Nowadays, liquid silicone rubber injection molding is the most versatile and dependable manufacturing process. As advance of LSR materials, consistent innovation of molding process, new injection molding machines appeal customer in different industries, we are able to fabricate LSR products with increasingly tighter tolerances and repeatability. Liquid silicone rubber become one of the best manufacturing process.
LSR injection molding gains a competitive advantage in various industries, such as electronics, automotive, aerospace, medical, food and beverage, and construction. Liquid silicone injection molding not only is a highly effective manufacturing process for accurate LSR parts and components, but also reduces production time and materials waste with higher accuracy. At the end of production, we can produce LSR parts very soft and flexible or as strong as steel.
In LSR 2-shot Molding process, both thermoplastic and LSR molding fabrication start from the mold. These molding systems are divided into two categories: cold runner molds and hot runner molds. Cold runners apply a closed thermoset mold, while hot runners use a screw nozzle with feed material from barrels. The primary task of injection runner systems is to guide material flow from sprue to mold cavities. These systems also require additional pressure to accelerate material flow through the runner. We need to confirm the proper sizing of the runner with the best payoffs in mold design.
Such as, large runners can facilitate material flow at low pressure. But these runners need longer cooling time, more material and scrap, additional clamping force. While the small runner system can achieve maximum application of raw materials with minimum energy consumption. Finally, the runner size normally depends on the pressure capability of injection machines.
Runner System Function
In conclusion, we should design the proper runner system to achieve the benefits as following:
Achieving the optimal cavity numbers
Delivering liquid materials to the cavities
Balancing multiple cavities filling
Balancing multi-gate cavities filling
Minimizing material waste
Allowing easy injection
Providing efficient energy consumption
Controlling time of filling, packing, cycle
In this article, we will explain the basic information of cold runner design and hot runner designs. With a better understanding of these two systems, we can make suitable decision on design and specification of LSR 2-shot molding parts.
Cold Runner Molds
Cold runner system is consisting of two or three plates within the mold. This system injects materials into the mold through the sprue, feeds the runners, and finally reach to the mold cavity. As simply, cold runner are unheated channels and molds to inject molten materials into mold cavities.
The simplest cold runner system is consist of two plate molds, this system has the runner and part attached. We can only design the gate on its perimeter, and the ejection system can separate both the runner and part from the mold.
Three-plate cold runner system is another common type. There are two parting planes to separate molded parts into three sections in ejection. This runner system provides greater design flexibility, and allows the flexibility of gate location with application requirements.
Sprues, Runners, Gates
Cold runner system has the critical flow path into the mold cavity through a sprue, runner, and gate. In the following image, we can find out the simulation and details of each portion.
Sprue
The sprue is a channel for molten materials to the mold. The sprue brushing is the location of nozzle to inject molten material towards the cavity. After injection molding, we need to remove the spure manually in a two-plate mold, or eject the spure from the mold. In order to minimize the materials waste, we should keep the spure diameter small and short, but sufficient to fully pack all cavities and avoid quality issues.
Runner
The runner is the channel to transmit molten materials from the sprue to the cavity gate. Similar to the sprue, we need to keep runner diameter as small and short, but large enough to fully pack all cavities.
Gate
The gate is the narrow intersection between the main runner and mold cavity. The molten materials enter into the cavities by this point. Gate location is critical for final parts performance and quality.
Advantages of Cold Runner System
Cold runner system has several advantages, this make it a viable option in LSR molding. The benefit of cold runner system including:
Inexpensive production and maintaining cost: this system is less complex than hot runner system.
Suitable for a wide range of polymers: especially for specialty and heat-sensitive polymers, which is unable to produce in hot runner system.
Easy to use: compare to hot runner system, cold runner system is more simplistic design with fewer components.
Quick and easy color change: the excess materials in feed channels are cured and ejected with molded parts.
Short cycle time: as heat-sensitive materials will cure in short time, cold runner system consumes less time in each cycle.
Considerations of Cold Runner Systems
The most critical consideration of cold runner system is to ensure the runner dimension is thicker than molded parts, in order to guarantee the proper filling without any limitation in cool situation. In most applications, the round runner is the best option for cold runner system, each mold half contains one-half of the runner diameter. Furthermore, in gate selection, we prefer Tunnel gates as mold opening or ejecting to will release the gate from components. There are other factors we should consider as following:
Nozzles
The nozzles are different from various vendors. We should deal with welded assemblies once particulates penetrate water jackets. Some nozzles have removable water jackets for easy cleaning.
Nozzle Tips
There are two type of nozzle tips. The first type mount the nozzle tips securely at the end of thenozzle. Another type are spring-loaded nozzle tips, it has thermal expansion once heated. In order to keep constant contact and prevent materials leakage.
Runner Manifolds
The gun drilled runner manifold will have dead spots at the channel end, where the materials are cured and hard to clean. However, split-plate runner design is easy to disassemble for cleaning, it is also quick for material and color changes.
Piston Assembly
Gun-drilled air passage ways can eliminate the air hoses and barb fittings in each nozzle, this can prevent the damage to air lines and connection errors.
Pneumatic Connections
Cold runner systems require a quick disconnect coupler by either external air lines or air terminal box. Therefore, the mating umbilical cord abuts with the molding machine, in order to make connection from the valve gate controller to the cold runner system.
As improvement technology in cold runner molding system, there are more available options for engineers and designers.
Hot Runner Molds
Hot runner is often referred as hot-manifold system or runnerless molding, it is a set of temperature control heating-devices in the mold. The hot runner system can maintain the molten flow of plastics or rubber from injection machine nozzles to gate in injection mold. This system must keep runner temperature above the melting point of plastic or rubber materials.
Hot runner molding system normally consists of two plates with a manifold system inside one half, it transmits the melted material to the nozzle, then feed the cavities. This hot runner system is conconsistingsist of two parts: hot manifold and drops. The host manifold transmits liquid form materials on a single plane above the cavity and parallel to the parting line. While the drops are perpendicular to the manifold, and transmit materials from the manifold to mold cavities.
Hot Runner System Types
There are two common types of hot runner systems: insulated runners and heated runners.
Insulated Runners
Insulated runners are unheated, this runner type requires thick runner channels to keep continuous cycling of molten materials. Therefore, these runners make extra-large size passages in the mold plate. In injection molding process, the passages size and heat in each shot result in open flow path of molten materials. This system is less expensive, it eliminates the cost of manifold and drops. However, it can provide flexible gates in system, and easy for color changes.
Heated Runners
Heated runners have two different design methods, externally heated and internally heated.
Externally heated: This method design apply a cartridge-heated manifold with interior flow passages. This manifold has insulating characteristics to prevent heat loss, in order to separate it from the rest part of the mold. As this runner doesn’t require a heater to block the materials flow, the externally heated runner channel have the lowest pressure drops. This system can work better for color changes, in reason of none of color materials are freezing is system. Furthermore, there is no surface where molten materials stick to and degrade, this make externally heated runner system suitable for thermal sensitive materials.
Internally heated: This system has annulus flow passages, it is heated by a probe and torpedo in the passages. In reason of insulating effect of melt plastics or rubber, this system reduce heat loss in the mold. However, it requires higher molding pressure, and is challenging for color change. In addition, because of materials degradation and sticking to the surfaces, this system is not suitable for thermal sensitive materials.
Hot Runner Benefits
Runner heating can apply a variety of devices, such as coils, cartridges heaters, heating pipes, heating rods, band heaters. We also require a complex control system to ensure a consistent flow and distribution of molten materials. Hot runner system has some unique advantages as following:
Fast cycle time
Improve consistency and quality of molded parts
Fewer materials waste than cold runner
Ideal for large size fabrication
Lower injection pressure
More process control and fine-tune in manufacturing
Hot runner system has high cost in part fabrication and system maintenance, it is not suitable for thermal-sensitive materials like resin, LSR, polymers.
How to Choose Hot Runners
In LSR injection molding design process, we should choose the most suitable runner system with careful consideration of different factors. These factors include not only price, quality and service, but also other technical aspects of other system. These technical requirements including:
Injection Pressure
As molten plastic materials stay hot in hot runners, the hot runner injection pressure is extreme lower than the cold runner system. Mold flow simulation is required to get necessary data for proper injection pressure design, especially for materials with poor melt-flow performance, and large parts with long melt flow and complete filling.
Heating
The main difference of hot runner systems is heating method. External heating systems transmit the molten materials flows through the runner without any carriers. This heating method provides reasonable melt shear force curves. On the other hand, internally heating system set on the melt channel and heat the materials inside.
Gate Types
The hot runner systems have different gate types, we should consider different aspects of gate marks, gate location, materials types. Crystalline and non-crystalline thermoplastic require different types of injection gates.
Standard or Custom
We should evaluate the injection molding process with a standard or custom-made hot runner system. Normally, standard solution is the best choice. The standard length, nozzles, gate inserts, runner boards are available in most time. This reduces delivery time and cost. In addition, it is easier to maintain.
Plastic Types
Once choose the hot runner system, we should consider the plastic types. Different molding materials with different properties require different runner system. Such as glass-reinforced plastic requires the gate inserts with reasonable wear resistance. For the easy discomposed plastic, we should apply external heat runner system to avoid the dead corners.
Runner Size
The runner size affects significantly on the overall performance of hot runners. Incorrect runner size give rise to degradation and incomplete filling of injection molded components. In order to determine the optimal runner size, we should consider factors of pressure drop, temperature, residence time, shear rate and frequency.
Multi-zone Temperature Control
We should apply a multi-zone temperature controlling system in large and complex hot runner system, especially for temperature-sensitive plastic materials with tight processing parameters. This system can account for heater quality and heat loss in precision.
Conclusion
In order to ensure the successful injection molding production, you require professional assistance from our engineering team to consider all the essential factors. Contact our engineering team now for more information in liquid silicone injection molding.
