Mould Polishing

Need for Polishing:

1. To provide efficient flow of material in sprue & runner. 

2. To avoid the scratch mark on the product.

3. East removable of component from core insert in ejection time. 

Polishing Accessories:

Files:

1. Needle file – Polish for different shapes

2. Riffle file – remove small amount burrs from profile edges 

3. Diamond file – for polish hard materials and critical surfaces.

Grades depend upon the abrasive particles Zero finishing emery sheets used for fine polishing. 

Oil Stone: This is made from carborandum with abrasive bonds used for remove cutter marks. 

Emery Sheets: The corundum is fixed in the emery sheets. It is used where more finishing required. This is available in 80 – 1200 grades. Emery has the following designations: Coarse, Medium, And Fine 

Diamond Paste: Used in polishing operation. This is hardest forms of abrasives. High finishing operations are done. Different grades are: Low grade, Medium grade, Fine grade

Silicon Carbide: This is second hardest form of abrasives.  Silicon carbides produced in the form of abrasive paper sheets. 

Boron Carbide paste: (Lapping paste) is used for polishing. 

Emery powders: are used for polishing. Emery powder has softest abrasive particles. 

Lubricants: can be used for most type of polishing materials.

Polishing core and cavity: Two things are important when judging the surface quality of the mould. First the surface must have a geometrically correct shape, without any long microwaves. This macro shape derives mostly from the early grinding and stoning steps. Second, the mirror finish of the mould surface is normally judged by naked eye.  the surface must be free from scratches, pores, orange peel, pilling (needle, sticks, pinholes), etc. There are certain difficulties involved in a visual evaluation. A flat surface can look perfect even though it is not geometrically completely flat. Thus eye can be fooled. The finish can be judged in a most sophisticated manner with instrumental methods, e.g., Optical Interference Methods. The choice of grinding and polishing sequences is determined by the experience of the operator and the equipment at his disposal.  The properties of the material can also affect the sequence. In polishing two methods are used. In first method, a paste with a certain grain size is selected and a hard polishing tool is used initially, after which softer and softer polishing tools are used. In second method, a medium hard polishing tool is selected and a coarse paste is used initially. Then pastes of finer and finer grain size are used. A combination of these two methods is recommended. 

The following sequence can be followed:

1. Start with a hard polishing tool and a coarse paste. 

2. Then change to a softer polishing tool with same paste. 

3. Then use a medium hard polishing tool and a medium coarse paste. 

4. Change to a soft polishing tool with same paste. 

5. Finally, use a soft polishing tool and a fine paste. 

These are some steps to be followed depending upon surface of the job and skill of the worker or operator. 

1. The grinding operation must not generate so much heat that the surface structure and hardness of material are affected. 

2. Use only clean and free cutting grinding tools!  Soft stones should be used for hard surfaces. 

3. Between each change of grain size, the workpiece and hands should be cleaned in order to prevent coarse abrasive particles and dust from being carried out into the next stage with a finer grain size.

4. The finer, the grain size used, the more important is the cleaning operation between each size of emery changed. 

5. When changing to the next finger grain size, grind in a direction approximately 45o to the previous grinding direction until surface shows only scratches from the present grinding step. After scratches from the previous step have disappeared continue grinding for about 25% longer before changing to the next grain size. This is to remove the deformed layer. 

6. Changing direction is also important to avoid the formation of irregularities and relief patterns. 

7. When grinding large, flat mould surfaces, avoid hand operation of grinding discs. The use of a stone reduces the risk of obtaining large shape irregularities. 

8. The polishing tools usually become “imp regulated” and improve with time. 

9. Paste should be applied to the polishing tool in manual polishing, whereas in machine polishing paste should be applied to the workpiece. 

10. The finer the grain size, the less the thinning liquid. 

11. Polishing pressure should be adjusted to the hardness of the polishing tool and grade of paste. For the finest grain sizes, the pressure should be only the weight of polishing tool. 

12. Removal of large amounts of material requires hard polishing tools and coarse paste. 

13. Polishing should start in corners, edges.

14. Be careful that sharp corners and edges are not rounded off. Preferably use hard polishing tools.