Injection Molding

Mold injection design Tips

17 design tips of mold injection molding

A. Mould opening direction and parting line

At the beginning of the design of each injection product, the mold opening direction and parting line should be determined first to ensure that the core-pulling slider mechanism is minimized and the influence of the parting line on the appearance is eliminated.

1. After the mold opening direction is determined, the reinforcing ribs, buckles, protrusions and other structures of the product are designed to be consistent with the mold opening direction as much as possible, so as to avoid core pulling and reduce seam lines 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, so as to improve the appearance and performance.

B. Demoulding slope

1. Appropriate demoulding slope can avoid product fluffing (pulling). The demoulding slope of the smooth surface should be greater than or equal to 0.5 degrees, the surface of the fine skin (sand surface) should be greater than 1 degree, and the surface of the coarse skin should be greater than 1.5 degrees.

2. Appropriate demoulding slope can avoid product top damage, such as top white, top deformation, and top rupture.

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

C. Product wall thickness

1. All kinds of plastics have a certain range of wall thickness, generally 0.5~4mm. When the wall thickness exceeds 4mm, it will cause the cooling time to be too long and cause shrinkage and other problems. Consider changing the product structure.

2. Uneven wall thickness will cause surface shrinkage.

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

D. Reinforcing ribs

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

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

3. The single-sided slope of the reinforcing rib should be greater than 1.5° to avoid top injury.

E, Rounded corners

1. Too small rounded corners may cause stress concentration in the product, resulting in product cracking.

2. Too small fillet may cause stress concentration in the mold cavity, resulting in cavity cracking.

3. Setting a reasonable rounded corner can also improve the processing technology of the mold. For example, the cavity can be directly milled with an R cutter to avoid low-efficiency electrical machining.

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

F. Holes

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

2. The axial direction of the hole is the same as that of the mold opening, which can avoid core pulling.

3. When the aspect ratio of the hole is greater than 2, the drafting slope should be set. At this time, the diameter of the hole 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. Anti-hole pin punching
5. The distance between the hole and the edge of the product is generally greater than the aperture size.

G. Core pulling, slider mechanism and avoidance of injection mold

1. When the plastic part cannot be demolded smoothly according to the mold opening direction, the 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 stitching and shrinkage of the product, and increase the cost of the mold and shorten the life of the mold.

2. When designing injection molding products, if there are no special requirements, try to avoid the core-pulling structure. For example, the direction of the hole axis and the direction of the rib is changed to the direction of the mold opening, and the cavity core is used to penetrate through the mold.

H, 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 the hinge should be less than 0.5mm and should be kept uniform.

3. When injecting an integrated hinge, the gate can only be designed on one side of the hinge.

I, Inserts

1. Inserting inserts into 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 the product cost.

2. The inserts are 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 structure. Such as: knurling, 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 the insert, the positioning method (hole, pin, magnetism) in the mold should be fully considered.

J. Identification

The product logo is generally set on the flat inner surface of the product and adopts a convex form. Select the surface where the normal direction and the mold opening direction ruler may be consistent to set the logo to avoid strain.

K. Precision of injection parts

Due to the non-uniformity and uncertainty of the shrinkage rate during injection molding, the precision of injection parts is obviously lower than that of metal parts, and the dimensional tolerance of mechanical parts cannot be simply applied. Appropriate tolerance requirements should be selected according to the standard. In 1993, GB/T14486-93 “Dimensional Tolerances of Engineering Plastics Moulded Plastic Parts” was released. The designer can determine the dimensional tolerances of the parts according to the requirements of the plastic raw materials used and the use requirements of the parts. At the same time, it is necessary to determine the appropriate design tolerance accuracy according to the comprehensive strength of the factory and the design accuracy of the products of the same industry.

L. Deformation of injection molded parts

Improve the rigidity of the injection molded product structure and reduce deformation. Try to avoid the flat structure, reasonably set the flanging and concave-convex structure. Set reasonable reinforcing ribs.

M, Deduction

1. The buckle device is designed to be shared by multiple buckles at the same time, so that the overall device will not be unable to operate due to the damage of individual buckles, thereby increasing its service life. In addition, it is necessary to filter and round corners to increase the strength.

2. The tolerance requirements of the relevant dimensions of the buckle are very strict. Too many undercut positions will easily cause damage to the buckle; on the contrary, if the undercut position is too small, the assembly position will be difficult to control or the combination part will be too loose. The solution is to reserve a way to change the mold and add glue easily.

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

1. Welding can improve the connection strength.

2. Welding can simplify product design.

O. Reasonably consider the contradiction between process and product performance

1 The contradiction between product appearance, performance and process must be comprehensively considered when designing injection molding products. Sometimes part of the manufacturability is sacrificed to obtain good appearance or performance.

2. When the structural design cannot avoid injection molding defects, let the defects occur in the hidden parts of the product as much as possible.

P. The relationship between the hole diameter of the screw column and the diameter of the self-tapping screw

Self-tapping screw screw column aperture M21.7mm M2.32.0mm M2.62.2mm M32.5mm

Q, BOSS design principles

1. The pillars should not be used alone as far as possible. They should be connected to the outer wall or used together with reinforcing ribs as much as possible. The purpose is to strengthen the strength of the pillars and make the rubber flow more smoothly.

2. The height of the pillar is generally not more than two and a half times the diameter of the pillar. Due to the high struts, the plastic parts will be trapped during molding (the length will cause air holes, 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 method to strengthen the strength of the pillar is to use reinforcing ribs.

4. The shape of BOSS is mainly circular, and other shapes are not easy to process.

5. The position of the 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. Part of the meat thickness can be removed around the BOSS (ie, crater opening) to prevent shrinkage and subsidence.

7. BOSS’s draft angle: usually take 0.5° outside and 0.5° or 1 inside.

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