1. Process principle

The core of this process is to firmly coat liquid silicone (LSR) onto a plastic substrate through chemical bonding or mechanical interlocking. Due to the significant chemical differences between LSR and most plastics, they cannot form a melt bond like different plastics, so achieving a strong bond is the key to this process.

2. Material selection: the foundation of success

Choosing the appropriate material combination is crucial and directly determines the success or failure of the bonding effect.

Liquid Silicone (LSR):

Usually, self-adhesive LSR is used. This LSR formula incorporates special bonding agents that can form strong chemical bonds with specific plastics during the heating and vulcanization process.

If there is no self-adhesive LSR, surface treatment of plastic parts (such as plasma treatment, primer treatment) is required, which is more complex and costly.

Plastic (hard glue):

Not all plastics are easy to bond with LSR. Common and easily adhesive plastics include:

PBT (polybutylene terephthalate), especially models containing fiberglass

PA (Nylon), especially PA6, PA66

PC (polycarbonate)

PPS (Polyphenylene Sulfide)

Some types of TPU (thermoplastic polyurethane)

Difficult to bond plastics include PP, PE, POM, TPE, etc. If product design requires the use of these materials, primer treatment agents must be used, but adhesive reliability and long-term stability will face challenges.

3. Process flow (taking secondary injection molding as an example)

Step 1: Injection molding of plastic substrate

Produce hard rubber parts using traditional injection molding machines (first shot).

This plastic part can be pre produced or completed in a set of molds through "in mold transfer printing".

Step 2: Place in the mold&surface treatment (if necessary)

Place the injected hard rubber parts into the cavity of the LSR encapsulation mold.

If using non self-adhesive LSR or difficult to stick plastic materials, it is necessary to manually or automatically spray primer at this stage and wait for it to evaporate and dry.

Step 3: Inject liquid silicone gel (second shot)

Close the mold.

LSR is transported into the injection molding machine barrel through a metering mixing device (A/B components are accurately mixed in a 1:1 ratio).

Inject the mixed LSR into the cavity covered with hard rubber parts under lower pressure (compared to traditional plastic injection molding). LSR will flow into the preset channel and wrap around a specific area of the hard rubber component.

Step 4: Heating vulcanization (curing)

Molds usually need to be heated to a higher temperature (170 ° C-200 ° C).

LR undergoes cross-linking reaction (vulcanization) at high temperatures, transforming from a liquid to a solid elastomer and forming a chemical bond with the hard rubber surface.

Step 5: Mold opening and ejection

After vulcanization is completed, the mold is opened and the robotic arm takes out the complete encapsulated parts.

Due to the fact that LSR does not stick to the mold after curing, demolding is usually smoother.

4. Key challenges and precautions

Adhesive strength: This is the biggest challenge. Strict testing (such as peel testing, tensile testing) must be conducted to verify the reliability of the adhesive.

Mold design: extremely critical.

Classification line design: It must be very precise to prevent LSR overflow (flying edges).

Exhaust system: LSR has excellent fluidity and is prone to gas leakage. Good exhaust design is the key to ensuring product quality.

Cold/hot runner system: For LSR, a cold runner is usually used to prevent early sulfurization and blockage of LSR in the runner. The mold requires a precise temperature control system, with high temperature in the mold cavity and low temperature in the flow channel.

Surface contamination: The surface of the plastic substrate must be absolutely clean, free of release agents, oil stains, dust, etc., otherwise it will seriously affect the bonding effect.

Equipment investment: A dedicated LSR metering and mixing feeding system and an injection molding machine compatible with LSR processing are required. Molds are usually very complex and expensive.

5. Application Fields

This process combines the structural strength of hard glue with the excellent characteristics of LSR, and is widely used in:

Automotive industry: steering wheel wrapping, buttons (such as window switches), sealed damping components, connector sealing.

Consumer electronics: waterproof smart watch case/strap, waterproof sealing ring for mobile phones, soft and hard combination buttons, drone seals.

Medical equipment: respiratory mask (soft silicone sealing edge combined with hard shell), valve components, surgical instrument handle (non slip and comfortable).

Home appliances: sealing rings for rice cookers and kettles (directly wrapped on the lid), soft rubber buttons.

6. Advantages

Function integration: Integrating sealing, shock absorption, insulation, comfortable touch and other functions with structural components to reduce subsequent assembly processes.

High design freedom: capable of achieving complex and aesthetically pleasing product structures.

High reliability: Chemical bonding is more reliable than mechanical assembly and can achieve long-term sealing and bonding.

Production efficiency: Once successfully debugged, large-scale automated production can be achieved.

summary

Liquid silicone encapsulation plastic molding is an advanced manufacturing technology with high technological content and complex processes. Its successful implementation relies on three core pillars:

Compatible material combination (preferred self-adhesive LSR+easy to stick plastic).

Precision professional molds (cold runner, exhaust, temperature control).

Appropriate production equipment (LSR metering mixing system and injection molding machine).