How to polish a mold

Mold Polishing

For manufacturing a component of high surface finish and accuracy from an injection mold, it is essential to polish core and cavity of the mold so that the component can be easily ejected out after cooling without any scratch marks or deformation in the final component so that the finished product is of superior quality and pays well. Other advantages of mold polishing are reduced risk of local corrosion, fracture or cracking due to temporary overloading or pure fatigue. For molding transparent objects such as fiber lens or window for covers of electric meters, water bottles, injections used by doctors, mobile screens, etc. the core and cavity must be highly finished and so well polished that it reflects like a mirror. Though polishing is necessary, but it must not be over polished because it may deteriorate the shape of the cavity.

In some cases where mirror finish is not required then we can simply use high precision machines to produce well finished surfaces and save the expenditure in polishing-
Surface grinding machine
Cylindrical grinding machine
Lapping
Honing
Spark erosion
Die sinking
Pantograph
Reaming
CNC milling and lathe

If the surface finish from these operations is not sufficient then further polishing is required which is not a very easy process. Polishing demands high confidence and years of experience from the polisher and if the right tool and method is not adopted then howsoever experienced the polisher may be he can’t polish the surface adequately.

Rules for polishing operation
·        Dedicate a dust free room specially for polishing. Keep away all the chips and abrasive particles.
·        Polishing area should be very clean and moisture free. All the tools and accessories should be well arranged so that right tool can be selected as per requirement.
·        New wood and felt should not be used in between to avoid cross contamination of grit sizes.
·        Test every polishing stone on a scrap piece before polishing.

 Method of polishing
·        At first closely examine the mold surface and determine the fastest and most economical way to polish.
·        Inspect every high spot area and remove the material carefully with a cartridge wheel wrapped with an emery paper.
·        After this begin polishing with a wood with the help of polishing pate of soft stone mixed with the thinning oil. Any lubricating, non-corrosive mineral oil can be used as thinning oil.
·        Continue polishing with the harder stone further and diamond polish is the best for final polishing.
·        For having a mirror finish, further high grades of diamond polish are used subsequently ending at #1 grade diamond compound.

Polishing tips for mirror finish
·        An oil soluble diamond compound should be used.
·        A thinning oil which serves as lubricant is also used.
·        A vide variety wooden tools of balsa wood, hard wood, maple wood or brushes or felts are used as laps to develop the required finish.
·        Light pressure must be applied during diamond polishing.

After the polishing is complete, microscopic inspection is required to check for imperfections and scratches. The surface finish standards are often specified by SPI-SPE mold finish comparison kit. It consists of six steel disks polished to different levels. The surface roughness of the disks measured in RMS micro inches are described below.

Finishing number
Polishing techniques
Surface roughness
1
#3 Diamond 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
26.0 to 32.0
6
24 Grit Dry blast
160.0 to 190.0

There are several defects that occur while polishing. Their knowledge is essential for successful polishing.
Orange Peel This defect occurs because of over polishing, the defective surface must be removed with hand with the stone with the grit size finer than the last grit used.
Pitting Due to high pressure of polishing, the hard non-metallic particles produce small pits in the surface. Light pressure must be applied and excess polishing should be avoided to eliminate this problem.


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