Surface Finishes Options of Plastic Injection Molded Parts
Toolingsun Surface Finishes
What Are Surface Finishes?
The appearance and tactile feel of a product greatly influence its overall appeal, regardless of the design efforts. Hence, the surface finish of plastic materials holds immense significance. Choosing the appropriate plastic surface finish is a pivotal aspect of the design journey. A well-selected plastic surface finish has the power to elevate a product from being unremarkable to exceptional. Consult with the knowledgeable professionals at Toolingsun to determine the most suitable plastic surface finish for your specific product.
Why Does Plastic Surface Finish Matter?
The success of a product often hinges on its design, encompassing not just aesthetics but the tactile experience it offers. While visual appeal matters, the way a product feels is equally—if not more—crucial. A flawlessly designed product may falter in the market if it doesn't feel pleasant to hold or interact with.
External attractiveness, surface quality, and tactile comfort are pivotal factors that sway customers toward your product rather than a competitor’s.
Furthermore, surface finish isn’t merely about looks; it serves a practical role too. The right finish can cleverly conceal imperfections from the manufacturing process like flow lines or weld marks. It's not just cosmetic—proper surface texture can enhance post-processing operations like paint adhesion, improving the overall quality of the final product.
Ultimately, surface finish plays a pivotal role in enhancing a product's functionality. Textured finishes, for instance, can boost grip and ensure the product feels just right in the hands of customers. Selecting the appropriate surface texture is a critical aspect of perfecting a product's finish.
Types of Plastic Surface Finish
There are two primary methods to attain the desired plastic surface finish. The first involves integrating the texture directly into the mold. This approach ensures consistency across all pieces, streamlining the process and reducing additional post-processing costs. The alternative method involves applying surface finishes after the molding process, which may involve techniques such as sanding, bead blasting, or etching.
In-mold texturing is ideal for ensuring a uniform plastic surface finish. Custom textures or specific finishes can be seamlessly integrated into the mold, whether it's across the entire surface or in select areas. On the other hand, post-processing techniques like sanding, etching, or blasting can be targeted to particular sections of the part, offering versatility in achieving various textures and finishes on a single piece. Frequently, a combination of in-mold texturing and post-processing methods is employed to achieve the precise plastic surface finish desired.
Mold Polishing and Mold Texture Standards
The final texture and appearance of a part are influenced by factors such as the mold tooling, plastic's inherent characteristics, and any subsequent post-processing methods employed. Texture refers to the pattern evident within the plastic, while finish pertains to the overall visual aspect of the plastic surface. Plastic surface finishes are broadly categorized into four main types: dull, matte, semi-gloss, and gloss.
Within the realm of injection molding finishes and textures, industry standards and designations are established by trade associations and companies. Prominent entities include the Society of the Plastics Industry (SPI), the Society of German Engineers (VDI), alongside companies such as Mold-Tech (MT) and Yick Sang (YS), each having formulated their own distinct standards. Consequently, terms like "SPI finishes" and "Mold-Tech textures" are commonly referenced during Design for Manufacturing (DFM) deliberations
Injection Molded Finish Options
In the realm of surface finish standards, there exists a multitude of options under each classification. Identification of specific surface finishes typically involves a callout comprising an organizational acronym followed by alphanumeric characters denoting the finish type. For instance:
SPI-A2: Part of the Society of the Plastics Industry (SPI) standard, indicating a high gloss or transparent finish.
MT-11010: A designation under Mold-Tech's system, specifying a light mold texture.
These alphanumeric codes serve as a means to precisely communicate and identify various surface finish standards, aiding in the selection and replication of specific textures or appearances for plastic parts.
Surface Finish for Plastic Parts
While various surface finishing standards exist, they universally categorize finishes into polished and textured types, further subdividing them based on either their visual appearance or the method used to achieve them.
Polished Finishes
High glossy or transparent
Semi-glossy
Matte
Textured Finishes
Bead blasting
Chemical etching
Mirror EDM
Keep reading to learn about some common types of mold polishes and mold textures, plus design tips that’ll help you get the best results when polishing and finishing your parts — Toolingsun’s Injection Molding Design Guide provides even more information and injection molding insights.
Polished Finishes
Injection molds undergo machining processes using CNC or EDM equipment. Upon completion, the mold core and cavity often bear milling or EDM marks. To eliminate these surface imperfections, the two halves of the mold undergo sanding and polishing. Various techniques are employed for finishing, with the industry commonly adhering to the SPI standard for mold polishing. This standard delineates nine levels of polished finishes, usually executed manually by proficient and highly skilled technicians.
High Glossy or Transparent Finishes
The SPI standards A-1, A-2, and A-3 represent the pinnacle of mold polishing excellence, resulting in either a high gloss or transparent finish on plastic parts. For opaque grades of plastics, these levels yield a glossy finish, while for clear plastics, they achieve transparency. All three SPI A-levels necessitate extensive mold polishing, involving the use of a rotary tool and diamond buffing paste.
Toolingsun commonly recommends the use of hardened steel molds to attain SPI levels A-1, A-2, or A-3. Specifically, stainless steel is suggested for A-1, whereas NAK80 steel is preferred for A-2 or A-3. Hardened steel exhibits superior resilience against potential ripples, sags, and crests resulting from the rotary tool, ensuring better polishing outcomes compared to molds made from softer materials like aluminum or softer steels.
The surface finish chart provided delineates the SPI A-level standards, outlining their respective finishing methods and surface roughness measurements. It's important to note that opting for higher levels of surface finishing and utilizing hardened steels can influence mold pricing and lead times.
Semi-Glossy Finishes
SPI finishes B-1, B-2, and B-3 represent intermediary levels of polishing, resulting in a semi-glossy finish. These finishes are achieved using different grades of sandpaper—600-grit for B-1, 400-grit for B-2, and 320-grit for B-3. The surface finishing process involves the use of various-sized filing tools, requiring back-and-forth sanding motions across surfaces. As a result of this sanding technique, more visible lines are evident compared to the smoother finishes achieved with SPI A-level polishing methods.
The provided surface finish chart outlines the specifics of these SPI B-level standards, detailing their respective finishing techniques and surface roughness measurements.
Certain intricate mold features, such as those tailored for accommodating slender, deep ribs, or diminutive bosses, present challenges during the polishing process due to their size, which renders them inaccessible to standard polishing tools. These features are commonly positioned on non-visible surfaces, typically on the cavity side of the mold. However, strategies exist in tooling construction to address this issue, notably through the utilization of sub-inserts. Integrating sub-inserts into the mold design allows improved access to these smaller features, enabling more effective and meticulous polishing.
For optimal outcomes, it is prudent to address these complexities at the Design for Manufacturing (DFM) stage of mold construction. By initiating discussions regarding these challenges during this phase, it becomes possible to incorporate specialized measures into the mold design, ensuring that these intricate features can be reached and polished effectively, thereby elevating the overall quality and consistency of the mold's surface finish.
Matte Finishes
SPI standards C-1, C-2, and C-3 are rougher and are the fastest and least expensive finishing methods. Applications include cosmetic or interior (non-visual) surfaces. For each C-level SPI finish, a sanding stone tool with a specific grit is used.
Textured Finishes
Injection molded parts can also have a textured finish instead of a polished finish. There are hundreds of texture options, but three main texture systems:
SPI and Mold Tech textures (United States)
VDI textures (European)
YS textures (Asian)
Apart from light, medium, and heavy textures, there are additional options available such as simulating wood or leather grain, as well as checkerboards or diamond patterns. Toolingsun primarily employs blasting, chemical etching, or mirror Electrical Discharge Machining (EDM) techniques to create mold textures. Nevertheless, the industry also commonly utilizes laser etching and standard sinker EDM methods for this purpose.
Bead Blasting
Textured finishes are achieved through bead blasting, a technique employing highly pressurized air to project diverse media onto mold surfaces. SPI finishes D-1, D-2, and D-3 are all accomplished using a dry blast method, each employing distinct media for the process. To streamline production timelines and minimize expenses linked to external services, prototype tool manufacturers commonly maintain in-house bead-blasting machines.
| Finish | SPI* standard | Finishing Method | Typical surface roughness Ra (μm) |
| Super High Glossy finish | A-1 | Grade #3, 6000 Grit Diamond Buff | 0.012 to 0.025 |
| High Glossy finish | A-2 | Grade #6, 3000 Grit Diamond Buff | 0.025 to 0.05 |
| Normal Glossy finish | A-3 | Grade #15, 1200 Grit Diamond Buff | 0.05 to 0.10 |
| Fine Semi-glossy finish | B-1 | 600 #Sand Paper | 0.05 to 0.10 |
| Medium Semi-glossy finish | B-2 | 400# Sand Paper | 0.10 to 0.15 |
| Normal Semi-glossy finish | B-3 | 320#Sand Paper | 0.28 to 0.32 |
| Fine Matte finish | C-1 | 600 Grit Stone | 0.35 to 0.40 |
| Medium Matte finish | C-2 | 400 Grit Stone | 0.45 to 0.55 |
| Normal Matte finish | C-3 | 320 Grit Stone | 0.63 to 0.70 |
| Satin Textured finish | D-1 | Dry Blast Glass Bead #11 | 0.80 to 1.00 |
| Dull Textured finish | D-2 | Dry Blast #240 Oxide | 1.00 to 2.80 |
| Rough Textured finish | D-3 | Dry Blast #24 Oxide | 3.20 to 18.0 |
| Machined Marks | – | – | 3.20 (with visible machining marks) |
Chemical Etching
Chemical etching involves applying a specialized film to the mold, followed by submersion in an acid bath. This process selectively erodes the unprotected surfaces, creating the desired texture on the mold. Given the intricate nature of these mold textures, this task is commonly outsourced to specialized finishing facilities. The duration for this work can vary, typically ranging between 2 to 5 days, depending on the facility's capabilities.
Toolingsun advises discussing chemical etching textures during the Design for Manufacturing (DFM) review, particularly if your design is undergoing Engineering Validation Testing (EVT) or Design Validation Testing (DVT) stages, or if engineering changes are anticipated. It's recommended to apply texture to the tooling post initial sampling and engineering changes (ECs), ensuring the design is finalized before incorporating texturing, to optimize the manufacturing process.
Mirror EDM
In scenarios where a product demands precise dimensional tolerances or a seamless appearance without any manual intervention on the tooling, special considerations become necessary. For instance, one of Toolingsun's clients developed a smartwatch requiring mating parts on the back cover and front bezel to seamlessly align when assembled.
To address such requirements, Toolingsun suggests employing mirror electro-discharge machining (EDM) to apply a light texture of Ra 0.2 (μm). This method involves using a meticulously machined and polished copper electrode, shaped to match the part's surface. Electrical sparks generated burn away the mold surface, replicating the part's shape and achieving the desired surface finish.
For mirror EDM, hardened steel is the preferred material due to its ability to yield a smoother surface finish. However, mirror EDM is more time-intensive compared to CNC machining or manual polishing. Typically, the machines utilized for this process are of German, Japanese, or Swiss origin. As mirror EDM is the most expensive texture application method, Toolingsun offers guidance to determine if this technique is necessary for your specific needs.
VDI (EDM) Mold Texture Values
Common VDI Injection Molding Finishes
| VDI Value | Description | Applications | Surface Roughness (Ra µm) |
| VDI 12 | 600 Stone | Low polish parts | 0.40 |
| VDI 15 | 400 Stone | Low polish parts | 0.56 |
| VDI 18 | Dry Blast Glass Bead | Satin finish | 0.80 |
| VDI 21 | Dry Blast # 240 Oxide | Dull finish | 1.12 |
| VDI 24 | Dry Blast # 240 Oxide | Dull finish | 1.60 |
| VDI 27 | Dry Blast # 240 Oxide | Dull finish | 2.24 |
| VDI 30 | Dry Blast # 24 Oxide | Dull finish | 3.15 |
| VDI 33 | Dry Blast # 24 Oxide | Dull finish | 4.50 |
| VDI 36 | Dry Blast # 24 Oxide | Dull finish | 6.30 |
| VDI 39 | Dry Blast # 24 Oxide | Dull finish | 9.00 |
| VDI 42 | Dry Blast # 24 Oxide | Dull finish | 12.50 |
| VDI 45 | Dry Blast # 24 Oxide | Dull finish | 18.00 |
Factors That Affect Injection Molding Surface Finish
Applying specific plastic finishes post-molding provides the highest degree of control over a plastic part's texture. Nevertheless, there are preliminary measures that can influence and modify the surface finish of a part even before the molding process initiates
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Tooling
The choice of material for the mold significantly influences the surface finish in injection molding. While various metals can be utilized, steel and aluminum stand out as the most prevalent options. However, these metals exert distinct effects on the final surface quality of molded plastic parts.
Typically, the smoothest plastic finishes can only be attained by employing molds crafted from hardened tool steel. Opting for a steel mold is often the preferred route, particularly for parts with aesthetic requirements necessitating an exceptionally minimal surface roughness.
Draft angle
| Draft Angle (Degrees) | Surface Texture | |||
| DA-PA | DA-PC | DA-ABS | SPI Grade | VDI Grade |
| 0 | 1 | 0.5 | A | 12 |
| 0.5 | 1 | 0.5 | A | 15 |
| 0.5 | 1 | 0.5 | B | 18 |
| 0.5 | 1 | 0.5 | 21 | |
| 0.5 | 1.5 | 1 | 24 | |
| 1 | 2 | 1.5 | 27 | |
| 1.5 | 2 | 2 | 30 | |
| 2 | 3 | 2.5 | 33 | |
| 2.5 | 4 | 3 | 36 | |
| 3 | 5 | 4 | 39 | |
| 4 | 6 | 5 | 42 | |
| 5 | 7 | 6 | 45 | |
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Material Choice
Injection molding encompasses a diverse array of material options, yet not all materials yield identical surface finishes. Some plastics naturally lend themselves to smoother finishes, while others are more conducive to achieving a textured surface.
Moreover, material additives such as fillers and colorants can impact the surface finish of the molded part. The following table outlines the compatibility of specific injection molding materials with particular SPI finish designations:
| Grade | Surface | Compatible Materials |
| A | Glossy | Acrylic, PC |
| B | Semi-glossy | ABS, HDPE, Nylon, PP, Polystyrene |
| C | Matte | ABS, HDPE, Nylon, PP, Polystyrene |
| D | Textured | ABS, HDPE, Nylon, PP, Polystyrene, TPU |
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Process variables
Surface finishes in injection molding can be influenced by various factors, including injection speed and temperature settings. Utilizing a faster injection speed along with elevated melt temperatures tends to result in glossier surface finishes for injection molded parts, often reducing the occurrence of visible weld lines.
Similarly, accelerating the filling process through mold cavities contributes to minimizing the visibility of weld lines, enhancing the overall appearance of the plastic finishes. When combining high mold and melt temperatures with increased injection speed, it tends to yield a smoother SPI surface finish
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Functional Reasons
Beyond the visual appeal, selecting an injection molding surface finish also involves functional considerations. In certain instances, opting for a textured finish becomes crucial to facilitate better paint adhesion to the molded part. A smooth SPI surface finish might fail to ensure proper paint adhesion, potentially leading to paint coming off.
Moreover, certain products necessitate a robust grip for improved performance. Textured plastic finishes significantly enhance the gripping quality, particularly in scenarios where slip-resistant features are vital. In such cases, custom injection molding surface finishes play a pivotal role.
Additionally, a textured SPI surface finish not only fortifies the part but also elevates its safety standards by providing added strength and reliability.
conclusion
By default, unless specified otherwise, an injection molded part typically comes with an SPI B-2 surface finish. This standard finish aids in better part release from the mold and helps eliminate tool marks. Surface finishes can be selectively applied to specific or all surfaces of a part. For instance, an opaque molded part might have a customized outer surface while maintaining a default, non-cosmetic interior. Moreover, a single tool cavity can accommodate multiple finishes and texturing if needed. For transparent parts, polishing both sides is standard practice to achieve optimal clarity.
Toolingsun offers an extensive range of surface finishes and diverse mold texturing options to ensure that your parts meet all structural and aesthetic requirements. Explore more about our injection molding service to find the right solutions for your needs.