Selecting a liquid silicone rubber injection molding machine

The criteria for selecting a Liquid Silicone Rubber injection molding machine are very similar to selecting a machine for standard thermoplastic injection molding, with some key differences. Let’s start with the clamping unit of the machine. The first thing to consider is the most obvious — the platen size must meet the dimensional space requirements of the mold. Secondly, when specifying a clamping system that utilizes tie-bars, you need to review the distance between tie-bars. When specifying a tie-bar-less clamping system, you must consider the frame or drop-out width. This specification is critical if the parts are brushed out of the mold or ejected and allowed to drop into a chute below the machine. A tie-bar-less system offers greater flexibility for auxiliary equipment, such as core pulls or robot movements into the molding area. This makes the tie-bar-less machine a perfect fit for molding rooms with low ceiling height constraints.

When specifying the correct size clamping unit, the most important thing to remember is that the selected clamping force must overcome silicone's expansion during vulcanization. The standard rule used for silicone most often in the industry is 2 to 3 tons per square inch of projected area of the part surface. Additional items to consider during clamping size specification are part cross-section and mold construction. As silicone material expands during part vulcanization, you may want to consider higher clamping forces if molding parts with a very thick cross-section. For parts with a thin cross-section (.005″ – .010″), such as diaphragms, the opposite is also true. Although the part geometry has a very large surface area, which may make you want to consider a larger clamping force, the expansion of the silicone with such a small thickness is limited. To a certain extent, the expansion rate can also be controlled by processing with lower mold temperatures and extended cure times.

Mold construction must also be considered when determining the required clamping force. The clamping force must be high enough to maintain the correct force distribution across the cavity land. This is particularly important to consider when specifying a machine for a multi-cavity mold, which is molding very small parts. In this case the total surface area of the part may be very small, so you must consider the cavity insert size instead. Also, the use of preloaded cavities and/or spring washers may have an impact on the required clamping force. It is best to consult with your toolmaker and machine supplier together. For these reasons, you will notice that most often in liquid silicone molding, the machine size is based on platen dimensional requirements instead of clamping force requirements — which makes the tie-bar-less design a good option.

Micro-injection Molding and its Challenges

Micro-injection molding means working with part weights of significantly less than one gram. Right now, some manufacturers are already achieving part weights below one milligram – although it makes a big difference here whether the raw material is a thermoplastic with a low specific weight or a liquid silicone with higher density (typically 1.10 to 1.50 g/cm3). Micro-parts made of silicone are generally heavier, but they can be smaller – and more challenging – in terms of dimensions and design.

Micro-injection molding comes with special challenges. These challenges exist independently of the raw material used because some engineering challenges in micro-injection molding arise solely from the small part dimensions and extremely low masses processed.

Such small quantities (as in the example shown in Fig. 1), now have to be uniformly and repeatably distributed over the cavities. The micro-injection unit, the sprue channel, and thermal management are enormously important here in terms of quality, precision, and repeatability.

One example that shows how much technical knowledge micro-injection molding requires is the volume of the sprue channel. This needs to be in a balanced ratio to the volume of the molded part(s). Several boundary conditions have to be weighed against each other and the overall process optimized. They result from process management, waste and energy statistics, and material price and cycle time. This poses the greatest challenges for the equipment manufacturer and the manufacturer of the micro-parts.

Precision and Repeatability are Essential for Micro-Injection Molding

This also makes the transition to high volumes one of the biggest challenges. Processes might have worked well under laboratory conditions, but during scale-up and high-volume production, the producers and users of micro-injection molding technologies face entirely new challenges. This applies above all to the parts' precision and dimensional accuracy. Factors that have virtually no effect on large parts are decisive for the reject rate in the case of delicate micro-parts. Deviations of a few thousandths of a millimeter or a few tenths of a milligram have a major impact on the quality of these tiny parts, and thus on their subsequent reliability in use.

Maximum precision in the interaction of all units of the system is indispensable. In the form of all-electric and tie-bar-less injection molding machines from the e-motion TL series, ENGEL offers a very good basis for this, not only for development, lab work, and prototyping or for short runs, but also for high-volume production (Fig. 2).

These precision machines are complemented by the flexible micro-injection unit, which ENGEL has developed with partner ACH Solution (Fischlham, Austria), as a dedicated solution for LSR micro-injection molding. The micro-injection unit makes manufacturing precision parts from Liquid Silicone Rubber with shot weights significantly below 0.1 grams possible (Fig. 3), with the use of a 6 mm plunger that is capable of a 2.3 cm3 maximum shot capacity. On its market launch at the end of 2020, connecting elements used in medical instruments were produced using a valve-gated cold runner mold with 32 cavities. The delicate parts measure 1.7 mm in length and 0.9 mm in diameter with a part weight of just 0.0005 g. The unit runs on an all-electric and tie-bar-less ENGEL e-motion 50/30 TL injection molding machine and is fed by an LSR metering pump from ACH Solution. An ENGEL viper 6 linear robot removes the parts and holds them in place using the EOAT for camera system-based quality control.

Repeatability of the injection movement has an enormous influence on the precision of the injection results. But high-quality micro-injection molding is not just a question of the injection unit. The machine has to offer the required degree of precision. Clamp positioning and traverse accuracy are essential for precise molding of a micro-part. For both materials, LSR and thermoplastic, all-electric, servo-driven machines are therefore a sensible choice in micro-injection molding.

The special features of the tie-bar-less machine solution have a positive effect in the injection molding of micro-parts. Due to their design, tie-bar-less machines inherently ensure great platen parallelism and uniform clamping force distribution. In addition, the tiny parts often need to be produced in a cleanroom. A compact machine design with a large clamping surface reduces the space requirements, and the clean machine concept without hydraulics, with encapsulated toggle levers and linear guidance also supports the processor in complying with GMP specifications.

Clamping Systems

The two most common clamping systems to choose from are hydraulic ram and electric toggle. Both systems offer unique advantages and disadvantages.

Hydraulic Clamping Systems

The hydraulic clamping system offers a simple construction with very few moving parts, as well as closed loop clamping force control using a hydraulic pressure transducer to maintain continuous clamping force throughout process settings. In addition, the clamping force setting can be changed without interrupting the process. The hydraulic clamping system offers greater flexibility for mold height as the entire opening stroke is available for mold installation, and the mold height set-up tends to be easier with this system.

A hydraulic machine system must be equipped with a double hydraulic pump to utilize the advanced processing method of running injection in parallel to clamping force build. Or, you can use a hybrid machine system with an electric servo-controlled injection unit and hydraulically operated clamp instead. Either of these options will allow you to maintain a low clamping force during the injection sequence, making the small mold vents used in LSR tooling more efficient, and then increase the clamping force during the holding pressure and curing sequence when it is most important. The hydraulic clamping system is available in tie-bar or tie-bar-less design.

Electric Toggle Clamping System

The electric toggle clamp provides more precise opening and closing positions for delicate demolding or robot take-off operations, and the ability to implement injection parallel to mold closing sequences with very precise clamp positioning. This differs from the hydraulic system, which is injection parallel to clamping force build-up. The electric system will allow you to close the mold to a precise position with an accuracy of less than 0.001″, begin the injection sequence, and then continue mold closing with precise closing speed — allowing you to process out trapped air or eliminate venting completely for very low viscosity materials. The electric system also can achieve faster dry cycle times due to its quicker acceleration and deceleration speeds.

One key functional difference between the hydraulic clamp and the electric toggle is how clamping force is monitored. Hydraulic machines monitor hydraulic pressure through a transducer, whereby the electric machine must measure stress on the tie-bars or toggle. For this reason, many machine manufacturers do not offer closed-loop clamping force control on electric toggle machines. The electric toggle clamping system is available in tie-bar or tie-bar-less design.

Injection Units

When selecting the correct injection unit, many things must be considered, such as injection pressure limits, shot capacity, and performance requirements. Liquid silicone molding can sometimes offer more flexibility in injection unit size than standard thermoplastic processing, but there are still many things to consider.

Pressure is often disregarded when specifying injection units for liquid silicone molding because the pressure requirement to inject the material and move the flow front is very low. However, injection pressure limits should be considered when specifying a machine running a multi-cavity valve gated cold runner system. All gates must open simultaneously in this type of system to maintain balance. In an open nozzle system, the gate shut-off is achieved by curing a small amount of material in the gate each cycle and injecting it into the next part. To achieve the required balance, it is essential to generate peak injection pressure very early in the injection sequence to ensure all nozzles open simultaneously. The injection profile would look similar to the following Fig.:

You will notice the first injection sequence is set at the maximum injection speed setting and then profiled down to a slower speed as it transitions into the hold sequence. Even though you may not be able to reach the velocity set points, it will ensure that all gates open at the same time.

In a valve-gated cold runner system, the opening of the valve gates is mechanical, so the ability to generate pressure very quickly is not critical. The injection sequence is based on the ideal speed to maintain flow and fill the cavity most efficiently. The normal injection profile for a valve-gated cold runner would look similar to the following Fig.:

The shot capacity is where you can see the most flexibility in liquid silicone injection machines. As LSR screw injection unit uses a zero-compression screw, there is no feed, melt, or compression zone. Also, liquid silicone molders do not consider residence time when sizing the screw diameter or injection capacity. The main consideration when selecting the correct screw diameter for LSR should be stroke resolution during the injection sequence. Many molders consider the ideal shot size to be 30% to 60% of the total shot capacity. Consideration of the screw stroke during the injection cycle versus the percentage of the barrel that is being used is a good rule of thumb. You must calculate the smallest stroke movement you can use, which will allow the machine to maintain speed and pressure control. This will differ depending on the drive system of the injection unit and how the injection unit is equipped.

Hydraulic Injection Unit

The hydraulic injection unit is a standard, proven system with very good repeatability and speed control – both acceleration and deceleration. Also, with the advances in controller scan times and hydraulic hardware, these systems have become more accurate. The performance of the standard hydraulic system can be improved by adding a dedicated servo-operated hydraulic valve located close to the injection unit; this will reduce the reaction time of the valve and the compression of the hydraulic fluid. Depending on the machine and controller ability, minimum strokes using a hydraulic system can be less than 0.500 inches, yet still maintain excellent control — this could be as low as 5% to 10% of the total shot capacity.

Electric Injection Unit

The electric injection unit provides increased accuracy, speed and position control, and shot-to-shot repeatability compared to the hydraulic unit. Whereby the hydraulic injection system measures pressure via a pressure transducer, the electric injection system provided by ENGEL measures direct injection and hold pressure via a force transducer located on the screw support. This means all pressure readings are absolute and not calculated. The maximum injection rates for the electric machine may not exceed the specifications of the standard hydraulic machine, but the dynamic acceleration and deceleration of the electric unit will outperform the hydraulic system. Start-up cycles and parameter changes take less time with the electric injection unit than on standard hydraulic systems.

Although it sounds as if there is no decision to be made and that you should always choose the electric injection unit, you must always consider your application and the capital cost of the equipment. Indeed, the electric injection will always be more accurate than a standard hydraulic system, but it can also add 20% to 30% to the equipment cost. Additionally, due to the compressible nature of liquid silicone in the barrel as well as in the cold runner and mold cavity, increased accuracy is not always realized in the finished part when looking at larger parts and high cavitation cold runner molds.

All discussions involving injection molding of liquid silicone come to the topic of micro-molding. Due to the low torque requirements to meter liquid silicone materials, and the low injection pressures required, it is possible to use screw injection units with diameters as small as 12 mm. This small diameter screw, coupled with the accuracy of an all-electric injection machine, can make shot sizes as small as 0.08 cm3 possible. In micro-molding applications, the injection molding machine is only one factor. The dosing equipment must also be sized appropriately to maintain consistent metering and mixing of small shot sizes. Micro-molding applications also highlight deficiencies in the injection mold so that quality tooling will be as important as the molding machine and dosing equipment.

Most of the details that differ between the liquid silicone molding machine and the standard thermoplastic machine relate to the melt screw and barrel assembly.

LSR Screw and Barrel Assembly

The LSR injection unit consists of a water-cooled barrel to maintain a consistent temperature of the material during metering and injection and to reduce the reaction of the material prior to molding. The screw is a zero-compression feed screw equipped with a plate-style spring-loaded check assembly. The check assembly is designed for the positive shut-off of low-viscosity materials, and the spring ensures the plates are in the closed position when static. This means the forward movement of the screw is not required to seat the check assembly as is necessary for the standard check ring used in thermoplastic molding. Also, the end of the barrel must be equipped with a shut-off nozzle. Based on reaction time, pneumatic operation is preferred over a hydraulically operated shut-off nozzle. There are many styles and manufacturers of shut-off nozzles available — it is important to understand your application when choosing. For cold runner molds, a diving tip is often used to eliminate air into the cold runner system. The diving tip is usually equipped with either a 15 mm or 25 mm tip with a perimeter seal. This tip penetrates the sprue bushing and stays engaged through the molding process. When used with a cold runner system, this tip can be water-cooled — but it is not a requirement. For hot runner molds, a ½” radius tip is usually used. In this case, water cooling is required to avoid material curing in the nozzle. This tip is usually retracted after the injection and hold sequence using a sprue break option.

Other liquid silicone options important for consistent processing are sufficient mold heating zones and control, cooling water manifold and flow control, vacuum pump and vacuum monitoring software, interface for integrated demolding device, and programmable sequence control.

As consistent mold heating is required to maintain continuous, fully automatic processes, the machine should be equipped with self-tuning software to optimize the heating profile per the specific mold requirements. The toolmaker should determine the number of mold heat zones required based on mold size and mass to maintain consistent temperatures. In standard, the minimum number of zones specified for a LSR machine over 50 tons should be eight. For a smaller machine four zones are sufficient.

As with mold heating zones, cooling water for cold runner systems is extremely important. The temperature and flow rate of the cooling water must be consistent, as inconsistent flow or temperature will cause the mold heating to be inconsistent. In open nozzle cold runner systems inconsistent flow or temperature may cause nozzles to leak or cure too much. It is also a good idea to add a water filter system to ensure clean even flow to the molding system.

A vacuum system with a dedicated vacuum source, valve, and software to monitor the vacuum level prior to injection is recommended. Due to the viscosity of liquid silicone, venting in silicone molding is very limited, and some materials will flash with vents less than 0.0005″ deep. This increases the importance of evacuating as much air as possible before injection. Vacuum monitoring software ensures the proper vacuum levels are reached during fully automatic processes.

With the flexible nature of silicone, many demolding options are not possible in thermoset molding. Because of this, many toolmakers have developed simple demolding devices that are integrated right into the mold — from a simple knock-out board or brush system to a more complex gripper system. The machine should be able to interface with this device and program it into the molding sequence.

The final piece of the liquid silicone cell is the dosing system. As with injection molding machines, there are many options for dosing equipment, ranging from systems fed by small reusable plastic cartridges to systems fed by 55-gallon drums. Pump operations range from servo-controlled to hydraulically and pneumatically operated systems. You should discuss your specific application with one of the industry’s quality dosing system providers.

Although you can never specify one machine to fit every requirement or opportunity that comes your way, the guidelines discussed in this article will help you specify an extremely versatile machine.

If you want to read more news articles about ENGEL, click here, or you can also visit our website. For any inquiries, feel free to call us at (717) 714-3235 or send an e-mail to [email protected]. We look forward to connecting with you!

Read more about the latest news and innovation from ENGEL