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Post-Finish for Injection Molding Parts

Injection molding is a popular manufacturing method to create plastic parts with extremely accuracy and consistence in high-volume production runs. Injection molding process is fast, cost-effective, and versatile. When the injection molding process is finished, there are many different post-processing and finishing options available to choose from. With injection molding post-processing, appearance of injection molding parts can be improved, aesthetic flaws can be removed and even additional mechanical proper can be provided. For example, post finish enhances strength or electrical conductivity. Anyway, without any injection molding surface finish or post-processing treatment, injection molding parts have smooth surface finishes are suitable for many final uses. Today, we will discuss some post-finish for injection molding parts that TEAM Rapid offers.


Injection Molding Surface Finish Standards

At TEAM Rapid, we offer post-finish options which match the Society of the Plastic Industry (SPI) standard. SPI standard defines the standards for the plastics industry which include the cosmetic quality of plastics. SPI standard includes the type of plastic injection molded parts polish or plastic injection molded parts finish.

The SPI surface finish standards go from most fine to least fine, below is an alphabetical grading system:

A – Glossy finish from Grade #3, Grade #6, or Grade #15 diamond buffing

B – Semi-glossy finish from 600, 400, or 320 grit paper

C – Matte finish from 600, 400, or 320 stone tooling

D – Textured finish from dry blast using glass beads, # 240 oxide, or # 24 oxide

Based on the SPI level, surface appearance, tactile quality, and production costs are different. For example, injection molding parts with highly polished finishes will cost more because of the increased tooling and precision required. At TEAM Rapid, we offer any SPI standard for a glossy, matte, rough injection molding surface finish.


What are the Available Injection Molding Surface Finish Options?

There are several surface finish options available for custom injection molded parts. Surface finishing help to increase or decrease the roughness of an injection molding part. Aesthetic parts like toys may prefer glossy texture. Certain mechanical parts may prefer rougher finishes. SPI establishes industry standards for plastic injection molding surface finishes and features. According to the SPI standard, there are 12 standard plastic surface finish types.



Grade A: Glossy injection molding surface finish

Min roughness is 0.012μm

Injection molding parts can be glossy by a special finishing process like diamond buff polishing. In polishing process, manufacturer uses a loose abrasive material to a work wheel. Then, apply this to the surface of injection molding parts with minimal aggression, which contributes to the glossiest injection molding part finishes.

Not all plastic materials can achieve a glossy texture. Acrylic and PC can have a very smooth finish by bluffing process. But it is impossible to apply such a finish to a material like TPU as these plastic materials have high abrasion resistance so they are resistant to the buffing process.


Grade B: Semi-Glossy Injection Molding Surface Finish

Min roughness is 0.05μm

If the injection molding parts need glossiness, B-grade finish with grit sandpaper is an ideal option. Semi-glossy process is compatible with a wide rang of injection molding plastic. Semi-glossy process creates parts with high aesthetic which are suitable for customer products.


Grade C: Matte Finish for injection molding surface

Min roughness is 0.35μm

Machining marks can be removed from the injection molding parts without making a glossy finish by grit sanding stones. Matte finishing process is suitable for injection molding parts that don’t have aesthetic value but need a consistent surface finish. Although providing high quality injection molding surface texture, SPI surface finish shows no noticeable divots or marks. Most injection molding plastic can use stone polish process for post finish.


Grade D: Textured surface finish for injection molding surface

Min roughness is 0.80μm

In some certain cases, injection molding parts need a very rough surface finish. Some injection molding parts need to increase friction for a mechanical purpose. Then, a rough surface finish by using sandblasting process is needed. During this roughing process, sandblasting use compressed air to direct abrasive material against the parts with force, the surface is roughed.


What affect injection molding surface finish?

Injection molding parts with finished is the good way to achieve maximum control over the texture of a plastic part. Here are some steps to manipulate the surface finish of an injection molding part before the molding cycle starts.

Injection Molded Parts


1, Tooling

When it comes to doing post finish to injection molding parts, the mold itself plays an important role. There are many metals available to make a mold, the most common materials are steel and aluminum. Steel and aluminum have different effect on the injection molding parts.


The smoothest surface finish can be achieved by using injection molds made from hardened steel. If an injection molding part have an aesthetic purpose, then, a low level of surface roughness is needed. An injection mold make from steel may be the best way to go.


2, Material

There are a wide range of injection molding materials available. But not all injection molding material can get the same injection molding surface finish. Some plastics are easier to rough up for a textured finish. And some are amenable to smooth finishes. Also, some material additive such as filler or color will affect the surface finish of injection molding parts.


3, Process

During injection molding process, speed and temperature can affect the surface finishes as well.  A fast injection molding speed and high melt temperature help to achieve glossier injection molding surface finishes with fewer weld lines. A fast filling through mold cavities will reduce weld line visibility which improve the overall appearance of injection molding surface finishes. A smoother SPI surface finish can be achieved by a combination of High mold and melt temperature and high injection speed.


4, Functional reasons

Some functional aspects will affect injection molding surface finish. If you want the paint sticks better to the injection molding parts, a texture finish is needed. In this case, a smooth SPI surface finish may cause the paint to peel off.


In some case, injection molding parts need solid gripping for better performance. Injection molding parts with texture finish will enhance the quality of a grip. Injection molding parts have texture have great slip resistance property. A textured SPI surface finish will add strength to the injection molding parts and enhance their safety levels.


Injection Molding Surface Finish for Functionality

There are several reasons to choose specific injection molding surface finishes. Injection molding surface finish can hide injection molding defects like weld lines, sinks, blush marks, flow lines, and more. Injection molding surface finish help injection molding parts to withstand contact damages during shipping or use. Especially for those with textured surface finishes. A textured injection molding surface finish also improves paint adhesion. Injection molding surface finish also improve the aesthetics of the products. It improves durability by preventing scuffs and scratches.



There are many advantages to choose the suitable injection molding surface finish for your injection molding parts. You need to do this as early as possible during design process as surface finish will affect the tooling process, material choice, cost and other decisions. It would be helpful to work with an injection molding expert to make sure you make the right choice. Contact our experienced team at TEAM Rapid today.

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