Inadequate design of plastic parts often leads to defects that compromise functionality or the appearance of the product, resulting in frequent quality issues or even complete design failure and rework.
Here are some basic guidelines for designing plastic parts:
1. Wall Thickness
Designing the wall thickness of plastic parts relies on two fundamental principles:
– Keep walls as thin as possible while ensuring mechanical performance and a good user experience (aesthetics or ergonomics):
If the wall is too thick: Risk of shrinkage and bubbles
If the wall is too thin: Filling issues and poor mechanical strength
– Maintain uniform wall thickness as much as possible.
If uniformity is difficult to achieve: thickness variation should not exceed 50% of the nominal wall thickness. Below 0.3 mm, filling becomes difficult (except with flexible plastics).
General wall thickness recommendations:
Plastic Material | Recommended Thickness |
|---|---|
ABS | 1.2 mm – 3.5 mm |
Polycarbonate (PC) | 1.2 mm – 3.0 mm |
Polypropylene (PP) | 1.5 mm – 2.5 mm |
Polystyrene (PS) | 1.2 mm – 3.0 mm |
Polyamide (PA / Nylon) | 1.2 mm – 5.0 mm |
Polyethylene (PE) | 1.5 mm – 2.5 mm |
Rigid PVC | 1.0 mm – 3.0 mm |
2. Reinforcing Ribs
Ribs increase rigidity without significantly adding weight. They are placed on non-visible surfaces and oriented according to mechanical stresses. General design guidelines for ribs include:
Rib thickness (t):
– No surface appearance requirement: t = 75% to 85% of the base wall thickness (T).
– With surface appearance requirement:
Textured surface: t ≤ 50% of T.
Smooth surface: t ≤ 40–45% of T.
Rib height :
– For effective reinforcement, h should be 2.5 to 3 times the base wall thickness (T).
Draft angle:
– Minimum draft angle: 0.5°. But, recommended in most cases: 1–1.5°.
– For very small draft angles (0.1–0.5°), rib height should be limited to 5–15 mm to ensure proper ejection.
Radius at rib base:
– A fillet is required where the rib joins the wall.
– Minimum radius: R ≥ 0.5 mm; typically R = 0.5 × rib thickness (t).
Spacing between ribs:
– Distance between adjacent ribs should be at least 2–3 times the base wall thickness (T) to avoid material accumulation or uneven cooling.
3. Draft Angle
To facilitate demolding, draft angles must be considered early in the design process:
– All draft angles must follow the actual direction of ejection
– For every 0.02 mm of surface texture depth, increase the draft angle by 1°
Here are some recommendations to follow:
Finish | VDI | SPI | Ra (µm) | Recommended Draft |
|---|---|---|---|---|
Very Smooth | VDI 12 | A1 | ~0.40 | 0.5° to 1° |
Satin | VDI 18 | B2 | ~0.80 | 1° to 2° |
Matte | VDI 24 | C3 | ~1.60 | 2° to 3° |
Very Matte | VDI 33 | D2 | ~4.50 | 3° to 5° |
Rough | VDI 45 | D3 | ~18.00 | 5° to 7° or more |
4. Radii
General principles for fillet design:
– Add radii whenever possible at junctions and at the base of features
– Use generous radii if design constraints allow
– Avoid unnecessary fillets along the parting line
– Recommended fillet size: 50% to 60% of wall or rib thickness
– Minimum fillet radius for visible surfaces: 0.1 mm; standard radius: 0.2–0.3 mm
– Base thickness of fillets should not exceed 125% of the nominal wall thicknes
5. Bosses, Chimneys, Posts
Design guidelines for bosses:
– Avoid isolated bosses; connect them to the main wall with ribs (see Figure 1) or reinforce them with triangular ribs (see Figure 2)
– Check for material accumulation at the boss base, which may cause sink marks. These can be reduced using a compensating “volcano” boss
– Rib thickness reinforcing the boss should ideally not exceed 0.5 × the wall thickness
– Boss diameter should be about twice the screw diameter (2D)
For long bosses (when the boss length exceeds 5 times its outer diameter):
6. Text and Marking
For recycling and traceability purposes, plastic parts must include markings indicating production date, material, and recycling symbol.
Markings can be either embossed or debossed. The recommended depth ranges from 0.2 mm to 0.5 mm depending on the size and function of the text, and the character height can range from 0.5 mm to 3 mm. However, debossed markings are generally preferred by users.
7. Threads
Plastic threads are not very precise and often require an unscrewing mechanism integrated into the mold. Threads can be replaced with threaded inserts, either through overmolding or by installing them using heat staking or even cold press-fitting.
8. Aesthetic Lines (Decoration Line)
Aesthetic lines serve two main purposes:
– Visual or functional separation:
Used when a surface has multiple colors or textures (e.g., glossy, matte)
– Seam between two housings:
When assembling two shells, a decorative seam is often added to ensure a clean visual alignment. These can include patterns or shapes to mask the mold parting line.
Design of aesthetic lines:
– Dimensions: Typically 0.2–1.0 mm depending on product size, wall thickness, mold precision, and material
– Design requirement: Add stop ribs to ensure proper fit and alignment
9. Weld Lines
Gate placement must be carefully chosen. Poor placement can result in weld lines, which can weaken the part or affect its appearance, as shown below:
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