6 Common Questions About Injection Molding Wall Thickness

Wall Thickness refers to the material thickness of the wall structure of an injection molded part with cavities or shell structures. It is one of the key parameters in the structural design of plastic products.

According to the statistics, about 40% of defects are directly related to incorrect wall thickness design. In the injection molding process, a scientific wall thickness ensures smooth melt filling of the mould cavity and uniform product cooling, reducing material consumption and mold maintenance costs.

This article will share 6 common questions and answers about injection molding wall thickness design.

1. Why is wall thickness design important?

Wall thickness design is one of the key factors that determine product quality, production efficiency, and cost control. Let’s look at it from four aspects below:

First, regarding molding quality, wall thickness directly affects melt flow and cooling. Excessively thin walls increase flow resistance, causing premature cooling and solidification before complete cavity filling, resulting in short-shot defects. Conversely, excessively thick walls can lead to uneven cooling rates, causing visible sink marks and internal voids, seriously affecting the appearance and mechanical properties of the product.

Second, regarding the cost of production, wall thickness directly affects material consumption and manufacturing efficiency. The industry data shows that every 0.1mm increase in wall thickness raises per-unit material costs by 3-5%. What is more important, cooling time increases proportionally to the square of wall thickness (e.g., 3mm walls require 125% longer cooling than 2mm walls), which extends cycle times, reduces equipment utilization rates.

Third, regarding the structural strength, insufficient thickness leads to product failure through fracture during use, while excessive thickness not only wastes material but may also induce cracking risks in stress concentration zones (e.g., corners).

Finally, wall thickness design directly impacts mold complexity and manufacturing costs. Too-thin walls increase the difficulty of mold making. While improper thickness distribution often requires more complex gating systems, cooling channels, and venting designs, all of which will increase the production costs and maintenance difficulty of the mold.

2. What are the recommended wall thicknesses for plastic parts?

Due to the differences in flowability, shrinkage rate, and mechanical properties among various plastics, their wall thickness are different too. Here we list recommended values for common plastics (see table below).

Tip: Thin-walled parts (<1 mm) require high-flow materials (such as PP, PC) and high-pressure injection machines.

These values may serve as a reference for preliminary design. However, we must note that the final wall thickness should be validated through mold flow analysis, taking into account factors such as product structure, functional requirements, and production processes.

3. Does wall thickness have to be completely uniform?

Wall thickness does not need to be uniform, but must follow transition principles.

Although thickness variation is allowed, it is recommended to use the gradual change mode, allowing the thick-walled area to transition to the thin-walled area gradually. Abrupt thickness changes can cause stress concentration, leading to warpage or sink marks. In practice, certain key areas (e.g., corners of phone cases, load-bearing joints) do require extra thickness, but reinforcement ribs should be used to provide added support rather than relying solely on thicker walls for strength.

The variation of wall thickness between adjacent areas should be limited to ±20% when designing, and transition areas should use rounded fillets or chamfers to ensure smooth connections. Special attention should be paid to rib-base junctions to avoid excessive thickness stacking. According to industry experience, the rib thickness is usually controlled at 50%-70 % of the main wall thickness, which ensures structural integrity while preventing molding defects caused by uneven wall thickness. By reasonably controlling the variation of wall thickness and optimizing transition design, it is possible to ensure the molding quality and structural reliability of the product while meeting functional requirements.

4. Does wall thickness variation affect mold life?

Yes. When there is obvious uneven wall thickness in the product, it will cause some areas of the mold to stay at a high temperature. This uneven heat load speeds up thermal fatigue in the mold steel, making the surface prone to cracks and other damage. Secondly, slower cooling rates in thick-walled sections often cause sink marks, which forces workers to frequently rework and polish, ​which increases labor costs and shortens mold life due to repeated modifications. 

More seriously, the uneven shrinkage caused by wall thickness variation can lead to a decrease in the accuracy of mold alignment, resulting in quality problems such as burrs or dimensional deviations. To prolong the mold life, it is recommended to unify the wall thickness standard as much as possible during the product design phase. If different wall thicknesses must be used due to functional requirements, the cooling system layout should be optimized through mold flow analysis as early as possible to keep all mold areas at balanced temperatures.

5. How to quickly determine if the wall thickness is appropriate?

In the injection molding part design process, wall thickness analysis is a critical step that requires repeated verification and optimization. Designers can efficiently complete this evaluation with professional 3D modeling software like SolidWorks.

The software’s wall thickness analysis feature can automatically scan the entire 3D model and visually display the thickness distribution via a color map. It allows designers to quickly identify potential structural weak points or material accumulation zones.

When the analysis result shows multiple areas with non-compliant wall thickness, it typically indicates the need to reconsider the overall design scheme. It is important to note that the analysis should be combined with material properties and actual injection molding process parameters to obtain the most accurate evaluation results.

6. How to reduce wall thickness while maintaining strength?

To achieve wall thickness reduction without compromising structural integrity in injection molded parts, engineers should pay attention to four aspects: structural design, material selection, molding processes, and simulation verification.

Structural Design: Adding reinforcing ribs at critical stress points can save materials and improve the overall rigidity. And fillets can be used at thickness transitions to avoid stress concentration.

Material Selection: Choosing reinforced materials, such as adding fibers, can significantly increase material strength and reduce wall thickness.

Molding Process: Microcellular foaming technology can achieve weight reduction while maintaining over 90% of the original strength by forming a closed bubble structure inside the material. For thick-wall areas, gas-assisted injection molding technology can form a hollow structure inside and achieve a thinner wall thickness.

Simulation Verification: The design should be validated with CAE simulation software. Ensure the product can still meet the strength requirements after reducing the wall thickness, while preventing defects like short shots and warpage.

At last, please feel free to contact RJCMOLD, a factory with over 20 years of injection molding experience. We can help analyze wall thickness and provide improvement suggestions.