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.

GLOSSY SURFACE, DIAMOND BUFF POLISH

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

NON-GLOSSY SURFACE, PAPER POLISH

SPI Finish B-1 — 600 Grit Paper

SPI Finish B-2 — 400 Grit Paper

SPI Finish B-3 — 320 Grit Paper

ROUGH SURFACE, STONE POLISH

SPI Finish C-1 — 600 Grit Stone

SPI Finish C-2 — 400 Grit Stone

SPI Finish C-3 — 320 Grit Stone

VERY ROUGH SURFACE, DRY BLASH POLISH

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.

341 Responses

  1. Gordon Crowder says:

    Pretty section of content. I just stumbled upon your site and in accession capital to assert that I acquire in fact enjoyed account your blog posts. Any way I will be subscribing to your augment and even I achievement you access consistently quickly.

  2. Jae Voto says:

    Excellent goods from you, man. I have understand your stuff previous to and you are just extremely great. I really like what you’ve acquired here, really like what you are saying and the way in which you say it. You make it enjoyable and you still care for to keep it smart. I can not wait to read much more from you. This is actually a tremendous site.

  3. Lamar Devera says:

    Wow! Thank you! I continually wanted to write on my site something like that. Can I take a portion of your post to my blog?

  4. Ignacio Harbor says:

    I actually wanted to jot down a simple word to say thanks to you for the precious tips you are writing on this website. My incredibly long internet research has finally been rewarded with reliable know-how to share with my close friends. I would assert that most of us website visitors actually are extremely blessed to dwell in a fine place with many wonderful individuals with interesting suggestions. I feel extremely fortunate to have discovered your web page and look forward to so many more exciting minutes reading here. Thank you again for everything.

  5. Natalia Carini says:

    I have been browsing online more than three hours today, yet I never found any interesting article like yours. It is pretty worth enough for me. Personally, if all website owners and bloggers made good content as you did, the web will be much more useful than ever before.

  6. Brynn Smithhart says:

    I recently came across your article and have been reading along. I want to express my admiration of your writing skill and ability to make readers read from the beginning to the end. I would like to read newer posts and to share my thoughts with you.

  7. Shandi Brasfield says:

    Wow, what a neat blog. I loved your narrating and will bookmark the article.

  8. Denver Tastet says:

    I just want to tell you that I am all new to blogging and absolutely enjoyed you’re website. Probably I’m likely to bookmark your website . You definitely have excellent articles and reviews. Thanks a lot for sharing with us your blog site.

  9. Annika Scifres says:

    Very efficiently written post. It will be supportive to everyone who employess it, as well as myself. Keep up the good work – can’r wait to read more posts.

  10. Alyson Gelineau says:

    Of course, what a splendid site and instructive posts, I will bookmark your blog.Best Regards!

Leave a Reply

Your email address will not be published. Required fields are marked *