Coinjection molding for liquid silicone rubber.

Coinjection molding is an exciting new process for the molding of liquid silicone rubber (LSR) that opens up new possibilities. This process was originally considered not to be feasible for LSRs, but recent developments have led to a patent application that now makes coinjection possible for LSR and other cross-linkable rubber materials. Coinjection as a technology is not new - it was originally developed in the plastics industry at about the same time as LIM molding of LSRs was being developed in the mid 1970s. It is, however, a major new technology only now made available for the LIM molding industry.

Coinjection involves injecting two or more similar materials into the mold cavity together. One material forms the skin and the other forms the core. This permits the use of material combinations where the skin material can be specified for perhaps greater abrasion or chemical resistance and the core can be of a lower cost LSR or of different durometer. This allows the product designer to develop products that have the better parts of both worlds, lower cost and greater performance. In some cases, completely new applications are foreseen that previously would not have been conceivable.

How does it work?

The process consists of three or four stages, as depicted in figures 1 to 4. In most cases, a specific amount of the first material (skin) is injected into the cavity, partially filling it. The amount depends on the ratio of skin to core required. This ratio is dependent on the processing and finished part properties required. Once the skin is injected, the second material (core) is injected through the existing sprue. (In some cases, at a certain stage during injection, both materials may need to continue being injected simultaneously.) The core material enters the still fluid center of the pre-injected skin material and displaces it into the as yet unfilled part of the cavity. The already cured outer layer of the skin material is not displaced and thus encloses the core.

[Figures 1-4 ILLUSTRATION OMITTED]

During the final stages of filling and cavity pressure build up, the core material injection is stopped and skin material is again injected. This purges the nozzle orifice free of core material ready for the next cycle.

The process is ideal for LSRs because of the rapid curing of the material at the interface between the material and the mold, which is typically held at a temperature between 180 to 200 [degrees] C. This quickly forms the skin, but most importantly, insulates the core material from the mold temperature for a sufficient length of time to permit material to continue flowing through the center (figure 5).

[Figure 5 ILLUSTRATION OMITTED]

The skin is formed when material flows into the cavity as the center material is forced to the flow front, and it splits to the outer cavity wall. This is called the fountain flow effect. When this material contacts the hot mold surface, it cures, developing the skin layer. Before the skin component reaches the end of the cavity, the core material is injected, forcing the remaining skin component from the center towards the flow front.

The relative viscosity of the core in relationship to the skin is important when developing a co-injected product. Figure 6 shows the effect of changing the core viscosity.

[Figure 6 ILLUSTRATION OMITTED]

The four photos in figure 7 show a very interesting view of a coinjection mold made out of glass. The photos were taken using time lapsed photographic technique. They show the sequence of two materials being injected from a point on the left of the mold. The flow line of the translucent skin material can be seen, which is then followed by the core material.

[Figure 7 ILLUSTRATION OMITTED]

Equipment requirements

The main requirements for the injection molding machine are that it must be capable of operating two injection units independently and simultaneously. The two injection units are connected together at the co-injection head (figure 8). The complexity of the coinjection process usually requires an advanced high-speed microprocessor based control system with closed loop injection capability for both units. Coinjection systems can also be incorporated as part of the mold. For LSR molds, the co-injection system would be handled best as part of a cold runner system.

[Figure 8 ILLUSTRATION OMITTED]

Machine configurations for 2K coinjection molding

Coinjection via a coinjection head

Figure 9a allows the use of the stationary platen for robot mounting which figure 9b does not permit. The H/V configuration is a flexible design that allows the vertical unit to be positioned over the mold (see figure 10b). Figure 9c is generally utilized on larger machines.

[Figures 9-10 ILLUSTRATION OMITTED]

Coinjection via the mold

The coinjection of the two components is completed in the mold to facilitate multiple gates and/or cavities.

Design considerations

There are several key points to remember regarding material, part design and gating locations when developing an LSR coinjection product.

Material

* The relationship of skin to core viscosity is important. Generally, the core material should be higher in viscosity than the skin component.

* The materials must be compatible.

* Materials must be self-adhering to each other.

Part design

* The skin is usually thicker near the gate because the first skin material in is exposed to the mold temperature the longest.

* Sharp bends and comers will accumulate skin material.

* When the flow path is split by core pins or other features in the mold that interrupt the flow, there will be an accumulation of skin material at the knit line where the flow meets (figure 11).

* Ribs, grooves and undercuts that fill with skin material are difficult to penetrate with core material.

* Depending on the processing technique applied, core material may be exposed when the gate is trimmed.

[Figure 11 ILLUSTRATION OMITTED]

Gating locations

* Gate location is one of the most critical, but also the most problematic consideration for a coinjected part, especially when parts are not symmetrical.

* Knit lines form with skin material opposite the gate when material must flow around an obstruction. Figure 11 shows the effect of relocating the gate on knit lines.

* Gate locations should be selected so that the skin material reaches the left and right side of a part at the same time.

* For more complex shaped parts, flow analysis may need to be completed before a mold is made, to avoid costly trial and error.

Applications

Applications for co-injection can be divided into two categories - those that are property driven and those that are economics driven. Property driven applications are those in which a co-injected part has performance properties that a single material does not. Economics driven applications are those in which a less expensive core material is combined with a more costly skin.

The main industries that will benefit from this technology are thought to be the medical, automotive and chemical industries. Specific areas of focus would be in those requiring an expensive grade of material for chemical or fuel resistance. The skin could be made of the high cost material and the core could be of a lower cost material. In sealing applications, it is sometimes difficult to combine the properties of good abrasion resistance with good sealing characteristics, but with two materials combined in one part, this may now be possible. Dual durometer parts can be designed with specific properties to provide low initial contact force that increases once the higher durometer material is actuated. Similar applications can now be developed in vibration control devices. In medical applications, a core material impregnated with barium sulfate could be used in devices that require silicone and that must be visible under x-ray. The skin material of silicone would prevent direct contact with the barium sulfate compound. There will be many more applications to add to the list as the technology becomes more wide spread.