Exploring Soft-on-Soft Overmolding in Two-Shot Injection Molding: Cases and Material Selection Principles
Time of issue:2025-05-15
In the field of two-shot overmolding, material compounding typically involves pairing rigid plastics with soft thermoplastic materials. However, in some two-shot injection molding cases, rigid plastics can also be compounded through special structural designs. This article explores a relatively unique material combination in two-shot overmolding: soft-on-soft bonding and overmolding.
The concept of "soft thermoplastic materials" does not have an absolutely strict definition. For example, PP has a lower hardness than ABS and can be regarded as a soft material, but it is much harder than TPR. In the context of two-shot overmolding, "soft thermoplastic materials" refer to a category of thermoplastic flexible materials with a hardness lower than 95A (this value is set by Subo based on industry experience and is not a unified standard). Common examples include elastomers such as TPE/TPR, TPU, TPV, and silicone. Due to environmental protection factors, soft PVC is less commonly used in two-shot molding.
In practical two-shot molding cases, more than 95% involve the compounding of soft thermoplastic materials and rigid plastics. Rigid plastics provide the basic functional properties of products, while soft thermoplastic materials enhance product functionality and added value. Although soft-on-soft two-shot molding applications are not widespread, Subo has encountered several practical cases in recent years.
There are many cases of soft-on-soft overmolding, such as mobile phone cases, tool handles, composite suction cups, medical teething toys, and toy products. Using soft-on-soft overmolding not only promotes product design innovation to provide users with a more comfortable experience but also enables micro-innovations to attract more user attention. At the very least, in a highly competitive market environment, it offers users new reasons to choose.
Take bicycle handlebar grips as an example: they can be bonded using soft TPE/TPR overmolded on rigid PP/PE, and some products also use two different hardness TPE/TPR for compound overmolding.
In the two-shot molding process of soft thermoplastic materials (TPE, TPR) overmolding onto soft thermoplastic materials, the selection of raw materials should follow two principles:First, the two soft thermoplastic materials for overmolding should preferably be of the same or similar compounding systems, such as both being TPR or TPU. Only with similar compounding systems can good bonding effects be ensured. If it is necessary to use two different soft thermoplastic materials, one of them must undergo material modification.Second, the hardness of the first-injected soft material should ideally be higher than that of the second-injected soft material, and it should also have better high-temperature resistance. This ensures that during the second injection molding, the soft component to be bonded will not deform in the mold cavity due to the high overmolding temperature and injection pressure.
During the actual molding process, it is necessary to adjust the injection molding temperature or pressure according to specific processing conditions, and in necessary cases, change the grade of the soft thermoplastic material.
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