For previous article, please refer to Small Home Appliance Structure - Plastic Part Screw Stud Design (Design Template).
In design and manufacturing of home appliances, wall thickness design of small home appliance plastic parts is a crucial step, affecting the overall performance, quality, and cost of product.

From a professional perspective, wall thickness of a plastic part refers to thickness between its outer and inner walls. In structural design software, extracted value is wall thickness. This parameter plays a fundamental role in the entire product design process and is an important basis for many subsequent design decisions.

 

Product Shell Parts: For product shell parts, outer wall is direct representation of product's appearance, determining user's first visual impression and tactile experience. Inner wall bears heavy responsibility of connecting other structural components and providing necessary structural strength, protecting internal electronic components and mechanical parts from external impacts and interference. Inner and outer walls are usually designed as a single unit to ensure integrity and stability of the entire shell.
Internal Product Parts: Most of plastic parts inside product function as load-bearing or connecting components. For example, some internal plastic supports need to support heavy components or act as bridges connecting different modules. To optimize production processes and improve efficiency, other functional structures are generally designed into inner walls, which helps reduce overall space occupation and improve structural compactness.
Combined Functions: An important and often overlooked function of wall thickness is providing a support surface for ejector pins. After injection molding, mold needs ejector pins to eject part from mold cavity. Appropriate wall thickness provides sufficient support area, ensuring even force distribution during ejection, smooth demolding, and preventing damage or deformation due to improper ejection.

 

Wall thickness directly affects mechanical strength of plastic parts.
Theoretically, increasing wall thickness can improve strength of parts, but in actual design, a larger wall thickness is not always better. Excessive wall thickness can easily lead to a series of problems,
such as shrinkage and porosity defects during cooling, which can reduce strength and reliability of part. Furthermore, excessive wall thickness significantly increases material costs and product weight. In design phase, geometric features should be fully utilized, such as rational placement of ribs and reinforcing ribs, to improve rigidity and strength of parts.
For parts with high strength requirements or complex structures, it may be necessary to use professional mechanical simulation software to accurately determine appropriate wall thickness by simulating stress conditions under actual working conditions.

Influence of Wall Thickness on Injection Molding Flow: Wall thickness directly determines thickness of mold cavity. During injection molding, flow of molten resin within cavity is approximately laminar.
When wall thickness is too small, flow resistance of resin within cavity increases, affecting filling effect and leading to defects such as incomplete filling.
To ensure complete filling, it is often necessary to increase injection pressure, but this may cause other problems, such as accelerated mold wear and flash on parts.
Viscosity and Flowability of Plastic Materials: Viscosity of plastic melt is a key factor affecting its flowability, usually characterized by melt flow index.
Different plastic materials have different melt flows, meaning their flowability varies.
For example, some crystalline plastics have relatively poor flowability, while amorphous plastics have better flowability. Therefore, material properties must be fully considered when selecting wall thickness.
Generally, wall thickness range for small household appliance plastic parts is between 0.6 and 6.0 mm, with a commonly used range of 1.5 to 3.0 mm.

 

Wall Thickness Affects Appearance: Uneven wall thickness can seriously affect product's appearance.
Uneven wall thickness leads to inconsistent shrinkage during cooling, causing problems such as surface shrinkage and warping.
When wall thickness is too large, internal cooling rate of material is uneven, easily resulting in shrinkage marks on the surface and formation of shrinkage cavities internally.
Conversely, insufficient wall thickness can lead to defects such as insufficient glue, ejector pin marks, and warping on part's surface. These problems significantly reduce product's appearance quality and user experience.
Shrinkage or Shrinkage Cavity Mechanism: Root cause of shrinkage or shrinkage cavities lies in uneven shrinkage of plastic melt during cooling.
In areas with thicker walls, melt cools relatively slowly, resulting in greater shrinkage stress internally. This stress concentration leads to surface depressions (surface shrinkage) and internal voids (internal shrinkage cavities).
Internal concentrated shrinkage is fundamental driving force behind these defects.
Causes of Warpage: For thermoplastic parts without added reinforcing fillers, warpage is primarily caused by uneven shrinkage during molding process.
Uneven wall thickness leads to different cooling rates in different areas, resulting in varying degrees of shrinkage. This difference in shrinkage is main cause of warpage.
In addition, mold design and injection molding processes also have some influence on warpage.
Tolerance for Wall Thickness Variation: During design process, different types of plastic materials have different tolerances for wall thickness variations.
For amorphous plastics, filled plastics, and pure semi-crystalline plastics, there are corresponding empirical values for tolerance variations available for reference.
Where space permits, a gradual transition should be used between thick and thin wall sections, with a transition length of at least twice wall thickness. This aims to achieve the highest possible wall thickness uniformity, effectively reducing stress concentration caused by abrupt changes in wall thickness and lowering risk of appearance defects.
Solving Warpage Deformation: To minimize impact of warpage deformation on product appearance and performance, ensuring uniform wall thickness is paramount. When variations in wall thickness are unavoidable, a symmetrical structure can be designed to balance shrinkage stress.
Additionally, methods such as localized material removal can adjust wall thickness distribution without compromising structural strength, thereby resolving warpage deformation.

 

Excessive wall thickness not only wastes raw materials but also negatively impacts production efficiency and costs. Since cooling time is approximately proportional to square of wall thickness, increasing wall thickness significantly prolongs cooling time, thus extending the entire injection molding cycle. A longer molding cycle translates to lower production efficiency and higher production costs.
Therefore, when designing wall thickness, it is necessary to comprehensively consider factors such as Design for Manufacturability (DFM), Design for Assemblability (DFA), and Design for Cost (DFC) to optimize wall thickness and reduce costs as much as possible while meeting product performance requirements.

Shelling Method: Shelling method is a commonly used wall thickness design method. Its advantages lie in its simplicity and directness, enabling rapid generation of a 3D model with a certain wall thickness. This method relies on quality of original surface, requiring high continuity and smoothness. If original surface has defects or discontinuities, it may lead to problems in shelled model.
Surface Method: Surface method is relatively complex, requiring multiple steps to complete wall thickness design, but it offers strong flexibility. It is particularly suitable for designing plastic parts with complex shapes, allowing for flexible adjustment of wall thickness in different areas according to specific needs. Surface method allows for precise control of wall thickness distribution, meeting diverse structural and functional requirements of product.
Thickening Method: Thickening method is commonly used in design of plastic parts with simple shapes. It generates wall thickness by thickening existing flat or curved surfaces. This method is simple to operate and relatively efficient, but it may have certain limitations when dealing with complex structures.
Wall thickness design of small household appliance plastic parts is a comprehensive issue that requires full consideration of factors such as mechanical properties, moldability, appearance, and cost. Only by using scientific design methods and advanced tools can plastic parts that meet product performance requirements while also possessing good economic efficiency and appearance quality be designed.
Below is a brief mind map for plastic part wall thickness design and a table of common material wall thickness selections. Feel free to save it!

 

 

What wall thickness issues have you encountered in your actual work with plastic parts?
For further read, please refer to Small Appliance Structure - Nine Principles for Reinforcing Ribs in Plastic Parts (with Diagram).