SPI Mold Finish – Practical Approach

Mold Finish – SPI Standard and the practical approach

SPI is the short form of the Society of Plastic Industry, who establish standards for plastic industry and the SPI Plastic Engineering Handbooks is one of the most important reference for engineers.

SPI defines the polishing standard on mold finish and widely applied in US markets. It helps to avoid grey areas and measure the finest of surface in the unit of Rough Average (Ra).

Nowadays, China’s standard is very much in line with SPI. However, there is little help using Ra value in China mold making industry because there is little chance to have precise equipment at site to do the precision measurement. What we needed is a practical guideline to more or less achieve the desired mold finish. Below guideline is widely accepted.


SPI Finish A-1 — Grade #3, 6000 Grit Diamond Buff

SPI Finish A-2 — Grade #6, 3000 Grit Diamond Buff

SPI Finish A-3 — Grade #15, 1200 Grit Diamond Buff


SPI Finish B-1 — 600 Grit Paper

SPI Finish B-2 — 400 Grit Paper

SPI Finish B-3 — 320 Grit Paper


SPI Finish C-1 — 600 Grit Stone

SPI Finish C-2 — 400 Grit Stone

SPI Finish C-3 — 320 Grit Stone


SPI Finish D-1 — 600 Stone Prior to Dry Blast Glass Bead #11

SPI Finish D-2 — 400 Stone Prior to Dry Blast #240 Oxide

SPI Finish D-3 — 320 Stone Prior to Dry Blast #24 Oxide

However, the above guideline falls short because it does not mention what steel should be used. For coarse surface, it may be fine. For demanding high glossy mold finish, the mold steel can be critical and the high glossy cannot be achieved without proper steel.  

For molding process, a melt plastic resin will be injected into a steel mold at high pressure and the steel will always face the issue of wear off. To produce a glossy product, the mold has to be significantly hard so that the shiny smooth surface will not be deteriorated quickly and the glossy surface can be remained for a longer period of time. The finer the surface is needed, the harder and the finer the steel is required.

Below is a supplementary guideline regarding steel to be used.

SPI Finish A1 – S136 Hardness at 54HRC

(Generally called “Super High Glossy” in China)

SPI Finish A2 – 718HH or NAK80 Hardness at 40HRC

(Generally called “High Glossy” in China)

SPI Finish A3 – 718H or S136 Hardness 32HRC

(Generally called “Normal Glossy” in China)

It is true that the steel made in China are somehow less consistent than from overseas. Therefore, it is suggested to use imported steel for demanding quality mold or at least use higher grade of steel from China to provide some buffer for the desired mold finish that you need.

Lastly, a high glossy mold finish is not equivalent to a high glossy finished product. It is significantly subject to other factors such as plastic resin used, molding condition and mold design. For example, ABS will provide higher glossy than PP, high temperature molding condition and non-flat surface will help to provide a much glossy finished product.

337 Responses to SPI Mold Finish – Practical Approach

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  3. BiobioLaf says:

    In this case, you should expect a significant cost increase because it is very time consuming to polish deep, narrow slots in molds.

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