What is plastic surface finishing? (Popular types 2020)
Most Common plastic surface treatment Methods
What is meant by surface finishing?
With the continuous development of technology, the surface treatment process of plastic products has been continuously upgraded. At present, after certain process treatment, the textured finish can even be comparable to metal, glass, and ceramics and it can also be both decorative and functional. Below we introduce several representative surface treatment processes.
In some applications, surface finishes refer to the process of removing any material residue visible on the top of a metal object following any production process (other than chemicals or soluble contaminants) in order to provide a smooth top that is safe to use and handle, for example, a component of an industrial system. The visual appearance of the product is of little importance, as long as it accomplishes its mission and does not present a risk for the operator.
In commercial applications or consumer products, the finished product can be enhanced with an aesthetic finish which can be perceived as adding value to the user. For example, a glossy finish on a laptop or the glossy look of a kitchen utensil.
What is the surface finish?
Surface finish refers to a wide range of industrial processes aimed at altering an object made of a non-ferrous material, typically made of stainless steel, plastic, wood, aluminum, glass, composite or other material. We do not consider painted surfaces since they use a covering material that is not of the same composition as the substrate.
Finish processes can be used to enhance the appearance of an object or to obtain specific properties: wear resistance, adhesion, wettability, weldability, electrical conductivity, hardness, friction, etc. Basically, plastic finish can simply refer to removing any material residue present on a manufactured item as a result of a previous process - like solder residue, laser cutting imperfections, burrs, edges, etc.
Types of surface finish
There are several types of surface finishes. Some finish processes are patented - which means that the recipe and parameters that allow a precise finish to be achieved are documented and protected by one entity - while others are only possible on certain substrates.
Here are the main categories of specialty surface finishes:
- Tarnished polish
- Satin polish
- Shiny polished
- With patterns
Some aesthetic finishes can be achieved with sandblasting processes. Here are a few examples:
Wet sandblasting - in which water is mixed with the abrasive media to soften the impact the top - produces a brushed or polished finish on electronic parts, equipment enclosures and various consumer goods.
Hammering with a glass marble or a steel ball are used to shot peen an object and give it a hammered appearance.
Sandblasting is also used to produce a frosted appearance on glass.
What is Sand Blasting?
By spraying a fine abrasive on the mold cavity to form irregularities, a cloudy texturing plastic is created. The surface finish roughness is controlled by the size of the abrasive particles and the strength of the shot. Sandblasting is generally used in sandblasting applications.
Surface finish preparation or from sandblast cleaning on steel products as a step prior to coating application. This technique can also be used to produce a specific finish on corrosion-resistant materials that do not necessarily require a protective coating, such as stainless steel.
Post-processing is performed after molding, but it is necessary to finish the mold in advance depending on the finished product.
Apply paint to the product and cover it with a paint film.
Immerse in an aqueous solution in which metal is dissolved to form a metal film on the product surface.
Thin film deposition
A thin film is formed on the product surface by evaporating metals and oxides, protective film is great for part that need be corrosion resistant.
Apply ink to the product and cover it with an ink film.
A special film is floated in a water tank, and the special film is transferred by the action of surface tension when the product is submerged.
Grindstone polishing is a polishing method in which the surface is polished by applying a workpiece to a rotating grindstone or by moving the workpiece while applying it to a fixed grindstone. Polishing machines such as sanders and grinders are mainly used, and sharpening a kitchen knife with a grindstone is also equivalent to grindstone polishing.
Electropolishing is a method of immersing a workpiece in an electrolytic polishing solution and applying an electric current to melt the surface and obtain a polishing effect. It utilizes the effect that the convex part is preferentially dissolved by electrolysis.
In addition to producing a high-precision flat surface finish, it is possible to polish parts that are too narrow to be polished by other polishing methods. However, it also has drawbacks such as limited applicable metals and high cost.
Laser engraving (laser marking): It is a process that uses optical principles to perform surfacing treatment. It is similar to screen printing and can be typed or patterned on the part surface through laser engraving.
This process uses soft-touch coating for features such as rubberized buttons and coatings, typically to improve the product’s ergonomic comfort.
The process that uses a laser beam to change the surface of an object and customize your parts
Screen printing is a printing technique used to transfer ink to a surface. An operator uses various instruments (paper, squeegee, etc.) to perform the process. Stencils may be used to prevent the transfer to certain areas. This technique is relevant to multicolored designs.
Pad Printing is a process of transferring a 2-D image onto a 3D part. The offset printing process requires the use of an etched plate (also known as a cliche) and pad to transfer the image
In-mold decoration technology (IMD)
IMD is also called in-mold decoration process. The printed film is put into the mold in different types, and after plastic injection molded parts, a molded part with a printed texture is obtained.
The injection-molded process technology combines product design, mold textures technology and traditional post-processing technology IMD is the current internationally popular surface finish decoration technology. It is mainly used in the decoration and functional panels of home appliances.
It is commonly used in mobile phone windows and casings, washing machine control panels, refrigerator control panels, air conditioning control panels, car dashboards, rice cooker control panels on appearance parts such as panels and signs in various fields.
Conceptually, IMD includes IML, IMF, and IMR. However, many domestic industry professionals regard IML as independent of IMD and call IMD the IMD process. IML and IMF processes are the same, which is classified into two categories:
The main reason is the difference between IML and IMR of the lens. IML has PET or PC sheets, while IMR has only ink on the top.
Norms and standards for surface finish
There are no specific quality standards or spi standards for surface finish as is the case with Surface roughness cleaning or from Surface preparation. However, it often happens that the finish to be obtained is documented and specified by the client in order to obtain consistency in quality.
In addition, certain processes are patented by manufacturers and cannot be reproduced or imitated for similar products.
However, it must be admitted that aesthetic finishes produced from Sandblasting lack documentation and consistency. The know-how behind these techniques rests more with the process or design engineers.
Abrasive media used for surface finishing
Spherical abrasive media like glass ball and steel ball are widely used for surface plastic finishes. Extra fine angular type abrasives can also produce interesting finishes on various metals, or glass.
The surface finish obtained from sandblasting is usually produced at relatively low pressure in order to avoid damaging the surface. Usually, a very fine EDM graining is used to evenly cover the surface and obtain a polished surface finish. Water is sometimes added to the mixture in order to soften the impact on the surface.
Metal surface finishing equipment
Several tools and machines can be used to obtain a specific metallic finish. Some hand tools such as sandpaper or hand sanders with a metal belt can be used to smooth sharp edges of metal parts. Polishers are excellent for producing polished finishes.
However, there is no other finishing equipment that is more efficient than sandblasting equipment when the application permits.
Metal finishes operations are generally carried out in sandblasting cabinet. Sandblasting cabinets are very practical for special surface finish processes since the operator can work freely close to his workpiece behind safety glass.
This type of equipment also has a very small footprint and can also be used for other types of surface treatment.
Parts manufacturing methods
Sheets and plates of thermoplastics are most often made by calendering - rolling on multi-roll mills. Corrugated sheets for honeycomb are also produced by rolling on profile rolls. Shaped products from sheets are obtained by pressing in dies with a rigid or elastic punch (air-filled rubber bag)
In pneumatic molding, a sheet blank, heated to a state, is clamped along the perimeter of the matrix, after which the blank is pressed onto the matrix by compressed air pressure. During vacuum forming, a vacuum is created inside the die, as a result of which the workpiece is drawn into the die, wrapping around its surface. In this way, shaped lids, open tanks, fairings, cocks, and other thin-walled products are made.
Pressing is used for the manufacture of shaped articles made of thermosetting plastics and durable thermoplastics. The starting material is tablets, granules, crumbs; for products with powder fillers - press powders.
The process is carried out in molds consisting of a matrix and a punch. Forming is carried out at an elevated temperature (the molds are heated), which provides material curing
A measured amount of preheated press material is poured into the matrix, after which a mechanical or hydraulic force is applied to the punch and the product is briefly held in the mold under constant pressure, as a result of which the material is cured. Then the punch is withdrawn, and the hardened product is removed from the matrix by pushers
Forming modes (preheating temperature, pressing temperature and pressure, holding time) depend on the formulation of the press material, on the size and configuration of the product, and are selected empirically. Typically, the preheating temperature is 130-180 ° C, the pressing temperature is 200-220 ° C, the pressing pressure is 100-300 kgf / cm 2 , the holding time is 15-30 s
The surface quality of the molded parts is high. With proper finishing of the forming surfaces of the matrix and punch (chrome plating, polishing), it is possible to obtain a roughness Ra = 0.080 - 0.160 μm
Plastic Injection molding process
Plastic Injection molding is used to mold thermoplastics. The starting material (granules, tablets) is heated until it softens completely. Casting mass of a fluid consistency is fed into a heated cylinder, from where it is squeezed out by a piston through sprue channels into cooled metal molds.
After cooling and solidification, the mold is opened and the castings are removed by pushers. Gates and burrs formed in the cavity of the mold part are chopped off and cleaned. The softening point of the casting compound depends on its composition. Pressing pressure 1000 - 1500 kgf / cm 2 . Mold temperature 20-40 ° С
Plastic Injection molded parts technology is more productive and provides a higher and more uniform product quality than pressing. It is possible to obtain surface roughness of Ra - 0.020 - 0.040 μm.
To eliminate internal stresses and increase the homogeneity of the structure, the castings are subjected to normalization: heating without air access (usually in mineral oil) at 140-160 ° C for 1.5-2 hours, followed by slow cooling
Rules for the design of plastic parts
When designing molded parts, start by identifying the parting plane that defines the configuration of the part, the direction of the molding slopes, the location of the holes and the placement of the reinforcement.
Typically, the shape should have only one parting plane.
The multi-plane connector complicates the design. Extra connectors in a direction perpendicular to the direction of the main connector are especially undesirable.
It should be borne in mind that the positive, i.e., convex parts of the shape that form the inner surfaces of the part, are easier to process than the negative ones (matrix nests) that form the outer surfaces of the part.
Therefore, it is recommended to transfer all complex profile elements of the part to the inner surfaces, trying to give the outer surfaces the simplest outlines. It is recommended that the profiled surfaces be positioned so that the forming surfaces can only be made in the matrix.
Choose the textured finish that suits your material
Different surface finishes can be applied to different materials. Below is a brief table for decision support on surface condition and material compatibility.
- Last name Material compatibility Approximate price increase *
- Smoothing All plastics and metals + 10%
- Shot blasting All metals + 5%
- Powder painting All metals + 15%
- Transparent anodization (type II) Aluminum and titanium alloys + 15%
- Color anodization (type II) Aluminum and titanium alloys + 20%
- "Hard coating" anodization (type III) Aluminum and titanium alloys + 30%
- Electrolytic polishing All metals + 15%
- Black oxidation Stainless steel and copper alloys + 10%
- Chromate conversion coating Aluminum and copper alloys + 10%
- Brushed finish All metals + 5%
What is plastic graining?
The graining of plastic parts is obtained directly in the plastic injection mold.
The surface of the injected plastic parts is either polished or grained. Most plastic parts have a grained surface. They are less sensitive to scratches and the result is often more qualitative than on a mirror-polished part.
The grain is obtained by directly processing the cavity or the cavity of the plastic injection molding. Depending on the treatment, by sandblasting or by chemical etching, different effects and states can be obtained. The grain chosen is contrary to the manufacture of the part by plastic injection. In fact, the more pronounced the pattern, the more the surface will have to present a significant relief.
The polishing procedure is roughly divided into four, and the flow is as follows: base, smoothing, polishing, and mirror finishing. It is easy to think that the surface finish is important, but the most important process is the base process with injection molding. No matter how carefully other processes are performed, if a large amount of unevenness remains on the surface of the product at the time of the grounding, a product with high accuracy cannot be produced.
In addition, the processing method must be selected according to the shape and surface finishes to be processed. For selection, it is also necessary to know the characteristics of other processing methods. First of all, let's check what kind of material you want to process.
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