The cover material is injected above, below, around or inside the substrate during the two shot injection molding process to form a complete part. This process can be done through multiple injections or insert injection molding, usually using an elastomeric covering material. During the two shot injection molding process, the core must be moved or moved to another cavity, or alternatively, the core must be fed into another injection molding machine.
Backlash, nozzle holes, air outlets and mold surface texture are key elements in the design of the two shot injection molding process.
Backstop between substrate and cover material is critical to bonding effectiveness, progressive thinning or linting of the projectile cover material should be avoided. Cover material that is too thin can lead to weak bonds, delamination and crimping. Excellent backstop design should isolate the cover material from the substrate.
Nozzle orifice design is equally important to the success of two shot injection molding. The ratio of runner length to wall thickness is the leading factor in the bonding process. Based on experience, the ratio should not exceed 150:1, and should be kept around 80:1 when developing two shot molding process.
In order to minimize the flow, the nozzle holes should be located at the point of maximum wall thickness. When using TPE resin, the size of the nozzle orifice should be paid attention to. Materials such as TPU require large bore nozzles to accommodate high viscosity, material degradation should be prevented due to high shear. Materials such as SEBS require higher shear velocities for optimal flow rates, It is better to use a small bore nozzle at the beginning and adjust the nozzle size after the first sampling.
As with the nozzle holes, the air outlet is an important factor in the bonding process. It is difficult to control the amount of air left, but if you don't know how to control it, the bond may not be strong enough, and there may be a furring phenomenon. The depth of the air outlet is extremely critical to preventing frizz. Depending on the viscosity of the covering material, the outlet depth should be between 0.0005 and 0.001 inch.