Injection molding thermoplastic elastomers.

Injection molding thermoplastic elastomers

Presently, all five classes of the commercially available thermoplastic elastomers (TPEs), can be injection molded into parts that have a high degree of elasticity. These five classes include the styrenics, polyolefins, copolyesters, polyurethanes, polyamides and the relatively new PVC based elastomers. While there is a certain amount of overlap in the applications for TPEs, each class has a certain area that it primarily dominates because of that material's unique advantage(s).

The styrenic materials introduced in 1965 offer the end-user the widest range of overall elastomeric properties. Durometers as low as 15 Shore A and as high as 70 Shore D have been molded successfully over the years. Applications for molded items include footwear, automotive goods, medical products, and general purpose items.

The thermoplastic polyolefin elastomers (TPOs) are the fastest growing market at an average rate of almost 20% per year (ref. 1). These elastomers, because of the various olefin classed type polymers used to make the material, and compounding technology, offer a wide range of processing characteristics and physical properties. The major applications include many different automotive items such as exterior body parts, body side molding, air ducts and industrial goods. The copolyester elastomers offer the molder a class of materials that exhibit physical properties that are in between those of an elastomer and an engineering plastic. They offer excellent flexibility and resistance to creep, and they are operational over a broad temperature range. Their major applications include automotive items such as constant velocity drive joint boots, electrical components and medical products.

A wise variety of thermoplastic polyurethane elastomers (TPUs) are available, with hardness values that range from 30 Shore A to 70 Shore D. Their major applications include automotive items, caster wheels and industrial goods such as drive belts, soft-faced hammer heads and gaskets.

The polyamide TPEs may be thought of as a non-plasticized elastomeric nylon. These materials have excellent low temperature impact properties and good abrasion resistance. The major applications for these materials include seals and gaskets, low temperature bellows and boots, and automotive parts.

Finally, a new class of TPEs based on PVC have recently been introduced. These materials exhibit much higher levels of elasticity and better compression set performance than conventional soft PVC materials. They also offer very good tensile and elongation properties, good abrasion resistance and weatherability. Applications include replacing products that traditionally use soft PVC materials that require higher levels of elasticity and products that need a "softer" TPE that is attractively priced.

Experimental

The injection molding process itself can be a difficult one, depending upon a number of factors. The major factors include materials and machine characteristics as well as a design of the part. This investigation deals with both the molding characteristics of the five major classes of thermoplastic elastomers and what affect some important molding parameters have on part quality.

Materials

Representative samples of the major thermoplastic elastomers (both high and low durometer) found on the market were studied. The materials that were molded are shown in table 1. All of the materials are dried in accordance to the manufacturers's recommendations. The materials were initially processed according to the manufacturer's recommendations. Subsequently, molding parameters were changed and observations on the change were noted. The molding parameters that were investigated include melt temperature, mold temperature, pressure and time. The effects of these molding parameters were then related to part quality.

Table : Table 1 - materials molded

Material Durometer

Styrenics

  SBS                          35A, 50D
  SEBS                         30A, 50D

Polyolefins

  Blended                      60A, 50D
  Thermoplastic vulcanizates   64A, 50D
  Melt processible rubber      60A, 50D
Copolyesters                   35D, 70D
Polyurethanes                  75A, 70D
Polyamides                     25D, 70D

Results and discussion

The molding of the six different classes of TPEs can be relatively easy if a few simple guidelines are followed. First, the rheological properties of almost all of the TPEs are different than most thermoplastic materials. Since all of the TPEs are quite rubbery in nature, the viscosity of the materials is generally more affected by shear than by temperature. As the material experiences higher shear rates, the viscosity decrease becomes more dramatic. Because of this, reciprocating type injection molding machines should be utilized whenever possible, since shear rates can be more easily controlled.

The styrenic TPEs are commercially available as two different classes, depending upon the midblock composition. Generally, the unsaturated styrenic materials (styrenebutadiene-styrene) do not require and should not experience high shear rates. However, the saturated midblock types (styrene-ethylene-butylene-styrene) do not require higher shear rates to insure adequate surface appearance and good physical properties. The cylinder temperatures for the unsaturated materials should range from 280 [Degrees] F-400 [Degrees] F, depending upon the grade of material. Generally, higher mold temperatures will result in a better surface appearance. The injection pressures for the unsaturated styrenics can range from as little as 3,000 psi to as much as 20,000 psi depending upon the surface area of the part and processing conditions. The injection pressure time should be as short as possible so that over packing of the part will not occur. The rate of injection should be from slow to moderate, depending upon runner lengths and part size. Normal screw speeds of 30-810 rpm should be used so that the screw will stop just prior to the next injection shot. Back pressures of 25-50 psi should be sufficient for developing a homogeneous melt.

The saturated styrenics have somewhat more processing stability and therefore are more forgiving. The cylinder temperatures for these materials are generally from 380 [Degrees] F-500 [Degrees] F with mold temperatures of 80 [Degrees] - 175 [Degrees] F. The injection rate should be fast so that freeze-off will not occur and surface appearance will be optimized. The screw speed back pressure can be somewhat higher than the unsaturated styrenics, depending upon the compound. Tables 2 and 3 give more complete breakdown on the preferred processing conditions for these two classes of styrenic materials. [Tabular Data Omitted]

The polyolefin elastomers or TPOs can be separated into three distinct classes, depending upon their particular morphology. Originally, TPOs were prepared by mechanically blending olefinic type materials together, particular polyethylene, polypropylene and EPDM. Another type of TPO, called a thermoplastic vulcanizate (TPV), consists of a compounding process that partially cures the elastomeric phase within a thermoplastic carrier. And finally, a single phase olefinic elastomer has been introduced recently (melt processible rubber-MPR) that has been reported to exhibit a single glass transition temperature (ref. 2). However, these three classes of materials do behave somewhat similarly during the molding process. The cylinder temperatures should vary from 375 [Degrees] F to a high of 450 [Degrees] F, depending upon the grade of material. The mold temperatures can be run from as low as 35 [Degrees] F for some grades to as high as 175 [Degrees] F. The injection rates should be from moderate to fast to insure adequate filling. The TPOs do not readily absorb water, but they should be stored in a relatively dry area. If the materials do become wet, they should be dried at 200 [Degrees] F for 1-3 hours. Table 4 reports the preferred conditions for this rapidly growing class of materials. The copolyester elastomer must be dry before molding for optimum properties (<0.1%). In general, safe copolyesters are not as shear sensitive as some of the other classes of TPEs. Their viscosity can be controlled by increasing or decreasing barrel temperatures. Injection pressures as low as 3,000 psi can be used when mold temperatures are high enough (150 [Degrees] F). Higher pressures can be used to reduce shrinkage since higher packing will result. The injection rates should be high for thin walled moldings, while a moderate rate from thicker sections will be sufficient. Table 5 suggests property molding conditions for copolyester TPEs. [Table Data Omitted]

The polyurethane TPEs (or TPUs) are generally thought of as being commercially available in two classes. The polyester based TPUs have higher tensile strength, better ozone, oxygen, oil and solvent resistance than the polyether type TPEs. However, the ether-based TPEs have better low temperature properties and better resistance to hydrolysis and microbial attack. Both classes do have similar processing conditions. The material is hygroscopic and must be dried prior to processing (<.05%). A mold temperature of between 90 [Degrees] F-150 [Degrees] F is sufficient for optimum surface appearance and physical properties. When the material is processed correctly, the melt should appear slightly off-white to a very light yellow color. If the melt contains bubbles, then moisture is probably present. Excessive melt temperatures will result in a "very transparent" purge shot. During shutdown, the barrel should be purged clean with a material such as polyethylene or polystyrene to prevent degradation of the material. Table 6 reports on the recommended processing conditions for the TPUs. [Table Data Omitted]

The polyamide TPEs are also very easy to injection mold if a few important guidelines are followed. The materials must be dry before molding to generally less than .08% moisture content. Insert molding can be accomplished quite easily without adhesives by using polymers such as Nylon 6, 11 or 12 as the structural portion of the part. A general purpose screw works quite well in generating complete melt homogenization. Regrind can be used safely at levels of 20-25% as long as the material has been properly dried. Table 7 gives a more complete breakdown of the preferred processing conditions for the polyamide TPEs. [Table Data Omitted]

The PVC TPEs have a fairly narrow processing window due to the materials' shear sensitivity and high temperature (>400 [Degrees] F) degradation. The molding of this material should be carried out at the lowest processing temperatures and shear conditions to produce an acceptable part. A general purpose screw with a sliding ring shutoff works fairly well with these materials. Molded products based on this material exhibit clarity, toughness, high gloss and excellent elasticity. Table 7 provides the general recommendations for molding these materials. [Table Data Omitted]

A few key molding parameters such as melt and mold temperature, pressure and time, were studied to relate their effects on overall part quality. The melt temperature by itself should be considered a major parameter during molding, and should generally be controlled by screw back pressure (figure 1). The effect of increasing or decreasing the melt temperature then has a direct influence on the part being molded. Increasing the melt temperature will lead to lower injection pressures (figure 2) and can also be used to alleviate shrinkage problems (figure 3). Increasing the melt temperatures may be beneficial in increasing weld tensile strength (figure 4) and will lead to an increase in gloss and surface finish (figure 5). However, care must be taken in that the melt temperature must not exceed a certain point or overall cycle time will increase, overfilling (flashing) the mold may result, and/or degradation of the polymer may occur. Increasing the mold temperatures will also decrease the pressure needed to fill the cavity (figure 2) but also may lead to longer hold pressures since gate seal-off will take longer (figure 6). Increasing the mold temperature will lead to a higher degree of gloss (figure 5) but may also lead to higher shrinkage. And finally, the holding pressure time versus part weight should always be monitored since this parameter has an effect on part weight.

Conclusions

* All of the commercially available TPEs may be molded quite easily in many conventionally designed systems.

* It should be noted on the processing guidelines tables that as the TPE grade becomes softer (within the same class) the processing parameters will fall into the lower ranges that have been reported. For example, a 35 Shore A styrenic TPE will have much lower injection pressures, processing temperatures, and subsequently higher flow properties than a 50 Shore D material.

* As with conventional thermoplastic material molding, the molding parameters (temperatures, pressure and time) will have a direct influence on the quality of a TPE molded part.

* And finally, this investigation was carried out to serve as a general guide for molding TPEs. Always consult the manufacturer's processing recommendations when molding a commercially available TPE.

References [1.] Walker, B.M., Rader C.P., Handbook of thermoplastic elastomers, 2nd Edition, "Thermoplastic polyolefin elastomers," Shedd C.D., p. 48, Van Nostrand Reinhold, New York (1988). [2.] Walker B.M., Rader C.P., Handbook of thermoplastic elastomers, 2nd Edition, "Single phase melt processible rubber," Wallace J.G., p. 143, Van Nostrand-Reinhold, New York (1988).