When designing each injection molding product, we must first determine its mold opening direction and parting line to ensure that the core pulling slider mechanism is minimized and the impact of the parting line on the appearance is eliminated.
1. After the mold opening direction is determined, the product's reinforcement ribs, buckles, protrusions and other structures should be designed to be consistent with the mold opening direction as much as possible to avoid core pulling and reduce the seam line and extend the mold life.
2. After the mold opening direction is determined, you can choose an appropriate parting line to avoid undercuts in the mold opening direction to improve the appearance and performance.
2. Demolding slope
1. Appropriate demolding slope can avoid product hair pulling (drawing). The demolding slope of the smooth surface should be ≥0.5 degrees, the surface of the fine leather grain (sand surface) should be greater than 1 degree, and the surface of the 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, the outer surface slope should be greater than the inner surface slope as much as possible to ensure that the mold core is not offset during injection molding, obtain uniform product wall thickness, and ensure the material strength of the product opening.
III. Product wall thickness
1. Various plastics have a certain wall thickness range, generally 0.5~4mm. When the wall thickness exceeds 4mm, it will cause too long cooling time and shrinkage problems. Consider changing the product structure.
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. The thickness of the reinforcement rib must be ≤(0.5~0.7)T product wall thickness, otherwise it will cause surface shrinkage.
3. The single-sided slope of the reinforcement rib should be greater than 1.5° to avoid top injury.
V. Fillet
1. Too small fillet may cause product stress concentration and lead to product cracking.
2. Too small a fillet may cause stress concentration in the mold cavity, leading to cracking of the cavity.
3. Setting a reasonable fillet can also improve the processing technology of the mold. For example, the cavity can be directly processed with an R cutter to avoid inefficient electrical processing.
4. Different fillets may cause the movement of the parting line. Different fillets or corner clearances should be selected based on actual conditions.
VI. Holes
1. The shape of the hole should be as simple as possible, generally a circle.
2. The axial direction of the hole is consistent with the mold opening direction to avoid core pulling.
3. When the aspect ratio of the hole is greater than 2, a demolding slope should be set. At this time, the diameter of the hole should be calculated according to the minor diameter size (maximum physical size).
4. The aspect ratio of the blind hole is generally not more than 4. Anti-hole needle bending
5. The distance between the hole and the edge of the product is generally greater than the hole diameter size.
VII. Core pulling, slider mechanism and avoidance of injection mold
1. When the plastic part cannot be demolded smoothly in the mold opening direction, a core pulling slider mechanism should be designed. The slider of the core-pulling mechanism can form complex product structures, but it is easy to cause defects such as product stitching lines and shrinkage, and 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, the direction of the hole axis and the ribs is changed to the mold opening direction, and the cavity core is penetrated.
VIII. Integrated hinge
1. Using the toughness of PP material, the hinge can be designed to be integrated with the product.
2. The size of the film used as a hinge should be less than 0.5mm and remain uniform.
3. When injection molding an integrated hinge, the gate can only be designed on one side of the hinge.
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. The part of the insert embedded in the plastic should be designed with a rotation-stop and anti-pullout structure. Such as: knurling, holes, bending, flattening, shoulder, etc.
4. The plastic around the insert should be appropriately thickened to prevent stress cracking of the plastic part.
5. When designing the insert, its positioning method in the mold (hole, pin, magnetism) should be fully considered
10. Identification
The product identification is generally set on the flatter surface of the product, and is in the form of a convexity. The surface where the normal and the mold opening direction may be consistent is selected to set the identification to avoid strain.
11. Injection molded parts accuracy
Due to the unevenness and uncertainty of the shrinkage rate during injection molding, the accuracy of injection molded parts is significantly lower than that of metal parts. The dimensional tolerance of mechanical parts cannot be simply applied. Appropriate tolerance requirements should be selected according to the standard. my country also issued GB/T14486-93 "Dimensional Tolerance of Engineering Plastic Molding Plastic Parts" in 1993. Designers can determine the dimensional tolerance of the parts according to the provisions in the standard according to the plastic raw materials used and the use requirements of the parts. At the same time, the appropriate design tolerance accuracy should be determined based on the comprehensive strength of the factory and the design accuracy of the products of the same industry.
12. Deformation of injection molded parts
Improve the rigidity of the structure of injection molded products and reduce deformation. Try to avoid flat structures, and reasonably set flanges and concave and convex structures. Set reasonable reinforcement ribs.
13. Buckle
1. Design the 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, and then consider adding fillets to increase strength.
2. The 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 the assembly position or the combined part is too loose. The solution is to reserve a way to easily add glue when modifying the mold.
14. Welding (hot plate welding, ultrasonic welding, vibration welding)
1. Welding can improve the connection strength.
2. Welding can simplify product design.
15. Reasonable consideration of the contradiction between process and product performance
1. When designing injection molded products, the contradiction between product appearance, performance and process must be comprehensively considered. Sometimes, sacrificing some processability can get a good appearance or performance.
2. When the structural design cannot avoid injection defects, try to make the defects occur in the hidden parts of the product.
16. The relationship between the hole diameter of the screw column and the diameter of the self-tapping screw
Self-tapping screw Screw column hole diameter
M2 1.7mm
M2.3 2.0mm
M2.6 2.2mm
M3 2.5mm
17. BOSS design principles:
1. Try not to use the pillar alone. Try to connect it to the outer wall or use it with the reinforcement ribs. The purpose is to strengthen the strength of the pillar and make the rubber flow smoother.
2. The height of the pillar is generally not more than two and a half times the diameter of the pillar. Because the pillar is too high, it will cause air to be trapped when the plastic parts are molded (when the length is too long, it will cause pores, burning, insufficient filling, etc.).
3. If the height of the pillar exceeds two and a half times the diameter of the pillar, especially the pillar far away from the outer wall, the way to strengthen the strength of the pillar is to use reinforcement ribs.
4. The shape of BOSS is mainly round, and other shapes are difficult to process.
5. The position of BOSS should not be too close to the corner or the outer wall, and should be kept at a distance from the outer wall of the product.
6) The thickness around BOSS can be removed (i.e., a crater is opened) to prevent shrinkage and sinking.
7) The draft angle of BOSS: usually 0.5° for the outside and 0.5° or 1 for the inside.
