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Several Basic Points Of Injection Mold Design
Jun 28, 2018

Core Tips: First, mold opening direction and parting line At the beginning of design, each injection molding product must first determine its mold opening direction and parting line in order to ensure that the core sliding is minimized.

 First, mold opening direction and parting line


For each injection molded product, the mold opening direction and parting line should be determined first at the beginning of the design (Shanghai Die Design Training School) to ensure that the core sliding block mechanism is eliminated and the influence of the parting line on the appearance is minimized.


1. After the mold opening direction is determined, the structure of the ribs, snaps, and protrusions of the product is designed to be consistent with the direction of mold opening as far as possible so as to avoid the core pulling and the reduction of the stitching line and prolong the life of the mold.


2. After the mold opening direction is determined, an appropriate parting line can be selected to avoid undercuts in the mold opening direction to improve the appearance and performance. Shanghai mold design training

Second, the draft angle


1, the appropriate draft angle can avoid product pull (pull). The draft of the smooth surface shall be 0.5 degrees, the surface of the fine striae (sand surface) shall be greater than 1 degree, and the surface of the rough striae shall be greater than 1.5 degrees.


2, the appropriate draft angle can avoid product top injury, such as top white, top deformation, top break.


3, deep cavity structure product design when the external surface slope as much as possible greater than the internal surface slope, in order to ensure that the mold core mold injection is not biased, get a uniform product wall thickness, and to ensure that the product of the opening of the material strength.


Third, the product thickness


1, all kinds of plastics have a certain range of wall thickness, generally 0.5 ~ 4mm, when the wall thickness exceeds 4mm, will cause cooling time is too long, resulting in a reduction and other issues, should consider changing the product structure.


2. Uneven wall thickness can cause surface shrinkage.


3, uneven wall thickness can cause porosity and weld marks.


Fourth, strengthening ribs


1. The rational application of stiffeners can increase product rigidity and reduce distortion.


2. The thickness of the stiffener must be (0.5-0.7)T product wall thickness, otherwise it will cause the surface to shrink.


3. The unilateral slope of stiffeners (Shanghai Die Design Training School) should be greater than 1.5° to avoid top injuries.


Fifth, fillet


1. Too small a fillet may cause product stress concentration, leading to product cracking.


2. If the fillet is too small, it may cause stress concentration in the mold cavity, leading to cracking of the cavity.


3, set a reasonable round, you can also improve the mold processing technology, such as the cavity can be directly R cutter milling, and avoid inefficient electrical processing.


4. Different fillets may cause movement of the parting lines. Different fillets or clear corners should be selected according to actual conditions.


Sixth, hole

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

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

3, when the aspect ratio of the hole is greater than 2, the draft angle should be set. The diameter of the hole at this time should be calculated according to the size of the minor diameter (maximum physical size).

4. The length-diameter ratio of blind holes is generally not more than 4. Needle punching

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


VII. Pull core, slider mechanism and avoidance of injection mold


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


2. When designing injection products, if there is no special requirement, try to avoid the core-pulling structure. If the axial direction of the hole and the direction of the ribs are changed to the direction of opening the mold, use the cavity core to touch through and other methods.


Eight, one hinge


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


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


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


Nine, insert

1. Inlay inserts in injection molded products can increase local strength, hardness, dimensional accuracy and set small threaded holes (shafts) to meet various special needs. At the same time, it will increase product costs.

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

3. The part of the insert embedded in the plastic should be designed with anti-rotation and anti-pullout structure. Such as: knurled, holes, bending, flattening, shoulders, etc.

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

5. When designing inserts, full consideration should be given to their positioning in the mold (holes, pins, magnetism)


Ten, logo


Product identification is generally set on the inner surface of the product is relatively flat, and the use of convex form, select the direction and the direction of the mold may be consistent with the direction of the direction of the logo, you can avoid strain.


Eleven, injection molding accuracy


Due to the non-uniformity and uncertainty of the shrinkage during injection molding, the accuracy of injection molded parts is significantly lower than that of metal parts, and the dimensional tolerances of mechanical parts cannot be simply applied. The appropriate tolerance requirements should be selected according to standards. China also issued GB/1993 in 1993. T14486-93 "Dimensional Tolerances for Plastics Molded from Engineering Plastics", the designer can determine the dimensional tolerances of the parts according to the requirements in the standard according to the requirements of the plastic raw materials and parts used. At the same time, according to the comprehensive strength of the factory, the precision of the design of the products of the counterparts to determine the appropriate design tolerance accuracy.


Twelve, deformation of injection molded parts


Improve the rigidity of injection molded products and reduce distortion. Try to avoid the flat plate structure, reasonable set flange, convex structure. Set reasonable reinforcements.


Thirteen, buckle position


1, the buckle device is designed to share a number of deduction at the same time, so that the overall device will not be unable to operate because of the individual deduction, so as to increase its service life, and then more examination filter fillet, increase the strength.


2. The tolerances of the relevant dimensions of the fasteners are very stringent. Excessive undercut positions are likely to cause damage to the fasteners. Conversely, if the undercut positions are too few, the assembly position is difficult to control or the assembly may be loose. The solution is to reserve the possibility of modifying the mold to add glue.


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


1, using welding, can increase the connection strength.


2, using welding, can simplify product design.


Fifteen, reasonably consider the contradiction between process and product performance


1 When designing injection molding products, the contradiction between product appearance, performance and process must be comprehensively considered. Sometimes sacrifice some of the craft, can get a good appearance or performance.

2 When the structural design can not avoid the defects of injection molding, the defects should occur in the concealed parts of the product as much as possible.


16. The relationship between the screw hole diameter and the diameter of the tapping screw

Self-tapping screws

M2 1.7mm

M2.3 2.0mm

M2.6 2.2mm

M3 2.5mm


Seventeen, BOSS design principles:

1. Struts should not be used alone. They should be connected to the outer wall or used together with stiffeners. The purpose is to strengthen the strength of the struts and make the rubber flow more smoothly.

2. The pillar height is generally no more than two and a half times the pillar diameter. Excessive pillars can cause air trapping during the molding of plastic parts (when the length is too long, it can cause pores, scorching, insufficient filling, etc.).

3. If the height of the pillar exceeds two and a half times the diameter of the pillar, especially the pillar that is far from the external wall, the method of strengthening the strength of the pillar is to use a stiffener.

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

5, the location of BOSS can not be too close to the corner or the outer wall, should keep a distance with the outer wall of the product

6, BOSS can be used around the removal of part of the flesh thickness (ie, the crater) to prevent contraction subsidence

7, BOSS dialing angle: usually take 0.5 °, take 0.5 ° or 1