Liquid Silicone Rubber Plastic Parts Overmolding

The liquid silicone rubber plastic parts overmolding products of Aobo Silicone, a factory and manufacturer and supplier, is a two-component injection molding process. It refers to a specialized manufacturing technology where liquid silicone rubber (LSR) and rigid plastic substrates (such as PC, ABS, PA, PBT, etc.) are sequentially molded and firmly bonded into an integrated piece in a single mold set via dedicated injection molding equipment.

The Operation Process of Liquid Silicone Rubber Plastic Parts Overmolding Products

The combined application of liquid silicone rubber (LSR) and plastic components is widespread across the electronics, electrical appliances, beauty equipment, and medical industries. We will elaborate on the comprehensive solutions covering material selection, design optimization, process improvement, and post-processing as follows:

Material selection

First of all, we need to conduct an in-depth analysis of the material properties of liquid silicone rubber (LSR) and plastics. LSR is widely used for its excellent high and low temperature resistance, electrical insulation, and biocompatibility. However, it features low surface energy, which makes it difficult to form strong intermolecular bonds with other materials.

There is a wide variety of plastics available. Different types of plastics vary significantly in hardness, toughness, weather resistance, and other properties, and their compatibility with LSR also differs greatly. Therefore, selecting the right plastic substrate is of critical importance.

It is recommended to choose plastic types with good compatibility with LSR, such as certain specially modified polyolefins or thermoplastic elastomers (TPEs). Meanwhile, consideration can be given to pre-treating the plastic surface—such as plasma treatment, chemical etching, or applying adhesion promoters—to increase its surface energy and enhance adhesion with LSR.

In addition, the type of LSR also affects its bonding performance. Different types of LSR have distinct molecular structures and chemical properties, so a comprehensive evaluation should be carried out based on actual application scenarios and requirements when making a selection.

Design Optimization

During the product design phase, rational structural design is equally crucial. The following key points should be noted during the design process:

Reduce Stress Concentration

Avoid designing sharp corners or weak structures at the bonding interface of silicone and plastic to prevent cracking and detachment caused by stress concentration. Adopting smooth transition designs can effectively disperse stress and improve the overall strength of the product.

Increase Bonding Area

Adhesion between silicone and plastic can be significantly enhanced by expanding their contact area. For example, structures such as wave-shaped or serrated interfaces not only increase the contact area but also enhance the mechanical interlocking effect of the bonding surface.

Reserve Expansion Space

Considering the potential expansion of liquid silicone under high temperatures or during service, a certain amount of space should be reserved between silicone and plastic. This prevents detachment caused by internal stress generated from expansion, which not only improves product reliability but also extends its service life.

Process Optimization

The process segment is the key to ensuring a firm bond between liquid silicone rubber (LSR) and plastics. Below are several key process improvement measures:

Precisely Control Molding Parameters

During injection or compression molding, precisely control parameters such as temperature, pressure and time. This is not only ensures the molding quality of plastic components, but also provides a favorable substrate condition for subsequent LSR injection or coating. Excessively low temperatures may lead to incomplete curing of LSR, while excessively high temperatures can cause over-curing and reduced adhesion.

Adopt Two-component Injection Molding Technology

For some complex structures, two-component injection molding technology can be adopted—i.e., molding LSR and plastic integrally in a single mold. This technology leverages the special structure of the injection molding machine to achieve seamless bonding of the two materials, greatly improving the overall performance and adhesion of the product.

Optimize the Vulcanization Process

The vulcanization process of LSR directly affects its adhesion strength to plastic. Optimize vulcanization parameters such as temperature, time and pressure to ensure full vulcanization of LSR while avoiding damage to the plastic substrate. During the vulcanization process, it is necessary to closely monitor the curing state of LSR and adjust process parameters in a timely manner to achieve optimal adhesion results.

Post-Processing

Even if the upfront design, material selection, and processes are all perfectly executed, post-processing cannot be overlooked. Below are several effective post-processing measures:

Surface Cleaning and Pre-treatment

Prior to the bonding of liquid silicone rubber (LSR) and plastic, the plastic surface must be thoroughly cleaned and pre-treated to remove oil stains, impurities, and oxide layers, thereby improving surface cleanliness and roughness. This step is critical to enhancing adhesion strength.

Use Specialized Adhesives

Select adhesives specifically formulated for bonding LSR and plastic, such as LSR-to-plastic bonding adhesives. These adhesives feature excellent fluidity and ease of application, and deliver strong bonding performance on a variety of plastic substrates. During the vulcanization process, the adhesives can stably exert their bonding properties, enabling LSR to tightly encapsulate the plastic components and preventing detachment or debonding during product service.

Regular Maintenance

For products intended for long-term use, a regular maintenance schedule should be established to inspect the bonding condition between LSR and plastic. If potential detachment risks are identified, immediate remedial measures should be taken to prevent the escalation of issues.