Mould Finishing-Nearly all cavity and punch forms require a Finish to be applied to them. This may be a Polished finish, a photo-etched finish or a plated finish. However, for some technical Components the surface finish is not important and they are often left with a Fine EDM Finish, Where special finishes are required, there are several processes available including:
- Chromium plating
- Photochemical etching
- Bead blasting
- Vapor blasting
The correct steel must be selected if a very high surface finish is required on the core/cavity. It must be free of impurities and stresses and have low content of sulfur (often added to improve machinability).
Electro slag refining (ESR) is a process used by suppliers of mold steels to reduce the amount of impurities in the steel. Vacuum de-gassed steels are also used in an effort to prevent the formation of localized impurities or pits in the surface of the steel, which can spoil a polished finish.
The polishing process is a slow laborious procedure, which involves three distinct stagers:
- Coarse: using abrasive papers, carborundum paste or a mechanical reciprocating polishing device such as a Diprofile.
- Intermediate: using fine abrasive stones
- Finish: hand polishing with diamond paste and orange sticks or with a powered mechanical reciprocating hand tool
All molding surfaces should be highly polished in line of draw for cavities and punches unless an alternative finish is required. The sprue bush and runners should also be highly polished in line of flow to assist streamlined flow of the polymer and to prevent any tendency for these to ‘stick back’.
Chrome plating can be used to improve hardness and corrosion resistance and improve release of moldings- The degree of finish is very much dependent on amount of polishing prior to placing. Plating will never improve a surface, it will only accentuate a poor finish.
With very corrosive materials like PVC and certain types of thermoplastic rubbers. it is usual to use stainless steel to combat the corrosion. Unfortunately even the best stainless steel alloys have impurities and localized ferrous (iron) areas in them. These will soon be attacked by corrosive materials, resulting in pitting on the surface of the steel.
With long- running tools it advisable to Chromium plate the cavity areas made from stainless steel to protect against this possibility. If a lion-stainless mold tool is to be used for molding corrosive materials, the molding surfaces must be chromed plated and the rest of the tool flash chrome plated to protect it.
|Finish||SPI* standard||Finishing Method||Typical surface roughness Ra (μm)|
|Super High Glossy finish||A-1||Grade #3, 6000 Grit Diamond Buff||0.012 to 0.025|
|High Glossy finish||A-2||Grade #6, 3000 Grit Diamond Buff||0.025 to 0.05|
|Normal Glossy finish||A-3||Grade #15, 1200 Grit Diamond Buff||0.05 to 0.10|
|Fine Semi-glossy finish||B-1||600 Grit Paper||0.05 to 0.10|
|Medium Semi-glossy finish||B-2||400 Grit Paper||0.10 to 0.15|
|Normal Semi-glossy finish||B-3||320 Grit Paper||0.28 to 0.32|
|Fine Matte finish||C-1||600 Grit Stone||0.35 to 0.40|
|Medium Matte finish||C-2||400 Grit Stone||0.45 to 0.55|
|Normal Matte finish||C-3||320 Grit Stone||0.63 to 0.70|
|Satin Textured finish||D-1||Dry Blast Glass Bead #11||0.80 to 1.00|
|Dull Textured finish||D-2||Dry Blast #240 Oxide||1.00 to 2.80|
|Rough Textured finish||D-3||Dry Blast #24 Oxide||3.20 to 18.0|
plastic mold polishing
With a wide range of high surface demmand plastic products‘applications increasing,especially such as optical lenses and CD, they require the appearance of the surface of the plastic mold cavity often to achieve the degree of mirror-polished, thus polishing requirements are very high. plastic mold polishing also has other advantages, such as ease of stripping , reduce injection cycles of plastic parts. Thus plastic mold polishing process is a very important step as to whole plastic mold manufacturing.Plastic mold polishing has 6 methods :
mechanical polishing is finished by cutting and removing the protrusions and get a smooth surface, polishing method generally uses Whetstone, wool wheel, sandpaper, etc., mainly by manual.some special parts with rotors surface need use the turntable and if the other tools require high surface quality ,it may need ultra-precision polishing. ultra-precision polishing adopt a special abrasive, the polishing liquid containing abrasive pressed against the surface of the workpiece to be machined with high speed rotation. this technique can achieve a surface roughness Ra0.008μm which is the highest precision polishing methods. Optical lens mold often use this method;
chemical polishing is to allow the material in the chemical medium with convex portion get dissolved firstly, thereby to obtain a smooth surface. The main advantage of this method is that this method does not require complicated equipment, the complex shape of the workpiece can be polished, many workpiece can be polished simultaneously , high efficiency. main problem is how to formulate the proper chemical polishing solution. Chemical polishing the surface roughness is general 10μm;
electrolytic polishing and chemical polishing have the same basic principle, namely by selectively dissolve the small convex portion on the surface , so as to smooth the surface. Compared with chemical polishing, it can eliminate the influence of cathodic reaction .
Electrochemical polishing process is divided into two steps: (1) macro-leveling :Solubility spreads to the electrolyte material ,refine rough surface geometry, Ra> 1μm. (2) Micro flat; anodizing, surface brightness increase, Ra <1μm;
the workpiece soaked into the abrasive solution and placed in an ultrasonic field together, for the ultrasonic oscillation, the abrasive on the workpiece surface could grind and polish. Ultrasonic machining force is small, so as not to cause deformation of the workpiece, but the tooling fabrication and installation are difficult. Ultrasonic polishing may combine with chemical or electrochemical method. After the etching solution and the electrolysis, the solution was stirred and then ultrasonic vibration is applied, so that the solute on the surface would fall off ;
fluid polishing is a method that high-speed flow liquid and its carrying abrasive wash surface to achieve the purpose of polishing. Common methods: abrasive jetting polishing, liquid jetting polishing, hydrodynamic grinding. Hydrodynamics grinding is driven by hydraulic fluid carrying the abrasive medium to reciprocate through the workpiece surface;
Magnetic polishing is to use magnetic abrasive form an abrasive brush in the magnetic field of the workpiece grinding. This method is with high processing efficiency, good quality, easy to control the processing conditions, . If using a suitable abrasive ,surface roughness can be achieved Ra0.1μm.