What is finishing?
Finishing involves altering the surface of a manufactured part to achieve a certain appearance, make it easier to bond with, or provide some type of durability. The finishing processes used on the manufactured part all depend on the part’s material, whether it be plastic or metal. The correct finishing processes for the part are chosen based on costs and part specifications.
What are the different processes for finishing plastics?
After the plastic parts have completed their plastic injection molding production run, there are three steps that need to take place. First, the parts need to be degated, second, they are deflashed, and finally, they are cleaned. Degating basically removes the marks from the gates where the thermoplastic entered the mold cavity on the part. Deflashing removes all the excess material around the part and reduces imperfections on the plastic part. Cleaning is the final step to make sure the surface is free of contaminants.
What are the types of plastic finishes?
Plastics can receive a wide variety of finishes based on their aesthetic needs or specifications. If the part needs labeling, such as branding or logos, there are many different options available for applying these markings to parts. Choosing a marking method depends on the durability of the markings as well as cost limitations because some methods can be more expensive. Some of the viable methods to achieve markings on plastic parts include:
Silk screening involves ink is directly transferring from a silk screen onto the plastic part.
Heat transfer is a process where a heat transferable label or marking is applied to a plastic part using heat and pressure.
During hot stamping, a hot metal stamping die is pushed into a plastic part leaving a marking in the shape of the die.
Engraving and Etching
Engraving and etching are both physically mark a plastic part, but engraving is done with either a laser or CNC, while etching is done using chemicals.
Pad printing is similar to heat transfer and silk screening but uses pads that pick-up ink from an etching and push the design onto the plastic part.
The coating process uses a UV powder that is electrostatically applied to the surface of the plastic part and cured using UV light.
How are finished plastic parts assembled?
If the plastic part is part of a larger assembly of pieces, then there are a few different options to attach the pieces together. The plastic can be mounted using traditional hardware, a chemical bonding agent, or ultrasonic welding.
How are surface finishes applied?
If the part needs surface finishing, there are a wide range of finishes available for many different forms and functions. Some ways to apply a surface finish are buffing, flame polishing, and painting. Painting is optimal for large volumes of parts because many manufacturing facilities have high-volume paint lines within their operations already. All of these finishes will enhance the part’s surface finish and appearance.
What are the different processes for metal finishing?
Metal finishing is a much more involved process than plastics because metal is more vulnerable to corrosion. Depending on the application of the metal part, some parts may need more extensive finishing than others. Metals can also be painted, etched, and engraved similarly to plastics. There are three main metal finishing processes and several other surface finishing processes including:
Electroplating is the process of plating a metal part by passing an electric current through a solution with dissolved ions of the desired metal inside. This process works because the metal part serves as a cathode; the metal ions are then attracted to and stick to the part. Electroplating is commonly used in a wide variety of products from jewelry to the automotive industry.
Electroless plating is very similar to electroplating with the exception that the metal coating is deposited onto the part using chemical reactions instead of electricity. Electroless plating is commonly used in a wide variety of products as well, such as hard drives.
Chemical/Electrochemical conversion deposits a protective or decorative coating onto the metal part. Chromating, anodizing, passivation, phosphating, and metal coloring all fall under this chemical conversion category. Chromating uses hexavalent chromium to deposit a protective coating on metal surfaces and is often used on car parts to make them reflective. Anodizing immerses that part in a solution with metal salts or acids that react and form an insoluble metal oxide, providing corrosion resistance. Passivation involves immersing the part into an acid solution to prevent corrosion as well as extend part longevity. Phosphating is used to prepare a metal surface for painting by coating the surface with a spray or immerses it in a phosphate material such as zinc phosphate. Metal coloring is exactly how it sounds, chemicals are applied to the metal part to change the color of the surface.
Cladding is a process that is used when a thicker outer coating needs to be applied to a metal part that cannot be done using electroplating. The most common example of cladding are coins, such as the quarter, where metal is squished between two pieces of alloy using a combination of heat and pressure.
Case hardening involves heating and quenching the surface of the metal, creating a hard surface over a malleable metal inside. Case hardening is best used in objects that will be subject to high pressures and impacts like engine camshafts.
Galvanizing is primarily applied to iron and steel parts to protect the metal from corrosion. It involves dipping the part in a molten bath of zinc compounds that immerse the part and create a coating. Galvanizing is one of the cheapest methods to finish metal because it is easy to do and does not provide a very polished finish.
Electropolishing uses acid or alkaline solution to smooth the surface of a metal part by conforming to the contours in a process similar to electroplating. The exception is that the part acts as the anode instead of the cathode. Electropolishing can enhance corrosion resistance and improve biocompatibility for medical applications.
Metallic coatings are applied to the surface of base metals to provide protection and corrosion resistance. The process can be applied using various methods from vapor deposition to cladding. The two materials interlace because they are brought together at high temperatures. Some common metallic coating metals include aluminum, tin, zinc, and lead.
How are metal parts painted?
Metal parts can be painted in three different ways, e-coatings, powder coatings, and liquid coatings. Each coating has its own advantages whether its cost, durability, or aesthetics.
E-Coating, also known as electrophoretic deposition, is a process where a metal part is dipped into a liquid paint bath and then electrified to adhere it to the part. The thickness of the coating is highly adjustable because the amount of adhesion depends on the amount of electricity applied to the coating. This coating method is most commonly used to reach hard to access spots as well as in topcoat and primer applications.
Powder coating is one of the most efficient painting methods out there because all painting material is evenly applied throughout the surface of the metal part. The powder is electrostatically charged to create adhesion to the surface. Powder coating has excellent durability and scratch resistance which means it is perfect for applications on tire rims and other automotive parts.
Liquid coating is essentially just regular paint that is applied wet and left to dry. Liquid coatings are ideal for situations where a specific color is needed, where the coating needs to be smooth, or there is a lot of color variation. Liquid coating continues to be the most popular method for finishing metal parts, but powder coating and e-coating are starting to catch up.
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