Structural Design Considerations for Injection Molded Containers

Structural Design Considerations for Injection Molded Containers

Injection molding is the process of injecting molten plastic into a closed mold at a lower temperature under high pressure through a nozzle to form the final product. Both thermoplastics and thermosetting plastics can be injection molded. In packaging container manufacturing, it is mainly used to produce various box-type packaging containers, such as various turnover boxes and small boxes. For box-type packaging containers, the following aspects should be considered in the structural design: 1. Container Wall Thickness
The design of the wall thickness mainly considers three aspects: thickness, uniformity, and smooth transition.

① Thickness: The wall thickness of the container is mainly determined by its purpose, type of plastic, and structure. For thermoplastics, it should generally not be less than 0.6 mm, and 2-4 mm is commonly chosen. For thermosetting plastics, due to their poor fluidity, the thickness should be greater, with small parts being 1.6-2.5 mm and large parts being 3.2-8 mm, with a maximum not exceeding 10 mm.

② Uniformity: The uniformity of the wall thickness has a significant impact on the quality of the container. If the thickness of a container varies too much, it will cause uneven shrinkage, resulting in defects such as deformation and cracks. Therefore, uniform thickness structures should be used as much as possible in structural design.

③ Gentle Transition: In some containers, where certain parts require different thicknesses due to structural needs, such as at container bends, a gentle transition should be used to avoid abrupt changes in cross-section.

2. Improving Rigidity Plastics are relatively flexible, resulting in containers with poor rigidity. However, turnover boxes sometimes bear significant loads. Therefore, various structural measures should be taken to improve rigidity.

① Adding Reinforcing Ribs: Reinforcing ribs enhance the strength and rigidity of the container. Basic requirements for reinforcing ribs are: a large number, thin walls, low height, sufficient slope, and a rounded bottom. The direction of the reinforcing ribs should be aligned with the material flow direction to improve the plastic's toughness.

② Improving Shape: Box-shaped containers are often rectangular thin-walled parts, easily deformed. Various reinforcement measures should be taken, such as slightly bulging the four side walls to improve rigidity; strip reinforcement on the side walls to prevent twisting; edge reinforcement at the container edges to prevent deformation of the opening; and bottom reinforcement by designing a corrugated or arched shape at the bottom.

③ Reasonable Support: Box-shaped containers have a large bottom area, but using the entire bottom surface as support is unreasonable because a slight arch at the bottom will prevent it from standing flat. Therefore, surface support should be changed to line support or point support, such as convex edge support (i.e., line support) or raised foot support (i.e., point support); the height of the convexity is generally 0.3~0.5mm.

④ Corners: Corners are the intersections of two or three surfaces. Regardless of the container's shape, all intersections must be rounded. This greatly improves the container's rigidity, enhances the plastic's mold filling properties, distributes stress, and reduces deformation. 3. Draft Angle
To facilitate demolding, a suitable draft angle must be considered when designing the container. A draft angle that is too small makes demolding difficult and damages the container surface; a draft angle that is too large affects dimensional accuracy.

The draft angle varies depending on the shape of the plastic part, the type of plastic, the mold structure, surface roughness, molding method, etc. Generally, the draft angle commonly used for plastic parts along the demolding direction is 1º~1.5º, and the minimum is not less than 0.5º.