Plastic parts are widely applied to various industries, like food, cosmetics, automotive, electronic consumer, and so on. Injection molding is a fast and inexpensive process. This process helps manufacturers create high volumes of identical plastic objects. The Surface of the molded part is important, an appropriate part surface improves good user experience. a designer will define the part surface before producing it.

Designers usually define the surface as per the product manager’s requirements. Some companies will demand an extremely smooth plastic surface for aesthetic reasons. Others may prefer a part with high roughness to obtain a longer lifespan or a plastic glossy texture. This article describes the most common surface options of SPI and VDI Surface Finish options for injection molded parts.

SPI Surface Finishing

There are a few finishing options for plastic injection molded parts. Surface finishing procedures are ways to increase or decrease the roughness of the mold to achieve the final effects. A glossy surface may be preferred for aesthetic parts such as cell phone cases.

SPI surface finishing standard is one kind of industry standard for plastic injection molding. The SPI standard sums up 4 grades of surface finishes, all these finishes relate to the aesthetic qualities of plastics. The categories range from smooth finishes to rougher ones. The following are the SPI surface finish types:

SPI divided tool finishes into 4 categories, ranging from glossy finishes to Matt ones, glossy means the tooling/molded part surface is smooth, on the contrary, that is rough. If you don’t have an aesthetic requirement for the plastic part, you may leave the part as molded, general polishing with abrasive paper #800. SPI finishes are beneficial for parts made from hard-steel tooling.

A Grades: Glossy Plastic part Finish

• A1 roughness: 0.016 μm, it is generally for Clear product
• A2 roughness: 0.032 μm, which is generally for Aesthetic product
• A3 roughness: 0.063 μm, it is generally for a visible part but not that important.

Polishing technicians utilize a special process(such as diamond buff polishing) to achieve a glossier surface finish. They use a loose abrasive material in the buff wheel. The steel surface is only with minimal aggression, to obtain the glossiest finishes thru this way.

The glossiest finish looks like a mirror surface, it is required on some optical products, like lenses. Only a few plastic materials can achieve a glossy texture, plastic acrylic and polycarbonate can be molded with a very smooth surface finish. As for soft materials like TPE or TPU, it is not possible to make glossy finishes, as they are abrasion-resistant materials.

B Grade : Semi-Glossy plastic part Surface Finish

• B1 roughness: 0.05 μm
• B2 roughness: 0.1 μm
• B3 roughness: 0.2 μm

If only slight glossiness is needed for the part, grade B finishes are available. This grade covers a wide range of injection molded parts. By polishing with different models of grit sandpaper to achieve the Grade B mold surface finishes. It also produces highly aesthetic parts suitable for mechanical products and other parts.

Grade C: Matte Injection Molding Surface Finish

• Min. roughness: 0.35 μm

For the ‘inside’ part, the surface finishes are not visible, these parts are hidden and work inside the products. The surface finish just needs to be without machining marks from molded parts. It still requires a consistent surface. Grade C is enough and easy for tooling. It looks matt and lackluster.

VDI Surface Finishing

On the other hand, rougher surface finishes may be more appropriate for certain mechanical components, the rougher finishes are able to reduce total costs.

Most of the current consumer electronics are with VDI texture or chemical texturing, some parts are made with coating. They have different features. VDI texture looks more consistent on the surface, VDI texture has better performance on scratch-resisting, but VDI is not suitable for large plastic parts, because EDM is not able to electrode-spark on large areas of mold. It is preferred for the small size of the plastic part. Chemical texturing has no limits on part sizes, but the texture is not that good on consistency and Scratch resistance.

It is a must to consider draft angles when a texture is decided for the plastic part, adding on draft angles will avoid scratch-broke during the de-mold process. Following we have a list that shows the VDI type of texture, Ra value, and recommended draft angle.

Why plastic material is important for the part surface finish

Most plastics are high-molecular polymers. Different molecular structures and heat-forming processes will lead to different material characteristics. There are dozens of commonly used injection molding materials. However, only a few types of plastic parts can get a similar injection molding surface finish. Some plastic parts are more amenable to glossy and smooth finishes, while some are ideal to roughen up for special touch-feeling of textured finish.

Plastic molded part Surface Finish for Functionality

A worthy surface finish of plastic molding is valuable, it will reduce molding defects like sink marks, welding lines, flow marks, and stress marks. The surface finish will look more homogeneous.

In addition, specific designation of plastic finishes promotes product impression. It also improves product durability by preventing scratches and scuffs. Meanwhile, high-level touch-feeling will give a good user experience.

What’s more, a proper plastic molding finish keeps part outer face withstanding contact damages in the process of transportation or work. Especially for the large molding part with textured surface finishes. For the painted/coated plastic part, a proper surface finish is good for paint adhesion and increases the product lifespan.

Conclusion

It is exquisite to choose an appropriate surface finish for the plastic component. Designers have to think thoroughly when he/she is at the design stage. Either functionality or aesthetics is the priority, it’s better to research and define before production. Surface finish will impact your tooling process, material options, and others.

Developlast is a plastic injection molding expert, contact the team for details discussion when you have any projects.