Designing Injection Molding Process for Medical Field: Key Considerations for Secondary Injection Molding

In the design of secondary injection molding, backflow prevention devices, nozzle holes, vents, and mold surface texture are key elements.

Backflow prevention devices in the secondary injection molding of medical devices

Backflow prevention devices between the substrate and the covering material are critical to adhesive effect, and should avoid the covering material becoming thinner or edge-raising. If the covering material is too thin, it will result in poor adhesion, delamination, and curling. A good backflow prevention device design should clearly separate the covering material from the substrate.

Nozzle holes in the secondary injection molding of medical devices

Nozzle hole design is also important for the success of secondary injection molding. The flow channel length-to-wall thickness ratio is the main factor affecting adhesion. According to experience, this ratio should not exceed 150:1, and the ratio should be kept at around 80:1 when developing a new process design.

To shorten the process as much as possible, the nozzle holes should be located at the thickest part of the wall. When using TPE resin, the nozzle hole size should be noted. TPU and other materials require large-diameter nozzles to adapt to higher viscosity and prevent material degradation caused by excessive shear force. SEBS and other materials require high shear rates for optimal flow rate. A good way is to use a small-diameter nozzle in the initial stage, and then adjust the nozzle size after the first sampling.

Vents in the secondary injection molding of medical devices

Like the nozzle holes, vents are also important factors affecting adhesion. How to control the air volume is a major problem. If not controlled properly, weak adhesion and fill edge defects may occur. The depth of the vent is critical to prevent edge-raising, and depending on the viscosity of the covering material, the vent depth should be between 0.0005-0.001 inches.

Mold surface texture in the secondary injection molding of medical devices

For some part designs, decorative surface textures can be used to facilitate product ejection. Most TPE materials adhere to the mold surface because such materials have metal affinity, or because a vacuum is formed between the material and the mold surface during molding. Since many materials have not formed a stable chemical bond after ejecting the products, if they adhere to the mold surface, it will greatly affect the adhesion effect. This means that after the parts are processed, they must be handled carefully. If adhesion testing is needed, it should be done 24 hours later, so that the material can form a stable chemical bond.

If the mold surface is parallel to the molding direction and the tensile force is insufficient, adhesion may also occur. In addition, the mold surface coating also helps the parts to eject.

Also, surface texture design requires careful consideration. Surface texture affects the softness, feel, and thickness of the covering material. Proper wall thickness and surface texture design can complement each other to achieve the desired processing characteristics. Generally, the lower the material hardness, the softer it is. Optimizing surface textures can reduce defects in injection molding and improve product feel, making it feel softer than the actual hardness.

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