A variety of surface finishes can be applied to injection molded parts. Parts made of plastics and other materials can have various finishes, either for aesthetic purposes or for functional ones. By utilizing different types of surfaces, one can influence the texture, feel, and look of the final product.
At a very early stage of the design process, the surface finish must be chosen. The finish type influences the tooling required to manufacture the part, which is why the finish type influences the tooling needed. To easily release the part from a mold, the draft angle needs to be higher when the finish is rough. It will also affect the material used to manufacture the part.
If the mold surface needs to be prepared accordingly, the right finish will also be achieved. Once the design and finish of the injection mold are determined, many of the vital details become apparent.
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Reasons of aesthetic appeal
It is often desirable to achieve a specific kind of surface finish on a product in order to enhance its look and feel. Potential customers are attracted to products that have a nice finish. A retail store is an example of this. An attractive finish makes products appear more valuable.
A textured finish can protect imperfections, which is another interesting function of surface finishes. Some types of finishes can easily conceal injection molding-related imperfections, such as line marks or sink marks.
Additionally, keeping with the aesthetic theme, surface finishes can be used to mask wear and tear that may occur during products’ transport or other supply chain processes. A product that comes in contact with hands quite frequently may benefit from a surface finish that masks fingerprint smudges.
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Reasons for functionality
Additionally, there are some functional aspects that influence the decision to use a certain type of surface finish after a part has been manufactured and used. The finish of the part can be textured in order to better hold the paint. For instance, flat surfaces could create problems for painting.
Likewise, sticker adhesion affects the finish on a surface just like paint adhesion does. It is desirable that a product have a finish that will enhance the adhesion of a label or sticker applied to its surface.
It is necessary to have a tight grip on some products in order to perform better. One example is sports equipment . Textured finishes can enhance grip on handles and bars. An anti-slip surface finish can benefit any part that needs to be slipresistant.
Surface finish choices are influenced by injection molding details. In the case where a part’s center of mass cannot be brought across to the moving half of a mold, a substitute is required to provide the necessary pull. Pull factor can be obtained by using undercuts that result from textured finishes.
Flow creases in plastic parts are often detrimental to quality when injection molding them. A textured finish can be used to hide plastic flow creases. As well as adding strength, textured finishes can increase safety.
| 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 Grit Paper | 0.05 to 0.10 |
| Medium Semi-glossy finish | B-2 | 400 Grit Paper | 0.10 to 0.15 |
| Normal Semi-glossy finish | B-3 | 320 Grit 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 |
| As machined | - | Finished to the machinist's discretion | 3.20 (with visible machining marks) |
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Surface finish and molding processes
Injection moulders and design engineers must work together to determine the best surface finish. In addition to chemical composition, the design aspect of the part and the surface finishing options determine the surface finish that can be applied to it.
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Materials used in the design
Physicochemical properties of materials vary. It is important to take into account other parameters, such as melting temperature, in order to determine the kind of surface finish that a material can deliver. In addition to additives, part finishes are also impacted by the materials themselves.
As an example, if a glossy finish is required on a part made of crystalline resin, a high melt temperature is necessary to reduce roughness and maximize gloss on the part. If one wants to know which additives to add to the mix, he or she must have deep knowledge of material science. Some materials or fillers can actually make the surface rougher.
You should be aware that a polished mold does not automatically mean a high level of gloss. Plastic resin used in the material, mold design conditions, and molding conditions all impact the gloss. A lot depends on the material.
There is more to a part than its material. Quality of finish can also be influenced by the material of the mold. The best finish can only be achieved by using a finely polished and hard tool steel mold. If you polish a mold simply, you will not obtain the desired result.
With today’s technology, you can simulate how the surface finish you want will be achieved when using the material you plan to use. If any defects occur during manufacturing, you can test for them. You can select the right material for your application with the help of mold flow simulation software.
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Injection velocity and temperature
Injection speed has a major influence on the ability to get a good finish. Fast injection speed is necessary for a glossy finish. During rapid filling through the mold cavity, weld lines can also be reduced and the surface finish is improved.
Combined with a high melt and mold temperatures, a high injection speed will produce an improved surface smoothness.