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Injection mold design principles and matters needing attention

2021-08-17

Injection mold design principles and matters needing attention

First,the direction of opening and parting line

Each injection molding product at the beginning of the design first to determine its opening direction and parting line, in order to ensure that as far as possible to reduce the core-pulling slider mechanism and eliminate parting line on the appearance of the impact.

1. After the direction of opening the die is determined, the stiffeners, buckles and protruding structures of the product are designed to be consistent with the direction of opening the die as far as possible, so as to avoid core-pulling, reduce stitching and extend the life of the die.

2. After the opening direction is determined, the appropriate parting line can be selected to avoid the overturning of the opening direction to improve the appearance and performance.

Two, demoulding slope

1, appropriate demolding slope can avoid product hair (drawing). The demolding slope of smooth surface should be more than 0.5 degrees, fine skin surface (sand surface) more than 1 degree, coarse skin surface more than 1.5 degrees.

2, appropriate demolding slope can avoid product top injury, such as top white, top deformation, top break.

3. The slope of the outer surface of the deep cavity structure product should be greater than the slope of the inner surface as far as possible to ensure that the mold core is not offset when injection molding, get uniform product wall thickness, and ensure the material strength of the opening part of the product.

Three, the product wall thickness

1, all kinds of plastics have a certain range of wall thickness, generally 0.5 ~ 4mm, when the wall thickness is more than 4mm, will cause cooling time is too long, produce problems such as shrinkage, should consider changing the product structure.

2, uneven wall thickness will cause surface shrinkage.

3, uneven wall thickness will cause pores and weld marks.

Four, strengthen the tendon

1. The reasonable application of stiffeners can increase the rigidity of products and reduce deformation.

2, the thickness of the reinforcing rib must be less than or equal to (0.5 ~ 0.7) T product wall thickness, otherwise cause surface shrinkage.

3. The slope of the reinforcing rib should be greater than 1.5° to avoid goating injury.

Five, the rounded corners

1, fillet is too small may cause product stress concentration, resulting in product cracking.

2, fillet is too small may cause mold cavity stress concentration, resulting in cavity cracking.

3, set reasonable fillet, but also can improve the processing technology of the mold, such as the cavity can be directly used R cutter milling processing, and avoid low efficiency of electrical processing.

4. Different rounded corners may cause the parting line to move, so different rounded corners or clear corners should be selected according to the actual situation.

Six, hole

1, the shape of the hole should be as simple as possible, generally take round.

2. The axial direction of the hole is consistent with the direction of the die, which can avoid core-pulling.

3. When the aspect ratio of the hole is greater than 2, the demoulding slope should be set. At this time, the diameter of the hole should be calculated according to the small diameter (maximum solid size).

4, the aspect ratio of blind hole is generally not more than 4. The anti-hole needle is bent

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

Core pulling and sliding block mechanism of injection mold and its avoidance

1. When the plastic parts can not be smoothly demounded according to the direction of opening the mold, the core-pulling slider mechanism should be designed. The sliding block of core-pulling mechanism can form complex product structure, but it is easy to cause defects such as stitching line and shrinkage, increase the cost of die and shorten the life of die.

2. When designing injection molded products, if there is no special requirement, try to avoid core-pulling structure. For example, the axial direction of the hole and the direction of the rib is changed to the direction of opening the mold, and the method of using the cavity core is touched through.

Eight, one hinge

1. By using the toughness of PP material, the hinge can be designed into one with the product.

2, as a hinge film size should be less than 0.5mm, and keep uniform,

3, injection molding integral hinge, gate can only be designed on one side of the hinge.

Nine, insert

1, insert insert in injection molding products can increase local strength, hardness, dimensional accuracy and set small threaded hole (shaft), to meet various special needs. It also increases the cost of the product.

2, insert is generally copper, can also be other metal or plastic parts.

3. The part of the insert in the embedded plastic shall be designed with stoppage and pull-out resistant structure. Such as: knurling, hole, bending, flattening, shaft shoulder, etc.

4. The plastic around the insert should be properly thickened to prevent the stress cracking of the plastic.

5. When designing inserts, the positioning mode (hole, pin, magnetic) in the mold should be fully considered.

Ten, identifying

Product identification is generally set in the product inner surface is relatively flat, and the form of convex, select the normal direction and the direction of the die ruler may be consistent with the surface to set the identification, can avoid strain.

11. Precision of injection parts

Due to the inhomogeneity and uncertainty of shrinkage rate during injection molding, the precision of injection molding parts is obviously lower than that of metal parts. It is not possible to simply apply the dimensional tolerance of mechanical parts, and appropriate tolerance requirements should be selected according to the standard. Designers can determine the dimensional tolerances of the parts according to the requirements of the plastic materials used and the parts used in the standards. At the same time, according to the comprehensive strength of the factory, the design accuracy of the same product to determine the appropriate design tolerance accuracy. 

Twelve, the deformation of injection parts

Improve the rigidity of product structure and reduce deformation. Avoid flat structure as far as possible, and set up flanging and concave and convex structure reasonably. Set up proper stiffeners.

Thirteen, buckle position

1. The buckle device is designed into a plurality of buckle positions at the same time, so that the overall device will not be unable to operate because of the damage of individual buckle positions, so as to increase its service life, and then more filter and fillet, increase strength.

2. The tolerance requirements of the relevant dimensions of the buckle are very strict, and the overturning position is easy to form the buckle damage; On the contrary, if the upside-down position is too little, the assembly position is difficult to control or the combination part is too loose. The solution is to reserve the easy way to change the mold glue to achieve.

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

1, the use of welding, can improve the strength of the connection.

2, the use of welding, can simplify product design.

15. Reasonable consideration of the contradiction between process and product performance

The contradiction between product appearance, performance and process must be considered comprehensively when designing injection molding products. Sometimes some workmanship is sacrificed to get good appearance or performance.

When the structural design is really unable to avoid the defects of injection molding, the defects should occur in the hidden parts of the product as far as possible.

BOSS design Principles:

1. The pillar should not be used alone as far as possible. It should be connected to the outer wall or used together with the reinforcing bars as far as possible, in order to strengthen the strength of the pillar and make the glue flow more smoothly.

2, the pillar height is generally not more than two and a half times the diameter of the pillar. Because of the high pillar will lead to the plastic parts forming gas trapping (too long will cause porosity, burning, insufficient filling, etc.).

3. If the pillar height 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 reinforcing bars

4, the BOSS shape is mainly round, other shapes are not easy to process

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

6, BOSS can be removed around part of the meat thickness (that is, open the crater) to prevent contraction and subsidence

7, BOSS dial die Angle: usually take 0.5° outside, take 0.5° or 1 inside