The VDI grade determines the coarseness of the mold surface texture, while the draft angle is essential to ensure that a textured part can be ejected smoothly without scuffing or dragging. These two factors are strongly correlated: the higher the VDI grade (the coarser the texture), the larger the required draft angle.
Below is a detailed breakdown:
1. VDI 3400 Standard (Surface Texture Standard)
VDI 3400 is a surface texture standard established by the Association of German Engineers, widely used in the mold industry and product design. It defines the roughness of mold surfaces using a series of standard grades.
- Representation: Typically denoted as VDI XX, e.g., VDI 18, VDI 24, VDI 30, VDI 42, etc.
- Meaning of Values:A smaller value indicates a smoother surface; a larger value indicates a rougher surface.
- VDI 0-15: Very smooth surfaces(close to a polished finish). For example, VDI 12 corresponds to a mirror polish.
- VDI 18-27: Slight surface texture, with a fine matte or velvety feel.
- VDI 30-48: Medium texture, common for finishes like fine leather grain or fine wood grain.
- VDI 54 and above: Very coarse textures, such as pronounced orange peel or anti-slip granules.
On product design drawings, the VDI grade for critical appearance surfaces is usually specified – for example, “Appearance Surface: VDI 24” – serving as the acceptance criteria for mold texturing (etching).
2. Draft Angle (Draft)
The draft angle is the taper angle applied to walls perpendicular to the parting line, designed to allow the part to be easily ejected from the mold.
- Primary Purpose: To prevent severe friction between the part and the mold cavity during ejection, which can cause “scuffing,” “drag marks,” or even the part becoming stuck.
- Influencing Factors: The required draft angle size depends on several factors, including:
- Surface Texture (VDI Grade): This is one of the most important factors.
- Part Depth: Deeper parts require larger draft angles.
- Plastic Material: Materials with high shrinkage rates or softer materials (e.g., PP, PE) require larger angles; rigid materials (e.g., PC, ABS) can use slightly smaller angles.
- Dimensional Tolerances: Areas with tight dimensional tolerances often require larger draft angles to ensure dimensional consistency.
3. Relationship Between VDI Grade and Draft Angle (Core Correlation)
Texture acts like countless tiny undercuts. The deeper the texture (higher VDI value), the more pronounced these “undercuts” become, increasing ejection resistance. Therefore, sufficient draft angle must be provided to overcome this resistance.
The following is a general reference table illustrating the minimum draft angle per side recommended for different VDI grades:
| VDI Grade | Surface Description | Approx. Texture Depth (Ra, μm) | Min. Draft Angle per Side (°) | Remarks |
|---|---|---|---|---|
| VDI 0 – 12 | High-Gloss Polish | < 0.8 | 0.5° – 1° | Smooth surface, low resistance |
| VDI 15 – 21 | Fine Satin / Slight Grit | ~ 1.0 – 2.0 | 1° – 1.5° | Fine matte feel |
| VDI 24 – 30 | Medium Texture (e.g., Fine Leather) | ~ 2.5 – 5.0 | 1.5° – 2.5° | Most common texture range |
| VDI 33 – 39 | Pronounced Texture (e.g., Std. Leather) | ~ 6.3 – 10.0 | 2.5° – 3.5° | Clearly visible texture |
| VDI 42 – 48 | Coarse Texture (e.g., Coarse Leather) | ~ 12.5 – 20.0 | 3.5° – 4.5° | Requires careful design |
| VDI 54+ | Very Coarse Texture (e.g., Anti-slip) | > 25.0 | ≥ 5° | Angle increases significantly per grade |
At ATC-Mould, we focus closely on our clients’ surface finish requirements. We consider the specific plastic material (including PP, ABS, PA66, PBT, PEEK, LCP) to determine comprehensive polishing specs and choose the most appropriate mold steel. We also guide clients in designing products with proper draft angles to ensure outstanding visual results. Additionally, we provide rapid prototype molding services for testing both the product’s structure and its surface quality.