The success of injection molding depends heavily on the design of the plastic part itself. This article provides a detailed guide to designing plastic parts for injection molding, covering key considerations, design principles, and best practices.
Injection molding involves injecting molten plastic material into a mold cavity under high pressure. The material cools and solidifies, taking the shape of the cavity. The process is ideal for producing high volumes of parts with consistent quality. Key elements include:
- Injection Unit: Melts and injects the plastic material.
- Mold: Consists of two halves (the cavity and core) that form the part shape.
- Clamping Unit: Holds the mold halves together during injection.
Plastic Part Design Guide for Injection Molding
Wall Thickness
Proper wall thickness is a fundamental requirement in plastic part design for injection molding. Inconsistent wall thickness can lead to defects such as sink marks, voids, stresses, and warping. As plastic cools, it shrinks, causing thicker sections to pull inward, creating stresses and defects.
- Material Considerations: For thermoplastics, wall thickness typically ranges from 1 to 6 mm, with 2 to 3 mm being most common. Larger parts may need thicker walls. For more information on material wall thickness, please refer to the table below.
- Uniformity: Consistent wall thickness helps avoid defects like sink marks and warping. If transitioning from thicker to thinner areas, keep the ratio gradual, ideally up to 3:1.
- Ribs: Using ribs can reinforce parts and reduce material usage without increasing wall thickness, which also shortens cooling time.
- Flow Path: The distance the molten material travels from the gate to the part affects filling efficiency. A longer flow path relative to wall thickness might require adjustments in thickness.
- Thinner Sections: Thinner sections cool quicker, reducing stress and warping between sections of different thicknesses.
| Plastic Material | Recommended Wall Thickness Range (mm) |
|---|---|
| ABS (Acrylonitrile Butadiene Styrene) | 1.5 – 4.0 |
| Polycarbonate (PC) | 2.0 – 4.0 (Thicker walls for larger parts to reduce warping) |
| Polypropylene (PP) | 1.5 – 3.5 |
| Polyethylene (PE) | 1.0 – 3.0 (Depends on grade, LDPE, HDPE, etc.) |
| Nylon (Polyamide) | 2.0 – 4.0 (Stronger grades may allow thinner walls) |
| Polyvinyl Chloride (PVC) | 1.5 – 3.5 (Flexible PVC may allow thinner walls) |
| Acrylic (PMMA) | 2.0 – 4.0 (Requires support for thicker walls to prevent sagging) |
| Polystyrene (PS) | 1.5 – 3.0 (HIPS tends to have slightly thicker walls) |
| PET (Polyethylene Terephthalate) | 1.0 – 3.0 (Commonly used for bottles and containers) |
| PBT (Polybutylene Terephthalate) | 2.0 – 4.0 (High strength and heat resistance) |
Draft Angles
Draft angles are necessary to facilitate the easy removal of the part from the mold. These angles are applied to the surfaces of the part that come into contact with the mold cavity, specifically in the direction of mold opening. This allows the part to easily separate from the mold without causing damage to the part or the mold itself during the ejection process.
The minimum requirement for draft angles can vary depending on several factors, but a range of 0.5° to 1° is generally considered the bare minimum. However, in most cases, a draft angle of 1.5° to 2° is widely accepted as the norm, providing a good balance between part releasability and manufacturing efficiency. This range offers sufficient clearance for the part to slide out of the mold without resistance, minimizing the risk of scratches, warping, or even breaking during ejection.
Sharp Angles
Sharp angles can increase the risk of stress concentrations within the plastic material as it cools and solidifies. These stress concentrations can lead to cracking, warping, or even failure of the part under normal operating conditions. Additionally, sharp angles can create sink marks or voids, which are surface imperfections that can detract from the part’s aesthetics and may compromise its structural integrity.
Moreover, sharp angles can make it difficult for the plastic to flow evenly during the injection molding process. This can result in uneven wall thickness, which can further exacerbate stress concentrations and increase the likelihood of defects. In extreme cases, sharp angles can even cause the plastic to freeze off prematurely, blocking the flow of material and preventing the mold from filling completely.
To mitigate these issues, designers often incorporate radii or fillets into sharp angles. These rounded transitions distribute stress more evenly and reduce the likelihood of cracking or warping. They also improve the flow of plastic during the molding process, helping to ensure consistent wall thickness and reducing the risk of defects. Furthermore, radii and fillets can enhance the part’s overall appearance and make it easier to handle and assemble.
Top-Out Direction and Parting Line in Injection Molding
- Top-Out Direction: Establishing the top-out direction early in the design process is essential. It helps minimize the need for complex core pulls and reduces the visual impact of parting lines. Align features like ribs, snaps, and protrusions with the top-out direction to avoid core pulls, lessen seam lines, and extend mold life.
- Parting Line: Selecting the right parting line is crucial for both aesthetics and functionality. The parting line should enhance the part’s appearance and ease of mold release. A well-placed parting line helps reduce visible seams and improves the overall quality of the part.
- Ejection Forces: During ejection, parts must overcome both top-out force and opening force. The top-out force is typically much higher due to cooling shrinkage and friction between the part and core. Excessive ejection force can cause part deformation, whitening, wrinkling, and surface wear.
Rib Design Considerations
Adding ribs increases the thickness at their junction with the main wall. This thickness is influenced by the maximum fillet radius, determined by rib thickness and root radius. For example, with a base material thickness of 4 mm, altering rib thickness and fillet radius changes the diameter of the maximum fillet radius. Proper rib design can reduce surface indentation and improve part quality.
Rib Shrinkage Areas:
- Rib Thickness: To maintain rigidity, rib thickness should be balanced. Thin ribs require increased height for stiffness but may lead to issues like deformation under pressure and difficulty in filling. Rib bases should not have too small a radius to avoid stress concentration. Typically, the rib base radius should be at least 40% of the rib thickness. Rib thickness should be 50% to 75% of the base material thickness, with higher ratios limited to materials with lower shrinkage rates. Rib height should be less than five times the base material thickness.
- Draft Angles and Spacing: Ribs must have draft angles and align with the top-out direction or utilize movable mold parts. Spacing between ribs should be greater than twice the base material thickness.
Enhancing Rigidity: For uniform rigidity in all directions, adding ribs both longitudinally and transversely at right angles is effective. However, this can increase wall thickness at intersections, leading to greater shrinkage. A common solution is to add a round hole at the intersection to create uniform wall thickness.
Holes in Plastic Part Design
Holes in plastic parts are commonly used for assembly or functionality. To maintain strength and simplify manufacturing, key factors include:
- Spacing: The distance between adjacent holes or from a hole to the nearest edge should be at least equal to the hole diameter. This is crucial for holes near edges to prevent cracking. For threaded holes, this distance should generally be more than three times the diameter of the hole.
Types of Holes:
- Through Holes: These are more common and easier to produce compared to blind holes. Structurally, through holes are simpler and can be formed using pins in either the movable or fixed parts of the mold. The former creates two short cantilever beams, while the latter forms a simple supported beam, both with minimal deformation.
- Blind Holes: Typically formed with cantilever beams, which can bend under the impact of molten plastic, leading to irregular hole shapes. Blind holes should not exceed twice their diameter in depth, and for diameters of 1.5 mm or smaller, depth should not exceed the diameter. The bottom wall thickness should be at least one-sixth of the hole diameter to avoid shrinkage issues.
Side Holes: Side holes are often created using side cores, which can increase mold costs and maintenance, especially if the cores are long and prone to breakage. Where possible, design improvements can be made to mitigate these issues.
Boss Design
Bosses are projections from the wall thickness of plastic parts used for assembly, separating objects, and supporting other components. Hollow bosses can accommodate inserts or threaded screws. To withstand pressure without cracking, bosses typically have a cylindrical shape, which is easier to mold and provides better mechanical performance.
Structural Integration:
- Connection: Ideally, bosses should not be designed as isolated cylinders. They should connect to the outer wall or be used with ribs to enhance strength and improve the flow of plastic material. The connection with the outer wall should be a thin-wall connection to prevent shrinkage issues.
- Radius and Thickness: The radius at the base of the boss should be 0.4 to 0.6 times the base material thickness. The boss wall thickness should be 0.5 to 0.75 times the base material thickness. The top of the boss should be chamfered to facilitate screw installation and should include a draft angle for ease of mold release. These requirements are similar to those for rib design.
Threaded Bosses for Self-Tapping Screws:
Threaded bosses often connect with self-tapping screws. Internal threads in these bosses are formed through cold flow processing, which deforms but does not cut the plastic. The size of the threaded boss must be sufficient to handle the insertion force and load carried by the screw.
Dimensions and Insertions: The boss diameter must withstand circumferential forces generated during screw tightening. Typically, the top of the boss is designed with a recess slightly larger than the nominal diameter of the screw for easy insertion. Calculating the correct dimensions can be complex, but simplified estimation methods based on screw nominal diameter and material type are available.
Snap-Fit Connections
Snap-fit connections offer a convenient, cost-effective, and eco-friendly method for assembling plastic parts. These connections are formed during the molding process, eliminating the need for additional fasteners like screws. Assembly is achieved by snapping parts together, simplifying the process.
Snap-Fit Mechanism: The snap-fit mechanism involves pushing a protruding part of one component over an obstacle on another component. This process requires elastic deformation; once the obstacle is cleared, the part snaps back to its original shape, locking the components together.
Angles and Calculations:
- Critical Angles: Two important angles in snap-fit design are the retraction angle and the entry angle. A larger retraction angle generally provides a stronger connection. When the retraction angle approaches 90 degrees, the snap-fit becomes permanent.
Snap-Fit Calculations:
- Maximum Deflection: For a snap-fit with a uniform cross-section, the maximum allowable deflection (Y) can be calculated using:
This formula assumes deformation occurs only within the snap-fit hook. Some deformation near the snap-fit can be considered as a safety factor.
- Force Required for Deflection: The force (P) needed to produce deflection Y is:
- Assembly Force: The assembly force (W) can be estimated using:
For releasable snap-fits, the same formula is used, substituting angle b for angle a.
The following table provides the coefficients calculated based on different materials.
| Materials | (e)(%) | GPa | Coefficient(s) of Friction |
|---|---|---|---|
| PS | 2 | 3.0 | 0.3 |
| ABS | 2 | 2.1 | 0.2 |
| SAN | 2 | 3.6 | 0.3 |
| PMMA | 2 | 2.9 | 0.4 |
| LDPE | 5 | 0.2 | 0.3 |
| HDPE | 4 | 1.2 | 0.3 |
| PP | 4 | 1.3 | 0.3 |
| PA | 3 | 1.2 | 0.1 |
| POM | 4 | 2.6 | 0.4 |
| PC | 2 | 2.8 | 0.4 |
Ring Snap-Fit Connections: Ring snap-fits use an internal projection on a ring to engage with a groove on a shaft. They can be either releasable or non-releasable, depending on the release angle. The ring expands elastically during insertion and removal, typically made from materials with good elasticity.
- Maximum Projection Size: The maximum size of the ring projection can be calculated with:
where S is the design stress, v is Poisson’s ratio, E is the modulus of elasticity, and K is the geometric coefficient, given by:
- Expansion Force: The force (P) required for expansion on the sleeve can be calculated using:
where μ is the friction coefficient.
Interference Fits
Interference fits are commonly used to connect holes and shafts, effectively transmitting torque and other forces. This method offers a direct and reliable connection. However, achieving the right interference fit is crucial, as insufficient interference can lead to unreliable connections, while excessive interference can make assembly difficult and increase the risk of cracking.
Key Considerations: When designing interference fits, it is essential to consider the tolerance of both the hole and shaft, as well as the working temperature, since temperature fluctuations can significantly affect the interference fit.
Common Practices:
- Surface Enhancements: To ensure a reliable connection, particularly with metal shafts, features like knurling or grooves are often added to the mating shaft.
- General Formula for Interference Fit:
where S is the design stress, v is Poisson’s ratio, E is the modulus of elasticity, and K is the geometric coefficient, calculated as:
- Assembly Force Calculation:
where μ is the friction coefficient and l is the engagement length.
- Materials and Poisson’s Ratio:
| Material | Poisson’s Ratio |
|---|---|
| ABS | 0.38 |
| PMMA (Acrylic) | 0.4 |
| LDPE (Low-Density Polyethylene) | 0.49 |
| HDPE (High-Density Polyethylene) | 0.47 |
| Polypropylene (PP) | 0.43 |
| Polycarbonate (PC) | 0.45 |
| PVC | 0.42 |
| POM (Polyoxymethylene) | 0.42 |
| PPS (Polyphenylene Sulfide) | 0.41 |
| Steel | 0.38 |
Alternative Joining Methods: Besides interference fits, other methods for joining plastic parts include hot-melt, welding, and ultrasonic welding.
Radii in Plastic Part Design
If a part has an internal radius but a sharp external corner, the area around the bend will be thicker than other sections, causing shrinkage issues. To solve this, both internal and external corners should be rounded to achieve uniform wall thickness. In this case, the outer radius should be the sum of the inner radius and the base wall thickness.
For cantilever snap-fits, the cantilever needs to bend and fit into place. If the radius (R) is too small, it will lead to excessive stress concentration, making the part prone to breaking during bending. Conversely, if R is too large, it may cause shrinkage marks and gaps. Therefore, the ratio between the radius and wall thickness should be kept between 0.2 and 0.6, with an ideal value around 0.5.
Gates and Ejector Pins in Plastic Part Design
Gates and ejector pins are crucial components in the molding process, enabling the strategic entry of plastic resin into the mold and effectively ejecting the finished part from the mold. Understanding different gate types and their locations is essential before preparing for processing.
Types of Gates:
- Pin Gates: Commonly used, tapering from the runner to the part surface, allowing heat dissipation and minimizing warping. Requires manual removal, leaving a small mark.
- Sub Gates: Includes tunnel and back gates, which reduce visible marks. Tunnel gates enter from the middle of the part, while back gates use pins near the part’s perimeter, potentially leaving decorative shadows.
- Hot Tip Gates: Ideal for balanced filling and minimal waste. Aesthetically pleasing and can be hidden in pits or around logos.
- Direct Gates: Larger and less attractive, used for high glass content materials or parts needing secondary processing. Difficult to remove manually.
Best Practices for Successful Injection Molding Design
- Collaborate with Mold Makers: Work closely with mold designers and manufacturers to ensure that the design is feasible and cost-effective.
- Optimize Design for Efficiency: Focus on reducing material waste, minimizing cycle times, and ensuring ease of part ejection.
- Continuous Improvement: Use feedback from prototypes and initial production runs to refine the design for better performance and cost-efficiency.
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Conclusion
Designing plastic parts for injection molding involves a deep understanding of both the design principles and the manufacturing process. By considering material properties, wall thickness, draft angles, and other key factors, designers can create parts that are not only functional and aesthetically pleasing but also cost-effective to produce. Following best practices and avoiding common pitfalls will lead to successful injection-molded parts and efficient manufacturing processes.
FAQ
Injection molding offers several benefits including high production efficiency, the ability to produce complex and detailed parts, excellent repeatability and consistency, minimal waste, and cost-effectiveness for high-volume production. It also allows for the use of a wide range of plastic materials with varying properties.
Choose a plastic material based on the desired mechanical properties (e.g., strength, flexibility), thermal resistance, chemical compatibility, and cost considerations. Common options include ABS for impact resistance, polycarbonate for high strength and transparency, and polypropylene for chemical resistance and flexibility.
Incorporate features such as ribs for added strength, bosses for mounting components, and snap-fits for easy assembly. Designing with functional areas in mind ensures the part performs its intended function effectively and integrates seamlessly with other components.
Gates are the entry points through which molten plastic flows into the mold cavity. Their placement affects the part’s filling and can impact the quality of the final product. Proper gate design helps ensure even filling, minimize defects, and avoid issues like weld lines and air traps.
Prototyping is a key step to test and validate your design. Techniques such as 3D printing or short-run injection molding can be used to create prototypes. These prototypes allow for testing and adjustments before committing to full-scale production.
This article was written by engineers from the BOYI team. Fuquan Chen is a professional engineer and technical expert with 20 years of experience in rapid prototyping, mold manufacturing, and plastic injection molding.
