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Main Points of Injection Mold Design

Date: Nov 06 2020 | By:LC Rapid
Tag:Plastic Injection Molding,

Die and mould industry is a basic industry in manufacturing industry, which is the basis of transformation of technological achievements.

plastic molding

Direction of mold opening and parting line

At the beginning of each injection product design, the mold opening direction and parting line should be determined to minimize the core-pulling slider mechanism and eliminate the influence of parting line on appearance.

1. After the direction of mould opening is determined, the structure of stiffener, clasp and bulge is designed to be consistent with the direction of mould opening as far as possible, so as to avoid core-pulling to reduce sewing thread and prolong the life of mould.

2. After the mould opening direction is determined, the proper parting line can be selected to avoid the reverse buckle in the mould opening direction so as to improve the appearance and performance.

Angle of demoulding

1. The proper demoulding angle can prevent the product from drawing. Smooth surface of the demoulding gradient should be ≥0.5 degrees, fine grain (sand surface) surface is greater than 1 degrees, coarse grain surface is greater than 1.5 degrees.

2. Proper demoulding angle can avoid product top damage, such as top white, top deformation, top break.

3. The slope of the outer surface should be larger than that of the inner surface in the design of the deep cavity structure, so as to ensure that the mould core is not misaligned, get uniform wall thickness and ensure the strength of the material in the open part of the product.

plastic molding

Product Wall thickness

1. All kinds of plastics have certain wall thickness range, generally 0.5 ~ 4mm, when the wall thickness is more than 4mm, it will cause the cooling time to be too long, resulting in shrinkage and other problems, we should consider changing the product structure.

2.The uneven wall thickness will cause the surface shrinkage.

3. Uneven wall thickness can cause blowholes and weld lines.

Rational application of stiffeners

1. The rational application of the stiffener can increase the rigidity of the product and reduce the deformation.

2. The thickness of the stiffener must be less than (0.5 ~ 0.7) T, otherwise the surface will shrink.

3. The one-sided slope of the stiffener shall be greater than 1.5 ° to avoid head injury.

Rounded corners

1. Too small rounded corners may cause stress concentration in the product, leading to product cracking.

2. Too small round corner may cause stress concentration in die cavity, resulting in cavity cracking.

3. Set a reasonable fillet, but also can improve the mold processing technology, such as cavity can be directly used r knife milling, and to avoid inefficient electrical processing.

4. The different fillet may cause the parting line movement, should combine the actual situation to choose the different fillet or the clear fillet.


1. The shape of holes should be as simple as possible, usually round.

2. The axial direction of the hole is the same as that of the die opening to avoid core pulling.

3. When the ratio of length to diameter of the hole is greater than 2, the demoulding slope should be set. At this point the diameter of the hole should be small diameter size (the largest physical size) calculation.

4. The ratio of length to diameter of blind holes should not exceed 4. Punch bending

5. The distance between the hole and the edge of the product is generally greater than the aperture size.

Core-pulling and slide mechanism of injection mould and its avoidance

1. When a plastic part can not be successfully demoulded in the direction of opening the mould, a core-pulling slide block mechanism should be designed. The slide block of core-pulling mechanism can form complex product structure, but it is easy to cause defects such as seam splicing, shrinkage and so on.

2. When designing injection molding products, if no special requirements, try to avoid core-pulling structure. For example, the axial direction of the hole and the direction of the rib are changed to the direction of the die opening, and the cavity and core are used to touch and pierce.

All in one hinge

1. Using the toughness of PP material, the hinge can be designed to be integrated with the product.

2. As the hinge of the film size should be less than 0.5 mm, and keep uniform.

3. When injection molded one-piece hinges, the gate can only be designed on one side of the hinge.


1. Insert parts in injection molding products can increase local strength, hardness, dimensional accuracy and set small thread holes (shafts) to meet various special needs. It also increases the cost of the product.

2. Inserts are usually made of copper, other metals or plastics.

3. The part of the insert embedded in the plastic shall have a stop-turn and pull-out resistant structure. Such as: Knurling, hole, bending, flattening, shoulder and so on.

4. The plastic around the insert shall be suitably thickened to prevent stress cracking of the plastic part.

5. When designing the insert, full consideration should be given to the positioning mode (hole, pin, magnetism) in the die


Product identification is usually set on the inner surface of the product, and in the form of a Convex, choose the normal direction and open die direction ruler may be consistent with the face set identification, can avoid pulling.

Precision of injection molded parts

Due to the inhomogeneity and uncertainty of shrinkage during injection molding, the precision of injection molded parts is obviously lower than that of metal parts, so the dimensional tolerance of mechanical parts can not be simply applied. In 1993, GB/T14486-93《 dimensional tolerance of engineering plastics moulded plastic parts 》 was published in our country. The designer can determine the dimensional tolerance of the parts according to the requirements of the plastic raw materials and parts. At the same time, according to the overall strength of the factory, the design accuracy of the products to determine the appropriate design tolerance accuracy.

Deformation of injection molded parts

Improve the rigidity of injection molding product structure, reduce the deformation. Avoid flat structure as far as possible, reasonable set flanging, concave-convex structure. Set up reasonable stiffener.


1. The buckle device is designed to share several buckle positions at the same time, so that the whole device can not be disabled because of the damage of individual buckle positions, thus increasing its service life.

2. On the contrary, the assembly position is difficult to control or the assembled part is too loose if the reverse position is too small. The solution is to reserve the mold easy to add glue way to achieve.

Welding (hot plate welding, ultrasonic welding, vibration welding)

1. The strength of the joint can be improved by welding.

2. The product design can be simplified by welding.

Reasonable consideration of the contradiction between process and product performance

1. The contradiction between appearance, performance and process must be considered in the design of injection molding products. Sometimes a good appearance or performance can be achieved by sacrificing some of the workability.

2. Structural design really can not avoid injection molding defects, as far as possible to allow defects in the hidden parts of the product.

Relationship between the hole diameter of the screw and the diameter of the self-tapping screw

Diameter of Stud for self tapping screw

M2:1.7 mm

M2.3:2.0 mm

M2.6:2.2 mm

M3:2.5 mm

Boss Design Principles

1. The support should not be used alone. It should be connected to the outer wall or used together with the stiffener to strengthen the support and make the rubber flow more smoothly.

2. The height of the struts is generally not more than two and a half times the diameter of the struts. Because of too high prop will lead to plastic parts molding air trap (too long will cause gas hole scorch filling insufficient) .

3. If the height of the pillar is more than two and a half times the diameter of the pillar, especially the pillar far away from the outer wall, the method of strengthening the strength of the pillar is to use stiffeners.

4. The shape of the BOSS to round-based other shapes are not easy to process.

5. The position of the BOSS should not be too close to the corner or the outer wall, and should be kept some distance from the outer wall of the product.

6. Around the BOSS can be used to remove part of the meat thick (that is, open crater) to prevent shrinkage subsidence.

Kill. Boss Angle: usually 0.5 ° outside, 0.5 ° or 1.0 ° inside.

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