Mold Polishing Tips and Guidelines

There are many technical institute conducting courses for mold and die making. But there are no dedicated courses for mold polishing as most of the experience polisher refuses to teach on this niche area.
Below are some guideline and tips for starting mold polishing:

 
1.
Closely examine the mold surface after the last machining; then determine the fastest and most economical way to polish the required surface finish.
Methods and sequence of mold Polishing greatly rely on the surface conditions and the correct usage of stones, abrasives and right equipment.

2
Inspect the “high spot” area and remove them carefully with a cartridge wheel containing an emery paper wrapped around a mandrel and mounted on a shank.

3
Begin stoning using “soft stones” containing Aluminum Oxide. Stoning must be performed wet with mineral lubricants. Perform the initial stoning using hand tools with 320 grit stone, followed by the higher grade 400 grit and 600 grit stones. Stoning must be done in the direction of ejection to create a draw finish.

4
On final Polishing, Use diamond polishing with #15 diamond compound, subsequently with #6 and finished with #3 diamond compound.

For optical quality finish, polish with #1 diamond compound.
Its highly recommend to use quality diamond compound like, KEMET, ENGIS.


 Polishing Tips for Diamond Finish


1.

An oil soluble diamond compound is preferred

2.
A thinning oil which serves as a lubricant can be used during diamond polishing

3.
A wide variety of tools are used for application of diamond compounds. In addition to brushes and felts, different type of wood ( eg. Balsa wood, hard wood , maple wood) serve as laps to develop the final finish required on the mold surface.

4.
Applied light pressure during diamond polishing so as not to produce a burnishing effect


Successful Mold Polishing Guidelines
1.
Dedicated a room specially for mold polishing. Keep away all dust and chips from grinders and blasting beads. A small grinding grits is enough to ruin the polish surface.

2
Practice high standard of cleanliness in polishing area, refrain loose stones and accessories laying everywhere on polishing table.

3
Discard and replace new felt, wood etc to avoid cross contamination of grit sizes.

4
To become a professional polisher, test every stone on a scrap piece of steel, press down as hard as you can and gradually work your way up to the finest polish you can get out of that stone. Teach yourself to know exactly what the total range is of each stone.

5
Organize your Tools, stone and accessories in an orderly manner. Have a dedicated storage compartment for different grits of stones and diamond compounds.

6
Make a chart of stone / diamond paste color in front of the polishing area. That is to make sure that correct grits is used at all time.

7  
Inspection with Microscope to check for imperfection and scratches.


Mold Finish Standards
The surface finish standard on molds is often specified by the SPI-SPE Mold Finish Comparison Kit. It consists of six steel disks polished to different levels. The polishing technique employed to obtain the six different mold finishes, and the surface roughness measured in RMS micro inches are shown in below :

Finishing numbers Polishing Techniques Surface Roughness/RMS
1 #3 Dimaond Polish 0.5 to 1.0
2 #15 Diamond Polish 1.0 to 2.0
3 320 Grit Abrasive Cloth 7.0 to 7.5
4 280 Grit Abrasive Cloth 12.0 to 15.0
5 240  Grit, Dry Blast
(5 inch distance 100psi)
26.0 to 32.0
6 24 Grit, Dry Blast
(3 inch distance 100psi)
160.0 to 190.0
 

             

Polishing Defects and Remendies

Defect Cause Remedies
Orange Peel
This surface irregularity results from over polishing with too much pressure


Polish by hand to remove the defective surface by stone with grit size finer than the last grit used.

Pitting
Small pits occur in a polish mold surface when hard non metallic inclusions are dislodged from the surface during polishing

Apply light pressure during polishing. Avoid polishing in excess of that which is required to remove marks made by the preceding grit.

Select clean mold materials to minimize the number of inclusion

 


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