August 18, 2023

How Thickness Injection Affects the Quality and Cost of Molded Products

Injection Molding tolerance

Thickness injection is a critical aspect of the plastic injection molding process. It refers to the practice of controlling the wall thickness of a plastic part during the molding process. The wall thickness of a plastic part can significantly influence its functionality, aesthetics, and manufacturability. Achieving the right wall thickness is crucial for the success of any injection molding project. The thickness of the walls in a plastic part can affect the flow of molten plastic, the cooling rate, and the overall quality of the final product. In this article, we will explore the importance of thickness injection in plastic molding, the challenges involved, and the best practices for achieving optimal wall thickness.

What Are the Challenges in Achieving Uniform Wall Thickness for Complex Injection Molded Parts?

Achieving uniform wall thickness in injection molded parts can be challenging, especially for complex designs. When the wall thickness varies significantly within a part, it can lead to a range of issues, including warpage, sink marks, and voids. These defects can compromise the structural integrity and appearance of the part. The flow of molten plastic in the mold cavity is influenced by the wall thickness. Thicker walls can slow down the flow, leading to short shots, while thinner sections may experience excessive shear stress, causing flow lines. Additionally, variations in wall thickness can result in different degrees of shrinkage during cooling, leading to warpage. To overcome these challenges, it is essential to consider design for manufacturability (DfM) principles and work closely with experienced injection molders.

Determining Optimal Wall Thickness: Key Principles

wall thickness injection molding
Source: Plastics Business Magazine

Determining the optimal wall thickness for an injection molded part is a delicate balance between design considerations, material properties, and manufacturability. A general rule is to aim for uniform wall thickness throughout the part. This ensures even cooling and reduces the risk of defects like warping. However, there are instances where varying wall thicknesses might be necessary due to part design or functionality requirements. In such cases, gradual transitions between thick and thin areas are recommended to avoid sharp corners and abrupt changes. Using tools like a plastic thickness chart or consulting with experts can provide insights into the recommended thickness for specific materials, ensuring both strength and manufacturability.

The Relationship Between Material Choice and Thickness Injection

Material choice plays a pivotal role in determining the wall thickness in injection molding. Different materials, from thermoplastics like nylon and polyester to more specialized resins, have unique flow characteristics and cooling rates. For instance, a material like polycarbonate might require a different wall thickness compared to silicone, given its specific properties. Understanding the material’s behavior during the molding process is crucial. Factors such as how the molten plastic fills the mold cavity, its susceptibility to shear stress, and its cooling rate all influence the optimal wall thickness. Collaborating with material suppliers and leveraging resources like the Aco Mold design guide can offer valuable insights into material-specific guidelines for thickness injection.

What Are the Common Defects Caused by Improper Wall Thickness in Injection Molding?

Thickness Injection acomold

Improper wall thickness can lead to a myriad of defects in injection molded parts. One common issue is warping, which occurs when different sections of a part cool at varying rates, causing distortion. Sink marks are another prevalent defect, appearing as depressions on the surface of the part. They often arise when thick sections cool and shrink, pulling the material inward. Short shots, where the molten plastic doesn’t fill the entire mold cavity, can occur if wall thickness impedes the flow of material. Additionally, excessive shear stress in thin sections can lead to flow lines, visible streaks on the part’s surface. By ensuring a consistent and appropriate wall thickness, many of these defects can be mitigated or entirely avoided.

How Wall Thickness Influences Aesthetics and Structural Integrity

Wall thickness isn’t just a technical specification; it has a profound impact on both the aesthetics and structural integrity of the molded product. Aesthetically, uneven wall thickness can lead to visible defects like sink marks or flow lines, detracting from the product’s appearance. From a structural standpoint, parts with inconsistent wall thicknesses can experience internal stresses, leading to premature failure or breakage. For instance, thick walls might seem like a good idea for added strength, but they can introduce challenges like longer cycle times and increased chances of defects. Conversely, very thin walls might not withstand the mechanical stresses the part is subjected to during its lifecycle. Striking the right balance is crucial for a product that looks good and performs reliably.

Gating Challenges: The Flow of Molten Plastic

Gating is the entry point where molten plastic enters the mold cavity. The design and location of gates play a pivotal role in how the material flows, especially when considering wall thickness. If the walls are too thick, the molten plastic might not flow smoothly, leading to short shots or incomplete filling. On the other hand, if the walls are too thin, the material might cool too quickly before filling the entire cavity, leading to voids or weak points. Proper gating ensures that the molten plastic fills the mold uniformly, respecting the desired wall thickness and minimizing defects. It’s a dance of precision, where the flow of molten plastic needs to be harmonized with the part’s design and wall thickness.

The Economic Implications of Wall Thickness in Injection Molding

Wall thickness doesn’t just affect product quality; it has economic implications too. Thicker walls require more material, increasing the cost per part. They also take longer to cool, extending the cycle time and reducing the production rate. On the flip side, walls that are too thin might lead to increased reject rates due to defects, driving up production costs. Furthermore, reworking or scrapping defective parts adds to wastage and overheads. By optimizing wall thickness, manufacturers can strike a balance between material usage, production speed, and quality, ensuring cost-effective production.

Shear Stress and Its Impact on Molded Product Quality

Shear stress is a force that acts parallel to the flow direction of the molten plastic during the injection molding process. When the molten plastic flows through the mold’s narrow sections or around sharp corners, it can experience different degrees of shear stress. This can lead to issues like flow lines, weld lines, or even burn marks on the molded part. The wall thickness plays a significant role in determining the amount of shear stress a particular section of the part will experience. Thinner sections can be more susceptible to high shear rates, which can lead to material degradation or discoloration. By understanding and optimizing wall thickness in relation to the part’s geometry and the material’s properties, molders can minimize the adverse effects of shear stress and produce high-quality parts.

Cooling Rate Variations: Thicker vs. Thinner Walls

Cooling rates are pivotal in the injection molding process. They influence the part’s final dimensions, appearance, and mechanical properties. Thicker walls take longer to cool than thinner ones. This difference in cooling rates can lead to internal stresses, especially if thick and thin sections are adjacent to each other. Such stresses can result in warping or even part deformation. Additionally, uneven cooling can lead to sink marks in thicker areas, as the inner material shrinks while cooling, pulling the surface inward. By maintaining a uniform wall thickness or ensuring gradual transitions between varying thicknesses, molders can achieve a more consistent cooling rate, reducing the risk of defects and ensuring the part’s dimensional stability.

Material-Specific Guidelines for Thickness Injection

Thickness Injection

Different materials have unique characteristics that influence their optimal wall thickness in injection molding. For instance, thermoplastics like nylon or polyester might have different recommended thicknesses compared to more rigid materials like polycarbonate. Factors such as the material’s flow behavior, shrinkage rate, and thermal conductivity all play a role in determining the ideal wall thickness. Resources like the plastic sheet thickness chart or the Aco Mold injection molding guide can provide valuable insights into material-specific recommendations. Collaborating with material experts and leveraging such resources can ensure that the chosen wall thickness aligns with the material’s properties, leading to successful molding outcomes.

MaterialWall Thickness (in)Wall Thickness (mm)Shrinkage Rate (%)
ABS0.045 – 0.1401.14 – 3.560.4 – 0.7
Acrylic (PMMA)0.025 – 0.1500.635 – 3.810.2 – 0.8
HDPE0.030 – 0.1500.76 – 3.811.5 – 4.0
POM0.030 – 0.1200.76 – 3.051.5 – 2.5
Polycarbonate0.040 – 0.1501.02 – 3.810.5 – 0.7
Polypropylene (PP)0.025 – 0.1500.635 – 3.811.0 – 2.5
TPU + TPE0.020 – 0.1200.51 – 3.051.0 – 3.0
Nylon + GF0.030 – 0.1150.76 – 2.920.5 – 1.5
PVC0.025 – 0.1500.635 – 3.810.2 – 0.6
PPS0.020 – 0.1300.51 – 3.300.5 – 1.0
Nylon Plastic (PA)0.030 – 0.1150.76 – 2.920.5 – 1.5
Silicon Rubber0.020 – 0.1200.51 – 3.051.0 – 3.0

The Role of Design for Manufacturability (DFM) in Thickness Injection

Design for Manufacturability (DFM) is a proactive approach that integrates the design and manufacturing processes. In the context of thickness injection, DFM plays a vital role. It encourages designers to consider how the wall thickness of a part will affect its manufacturability, cost, and performance. For instance, DFM might involve using a consistent and minimum wall thickness to reduce material usage and cycle time, thereby lowering costs. It could also mean adding gussets to maintain strength without significantly increasing wall thickness. DFM is not just about avoiding problems; it’s about optimizing the design for efficient and cost-effective production. By embracing DFM principles early in the design process, engineers can create parts that are not only functional and aesthetically pleasing but also economical to produce.

Balancing Cost, Strength, and Aesthetics in Injection Molding

In injection molding, wall thickness is a critical factor that requires a delicate balance. Too thick, and the costs can escalate due to increased material usage and longer cycle times. Too thin, and the part may lack the necessary strength and could be prone to defects like warping or cracking. Furthermore, the wall thickness must be considered in relation to the part’s aesthetic requirements. For example, thick walls might lead to visible sink marks, while thin walls might not allow for the desired surface finish. Achieving this balance is an art and science, requiring a deep understanding of materials, the injection molding process, and the end-use requirements of the part.

Advanced Techniques for Ensuring Uniform Wall Thickness

To achieve uniform wall thickness in complex parts, advanced techniques, and technologies are often employed. Computational tools, such as flow simulation software, can predict how the molten plastic will flow through the mold, allowing engineers to identify and address potential issues before the mold is even made. Other techniques include the use of conformal cooling channels, which are designed to follow the shape of the mold cavity closely, providing more uniform and efficient cooling. Additionally, employing high-quality molds with precise machining ensures that the actual mold cavity aligns perfectly with the design specifications, further promoting uniform wall thickness.

Conclusion: Best Practices for Thickness Injection in Future Projects with Aco Mold

Thickness injection is undeniably a cornerstone of successful plastic injection molding. It’s a multifaceted consideration that intertwines with material choice, design intricacies, and manufacturing processes. Achieving the right balance in wall thickness is pivotal for both the functional and aesthetic aspects of a molded product.

For future projects, it’s essential to prioritize early collaboration between design and manufacturing teams. This synergy ensures that wall thickness considerations are integrated from the outset, minimizing costly redesigns or production issues. Utilizing advanced tools, like flow simulations and material-specific guidelines, can provide invaluable insights into achieving optimal wall thickness.

At this juncture, Aco Mold’s injection molding services stand out as a valuable partner. With a team of experts, Aco Mold can help customers choose the suitable and cost-effective solutions for their projects. Whether it’s material selection, design optimization, or manufacturing support, Aco Mold’s expertise ensures that the right balance in wall thickness is achieved, leading to high-quality, cost-effective, and durable molded parts.

In essence, while challenges in thickness injection are inevitable, with the right knowledge, tools, collaboration, and the support of experienced partners like Aco Mold, they are surmountable. By embracing best practices and prioritizing thickness considerations, manufacturers and designers can ensure the production of top-tier molded products.

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