SPI Surface Finishes Basic Guide

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SPI Surface Finishes Basic Guide

SPI Finishing

The SPI, founded in 1937, has been instrumental in developing industry standards and guidelines for various aspects of the plastics industry.

Over time, the SPI surface finish standards have been refined and widely accepted as a crucial part of communication between designers, manufacturers, and clients. They provide a clear and standardized way to specify the desired appearance and texture of injection-molded plastic parts.

What is SPI Surface Finish Standards?

Acorn-Gencon Plastics uses standards developed by The Society of the Plastics Industry (SPI) to define the type of polishing for plastic products. SPI is a plastics industry association in the United States that sets industry standards. Among them, a standard specifies the polishing method of molded plastic products.

The American Plastics Industry Association (SPI) specifies 12 polish grades and divides them into four grades, from the brightest to the faintest. Each level has a different allowable deviation, with lower numbers allowing less deviation and higher numbers allowing more deviation.

The following list shows all the polished surfaces specified by SPI (Figure 1).

spi surface finishes chart.jpg

The purpose of mold polishing

Mold polishing is a very important process step, it can effectively improve the surface finish of the die, and then make the surface of the product more smooth, beautiful and beautiful. At the same time, mold polishing can also greatly improve the production efficiency and production quality of products. This is because in the plastic injection production, if the mold surface is not smooth enough, the product is easy to get stuck and cannot be demoulded, resulting in a decline in production efficiency and product quality.

The process of polishing the mold is usually to use tools such as whetstone, sandpaper, polishing paste to polish the surface of the mold cavity so that its surface can achieve the effect of bright as a mirror. The specific polishing method varies according to factors such as the material, shape and surface condition of the mold.

How to judge the surface quality of the mold?


When judging the polishing quality of the mold surface, we need to pay attention to two points, which are crucial to ensure the correct operation of the mold.

First of all, the mold surface must have an accurate geometry, and there must be no undulating long wavy patterns.

This phenomenon is usually caused by defects left by early grinding with grinding wheels or whetstones. These defects may have an adverse effect on the flatness of the mold surface, thus affecting the appearance and quality of the product.

Secondly, after the mirror treatment, the mold surface must be free of scratches, small holes, orange peel lines (orange peel) and micro-pits (pinholes) and other defects. These defects are usually caused by damage to the mold during the mirror treatment process. If there are these defects on the mold surface, it will affect the appearance and quality of the product. Therefore, when performing the surface treatment of the mold, extra care must be taken to ensure the integrity and flatness of the surface.

Normally, we use the naked eye to judge the surface quality of the mold, but sometimes there are difficulties. This is because the visual judgment will be biased, and the plane that looks very smooth is not completely flat in geometry. In this case, we need to use instruments for testing to ensure the flatness and quality of the mold surface. For example, optical interference technology can be used to detect the flatness and shape of the mold surface to ensure that the quality of the mold surface meets the standard requirements.

Common polishing methods

SPI surface finish

Mechanical polishing

Chemical polishing

Electrolytic polishing

Fluid polishing

Mechanical polishing

Mechanical polishing is a process of using mechanical equipment and abrasive materials to rub and polish the surface of the workpiece to improve the finish and beauty of the surface of the workpiece. This method improves the appearance quality and performance of the workpiece by reducing the surface roughness.

In the mechanical polishing process, different types of grinding materials are usually used, such as grinding wheels, grinding pastes, polishing wheels and sandpaper. These abrasive materials are driven by mechanical equipment to produce friction on the surface of the workpiece, and gradually remove the surface roughness and defects, making it more smooth.

Mechanical polishing is suitable for a variety of metal and non-metal materials, such as steel, aluminum, copper, plastics and ceramics. It is widely used in various industries, such as aviation, automotive, mold manufacturing, hardware processing and medical equipment.

Chemical polishing

Chemical polishing is a method that uses the principle of chemical reaction to treat the surface of metal or other materials to improve the finish and beauty of the workpiece surface. In the chemical polishing process, the workpiece is immersed in a specific chemical agent, which will chemically react with the uneven part of the surface, so as to remove surface defects and defects.

The main advantage of this method is that it does not require complex equipment, can polish the workpiece with complex shape, and can polish many workpieces at the same time, with high efficiency. The core problem of chemical polishing is the preparation of polishing liquid.

Electrolytic polishing

The basic principle of electrolytic polishing is the same as that of chemical polishing, that is, the selective dissolution of small protruding parts on the surface of the material makes the surface smooth. Compared with chemical polishing, the effect of cathode reaction can be eliminated and the effect is better.

Fluid polishing

Fluid polishing is a material surface treatment technique used to improve the surface smoothness and overall appearance of an object. It is driven by hydraulic pressure to make the liquid medium carrying abrasive particles flow through the surface of the workpiece at high speed, so that the particles produce turbulence and friction on the surface of the object, so as to achieve the polishing effect.

The medium is mainly made of special compounds (polymer-like substances) with good fluidity under low pressure and mixed with abrasives, which can be made of silicon carbide powder.

Common polishing methods

There are many factors affecting the polishing of the die surface, and the following are some of the main factors:

(a) Choice of mold material.

(b) Heat treatment process.

(c) Roughness of the die surface.

(d) The shape of the die surface.

(e) Polishing process.

(f) Selection of polishing tools

It is generally considered that the polishing technique is the most important influencing factor. Proper polishing technology with good quality tool steel and the correct heat treatment process to obtain a satisfactory finish. On the contrary, if the technology is not proper, even the use of the best quality steel will not achieve the effect of a high mirror.


Gavin Leo is a technical writer at Aria with 8 years of experience in Engineering, He proficient in machining characteristics and surface finish process of various materials. and participated in the development of more than 100complex injection molding and CNC machining projects. He is passionate about sharing his knowledge and experience.