What are the principles and core of injection mould design?

Thu May 04 22:01:06 CST 2023

One,mould opening direction and parting line

      When designing each injection moulded product, the mould opening direction and parting line should be determined to ensure that the core pulling slider mechanism is reduced as far as possible and to eliminate the impact of the parting line on the appearance.

1. Once the mould opening direction has been determined, the reinforcement bars, clips, bumps and other structures of the product should be designed to coincide with the mould opening direction as far as possible, in order to avoid core extraction and reduce the parting line and prolong the life of the mould.

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

Two,  the slope of the mold release

1、Appropriate release slope can avoid product pulling (pulling flower). Smooth surface of the release slope should be ≥ 0.5 degrees, fine grain (sand surface) surface is greater than 1 degree, coarse grain surface is greater than 1.5 degrees.

2, the appropriate slope of the 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 greater than the slope of the inner surface as far as possible to ensure that the core of the mould does not deviate during injection, to obtain a uniform wall thickness of the product and to ensure the material strength of the open part of the product.Injection moulds

Three, product wall thickness

1, all kinds of plastics have a certain wall thickness range, generally 0.5 ~ 4mm, when the wall thickness exceeds 4mm, will cause excessive cooling time, resulting in shrinkage and other problems, 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.

Four, reinforcement

1, the reasonable application of reinforcement, can increase product rigidity, reduce deformation.

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

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

Five, rounded corners

1、 Too small rounded corners may cause stress concentration in the product, leading to product cracking.

2、 Too small rounded corners may cause stress concentration in the mould cavity, leading to cavity cracking.

3, set a reasonable rounded corner, but also can improve the mould processing, such as cavity can be directly with R tool milling, and avoid inefficient electrical processing.

4, Different rounding angles may cause the movement of the parting line, should be combined with the actual situation to choose a different rounding or clear angle.

Six, hole

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

2, the axial direction of the hole and the direction of the mold opening, can avoid core extraction.

3, when the hole length-diameter ratio is greater than 2, should be set to release the slope of the mold. In this case, the diameter of the hole should be calculated according to the small diameter size (maximum solid size).

4、The length to diameter ratio of the blind hole should not exceed 4. Prevent the hole pin from punching and bending.

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

Seven, the injection mould core, slide mechanism and avoid

1, when the plastic parts can not be smoothly released according to the direction of the mold opening, should be designed to draw the core slider mechanism. The core extraction mechanism slider can shape complex product structures, but it is easy to cause defects such as product stitching lines and shrinkage, and increase mould costs and shorten mould life.

2, design injection molding products, such as no special requirements, try to avoid the core structure. Such as hole axial and tendon direction to open the direction of the mold, the use of cavity core touch through and other methods.

Eight, one piece 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 it should be uniform.

3、When injection moulding one-piece hinges, the gate can only be designed on one side of the hinge.

Nine, inserts

1、Inserts can be inserted into the injection moulded product to 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. 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 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 suitably thickened to prevent stress cracking of the plastic part.

5、When designing the insert, full consideration should be given to its positioning in the mould (hole, pin, magnetic)

Ten, marking

       Product marking is generally set at the flatter inner surface of the product, and in the form of a projection, choosing the normal direction and the opening direction ruler may be consistent with the face of the marking, can avoid strains.

Eleven, the accuracy of the injection moulded 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, and cannot simply apply the dimensional tolerances of mechanical parts should be selected according to the standard appropriate tolerance requirements. China also issued in 1993 GB/T14486-93 "engineering plastics moulding plastic parts size tolerance", the designer can be based on the plastic raw materials used and the requirements of the use of 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 determine the accuracy of suitable design tolerances.

Twelve, the deformation of injection moulded parts

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

Thirteen, buckle position

1、The buckle bit device will be designed to share multiple buckle bits at the same time, so that the overall device will not be unable to operate because of damage to individual buckle bits, thus increasing its service life, and then more ko filter plus rounded corners to increase the strength.

2. The tolerance requirements of the relevant dimensions of the buckle are very strict, too many inverted buckle positions are likely to form the buckle damage; on the contrary, too few inverted buckle positions are difficult to control the assembly position or the combination of parts appear too loose phenomenon. The solution is to set aside to change the mould easy to add glue to achieve the way.

Fourteen, welding (hot plate welding, ultrasonic welding, vibration welding)

1, the use of welding, can improve the strength of the joint.

2 、 The use of welding, can simplify product design.

Fifteen, reasonable consideration of the contradiction between the process and product performance

1, the design of injection moulded products must take into account the contradictions between product appearance, performance and technology. Sometimes sacrifice part of the process, can get a good appearance or performance.

2 Structure design really can not avoid injection molding defects, as far as possible to let the defects occur in the hidden parts of the product.

Sixteen, the design principles of BOSS:

1、 The pillar should not be used alone, but should be connected to the outer wall or used together with the reinforcement as far as possible, in order 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. A pillar that is too high can lead to air trapping when moulding plastic parts (too long 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 if the pillar is away from the outer wall, the pillar should be strengthened by using reinforcement bars.

4. BOSS shapes are mainly round, but other shapes are not easy to work with.

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

6、Some of the flesh around the BOSS can be removed (i.e. cratering) to prevent shrinkage and sagging.

7、The angle of BOSS: usually 0.5° outside, 0.5° or 1° inside.