The polishing described in the processing of Pet Bottle Preform is very different from the surface polishing required in other industries. Strictly speaking, the polishing of the mold should be called mirror processing. Not only does it have high requirements for polishing itself, but it also has a high standard for surface flatness, smoothness and geometric accuracy. Surface polishing generally requires only a shiny surface. The standard of mirror processing is divided into four levels: AO=Ra0.008μm, A1=Ra0.016μm, A3=Ra0.032μm, A4=Ra0.063μm. It is difficult to accurately control the geometric accuracy of parts due to methods such as electropolishing and fluid polishing. The surface quality of chemical polishing, ultrasonic polishing, magnetic polishing and other methods does not meet the requirements, so the mirror processing of precision molds is mainly based on mechanical polishing.

1 Mechanical polishing basic program

For high-quality polishing, the most important thing is to have high-quality polishing tools and accessories such as oilstone, sandpaper and diamond paste. The choice of polishing procedure depends on the surface condition of the previous processing, such as machining, EDM, grinding and so on. The general process of mechanical polishing is as follows:

(1) Rough throwing The surface after milling, EDM, grinding, etc. can be polished by a rotary surface polishing machine or an ultrasonic grinder with a rotational speed of 35 000-40 000 rpm. A commonly used method is to remove the white spark layer by using a wheel having a diameter of Φ3 mm and WA #400. Then there is a manual oil stone grinding, strip of oil stone plus kerosene as a lubricant or coolant. The general order of use is #180 ~ #240 ~ #320 ~ #400 ~ #600 ~ #800 ~ #1000. Many mold makers choose to start with #400 in order to save time.

(2) Semi-finishing Semi-finishing mainly uses sandpaper and kerosene. The number of sandpaper is: #400 ~ #600 ~ #800 ~ #1000 ~ #1200 ~ #1500. In fact, #1500 sandpaper only uses die steel suitable for hardening (above 52HRC), and is not suitable for pre-hardened steel, as this may cause burns on the surface of pre-hardened steel.

(3) Fine polishing Fine polishing mainly uses diamond polishing paste. If the polishing powder is mixed with a diamond abrasive powder or a polishing paste, the usual polishing order is 9 μm (#1800) to 6 μm (#3000) to 3 μm (#8000). The 9μm diamond paste and polishing cloth wheel can be used to remove the hair marks left by #1200 and #1500 sandpaper. The polishing is then carried out using a felt and a diamond paste in the order of 1 μm (#14000) to 1/2 μm (#60000) to 1/4 μm (#100000).

A polishing process with an accuracy of more than 1 μm (including 1 μm) can be performed in a clean polishing chamber in the mold shop. For more precise polishing, an absolutely clean space is required. Dust, smoke, dandruff and saliva can all leave high-precision polished surfaces that are obtained after hours of work.

2 Problems to be noted in mechanical polishing

Polishing with sandpaper should pay attention to the following points:

(1) Polishing with sandpaper requires the use of soft wood sticks or bamboo sticks. When polishing a round or spherical surface, use a cork stick to better match the curvature of the round and spherical surfaces. The harder wood strips are like cherry wood, which is more suitable for the polishing of flat surfaces. The end of the trim strip is adjusted to conform to the shape of the steel surface so that the acute angle of the strip (or bamboo strip) is prevented from contacting the surface of the steel and causing deep scratches.

(2) When changing to different types of sandpaper, the polishing direction should be changed from 45° to 90°, so that the stripe shadow left by the polishing of the former type of sandpaper can be distinguished. Before changing the different types of sandpaper, the polished surface must be carefully wiped with 100% pure cotton and a cleaning solution such as alcohol, because a small gravel left on the surface will destroy the entire polishing work. This cleaning process is equally important when polishing from sandpaper to diamond polishing. All pellets and kerosene must be completely cleaned before polishing continues.

(3) In order to avoid scratching and burning the surface of the workpiece, special care must be taken when polishing with #1200 and #1500 sandpaper. It is therefore necessary to load a light load and polish the surface using a two-step polishing method. When polishing with each type of sandpaper, it should be polished twice in two different directions, with each rotation between 45° and 90°.


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