chrome is the mirror-like finish used on furniture, automobiles, toys, and more. It gives surfaces a good wear life, corrosion protection, and a good appearance. Satin chrome has the same characteristics as the polished one but has a duller finish. 

Chrome plating can be used in various products such as linear guides, shafts, and rollers in the automotive industry.

7. Copper-nickel-tin

Copper-nickel (CuNi) is an alloy of copper that contains nickel and strengthening elements, such as manganese and iron. The copper content typically varies from 60 to 90%. (Monel is a nickel-copper alloy that contains a minimum of 52% nickel.) It provides solderability to the base metal substrate.  

It is required in landing gears, breaks, bearings, and bushings and increasing hydraulic systems pressure. 

8. Cobalt tin

Cobalt tin is similar to chrome in its application and appearance. To get an appearance that is identical to a chrome-plated part, first plate the cobalt-tin components with bright nickel. You will then flash the components with cobalt-tin.

Cobalt tin is used to decorate objects, improve wearability, harden, improve solderability, reduce friction, improve IR reflectivity, improve paint adhesion, alter conductivity for radiation shielding, corrosion inhibition, and other purposes.

9. Electroless nickel

Electroless nickel plating falls under chemical plating techniques. You use electroless nickel plating to deposit a metal plating layer without using an outside electrical energy source to charge the plating bath. This method allows depositing of a relatively thick layer that provides uniform plating stiffness and is unique.

It has good adhesion to special materials such as ITO (Indium Tin oxide), glass, and polyimides. It is used on composite plating, electronic components, and valves.

10. Gold plating

Gold plating is an extremely dependable material used in medical, aerospace, and electronics, due to gold’s resistance to oxidation, reflective nature, good electrical conductivity, and solderability properties. 

Since the gold plating process requires a controlled environment, operations are in an area with independent air systems, separated from other plating processes. It is expensive but worthwhile.

Because of gold’s ability to improve and sustain electrical connectivity, gold plating is commonly used in electronics such as laptops, smartphones, and desktop computers.

11. Silver

Silver plating offers greater resistance to tarnish whenever you compare it to sterling silver. When used over sterling, silver gives it a bright appearance that is easy on the eyes. 

Some people consider using fine silver for plating to be more effective. This is when they compare it with surface finishing with ultrasonics, abrasive, and tumblers. It is cost-friendly, but it is easy to tarnish when exposed to the atmosphere. It is more of a decorative finish.

Silver is commonly used in the jewellery industry to give a shiny look. It is also used in making currencies.

12. Polyurethane paint

Polyurethane wood coatings are incredibly long-lasting, durable, and versatile. Moreover, they are resistant to many types of physical and chemical damages. This coating is bound to produce a hard coating that will be hard to break and comes in non-aromatic and aromatic solvents. Its purpose is for interior and exterior use.

It is used in the furniture finishing industry on wood materials like tables, wardrobes, cabinets, chairs, and even doors to protect them against wearing out.

13. Nickel-Free Surface finish

It has excellent adhesion properties and acts as a barrier for the skin, hence it is great for wearables and consumer electronics. When the coating scratches off, there is a concern of nickel-to-skin contact that creates challenges for suppliers and the consumer in the electronics industry. Buyers with sensitive skin can develop a rash from metal wearables containing nickel.

Nickel-free coating is used in the cosmetic industry on jewels to give them a furnished and decorated look.

14. Impregnation

It is a great option to use after machining to remove the “skin” of the casting to create a leak-free component, seal porosity and create watertight components. It is a viable option to reduce scrap and improve yields. 

Since impregnation prevents leakages, it can be used in plumbing for water pipes or the automobile industry to fix engine parts.

15. Teflon

Teflon surface protector ensures walls remain protected from stains, bacteria, fungus and have a smooth finish that lasts longer; it utilizes a racking process and is corrosion resistant that encompasses all the properties essential for a useful sealing element, mainly:

• High wear resistance
• Low friction
• Moderate hardness (allowing for better overall mating with metal parts)
• Durability – both with temperature as well as corrosion

The original Teflon coating is used worldwide to give pans, pots, and other cookware a durable, nonstick surface. It also evolved into a new family of fluorochemicals designed to prevent stains in carpets.

16. Bright nickel

Bright nickel plating contains a high amount of sulfur which gives it a mirror-like finishing. In as much as it has conductivity when compared to other types of nickel plating, it is less ductile. 

Bright nickel hides polish lines as well as any other imperfections of a material’s surface. It must pass through an electric current to add bright nickel plating.

Bright nickel plating by itself or with other nickel deposits is used in automotive applications such as on bright trim, plated wheels, truck exhausts, restorations, and bumpers.

17. Physical vapour deposition (PVD)

It is the process of vaporizing a solid material in a vacuum and depositing it onto the part’s surface, known as a thin-film coating. These coatings are not merely metal layers. 

This technique is to deposit a thin layer of material on a substrate surface in the presence of a vacuum. There are various types, such as magnetron sputtering, ion plating sputtering, and electron beam sputtering. 

PVD can be applied to medical devices to differentiate their appearance from similar products in cutting tools to improve their performance.

Surface finishing process

Following are the key steps involved in a surface finishing process:

Surface cleaning

Surface cleaning is a vital manufacturing step. It is useful to decrease the bioburden (living organisms) before sterilization and eliminate contaminants originating from manufacturing processes, such as polishing fluids, particle cutting, and mould releasing agents.

Conventional cleaning methods generally employ wet chemical techniques, such as using an aqueous surfactant solution with/without acids or organic solvents to dissolve and eliminate contaminants.

Deburring

Deburring is the process of rounding sharp corners chamfering formed on a metallic part during the machining process. It can remove the small pieces and raised edges that may remain attached to a workpiece. 

Surface finishing can also enhance the part’s function or appearance and prepare it for subsequent coating processes such as painting, plating, bonding, and plating. Deburring is a necessary process during furnishing.

Vibratory Shot Blasting

After a metal workpiece goes through pressing, punching, cutting, and forming, there are often remnants on the piece that affects its quality. These remnants include sharp edges, sharp points of metal along the radius of a component, and roughness on a component’s surface. 

In mass production, manufacturers use shot blasting to remove the flaws instead of using hands. It is a crucial part of the metal manufacturing process. 

During vibratory finishing, the components in a massive bowl hold abrasive media and cleaning. In operation, the bowl vibrates at an exaggerated rate, causing the media to turn up and down while rotating in a circular motion. 

The components travel with the abrasive media. Over time, the combination of rotation and vibration smoothes the edge and surface of an element while removing burrs. 

Factors affecting the cost of Surface Finishing

Check out the list of factors that influence the overall cost of surface coating for a part.

Raw Material

The type of material to use when furnishing is one of the first decisions one should make because it affects the resulting cost. The good thing here is that this is the most manageable expense to predict and control. 

The cost, with few exceptions, is based on material volume consumed and will not vary a great deal no matter where production takes place. Price quotations for commodities are easy to find on the internet and are always updated so developers can easily access them.

Labour cost

The cost varies most depending on the geographical location of the people doing the work. As much as labour cost directly affects the price of furnishing, paying for highly skilled labour is much more cost-effective than paying for cheap labour and getting a bad quality product. Hence, this is the variable expense most worth investing in for the best long-term results.

Part Complexity

Complexity relates to how sophisticated a particular part is and the number of production steps and separate processes it takes to achieve the final design.

With each process, the cost will increase because of the additional manual labour involved in the set-up, testing, measurement, and extra care and attention to detail required to hold tight tolerances on multiple features. 

Wherever possible, it is usually the best practice to keep designs simple, therefore reducing expenses and speeds up manufacturing.

Accuracy

Accuracy means how close the measurement of a feature conforms to the exact value one is striving to achieve. It can be a meter or an inch since accuracy is indifferent to the measurement system used. 

Whatever the nominal value, the closer you can make a feature to that value, the more accurate you are. The more precise the measurements during manufacturing, the lesser the cost incurred in furnishing faults made.

Tooling

Some parts require special and complex tools to furnish, which directly affects the cost incurred for furnishing. Parts that need complex machines will lead to a higher price compared to those requiring easily accessible tools.  

Final Words

In light of this discussion, it’s clear that surface finishing is critical in the die casting manufacturing process since it influences the parts’ look and durability. You need to be aware of the factors affecting the cost, the material to use, and the best processes for producing quality parts.

This article is a part of our comprehensive guide on “Die casting: Process, Equipment, Uses, and More”