Wall thickness in injection molding refers to the distance between the two opposite surfaces of a molded part. It plays a crucial role in the design and production of plastic parts. Choosing the proper wall thickness for your part can have a dramatic impact on the cost and speed of manufacturing. In this post, we will explain the importance of uniform wall thickness, what factors will affect the product wall thickness design, and some principles of wall thickness design.
Importance to Maintain Uniform Wall Thickness
Maintaining uniform wall thickness is of paramount importance in injection molding due to its profound impact on the structural integrity, aesthetics, and manufacturability of plastic parts.
- Avoiding Warping and Deformation:Uneven cooling caused by varying wall thickness can lead to differential shrinkage, warping, and deformation in the final part. This can compromise the functionality and aesthetics of the product.
- Minimized Sink Marks:Consistent wall thickness reduces the likelihood of sink marks or depressions on the surface of the part. These aesthetic defects can arise from variations in cooling rates.
- Affect Mold and Gate Design:Tooling and mold design are influenced by the desired wall thickness. Uniform thickness simplifies mold design and ejection systems, reducing complexity and minimizing the risk of part ejection challenges. And the uniform wall thickness also makes it easier to determine gate location.
- Reduce Shear Stress and Smoother Material Flow:Varying wall thickness can create differences in shear stress during the material flow process. Shear stress occurs when molten plastic is pushed through narrow channels or changes in geometry. Inconsistent wall thickness exacerbates shear stress differences, potentially causing material degradation, flow imbalances, and defects. Uniform wall thickness promotes smoother and more predictable material flow.
- Cooling Rate and Cycle Time:Inconsistent wall thickness affects cooling rates, potentially leading to parts with varying levels of residual heat. Uniform wall thickness enables more predictable cooling rates, reducing the risk of non-uniform shrinkage and the associated challenges of dimensional accuracy and warping. Moreover, uniform cooling contributes to shorter cycle times, boosting production efficiency.
- Assembly Compatibility:Parts with uniform wall thickness fit together more seamlessly during assembly. Inconsistent thickness can lead to misalignments, interference, or poor functionality in the final product.
Factors Affecting the Design of Product Wall Thickness
When designing the product wall thickness, it is necessary to comprehensively consider the product structure, material selection, and mold design.
—structural factors, there are many factors to consider, including determining functional requirements, and structural complexities. Structural components might require thicker wall sections to bear loads and stresses. Aesthetic parts could have thinner walls to reduce material usage and enhance visual appeal. Simple geometries can often maintain more consistent wall thickness, contributing to easier manufacturing and better part quality.
—In terms of materials, Materials with good flow properties might allow for more uniform wall thickness, while less flowable materials could require additional considerations. Materials with high shrinkage might need slightly thicker walls to compensate for post-molding dimensional changes. Those material properties are all we need to consider when designing product wall thickness.
—In mold design, Designing ribs or support structures can enable maintaining uniform wall thickness while reinforcing critical areas. In addition, gate system design, cooling system, ejector system, etc. are also factors that can affect wall thickness.
In the next sections, we will introduce in detail the factors that need to be considered in the design to ensure uniform product wall thickness, avoid uneven product wall thickness, and ensure that the product reaches higher quality during the injection molding manufacturing process.
Basic Principles of Wall Thickness Design
Designing the optimal wall thickness for injection molded parts is very important. It directly impacts both the structural integrity and manufacturing efficiency. To achieve uniform material flow, balanced cooling, and high-quality outcomes, we can make some efforts on the following sides.
Avoid Sharp Corners
Avoid sharp corners in the design, as they can lead to material flow and cooling challenges. Incorporate fillets or radii to smooth out corners. Rounded corners facilitate material flow during injection and reduce stress concentration points.
Adjacent Wall Thickness
Maintain consistent wall thickness between adjacent sections to promote even material flow and cooling. Sudden changes in thickness can cause uneven filling and cooling, leading to defects like sink marks and warping.
Draft Angles
Incorporate draft angles on vertical walls to facilitate easy part ejection from the mold. Draft angles prevent friction between the part and mold surfaces, reducing the risk of damage and ensuring uniform wall thickness.
Click here to learn more about how draft angles affect part quality.
Bosses
Design bosses (projections) with proper wall thickness to avoid sink marks on the opposite side. Adding ribs or gussets can strengthen bosses while minimizing variations in wall thickness.
Rims and Bends
Maintain uniform wall thickness along rims and bends to prevent material accumulation and improve structural integrity. Variations in thickness can lead to non-uniform cooling and warping.
Ribs
Integrate ribs to enhance structural integrity without sacrificing uniform wall thickness. Ribs distribute stress and load across the part, promoting consistent performance while avoiding overly thick sections.
Holes
Design holes with consistent wall thickness around their perimeters. Proper wall thickness ensures hole integrity and prevents distortion due to uneven cooling. Counterbore or chamfer holes to reduce stress concentration.
Select Suitable Material to Maintain Uniform Wall Thickness
Different materials have different characteristics. We could consider material properties when we design products and select materials. Here are the main properties we should consider.
Material Flow Characteristics
Material viscosity and flow behavior affect how well the molten plastic fills the mold cavity. Materials with good flow properties might allow for more consistent wall thickness, while less flowable materials could require additional considerations.
Material Strength
Stronger materials might allow for thinner wall thickness, especially in non-load-bearing areas. Weaker materials might necessitate thicker walls for structural integrity.
Cooling Rate
Materials with different thermal conductivities cool at different rates. Materials that cool quickly might allow for thinner walls, while slower-cooling materials might require thicker walls to prevent warping.
Shrinkage
Different materials exhibit varying degrees of shrinkage as they solidify. Materials with high shrinkage might need slightly thicker walls to compensate for post-molding dimensional changes.
Usually, each material has a recommended wall thickness range. Here’s a table that provides recommended wall thickness ranges for common injection molding materials.
Material | Recommended Wall Thickness Range (mm) | Recommended Wall Thickness Range (inches) |
ABS | 1.5 – 4 | 0.06 – 0.16 |
Polycarbonate (PC) | 1.5 – 3.5 | 0.06 – 0.14 |
Polypropylene (PP) | 1 – 4 | 0.04 – 0.16 |
Polyethylene (PE) | 1 – 4 | 0.04 – 0.16 |
Acrylic (PMMA) | 1.5 – 3.5 | 0.06 – 0.14 |
Nylon (PA) | 1.5 – 3.5 | 0.06 – 0.14 |
Polystyrene (PS) | 1 – 4 | 0.04 – 0.16 |
Polyoxymethylene (POM) | 1.5 – 3.5 | 0.06 – 0.14 |
Selecting the Optimal Wall Thickness
Injection molded parts typically have wall thicknesses between 2mm – 4mm (0.080″ – 0.160″). Thinner walls use less material, which reduces costs and requires less cooling time, resulting in shorter cycle times. Thicker parts require longer cooling times than thinner parts. Selecting the optimal wall thickness during the design phase of a product is crucial to ensure both structural integrity and efficient manufacturing. It is indeed a delicate balancing act that requires a deep understanding of various factors and real-world experience. Such as part cost, structural strength, cycle time, material cooling rates, shrinkage, etc.
What is the Minimum Injection Molding Wall Thickness?
Minimum injection molding wall thickness is the thinnest recommended wall for a specific plastic type. Factors to consider include expected loads – cosmetic vs. structural. Some start with the lowest thickness for the material and use FEA to assess. If too thin, adjust or change materials. Typically, 1 mm is the minimum, but it can be less for specific materials.
Click here to learn more about thin wall injection molding design tips and considerations.
What is the Maximum Injection Molding Wall Thickness?
Maximum injection molding wall thickness is the thickest recommended wall for a specific plastic type. Considerations include avoiding excessively thick walls, which can lead to defects. Sometimes, thicker walls are needed due to high loads, insulation needs, or desired weight. Most plastics have an upper limit, often 5 mm, to prevent issues like underfilling, warping, and dimensional deviations. The limit varies by material.
Conclusion
In conclusion, mastering the tips of wall thickness design in injection molding is a key factor in achieving exceptional part quality, structural integrity, and manufacturing efficiency. By embracing principles that ensure uniformity, careful consideration of material properties, and meticulous mold design, manufacturers can navigate the intricate balance between functionality and manufacturability. Using these strategies leads to the creation of injection-molded parts that meet top performance standards and make production smoother. As injection molding advances, sticking to these principles lets designers and manufacturers achieve better precision, consistency, and excellence in their products.
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