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Structural design considerations for injection molded containers
May 21, 2018

Injection molding is the formation of molten plastic under high pressure through a nozzle into a closed mold with a lower temperature. Thermoplastics and thermosetting plastics can be injection molded. It can be used to make various kinds of box-type packaging containers, such as various turnover boxes, small boxes and so on. For box-type packaging containers, the following aspects should be considered when designing the structure.

 

Wall thickness of the container


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


1 Thickness: The wall thickness of a container is mainly determined by its use, plastic type, and structure. For thermoplastics, generally not less than 0. 6mm, often choose 2 ~ 4mm; for thermosetting plastics, because of its poor mobility, the thickness should be larger, small pieces of 1. 6 ~ 2. 5ram, large pieces of 3. 2 ~ 8mm, the maximum does not exceed 10mm.


2 Uniformity: The uniformity of the wall thickness has a great influence on the quality of the container. If the thickness difference of a container is too large, the shrinkage will not be uniform, resulting in defects such as deformation and cracks. Therefore, equal thickness structures should be used as far as possible in structural design.


3 gentle transition: In some containers, some parts due to structural needs and thickness must be different, such as the container transition, then you should use a smooth transition to avoid cross-sectional mutations.


2. Improve the stiffness


Plastics are relatively flexible, and the stiffness of the manufactured containers is poor, and the container-type containers sometimes have a large carrying capacity. Therefore, various measures for increasing the rigidity should be adopted in the structure.


1 Adding stiffeners: Ribs can increase the strength and stiffness of the container. The basic requirements for reinforcing ribs are: the number should be more, the wall thickness should be thin, the height should be short, there should be enough slope, the bottom of the ribs should be a circular arc transition, and the direction of the ribs should be consistent with the material flow direction to improve the plastics toughness;


2 to improve the shape: box-shaped containers are mostly thin-walled parts, easy to deform, should take various enhancements, the four sides of the container slightly convex outward to increase the stiffness; is in the side to prevent distortion of the ribbon 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 or arched.


3 Reasonable support: The bottom area of the box-shaped container is relatively large, but it is unreasonable to use the entire bottom surface as a support. Since the bottom part is slightly arched, it should be uneven. Therefore, the surface support should be changed to line support or point support, and the flange support (ie, line Support); raised foot support (ie, point support); raised height is generally 0. 3 ~ 0. 5mm.


4 Corner: Corner is the intersection of two sides or three sides. Regardless of the shape of the container, the intersection of each side must be rounded. This can greatly improve the rigidity of the container and improve the filling of the model. Sex, and can disperse stress and reduce distortion.

 

3. Demolding slope


In order to facilitate the demolding of the molded part, a proper draft angle must be considered when designing the container. If the draft is too small, mold release is difficult and the surface of the container is damaged. If the draft is too large, the dimensional accuracy is affected.


The draft angle differs depending on the shape of the plastic part, the type of plastic, the mold structure, the surface roughness, and the molding method. In general, the commonly used slope of plastic parts along the stripping direction is 1º~1.5º, and the minimum is no less than 0.5º.


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