Silicone Transfer Molding

Silicone Transfer Molding

What is silicone transfer molding?

Silicone transfer molding is a processing technology for silicone. High temperature and high pressure are often used in the production process. First, put the preheated materials into a separate transfer pot. After the mold is closed, the plunger presses down to squeeze the material into the runner, which is then injected into the closed mold cavity.The heat cures the material into the shape you want.

This production method is between silicone injection molding and compression molding. It combines the precision of injection molding with the simplicity of compression molding. It is highly suitable for manufacturing complex and delicate silicone components. Its products fully meet the application requirements in the sealed box packaging scenario.

Process Features

High precision: The system accurately measures the amount of compound injection, and the dimensional stability of the product is better than compression molding.

Excellent insert Molding: For some metal inserts, the low flow rate of compound filling reduces the impact on the insert, avoiding displacement and damage.

High material utilization rate: The flow channel system design is flexible, with a high material utilization rate and reduced material waste

High degree of automation: It is easy to achieve system integration and can realize fully automated production.

Suitable Material

Liquid Silicone Rubber(LSR): This material features high transparency, short curing time and good bio-compatibility.

High Consistency Rubber(HCR): This material can have high strength and hardness, and excellent tear resistance, but the curing time is relatively long.

Specialty Silicone Materials: It refers to silicone with modified fillers added, such as conductive silicone, thermal conductive silicone, flame-retardant silicone, etc.

Silicone Transfer Molding Process

Preparation

Engineers need to design a three-dimensional mold including the transfer pot, runner system and ejection system based on your product designs. And through software simulation of the production process, predict possible problems that may occur during production and optimize the molds.

Before production, the silicone that is evenly mixed and free of bubbles needs to be prepared. Manufacturers need to precisely measure the amount of compound injected into each of your individual products to guarantee consistent product weight and reduce material waste.

If you are customizing insert products, the inserts need to be cleaned and dried before production. Sometimes, they may also need to be preheated. The processed inserts need to be placed in precise positions inside the mold.

Material Loading

Put the pre-treated compound into the transfer pot. Usually, LSR is injected into the transfer pot through the injection nozzle, while HCR is placed automatically by the equipment or manually put in.

Mold Closure

Molds are usually made of metal. The mold will be designed according to the shape, details and texture of your product. After the compound is placed in the transfer pot, the upper and lower molds close and lock. The pressure for mold locking needs to be precisely controlled to prevent compound overflow during silicone injection.

Material Transfer

After the mold is closed, the plunger, driven by pressure, squeezes the compound in the transfer pot downward. The compound is injected into the cavity through the runner and gate under high pressure until it is completely filled. Under high temperature and high pressure, the viscosity of silicone decreases and its fluidity improves. This allows the silicone to better fill every detail of the cavity.

At this stage, the manufacturer needs to set precise parameters for injection pressure and speed. Both excessive and insufficient injection pressure and speed will affect the quality of your products.

Curing

After the compound fills the cavity, it needs to stay in the heated mold for a specific period of time and keep a certain pressure. At this stage, silicone will gradually change from a viscous flow state to an elastic solid.

The curing process determines the final physical and chemical properties of your product. The curing temperature and time need to be matched with your product. Otherwise, it will lead to a decline in product performance, such as the product becoming sticky or prone to aging.

Mold Opening and Ejection

After the product is cured, the upper mold rises to open the mold. The ejection device pushes the ejector pin to push the product out of the lower model cavity. The removed components represent the completion of one molding cycle.

Flash Trimming

There may be some flash in the final product. This requires manual trimming or mechanical cutting to remove the tiny flash on the product.

Clean Up

The product still needs to clean its surface or undergo plasma treatment and other treatments. This is to improve the appearance and adhesion of the product. The final product needs to coincide with your product and appearance requirements.

Quality Check

The product still needs to be inspected before it can be delivered to you. Generally, online inspection usually uses calipers and optical image inspection instruments to measure the dimensions of products. Moreover, other equipment is also needed to test the performance of the product, such as hardness, tensile strength, sealing performance and electrical performance. What’s more, It is also necessary to check the product under light for any bubbles, impurities or stains.

Qualified products will be packaged according to your requirements for dust and scratch prevention after inspection. We will establish a quality file for each batch of products to facilitate subsequent traceability.

Advantages of Silicone Transfer Molding

High Precision and Consistency

Compared with compression molding, the injection volume of transfer molding can achieve minimal waste of compound through precise measurement in the transfer pot and plunger. Its products can achieve high precision and high dimensional stability. Therefore, it is very suitable for manufacturing precision parts with strict dimensional requirements, and the differences between batches of parts are extremely small.

Superior Insert Molding Capability

The low-pressure and low-flow compound is gently filled from the bottom or side of the mold without damaging the pre-placed inserts. It can cover complex and precise inserts and achieve one-piece molding with silicone. This is also the core advantage of transfer molding.

Production of Complex Products

The injection pressure of transfer molding can guarantee that the material fills tiny holes and thin-walled areas. Therefore, it is used to produce products with complex structures, featuring multiple steps, deep grooves, and fine textures. Usually, silicone is evenly filled inside the cavity with a consistent density.

Low Material Waste

Compared with silicone compression molding, the material utilization rate of transfer molding is higher. On the one hand, it is because of the precise control of compound filling; on the other hand, it is because the volume of waste in the runner system of transfer molding is small and often keeps stable. Besides, transfer molding can recycle and reuse excess materials more easily.

High Automation

This process can achieve fully automated production. From insert placement, compound injection to ejection, the entire production process can be mechanized. This greatly reduces the possibility of errors caused by manual operations and eliminates labor costs. Production efficiency has been greatly elevated and the quality of products more stable.

Disadvantages of Silicone Transfer Molding

Long Production Cycle

Compared with injection molding, the molding cycle of transfer molding requires multiple steps. It requires manually adding compound to the transfer pot. What’s more, the curing stage of transfer molding also requires sufficient time. This results in the cycle time of a single part being slower than injection molding. Therefore, its production efficiency is not the highest.

High Mold Costs

Unlike silicone compression molding, the molds for transfer molding are more complex. It contains the transfer pot, plunger,precise runner and gate system. Molds usually adopt high-quality mold steel, which is generally more difficult to design and process, and the mold design cycle is also longer.

If you choose this production process, it usually requires a higher initial investment. Moreover, for small and medium-sized batch product orders, the economic efficiency of this process is not high.

Limited Part Dimensions

Due to the limitations of the volume of the transfer pot and the pressure generated during compound injection, there are upper limits on the size and weight of the single product produced. Therefore, large sealing strips and gaskets may not be suitable for this production process.

Critical Parameter Control in Production

Silicone transfer molding is highly sensitive to the process parameters that control the production process. Precise process parameters are the core of high quality, high consistency and high production efficiency. The following are the parameters that require precise control.

Temperature

Temperature is the most crucial factor influencing the chemical reaction of silicone curing.

Mold:The temperature of the mold affects the curing rate and degree. The temperature of the mold needs to reach the vulcanization temperature of the material and be kept for a certain period of time. Excessively high or low temperatures can lead to a deterioration in the physical properties of the product or cause it to age and become brittle.

The temperature of the mold can also affect the fluidity of the material. Preheated molds can make it easier for the compound to fill complex cavities. The uniformity of mold temperature also affects the curing rate and effect of the product.

Transfer Mold:The temperature of the transfer pot is usually sufficient to preheat the compound. Make the compound fluid and reduce the injection pressure. The precisely controlled temperature can not only soften the compound in advance but also prevent it from cross-linking prematurely.The above content typically applies to HCR solid compounds.

Insert Preheating Temperature:Preheated inserts can remove surface moisture, improve the bonding strength between the silicone and the inserts, and reduce the problem of local curing of the compound.

Pressure

Injection Pressure:Pressure can provide power for compound injection. Pressure can also affect the filling speed of the compound. Insufficient pressure can lead to slow filling. Excessive pressure can cause the compound to fill too quickly, which may lead to local coking.

Holding Pressure:When the compound begins to cure, continuously applying a low pressure can prevent the product from having voids or unstable dimensions. It should be noted that the holding time needs to be the same as the curing characteristics.

Mold Locking Pressure:During the injection and holding pressure stages, a strong clamping force can guarantee that the mold is not stretched by the pressure of the compound and effectively prevent the product from having flash.

Time

Injection Time:The time of compound injection and the pressure of compound injection have the same effect. If the time is too short, it may draw in air. Too long a time may cause the compound to start curing prematurely.

Curing Time:The time of curing determines the degree and effect of the chemical reaction of silicone in the mold cavity and the final physical properties of the product. This directly determines production efficiency and product quality.

Time of Holding Pressure:The holding pressure time can effectively compensate for the shrinkage. It is usually related to the wall thickness of your product and the material shrinkage rate. The specific duration needs to be determined through experiments.

Overall, these parameters in the production process are closely related and influence each other. On modern production equipment, these parameters can be closed-loop controlled by the equipment. Real-time monitoring and data feedback will guarantee that each molding cycle operates within the optimal and most stable parameter range, thereby achieving high-quality and highly consistent large-scale production.

Comparison with Silicone Injection Molding

Transfer molding is usually considered a simplified version of silicone injection molding. The main difference is that silicone injection molding uses a screw injection molding machine. The production processes such as mixing, plasticizing and injection of the compound are completed through the screw. The silicone transfer molding uses a plunger press. The injection of its compound is mainly accomplished by the downward squeezing of the plunger.

For molding cycles, the efficiency of injection molding is significantly higher than transfer molding. If you want to carry out mass production, injection molding is obviously a good choice.

For the degree of automation, injection molding is more likely to achieve fully automatic and continuous mass production. However, in transfer molding, the presetting of compounds may sometimes rely on manual labor.

For the variety of products, silicone injection molding is more suitable for manufacturing high-precision metal or electronic components and thin-walled products. It is particularly suitable for manufacturing products with complex three-dimensional structures. It also does not restrict the size of the product.

Silicone transfer molding is more suitable for insert molding. It tends to produce more complex components. For extremely complex structures, its processing capacity is inferior to the former. The size of the products it can produce is also limited.

Both can be produced in large quantities, but transfer molding is more economical for medium-sized orders.Your production method should take into account your cost, delivery time, product size, accuracy, output and so on.

Comparison with Silicone Compression Molding

For the production process, compression molding involves placing the compound into the mold cavity. And the closed mold fills the cavity with the compound through extrusion. Transfer molding involves first closing the mold and then extruding the compound into the cavity through a plunger.

When comparing the molds of the two, the mold for compression molding is simpler. The mold for transfer molding is more complex, which includes structures such as the transfer pot, plunger and runner.

For compound, compression molding usually uses HCR. Transfer molding uses LSR more widely, but it can also be compatible with solid compounds.

For the production cycle, the compression molding time is longer. The processes such as mold closing, curing and trimming require a considerable amount of time. The production cycle of transfer molding is shorter. Fully automated production, shorter curing time and simple post-treatment stages all shorten the production cycle.

For product precision, the consistency of compression molding is relatively poor, and the tolerance is usually above ±0.3mm. While transfer molding can achieve high dimensional accuracy. The precisely controlled amount of compound injection can achieve uniform filling and high consistency of the product.

Therefore, compression molding is more suitable for manufacturing simple products with low precision requirements and small batches. Transfer molding is more suitable for the production of inserts, medium and large-batch products with complex structures and high-precision requirements.

Applications

Electronics

Transfer molding can well meet the precise protection and insulation requirements of electronic components. This is also the most widely used and valuable application field for transfer molding.

It is capable of manufacturing connectors and encapsulating them. Transfer molding can form a permanent seal and can withstand high-temperature welding. It can also provide a corrosion-resistant and media-resistant housing for the sensor.

It can also produce semiconductor modules. It provides reliable electrical insulation and mechanical protection for high-power devices, and is capable of withstanding high and low temperatures.

Automotive Industry

Transfer molding can satisfy the usage requirements of automotive industrial components like aging resistance, high-temperature resistance, oil resistance and insulation. The products produced include transmission valve body seals, sensor sealing sleeves, motor controller seals for new energy vehicles, and lamp seals, etc.

Healthcare Industry

Medical equipment has extremely high standards for product cleanliness, biocompatibility and sealing performance. Some ventilators’ tubes and valve diaphragms need to use medical-grade LSR and also have relevant certifications. Additionally, there are also some surgical knife handles and instrument sealing parts. These products can withstand high-temperature disinfection and sterilization and provide a comfortable touch.

Consumer Products

For consumer products, it needs to offer an aesthetically pleasing, durable and comfortable user experience. Waterproof earphone plugs and camera waterproof sealing rings, these products achieve a high transparency and a close combination of colored silicone and metal, realizing the integration of functionality and aesthetics. The sealing valve of the coffee machine and the pressure sealing ring of the rice cooker made of food-grade raw materials can withstand high temperatures and have a long service life.