Things You Need To Know About Silicone Injection Molding China

Silicone injection molding usually includes using liquid silicone rubber to produce durable and pliable parts or components in high volumes. Silicone rubber has great stability, resisting cold and heat for manufacturing high-quality parts.    

Compared to thermoplastics, silicone parts offer more options and make excellent final products, so many manufacturers prefer to use this method instead. When choosing a fabrication technique for making silicone prototypes, the key criteria include speed, approximation, and cost. 

Below are the important things you need to know about silicone injection molding:

Silicone Injection Molding Is Used to Create Prototypes Fast

Silicone has a thermosetting nature that requires intensive distributive mixing. Creating shapes out of silicone rubber to make product prototypes include various methods, such as room temperature vulcanization (RTV) molding, 3D printing, fused deposition modeling, selective laser sintering, and injection molding. An oven is usually used in accelerating the material curing time with the RTV method. 

Raw materials for RTV molding are widely available, and the capital requirement is low, making the in-house manufacturing of tools from soft metal and plastic materials possible. The disadvantage of this method is a slow cure time, which extends from 20 minutes to several hours. But a skilled and experienced technician can create the first component prototype within the day.

Because of the availability of new elastomeric materials, 3D printing is now possible. This method doesn’t require mold machining a mold. However, elastomeric materials are not real silicone rubber. 

On the other hand, injection molding uses liquid silicone rubber (LSR). With this method, soft metal tooling is used to produce molded grade materials. With rapid silicone molding, the production of parts is faster than other methods because tooling is quick. Learn more about rapid injection molding at https://prototechasia.com/en/from-mockup-to-prototype-the-benefits-of-rapid-silicone-molding.

Here are the advantages of silicone injection molding:

  • Batches stability or ready-to-use material
  • Direct injection (zero waste)
  • Process repeatability
  • ‘Flashless’ technology (zero burrs)
  • Short cycle time
  • Automated demolding systems 
  • Automated process

Principles of LSR Mold Design

The design of silicone injection molds for LSR is similar to thermoplastic parts mold design. Because of the low viscosity levels of LSR, the cavity filling times tend to be very short even at very low injection pressures. A good venting mold avoids air entrapment. Unlike thermoplastics, LSR expands in the hot mold and shrinks. 

1. Shrinkage 

Rubber parts don’t remain on the core, and rubber parts should remain in the cavity with larger surface areas. Rubber parts don’t shrink in the mold and will just shrink after demolding following the cooling process. 

LSR shrinkage depends on the following factors: 

  • Tool temperature or demolding temperature
  • Cavity pressure and material compression
  • Injection point location or shrinkage in a material flow direction 
  • Part dimension (thicker LSR parts shrink lower than thinner rubber parts) 
  • Post-curing (causes additional shrinkage) 

2. Parting Line

One of the initial steps in injection mold development is deciding where the location of the parting line. Venting is necessary for the proper flow of material, which takes place via special channels. That’s why the best location of the parting line is the last area that the LSR reaches. 

This will avoid air entrapments. Also, the mechanical strength isn’t reduced with the separation of the molded part that’s easily recognizable later on. Because of low silicone viscosity, the area becomes sensitive and easily brings about flash. The parting line location and the rubber part’s geometry influences the demolding of rubber parts. 

3. Venting

The enclosed air in the heated cavity is compressed by the LSR and expelled via the venting channels. Air entrapments occur if the air can’t escape entirely. When this happens, the rubber part will have white edges, which is not good for your final product. 

A vacuum is needed for optimum venting, so the mold should stop in the closing movement before it’s completely closed to produce a vacuum. In the parting line, a gasket is built and a vacuum pump draws the air from the heated cavities. Once the vacuum reaches reduced pressure or the time cycle ends, the machine closes the mold completely, which marks the restart of the injection process.

Modern silicone injection molding machines usually offer different clamping forces. A low clamping force permits the easy escape of air through the parting line. It avoids flash that is usually caused by silicone expansion. 

4. Injection Point

The injection point or type of gate highly depends on several factors. With the use of a cold runner system, you’ll benefit from LSR’s technical advantages, which also maximizes productivity. The main goal is to produce rubber parts without removing any sprue, which is labor-intensive. Be eliminating the sprue removal process, it also saves you a considerable amount of material with faster cycle times.

Liquid Silicone Rubber Has a Wide Range of Applications 

Silicone rubber belongs to the thermoset elastomers family with alternating oxygen and silicon atoms and vinyl or methyl side groups used in different applications. Silicone rubbers are the largest group, which makes about 30 percent of the silicone family, which is widely manufactured and used in China. It maintains its mechanical properties at different temperatures. 

Also, silicone rubbers are extremely hydrophobic, which makes them perfect for making electrical surface insulation. Whether you prioritize an online business or a full-blown manufacturing business, silicone injection molding can help you speed up your production to address customers’ demands.

Here are the most common applications of silicone rubber products produced via silicone injection molding:

  • Seals 
  • Electric connectors 
  • Sealing membranes 
  • Infant products 
  • Multi-pin connectors 
  • Medical applications 
  • Kitchen goods 

Silicone Injection Molding Equipment

Liquid injection molding involves several mechanical components. A molding machine is required with a metered pumping device with a static or dynamic mixer that makes the injection unit. By having an integrated system, silicone injection molding is made faster. 

Here are the important components of a silicone injection molding machine:

  • Injectors: These pressurize the liquid silicone, which aids in material injection into the pumping section of the silicone injection machine. 
  • Metering Units: Pump the base and the catalyst to ensure that the materials maintain a simultaneously released constant ratio.
  • Supply Drums or Plungers: They serve as the main containers for mixing various materials. 
  • Mixers: A dynamic or static mixer combines the materials after leaving the metering units. The pressure is used once combined, driving the mixture into the mold.
  • Nozzle: Facilitates the mixture deposition into the mold. It prevents overfilling and the leaking of mold because of the automatic shut-off valve feature.
  • Mold Clamp: It secures the mold during the silicone injection molding process. 

Silicone Injection Molding Process

As silicone injections are done at a high temperature, natural shrinkage and steel dilation must be considered at the LSR injection tooling design stage. When it comes to the silicone injection molding process, LSR comes in barrels and is pumped through tubes and pipelines, going towards the vulcanization equipment. There are two components involved with this process. One component usually contains a platinum-based catalyst. 

Here is the step-by-step silicone injection molding process:

  • Before entering the static mixer, additives and coloring paste can be added. Components are mixed and transferred to the metering section (cooled) of the silicone injection molding machine. 
  • Use the static mixer and the material becomes homogeneous and very consistent from one part to another. 
  • From the injection molding machine’s metering section, the material is pushed via a cooled runner system into the vulcanization area (heated cavity). General cooling preserves the material. It means zero waste and eliminates trimming operations, and yields significant material cost savings. 

Understanding the Liquid Silicone Rubber or LSR Vulcanization Process 

Because of the thermosetting nature of liquid silicone injection molding, it requires special treatment, such as intensive distributive mixing. Before being pushed into a heated cavity, a low temperature is maintained, after which it undergoes the vulcanization process. 

The speed of vulcanization for liquid silicone rubber depends on several factors, such as the following:

  • Mold temperature or possible inserts temperature
  • Silicone temperature upon reaching the heated cavity 
  • Part or component geometry 
  • Curing chemistry 
  • General vulcanization behavior 

If the cycle times are reduced, productivity is increased by raising the temperature of the mold using a quicker curing material, and pre-heating the cold runner and injection barrel between 40 and 80 degrees Celsius using a temperature controller.

Here are some good-to-know facts about the liquid rubber silicone vulcanization process:

  • Injecting pre-heated silicone is highly recommended for fully automated processes. It makes larger items with small surface-volume ratios.
  • The cooling system should be activated to prevent the curing of the material. It must be done immediately if the machine’s cycle is interrupted. 
  • The vulcanization time is not a decisive factor when it comes to the total cycle time. The rubber part is longer to demold, and the dosage of LSR in the injection barrel is a major key element in the cycle time. 
  • If there are several numbers of mold cavities being run, the dosing unit is less likely to load enough. If it does, the dosing time extends the cycle. This is why the machine should be adjusted precisely to achieve maximum productivity. 
  • More often than not, the curing time is assessed during the planning phase. The total vulcanization time used is around 4 to 6 seconds per millimeter, which becomes longer with parts with larger thicknesses. Simulation software can help achieve more precise calculations.

Understanding the Curing Behavior of Liquid Silicone Rubber

With the use of a catalyst, liquid silicone rubber is cured, which takes place faster at high temperatures. Once mixed, liquid silicone rubber has a pot life or minimum usage time of three days. LSR curing is measured on torque or on a rheometer. 

Plastic has low deformation resistance caused by the testing equipment. However, liquid silicone rubber absorbs the chamber heat, and the torque starts to rise upon the beginning of vulcanization. Once the maximum torque has been reached, curing starts. 

Here are the good-to-know facts about the curing behavior of LSR:

  • A key factor in curing LSR is temperature. The time should be short enough to permit injection, but high enough to reduce cure time. 
  • At 25 degrees Celsius, the material takes several weeks to become totally vulcanized. However, if the temperature is above 120 degrees Celsius, vulcanization only takes a few seconds. Most high-quality LSR cure quickly because of using temperatures from 170 degrees Celsius to 210 degrees Celsius, which lead to a very productive process.
  • By choosing an LSR with a very fast curing time at lower temperatures, it significantly increases your production output. 

When it comes to post-curing, finished silicone rubber parts should meet certain guidelines (imposed by the Federal Institute for Risk Assessment or the Food and Drug Administration). Post-curing helps achieve particularly high mechanical properties that are strictly required from the silicone rubbers. 

The post-curing process usually takes place at 200 degrees Celsius in four hours with a supply of fresh air. Post-curing temperature and times can be reduced depending on the properties required for the finished products. With the use of experimental trials, post-curing is fine-tuned. 

Other Silicone Injection Molding Technologies

The most commonly used technology in silicone injection molding is the two-component silicone injection molding. It involves creating parts from LSR after production or directly in place for final use. For instance, producing a silicone rubber gasket on a nylon housing entails creating both parts, which are the gasket and the housing on separate silicone injection molding machines.

Because faster curing is most sought-after for maximum production, you can also consider other silicone injection technologies. Depending on your production needs or product requirements, you can always find the best silicone molding technologies to use for your industrial business.

Here are the other silicone molding technologies you may consider:

  • Multiple color rubber silicone parts productions 
  • Electrically conductive LSR in combination with other insulating ones used in high-voltage applications
  • Silicone gaskets injection produced on aluminum housings 

Conclusion

Silicone is a durable, pliable, and safe material in creating prototypes and final products in various applications. Now you have a better understanding of silicone injection molding, the machine used, the process, the advantages, and the products you can manufacture using liquid silicone rubber. Indeed, silicone injection molding promotes rapid production for a more successful manufacturing for industrial businesses.

Adam Torkildson