As a seasoned injection molding engineer, I've witnessed countless cases where the ideal design drafts are rosy, only to be met with a harsh reality after mold creation. Injection molding isn't like painting; simply looking good isn't enough. It must be practical, productive, and cost-effective. Today, I'll share several common design errors and our solutions to help you.
What is Injection Molding Design?
Injection molding is a highly efficient plastic part manufacturing process widely used across various industries. However, even minor errors in the injection molding design process can lead to product defects, reduced performance, and even production delays. This report explores common errors in injection molding design, analyzes their causes, and provides detailed solutions and best practices to help designers and engineers avoid costly rework and production issues.
Common Injection Molding Design Errors and Solutions
1. Inconsistent or Incorrect Wall Thickness
Error Description: Excessive wall thickness variation within the part, or improper wall thickness selection.
Cause: Uneven cooling rate: Thick wall areas cool slowly, while thin wall areas cool quickly, resulting in uneven shrinkage.
Restricted material flow: Plastic has difficulty flowing through areas of varying thickness.
Potential Consequences:
Sink Marks: Thick areas sink inward as they cool and shrink.
Warp: Part deformation caused by differential cooling rates.
Incomplete Fill: Plastic fails to completely fill the mold cavity.
Stress Concentration: This can lead to premature failure during part use.
Solutions and Best Practices:
Maintaining Uniform Wall Thickness: Design for uniform wall thickness whenever possible, avoiding sudden thickness changes. Ideally, thin sections should account for approximately 40%–60% of thicker sections.
Choosing the Appropriate Wall Thickness: Refer to the recommended minimum wall thickness based on the properties of the selected material. For example:
Utilizing Ribs and Gussets: Use ribs and gussets where needed to increase structural strength while maintaining uniform wall thickness.
2. Insufficient Draft Angles
Error Description: Insufficient draft angles are designed into vertical walls.
Cause: Ignoring demolding requirements: Failure to adequately consider ease of part removal from the mold.
Plastic Shrinkage: Plastic shrinks toward the center of the part during cooling, causing the part to cling to the mold.
Potential Consequences:
Difficulty in demolding: The part becomes stuck in the mold, making it difficult to remove.
Part Damage: The part surface is scratched, deformed, or cracked during demolding.
Mold Wear: Accelerated mold wear due to forced demolding.
Solutions and Best Practices:
Designing for Sufficient Draft Angles: Standard industry practice is to provide at least 1 degree of draft angle per side. For textured surfaces, an additional 0.001 degree is recommended for every 1.5 mm (0.025 in) of texture depth.
Considering Material Properties: Different materials have different shrinkage rates, which may require adjustment of draft angles.
3. Using Sharp Corners
Error Description: The part design contains sharp internal and external corners. Causes:
Design Habits: Failure to fully consider the geometric requirements of the injection molding process.
Simplified Design: Ignoring corner fillets to simplify the design.
Potential Consequences:
Stress Concentration: Sharp corners are prone to stress concentration, leading to cracking or breakage during use.
Uneven Filling: When plastic flows into sharp corners under high pressure, flow is hindered, potentially resulting in incomplete filling or air pockets.
Mold Manufacturing Difficulties: Manufacturing molds with sharp corners is costly and difficult.
Solutions and Best Practices:
Use Corners: Round all sharp corners and edges whenever possible to distribute stress and promote smooth plastic flow.
Corner Radius Recommendations:
Internal Corners: The corner radius should be approximately 0.5 times the adjacent wall thickness.
External Corners: The corner radius should be approximately 1.5 times the adjacent wall thickness.
4. Improper Rib Design
Error Description: The height, thickness, or location of the reinforcing ribs is not designed appropriately.
Causes:
Excessive pursuit of strength: Ribs are designed to be too high or too thick to reinforce the structure.
Ignoring process limitations: The effect of ribs on cooling and shrinkage is not considered.
Potential consequences:
Sink marks: Excessive rib thickness causes depressions in the surface beneath it.
Voids: Voids may form within the rib area.
Mold filling difficulties: Plastic has difficulty completely filling the rib area.
Stress concentration: Stress concentration may occur at the junction of the rib and the main wall.
Solutions and best practices:
Control rib height: The rib height should not exceed three times the nominal wall thickness.
Control rib thickness: The recommended rib thickness is approximately 40%–60% of the nominal wall thickness to prevent excessive shrinkage and stress concentration.
Optimize rib location: Arrange ribs so that they provide structural reinforcement without affecting material flow and cooling.
5. Undercut Issues
Error Description: The part design contains undercuts (undercuts) that are not properly addressed. Causes:
Functional or Aesthetic Requirements: Undercuts are introduced to achieve specific functions or appearances.
Design Complexity: The impact of undercuts on mold design and manufacturing costs is not fully considered.
Potential Consequences:
Mold Complexity: Additional mold mechanisms (such as slides or lifters) are required for demolding.
Increased Manufacturing Costs: Mold costs and production cycle times increase significantly.
Mold Demolding Difficulties: Parts may be damaged during demolding.
Solutions and Best Practices:
Avoiding Unnecessary Undercuts: Prioritize designing parts that do not require undercuts.
Designing Undercuts Properly: If undercuts are necessary, they should be parallel to the demolding direction and their structure should be as simple as possible.
Utilizing Special Mechanisms: Mold mechanisms such as slides, lifters, or collapsible cores can be used for demolding.
6. Improper Material Selection
Error Description: The selected material is not suitable for the part's function, operating environment, or injection molding process requirements.
Causes:
Lack of Material Knowledge: Insufficient understanding of the properties of different plastic materials (flowability, shrinkage, mechanical properties, weather resistance, etc.). Cost Prioritization: Excessive pursuit of low cost at the expense of material performance.
Potential Consequences:
Dimensional Deviation and Warpage: Material shrinkage is not accounted for, resulting in inaccurate or deformed part dimensions.
Degraded Performance: Parts fail to meet expected mechanical strength, heat resistance, weather resistance, and other requirements.
Surface Defects: Surface defects appear due to poor material flow or incompatibility with impurities.
Premature Failure: Parts rapidly deteriorate during use due to inappropriate materials.
Solutions and Best Practices:
Comprehensive Considerations: When selecting a material, consider the part's functional requirements, operating environment (e.g., outdoor, high-temperature, load-bearing), mechanical performance requirements, cost, and the characteristics of the injection molding process.
Understanding Material Properties: Be familiar with the flow characteristics, shrinkage (crystalline materials like PP and PE have high shrinkage, while amorphous materials like ABS and PS have low shrinkage), and additive requirements (e.g., UV stabilizers, glass fiber, lubricants) of different materials.
Avoiding Incompatible Materials: Ensure the material is pure and avoid mixing with dust, moisture, oil, or incompatible recycled materials, as these contaminants can degrade the polymer matrix.
7. Improper Gate Size or Position
Error Description: The gate (the entrance for plastic into the mold cavity) is too large or too small, or improperly positioned.
Causes:
Inexperience: Lack of understanding of the impact of gate design on material flow and product quality.
Simplified design: Failure to fully optimize the gate design to suit part geometry and material properties.
Potential Consequences:
Incomplete Fill (Short Shot): A gate that is too small restricts material flow, resulting in incomplete cavity filling.
Excessive Shear Stress: A gate that is too small causes excessive shear stress during plastic flow, affecting material properties.
Weld Marks: Visible marks where plastic meets within the mold cavity, affecting appearance and strength.
Flash: An oversized gate causes plastic to seep into the mold parting line or other gaps, forming excess material.
Surface Marks: Improper gate placement leaves visible marks on the part, affecting aesthetics. Solutions and Best Practices:
Adjust Gate Size: Adjust the gate size to 50%–80% of the part's nominal wall thickness. For example, a 2mm wall thickness typically uses a 1–1.6mm gate.
Select the Appropriate Gate Type: Select the appropriate gate type (e.g., side gate, pinpoint gate, or latent gate) based on the mold geometry and material used.
Optimize Gate Location: The gate should be placed away from critical aesthetic surfaces, in areas with thicker walls, and conducive to uniform material filling.
8. Inadequate Venting
Error Description: An incomplete venting system in the mold design prevents air and gas from being effectively vented from the mold cavity.
Causes:
Ignoring Venting Needs: Failure to fully understand the importance of venting to the injection molding process.
Improper Vent Design: Vents are too small, insufficient in number, or improperly positioned.
Potential Consequences:
Air Cavitation: Air trapped in the mold cavity forms cavities, affecting part strength and appearance.
Burn Marks: Trapped air is compressed under high temperature and pressure, causing the plastic to burn and discolor. Incomplete Fill: Air obstructs plastic flow, resulting in incomplete mold cavity filling.
Mold Release Issues: Negative pressure within the mold cavity makes it difficult to demold the part.
Solutions and Best Practices:
Add Vents at the End of Fill: Add vents at the ends of the plastic flow path, at ribs, corners, and along the parting line.
Appropriate Vent Size: For most materials, use vents 0.01 to 0.03 mm deep.
Clean the Vent System Regularly: Ensure the vents are unobstructed.
9. Not Considering the Parting Line
Error: The parting line (the joining line between the two halves of the mold) is improperly designed.
Causes:
Simplified Mold Design: The parting line is arbitrarily set to simplify the mold structure.
Ignoring Appearance: The impact of the parting line on the product's appearance is not fully considered.
Potential Consequences:
Flash: Molten plastic escapes through the gap at the parting line, forming thin, excess material that affects appearance and functionality.
Appearance Defect: The parting line is too noticeable on the part's surface, affecting its aesthetics. Increased Clamping Force Requirement: An improper parting line may result in the mold requiring greater clamping force.
Solutions and Best Practices:
Optimize Parting Line Location: It's best to place the parting line at sharp edges or subtle transitions in the geometry to minimize visual artifacts.
Utilize CAD Tools: Use the parting line analysis capabilities of modern CAD tools to optimize layout early in the design process.
10. Creating Features That Cannot Be Molded or Machined
Error Description: The designed part features are too complex to be molded or machined using conventional injection molding processes.
Causes:
Lack of Manufacturing Knowledge: Insufficient understanding of the limitations of the injection molding process.
Overdesign: Pursuing complex functionality or aesthetics while ignoring manufacturability.
Potential Consequences:
Mold Manufacturing Difficulties: Requires highly complex, expensive, and time-consuming mold solutions.
Inefficient Production: Complex mold structures lead to long production cycles and high maintenance costs.
Product Quality Issues: Complex features can make molding difficult and prone to defects. Solutions and Best Practices:
Simplify Design: Prioritize features that are simple and easy to mold, and avoid unnecessarily complex geometries.
Collaborate with Moldmakers: Work closely with mold designers and manufacturers experienced in injection molding early in the design process to ensure the manufacturability of your design.
Understand Mold Capabilities: Familiarize yourself with mold manufacturing capabilities to avoid designing difficult-to-mold features such as deep and complex undercuts, complex internal geometries, or extremely thin walls.
11. Specifying Unnecessarily Tight Tolerances
Error: Specifying overly tight tolerances on part dimensions or geometry.
Causes:
Pursuit of Perfection: Excessive pursuit of high precision without considering actual needs and manufacturing costs.
Lack of Cost Awareness: Failing to fully assess the additional costs and manufacturing challenges associated with tight tolerances.
Potential Consequences:
Increased Manufacturing Costs: Tight tolerances require more precise and expensive molds, stricter process controls, and longer production cycles.
Reduced Production Yield: Difficulty achieving tight tolerances leads to increased scrap rates.
Longer Lead Times: The production process requires more adjustments and testing, extending lead times. Solutions and Best Practices:
Evaluate tolerances appropriately: Set reasonable tolerances based on the actual functional and assembly requirements of the part, avoiding unnecessarily tight requirements.
Refer to industry standards: Adhere to industry-standard tolerance standards for injection molded parts.
Communicate with manufacturers: Communicate with manufacturers during the design phase to understand their process capabilities and recommended tolerances.
Injection molding design is a complex process that integrates knowledge of materials science, mold engineering, and product functionality. By avoiding the common mistakes mentioned above and following best practices, designers and engineers can significantly improve the quality of injection molded parts, reduce production costs, shorten development cycles, and ultimately meet customer needs. Maintaining close collaboration with moldmakers and injection molding experts throughout the design process is crucial to project success.
