Mastering Surface Finishes in Injection-Molded Part Design

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Injection molding surface finish is critical to a successful part design and used for aesthetic and functional reasons in injection molded parts for engineering products. The surface finish improves the look and the feel of a product as the perceived value and quality of the product increase with a suitable surface finish.

Many product designers would agree that the product’s aesthetic appeal heavily relies on the product’s look and how it feels in the consumer’s hands. Identifying the right surface finish is dependent on a few important elements. These elements include the development or production stage that your parts are in, the materials they’re being manufactured in, and their end-use application.

That said, understanding surface finishes is vital for your injection molded project. In this article, we will look at surface finishes in injection molded parts. 

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Injection Molded Plastic Caps with Smooth Surface Finish (Image Source: Shutterstock) 

The Importance of Surface Finishes

There are various reasons as to why you need to use surface finishes in injection molding. Here are some of them: 

1. To increase part aesthetics

Part designers may use textures for a variety of aesthetic purposes. A smooth or matte surface texture improves its appearance and gives it a polished aspect. It also covers faults generated by injection moldings, such as tool machining marks, sink marks, weld lines, flow lines, and shadow markings. Parts with excellent surface quality appeal more to customers from a business standpoint.

2. To Improve Part Functionality

Besides the aesthetic considerations that go into selecting an injection molding surface finish, there are also important practical considerations.

The design may necessitate a firm grip for optimal functioning. Textured plastic finishes improve grip quality. Hence injection molding surface treatments are frequently used on slip-resistant products. A textured mold can also aid in the escape of trapped gasses.

A smooth SPI surface finish may cause the paint to peel off. However, a rough surface can ensure that the paint adheres better to the molded item. A textured SPI surface treatment also increases the part’s strength and safety.

Texture has several advantages, including:

  • Plastic flow creases – These creases may be removed by adding textured thickness while increasing strength and non-slip properties.
  • Improved grip – Adding texture to the component makes handling easier, increasing usefulness and safety in specific applications.
  • Paint adhesion – Paint adheres firmly to a textured object during subsequent molding.
  • Making undercuts – If you have a portion that will not consistently come over to the moving half of a mold, texturing on any surface may provide the necessary pull.To improve part functionality

Surface Finishes for Thermoplastics

Injection molding offers hundreds of thermoplastic resins that are flexible, strong, and can have fine cosmetic finishes.  Molds can be quickly produced by three-axis molding and easily hand polished or textured to one of eight surface finishes, which range from non-cosmetic to high-gloss polish.

Material color and hardness significantly affects the visual aesthetics of finished molded parts. For example; harder materials pick up textures more than softer ones. 

Polycarbonate shows the difference between an F1 surface finish and a B1 more than on a part made of TPV. Darker colors show textures more than lighter colors, regardless of part hardness.

All polish and texture is applied directly to the mold itself and transferred to the parts during the injection molding process. For this reason, secondary finishing is not available, which makes material selection critical in some cases. 

Typically, the link between the surface finish and material is small, but glass, mineral, or soft and high-shrink materials can produce parts that don’t always reflect what you may be looking for in a finish.

Below are the eight surface finishes available at Protolabs:

Surface Finish SpecificationDescription
SPI-C1600 grit stone
SPI-B1600 grit paper
SPI-A2Grade 2 Diamond Buff
PM-T1Light Bead Blast
PM-T2Medium Bead Blast

Factors Influencing Surface Finish Selection

Several variables determine the approach you use to achieve the desired surface finish. In general, the most important thing is to consider the factors below:

Production Speed Of Surface Machining Methods

Consider the needed speed of delivery. Examine the time necessary to perform the various procedures involved in the mixed metal finishing alternatives.

Degree Of The Hardness Of the Plastic

This is especially important when applying techniques from the second category. You will need a more intense or vigorous plastic when you access a harder plastic. On the other hand, pressing too hard with softer materials may create damage to the parts that are difficult or impossible to fix.

Costs And Financial Projections

There are undoubtedly several processes that, when applied to your specific scenario, seem to be the best option. However, some more challenging jobs may require specialized tools and equipment, causing you to exceed your budget. Check to see whether your decision will save you money in the long term.

Fillers impact the overall appearance of texture. A non-filled nylon part will pick up more than a 30% glass filled nylon part, for example.

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Injection Molded Surface Finish (Image source: kaysun)

Choosing the Right Finish Type for Plastic Injection Molds

Corner radius mold is a cutting tool used in injection molding. It features a rounded or curved edge on the corner, instead of a sharp 90-degree angle, which provides a smoother finish and reduces the likelihood of chipping or breaking the tool.

The mold works by rotating and cutting into a material, such as metal or plastic, to remove material and create the desired shape or profile. The corner radius of the tool allows for a more gradual entry into the material, reducing stress on the tool and the workpiece, and resulting in a smoother finish.

Corner radius mold is commonly used in a variety of molding applications, such as creating filets, chamfers, and rounded corners on parts and components. They are available in a range of sizes and configurations to suit different machining requirements, and can be made from various materials, such as high-speed steel, carbide, or cobalt. 

The appropriate selection of the tool material and geometry is crucial to achieving the desired performance and longevity of the tool.

Corner Radius Mold Benefits

The benefits of using corner radius mold include:

  • Improved Surface Finish: The curved corner radius of the tool creates a smoother finish on the machined surface, reducing the need for additional finishing operations.
  • Reduced Tool Wear: The gradual entry into the material reduces stress on the tool, resulting in less wear and longer tool life.
  • Reduced Chipping and Breakage: The curved corner radius helps to distribute cutting forces evenly, reducing the likelihood of chipping or breaking the tool.
  • Increased Efficiency: The ability to mold complex shapes and contours in a single operation can save time and improve efficiency in the machining process.

Overall, the choice of corner radius mold will depend on the specific application and material being machined, as well as the desired finish and accuracy of the final product.

Surface Finish Options for Silicone Rubber Parts

Surface finishes are used to create the final look of a given part. Its primary goal is to make the part look appropriate while also reducing any kinds of blemishes or inconsistencies that might be visible to the naked eye.

These finishes are applied using different materials and formats, depending on the precise need. Various finishes are categorized into different grades by the Society of Plastics Industry (SPI), which technically has 12 separate grades it can use for this purpose.

However, we will look into the four general groups that these grades can usually be placed in, plus what to know about each one and why you might consider it below.

Finish Categories and Their Applications

1. Shiny Finish

For those looking for a smooth, shiny finish to a given part, the “shiny” category is the way to go. Referring to finish grades A-1, A-2 and A-3 within the SPI’s grading list, these finishes create a mirrored-like look that’s very aesthetically pleasing.

However, they are also more expensive than other kinds of finishes due to the amount of work and additional processes employed in their creation. In addition, these particular grades tend to be less durable than many others, despite the lack of imperfections.

For those wondering how this is done, the shiny finish is accomplished using a diamond powder suspended in an oil solution. You’ll often see these kinds of finishes used in products like mirrors, optical parts, jewelry and even some watches.

2. Semi-Gloss Finish

A finish that’s similarly smooth to the shiny option, but does not have the same level of shine or reflection, is the semi-gloss finish, which covers grades B-1, B-2 and B-2 from SPI. These grades range from medium to medium-low in polish, whereas shiny finishes will be high in polish factor.

Semi-gloss is a very popular style because it meets multiple needs and comes at a relatively low price compared to the shiny finish. It’s also very durable and versatile, able to stand up to many conditions without losing its overall appeal.

It’s usually applied by using some of the same methods as other finishes – an oil-based material suspended with a similar solution will be used in order to get this kind of finish. It’s commonly used in areas like appliances, automotive parts and even certain items of clothing and upholstery.

3. Matte Finish

Created using a fine stone powder rather than any kind of liquid solution, matte finishes are categorized from C-1 to C-3 by SPI. These are known for their price and durability, as well as the fact that they’re generally very resistant to staining.

This is because matte finishes tend to cover a wider range of applications than other grades. They are often used in areas like labels or identification cards, package inserts, printers and even some electronics or medical devices.

4. Glossy Finish

Similar to the semi-gloss finish, but with even greater smoothness and shininess, gloss finishes are also a good option for most plastic injection molded products. Many of these products will be similar to shiny finishes in several ways.

While somewhat more expensive than other kinds of finishes, glossy is still a very versatile option that’s often used in areas like furniture, automotive parts, bath and kitchen fixtures and even some toys. It’s also commonly found on items of clothing, cosmetics packaging and other products where a high-end appearance is desired.

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Glossy Surface Finish on Plastic Plugs (Image Source:blogspot)

5. Dull Finish

Finally, using processes like sandblasting and others meant for roughing up a given product, the dull finish is considered to be an excellent choice for plastic products that will be used in areas like electronics. This type of finish can also vary from very low to high in terms of its level of smoothness, depending on what was done during the production process. It’s often found on medical devices and some kinds of packaging or cooking appliances.

Which kind of finish you choose for your plastic injection molded product will depend on a number of factors, including the specific use for the piece, how much durability it needs and how much shine is desired. It’s also important to keep in mind which types are most popular in certain applications so that you can stay up-to-date on current trends.


Surface finishes play a huge role in injection-molded part design. It helps in achieving both aesthetic and functional needs of the part. Basically, you can use surface finishes to mask part imperfections such as tool marks and flow lines, thereby improving aesthetic quality and value of the product. 

As a manufacturer, you need to select surface finishes that are tailored to your project specifications. This will ensure that the parts you produce possess the desired look and tactile sensation, catering to consumer preferences and functional requirements alike. 

Factors like production speed, plastic hardness, and cost are pivotal in determining the most appropriate finish for a given application. Moreover, leveraging corner radius molds will give you added benefits. These benefits include enhanced surface finish and diminished tool wear, thereby enhancing the overall efficiency of the molding process, particularly in crafting intricate shapes and achieving smooth contours.


Gary Liao

Gary Liao

Gary Liao is the Engineering Manager of TDL Company and has more than 20 years of mold design experience.

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