Rubber World — December 2011
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Self-adhesive LSR in 2K injection molding
Vic Wilcik

Applications for thermoplastic polymers keep growing, especially in overmolding usage. However, as with any polymer, thermoplastics have limitations dependent on their particular structure and molecular make up. One of the major limitations is heat resistance because thermoplastics soften (melt) when heated and harden when cooled, thus allowing them to be molded into a new shape by the addition of heat and a form. However, this same property means that a thermoplastic part may lose functionality when exposed to temperatures high enough to cause deformation. Thermoset materials continue to compete and fill the voids where thermoplastics cannot. Silicone rubber is one of these materials. Finding a broad range of applications across many differing markets, silicone is one of the most versatile of all thermosets.

Silicone comes in several forms, from fluids and polymers to finished rubber bases and compounds. One such form, liquid silicone rubber (LSR), has some commonalities to thermoplastics in its processing, while maintaining many of the differences that are inherent in a thermoset material.

Like all polymers, LSR has a main defined chemical structure; in this case, an alternating backbone of silicon and oxygen. This backbone is surrounded by side units of hydrogen, hydroxyl and vinyl groups which allow for crosslinking, which defines it as a thermoset (figure 1). Once cured, LSR cannot be reshaped or easily re-used or recycled. To do so requires a great deal of energy to overcome and break apart the crosslinks and/or the backbone itself. This is in contrast to thermoplastic materials which are easily recycled for reuse. This same molecular structure imparts the benefit of LSR being useful over a wide range of temperatures. Since the polymer structure does not constrict as easily as other polymers and plastics, the point at which it becomes brittle is very low. At the same time, the bond affinity of the silicone and oxygen molecules means that the polymer does not degrade until temperatures above most other polymers. Another of LSR’s major attributes is its chemical inertness, or purity. Needing no organic additives, LSRs are crosslinked utilizing a platinum metal catalyzed reaction that produces no by-products. Coupled with their ability to withstand sterilization processes, this makes LSRs ideal for many medical uses, as well as for baby care.

With this in mind, we should look at some of the similarities and differences of how LSR is processed compared to thermoplastics. From a machine perspective, a first glance may lead to mistakenly confusing an LSR injection press with a thermoplastic injection press. Both have feeding systems that move the material into the injection barrels, which then move the material into the mold in order to take the shape of the finished part. One of the largest differences occurs within the mold. As stated earlier, thermoplastics are heated to their melting point in order for them to take the shape of the end part within the mold. To do so, the injection barrel is heated, while the mold is held at a temperature below the melt point. For LSR, this set-up is exactly opposite. As a thermoset, heat will cause the material to cure – at which point it can no longer take on a new shape. Therefore, the injection barrel is water-cooled to keep the temperature below the point where this reaction will progress. Conversely, the mold is heated (normally 180°C-200°C) to make that reaction happen and establish the crosslinks that will hold the LSR in the intended end part shape.

The initial feed system between these two types of materials is another major difference in their processing. Thermoplastics come in the form of pellets and, as stated, they are melted to enable the injection into the mold. This pellet form imparts large surface area in which to absorb the heat of the injection barrel. To get to this point, the pellets are loaded in a hopper or vacuumed up through a hose or pipe to the right of the shipping container. LSR comes as a paste with a consistency similar to that of peanut butter. To get to the injection barrel it needs to be pumped at a 1 to 1 ratio, as the crosslinker and catalyst are kept apart in A and B sides to keep the crosslinking reaction from progressing spontaneously, though very slowly. They also need to be blended prior to injection. This is done when the A and B materials are pumped through a section referred to as a “static mixer.” This is a chamber prior to the injection unit that contains a metal rod with flights designed for cavitation, resulting in the A and B mixing together. This area is also water cooled to keep the crosslinking reaction from starting prior to the material getting into the mold.

Multiple cavitation within the same mold is common practice for both molding techniques. To deliver the material to multiple cavities a runner system is used. For thermoplastics runners from the mold need to be separated and reground. To eliminate this step and realize a more efficient process thermoplastics incorporate a hot runner manifold where the melt property of the thermoplastic allows the runners to be maintained hot (melted) in the manifold and utilized in the next injection cycle. For LSR, runners in the mold proper are waste with the added difficulty of demolding and separating them from good parts. For a LSR process without waste the same manifolding technique is incorporated. But, in contrast to thermoplastic, the LSR manifold system is kept cold (55-75ºF). This allows the LSR to be maintained in the cold runner manifold and utilized in the next injection cycle.

An additional processing difference related to thermoplastics being in melted form is that in order to maintain part quality and consistency, the material needs to be “packed” into the mold to assure proper fill-out of the entire shape of the part. The packing pressure allows for additional material to be injected, accounting for shrinkage so dimensions and whole part integrity are maintained. Using this molding technique will produce defective LSR articles. This is due to two main factors: LSR materials are compressible; and during the LSR vulcanization (curing) process, there is thermal expansion taking place. If LSR mold cavities are filled to 100%, or “overpacked” beyond 100% due to the compressability of the material, when the thermal expansion takes place during mold heat either flash or surface defects and irregularities will occur. This will result in unusable and scrap parts.

Based on these differences, it is important to also realize that the processing of LSR is looked upon as a four-component system. The type/grade of LSR used is only the first of these. In order to gain full efficiency, consistency and quality, the injection machine, pumping unit and the mold all play a vital role. With proper planning and considerations, it is possible to run an LSR part as fully automatic as a general thermoplastic part.

Like the many thermoplastics available for different applications, there are a number of differing types/grades of LSR. General purpose grades are not highly filled and are suitable for various applications with the need for only basic physical properties. With the addition of additives and other fillers, LSR can be made to push even higher temperature applications or oil and other fluid environments. The addition of phenyl units stretches the low temperature capabilities of LSR. Phenyl fluid can be added as a mechanism for low coefficient of friction, resulting in the surface of the part becoming slippery as the fluid bleeds out. There are also versions available that impart this lower coefficient of friction (COF) chemically, without the need for fluid on the surface. Of great importance to many plastic molders and end use designers are the grades of self-adhesive LSR. These materials no longer require a separate primer to bond to many of the most commonly used thermoplastics. This not only eliminates the priming operation, but the cure cycles are such that they closely mimic the cooling cycle for the thermoplastic, allowing for in-mold bonding of the LSR to the thermoplastic, The K in 2K is derived from the German spelling of Component (Komponent) and comes from our German colleagues as the initial users of this technology, wherein a finished product (thermoplastic substrate overmolded with LSR) can be made in a single step. This is similar to the overmolding currently done with multiple thermoplastics, with the major difference being the rubber side of the mold requires heat to crosslink. This must be taken into account when choosing the thermoplastic to be mated to, in order to avoid deformation of the thermoplastic portion.

The ability to mate LSR to various thermoplastics without a priming step also allows for the reevaluation of part design. Without the need for having to make the thermoplastic sections thick enough to absorb more heat during the LSR overmold, and without the need to add LSR to surfaces just to assure the overmold will hold, parts can be designed more specifically to contain the LSR only where it is needed and only as much as needed to function. This can result in a more economical part based on using only as much material as needed, and getting it bonded where it is most important. It can also result in smaller overall parts for the many increasingly smaller end products. It is, however, important to point out that current self-adhesive LSR technology is designed only to provide enough initial bond strength for part removal (no one wants the LSR to bond to the metal of the mold). The overall bond strength continues to increase over a couple of weeks (figure 2), normally well before arriving at the end user. If quicker maximum bond strength is required, this can be obtained with a one hour/100ºC post-bake.

Processing self-adhesive LSR in 2K applications Material selection and compatibility Based on prior work with a number of customers, the best adhesion between self-adhesive LSR and various plastic groups is highlighted in black in figure 3. The photo displays how the test specimens to test the bond are produced, showing specimens injection molded using the 2K process. The substrate thermoplastic (white) is shown below the curved silicone rubber. Adhesive failure (D) is obtained when the LSR pulls away from the substrate cleanly, with no LSR residue on the thermoplastic. Cohesive failure (R) is obtained when the LSR breaks before the bond between itself and the substrate does (rubber tearing bond).

Table 1 warns about evaluating a potential 2K process via a pre-molded plastic test strip and then compression molding the potential LSR onto it. Evaluation via this method would suggest there is no bond between the highlighted plastic (PBT) and the LSR.

Table 2 shows why you should not use the pre-molded plastic, compression molded LSR method. Table 1 indicated no bond between the highlighted plastic (PBT) and the self-adhesive LSR, when in fact this combination is one of the best obtainable when done in true 2K molding.

Finally, table 3 indicates why we cannot make blanket statements about plastic types. As displayed by the test results posted, generally, we bond well to the polyamide (Nylon), PA6, PA6.6 and PA12. But something in the Nylon 6.6 highlighted actually inhibited the vulcanization process of the LSR used in combination with it.

Mold preparation – plastic substrate
In order to begin discussing 2K processing, we start by developing the plastic part of the process. First, use the thermoplastic process guide for the material in question. Processing self-adhesive LSR is no different than standard grade LSR. The same principles apply, but there will be the added complication of a plastic that wants the exact opposite of the LSR, as far as mold heating goes, to complete the final molded article.

From an LSR supplier’s viewpoint, all processing points (cycle time, venting, injection profile and ejection) are going to be dependent upon what temperature at which you are able to run (cool) the plastics portion of the 2K mold. LSR cycle time will be directly related to the plastics cooling temperature used. For practicality (profitability) it will be more than likely to push the limits of the plastic. Remembering that in a 2K application, the thermoplastic will be in the mold twice as long as typical, higher than recommended cooling temperatures should be considered. One note of caution, if the plastic is is kept too hot, to the point of being pliable when the LSR is injected, the LSR will push the plastic out of the way and go places in the mold it is not intended to. The typical result is self-adhesive LSR on untreated mold surfaces (it will stick to the steel), and a long, tedious clean-up.

But, it is still necessary to get the best cycle time for the LSR. Figure 4 demonstrates why it is not always possible to run at the recommended mold temperature for the plastic. Low mold temperatures (as far as the LSR is concerned) result in unrealistic and/or unprofitable cycle times for the LSR.

A typical cure rate for LSR is ~5 seconds per mm of wall thickness at 180-190ºC as shown in figure 5. The cure rate of the LSR is not linear. Figure 5 demonstrates the expected effect on the cycle time of an LSR as the temperature is reduced, and compares 3mm and 6mm wall thicknesses.

Mold preparation – LSR self-adhesive
When it is time to run the self-adhesive LSR, the mold will need to be “seasoned” to avoid the LSR sticking to the mold surface. Depending upon final molded part geometry there are at least two methods to season a mold. Run standard LSR in the mold first or use a soap based mold release as a start-up assist. A note of caution, do not use mold release with silicone in it (ref. 1).

Processing with LSR self-adhesive
To vent but remain flashless with LSR, vent depth should not exceed 0.00045”. Trying to maintain this vent depth in warm plastic that still has the capability to move, and will be clamped on is impossible. Vacuum and a well thought out mold design are the solution.

The injection sequence for self- adhesive LSR is no different from that of standard grade LSRs. All the same trouble-shooting techniques should be incorporated. If injecting directly onto a plastic substrate measures must be used to ensure the incoming LSR does not deform the still warm thermoplastic at the injection point. As opposed to a standard LSR mold where temperatures are typically set at one given point, with self adhesive molds the LSR will be influenced by three different tempetatures. The set point of the plastics cooling portion of the mold, the ever changing temperature of the plastic itself, and the set point of the LSR portion of the mold. Because of this it is possible to realize a LSR flow front shaped differently than the most common convex shape. If this does occur working and an odd shaped flow front results in surface defects and/or irregularities working with the heat differences has proved beneficial.

Ejecting 2K parts becomes much easier than just an LSR part because of the hard surface of the plastic. But, since the plastic is probably hotter than recommended, you must watch for witness marks left on the plastic from the knock-out pins/ rods. If the plastics portion of the mold is hotter than the recommended mold temperature for processing the plastic, ejector speed and force have the potential to leave depressions, warp or bend the final plastic molded article.

If the LSR will in contact with the substrate over a large surface area shrinkage differences between it and plastic should be investigated. Due to the elasticity of the LSR, large shrinkage differences have deformed the LSR at the contact area. Other considerations:
• Shutdown;
• LSR shutdown is a normal procedure;
• ensure no residual heat from the plastics portion of the mold migrates to the LSR part during shutdown;
• restart;
• a very light coating of soap style mold release appears to be the best procedure.

These types and grades of LSR find themselves applicable for many applications and markets, from automotive to healthcare and everywhere in between. In many of these applications, the LSR part comes in contact or is designed to work in concert with thermoplastic parts. For this reason, many thermoplastic part fabricators are looking at LSR molding as a way to increase their business and overall offering to their customers. Luckily, there are many similarities between LSR processing and that of thermoplastics. With the proper training and attention to the differences that do exist, thermoplastic part fabricators can find that the transition to LSR molding can be a successful one.