Liquid injection molding versus compression molding: effect on LSR elastomer mechanical properties?

Over the past decade, medical device fabricators have increasingly turned to liquid silicone rubber (LSR) over the well-established high consistency rubbers (HCRs) for a variety of performance and economic reasons.

For example, the inherent flow characteristics of LSRs enable them to fill complex molds to create intricate parts. Also, LSRs provide notably higher tensile moduli than their HCR counterparts, even though the ultimate tensile strengths of the liquid-based elastomers are comparable with those of their gum-based cousins (figure 1). Compared to an LSR, the crosslink density of an HCR is much lower, which means there are many more siloxane units in the polymer chain between links, as many as 20-fold more. These longer polymer chains can coil like a spring, giving HCR the ability to stretch further than LSR given the same load. From the opposite perspective, it takes more force to stretch an LSR a given distance compared to an HCR; hence, LSRs offer higher tensile moduli.

[FIGURE 1 OMITTED]

Another factor, and one that is often overlooked, is the material's relative density. Designers usually seek lighter components, and silicone prices are typically based on weight. Dow Coming Healthcare elastomers are typically reinforced with high surface-area fumed amorphous silica that is well incorporated into the matrix to ensure excellent physical properties. Consequently, these materials have lower densities than silicones that contain extending fillers. Dow Coming LSRs, particularly those in the mid and higher durometer ranges, have even lower specific gravities than their Dow Coming HCR counterparts. Elastomer cost is generally the same per kilogram, so these LSRs can be more cost-effective on a per part volume basis, assuming equal scrap lost to runners and flash.

Figure 2 shows the number of parts per pound (or kilouam) of elastomer at a hypothetical 1 cc dosage (i.e., 1 [cm.sup.3] per cavity per cycle). Notice that 50 durometer and harder LSRs yield more parts per pound than HCRs.

[FIGURE 2 OMITTED]

Of course, for most molders, there is one overriding reason to consider LSRs for fabrication of their medical device components; efficiency of part processing. LSRs are designed for liquid injection molding (LIM), which is generally faster, cheaper (in the long run) and more reliable than the older, more labor-intensive molding methods used for HCRs, such as compression molding (CM). LSRs typically arrive in two-part kits. The two parts are very easily combined at a 1:1 ratio as they are pumped through metered static mixer equipment. Consequently, they do not require laborious material handling and (potentially dangerous) two-roll milling. Put simply, LSRs (and capital investment in the proper LIM equipment) make possible the easy handling, short cycle times and consistently high yields silicone converters once only dreamed of.

Even fabricators who cannot afford the high initial investment in LIM equipment, or already have suitable compression molds, can take advantage of LSR properties. Although ideal for LIM, LSRs are readily processed by other techniques such as CM, and even "supported" or co-extrusion.

Because silicone elastomer mechanical properties might conceivably be influenced by cure profile or "thermal history," an examination of the effects of molding process on these properties was undertaken with elastomer slabs of 10 popular Dow Coming LSRs using liquid injection and compression molding. The thermal history for samples prepared by CM was five minutes at 150[degrees]C (302[degrees]F). LIM conditions were three minutes at 171[degrees]C (340[degrees]F). The molding occurred contemporaneously, and no post cure was performed. Using standard test methods, mechanical testing was conducted to compare the elastomer properties resulting from the two molding methods.

Results are shown in figures 3 to 9. Taken as a whole, the elastomer slabs prepared by CM have slightly decreased durometer, tensile strength and moduli, and increased elongation compared to the slabs of the same materials prepared by LIM. This may be due to factors such as crosslink matrix formation or degree of cure; however, these slight differences are within the test reproducibility values published by ASTM for the test methods. No consistent difference was seen for tear strength.

In summary, while the growing popularity of pumpable liquid silicone rubbers over millable high consistency rubbers is mostly related to long-term economics, there are other reasons designers might specify LSRs, irrespective of part molding method. LSRs allow more intricate parts, give higher moduli, comparable tensile strength and may have lower density than HCRs. Although silicone elastomer properties can be affected by molding conditions, our study of ten LSRs found little, if any, difference comparing mechanical properties of compression molded versus liquid molded specimens. Because every molding process is unique with respect to machine parameters and part design, application-specific testing is necessary to ensure molded parts are suitable for their intended use.