17 rules for injection molding mold design to avoid product defects

The mold industry is a basic industry in the manufacturing industry, is the basis for the transformation of technological achievements, and at the same time itself is an important area of high-tech industry, in Europe and the United States and other industrial countries are known as "iron into gold" "magnetic industry".

U.S. industry that "mold industry is the cornerstone of the United States industry"; Germany is that it is the "key industry" in all industries; Japan Mold Association also believes that "mold is to promote social prosperity and wealth of the power ", but also "the secret of the whole industrial development", is "into the rich society of the driving force".
 

I. Mold opening direction and parting line

When each injection product is designed, the mold opening direction and parting line should be determined first to ensure that the core pulling slider mechanism is minimized and the effect of the parting line on the appearance is eliminated.

1). After the mold opening direction is determined, the structure of the product such as reinforcement, clips, and bumps should be designed to be consistent with the mold opening direction as much as possible to avoid core extraction to reduce the parting line and prolong the mold life.

2). After the mold opening direction is determined, appropriate parting lines can be selected to avoid the existence of backlash in the mold opening direction to improve the appearance and performance.
 

II. Mold release slope

1). A proper release slope can avoid product pulling (pulling). The release slope of the smooth surface should be ≥ 0.5 degrees, fine grain (sand surface) surface is more than 1 degree, coarse grain surface is more than 1.5 degrees.

2). The appropriate scope of the mold release can avoid product top injury, such as top white, top deformation, top broken.

3). The slope of the outer surface of the deep cavity structure product design should be larger than the slope of the inner surface as much as possible to ensure that the core of the mold does not deviate during injection, to get a uniform product wall thickness, and to ensure the material strength of the open part of the product.
 

III. Product wall thickness

1). All kinds of plastics have a certain wall thickness range, generally 0.5~4mm, when the wall thickness exceeds 4mm, it will cause long cooling time and problems such as shrinkage, and should consider changing the product structure.

2). Uneven wall thickness will cause surface shrinkage.

3). Uneven wall thickness will cause air holes and fusion marks.
 

IV. Reinforcement

1). The reasonable application of reinforcement can increase product rigidity and reduce deformation.

2). The thickness of the reinforcement must be ≤ (0.5 ~ 0.7) T product wall thickness, otherwise, it causes surface shrinkage.

3). The one-sided slope of the reinforcement should be greater than 1.5° to avoid top injury.

V. Rounded corners

1). Too small rounded corners may cause stress concentration in the product and lead to cracking.

2). Too small rounding may cause stress concentration in the mold cavity, leading to cavity cracking.

3). Setting a reasonable rounding angle can also improve the processing of the mold, such as the cavity can be directly milled with R cutter, and avoid inefficient electrical processing.

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

VI. Hole

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

2). The axial direction of the hole and the opening direction are the same, which can avoid core extraction.

3). When the hole's length-diameter ratio is greater than 2, the release slope should be set. At this time, the diameter of the hole should be calculated according to the small diameter size (maximum solid size).

4). Blind hole length to diameter ratio generally does not exceed 4. Anti-hole pin punching bend

5). The distance between the hole and the edge of the product is generally larger than the size of the hole diameter.
 

VII. Injection mold core extraction, slider mechanism and avoid

1). When the molded part can not be released smoothly according to the opening direction, the core extraction slider mechanism should be designed. The core pulling mechanism slider can shape complex product structures, but it is easy to cause defects such as product stitching line and shrinkage, and increase mold cost and shorten mold life.

2). When designing injection molded products, if there are no special requirements, try to avoid the core pulling structure. Such as the direction of the hole axial and tendons to open the mold direction, the use of cavity core touch through and other methods.
 

VIII. One-piece hinge

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

2). The film size as hinge should be less than 0.5mm and keep uniform.

3). When injection molding a one-piece hinge, the gate can only be designed on one side of the hinge.
 

IX. Insert

1). Insert in the injection molding product can increase the local strength, hardness, dimensional accuracy and set small threaded holes (shafts) to meet various special needs. At the same time, it will increase the cost of the product.

2). The insert is usually copper, but it can also be other metal or plastic parts. Injection Molding Exchange focuses on providing injection molding technology training and improvement consulting for injection molding factories.

3). The part of the insert embedded in the plastic should be designed to stop the rotation and anti-pull-out structure. 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 part.

5). When designing the insert, its positioning in the mold should be fully considered (hole, pin, magnetic)
 

X. Marking

Product marking is generally set at the flatter inner surface of the product, and in the form of projection, and the marking is set at the face where the normal direction and the ruler of the mold opening direction may be consistent, which can avoid strain.
 

XI. Precision of injection molded parts

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, can not simply apply the dimensional tolerances of mechanical parts should be selected according to the standard Huai appropriate tolerance requirements. China also released in 1993 GB/T14486-93 "engineering plastics molded plastic parts size tolerance", the designer can be based on the plastic raw materials used and the requirements of the use of the parts, according to the provisions of the standard to determine the size tolerance of the parts. At the same time, according to the comprehensive strength of the factory, the design accuracy of the peer's products to determines the accuracy of the suitable design tolerance.
 

XII. Deformation of injection molded parts

Improve the rigidity of the structure of injection molding products to reduce deformation. Try to avoid flat structure, set reasonable flanges, concave and convex structure. Set reasonable reinforcement bars.
 

XIII. Buckle position

1). The buckle device will be designed to share multiple buckles at the same time, so that the overall device will not be unable to operate because of damage to individual buckles, thereby increasing its service life, and then more consideration is given to adding rounded corners to increase the strength.

2). Is the tolerance requirements of the relevant size of the buckle is very strict, too many buckle position is easy to form the buckle damage; on the contrary, too few buckle position it difficult to control the assembly position or the combination of parts appear to lose phenomenon. The solution is to set aside to change the mold easy to add glue to achieve the way.
 

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

1). The use of welding, can improve the strength of the connection.

2). The use of welding can simplify product design.
 

XV. 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 considered. Sometimes sacrifice part of the process, can get a good appearance or performance.

2. When it is impossible to avoid injection molding defects in structural design, let the defects occur in the hidden parts of the product as far as possible.
 

XVI. Screw column hole diameter and the relationship between the diameter of self-tapping screws

Self-tapping screw Screw column hole diameter

M2 1.7mm

M2.3 2.0mm

M2.6 2.2mm

M3 2.5mm
 

XVII. Design principles of BOSS.

1). The pillar should not be used alone as far as possible, it should be connected to the outer wall or used together with the reinforcement as far as possible, the purpose is to strengthen the pillar and make the rubber flow more smoothly.

2). The height of the pillar should not exceed two and a half times the diameter of the pillar. Too high pillars can lead to air trapping when molding plastic parts (too long length can cause air holes, scorching, underfilling, 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 outer wall, the way to strengthen the pillar is to use reinforcement.

4). The shape of BOSS is mainly round, but other shapes are not easy to process.

5). The BOSS should not be located too close to the corner of the outer wall but should be kept at a distance from the outer wall of the product.

6). Around the BOSS, we can remove part of the flesh thickness (i.e. open crater) to prevent shrinkage and sagging.

7). BOSS dialing angle: usually 0.5° outside, 0.5° or 1 inside.