Plastics Part Structural Design For Electronic Products
Designing plastic parts for electronic products involves balancing aesthetics, functionality, manufacturability, and structural integrity. Here’s a detailed guide covering critical aspects like wall thickness, reinforcing ribs, draft angles, corners, holes, bosses, inserts, surface textures, and patterns. We’ll also discuss the role of Finite Element Analysis (FEA) in optimizing designs.
1. Wall Thickness
The wall thickness of plastic parts (Figure 1) significantly influences strength, weight, and manufacturability. For larger components, the design generally ranges from 2.4 to 3.2 mm, and 1 mm for smaller components.
Uniformity: Ensure walls are uniform to avoid warping and uneven cooling during injection molding. the thikness design also depends on the material used.
Following are the recomended wall thickness based on materials:
Table 1. Wall Thickness (mm) Recomendation Based on Materials
| Material | min. Thickness | Small Components | Medium Components | Large Components |
|---|---|---|---|---|
| Nylon | 0.45 | 0.76 | 1.5 | 2.4-3.2 |
| PE | 0.6 | 1.25 | 1.6 | 2.4-3.2 |
| PS | 0.75 | 1.25 | 1.6 | 3.2-5.4 |
| PMMA | 0.8 | 1.5 | 2.2 | 4-6.5 |
| PVC | 1.2 | 1.6 | 1.8 | 3.2-5.8 |
| PP | 0.85 | 1.54 | 1.75 | 2.4-3.2 |
| PC | 0.95 | 1.8 | 2.3 | 3.4.5 |
| POM | 0.8 | 1.4 | 1.6 | 3.2-5.4 |
| ABS | 0.8 | 1 | 2.3 | 3.2-6 |
These recommendations are based on the general use plastic parts, for special case like pressure resistance, flammability, etc, further detailed calculation or refering to specific standards is required.
Figure 1. Plastic Parts Thickness
2. Reinforcing Ribs
Ribs enhance structural integrity without increasing material usage without increasing overall wall thickness (reducing cost and weight). The thickness of reinforcing ribs is typically about 50% to 75% of wall thickness (recomended to be less than 60%), this is to avoid shrinkage. For glossy surface plastic parts, the bottom thickness of the reincorcing ribs is recomended to be less than about 50% of the thickness.
2.1. Multipple Ribs Intersection
If multiple Reinforcing Ribes intersect or connect, designer should consider the local material accumulation and shrink marks on the back as illustrated in the Figure 2. Lower number of ribs intersected is better.
Figure 2.a. 6 Crossed Ribs (poor)
Figure 2.b. 4 Crossed Ribs (Better)
Figure 2.c. 3 Crossed Ribs (Best)
2.2. Ribs connecting to the Outer Wall
Designer should try to make the ribs perpendicullar to the outer wall if possible (Figure 3).
Figure 3.a Ribs to Outer Wall (poor)
Figure 3.b Ribs to Outer Wall (better)
Figure 3.c Ribs to Outer Wall (best)
2.3. Ribs On Steep Slopes
Avoid to design the ribs or bosses on the steep slopes to avoid shrinkage (Figure 4).
Figure 4.a Ribs On Steep Slopes (poor)
Figure 4.a Ribs On Steep Slopes (better)
2.4. Ribs And Surface Deffects
Thick ribs on the surface can induce shrinkage. If reduction of ribs thickness is not possible, try to alter the external appearance to reduce the shrink marks visibility. Use this method cautiously, sometimes it is difficult to control.
3. Draft Angle
Draft angles facilitate easy removal of parts from molds. Draft angle ranging from 0.5° to 5° with typical angle about 2° per side. For textured surfaces, angles may increase to 3° or more.
For the mold cavity, the draft angle is generally 0.5° higher than the mold core.
Table 2 below illustrates the recommended draft angle for variety of materials:
Figure 5. Draft angle in plastic part design
Table 2. Recomended draft angle for various materials
| Material | Core | Cavity |
|---|---|---|
| ABS | 0.583° ~ 1.0° | 0.667° ~ 1.333° |
| PS | 0.5° ~ 1.0° | 0.583° ~ 1.5° |
| PC | 0.5° ~ 0.833° | 0.583° ~ 1.0° |
| PP | 0.417° ~ 0.833° | 0.5° ~ 1.0° |
| PE | 0.333° ~ 0.75° | 0.417° ~ 0.75° |
| PMMA | 0.5° ~ 1.0° | 0.583° ~ 1.5° |
| POM | 0.5° ~ 1.0° | 0.583° ~ 1.5° |
| PA | 0.333° ~ 0.667° | 0.417° ~ 0.667° |
| HPVC | 0.833° ~ 1.75° | 0.833° ~ 2.0° |
| SPV | 0.417° ~ 0.833° | 0.5° ~ 1.0° |
| CP | 0.333° ~ 0.75° | 0.417° ~ 0.75° |
4. Radii (Rounded Corners)
Sharp corners are stress concentrators and can lead to cracking and can be dangerous if the part contact with skin (minimum must be not less than 0.3 mm). To design internal and external radius, make the thickness at the corner the same thickness as the wall. Figure 6 illustrates the poor (left) design, and the best (right) design.
Figure 6. (a) internal: sharp, external: sharp, (b) internal: r = 0.6t, external: sharp, (c) internal: r=0.6t, external: r=0.6t, (d) internal: r=0.6t, external: r=0.6t+t
5. Holes
Holes are a common feature in product structure design and are generally classified as either circular or non-circular. Proper positioning of holes is crucial to ensure ease of mold processing while maintaining the structural integrity of the plastic part. The distance between holes should follow specific guidelines: if the hole diameter is less than 3.00 mm, the spacing should be at least equal to the hole’s diameter, whereas for diameters exceeding 3.00 mm, the spacing can be reduced to 70% of the diameter. Additionally, the distance from a hole to the edge should always be equal to or greater than the hole’s diameter to avoid compromising the part’s strength.
6. Bosses
Bosses are commonly used for assembling two plastic components through a shaft-hole fit or for securing self-tapping screws. If a boss is relatively short and ejected using an ejector sleeve during molding, it may not require a draft angle. However, for taller bosses, external reinforcements such as cross ribs are often added. These cross ribs generally have a draft angle of 1–2 degrees, and the boss itself might need a draft angle depending on the design requirements.
When a boss is paired with another boss or a post, a fitting gap of 0.05–0.10 mm is typically maintained to account for positional deviations during processing. For self-tapping screw assemblies, the inner hole of the boss should be 0.1–0.2 mm smaller than the screw’s diameter to ensure a secure fit. For example, when using an M3.0 self-tapping screw, the inner hole diameter of the boss is usually designed to be between 2.60 mm and 2.80 mm.
7. Inserts
In the plastic molding process, inserts refer to metal or other material components, such as bolts or terminals, embedded into plastic parts during or after molding. These inserts enhance functionality, act as fasteners or support elements, and can also serve decorative purposes. Commonly used in designs requiring easy repair, replacement, or reusability, inserts must be carefully designed to minimize their impact on production costs, as their inclusion adds extra processing steps. Typically made from metals like copper, inserts must be manufactured in shapes conducive to cutting or stamping, with adequate mechanical and bonding strength to prevent issues like pull-out or rotation. Features such as annular grooves or crosshatching improve bonding, while rounded or symmetrical shapes help ensure uniform shrinkage. Inserts should also include cylindrical extensions for easier placement within molds, sealing structures to prevent flash, and designs accommodating secondary processing like threading or flanging. In structural designs, inserts must be positioned precisely and securely within the mold while ensuring sufficient encapsulating material for a strong connection and preventing any plastic leakage.
8. Surface Texture
The surface of plastic products can feature different finishes, such as smooth (achieved with a polished mold surface), spark-etched (produced using copper EDM processing), patterned (various etched designs), or engraved textures. When the texture depth is substantial or the surface includes intricate patterns, the resistance during demolding increases, requiring a larger demolding angle to ensure successful ejection from the mold.
9. Text and Patterns
Text and patterns on plastic products can be either raised or recessed, with two common processing methods: small text and patterns are created through mold etching, while larger ones are machined directly into the mold. To ensure successful molding, text and patterns should avoid sharp angles and be designed with appropriate dimensions. Raised text and patterns are generally preferred as they simplify mold processing by being recessed on the mold surface. If raised features are not suitable for the product’s surface, a recessed area can be added to the desired depth, with the text or pattern raised within it, meeting structural and molding requirements.
For plastic products, the typical height of raised text and patterns ranges from 0.15 to 0.30 mm, while recessed text and patterns are usually 0.15 to 0.25 mm deep. The design of text should also follow size specifications: the stroke width should be at least 0.25 mm, the spacing between characters no less than 0.40 mm, and the distance from characters to the edge should be at least 0.60 mm. These dimensions ensure ease of molding and maintain structural integrity.
10. Finite Element Analysis (FEA)
FEA is a powerful tool for validating and optimizing plastic part designs before manufacturing.
- Stress Analysis: Identify high-stress areas and adjust features like ribs or fillets.
- Thermal Analysis: Evaluate cooling and warping tendencies during molding.
- Dynamic Analysis: Assess how the part withstands vibrations and impacts.
Figure 7. Finite Element Analysis of Plastic Component