As a product structure engineer, are you struggling with plastic part design? Carefully designed shell warping, fracture of slender ribs under pressure, obvious shrinkage marks in thick-walled areas, stress cracking at sharp corners… these problems may seem trivial, but they directly affect product's appearance, assembly accuracy, and reliability. Plastic part design is far more complex than simply "drawing a shell"—it requires consideration of material properties (shrinkage rate, flowability), injection molding processes (cooling, demolding), and, more importantly, adherence to structural mechanics logic.

Core Logic: Essence of plastic part warping is uneven shrinkage during cooling (differences in wall thickness, inconsistent molecular orientation, different cooling rates), leading to an imbalance of internal stress and causing bending deformation.

 

Practical Methods:
Wall Thickness Uniformity: Avoid abrupt changes in wall thickness; use a sloping transition (slope ≤ 1:3); reduce material at thicker sections (e.g., boss thickness ≤ 60% of main body wall thickness);
Symmetrical Reinforcing Rib Layout: Rib spacing ≥ 2 times rib height, and symmetrical distribution along shrinkage direction (e.g., ribs at four corners of a rectangular shell to offset diagonal shrinkage force);
Draft Angle Compensation: For large, easily warped flat surfaces (e.g., panels), set a 0.5°-1° draft angle to reduce "tensile stress" during demolding;
Gate Location Optimization: Place gate in a location with low shrinkage (e.g., center of symmetry) to avoid uneven shrinkage caused by unilateral feeding (e.g., gate for a mobile phone back cover is placed in the center, not edge).

Core Logic: Slender ribs (height-to-diameter ratio > 5) in plastic parts are prone to instability and bending under compression (similar to bar instability), but have higher strength under tension (plastic tensile properties are better than compressive properties). In design, ribs should be prioritized to bear tensile forces to avoid pure compressive conditions.

 

Design Tips:
Rib Direction Matching to Stress: For example, reinforcing ribs of equipment supports should be arranged along direction of tensile load (e.g., vertical ribs in horizontal supports work together to bear load under tension);
T-shaped ribs instead of I-shaped ribs: T-shaped ribs are wider at the bottom (increasing pressure area) and narrower at the top (reducing material accumulation), making them more resistant to instability than I-shaped ribs;
Rib Thickness Control: Rib thickness ≤ 0.4 times main body wall thickness (e.g., if main body is 3mm thick, rib thickness ≤ 1.2mm), avoiding uneven shrinkage due to excessive rib thickness;
Adding Triangular Supports to Prevent Instability: For ultra-high ribs (height > 50mm), add triangular support blocks to back of rib to transform "pure compression" into a combination of "compression + bending" stress.

Core Logic: "Internal cutting-off" refers to edge of internal structure (such as holes, grooves, ribs) forming a concave sharp angle with main body (e.g., inner right angle of a square hole). Material flow is obstructed here, and "vacuum negative pressure" is easily generated during cooling, leading to stress concentration, shrinkage cavities, or difficulty in demolding.

 

Potential Avoidance Methods:
Convert Inner Right Angles to Rounded Corners: All concave corners should have a radius (R≥0.5mm) (depending on wall thickness; R≥1mm for thicker walls), such as rounding four corners of a square through-hole with a radius of 1mm.
Convert Outer and Inner Concave Structures: Change concave structures to convex structures (e.g., change internal grooves to external bosses) to avoid internal cutting.
Use Rounded Arcs to Connect Ribs to Walls: Use rounded arcs (R≥0.3mm) at connection points between ribs and main body to replace right angles, reducing material buildup and stress concentration.

Core Logic: Sharp edges (convex and concave angles) are major stress concentration areas (stress concentration coefficient Kt can reach 5-10 times). They also cool faster than flat surfaces, easily generating microcracks, especially prone to fracture under low temperatures or dynamic loads.

 

Optimization Scheme:
External Corner Rounding: All exposed sharp corners should have a radius of ≥0.3mm (e.g., shell edges, button edges), with a recommended radius of 0.5-1mm (balancing aesthetics and strength);
Internal Corner Rounding Preferred: Concave sharp corners (e.g., assembly slots) should have a radius of ≥1mm to prevent sharp corners from scratching components;
Avoid Thin-Walled Sharp Corners: Corners of thin-walled parts (wall thickness <2mm) need to be more rounded (R≥0.5mm) to prevent local stress from exceeding material strength.

Core Logic: Local material accumulation (thick-walled areas, intersections of multiple gates) leads to slow cooling, increased shrinkage, resulting in shrinkage marks, pits, or internal voids, while also wasting material and prolonging injection molding cycle.

 

Methods to Reduce Material Accumulation:
Prefer Thinner Walls: Main body wall thickness should be controlled between 1.5-3mm (for common plastics such as ABS/POM). For thicker walls, create "hollowed-out grooves" (e.g., hollowing out inside of bosses).
Gate Number and Location: Avoid convergence of multiple gates (e.g., use "overlapping gates" instead of "side gate convergence"). Single gates are preferred (to reduce material flow).
Combining Ribs and Bosses: Combine bosses and ribs (e.g., screw posts also function as ribs) to avoid material accumulation on individual bosses.
Process Compensation: Install "overflow channels" (to collect excess material) at thicker walls, or increase holding pressure during injection molding (but avoid excessive holding pressure to prevent flash).

Core Logic: For large flat surfaces (area > 100cm²) or thin-walled surfaces (wall thickness < 1.5mm), insufficient injection pressure or short holding time during injection molding can easily lead to surface collapse and depressions due to incomplete material filling or excessively rapid cooling.

 

Anti-collapse Design:
Add supporting ribs or reinforcing columns: Add sparse ribs (rib spacing ≤ 50mm) below large surface, or add a "support column" (diameter ≥ 5mm) in the center;
Zoned injection molding approach: Divide large surface into multiple smaller surfaces (separated by shallow grooves) to reduce the area filled in a single operation;
Matching process parameters: Increase injection pressure (but not exceeding mold's capacity), extend holding time (holding time ≥ 50% of cooling time);
Avoid isolated structures: Isolated protrusions (such as small bosses) on large surface need to be connected to large surface with ribs to prevent protrusions from collapsing due to lack of support.

Six principles of plastic part design mentioned above are essentially about "following nature of material and avoiding process pitfalls": avoiding warping is "controlling shrinkage," slender ribs under tension is "preventing instability," avoiding internal sharp corners is "reducing stress," reducing material accumulation is "promoting uniformity," and preventing surface collapse is "strengthening support." These details are interconnected—without uniform wall thickness, warping is unavoidable; without proper rib orientation, instability can occur at any time.