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 and the polyamides. 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 items 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 olefinic based 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, resistance to creep, and 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 wide 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.
And finally, the polyamide TPEs may be thought of as a nonplasticized 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.
The injection molding process
The injection molding process itself can be a difficult one depending upon a number of factors. These major factors include material and machine characteristics as well as the design of the part. The process can be described as one that discontinuously produces three dimensional parts from polymeric materials, although other materials are injection molded. The production of parts is accomplished by first melting the material (generally referred to as plasticating) and then injecting the material under pressure into a mold where it solidifies in the form of the cavity. The history of injection molding dates back to 1872 when a U.S. patent was issued for a machine design that used a plunger (ref. 2). TPE materials may be molded on plunger machines today, however, melt homogenization is much better utilizing reciprocating screw type machines. It was not until 1956 that the first reciprocating screw machine became available and are the machines of choice today.
Generally, the machine can be broken down into three pieces. These three pieces consist of an injection unit, the mold, and the process control system. The polymer, either in pellet or powder form, is supplied to the machine through a hopper. It is essential that the material feeds continuously through the hopper and down into the injection unit. As the screw rotates, the material is conveyed through the barrel to the front of the screw. As it moves along, the material is heated by a series of heater bands found on the outside of the barrel. A combination of heat and shear should develop a homogeneous melt. As the material is conveyed, pressure builds in front of the screw tip. This pressure forces the screw back until a predetermined amount of material is developed. The screw will stop momentarily (the amount of time depends upon the cooling time) and then will come forward (non-rotating) forcing the material into the mold cavity. The material then cools under decreasing internal pressure. When the part has solidified enough, the clamping unit will open and the part will fall out. Figure 1 follows the cavity pressure profile over time during the molding process.
Molding thermoplastic elastomers
The molding of the five different classes of TPEs can be relatively easy if you follow a few simple guidelines. 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 are generally more affected by shear than by temperature (figure 2). 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. Table 1 compares the differences between these two classes. 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 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 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 overpacking 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 30-80 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] F - 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 and back pressure can be somewhat higher than the unsaturated styrenics, depending upon the compound. Table 2 gives a more complete breakdown on the preferred processing conditions for these two classes of styrenic materials.
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, particularly 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. 3). However, these three classes of materials do behave somewhat similar 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 3 reports the preferred conditions for this rapidly growing class of materials.
The copolyester elastomers must be dry before molding for optimum properties ([is less than] 0.1%). In general, the 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 hot enough (150 [degrees] F). Higher pressures will reduce shrinkage because of the higher packing that will result. The injection rates should be high for thin walled moldings, while a moderate rate for thicker sections will be sufficient. Table 4 suggests proper molding conditions for copolyester TPEs.
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 ([is less than] .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 5 reports on the recommended processing conditions for the TPUs.
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 6 gives a more complete breakdown of the preferred processing conditions for the polyamide TPEs.
Finally, all of the TPEs that are commercially available today may be molded quite easily in many conventionally designed molds. Processing the softer durometer compounds ([is less than] 45A) will sometimes require a modified sprue design that will allow for easier removal. Draft angles of 3.0 [degrees] are generally satisfactory for most of the softer compounds. Also, more "aggressive" sprue puller pins are sometimes required in order to increase the pulling force. For deeper draw and softer compounds a stripper plate and/or air may be required to break the vaccum that can be created by shrinkage. Ejector pins should also be as large as possible so that they will not penetrate the part. The runners should be of a full round configuration and as short as possible so that adequate flow will occur on all shots. Gate sizes should always begin small (the size depends upon the part) and enlarged when necessary. The mold cavity surfaces should be vapor honed for optimum release from the cavities. Highly polished or chrome plated mold surfaces will cause sticking and are not recommended. Insulated runners can be utilized as long as the runner itself is "fairly short" and at least one inch in diameter. Hot runners are also being successfully used in systems that have runner diameters greater than .5 inch.
Injection molding TPEs can be relatively easy if certain guidelines are followed. Some of the more major guidelines are as follows:
* The majority of TPEs are shear sensitive. Increasing or decreasing the back pressure on the material will change the viscosity of the material.
* Since the materials are shear sensitive, the screw retraction time should be adjusted so that it ends just before the next shot.
* The injection pressure time should be as short as possible so that overpacking will not occur.
* The holding pressure should be 1/3 to 1/2 that of the injection pressure so that the gate will seal off and overpacking will not occur.
* The mold cavities should be vapor honed for better part ejection.
* A general purpose screw will work adequately for all TPE materials.
* Always follow the manufacturer's recommendations on drying the material. Generally, the copolyesters, polyurethanes, and the polyamides are hygroscopic while the styrenics and polyolefins are not. Note 1: It should be noted on the processing guideline 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.
References Walker, B.M., Rader C.P., Handbook of Thermoplastic Elastomers, 2nd Ed. "Thermoplastic Polyolefin Elastomers," Shedd C.D., p. 48, Van Nostrand Reinhold, New York (1988). Rubin, I.I., Injection Molding; Theory and Practice, SPE Monographs, Wiley, New York (1973). German Patent DRP 858,310. Walker B.M., Rader C.P., Handbook of Thermoplastic Elastomers, 2nd Ed., "Single Phase Melt Processible Rubber," Wallace J.G., p. 143, Van Nostrand Reinhold, New York (1988).
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|Author:||Hudson, John A.|
|Date:||Jul 1, 1989|
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