Injection Molding With Liquid Silicone Rubbers: Using Process Design To Maximize Results

Medical Plastics and Biomaterials Magazine
MPB Article Index

Originally published November 1997

SILICONES

Following the introduction of liquid silicone rubber (LSR) in the early 1970s, decisive improvements in process and engineering techniques appeared in relatively rapid succession. Today, injection molding with LSRs is widely used as an alternative to manufacturing with traditional solid silicone-rubber elastomers.

Liquid silicone rubber is supplied to the injection molding press by a meter/mix system that combines equal portions of the two elastomer components. Photo: Dow Corning Corp.

This article focuses on the following key parameters in the LSR fabrication process: injection pressure, core temperature, cure time, and metering pressure. Each was evaluated in conjunction with mold design, another factor that is critical to successful injection molding with LSRs. In addition, the advantages and disadvantages of electric injection molding machines and meter/mix equipment were assessed as part of an overall LSR implementation strategy.

The components of two-part LSRs are typically supplied in 5-gal pails or 55-gal drum kits, and fabricators design their molding processes based on this choice of packaging. For the purposes of this article, process design for LSR injection molding is divided into four basic topics:

METERING, MIXING, AND DISPENSING

The objective of the meter/mix/dispense phase of the molding process is to introduce equally proportioned and mixed LSR to the injection molding machine. The equipment associated with these steps, though independent of the actual injection molding machine, definitely affects the processability of the LSR.

It is important to have a positive shutoff valve at the point where the mixed material is introduced into the injection molder. This is particularly true if the molding press will ever be temporarily unattended, since the valve prevents the injection screw from being pushed backward, which could result in overfill of the barrel and subsequent flashing or overpacking in the mold. Such an occurrence can influence final part size: with the additional packing in the cavity, the part tends to shrink less than under normal circumstances.

The fabricator should regularly monitor the flow control valves in the impingement block. Over time, cured elastomer can build up in the check valves, throwing off the ratio of the components in the material. One way to eliminate this potential problem is to position two pressure gages—one on the A side and the other on the B side—next to the check valves (see Figure 1). Any obstruction lodged in a check valve will be apparent on the adjacent gage by a slow response of the needle, since response time on both the A- and B-side needles should be the same. In most cases, "off-ratio" material will fully cure, but parts may stick to the mold or exhibit physical properties that fall outside the acceptable specification range.

Figure 1. Schematic of equipment configuration for LSR processing.

THE INJECTION MOLD

Since the introduction of LSRs, the injection molding machine has become the equipment of choice for converting them into finished parts, and a detailed understanding of the requirements for producing quality components has gradually evolved. Three of the most important factors are injection pressure, vulcanization temperature, and vulcanization time.

Injection pressures for molding LSRs can vary from approximately 200 to 1200 psi, depending on process requirements, with the majority of applications requiring pressures from 300 to 700 psi. It is important to note that liquid silicone rubber is slightly compressible, and that injection molding with this type of material is somewhat like molding a spring. The compressibility of the product depends on the type of silicone fluid and on the amount of filler used in the particular LSR formulation.

Physical properties of finished products made from LSR will be affected by changes in molding conditions, especially injection pressure. Shrinkage of finished parts is normal and is also related to injection pressure. In general, higher injection pressure results in lower part shrinkage. (It is for this reason that LSR suppliers provide a range of shrinkage values with their products.) The data in Table I illustrate the trend toward lower shrinkage with the use of higher injection pressures.

Table I. Representative LSR shrinkage values at 300°F. (Material was Dow Corning Medical-Grade Silastic #6830; x and y are axes in pressure/shrinkage graphs.)

Cavity temperature will also vary depending on process requirements, and, like other elements in molding, will affect the quality of the finished product. Fabricators typically operate at vulcanization temperatures between 250° and 375°F. Actual results will depend on the design and size of the part. Table II summarizes typical LSR shrinkage results obtained when vulcanization temperature is adjusted, while Table III illustrates physical properties for a typical LSR based on a vulcanization time of 60 seconds at different temperatures.

Table II. Representative LSR shrinkage values after vulcanization for 60 seconds. (Material was Dow Corning Medical-Grade Silastic #6830; x and y are axes in pressure/shrinkage graphs.)

Table III. Representative LSR physical properties after vulcanization for 60 seconds. (Material was Dow Corning Medical-Grade Silastic #6830.)

Prior to vulcanization, the LSR is injected into the cavity at room temperature. As the material is subjected to shear and heat in the runner system, the temperature increases but is still much lower than the cavity temperature. Once the cavity is filled and the heat of the LSR begins to increase at a rapid rate, the material is prone to expansion. At this point, the clamp pressure should exceed the cavity pressure, but backrinding may occur in the gate area as the expanding elastomer is forced out of the runner system.

This problem normally occurs at high vulcanization temperatures with parts that have large cross sections. There are three basic remedies in such cases: to decrease the injection pressure, to decrease the vulcanization temperature, or to move the gate location to a smaller cross section in the tool. Although the first two recommendations are easily accomplished, they negatively affect the productivity of the process and also influence the final shrink size. For these reasons, the third option may be the best choice. In moving the gate to a smaller cross section, the objective is to fill the cavity and vulcanize the rubber in the thin sections first. This approach allows for more sealing around the gate area while the large cross section is still vulcanizing.

To facilitate removal of parts in both manual and automated operations, it is often desirable to ensure that the part sticks to a specific side of the mold as the mold opens. In some cases, if the mold temperature between the fixed and the movable platen varies by 5° to 10°F, the part tends to stick to the surface with the lower temperature. Note that as the variance increases, stress on the guide pins also increases, which could result in premature wear on the guide pins and bushings.

Vulcanization time is the next important consideration. As with other aspects of fabrication, the individual process will determine if it is permissible to adjust vulcanization time. Under normal conditions, the part is vulcanized sufficiently to allow for easy removal from the mold without any deformation. However, if the final dimensions or physical properties of the part might be compromised during removal, a postcure may be appropriate. Unlike peroxide postcures that drive off residual materials from rubber, the addition-curing process for LSRs is used to optimize physical properties such as tensile strength, modulus, elongation, and durometer. The length of time and temperature required for postcure will depend on the precure characteristics of the part and the targeted properties. The curing temperature most commonly used in the industry is 175°C, with cure time depending on desired final properties. The curing oven should be monitored for uniform heating, since hot spots or poor air circulation may cause uneven curing, which can result in inconsistent part performance.

When a postcure for LSR components is implemented, it is important that the parts reach the temperature uniformly and that no stress is applied. Parts should be individually placed on an open-mesh tray so that they do not touch. If a mesh version is not available to facilitate air movement around the parts, a heat-conductive tray (e.g., PTFE-coated aluminum) is an acceptable alternative.

TOOL DESIGN

Several aspects of tool design can have an impact on cavity pressure and vulcanization temperature within the tool. Even given successful filling, it is still possible that end products may not meet specifications and that additional work on the tooling may be necessary. These potential problems can be avoided by improved tool design that focuses on cavity pressure and on attaining a uniform vulcanization temperature.

After a tool is completed and a few trial shots have been made, it often becomes apparent that the cavity will not fill without creating a void. Attempts to counteract this problem are sometimes made by providing an overfill area to help push out the void. Unfortunately, the overfill area occasionally fills to compaction, and it becomes very difficult to control the shrinkage and modulus of the molded part. Filling the mold to compaction, evaluating where voids are located, and then adding a vent is a normal corrective practice, with the size of the vent critical to maintaining cavity pressure and minimizing flash. The typical vent depth for processing LSRs is 0.0003 to 0.0005 in., with appropriate land length of approximately 1Ž8 in. (see Figure 2).

Figure 2. Typical vent employed for molding LSRs.

Another helpful technique for venting LSR molds is through the use of a tear tab. This approach involves creating a vent around the entire periphery of the cavity; the depth of the vent changes to allow flow through it. Vent depth typically ranges from 0.002 to 0.005 in., and land length is approximately 0.010 in. The depth of the tear tab will vary depending on the part, but it should be significantly deeper than the vent area to lend the tab sufficient strength during its removal. Care must be taken to prevent the material from flowing too easily into the tear tab area, because this could prematurely seal off the venting.

A final consideration for tool design is the issue of heaters and their location inside the tool. It is important to maintain a uniform temperature profile across the mold surface. However, the continual opening and closing of the mold, the injection of cold raw material, and the cold nozzle against the sprue bushing all work against a constant mold temperature. Because poor heat distribution within the mold can affect the physical properties of the molded part, it is imperative to have a good distribution of heaters within the mold.

In many cases, designers distribute heaters evenly throughout the mold without considering where heat loss will occur. For this reason, the center portion of the mold often operates at a significantly higher temperature than does the rest of the tool. When designing a tool, it is important to review the vendor's specification to determine how close the heaters can be positioned to the exterior surfaces of the mold. Begin with a location from the exterior and work toward the center section of the mold. If the heaters are positioned 2 in. apart, the exterior heater should be no more than 1 in. from the exterior mold surface, if not closer. This arrangement will help promote even heat distribution within the tool, resulting in a uniformly vulcanized part.

ELECTRIC INJECTION MOLDING MACHINES

Plastic injection molding machines have proven to be very successful for molding LSRs, and many processing hurdles related to the nature of silicone materials have been overcome by the precision control capabilities of current molding equipment. However, the ability to accurately control two variables—injection pressure and shot size—still eludes many processors of LSRs. Because standard machines were developed to handle thermoplastics, their injection systems were designed for materials more viscous and less compressible than LSRs.

One solution to this problem may lie in the use of electric injection molding machines. Instead of employing hydraulics or pneumatics for injection, electric machines use a direct-current servomotor to drive a ball and screw, and can achieve especially tight control of the volume and speed of the injection system. Because the injection pressure necessary to process LSRs is much lower than that for plastics, a large motor is not required. Electric machines also offer less potential for contamination since the only oil needed is for lubrication—a feature particularly desirable for cleanroom molding. One disadvantage of current electric machines is their higher cost—at least 25% more than a hydraulic machine of the same size. To date, electric injection molding machines have not been fully evaluated for processing LSRs.

CONCLUSION

When metering, mixing, and dispensing liquid silicone to an injection molding machine, it is helpful to locate a shutoff valve between the two systems. This valve should be actuated by the screw recharge sequence in the injection molding machine in order to guard against overcharging the screw. In addition, pressure gages should be positioned next to the mixing block to help detect plugged flow control valves.

During the molding process, higher injection pressure results in less shrinkage, and higher vulcanization temperature causes greater shrinkage. Therefore, if the fabricator's objective is to minimize shrinkage, injection pressure should be increased or vulcanization temperate reduced. The fabricator must allow sufficient vulcanization time to be able to remove the molded part without deforming it. Upon removal from the mold, the part may require a postcure to optimize its physical properties.

The design of the tool should allow for packing out the cavity, which will help in producing a more uniform part. Placement of a 0.0003-in.-deep vent located where voids normally appear will allow for outgassing. A tear tab may also be used to remove air from the part, but if the LSR flows too easily into the tear-tab area, some air is likely to be trapped. When considering the location of heaters in a mold, designers should ensure that the exterior portion of the tool receives enough heat to match the temperature of the interior. This can be accomplished by positioning heaters closer to the outside surface of the mold.

Electric injection molding machines may be the equipment of the future for fabricators of medical LSR products. These machines are especially suitable for cleanroom manufacturing, given their lower potential for contamination compared with hydraulic units. In addition, the direct-current servomotors used to drive electric machines can control the rate and position of the injection system more closely than can hydraulics, resulting in better shot control for LSRs.

Virgil J. Johnsonis a senior industries specialist with Dow Corning's Rubber Business S&T (Midland, MI). He has more than 23 years' experience in developing processes for liquid-silicone and high-consistency rubbers, and holds four patents in silicone processing. A member of the Society of Plastics Engineers, he also serves on the industrial advisory committee for Michigan Tech University.

Copyright ©1997 Medical Plastics and Biomaterials