Increasing profitability of silicone molding.

With the growth of custom compounding in the silicone rubber industry, material suppliers and fabricators are cooperating more closely than ever before to meet application challenges. The technique of marketing a new and innovative silicone material as a universal design solution is giving way to the custom compound, as specific application requirements drive material advancements. Custom compounds produce the optimum mix of physical properties in a finished part, minimizing compromises and raising process efficiency.

From an economic standpoint, material suppliers have had limited opportunity to affect processing, except as it directly relates to the molding operation: reducing material variation, making recommendations for handling and finishing techniques, etc. Material cost remains the most obvious component of a finished part's selling price, and most fabricators equate raw material prices with manufacturing costs.

In reality, material costs contribute 25% or less of a finished product's selling price in most cases. Other factors such as equipment costs, labor and energy account for the rest. When fabricators concentrate on reducing material prices without looking for ways to influence process expenses, they overlook the real opportunity to reduce costs: maximized labor efficiency, lower overhead and minimized scrap.

Raising output levels

One way to reduce the molder's cost per part and improve profitability is to increase output, especially in compression- and transfer-molding operations. A common technique is to design more complex molds that produce more parts per cycle. Unfortunately, temperature and flow are more difficult to control in multi-cavity molds, and part quality can suffer as a result.

Some molders are turning to liquid silicone rubbers (LSRs) to achieve higher output, as the injection molding of LSRs is often 50-100% faster than processing high-consistency material. The strategy is a viable approach to increasing throughput, although the capital equipment investment can be prohibitive, and molding techniques are substantially different.

Cure technology

In the past, materials suppliers were rarely asked to assist a fabricator's efforts to improve production rates, but molders are finding that material advancements sometimes improve output significantly, and thus have a direct effect on profitability. In processing high consistency silicone rubber, recent cure technology breakthroughs are reducing cycle times and helping fabricators to raise production levels dramatically.

Crosslinking of silicone polymers by peroxide chemistry has been a standard technique in molding operations for many years. Chemically, the process is quite simple -- there are essentially just two components: peroxide and polymer. The polymer may be methyl or vinyl based. Organic peroxides such as 2,5 dimethyl-2,5 di(t-butylperoxy) hexane or 2,4 dichlorobenzoyl peroxide are commonly used with silicone rubber, as is dicumyl peroxide.

Despite common use of the term "catalyst," peroxide is really a cure initiator, not a true catalyst; it is consumed during the reaction. Peroxide-initiated cure has three steps. When the elastomer is placed in the heated mold, the peroxide thermally decomposes to form free radicals. These free radicals attack either a methyl or vinyl group, depending on the specific polymer and peroxide, to form active crosslink sites. The active sites on the silicone polymer then combine, forming a carbon-carbon bond. The rate of cure with peroxide is determined by a slower (second order) reaction (figure 1).

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The cure rate depends on the individual peroxide and the temperature of the mold. The crosslink density is a function of both the vinyl level of the compound and the amount of peroxide in the formulation. One drawback to peroxide cure is the inability to adjust only the speed of the reaction (cure rate), without other tradeoffs. For example, molders often attempt to accelerate cure or retard scorching by varying the peroxide level. However, changes to the peroxide level also affect the crosslink density and physical properties of the cured elastomer (figure 2).

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Lowering the mold temperature will improve induction time and allow the material greater opportunity to completely fill the mold cavity in a peroxide-initiated reaction. Unfortunately, temperature reduction also lengthens molding time and reduces production output. Attempts to accelerate cure by raising the molding temperature increase the risk of scorching.

Platinum cure

New platinum cure technology has become widely used as a substitute for peroxide systems. Also called "addition cure," the formulation is more complex than peroxide compounds and requires that the base polymer contain vinyl groups.

Unlike the peroxide cure mechanism, platinum is a true catalyst, and is not consumed during the reaction. Since the platinum-catalyzed reaction will occur even at room temperature, an inhibitor is necessary in the platinum formulation. This first order reaction takes place much more quickly than one initiated by peroxide, as it does not depend on a second order, rate-determining step (figure 3).

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The rheometer curves shown in figure 4 represent identical polymers cured by both peroxide and platinum mechanisms. The curves illustrate the difference between the two reaction rates. The platinum curve rises steeply as the material cures very rapidly, until it flattens out approaching maximum torque. The peroxide curve, by comparison, rises more gradually, indicating its slower rate of cure.

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Faster processing is an important advantage of the platinum system, bringing a corresponding saving in labor and utility expenses per part. These can combine to produce a significant reduction in the fabricator's cost per finished unit. Molders report an average output improvement over peroxide cured materials of around 50%, and many have realized gains of 100% or more, depending on the size and complexity of the parts being produced.

The slower-curing peroxide materials may require large, multi-cavity molds to attain reasonable production output, at some expense to overall quality. The faster cure of the platinum materials allows the use of smaller molds with fewer cavities (and a lower tooling cost). Better temperature control and enhanced flow characteristics make it possible to hold much tighter tolerances on finished parts. The ultimate result is higher part quality using less energy, with less scrap.

Faster throughput may also permit the use of fewer (or smaller) presses to complete a production run, which in turn reduces floor space requirements or frees up equipment to be used for other runs. Fixed costs go down, and the formulator cuts the cost per part and raises profit levels.

Manipulating induction time

The induction time of the platinum cure system is controlled by formulation, permitting the compounder to optimize a material's processing characteristics for a fabricator's individual molding conditions. The inhibitor in the compound serves only to delay the beginning of the cure cycle; it does not alter cure speed or crosslink density.

By manipulating the formulation, the induction time can be changed to suit specific molding requirements. The actual reaction speed itself remains constant. The slope of the rheometer curve does not change, as seen in figure 5.

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In an attempt to provide improved scorch safety, it is common to vary the peroxide level. As these rheometer curves show, however, the small gain in scorch safety is accompanied by a much larger change in both cure rate and final cured properties. Because platinum is a true catalyst, the quantity has little effect on cure rate as long as there is a sufficient amount to allow curing. In the platinum system, the scorch time is adjusted by the level of inhibitor -- more inhibitor improves scorch time. While T90 is increased with additional inhibitor, both the cure rate and final cure are only slightly affected. Although a bit more complex chemically, inhibitor is most easily thought of as a material that prevents platinum from reacting until it's volatized.

Temperature variation

The platinum system is also much more forgiving under varying temperatures. Depending on individual designs and press conditions, temperatures in the mold may vary from the center to the perimeter by as much as 20[degrees]F. Temperature differnces within the mold can cause a wide variation of physical properties in a peroxide compound, as these are dependent on cure.

Figure 6 illustrates the effect of temperature variation on final torque. In a peroxide compound, material in hotter areas of the mold cures more quickly, and usually reaches higher maximum values. In a large, multi-cavity tool, some sections of the material may scorch, while others are undercured.

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With a platinum system, the effects of temperature variation are less significant. Cure rate changes only slightly, and there is little difference in the ultimate state of cure. Platinum compounds generally exhibit less variation, yet their lower scrap rates and faster cure reduce the cost per finished part.

Platinum cured silicone is more robust to mold temperature variation than peroxide. Those who use mold temperature as a tool to prevent scorch with peroxide cure systems should note that little benefit is obtained by lower mold temperature; however, a large penalty is paid in the final state of cure.

In some operations, the peroxide system experiences incomplete curing of flash, runners and sprue due to oxygen inhibition. The uncured material causes problems by sticking to either the molded part or mold. In contrast, platinum cure is not inhibited by oxygen. Flash cures completely and does not stick to the part or the mold This feature alone can shorten mold-open time for between-heat cleanouts, saving time, reducing temperature variation and conserving energy.

Fully cured platinum compounds tend to be less tacky than peroxide materials. This phenomenon further improves handling of the molded part, which reduces scrap in demolding, deflashing, inspection and packaging operations.

Physical properties

Along with the processing advantages cited, fabricators employing the platinum cure system report physical property improvements over peroxide-initiated materials. Increased values for elongation, tensile, tear and hot tear are common. In some cases, a 40% gain can be realized.

Table 1 shows a comparison of physical profiles for two general-purpose, low-strength compounds. The formulations are the same, except for the cure systems used. They were press-cured and post-cured identically, according to ASTM test procedures.

Table 1 - physical property profiles

Peroxide cure
                                             Post cure
                     10 min./350[degrees]F   1 hr/400[degrees]F
Hardness, Shore A                        39                   41
Tensile, psi                            714                  855
Elongation, %                           520                  550
Tear B, ppi                              54                   61
Specific gravity                       1.17                 1.17
Modulus @ 100%, psi                     141                  141
Platinum cure
                                                Post cure
                     10 min./350[degrees]F(*)   1 hr/400[degrees]F
Hardness, Shore A                           37                39
Tensile, psi                             1,136             1,050
Elongation, %                              890               820
Tear B, ppi                                104               105
Specific gravity                          1.18              1.18
Modulus @ 100%, psi                        106               113

(*)Under actual molding conditions, the platinum compound would require a much shorter cycle time

Post cure

All silicone compounds presently require some degree of post cure to obtain maximum physical properties, especially compression set resistance. Post curing of a platinum compound serves to complete the crosslinking process. Peroxide compounds, on the other hand, require post cure primarily to drive off the residual products of peroxide decomposition which can potentially revert the polymer to a reduced state of cure.

As environmental regulations continue to tighten, these by-products may come under increased scrutiny. Platinum systems are free of such concerns during molding or post cure, a fact that is likely to increase in importance over time.

Improved consistency after post cure is another benefit of platinum-cured compounds, as post cure operations continue the curing process in a platinum material. The additional crosslinking produces minor physical changes that further reduce variation in finished parts. In a peroxide compound, there is no additional crosslinking during post cure, and any variation from the mold will also be apparent after post cure.

Compared to their peroxide counterparts, platinum cured materials also exhibit greater resiliency. Further, because of the chemistry involved, platinum materials are inherently flame retardant.

New materials development

In the past, platinum cured silicone was available only as a two-part material. Advancements in compounding technology at Dow Corning STI have produced custom-mixed, one-part materials that are supplied to the fabricator fully compounded and ready to run. Unlike two-component products and previous one-part materials, these compounds have a shelf life that is measured in months rather than days or even hours.

One part platinum cured compounds have greatly simplified molding. The new generation of materials can be supplied in sheet, preforms or bulk, and overall processing speeds rival those of the LSRs. Compression- and transfer-molders can now attain production levels with a high consistency silicone that were once possible only through injection molding. Fabricators, who currently mold LSRs, have found that platinum high consistency silicone can be processed on injection equipment, at the same speed as the liquid materials. This is accomplished by simply disconnecting the pumping and meter mixing system and attaching a stuffer box.

There are notably few disadvantages to the platinum system. Platinum-based compounds tend to be more susceptible to cure inhibition than peroxide-based materials, and exposure to sulfur, tin or amine groups can inhibit complete cure. These tendencies are well understood, however, and conscientious handling prevents serious problems. As operators become familiar with the new compounds, they are able to integrate them into an optimized process flow.

Initial material prices for platinum compounds are higher than peroxide materials, but these are more than offset by production gains. An actual application is illustrated in table 2. In this situation, a platinum cure material costs about 40% more than a comparable peroxide compound, yet the cost per finished part in platinum is reduced by more than 20%. The profit improvement is dramatic.

Table 2 - cost comparison

Item
                           Material 1    Material 2
Cost/pound                      $3.50         $5.00
Cure time (seconds)               120            60
Overhead ($/hr)                $60.00        $60.00
Labor                          $12.00        $12.00
Hours/shift                        10            10
Shifts/week                         8             8
Cavities/mold                       3             3
Part weight + 10%                59.5          59.5
(flash/sprue)
Parts/week                      7,200        14,400
Material usage/week               885         1,770
Material cost/week          $3,097.31     $8,849.45
Total cost/week             $8,857.31    $14,609.45
Cost/unit                       $1.23         $1.01
Savings/unit                       0%        21.25%
Profits/yr. (assume 10%)   $44,286.55   $230,563.76
Profit improvement                 0%       420.62%

Conclusion

It's rare for a new elastomer development to have a major impact on fabrication costs. More common is the rising price of source materials and labor, coupled with stricter environmental legislation. In a time when it seems ever more difficult for the fabricator to profit, the increasing popularity of platinum as an alternative to peroxide cure technology has been augmented by the development of new one-part materials.

The new platinum-cure compounds are both application and process specific. They meet or exceed physical properties of peroxide materials and maximize productivity from existing production equipment. The process gains translate to lower costs to OEMs and improved profit levels for molders.