Injection molding vs transfer molding are two common manufacturing methods for liquid silicone rubber. In silicone parts fabrication process, we need to determine the most suitable manufacturing process, in order to balance the efficiency and quality. Both injection molding and transfer molding of liquid silicone rubber have distinct advantages and disadvantages. Before we determine which process is the best suitable for the particular LSR products, we should compare the key features and benefits of injection molding and transfer molding.
Injection Molding
Liquid silicone Injection molding is the most common fabrication method for silicone products. This technology encourages the increase demand of inexpensive and mass-production liquid silicone parts. As advanced development in liquid silicone injection molding, LSR injection molding has achieved a new level. It can produce cubic, cylindrical or complex three-dimensional structure in size from 6 mm2 to 1 m2. Therefore, we normally apply liquid silicone injection molding to produce housings, electronics, power tools, automotive interiors, medical devices like syringes and valves.
Injection Molding Process
The normal injection molding process including four steps:
- Material selection: The materials are often in the form of pellets or granules, then fed into heated barrels and molten into fluid form. However, in liquid silicone injection molding, the materials are in the form of liquid state.
- Injection: Molten materials or liquid silicone are forced through the spures into the mold cavities by a reciprocating screw or ram injector.
- Solidification: The melt or liquid materials are left inside the molds, then solidify in cool down (thermoplastics) or heated up (thermosets) situation, finally create molded parts with required structure.
- Ejection: Once the curing process finish, the mold will open and eject molded parts.
Common Materials
Injection molding is suitable for both thermoplastic and thermosetting materials, the common materials include:
- Liquid Silicone Rubber (LSR)
- Polystyrene (PS)
- Polyamide (PA)
- Polyethylene (PE)
- Polyvinylchloride (PVC)
- Acrylonitrile Butadiene Styrene (ABS)
- Polypropylene (PP)
Injection Molding Advantages
Injection molding is the most cost-effective molding methods for both plastic and silicone products. This technology has unique advantages.
- Perfect Materials: Molten thermoplastics or liquid thermosets can flow into the mold cavities easily. This result in faster cavity filling and rapid curing.
- Higher Capacity: Injection molding machines can accommodate more cavities in one mold, this result in additional units in each production cycle.
- Minimal Waste: Comparing to transfer molding and other molding methods, there are less material waste in reason of smaller sprues and lack of overflow channels.
- Fast: The common cycle time of injection molding is 2 seconds to 2 minutes. This is faster than other molding technologies.
Injection Molding Disadvantages
The injection molding technology is suitable for large-volume production, but there are some certain limitations.
- Thin & Uniform Walls: Injection molds require uniform thin walls with thickness between 0.4 mm and 12.5 mm. Otherwise, too thick parts or non-uniform molds will result in various defects in molding process.
- Rounded Corners: It is difficult to produce parts with sharp corners in injection molds, these sharp corners will also give rise to other defects in parts ejection. Therefore, we need to make the rounded corners in molds.
- High Setup Cost: Injection molding machines are more expensive than transfer molding machines. In addition, its setup is much more complex.
Potential Defects
Any molding process have the risk of potential defects. The normal defects in injection molding, including:
- Flash: This is the occurrence of molten or liquid material seeping from the mold cavity. It will happen once the injection pressure is too high, or the mold has too much clearance.
- Warping: If the molded parts cool at non-uniform rate, the internal stress will give rise to permanent bending in parts structure.
- Bubbles: Injection molding machines are more expensive than transfer molding machines. In addition, its setup is much more complex.
- Unfilled Sections: Once molded parts solidify certain sections firstly, these voids will occur on parts surface.
- Sink Marks: Once molded parts solidify certain sections firstly, these voids will occur on parts surface.
- Ejector Marks: Ejector marks will occur once the cooling time isn’t enough, or the ejection force is too high.
Transfer Molding
Transfer molding is s simplified variation of injection molding, it has much in common with injection molding. The key differences between these two molding lie in the processes. Transfer molding pushes materials into molds by plungers, while injection molding by screw injectors.
Transfer molding is the most suitable manufacturing method for electronic components with rubber or plastic encasing. Such as pins, studs, connectors and molded terminals. Transfer molding has unique advantages, this makes it wide applications in different industries. Like metal-to-rubber face seals for gas vales in gas industry, mold connector seals in electrical industry, sharper lip seals in hydraulic industry.
Transfer Molding Process
Transfer molding process has a few differences with injection molding, its normal process is as following:
- The raw material is placed into a holding chamber as a transfer pot.
- The hydraulic plunger will push materials through sprues into the mold cavity.
- Materials inside the molded will solidify by cooling down or heating up.
- Open mold and eject parts once curing finish.
- Trim off extra material for the sprues.
Common Polymer Materials
Transfer Molding can apply both thermosetting and thermoplastic materials, the most common polymer materials include:
- Unsaturated Polyester
- Epoxy
- Phenol-formaldehyde Plastics
- Silicone Rubber
Transfer Molding Advantages
Transfer Molding has simplistic machines design and process, this provides some unique advantages as following:
- Inserts Permission: Transfer molding is perfect for metal inserts molding.
- Fast Set-up: Transfer molding is easily assembly with inexpensive machinery, it is suitable for a quick start-up.
- Low Maintenance Cost: Transfer molding has lower machinery cost than injection molding, so its maintenance cost is cheaper.
- Sharp Edges: Transfer molding has higher injection pressure than injection molding. Therefore, it can achieve sharper edges and cutoffs.
Transfer Molding Disadvantages
The simplicity and fastness of transfer molding give rise to a few disadvantages:
- Material Waste: Transfer molding produces more materials waste, because of larger size of sprues and overflow channels.
- Slow Production: Because of materials preheating before transfer molding, its production speed is slower than injection molding.
- Low Quantity: Transfer molding machines cannot accommodate multiple cavities as injection machines, it produces fewer units per cycle.
Potential Defects
Some potential defects in transfer molding include:
- Flash: Similar to injection molding, too high injection pressure or too much mold clearance will give rise to flash issues.
- Warping: Non-uniform cooling rate of molded parts result in permanent bending.
- Voids: Injection molding machines are more expensive than transfer molding machines. In addition, its setup is much more complex.
- Ejector Marks: Ejector marks will occur once the cooling time isn’t enough, or the ejection force is too high.
Comparison of Injection Molding vs Transfer Molding
Injection Molding vs Transfer Molding have numerous differences to determine the final production methods for specific products. There are the most critical differences to consider as following:
Initial Investment
Both injection molding and transfer molding require the expensive mold tooling process. However, the machinery cost is the main factor to determine the startup costs. The injection molding machine is more expensive than the transfer molding machine, because of the complexity and specialization within the injection molding machine.
Furthermore, injection molding machine requires more time to set up than transfer molding machine. The high cost and complexity of injection molding machines result in high maintenance costs.
Production Speed
Injection molding has very short cycle time, it varies from 2 seconds to 2 minute depending on different parts size. Additional process of excess material removal, like flash or the sprue, will increase the overall production time. However, injection molding still has shorter production time than transfer molding. The preparation of raw materials, before placement in the machine, can increase the cycle time significantly.
Production Cost
Normally, the production cost depends on geometry of final products, not just the production methods. Materials with high injection pressure require more powerful injection machines, this result in higher cost to apply injection machines than transfer machines. However, injection machines accommodate multiple cavities to increase production per cycle, and automated injection molding also reduce labor cost in long-term.
Geometry and Accuracy
Both injection molding and transfer molding can provide high accuracy with consistent results. But injection molding has potential issues to create sharp edges features, it only can end up rounding off edges. Furthermore, even both methods can produce complex units, the transfer molding has significant lower cost than injection molding. Because the complex molds need additional intricate and expensive injection system to produce.
Waste Amount
Although injection molding has flash and sprue waste, transfer molding creates more waste on average due to wider sprue, air holes and overflow grooves. If we apply thermoset materials, this will result in substantial material waste in transfer molding.
Product Size
These two methods can produce both small to medium size products, but injection molding can produce much larger products. Transfer molding is more suitable for small part sizes, due to its limitations in press sizes.
Product Volume
Injection molding is superior to transfer molding in terms of production quantity. The relative automated system in injection molding, combine with faster cycle time. In order to make injection molding cost-effective in long-term production with high quantity.
Both these two methods can produce high-quality products. However, injection molding is much more suitable for higher quantities of large, thin wall parts. While transfer molding is better suited to small quantities of simple parts.
Liquid Silicone Injection Molding vs Transfer Molding
In silicone products production, we must maximize efficiency with high quality maintenance. There are different technologies to satisfy the fabrication requirements.
Liquid Silicone Injection Molding
Injection molding is an advanced technology for high volume production. Comparing to common thermoplastics, liquid silicone injection molding requires special treatments, such as intensive even mixing of compound A&B, low temperature in injection system, high temperature in molds for solidification promotion.
Silicone Rubber Transfer Molding
Transfer molding is considered a simplified injection molding process. The prepared amount of silicone rubber is preheated and located in the chamber, then the piston forces silicone rubber into the mold cavity through the sprue, finally silicone rubber vulcanizes and solidifies in the mold.
Molding Method Selection
Once we are determining which molding method is much more suitable for parts fabrication, there are several factors we should take into accounts.
Different Machinery: Transfer molding applies a press to transmit pressure to the piston, this press has low cost than injection molding machine. Therefore, the initial investment in transfer molding is lower. As the press create lower pressure than an injection molding machine, transfer molding is better suited for small and medium size parts.
Cavitation Limitation: The cavitation in molds are limited by the capacity of the press or injection molding machine. Typically, the injection molding machine can accommodate more cavities. This will reduce the unit price and improve efficiency in high-volume production.
Raw Material Preparation: In injection molding, we can prepare and mix raw materials (compound A&B) immediately. While in transfer molding, we must prepare the raw materials in many steps before placing it in pots. This will increase the process cycle time in transfer molding, and finally the production cost.
Material Waste: Because of wider sprue, additional air holes and overflow grooves in transfer molding, there are more materials waste than injection molding. Furthermore, we need to remove these waste by additional process, like trimming, cryogenic tumbling, precision grinding.
We summarize the main information of liquid silicone injection molding vs transfer molding as following:
| Characteristic | Liquid Silicone Injection Molding | Silicone Rubber Injection Molding |
| Initial Investment | High | Low |
| Tooling Cost | High | Low |
| Production Speed | High | Low |
| Tolerance | High | Medium |
| Material Waste | Low | High |
| Optimal Product Size | Large to Medium | Medium to Small |
| Production Cost | Depends | Depends |
Conclusion
In conclusion, both injection and transfer molding can produce high quality products. However, injection molding is more suitable for consistent parts with precise intricacy, while transfer molding specializes in simple mold structure with faster production. In silicone molding, we should consider all factor in final parts fabrication.
Contact our engineering team, we will find the most cost-effective method for your specific project.
