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Structural Design Considerations for Injection-molded Containers

April 28,2022

Core abstract: Injection molding is to inject molten plastic into a closed mold with a lower temperature through a nozzle under high pressure. Both thermoplastics and thermosetting plastics can be injection molded. In the production of packaging containers, it can mainly produce various box-type packaging containers, such as various turnover boxes, small boxes, etc. For box-type packaging containers, the following aspects should be considered in the structural design.

1. The wall thickness of the container

The design of wall thickness mainly considers three aspects: first, thickness; second, uniformity; third, smooth transition.

①Thickness: The wall thickness of the container is mainly determined by its use, plastic type, structure, etc. For thermoplastics, it is generally not less than 0.6mm, usually 2~4mm; for thermosetting plastics, because of its poor fluidity, the thickness should be larger, the small part is 1.6~2.5mm, the large part is 3.2~8mm, and the size does not exceed 10mm.

②Uniformity: The uniformity of the wall thickness has a great influence on the quality of the container. If the thickness difference of the container is too large, the shrinkage will be uneven, resulting in defects such as deformation and cracks. Therefore, an equal-thickness structure should be used as much as possible in the structural design.

③ Smooth transition: In some containers, when some parts must have different thicknesses due to structural requirements, such as the turning point of the container, a smooth transition should be used to avoid sudden changes in cross-section.

2. Improve stiffness

Plastics are relatively flexible, and the rigidity of the containers made of them is poor, and the turnover box containers sometimes have a large bearing capacity, so various measures to improve the rigidity should be taken in the structure.

①Adding reinforcing ribs: The reinforcing ribs can enhance the strength and rigidity of the container. The basic requirements for reinforcing ribs are: the number of ribs should be large, the wall thickness should be thin, the height should be short, and there should be sufficient slope, the bottom of the ribs should be in a circular arc transition, and the direction of the ribs should be consistent with the direction of the material flow to improve the plasticity. toughness;

②Improve the shape: Most of the box-shaped containers are rectangular thin-walled parts, which are easy to deform. Various strengthening measures should be taken to slightly bulge the four side walls of the container outward to improve the rigidity; Reinforcement; edge reinforcement at the edge of the container to prevent deformation of the mouth; bottom reinforcement at the bottom of the container designed to be corrugated and arched.

③ Reasonable support: The bottom area of the box-shaped container is large, but it is unreasonable to use the entire bottom surface as a support. Because the bottom is slightly arched, it will not be placed flat, so the surface support should be changed to a line support or a point support, and the convex edge support (that is, the line support). Support); raised foot support (ie point support); the height of the protrusion is generally 0.3~0.5mm.
④ Corner: The corner is the intersection of two or three sides. Regardless of the shape of the container, rounded transitions must be adopted at the intersection of each surface, which can greatly improve the rigidity of the container and improve the plastic filling model. It can disperse stress and reduce deformation.

3. Demoulding slope

In order to facilitate the demoulding of the molded part, a suitable demoulding slope must be considered when designing the container. If the demoulding slope is too small, demolding is difficult and damages the surface of the container; if it is too large, the dimensional accuracy will be affected.

The demolding slope varies with the shape of the plastic part, the type of plastic, the mold structure, the surface roughness, the molding method, etc. Generally speaking, the commonly used slope of plastic parts along the demolding direction is 1°~1.5°, and the minimum is not less than 0.5°