What Is Polishing Process?
Polishing Process
The surface quality of plastic products includes defects such as blemishes, streaks, indentations, air bubbles, and discoloration, as well as surface gloss and surface roughness. Many factors affect the surface quality of plastic products; the main factors are the machined surface quality of the mold and the molding process conditions of the plastic.
The machined surface quality of the mold, specifically the surface roughness of the cavity, is the main factor affecting the surface quality of plastic products. The relationship between cavity surface roughness and the surface roughness Ra of the part is mainly that the surface roughness Ra of the mold cavity is generally 1 to 2 grades higher than the surface quality of the plastic product. Currently, the surface roughness Ra of injection molded parts is generally 0.04 μm to 1.25 μm, and the surface roughness Ra of the mold cavity is generally 0.02 μm to 0.63 μm.
Transparent parts require the same surface roughness for both the cavity and the core. For opaque parts, the surface roughness may be greater for non-mating surfaces and hidden surfaces, depending on the application.
Depending on the requirements of the part, sometimes the molding parts of the mold need to be polished. Polishing is a very important process in mold manufacturing and is also the last process in mold part processing. As the demands for the appearance of plastic products increase, the quality of mold cavity surface polishing has also increased, especially for molds requiring mirror-like and high-gloss surfaces, which impose higher demands on mold surface roughness and thus on polishing. Polishing not only increases the cost of the mold but also improves the material's surface corrosion resistance and wear resistance, and facilitates subsequent injection molding production, such as making plastic products easier to demold and reducing the production cycle for injection molding processing factories.
The commonly used polishing methods currently include the following:
Mechanical Polishing
Mechanical polishing is a polishing method that achieves a smooth surface by cutting, plastic deformation of the material surface, and subsequent flattening of the raised areas. It mainly uses oilstones, wool wheels, sandpaper, etc., primarily by manual operation, with auxiliary tools like turntables for special parts such as rotating bodies. For high surface quality requirements, superfine grinding methods can be used. This technology can achieve a surface roughness Ra of 0.008 μm and is the best of all polishing methods. This method is often used for optical lens molds by professional injection molding mirror polishing suppliers.
To achieve high-quality polishing results, it's crucial to have high-quality polishing tools and aids such as oilstones, sandpaper, and diamond abrasive pastes. The choice of polishing procedure depends on the initial surface condition after prior processing, such as machining, electrical discharge machining (EDM), or grinding.
The general process for mechanical polishing used by most injection molding mold polishing factories is as follows:
Coarse Grinding
Surfaces after milling, EDM, grinding, and other processes can be polished using a rotary surface polishing machine or an ultrasonic grinding machine rotating at 35000 to 40000 r/min. The common method is to use 3mm or 4mm diameter wheels to remove the white layer of EDM, and then use manual sandpaper and abrasive oilstone for grinding, starting with stick-shaped oilstones. Oilstones are generally dipped in kerosene for lubrication or cooling. The typical sequence for sandpaper and oilstones is #180 → #240 → #320 → #400 → #600 → #800 → #1000. Many injection molding mold manufacturers choose to start from #400 to save time.
Semi-fine Polishing
Mainly uses sandpaper and kerosene, with sandpaper grits sequentially being: #400 → #600 → #800 → #1000 → #1200 → #1500. In reality, #1500 grit sandpaper is only suitable for hardened mold steel (≥52HRC and above), and is not suitable for pre-hardened steel, as it may damage the surface of pre-hardened steel parts.
Fine Polishing
Mainly uses diamond abrasive paste. Generally, a polishing cloth wheel mixed with diamond abrasive powder or abrasive paste is used for grinding. The usual grinding sequence is 9 μm (#1800) → 6 μm (#3000) → 3 μm (#8000). The 9 μm diamond abrasive paste and polishing cloth wheel are used to remove the scratch marks left by the #1200 and #1500 grit sandpaper. Then, polishing is performed with sticky felt and diamond abrasive paste, in the sequence of 1 μm (#14000) → 0.5 μm (#60000) → 0.25 μm (#100000).
Mirror Finish
Polishing processes above 1 μm (#15000 grade) can be carried out in a clean polishing room within the mold processing workshop. For more precise polishing, an absolutely clean space is required. Dust, fumes, dandruff, and saliva can potentially ruin the precision polished surface achieved after several hours of work.
The polishing of the cavity and core sections of an insert-type injection mold is done using the mechanical polishing method, grinding with manual sandpaper and abrasive oilstone. The sandpaper sequence for polishing is #240 → #320 → #400 → #600 → #800 → #1000.
Oilstones are dipped in kerosene for polishing.
The core before polishing is shown in Figure 6-11, where machining marks are obvious. Manual polishing is shown in Figure 6-12. The polishing process will not be described in detail here.
The core after polishing is shown in Figure 6-13. The production site processing video can be found in the resource library of China University MOOC "Mold Manufacturing Technology" (Course No.: 0802SUST006), in the "core polishing.mp4" file under the "case videos" folder.
Chemical Polishing
Chemical polishing is a process where the microscopic protrusions on the material surface preferentially dissolve compared to the depressions in a chemical medium, resulting in a smooth surface. The main advantage of this method is that complex equipment is not required; it can polish workpieces with complex shapes, polish many workpieces simultaneously, and has high efficiency. This method is often adopted by injection molding mold chemical polishing suppliers. The surface roughness obtained by chemical polishing is generally in the range of tens of μm.
Electropolishing
The basic principle of electropolishing is the same as chemical polishing, meaning it selectively dissolves the microscopic protrusions on the material surface to make the surface smooth. Compared to chemical polishing, it can eliminate the influence of anode and cathode reactions, resulting in a better effect. The electropolishing process is divided into two steps:
(1) Macro-leveling: Dissolved products diffuse into the electrolyte, and the material surface roughness value increases, Ra > 1 μm.
(2) Micro-brightening: Anodic polarization occurs, the surface brightness increases, Ra < 1 μm.
Ultrasonic Polishing
The workpiece is placed in an abrasive suspension and placed together in an ultrasonic field. The vibration of the ultrasonic waves causes the abrasive to polish and grind the surface of the workpiece. Ultrasonic processing has a small macroscopic force and will not cause deformation of the workpiece, but the manufacturing and installation of the tooling are more difficult. Ultrasonic processing can be combined with chemical or electrochemical methods.
Fluid Polishing
Fluid polishing achieves its purpose by relying on high-speed flowing liquid and the abrasive particles it carries to scour the workpiece surface. Common methods include abrasive jet machining, liquid jet machining, and hydrodynamic grinding.
Magnetic Abrasive Polishing
Magnetic abrasive polishing uses magnetic abrasives to form an abrasive brush under the action of a magnetic field, which grinds and polishes the workpiece. This method has high processing efficiency, good quality, easy control of processing conditions, and good working conditions. With suitable abrasives, the surface roughness Ra can reach 0.1 μm, and is increasingly used by injection molding precision mold polishing service providers.