Structural Design of Plastic Products

Plastic products are everywhere, but designing them isn’t simple. How do engineers balance strength, cost, and production efficiency? This article will uncover the complexities behind the structural design of plastic products. You'll learn key factors, like wall thickness, reinforcing ribs, and more, that make durable, cost-effective plastic parts.

Characteristics and Procedures of Plastic Part Structural Design

Plastic materials offer unique properties and versatile shaping options, setting them apart from conventional engineering materials like steel, copper, aluminum, and wood. This distinctive combination of material composition and formability grants plastics a higher degree of design flexibility compared to their counterparts.

Unique Material Composition and Versatile Shapes

The diverse range of plastic materials, each with its specific properties, allows designers to tailor their choice according to the product's requirements. This variety, coupled with the ability to mold plastics into intricate shapes, enables the creation of complex geometries and functional features that would be challenging or impractical with other materials.

General Procedure for Plastic Part Design

To leverage the advantages of plastics and ensure optimal structural design, it is essential to follow a systematic approach. The general procedure for plastic part design involves several key stages:

1. Determine functional requirements and appearance of the product:

• Identify the product's intended use and necessary functions

• Define the desired aesthetic appeal and visual characteristics

2. Draw preliminary design drawings:

• Create initial sketches and CAD models based on the functional and aesthetic requirements

• Consider the chosen plastic material's properties during the design process

3. Prototyping:

• Produce physical prototypes using methods like 3D printing or CNC machining

• Evaluate the prototype's functionality, ergonomics, and overall design

4. Product testing:

• Conduct rigorous tests to assess the product's performance under various conditions

• Verify if the design meets the specified functional requirements and safety standards

5. Design recalibration and revision:

• Analyze test results and identify areas for improvement

• Make necessary design adjustments to enhance performance, reliability, or manufacturability

6. Develop important specifications:

• Create detailed specifications for the final product, including dimensions, tolerances, and material grade

• Ensure the specifications align with the manufacturing process and quality control standards

7. Open mold production:

• Design and fabricate the injection mold based on the finalized product specifications

• Optimize the mold design for efficient material flow, cooling, and ejection

8. Quality control:

• Establish a robust quality control system to monitor and maintain product consistency

• Regularly inspect manufactured parts to ensure they meet the specified requirements

Fundamental Factors in Plastic Product Structural Design

Wall Thickness

Wall thickness plays a crucial role in plastic product design. Proper thickness ensures optimal performance, manufacturability, and cost-effectiveness.

Recommended Wall Thickness Values

Plastic Material	Minimum (mm)	Small Parts (mm)	Medium Parts (mm)	Large Parts (mm)
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

Factors Affecting Wall Thickness Selection

1. Plastic material properties

• Shrinkage rate

• Fluidity during injection molding

2. External forces endured

• Greater forces require thicker walls

• Consider metal parts or strength checks for special cases

3. Safety regulations

• Pressure resistance requirements

• Flammability standards

Reinforcing Ribs

Reinforcing ribs enhance strength without increasing overall wall thickness, prevent product deformation, and improve structural integrity.

Design Guidelines for Reinforcing Ribs

• Thickness: 0.5-0.75 times overall wall thickness (recommended: <0.6 times)

• Height: Less than 3 times wall thickness

• Spacing: Greater than 4 times wall thickness

Aspects of Reinforcement Design Needing Attention

1. Avoid material accumulation at rib intersections

2. Maintain perpendicularity to outer walls

3. Minimize reinforcing ribs on steep slopes

4. Consider appearance impact of sink marks

Draft Angles

Draft angles facilitate easy part removal from molds, ensuring smooth production and high-quality parts.

Recommended Draft Angles for Different Materials

Material	Mold Core	Mold Cavity
ABS	35'-1°	40'-1°20'
PS	30'-1°	35'-1°30'
PC	30'-50'	35'-1°
PP	25'-50'	30'-1°
PE	20'-45'	25'-45'
PMMA	30'-1°	35'-1°30'
POM	30'-1°	35'-1°30'
PA	20'-40'	25'-40'
HPVC	50'-1°45'	50'-2°
SPV	25'-50'	30'-1°
CP	20'-45'	25'-45'

Aspects of Draft Angle Selection Needing Attention

1. Choose smaller angles for glossy surfaces and high-precision parts

2. Use larger angles for parts with high shrinkage rates

3. Increase draft for transparent parts to prevent scratches

4. Adjust angle based on texture depth for textured surfaces

R Corners (Rounded Corners)

Rounded corners reduce stress concentration, facilitate plastic flow, and ease demolding.

Design Guidelines for R Corners

• Internal corner radius: 0.50 to 1.50 times material thickness

• Minimum radius: 0.30mm

• Maintain uniform wall thickness when designing rounded corners

• Avoid rounded corners on mold parting surfaces

• Use minimum 0.30mm radius for edges to prevent scratching

Holes

Holes serve various functions in plastic products and require careful design consideration.

Design Requirements for Holes

• Distance between holes (A): ≥ D (hole diameter) if D < 3.00mm; ≥ 0.70D if D > 3.00mm

• Distance from hole to edge (B): ≥ D

Relationship Between Hole Diameter and Depth

• Blind hole depth (A): ≤ 5D (recommended A < 2D)

• Through-hole depth (B): ≤ 10D

Design Considerations for Special Hole Types

1. Step holes: Use multiple coaxially connected holes of different diameters

2. Angled holes: Align axis with mold opening direction when possible

3. Side holes and indentations: Consider core pulling structures or design improvements

Bosses

Bosses provide assembly points, support other parts, and enhance structural integrity.

Basic Design Guidelines for Bosses

• Height: ≤ 2.5 times boss diameter

• Use reinforcement ribs or attach to outer walls when possible

• Design for smooth plastic flow and easy demolding

Design Points for Different Materials

• ABS: Outer diameter ≈ 2x inner diameter; use beveled ribs for strengthening

• PBT: Base design on rib concept; connect to sidewalls when possible

• PC: Interlock side bosses with ribs; use for assembly and support

• PS: Add ribs for strengthening; connect to sidewalls when nearby

• PSU: Outer diameter ≈ 2x inner diameter; height ≤ 2x outer diameter

Inserts

Inserts enhance functionality, provide decorative elements, and improve assembly options in plastic parts.

Shape and Structural Requirements for Inserts

1. Manufacturability: Compatible with cutting or stamping processes

2. Mechanical strength: Sufficient material and dimensions

3. Bonding strength: Adequate surface features for secure attachment

4. Positioning: Cylindrical extending portions for easy mold placement

5. Flash prevention: Include sealing boss structures

6. Post-processing: Design for secondary operations (threading, cutting, flanging)

Design Considerations When Using Inserts

• Ensure precise positioning within molds

• Create strong connections with molded parts

• Prevent plastic leakage around inserts

• Consider thermal expansion differences between insert and plastic material

Product Surface Texture and Text/Pattern Design

Surface Textures for Plastic Products

Plastic product surfaces can be designed with various textures to enhance aesthetics, functionality, and user experience. Common surface textures include:

1. Smooth

2. Spark-etched

3. Patterned etched

4. Engraved

Smooth Surfaces

Smooth surfaces result from polished mold surfaces. They offer:

• Clean, sleek appearance

• Easier part ejection from the mold

• Lower draft angle requirements

Spark-Etched Surfaces

Created through copper EDM processing of the mold cavity, spark-etched surfaces provide:

• Unique, subtle texture

• Improved grip

• Reduced visibility of surface imperfections

Patterned Etched Surfaces

These surfaces feature various patterns etched into the mold cavity, offering:

• Customizable designs

• Enhanced product differentiation

• Improved tactile properties

Engraved Surfaces

Engraved surfaces are created by directly machining patterns into the mold, allowing for:

• Deep, distinct textures

• Complex designs

• Durability of surface features

Draft Angle Considerations for Textured Surfaces

When designing textured surfaces, consider increasing draft angles to facilitate part ejection:

Texture Depth	Recommended Additional Draft Angle
0.025 mm	1°
0.050 mm	2°
0.075 mm	3°
> 0.100 mm	4-5°

Text and Pattern Design

Plastic products often incorporate text and patterns for branding, instructions, or decorative purposes. These elements can be either raised or recessed.

Raised vs. Recessed Surfaces

Recommendation: Use raised surfaces for text and patterns when possible.

Benefits of raised surfaces:

• Simplified mold processing

• Easier mold maintenance

• Enhanced legibility

For designs requiring flush or recessed features:

1. Create a recessed area

2. Raise text or pattern within the recess

3. Maintain overall flush appearance while simplifying mold design

Text and Pattern Dimensions

Feature	Recommended Dimension
Height/Depth	0.15 - 0.30 mm (raised)
	0.15 - 0.25 mm (recessed)

Text Size Specifications

Follow these guidelines for optimal text design:

• Stroke width (A): ≥ 0.25 mm

• Spacing between characters (B): ≥ 0.40 mm

• Distance from characters to edge (C, D): ≥ 0.60 mm

Additional Text/Pattern Design Considerations

1. Avoid sharp angles in text or patterns

2. Ensure size is conducive to molding process

3. Consider the impact of text/pattern on overall part strength

4. Evaluate the effect of text/pattern on material flow during molding

Additional Structural Design Considerations

Reinforcement Structure Design Principles

Reinforcement structures play a crucial role in enhancing the overall performance of plastic products. They significantly improve strength, stiffness, and dimensional stability.

Key Objectives of Reinforcement Design:

1. Strength enhancement

2. Stiffness improvement

3. Warping prevention

4. Deformation reduction

Proper Positioning and Sizing of Reinforcements:

• Wall thickness: 0.4-0.6 times main body thickness

• Spacing: > 4 times main body thickness

• Height: < 3 times main body thickness

• Screw column reinforcement: At least 1.0mm below column surface

• General reinforcement: Minimum 1.0mm below part surface or parting line

Advanced Reinforcement Techniques:

1. Misaligned reinforcement bars to prevent material buildup

2. Hollow structures at reinforcement intersections

3. Tension-based designs for slender reinforcements

Avoiding Stress Concentration

Stress concentration can significantly impact the structural integrity and longevity of plastic products. Proper design techniques can mitigate these issues.

Importance of Avoiding Sharp Corners:

• Reduced part strength

• Increased risk of crack initiation

• Potential for premature failure

Measures to Reduce Stress Concentration:

1. Chamfers

2. Rounded corners

3. Gentle slopes for transitions

4. Inward hollowing at sharp corners

Technique	Description	Benefit
Chamfers	Beveled edges	Gradual stress distribution
Rounded corners	Curved transitions	Eliminates sharp stress points
Gentle slopes	Gradual thickness changes	Even stress distribution
Inward hollowing	Material removal at corners	Localized stress reduction

Designing Suitable Draft Angles

Draft angles are essential for successful part ejection from molds. They significantly impact part quality and production efficiency.

Principles for Determining Draft Angles:

1. Use whole number angles (e.g., 0.5°, 1°, 1.5°)

2. Exterior angles > interior angles

3. Maximize angles without compromising appearance

Factors Influencing Draft Angle Size:

• Part depth

• Surface finish

• Material shrinkage rate

• Texture depth

Draft Angle Design Points for Different Materials:

Material	Recommended Draft Angle Range
ABS	0.5° - 1°
PC	1° - 1.5°
PP	0.5° - 1°
PS	0.5° - 1°
PET	1° - 1.5°

Structural Design from Mold Structure Perspective

Efficient mold design is crucial for successful plastic part production. Consider these aspects to optimize both part and mold design.

Avoiding Complex Structures:

• Simplify part geometry

• Reduce undercuts

• Minimize side actions

Avoiding Internal Cutting Structures:

• Eliminate features requiring complex core pulls

• Design for split-line accessibility

Considering Lateral Release Requirements:

• Allow sufficient space for slider movement

• Design appropriate shut-off surfaces

• Optimize part orientation in the mold

Designing for Non-Isotropic Characteristics of Plastics

Many plastics exhibit non-isotropic properties, requiring special design considerations to maximize performance.

Aligning Material Flow Direction with Load-Bearing Direction:

• Orient mold gates to promote favorable flow patterns

• Consider fiber orientation in reinforced plastics

Force Direction Relative to Fusion Lines:

• Design for forces perpendicular or angled to weld lines

• Avoid parallel forces to fusion lines to prevent weakness

Structural Design from Assembly Perspective

Effective assembly design ensures product functionality, longevity, and ease of manufacture.

Avoiding Large Sizes with Small Tolerances:

• Break large parts into smaller components

• Use appropriate tolerance stacks

Bonding Interface Design:

• Prioritize shear force over tearing tension

• Increase bonding surface area

• Consider chemical compatibility of adhesives

Bolt Connection Considerations for Plastic Parts:

• Use inserts for high-stress connections

• Design appropriate boss structures

• Consider thermal expansion differences

Summary

In plastic product design, key structural factors like wall thickness, reinforcing ribs, and draft angles are essential for durability and performance. It's crucial to consider material properties, mold structure, and assembly needs throughout the process. Proper structural design not only enhances product functionality but also reduces defects and manufacturing costs. By focusing on these design elements, manufacturers can ensure high-quality, cost-effective plastic parts that meet both functional and aesthetic requirements.