17 mold design considerations and design points

17 mold design considerations and design points

Commonly used calculation formulas for injection molding work (1)

1. 3D and 2D analysis

1. Analysis of 3D structure.

2. 2D drawing tolerance, appearance, and material analysis.

2. Mold opening direction and parting line

When starting the design of each injection molded product, the mold opening direction and parting line must first be determined 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 reinforcing ribs, buckles, bulges and other structures should be designed to be consistent with the mold opening direction as much as possible to avoid core pulling, reduce stitching lines, and extend 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 appearance and performance.

3. Draft angle

1. Appropriate demoulding slope can avoid product nap (drawing). The draft angle of the smooth surface should be ≥0.5 degrees, the surface with fine texture (sand surface) should be greater than 2 degrees, and the surface with coarse texture should be greater than 3 degrees.

2. Appropriate draft angle can avoid product top damage, such as top whitening, top deformation, and top breakage.

3. When designing products with deep cavity structures, the outer surface slope should be larger than the inner surface slope as much as possible to ensure that the mold core is not deflected during injection molding, obtain uniform product wall thickness, and ensure the material strength of the product opening.

4. When doing drafting, pay attention to the tolerance size requirements of the 2D drawing, and control the drafting within the tolerance range.

4. 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 problems such as long cooling time and shrinkage. The product structure should be changed.

2. Uneven wall thickness will cause surface shrinkage.

3. Uneven wall thickness will cause pores and weld marks.

4. When the product wall thickness is uneven, the structure needs to be differentiated to prevent stress marks on the surface product.

5. Reinforcement

1. Reasonable application of reinforcing ribs can increase product rigidity and reduce deformation.

2. The thickness of the reinforcing rib must be ≤ (0.5~0.7) T product wall thickness, otherwise it will cause surface shrinkage.

3. The single-sided slope of the reinforcing rib should be greater than 0.5° to avoid top injuries.

4. Under normal circumstances, in order to exhaust the gas, the reinforcing ribs are processed into inserts. If the height of the reinforcing ribs is above 15MM, the reinforcing position is placed on the insert. If it can be processed with a grinder, demoulding is not required. Slope, but the slope on one side of the mold core can be larger.

6. Rounded corners

1. If the fillet is too small, it may cause stress concentration on the product, leading to product cracking.

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

3. Setting reasonable round corners can also improve the processing technology of the mold. For example, the cavity can be directly milled with an R cutter to avoid inefficient electrical machining.

4. Different fillets may cause the parting line to move. Different fillets or clear corners should be selected based on the actual situation.

5. When the product has a buckle area that is deformed during assembly, an R angle of 0.2~0.3MM can be added to the root to prevent the buckle from breaking during the assembly process.

7. Hole

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

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. At this time, the diameter of the hole should be calculated based on the small diameter size (maximum physical size).

4. When the aspect ratio of the blind hole exceeds 10MM, control the effective assembly and positioning length, and add a slope to the other lengths to increase the strength, and try to avoid direct filling with glue.

5. The distance between the hole and the edge of the product is generally larger than the hole diameter to prevent the presence of sharp iron and thin iron on the mold.

6. For through holes with a diameter greater than 10MM, it is best to make vent holes.

7. There will be a bonding line in the area with holes. If conditions permit, inserts can be made to increase air.

8. Core pulling and slider mechanisms of injection molds and their avoidance

1. When the plastic parts cannot be demoulded smoothly according to 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 can easily cause defects such as seam lines and shrinkage of the product, increase the cost of the mold and shorten the life of the mold.

2. When designing injection molded products, if there are no special requirements, try to avoid core-pulling structures. For example, the direction of the hole axis and ribs is changed to the direction of mold opening, and methods such as cavity core collision are used.

9. Inserts

1. Inserting inserts into injection molded products can increase local strength, hardness, dimensional accuracy and set up small threaded holes (shafts) to meet various special needs. It will also increase product costs.

2. The insert is generally copper, but 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-pull-out structures.

Such as: knurling, holes, bending, flattening, shoulders, etc.

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

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

10. Logo

1. Product logos are generally placed on the flat inner surface of the product in a raised form. Select a surface where the normal direction may be consistent with the mold opening direction to avoid strain.

2. It is best for the year and month stamps to be independent so that they can be replaced every year.

11. Precision of injection molded parts

Due to the unevenness and uncertainty of shrinkage during injection molding, the accuracy of injection molded parts is significantly lower than that of metal parts. The dimensional tolerances of mechanical parts cannot simply be applied and appropriate tolerance requirements should be selected according to standards. my country also issued GB/ T14486-93 "Dimensional Tolerances of Engineering Plastic Molded Plastic Parts", the designer can determine the dimensional tolerance of the parts according to the plastic raw materials used and the requirements for the parts, and according to the provisions of the standard.

At the same time, the appropriate design tolerance accuracy must be determined based on the comprehensive strength of the factory and the design accuracy of peer products.

12. Deformation of injection molded parts

1. Improve the rigidity of the injection molded product structure and reduce deformation.

2. Try to avoid flat structures and rationally set flanges and concave-convex structures.

3. Set up reasonable reinforcement ribs.

Thirteen, buckle position

1. The buckle device is designed to be shared by multiple buckles at the same time, so that the entire device will not be unable to operate due to damage to individual buckles, thereby increasing its service life. Furthermore, multiple filters and rounded corners can be added to increase strength.

2. The tolerance requirements for buckle-related dimensions are very strict. Too many undercuts can easily cause buckle damage; on the contrary, too few undercuts can make it difficult to control the assembly position or the assembled parts may be 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. Using welding can improve the connection strength.

2. Using welding can simplify product design.

3. In the case of ultrasonic welding of exterior parts, to prevent the step difference between the upper and lower parts and ultrasonic glue overflow, a plane of more than 0.25MM should be reserved on one side during design.

15. Reasonably consider 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 part of the craftsmanship can result in good appearance or performance.

2. When the structural design cannot avoid injection molding defects, try to make the defects occur in hidden parts of the product.

16. Relationship between screw column hole diameter and self-tapping screw diameter

Self-tapping screws screw post hole diameter

M2 1.7mm

M2.3 2.0mm

M2.6 2.2mm

M3 2.5mm

17. BOSS design principles:

1. The pillars should not be used alone. They should be connected to the outer wall or used together with reinforcing ribs. The purpose is to strengthen the strength of the pillars 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 pillars that are too high will cause air to be trapped during molding of plastic parts (if the length is too long, it is easy to deform, not fully filled, and easy to crack during assembly).

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 method to strengthen the pillar is to use reinforcing ribs

4. The shape of BOSS is mainly round, and other shapes are difficult to process.

5. The location of the 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. You can remove part of the flesh around the BOSS (that is, open a crater) to prevent shrinkage and subsidence.

7. BOSS’s mold dialing angle: the outer diameter should be controlled at 0.5~1°, and the effective assembly surface of the inner diameter should be as zero as possible.