For previous article, please refer to Mechanical Structure Design Specifications – Plastic Part Design Guidelines (Part 1).
In previous article, we focused on "deformation and failure prevention" (warping, sharp corners, material buildup, etc.) in plastic part design, solving "basic stability" problem of injection molding. Core of this second article is to overcome unique "material limitations" of plastic parts—tolerance control failure caused by shrinkage fluctuations, anisotropy caused by molecular orientation, and connection failure due to low strength. Today, we will teach you how to use "material property adaptation thinking" to make plastic parts "reasonably accurate, scientifically stressed, and reliably connected," starting with five advanced guidelines: avoiding tolerance accuracy traps, adapting to anisotropy, optimizing shear force at bonding surface, strengthening bolts with backing plates, and controlling minimum wall thickness.
What is design thinking mode for consumer electronics products?

Core Logic: Plastic parts are affected by shrinkage fluctuations, mold wear, and injection parameter drift, making it difficult to consistently guarantee high-precision tolerances, significantly increasing mold costs and scrap rates. Design should prioritize critical dimensions, allowing for more lenient requirements on non-critical dimensions.

 

Three major factors influencing tolerances in plastic parts:
1. Shrinkage variation: Different materials (ABS shrinkage 0.5%~0.7%, PP shrinkage 1.5%) and wall thicknesses (thicker walls shrink more) lead to dimensional fluctuations.
2. Mold precision limits: Ordinary injection molds have a tolerance of approximately ±0.1mm, while precision molds (such as mirror molds) can reach ±0.03mm, but cost is 3~5 times that of ordinary molds.
3. Process fluctuations: Changes in injection pressure, temperature, and cooling time can cause dimensional deviations of over ±0.08mm for parts in same batch.
Practical Method: Graded Tolerance Control
Critical Dimensions: Dimensions affecting assembly, sealing, and movement (e.g., bearing bore inner diameter, snap-fit clearance), controlled at IT8~IT9 level (e.g., Φ10mm hole tolerance ±0.1mm), using "functional tolerance" instead of "drawing absolute tolerance" (e.g., "fit clearance 0.2~0.4mm" instead of "Φ10±0.05mm");
Non-Critical Dimensions: External surfaces, non-mating surfaces (e.g., housing sidewalls), relaxed to IT10~IT12 level (e.g., length 100mm tolerance ±0.3mm), using "Maximum Material Ratio" (MMR) designation;
Reference Industry Standards: Follow ISO 20457 (dimensional tolerances for plastic parts) and GB/T 14486 (dimensional tolerances for engineering plastic molded parts), avoiding custom-defined excessively strict tolerances.

Core Logic: During injection molding of plastic parts, molten material fills along Mold Flow Direction (MFD), and molecular chains align along MFD, resulting in anisotropy—strength/stiffness along MFD direction is 30%~50% higher than perpendicular direction, making it more prone to fracture. Designs must ensure "parallel force" to avoid unidirectional loads perpendicular to MFD.

 

Two Major Manifestations of Anisotropy
Mechanical Property Differences: For example, tensile strength of PP material along MFD is 40% higher than that along perpendicular direction, and impact strength is 25% higher;
Shrinkage Differences: Shrinkage rate along MFD direction is 0.1%~0.2% lower than that along perpendicular direction (due to molecular chain orientation limiting shrinkage).
Practical Methods: Forward Design
Ribs/Reinforcing Ribs Arranged Along MFD: Ensure stress direction of ribs aligns with MFD (e.g., for long, narrow shells, longitudinal ribs are placed along injection flow direction), avoiding transverse ribs (perpendicular to MFD, they are prone to tensile fracture);
Avoid Tensile Loads Perpendicular to MFD: For example, "insertion/removal direction" of snap-fits should be parallel to MFD (reducing vertical peeling forces), or design "double snap-fits with symmetrical stress" (counteracting anisotropy);
Symmetrical Structures Balance Anisotropy: Complex plastic parts should be designed with "central symmetry" or "axisymmetrical symmetry" (e.g., circular shells) to allow differences in MFD values in different directions to cancel each other out.

Core Logic: When bonding plastic parts (adhesive bonding, ultrasonic welding, hot plate welding), stability of shear force (parallel to adhesive surface) is far higher than that of tensile force/peel force (peel force easily causes adhesive layer to "lift at the edges," while shear force is borne by intermolecular cohesive forces). Design should ensure that adhesive surface primarily bears shear force, avoiding peeling force.

 

Main Cause of Adhesive Failure: In failure mode of adhesively bonded parts dominated by peel force, 70% originates from peel force (such as "face-to-edge" bonding), while only 30% originates from shear force. For example, when two flat plates are "face-to-face bonded" and subjected to shear force, adhesive layer is evenly stressed; however, if "edge-to-edge bonded," even slight misalignment will generate peel force, leading to delamination.
Practical Methods: Prioritize Shear Surfaces
Increase Bonding Area: Use surface bonding instead of line bonding (e.g., a 10cm² bonding area at shell joint is 5 times stronger than a 2cm line bond);
Design Interlocking Structure: Add tenon and serrations to bonding surface (e.g., 0.5mm high serrations at upper and lower shell bonding joint) to convert some shear force into mechanical interlocking;
Control Bonding Surface Roughness: Surface roughening (Ra 3.2~6.3μm) can increase adhesive adhesion, but avoid excessive roughness (>Ra 12.5μm) which leads to uneven adhesive thickness;
Prioritize Shear Force: For example, on battery compartment cover bonding surface, ensure opening force is parallel to bonding surface (shear), rather than perpendicularly upwards (peeling).

Core Logic: Plastic parts have low strength (e.g., ABS tensile strength is only 40-50 MPa, far lower than steel's 200-400 MPa). When bolts are directly connected, bolt hole edges are prone to crushing due to excessive pressure (bearing failure) or bolt loosening (preload reduction). Metal inserts (inserts) are needed to distribute pressure and strengthen connection.

 

Two Major Pain Points of Bolt Connections in Plastic Parts
Bearing Failure: Bolt preload concentrates at hole edge, with local pressure exceeding material's allowable bearing capacity (e.g., ABS's allowable bearing capacity is 30 MPa), leading to cracking at hole edge;
Loosening Failure: Plastic creep (deformation under long-term stress) leads to preload reduction, causing bolt loosening (especially in high-temperature environments).
Practical Methods: Reinforcement with Liners
Adding Metal Inserts/Liners: Embed stainless steel/aluminum alloy liners (thickness ≥ 2mm) around bolt holes. Liner hole diameter should be 0.2~0.5mm larger than bolt (to avoid interference fit). Round hole edges with a 1mm radius (to reduce stress concentration).
Counterhead Bolts + Large Flat Liners: Embed countersunk bolt head into liner to increase bearing area (e.g., for a Φ5mm bolt, using a 20mm×20mm liner increases bearing area from 19.6mm² to 400mm²).
Avoiding Single-Bolt Cantilever Connections: When using multiple bolts, bolt spacing should be ≥ 3 times hole diameter (e.g., for Φ5mm bolts, spacing should be ≥ 15mm) to prevent localized deformation.

Core Logic: Thicker wall thickness is not always better for plastic parts—excessive thickness leads to uneven shrinkage (resulting in shrinkage marks), slow cooling (longer injection molding cycle), and material waste; insufficient thickness results in incomplete filling and insufficient strength. Minimum wall thickness needs to balance "process feasibility" and "strength requirements," using rib reinforcement instead of blindly increasing thickness.

 

Two major constraints on minimum wall thickness:
Process constraints: Material flowability determines minimum wall thickness (e.g., PE with good flowability has a minimum wall thickness of 0.8mm, while PC with poor flowability has a minimum wall thickness of 1.5mm);
Strength constraints: Wall thickness bearing load must satisfy σ=F/A≤[σ] ([σ] is allowable stress of material, such as 20MPa for ABS).
Practical Method: Thin-walled Structure + Rib Reinforcement
Reference Material Minimum Wall Thickness Table: ABS/PC minimum wall thickness 1.0~1.5mm, PP/PE minimum wall thickness 0.8~1.2mm, POM minimum wall thickness 1.2~1.5mm;
Reinforcement at Thin-walled Areas: When wall thickness is <1.5mm, reinforce with ribs (rib thickness ≤ 0.4 times main wall thickness, rib height ≤ 3 times rib thickness). For example, adding a 0.4mm thick rib to a 1mm thin-walled shell increases strength by 50%;
Avoid Localized Excessive Thinness: Use a slope transition (slope ≤ 1:3) at wall thickness changes to prevent "flow stagnation" during filling (e.g., when connecting a thick-walled boss to a thin-walled main body, use a slope transition to avoid material shortage).
Conclusion
Second part of design principles for plastic parts focuses on "overcoming material limitations": avoiding tolerance and precision traps is "rational cost control"; adapting to anisotropy is "leveraging existing strengths"; optimizing shear force at adhesive surfaces is "scientific connection"; bolts with backing plates are "local reinforcement"; and minimum wall thickness control is "lightweighting and cost reduction". These principles, combined with principles from the first part such as "uniform shrinkage" and "avoiding sharp corners", constitute a complete design logic "from materials to processes, from structure to connection".