How to design stronger plastic parts is always a subject of concern for product design engineers. Compared with metal parts, plastic parts generally have lower strength. However, through reasonable parts design, such as adapted ribs and gussets, the strength of plastic parts can be significantly improved, thereby expanding their application range.
The following are some practical ways to design stronger plastic parts.
1. Adding Ribs Instead of Increasing Wall Thickness
Increasing wall thickness can enhance part strength in part design, but this is often impractical. (Thin wall design is a must, read our previous blog Thin Wall Injection Molding Design Tips) The increasing wall thickness not only adds plastic parts’ weight, but also tends to cause defects like shrinkage and air bubbles, meanwhile, increasing injection molding cycle time and reducing production efficiency. The correct approach to design stronger parts is to add ribs rather than increasing wall thickness. Adding ribs can both make parts stronger and avoid issues like shrinkage, air bubbles, and low production efficiency. Of course, always remember to follow the plastic ribs design guidelines when designing ribs; improper ribs design, like overly thick ribs, can also lead to shrinkage and air bubbles.
The Figure shows two methods to design double-part strength. The first is to increase wall thickness, and the second is to keep the wall thickness dimension and add ribs. To achieve a doubling of part strength, the first method increases 25% in part volume, whereas the second method only increases 7% in part volume. From this, it is evident that adding ribs is the best method to improve part strength.
Please note, adding ribs can reduce overall shrinkage, but can cause sink marks that affect the appearance. For the appearance part, consider thickening the part instead of adding ribs.
However, sink marks can be avoided in some cases. Learn what causes sink marks and the solutions.
2. Consider Loading Direction When Adding Ribs and Gussets
It should be noted that ribs and gussets can only enhance strength in one direction. It is important to consider load direction when designing the orientation of the ribs and gussets; otherwise, the ribs or gussets cannot resist the load, as shown in the Figure.
If the part is subjected to loads in multiple directions or torsional loads, consider adding X-shaped ribs or radiating ribs as reinforcement. For example, plastic stools in our daily life are often designed with X-shaped or radiating ribs at the back for a stronger purpose.
3. Multiple Ribs are Better Than Single Thicker or Taller Ribs
To design stronger plastic molded parts, it is more effective to add multiple ribs than one single, thicker or taller rib. Furthermore, it can prevent troubleshooting such as shrinkage or short shots at the rib tips. Therefore, when a single rib is too high or thick, it is better to replace it with two smaller, shorter, and thinner ribs, as illustrated in the Figure.
4. Design Reinforcement Cross-Section
In some cases, it is a good method to design stronger parts by designing the shape of the part’s reinforcement cross-section. Common part reinforcement cross-sections include V-shaped, serrated, and arc-shaped, as shown in the following Figure. However, the part cannot provide a flat surface, making it unusable in some cases.
5. Add Side Walls and Optimize Their Cross-Section
It is advisable to avoid designing plastic parts as flat sheets, as they exhibit very low strength. To improve strength, side walls can be added around the perimeter of the part, as illustrated in the Figure.
Generally, try to avoid designing plastic parts in flat sheets, as they exhibit very low strength. Consider adding side walls around the perimeter of the parts to improve strength, as illustrated in the following Figure.
The side wall can be designed as simple straight walls, using curved side walls, or side walls with reinforced cross-sections, if acceptable, which can further enhance the part’s strength.
The following left Figure is an example of a box design that incorporates side walls, and the right Figure demonstrates how a corrugated structure can be employed to increase the strength of a box.