17 key points to pay attention to when designing injection molds

Time:2024-12-23 08:07:30 / Popularity: / Source:

Mold industry is a basic industry in manufacturing industry, basis for transformation of technological achievements, and an important field of high-tech industries. It is called "magnetic industry" that "turns iron into gold" in industrialized countries such as Europe and United States.
American industrial community believes that "mold industry is cornerstone of American industry"; Germany believes that it is "key industry" in all industries; Japanese Mold Association also believes that "molds are driving force for promoting social prosperity and wealth", are also "secret of the entire industrial development" and "driving force for entering a wealthy society."
injection molding product 

I. Mold opening direction and parting line

When designing each injection molding product, mold opening direction and parting line must be determined first to ensure that core pulling slider mechanism is minimized and impact of parting line on appearance is eliminated.
1. After mold opening direction is determined, product's reinforcement ribs, buckles, protrusions and other structures should be designed to be consistent with mold opening direction as much as possible to avoid core pulling and reduce stitching line, extending mold life.
2. After mold opening direction is determined, you can choose an appropriate parting line to avoid undercuts in mold opening direction to improve appearance and performance.
injection molding product 

II. Demolding slope

1. Appropriate demolding slope can avoid product hair pulling (drawing). Demolding slope of smooth surface should be ≥0.5 degrees, surface of fine leather grain (sand surface) should be greater than 1 degree, and surface of rough leather grain should be greater than 1.5 degrees.
2. Appropriate demolding slope can avoid product top damage, such as top whitening, top deformation, and top breakage.
3. When designing deep cavity structure products, outer surface slope should be greater than inner surface slope as much as possible to ensure that mold core is not offset during injection molding, obtain a uniform product wall thickness, and ensure material strength of product opening.
injection molding product 

III. Product wall thickness

1. All kinds of plastics have a certain range of wall thickness, generally 0.5~4mm. When wall thickness exceeds 4mm, it will cause too long cooling time and shrinkage. Product structure should be changed.
2. Uneven wall thickness will cause surface shrinkage.
3. Uneven wall thickness will cause pores and weld marks.

IV. Reinforcement ribs

1. Reasonable application of reinforcement ribs can increase product rigidity and reduce deformation.
2. Thickness of reinforcement rib must be ≤(0.5~0.7)T product wall thickness, otherwise it will cause surface shrinkage.
3. Single-sided slope of the reinforcement rib should be greater than 1.5° to avoid top injury.
injection molding product 

V. Fillet

1. Too small fillet may cause product stress concentration and cracking.
2. Too small fillet may cause mold cavity stress concentration and cracking.
3. Setting reasonable fillets can also improve processing technology of mold. For example, cavity can be directly processed by R cutter milling to avoid inefficient electrical processing.
4. Different fillets may cause movement of parting line. Different fillets or corner clearances should be selected according to actual situation.

VI. Holes

1. Shape of hole should be as simple as possible, generally a circle.
2. Axial direction of hole is consistent with mold opening direction to avoid core pulling.
3. When aspect ratio of hole is greater than 2, demolding slope should be set. At this time, diameter of hole should be calculated according to minor diameter size (maximum physical size).
4. Aspect ratio of blind hole is generally not more than 4. Anti-hole needle punching
5. Distance between hole and edge of product is generally greater than hole diameter size.
injection molding product 

VII. Injection mold core pulling, slider mechanism and avoidance

1. When plastic part cannot be demolded smoothly in mold opening direction, a core pulling slider mechanism should be designed. Slider of core-pulling mechanism can form complex product structures, but it is easy to cause defects such as product stitching lines and shrinkage, increase mold costs and shorten mold life.
2. When designing injection molded products, try to avoid core-pulling structures if there are no special requirements. For example, direction of hole axis and ribs is changed to mold opening direction, and cavity core is penetrated.

VIII. Integrated hinge

1. Using toughness of PP material, hinge can be designed to be integrated with product.
2. Size of film used as a hinge should be less than 0.5mm and remain uniform.
3. When injecting an integrated hinge, gate can only be designed on one side of hinge.
injection molding product 

IX. Inserts

1. Inserting inserts in injection molded products can increase local strength, hardness, dimensional accuracy and set small threaded holes (axles) to meet various special needs. At the same time, it will increase product costs.
2. Inserts are generally copper, but can also be other metals or plastic parts.
3. Part of insert embedded in plastic should be designed with a rotation-stop and anti-pullout structure. Such as: knurling, holes, bending, flattening, shoulder, etc.
4. Plastic around insert should be appropriately thickened to prevent stress cracking of plastic part.
5. When designing insert, its positioning method in mold (hole, pin, magnetism) should be fully considered
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X. Identification

Product identification is generally set on flatter surface of product, and is in the form of a convexity. Surface where normal and mold opening direction may be consistent is selected to set identification to avoid strain.

XI. Injection molded parts accuracy

Due to unevenness and uncertainty of shrinkage rate during injection molding, accuracy of injection molded parts is significantly lower than that of metal parts. Dimensional tolerance of mechanical parts cannot be simply applied. Appropriate tolerance requirements should be selected according to standard. China also issued GB/T14486-93 "Dimensional Tolerance of Engineering Plastic Molding Plastic Parts" in 1993. Designers can determine dimensional tolerance of parts according to plastic raw materials used and use requirements of parts according to provisions in standard. At the same time, appropriate design tolerance accuracy should be determined based on comprehensive strength of factory and design accuracy of products of same industry.

XII. Deformation of injection molded parts

Improve rigidity of structure of injection molded products and reduce deformation. Try to avoid flat structures, reasonably set flanges and concave and convex structures. Set reasonable reinforcement ribs.
injection molding product 

XIII. Buckle

1. Design buckle device to share multiple buckles at the same time, so that the overall device will not be unable to operate due to damage to individual buckles, thereby increasing its service life, then consider adding fillets to increase strength.
2. Tolerance requirements for buckle-related dimensions are very strict. Too many undercut positions are prone to buckle damage; on the contrary, too few undercut positions make it difficult to control assembly position or combined part is too loose. Solution is to reserve a way to easily add glue for mold modification.

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

1. Welding can improve connection strength.
2. Welding can simplify product design.
injection molding product 

XV. Reasonable consideration of contradiction between process and product performance

1 When designing injection molded products, contradiction between product appearance, performance and process must be comprehensively considered. Sometimes, sacrificing some processability can get a good appearance or performance.
2 When structural design cannot avoid injection defects, try to make defects occur in hidden parts of product.

XVI. Relationship between hole diameter of screw column and diameter of self-tapping screw

Hole diameter of the self-tapping screw screw column
M21.7mm
M2.32.0mm
M2.62.2mm
M32.5mm
injection molding product 

XVII. BOSS design principles:

1. Try not to use pillar alone. Try to connect it to outer wall or use it with reinforcement ribs. Purpose is to strengthen strength of pillar and make rubber flow smoother.
2. Height of pillar is generally not more than two and a half times diameter of pillar. Because pillar is too high, it will cause air to be trapped when plastic part is molded (when length is too long, it will cause pores, burning, insufficient filling, etc.).
3. If height of pillar exceeds two and a half times diameter of pillar, especially pillar far away from outer wall, way to strengthen strength of pillar is to use reinforcement ribs.
4. Shape of BOSS is mainly round, and other shapes are difficult to process.
5. Position of BOSS cannot be too close to corner or outer wall, and should be kept at a distance from the outer wall of product.
6) Thickness around BOSS can be removed (i.e., a crater is opened) to prevent shrinkage and sinking.
7) Draft angle of BOSS: usually 0.5° for outside and 0.5° or 1 for inside.

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