Knowledge Hub
20/02/2024
Surface Finish for Injection Moulded Parts
What is Surface Finish in Injection Moulding?
Surface finish for injection moulded parts refers to the final appearance and texture of the part’s surface after the moulding process.
It can be influenced by several factors, such as the type of mould used, the material being injected, and any additional post-processing techniques applied.
What are the Different Types of Surface Finish for Injection Moulded Parts?
There are several types of surface finish for injection moulded parts, including:
Smooth or high gloss finish: This is the most common surface finish and provides a smooth and shiny appearance to the part. It is achieved by using a polished mould surface.
Textured finish: Textured finishes are used to create specific textures or patterns on the surface of the part, ranging from fine textures to coarse textures.
Matte finish: A matte finish provides a low-gloss appearance to the part. It is achieved by using a bead-blasting or chemical treatment on the mould surface.
Leather grain finish: This finish replicates the texture and appearance of leather. It is commonly used in automotive interiors and consumer goods.
Metallic finish: Metallic finishes are used to create a metallic appearance on the surface of the part. It can be achieved by using a paint or coating specifically designed to mimic metal.
Polished finish: A polished finish provides an extremely smooth and reflective surface to the part. It is achieved by using high-speed polishing techniques on the mould surface.
Patterned finish: Patterned finishes are used to create a specific pattern or design on the surface of the part. It can range from geometric patterns to custom designs.
Moulded-in colour finish: The desired surface finish can be achieved by incorporating colour directly into the part during the injection moulding process. This eliminates the need for secondary painting or coating.
Grainy or stippled: This type of surface finish has a rough texture with small bumps or indentations, often resembling a grainy or stippled pattern. It is commonly used for parts that require a better grip or to reduce the appearance of scratches or wear.
Painted or coated: In some cases, injection moulded parts are coated or painted after moulding. This allows for endless possibilities in terms of colours, patterns, or textures.
These are just a few examples of the many types of surface finish which are available for injection moulded parts. The choice of surface finish depends on the specific requirements of the part and its intended application.
Why is Surface Finish Important in Injection Moulding?
Choosing the right surface finish is important in injection moulding for several reasons:
Aesthetic Appeal: The surface finish of a part plays a significant role in its visual appeal. A smooth, polished surface finish enhances the overall appearance of the part, making it visually pleasing to customers and end users.
Functionality: Different surface finishes can affect the functionality of a part. For example, a textured surface finish may improve grip or reduce friction, making the part easier to handle or improving its performance in specific applications.
Part Quality: The surface finish can directly impact the quality of a part. Imperfections such as rough surfaces, scratches, or blemishes can negatively affect the part’s performance, integrity, and durability.
Mould Release: Mould release is an essential aspect of injection moulding. The right surface finish can facilitate the easy release of the part from the mould, reducing the potential for damage or defects during the ejection process.
Ease of Cleaning: Certain surface finishes can make a part easier to clean. Smooth finishes, for example, may prevent the accumulation of dirt or bacteria, making the part more suitable for applications with strict cleanliness requirements, such as medical devices or food packaging.
Mould Life: Surface finishes can impact the lifespan of the mould. Unnecessarily rough or abrasive finishes can cause wear and tear on the mould, requiring frequent maintenance or replacement. Choosing the appropriate surface finish can help extend the life of the mould, thereby reducing the costs associated with repairs or replacements.
How is Surface Finish Achieved in Injection Moulding?
Surface finish in injection moulding is achieved through a combination of mould design and process parameters.
Here are some key pointers to follow to ensure that the best possible surface finish is achieved:
Mould Material: Use high-quality mould materials, such as hardened steel or aluminium. These materials have a smooth surface and will impart a better finish to the moulded part.
Mould Texture: Apply texture to the mould cavity or core surfaces if a specific surface finish is required. This can be achieved by sandblasting, etching, or EDM (Electrical Discharge Machining).
Mould Venting: Adequate venting in the mould design helps to release trapped air and gases during the moulding process. Insufficient venting can cause surface defects or blemishes.
Injection Pressure and Speed: Injection pressure and speed should be optimised to ensure proper filling of the mould cavity, minimising any flow marks, uneven surfaces, or weld lines.
Cooling and Ejection: Proper cooling channels in the mould design should be implemented to achieve uniform cooling and solidification of the molten plastic. This helps in minimising warpage and achieving a better surface finish. Additionally, a well-designed ejection system should be present to ensure smooth part release, so that parts are ejected without damage.
Material Selection: Choosing the appropriate resin with good flowability, low shrinkage, and good surface finish properties will help to achieve the desired surface finish. Fillers or additives can also be incorporated, specifically for improved surface quality.
Process Optimisation: Fine-tuning process parameters like melt temperature, mould temperature, injection time, and holding pressure is crucial.
By combining these factors and optimising the injection moulding process, manufacturers can achieve the desired surface finish for moulded parts, ensuring that the final component meets its functionality, design, and aesthetic requirements.
