Injection molding

Overmolding，also known as secondary injection molding or two-color injection molding, is a molding technique that combines two different materials into one. Its basic principle is: first, a hard plastic part is injection molded as the framework, and then the component is placed in another mold, where a layer of soft rubber material (usually a thermoplastic elastomer, or TPE) is injected onto its specific surface. Under high temperature and pressure, the molecules of the two materials fuse at the contact surface, forming a strong and inseparable whole.

The core advantages of Overmolding

• Enhanced grip experience: The soft rubber layer provides a smooth and non-slip touch, effectively absorbing hand pressure, and is not prone to 

        fatigue even when used for a long time.

• Improved product durability: The soft rubber layer can buffer external impacts, protect the internal hard structure, and also serve as a sealing 

        and dust-proof function.

• Increased design possibilities: By combining different colors and textures, unique visual and tactile effects can be created, enhancing the quality 

        perception of the product.

Case of products produced by Haina Company:

This is a massage stick product produced by our company. If the entire stick is made of hard plastic, when users hold and use it, they will experience 

a cold sensation. Long-term operation can cause hand numbness and soreness. Moreover, when sweating or applying massage oil, there is a risk of 

slipping on the hard and smooth surface. Through the use of the overmolding process, this problem has been well resolved. 

The final effect is as follows: The massage stick is sturdy and durable throughout, while the hand-holding area is soft, non-slip, and skin-friendly. The 

transition between hardness and softness is smooth and natural, and the entire machine has a strong sense of unity, without any seams or traces of 

glue application. This is precisely the quality upgrade brought about by the rubber-molding and injection process. 

Technical Points and Precautions

To successfully achieve the process of rubber coating injection molding, several key technical points need to be considered:

• Material Compatibility: The hard core material and the soft rubber TPE must have good compatibility to achieve a strong chemical bond. For 

      example, the ABS/PC core is usually paired with TPE based on SEBS.

• Mold Design: The secondary injection mold needs to be precisely positioned to ensure that the soft rubber layer covers the designated area and 

      has uniform thickness.

• Process Parameters: The temperature of the core component, injection pressure, holding time, etc., all affect the bonding strength.