General Principles of Plastic Structure Design

Plastic structure design: Uniform thick wall

Uniform wall thickness is the first principle of Plastic structure design, and excessive or thin wall thickness should be avoided as much as possible. Pay attention to this even in corners. Because the uneven wall thickness will cause uneven shrinkage of the plastic part after cooling, resulting in shrinkage depression, internal stress, deformation, cracking, etc. In addition, the cooling time of the molded plastic part depends on the thicker part, and the uneven wall thickness will cause The molding cycle to be prolonged and the production efficiency to be reduced.

When the plastic part has a large wall thickness, the structure of the plastic part should be improved. When reducing glue, the inner mold core should be enlarged as much as possible, because the strength of the inner mold core is poor if the inner mold core is too small, and it is not easy to cool. After thick-wall glue is reduced, if strength or assembly problems are caused, ribs or protrusions can be added to solve them. If thick walls are unavoidable, sudden changes in wall thickness should be replaced by gradual changes.

Plastic structure design: Fully guaranteed strength and rigidity

One of the disadvantages of plastic parts is that their strength and rigidity are far inferior to steel products. How to improve the strength and stiffness of plastic parts to meet the requirements of product functions is a must for designers. The simplest and most practical way to improve the strength and stiffness of plastic parts is to design stiffeners instead of simply increasing the wall thickness. Because increasing the wall thickness not only greatly increases the weight of the plastic part, but also easily produces defects such as shrinkage cavities and dents. Setting ribs can not only improve the strength and rigidity of the plastic part but also prevent and avoid plastic deformation and warping.

The direction of setting the ribs should be as consistent as possible with the direction of the material flow, to prevent the material flow from being disturbed during mold filling, reducing the toughness of the plastic part, or affecting the appearance quality of the plastic part. Reinforcement methods include sidewall reinforcement, bottom reinforcement, and edge reinforcement, etc.

For container plastic parts, the method to improve strength and rigidity is usually to strengthen the edges, and at the same time add round bones or make arches and other structures at the bottom.

Plastic structure design: Strives to be simple and easy to form

Compared with other materials such as metals, plastics are easy to mold and can be molded in a variety of ways. Therefore, the structure and shape of plastic parts can be more complex and changeable than metal parts. However, complex plastic parts will increase the difficulty and cost of mold manufacturing, and will also increase the injection molding cost of plastic parts, which is of course not good for the control of product development costs.

If the lateral concave-convex structure is unavoidable, the lateral concave-convex structure should be simplified as much as possible, and the lateral core-pulling mechanism should be avoided.

When designing plastic parts, we should strive to make the structure of the plastic parts simple under the premise of meeting the functional requirements of the product, especially avoiding the lateral concave-convex structure as much as possible. Because the lateral concave-convex structure requires a lateral core-pulling mechanism to be added to the mold, the mold becomes complicated and the manufacturing cost is increased.

In addition to avoiding lateral core pulling as much as possible during the design of plastic parts, we must also strive to make other structures of the mold simple and durable, including the following aspects:

• There must be no sharp corners or weak structures on the molded parts. Because sharp corners or weak structures on the mold will affect the strength and service life of the mold.

• Make the parting surface as easy as possible, because a simple parting surface can make mold processing easy, it is not easy to produce flash during injection molding, and gate removal is also easy.

• Make the molded parts as simple and easy to process as possible.

Plastic structure design: Reasonable fit clearance

The assembly gap between each plastic part should be uniform and reasonable, and the general plastic part gap (one side) is as follows.

• The fit gap between the fixing parts is generally 0.05 ~ 0.1mm.

• The gap between the surface and the bottom cover is generally 0.05 ~ 0.1mm.

• For buttons with a diameter of φ≤15mm, the movable gap (one side) is 0.1~0.2mm, for buttons with a diameter of φ>15mm, the movable gap (one side) is 0.15~0.25mm, for special-shaped buttons, the movable gap is 0.3~0.4mm.

Plastic structure design: Other principles

1. Determine the shape, size, appearance, and material according to the required function of the plastic part. When the appearance of the plastic part is high, the internal structure should be designed after the appearance modeling.

2. Try to design the plastic part as a body of revolution or a symmetrical shape. This shaped structure has good manufacturability and can withstand large forces. It is easy to ensure temperature balance during mold design, and plastic parts are not easy to warp and other deformations.

3. When designing plastic parts, the fluidity, shrinkage, and other characteristics of plastics should be considered. Under the premise of meeting the requirements of use, all corners of plastic parts should be designed as rounded corners as possible, or transitioned with arcs.