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Everything You Should Know About Surface Molding and Finishing

When you talk about making plastic parts, additional processing may be needed before the completion of these parts. These processing requirements are needed so that the overall aesthetics and the appearance of the product can be improved and made as per the set industry standards. One of the many processes that are used for this purpose is the use of surface textures for mold surface. If you want a product with a good appearance surface finish is one of the most important factor.

Surface finishes are also usually responsible for the overall performance of the plastic product. As such surface finishes are responsible for both aesthetic and functional aspects of your product. It is therefore important that you understand that mold texture is a very crucial factor in determining the look, feel and the overall performance of your products. In this piece we will walk you through everything that you need to know about surface molding and finishing.

Let us begin by understanding what Surface Finishes really are.

surface molding and finishing

What is Surface Finish?

Surface finish is also known as surface topography or surface texture and is used to describe the nature of any surface. The surface finish is about defining the surface by allowing certain deviations in the surface and from being a true plane or a perfect surface. In simple terms it is the three dimensional form of a given product or solid. As such you will always find some kind of texture on every kind of surface.

The texture that is seen on the surface is formed as a result of some repeating patterns that are either regular or discreet. Texturing generally involves creating an array of certain recognizable patterns and these patterns in fact form the surface of these products. The various techniques that are followed in order to create these surface textures are known Surface Finish processes. The surface texture or surface finish are usually described using the following terms that are:

Lay – This is used to describe the general direction of the pattern. This is achieved using the various processes such as sanding, grinding, milling, etc.

Waviness – The next term that is generally used to describe a surface texture is waviness. This is achieved when a machine tool used to create the texture moves over a large area of the product. As such a waveform kind of texture is created on any piece of work due to the repetitive cyclical movement. This is usually achieved from a CNC machine as it is rolled over the product surface.

Roughness – You may get a product surface that is rough. This is either obtained by deliberate cutting to create a rough effect or can be result of chaotic, random cutting.

SPI Surface Finish Standards Explained

SPI is the Society of Plastics Industry, that is responsible for setting standards for the plastic industries in the US. The surface of any plastic product manufactured in the United States need to be made as per the SPI standards for surface finish. These standards are set to help determine and identify the aesthetic aspects of the plastic parts and materials used.

There are 12 different grades defined by SPI for surface finish of plastics. These 12 grades are further divided or categorized under four different categories. Each of these categories have a permissible extent or deviation from the perfect. These categories can further be categorized into:

Shiny Finish

This category includes the top grades which are A-1, A-2, and A-3. These grades mean the product is supposed to have a high gloss finishing and an extremely hardened mold. The molds are made of steel which are buffed in order to lend a mirror luster using diamond powder in oil.

The surface finishing that is obtained in this category and the given grades are smooth, shiny and are usually expensive. The plastic products made in this grade include visors, optical parts or plastic mirrors.

Semi-Gloss Finish

The grades in these category include SPI grades that are next to the A grades, which are B-1, B-2, and B-3. This category provides the specification for products that need to be manufactured with semi-gloss surface finish. The semi-gloss finish is obtained using some hardened tools made from steel. These molds are polished using super-fine sandpaper. Some of the manufacturers also use emery cloth to manufacture these semi-gloss finish products. Plastic parts that are finished using this process do not show any molding defects.

Matte Finish

The products given a Matte finish include SPI grades C-1, C-2, and C-3. This is the category that specifies matte finish in the steel molds. The surfaces are polished using finely ground stone powder. The finish obtained in these grades do not show any machining or tool defects. They usually find their applications majorly using thermoset and die cast industrial parts. Matte finish is a pretty popular finish in most plastic products.

Textured Finish

The last type of finish is the textured finish that is categorized under SPI grades D-1, D-2, and D-3. Under this grade the molder is first used to give a fine finish using the stone powder. The molder is then used to do dry blasts using aluminum oxide. Glass beads are also used at times to lend a textured finish. Such type of finish gives a very good grip to the product. Textured finish is also good in terms of hiding any injection molding surface finish defects. The textured finished products are usually also sold well for their fingerprint and smudge free feature.

Now that we know about the various standards for surface finishes, let us now look at how the surface finish type affects the cost of the finish.

How Does Surface Finishes Affect Cost?

It is no surprise that various types of surface finishes are obtained using various types of processes. And the cost of obtaining the desired surface finish depends directly on the process, the machines and the materials used. So, in other words it is clear that finishing techniques that use more men, machines, or materials will cost more and others will cost less.

A typical example for the same can be Shiny finish. The materials used to obtain a shiny finish include things such as diamond powder. As such it is expensive to manufacture products with a shiny finish. Similarly, textures and surface finish obtained through the polishing process will be expensive as it does involve a good amount of human expertise and machinery. On the contrary, if you wish to go for textured finish, the products may not be that expensive. That is because textures can be obtained using s simple surface molding process using something as inexpensive as a sandpaper.

As such the surface finish and the processes used to obtain the finishing does affect the cost of the final product. Let us discuss the factors that you should consider while choosing mold surface structures.

Factors to Consider When Choosing Mold Surface Textures

When you are looking to manufacture high quality plastic products with the right kind of surface finish as per their usage, you must make sure you use the right mold to give the desired surface texture to your products. This may depend on a lot of factors and as such you must speak to your design expert or engineer to ensure you know what texture you need for the product you are manufacturing. Knowing what is actually required ensures you get the desired outcomes in terms of functionality as well as the design aesthetics.

To make it easier here are certain factors that will help you choose the right mold texture for your products.

Appearance and Function of the Product/Part

The mold texture should help you to improve the appearance of your products along with the functionality. It is therefore important to understand the level of aesthetic improvements you may need for the parts you are making. This will help you choose the grade you want while making sure the functional part of it is also taken care of.

Choice of Material

Different materials and chemicals used will have different effect on the surface texture. Properties such as the melting point of the material used can have a good impact on your product’s surface finish. Then there are certain additives that can improve the quality of your finish. This makes it important to that you choose the right materials that can be used to obtain the desired surface texture.

Injection Speed and Temperature

The injection speed of the material in its molten form also is a major factor in determining the surface finish quality. If you need a glossy finish, a fast injection speed would be needed. The temperature at which the mold is used is another factor to consider. Based on the surface texture you need these requirements cane be determined accordingly.


Surface textures and surface finish of any plastic product or part is extremely crucial when it comes to the overall look and feel and functionality of your product. It is therefore important to keep in mind the various surface textures, finish types and standards that can be used as specifications of your parts. Use this piece as a guide to understand what works best for your requirement and how you can go about making the perfectly finished plastic parts.

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