Liquid silicone rubber (LSR) coated polycarbonate (PC) is commonly used to manufacture components that require a combination of soft touch, sealing, weather resistance, and structural strength, such as:

Consumer electronics: smart watch straps, phone case borders, waterproof buttons.

Automotive industry: steering wheel encapsulation, sealing gaskets, sensor housings.

Medical equipment: respiratory mask, valve housing, soft rubber grip for handheld devices.

To achieve a successful LSR package PC, the core lies in solving the adhesion problem between the two. Below, I will provide you with a detailed analysis from the aspects of principles, processes, common problems, and solutions.

1. Core challenge: Adhesive properties

Significant differences in material chemical polarity:

PC: It is a polar material with high surface energy.

LSR: It is a non-polar material with very stable chemical properties and extremely low surface energy. It has excellent demolding properties (non stick), but this also means that it is difficult to bond with other materials.

Due to this inherent incompatibility, if left untreated, LSR cannot form a strong bond with PC, which can easily cause the encapsulated area to detach.

2. Solution: How to achieve strong adhesion

a) Physical method: Micro Mechanical Interlock

By changing the surface structure of PC components, an "anchor point" is provided for the injection and curing of LSR.

Design inverted hooks, grooves, and holes: During the design phase of PC components, small inverted hooks, grooves, or through holes are designed in the areas that require encapsulation. After LSR injection, these structures will be filled and solidified to form mechanical interlocking, thereby achieving physical fixation.

Advantages: Low cost, no involvement of chemical agents.

Disadvantages: The bonding strength is relatively weak compared to chemical bonding, and the design is complex, which may affect the structural strength of the PC component itself.

b) Chemical method: surface treatment/use of primer

This is the most commonly used and effective method. By changing the chemical properties of the PC surface or acting as an intermediary to promote the combination of the two.

Plasma Treatment:

Principle: By bombarding the surface of PC with plasma, on the one hand, the surface can be cleaned and oil stains can be removed; On the other hand, it can activate the surface of PC at the molecular level, increase its surface energy, and make it easier to bind with LSR.

Advantages: environmentally friendly, evenly processed, and effective.

Disadvantages: Special equipment is required, and the treated surface has "timeliness" (usually effective within a few hours), requiring immediate injection molding.

Flame Treatment:

Principle: Similar to plasma treatment, the surface of PC is activated by instantaneous high-temperature oxidation of flame.

Advantages: The equipment is relatively simple.

Disadvantages: The control difficulty is high, which can easily cause product deformation or burns, and the uniformity is not as good as plasma treatment.

Using Primer/Adhesive Promoter:

Principle: Before gluing, spray a layer of specialized silicone adhesive primer on the surface of the processed PC. This layer of primer binds to PC molecules at one end and LSR molecules at the other end, playing a bridging role.

Advantages: The bonding effect is very reliable and stable, and it is currently the mainstream method in the industry.

Disadvantages: Increased processes and costs, requiring control over the storage, spraying uniformity, and drying time of the primer.

Usually, a combination of physical and chemical methods (such as designing a slight flip and spraying primer) is used to ensure foolproof bonding strength.

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

Preparation of PC inserts: Use traditional injection molding machines to pre inject PC parts.

PC surface treatment (optional): If plasma or flame treatment is used, it should be carried out at this stage.

Spray primer: Spray the primer evenly on the area where the PC needs to be coated, and then place it in an oven or conveyor belt to dry it thoroughly (solvent evaporation).

Insert into mold: Use the processed PC parts as inserts and place them into the LSR injection mold.

Combined injection molding: The mold is closed, and A/B two-component liquid silicone is injected into the mold cavity through a dedicated LSR injection molding machine and wrapped around the PC part.

Heating solidification: The mold remains heated (usually 170 ° C-200 ° C), and LSR undergoes an addition reaction (platinum catalyzed) to rapidly solidify.

Mold opening and retrieval: Open the mold and retrieve the product that has been coated with adhesive. Note: LSR has almost no shrinkage after curing and excellent demolding properties, so the ejection process is usually smooth.

4. Common problems and countermeasures

Common problems, possible causes, and solutions

Poor adhesion and detachment: 1. Insufficient cleanliness of PC surface (oil stains, release agent).

2. Failure to use primer or primer failure.

3. Uneven or not dried primer spraying.

4. Insufficient injection temperature/pressure. 1. Clean the PC parts (it is recommended to use IPA for wiping).

2. Check and use the primer correctly.

3. Optimize the spraying process to ensure dryness.

4. Improve injection molding process parameters.

PC deformation and cracking: 1. LSR injection molding temperature is too high (exceeding the glass transition temperature Tg of PC, which is about 145 ° C).

During the gluing process, the PC was subjected to excessive force. This is the key! It is necessary to optimize the mold design (such as using cold runner) and LSR curing temperature, or add high-temperature resistant additives in PC.

2. Optimize the structure of PC components and the fixing method of mold inserts.

Overflow and burrs: 1. Insufficient mold accuracy and loose mold fit.

2. The injection pressure is too high.

3. The size tolerance of PC embedded parts is too large, and there is a gap between them and the mold. 1. Improve the precision of mold processing.

2. Adjust the injection pressure.

3. Strictly control the dimensional tolerances of PC inserts.

Surface defects (bubbles, shrinkage marks): 1. There is air in LR material.

2. Poor exhaust of the mold.

3. Insufficient curing time or temperature. 1. Check the LSR feeding system to ensure defoaming.

2. Add mold exhaust slots.

3. Adjust the curing process.

5. Design points and material selection

PC selection:

High temperature resistant PC must be selected to withstand the curing temperature of LSR. Ordinary PCs will soften and deform at high temperatures.

It is possible to consider using fiberglass reinforced PC to improve structural strength and heat resistance, but it should be noted that fiberglass may cause changes in surface appearance.

LSR selection:

Choose LSR grades with high adhesion levels, which typically have better response to primer.

Choose LSR hardness (commonly Shore A 30-50), color, medical grade or food grade according to product requirements.

Mold design:

The cold runner system is the preferred choice as it can prevent LSR from solidifying prematurely in the runner, making it particularly suitable for multi cavity molds and preventing PC overheating.

A well-designed exhaust system is necessary because LSR has excellent fluidity and is prone to trapping air.

Accurate embedded positioning and fixing design are crucial.

summary

Liquid silicone encapsulated PC is a mature but highly demanding technology. The key to success lies in:

Admitting differences: understanding the significant differences in polarity between the two.

Strong adhesion: Without hesitation, use primer as the primary bonding guarantee, combined with mechanical interlocking design.

Control thermal history: Carefully manage process temperature to prevent deformation of PC components at high temperatures.

Precision manufacturing: strict control of mold accuracy, insert tolerances, and process parameters (pressure, time, temperature).