Contrasts between molding TSEs and TPEs.

While thermosetting elastomers (TSEs) have been and continue to be the most used elastomers today, thermoplastic elastomers (TPEs) are increasingly important because of their rapid growth rate and their inherent processing advantages. Injection molding is used here as a basis of comparing TSEs and TPEs because of its extensive use for molding both elastomers. In contrast, blow molding is nearly the exclusive domain of TPEs.

Prior to the time that TPEs became a significant force in the elastomer industry, there was a relatively sharp divide between markets for these elastomers and the plastics industry. Increased availability and use of TPEs made them attractive to both the elastomers and the plastics industries, with each industry having its own strengths and weaknesses. TSE molders had a long history of supplying molded articles to the automotive industry--the major user of TSEs; the weakness of the TSE molders was initially a general lack of processing technology required for TPEs. This knowledge resided primarily in the plastics industry and was in daily use by them. During processing, TSE articles are heated and cured in a mold at a high temperature to produce a product. In contrast, traditional molders of plastic products cool a hot plastic melt in a mold for a sufficient time for stiffening to occur before a product can be removed from its mold. In addition to significant material differences, there are also significant equipment differences.

While the plastics industry possessed the equipment and processing knowledge for TPEs, they were much less familiar with TSEs traditionally used in automotive and other applications. Hence, it is not surprising that both the TSE and plastics industries competed for TPE business in a market where the TPE growth rate is more than twice that of TSEs (ref. 1). TPEs and TSEs can be compared using a number of factors as summarized (ref. 2) in table 1.

Materials

Although TSEs are much more difficult and costly to recycle, improvements are being made in these areas. For example, vulcanized TSE particles from scrap vulcanizate can be molded using a high-pressure sintering process (ref. 3). Another TSE, EPDM, will likely lose market to TPEs in door sealing systems for automobiles, partly because cycle times can be reduced significantly from about two minutes to 30 seconds for corner moldings (ref. 4). The last item in table 1, permissible upper service temperature, is of special importance. While TPEs offer significant advantages relative to TSEs, TPE tipper service temperature can limit its use in many applications (ref. 5). For example, the low [T.sub.g] of about 100[degrees]C lot the polystyrene in SBS copolymers significantly limits use at higher temperatures because the polystyrene domains soften. Santoprene materials are much better on this account, but they still tend to soften above 135[degrees]C. Hence, TSEs are likely to remain the materials of choice in demanding high temperature applications.

Figure 1 shows three temperatures of interest for TPEs, namely [T.sub.g] for both the rubber and plastic domains, and [T.sub.m] for the plastic domain in TPE with crystalline domains (ref. 6).

[FIGURE 1 OMITTED]

The dashed line applies to a TPE like SBS that does not contain a crystallizing domain; [T.sub.m] applies to Santoprene that contains a crystallizing domain, namely polypropylene. The wider the separation between [T.sub.g] of the rubber domain and [T.sub.g] of the plastic domain in a non-crystalline TIDE, the greater the range of service temperatures. For a TPE with a crystalline domain, the wider the separation between the [T.sub.g] of the robber domain and the crystalline domain ([T.sub.m]), the greater the range of service temperature (ref. 7).

Raising the softening point of the plastic domains increases the upper service temperature capability of TPEs. However, doing this necessitates the use of higher processing temperatures, a factor that may cause degradation of TPEs at the higher temperatures required during molding. In TSEs, thermal stability of the backbone and crosslinks mainly determine upper service temperature limit.

Performance and cost are important considerations for each intended application. Comparisons of several molded products show that either a TSE or a TPE may provide cost advantages or disadvantages (ref. 8). For example, a molded TPE grommet costs less than its TSE counterpart did, even though the mold for the TPE costs significantly more than that for the TSE. Other advantages were the ease of coloring the TPE a specified gray color, and ease of installation of the TPE grommet. Another example is a small plug that is exposed to occasional contact with motor oil. Comparison between an oil-resistant TPE (Geolast) and NBR demonstrated a lower cost for the NBR plug. The TPE industry continues to innovate and improve the properties and processing capabilities of their products.

Processing

Since the introduction and use of TPEs in both the traditional TSE and plastics industries, personnel have had to adapt to the design, materials, processing and application differences between TPEs and TSEs. This adaptation may be required when personnel transition from one industry to the other, or when a new elastomer is introduced into an existing manufacturing operation. The difference in the processing temperature profile for TPE and TSE accounts in a major way for the significant differences in molding behavior between these materials. During injection molding, TSEs progress from a warm barrel to a hot mold and solidify; TPEs progress from a hot barrel to a cool mold and solidify.

Other differences are the approaches taken in working with these different elastomers. Chemistry plays a more dominant role in the molding of TSEs because the majority of TSEs are unsaturated and therefore more subject to oxidation than most TPEs. This unsaturation also serves as a crosslinking site, and TSEs depend upon chemical crosslinks for stability, while TPEs depend upon physical crosslinks for stability. Hence, relative to TSEs, physics and engineering play a larger role than chemistry in the molding of TPEs.

The temperature of a TPE in the barrel of an injection molding machine must significantly exceed the [T.sub.g] and [T.sub.m] (figure 1) of the plastic domains to lower viscosity and convert a TPE into a melt with desirable flow characteristics. After melt fills a cool mold, viscosity increases sufficiently to permit removal of the molded article from the mold. Selection of mold temperature for a TPE represents a compromise. The temperature must be high enough to ease mold filling and produce an article of the desired quality. On the other hand, the temperature must be low enough to minimize cooling time and make the molding cycle economical.

Similarities and differences in behavior occur during the injection molding of TPEs and TSEs. Both TPEs and TSEs increase in temperature, the TPEs much more so, as they progress through the barrel of an injection molding machine toward their respective molds. Although limited degradation of a TPE occurs in its hot barrel, the TPE molding process is essentially physical only. This is in contrast to a TSE, where unintentional chemical crosslinking (scorch) can occur in the barrel and inhibit flow to the mold. The residence time and temperature for TSE in the barrel must be limited to avoid scorch. While scorch is not a concern with TPEs, upper temperature and residence time must be controlled to a level that acceptably limits TPE degradation.

Differences ha molding conditions can cause confusion when comparing flow behavior of TSEs and thermoplastics (TPs). For example, some technologists believe that TSEs will enter and then flow into very small gaps of 0.001 inch and less, while TPs will not. This belief does not consider the large differences in mold temperatures used for TSEs and TPs. The viscosity of a TSE decreases upon entering its hot mold; in contrast, the viscosity of a TP increases upon entering its cold mold, and with further cooling the TP solidifies. Viscosities of thermoplastics are generally lower than viscosities of typical TSEs (ref. 9). Hence, when compared at the same temperature and under equivalent conditions, thermoplastics should enter small gaps easier than TSEs, the opposite of the behavior described above.

Specific features and processing techniques for TSEs and TPEs are now compared using injection molding as the primary basis of comparison.

Feeding the injection molding machine

Different forms of elastomers are used to feed injection molding machines, with pellets being dominant for feeding TPEs. While either pellets or strips can be used with TSEs, strip feed dominates. Some TSEs, silicone elastomer for example, feed with difficulty and may require auxiliary equipment to facilitate feeding. Difficulties can also be experienced with TPEs, so cooling of the hopper may be necessary to prevent agglomeration of TPE pellets in the hopper of an injection molding machine. The rotating screw below the hopper transports individual pellets toward the compression zone where increasing pressure and temperature agglomerate or mass the pellets. The massed TPE subsequently passes through compression and metering zones of the injection molding machine and ideally exits the nozzle with a uniform temperature.

Most TSEs, because of their low [T.sub.g], tend to mass at room temperature. Relative to pellets, the use of strip form minimizes this difficulty, especially if the strip is coated with an effective release agent. Minimum massing or sticking problems are experienced with TSEs with higher [T.sub.g] values, for example high-acrylonitrile NBR.

TSE strips must possess sufficient green strength to avoid strip breakage during feeding. Feed strip dimensions must be suitable for easy entry of the strip into the feed throat and subsequent entry into the opening between the barrel and the rotating screw. Assuming a TSE compound feeds a machine satisfactorily and scorch time is acceptable, machine variables are adjusted to provide the highest injection temperature consistent with scorch-free mold filling.

Screw and barrel

Conditions during processing of TSEs and TPEs in the barrels of their respective injection molding machines are both similar and different. Both elastomers encounter a high temperature environment, with the TPE temperatures generally being substantially higher to soften or melt plastic domains and to insure delivery of a uniform melt to the nozzle. The temperature required to soften or melt a hard domain in a TPE depends significantly on the composition of the domain. For example, the hard crystalline domains in a copolyester elastomer require quite high temperatures for melting compared to the glassy polystyrene domains in an SBS.

Length/diameter ratio (L/D) and compression ratio are two important screw parameters for injection molding machines, and table 2 compares typical ranges of these parameters for TPE and TSE machines.

Screws for both types of machines convey their respective elastomers from the hopper or feed throat to the nozzle. Deeper channel depth below the hopper of a screw for TPEs pro vides the volume required to accept TPE pellets before they are subsequently compacted. In contrast, the TSE in a feed strip is compacted before it enters the feed zone.

The increased work associated with the higher L/D of a TPE screw results in the generation of substantially higher temperatures needed for softening glassy and crystalline domains (figure 1) in a TPE. Processing factors such as screw speed and back pressure are also important. Because scorch is not a concern with TPEs. thermal stability mainly determines their allowable upper processing temperature. The combination of temperature and residence time determines the degree to which a TPE will degrade in the barrel during processing. With TSEs, in contrast, scorch time mainly determines the allowable processing conditions. Lower L/D results in a substantial but allowable temperature increase in a TSE, while avoiding scorch.

Supplemental heat for the barrel of a TSE injection molding machine is not usually required after the machine reaches stable operating conditions. Cooling may be required, depending upon factors such as machine size and TSE viscosity. It is unlikely that insulation on the barrel of a TSE machine will be effective. In contrast, insulation surrounding the substantially hotter barrel of a TPE machine can result in significant energy savings.

Nozzle

The suppliers of TPEs often provide recommendations for nozzles for their specific elastomers. For example, conventional nozzles, nozzles with reverse taper or nozzles with positive shut-off are said to be satisfactory for injection molding styrenic TPEs (ref. 10). The characteristics of nozzles can vary substantially and some might be equipped with a separately controlled heater band to prevent material from freezing or drooling. For TSEs, desirable nozzle characteristics are generally determined by the molder because of the wider range of TSE compounds used relative to TPEs.

Nozzle diameter for TSEs should provide a temperature increase of about 20 to 30[degrees]C, combined with an injection time short enough to avoid scorch. Excessive nozzle temperatures can cause scorched material that could enter a subsequent shot. To minimize heat transfer from sprue to nozzle, nozzles can be separated from their spree bushing after mold filling, or an effective insulator can be located between the sprue plate and its mold.

Molds

The high mold temperatures used to shorten cure times for typical TSEs place additional demands on the selection of materials of construction for molds relative to TPEs. Lower mold temperatures used with TPEs allow for the wider use of movable mold components such as sliding cores, stripper plates and sleeves, with significantly fewer problems with galling of metals relative to TSEs. Higher TPE viscosity associated with lower mold temperature inhibits flow of elastomer into the clearance required for movable components; hot TSE of a lower viscosity might more easily enter an equivalent clearance. In a similar manner, high viscosity also inhibits flow of TPEs into mold parting lines and thus minimizes problems with flash relative to TSEs.

Runners

Runners, whether for TPE or TSE, must have a cross section sufficiently large to deliver elastomer to gates at a pressure adequate to fill mold cavities. TSEs generally have higher viscosities that require larger runner cross sections than thermoplastics (ref. 11). Rheological differences between TPEs and TSEs vary substantially, depending upon compounding, their viscosity response to temperature changes and other factors that affect flow in runners.

After exiting the nozzle of an injection molding machine, TPEs and TSEs encounter different thermal environments. Hot runner systems are used mainly for TSEs, wherein elastomer in the runner system cures along with the molded article. Of course, this produces crosslinked runner scrap that is often substantial, especially for a mold with many runners that feed small cavities. Disposal of crosslinked runners and other scrap, expensive now, is becoming increasingly so because of reduced landfill availability and other factors.

A cold-runner system maintains a TSE in its uncrosslinked state in a runner and considerably minimizes or eliminates runner scrap. Compounds for cold-runner molding must possess sufficient scorch resistance to remain uncrosslinked after residing in the runner system for at least one molding cycle. Although cold-runner systems are significantly more complex and expensive than are their hot-runner counterparts, significant elastomer savings tend to offset the increased expense.

In contrast to TSEs, TPEs are most frequently molded using a cold runner system and the resulting solidified runner scrap can be easily ground and blended with virgin TPE for reuse. Again, differing from TSEs, hot runners minimize or eliminate TPE runner scrap. A range of hot-runner designs is available, each with desirable and undesirable features, e.g., externally heated manifolds for hot runners are generally preferred to internally heated manifolds.

Gates

Among the three dimensions for gates--gate depth, gate width and land length--gate depth is especially important. Gate depth mainly determines the time needed for a TPE melt to cool, solidity and seal the gate, or for a TSE to heat, crosslink and seal the gate. The other dimensions, in combination with gate depth, affect the cavity-filling rate and the increase in temperature as elastomer flows through the gate.

Small gates are desirable because they produce only a small blemish at the junction of the gate and the molded article. Locating gates to avoid impingement of elastomer melt directly onto adhesive-coated inserts is important to avoid adhesive displacement. A wide range of gate designs is available and their choice and location often depends more upon the design of a molded article than on the use of a TSE or TPE.

Cavities

TPEs and TSEs encounter substantially different environments in their respective mold cavities, the main difference being the temperature profile. TPEs cool upon contacting cold cavity walls, where they then solidify adjacent to the wall surface. Shrinkage in the mold cavity associated with solidification can create a gap between the cooled TPE if holding pressure on the residual molten TPE is insufficient to press solidified TPE layers against cavity walls. The result is slower cooling across the gap and shrinkage of the cooled melt from the mold wall. With both TPEs and TSEs, excessive pressure can cause molds to open and form flash.

TSEs, in contrast to TPEs, expand upon entering the higher temperature environment of their mold cavity. This expansion forces TSE against the hotter mold wall where contact is maintained between elastomer and cavity wall throughout the molding cycle. Hence, heat transfer in the mold cavity is much more favorable for TSEs than for TPEs.

Unfortunately, TSE molds lose much more heat to atmosphere because their temperature is significantly higher than TPE molds. Thus, insulation used to reduce heat loss from TSE molds is much more effective than that for TPE molds. This situation contrasts with the effective use of insulation on the very hot barrels of TPE injection molding machines, compared with the lower barrel temperatures used for TSEs.

Advancing flow fronts in a mold cavity can fail to knit properly for different reasons. Too long a flow path for a TPE in its cooled cavity can partially solidify the flowing front of the elastomer, with resultant poor knitting. In the absence of scorch, higher temperatures promote good knitting of flowing TSE in a cavity because of increased molecular mobility of the TSE.

With either type of elastomer, effective venting of mold cavities is imperative because rapid release of air during cavity filling minimizes trapped air and shortens cycle times. The savings in cycle time is proportionately more important for TPEs because of their substantially shorter molding cycles.

Cycle times

TSEs and TPEs encounter significant temperature changes during their molding cycle because their temperature profiles are reversed. It is necessary only to cool a TPE melt in its mold to produce a molded product, but a TSE must be heated in its mold cavity and then maintained at a high temperature sufficiently long to obtain the desired degree of crosslinking. The time required for the TSE crosslinking reaction mainly accounts for differences in cycle times, typically minutes for TSEs vs. seconds for TPEs. Hence, an equal decrease in cycle time for a TPE and a TSE results in a higher percent reduction for the TPE. For example, shortening a thirty-second TPE cycle time by three seconds results in a 10% reduction in cycle time. A three second reduction for a TSE with a five-minute cycle time results in only a 1% reduction, an amount that would affect manufacturing costs significantly less.

Secondary effects can also influence cycle times. For example, a corrosive compound may require the use of a stainless steel mold rather than a conventional one to avoid corrosion. Stainless steel possesses poorer heat transfer characteristics, and its use may increase mold cycle times.

Clamp opening and closing times on an injection molding machine also affect the mold cycle time. Toggle clamps are most frequently used on TPE machines because they open and close much faster than the hydraulic clamps used for TSE machines. Because TPE cycle times are short, clamp opening and closing times are proportionately more important for TPEs. Although it is possible to considerably reduce cycle times for hydraulic clamps, doing so increases their cost significantly.

A new injection molding machine is said to be capable of molding either TPE or TSE with no modification to the press (ref. 12). The machine is directed primarily toward the manufacture of sealing systems for automobiles. The significantly shorter cycle times associated with molding TPE necessitated substantial increases in the speed of operation of the clamping and injection units.

Ejection

An elastomer article, prior to ejection of from its mold cavity, must become sufficiently stiff to resist permanent deformation that results from stresses encountered during ejection. Cooling a TPE melt in its mold cavity causes the melt to stiffen as physical crosslinks are formed; heating a TSE in its mold cavity causes it to stiffen through formation of chemical crosslinks. Either elastomer must stiffen and solidify sufficiently prior to ejection if distortion and other properties are to be acceptable. A rigid insert affixed to a molded TSE or TPE article facilitates ejection if an ejector pin(s) bears upon the insert and thus distributes stress in the elastomer during ejection.

Ejection with pressurized air, with or without the use of ejector pins, is very effective with both TPEs and with TSEs. The air distributes the ejection force over the surface of an article and thus lowers the stress during ejection. Uniform cooling of a TPE mold dissipates heat from the article and facilitates the ejection process.

Mold fouling

Fouling, the process by which undesirable material deposits on a mold surface, is important because it affects both the appearance of molded articles and their manufacturing costs. Fouling appears initially as a stain that progressively thickens. In severe cases, the fouled material can adhere to and ultimately pull fragments from a molded product. The large number of factors that contribute to fouling is sometimes unrecognized. Frequently, the compound is exclusively blamed for mold fouling when in fact other factors, e.g., very high cure temperatures, and combinations of factors may cause or aggravate fouling.

Fouling tendencies differ significantly among different compounds, both within an elastomer class, and among types within a class. Fouling problems are generally much more severe with TSEs than with TPEs, and dissimilarities in processing temperatures account liar much of the difference. The substantially higher mold temperatures used with TSEs promote oxidation of compounds, and high temperature can alter the nature of chemical crosslinks. Especially oxidation-prone are unsaturated TSEs such as NR, BR, SBR and NBR that are among the most widely used elastomers. Long-time exposure of fouled material at high temperatures, and depletion of antioxidants, aggravates fouling.

Since TPE molds operate at substantially lower temperatures than TSE molds, significantly less fouling occurs in TPE mold cavities. When TPE fouling does occur, it is most common in the hot runner(s) of a TPE mold, where temperatures of hot runners can equal or exceed those of TSE molds. For example, melt temperatures (ref. 13) for Hytrel might be as high as 450[degrees]F. However, overall fouling problems with TPEs are generally substantially less than fouling problems with TSEs.

Additional considerations

Fillers may be incorporated in TPEs and TSEs to lower cost rather than to improve strength. Fillers such as carbon black are often incorporated in TSEs such as SBR and NBR to improve strength and other properties. The resultant black-filled articles are less easily identified than non-black articles. TPEs are generally easier to color than TSEs because sulfur and discoloring antioxidants that are commonly incorporated in TSE compounds can produce reaction products that interfere with obtaining the desired color in a TSE compound (ref. 14). The use of colored TPEs can reduce errors in subsequent assembly operations, e.g., in automotive plants (ref. 15).

Hence, many contrasts can be made between the processing behavior and properties of TSEs and TPEs. It appears likely that TPEs will continue to displace TSEs in lower temperature applications that involve only an acceptable level of stress, while TSEs will continue to dominate in more demanding applications. Continuing technological improvements are expected to result in TPEs making increased inroads into established TSE markets.

Table 1--comparison of TPEs and TSEs

Item              TPE

Recycle           Blend of ground scrap and virgin TPE shows good
                  properties, along with cost savings. Easy to recycle.

Energy            Fewer recycling steps use significantly less energy.

Automation        Capitalizes on robotics technology used in the
                  plastics industry.

Bonding           Heat welding feasible along with other bonding
                  methods.

Molding           Required equipment is usually available in the
  equipment       plastics industry.

Elastomer cost    Generally higher.

Density           Often lower; elastomer products are sold on a volume
                  basis.

Cycle times       Typically measured in seconds.

Quality control   Fewer manufacturing steps reduce QC costs.

Moisture          Drying often required before molding.

Upper service     Limited to less than [T.sub.g] or Tm of hard domain in
  temperature     the TPE

Item              TSE

Recycle           Crosslinked materials very difficult to recycle.
                  Disposal is costly.

Energy            Many processing and recycling steps require
                  considerable energy.

Automation        TSEs less amenable to use of robotics.

Bonding           Wide range of effective adhesives is commercially
                  available.

Molding           Injection machines for TSEs not generally useful for
  equipment       molding TPEs

Elastomer cost    Generally lower.

Density           Variable, but often higher--especially for low cost
                  compounds containing high filler levels.

Cycle times       Typically measured in minutes.

Quality control   Many processing steps increase QC costs.

Moisture          Moisture generally less of a problem.

Upper service     Generally higher, but limited by thermal stability of
  temperature     TSE and its crosslinking system.

Table 2--screw parameters for injection
molding machines

Range               TPE            TSE

L/D ratio           12:1 to 25:1   10:1 to 12:1
Compression ratio   2:1 to 2.5:1   1:1 to 1.2:1

This article is excerpted from the book "Elastomer Molding Technology" by the author that is in press.

References

(1.) M.S. Reisch, Chemical and Engineering News. 74 (32) 10 (1996) August 5.

(2.) C.P. Rader, Plastics Rubber and Composites Processing and Applications, 27 (7) 332 (1998).

(3.) D.E. Williams, J.E. Morin, A.R. Tripathy and R.J. Farris, Rubber World, 226 (3) 40 (2002) June.

(4.) D. Shaw, European Rubber Journal, 184 (9) 22 (2002) September:

(5.) J.D. Destefani, Molding Systems, 55 (12) 32 (1997) Nov./Dec.

(6.) J.G. Sommer, "Elastomer Molding Technology." Elastech, Hudson, OH, in press.

(7.) W. Hofmann, Kunstoffe German Plastics, 80 (10) 88 (1990).

(8.) D.R. Mathias, presented at a meeting of the Rubber Division, ACS., Nashville, TN, Nov. 3-6, 1992.

(9.) J.G. Sommer, "Rubber Molding Methods," in Handbook of Polymer Science and Technology, N.P. Cheremisinoff Ed., Marcel Dekker, Inc., New York, 1989, Vol. 3, Ch. 7, p.312.

(10.) Technical Literature, Kraton, Shell Chemical Company.

(11.) J.D. Byam, R.D. Souffie and K.D. Ziegel, Elastomerics, 113 (3) 23 (1981) March.

(12.) Anon., European Rubber Journal, 183 (11) 27 (2001) November.

(13.) Technical Bulletin HYT-402(R2), "Injection Molding of Hytrel," DuPont Company, Wilmington, Delaware.

(14.) D.A. Smith, Rubber Journal 153 (11) 19 (1971) November

(15.) M. W. Bednarik and R.L. Arnold, Automotive Polymers and Design, 7 (6) 7 (1988) August.