The moulding of silicone phone cases mainly relies on its thermosetting properties, where liquid or semi-solid silicone is cured and moulded by heating. The following are the detailed steps and characteristics of the two mainstream moulding processes:
I. Liquid Silicone Rubber Injection Molding (LSR, Liquid Silicone Rubber Injection Molding)
1. Raw material preparation
- Liquid Silicone Rubber: Two-component (A+B) material, mixed and cured by heating vulcanisation, with high fluidity and low viscosity.
- Pre-treatment: the raw material should be kept refrigerated, return to temperature and vacuum to eliminate air bubbles before use.
2. Mould design
- Precision moulds: usually steel moulds, high temperature and pressure resistant, with complex parting surface design to match the details of camera holes and key slots of mobile phone cases.
- Cold runner system: avoid silicone curing in advance and reduce raw material waste.
3. Injection moulding process
- Mixing and Injection: Components A and B are precisely mixed by metering pump and injected into the mould cavity.
- Curing (Curing):
- Temperature: The mould is heated to 160~200℃, and the silicone is vulcanised within 1~5 minutes.
- Pressure: High pressure (10~30MPa) to ensure complete filling and avoid air bubbles.
- Stripping: Ejector pin pushes out the finished product after opening the mould, no need to cool down (Silicone has poor thermal conductivity, it will be stripped off directly after curing).
4. Post-processing
- De-burr: Manual or automated trimming edge overflow.
- Surface treatment: spraying anti-fouling coating, laser engraving Logo, etc.
Features:
- Advantages: high precision, fast production efficiency (30~60 seconds/cycle), suitable for complex structures and mass production.
- Disadvantages: high mould cost (hundreds of thousands to millions of dollars), uneconomical for small orders.
II. Moulding (Compression Molding)
1. Preparation of raw materials
- Solid silicone: pre-moulded into flakes or granules, need to add vulcanising agent.
- Weighing: weigh accurately according to the volume of the mould cavity to avoid lack of material or overflow.
2. Mould and equipment
- Simple mould: aluminium or steel mould, low cost.
- Flat plate vulcanising machine: provide heating and pressure, high temperature uniformity is required.
3. Forming process
- Loading: Put the silicone into the mould cavity.
- Pressurised and heated:
-Temperature: 120~180℃, time 5~10 minutes (depends on thickness).
- Pressure: 5~15MPa to make silicone flow to fill the mould.
- Demoulding: Remove it after vulcanisation is finished and cool it naturally.
4. Post-treatment
- Secondary vulcanisation: Some products need to be further cured in 200℃ oven to enhance the performance.
- Quality control: check the dimensional stability, hardness (commonly used Shore A hardness 40~60).
Features:
- Advantages: low mould cost (thousands to tens of thousands of dollars), suitable for small batch or customised production.
- Disadvantages: long cycle time (5~15 minutes for a single piece), lower detail accuracy than injection moulding.
III. Other moulding processes
1. 3D printing silicone:
- Light-curing silicone: curing layer by layer using DLP technology, high precision but expensive, mostly used for prototyping.
- Limitations: weak mechanical properties, not yet massively applied to the production of mobile phone cases.
2. Drip gluing process:
- Manual drip gluing: mixed silicone is poured into open moulds and cured naturally, suitable for DIY or very small quantity production.
- Disadvantages: low efficiency, easy to leave bubbles on the surface.
Fourth, the core elements of silicone phone case moulding
1. Temperature control: the curing temperature directly affects the curing speed and hardness of the finished product.
2. Mould precision: to determine the details of the hole position, key fit and so on.
3. Raw material selection: medical grade silicone (non-toxic), food grade silicone (dirt-resistant) or common industrial grade.
Finished product characteristics and process related
- Soft and drop-proof: the high elasticity of silicone comes from the structure of vulcanised cross-linking network.
- High temperature resistance: -50℃~250℃ after curing, adaptable to daily use environment.
- Environmentally friendly: no plasticiser, can be degraded by incineration (generating silica and carbon dioxide).
Summary: Process selection basis
- Mass production: Priority is given to liquid silicone injection moulding (high efficiency and excellent details).
- Small quantities/customisation: moulding is more economical.
- Special needs: 3D printing for prototyping, drop moulding for handmade products.