Blow molding of melt processible rubber.

Blow molding of melt processible rubber

Today's thermoplastic elastomers span an extensive performance range, from the SBR-like behavior of the styrenics to the engineering properties of the copolyesters. Thermoplastic elastomers can match or exceed the performance of most of the thermoset rubbers in the parameters critical to a wide range of applications.

Beyond functional performance, thermoplastic elastomers are clearly superior to thermoset rubbers in the ease, versatility and cost of part fabrication. One obvious demonstration of this fact is the ability of thermoplastic elastomers to form hollow parts by blow molding, as opposed to injection molding or the antiquated transfer molding processes, which are the techniques available for making such parts from thermoset rubber.

Melt processible rubber (MPR), is an excellent example of a thermoplastic elastomer, which meets the performance requirements of a wide variety of rubber applications, while affording the part fabricator all the advantages of making hollow parts by the blow molding process. MPR was commercially introduced by Du Pont under the trademark Alcryn in late 1985. MPR provides parts with the look and feel of rubber, plus the weather resistance of EPDM, the fluid resistance expected of a medium-AN NBR and colorability/color stability better than that of any thermoset rubber. All grades of this product line can be blow molded by one or more of the commercial blow molding processes. However, Du Pont recently expanded the product line to include translucent, neutral and black grades, spanning a hardness range of 55 to 80 Shore A, with improved flow characteristics. These new grades are especially suited for fabrication into hollow parts using any of the standard blow molding processes.

This article will discuss the advantages of making hollow rubber parts by blow molding TPEs versus conventional rubber processing. It will describe the various types of blow molding processes and it will provide some insight into the rheological properties of MPR and how MPR should be molded by each of these processes. A number of blow molded applications for MPR will also be discussed.

TPEs as an opportunity for rubber molders

Thermoplastic elastomers (TPEs) are regarded by many in the rubber industry as simply the most recent "invasion" of traditional rubber markets by the "plastics industry." Rubber parts manufacturers often view TPEs as functionally inferior to thermoset rubber and therefore a limited threat to their existing businesses. However, the currently available thermoplastic elastomers are more than adequate for a very high percentage of applications now employing thermoset rubber, as their growth, at the expense of thermoset rubber, clearly demonstrates.

The growth of rubber parts in most market segments is, at the least, keeping up with the growth of the economy, but much of that growth is going to the thermoplastic elastomers. And advancing technology is slowly eliminating the functional advantages of thermoset rubber. Thermoset rubber fabricators should seriously consider the TPEs as an opportunity, rather than as a threat. The unique processing of TPEs could be exploited to secure existing business and provide an opportunity to expand into markets not now open to thermoset rubber molders. An investment in blow molding equipment could be an important first step in using TPEs to complement existing thermoset rubber business, enabling the thermoset rubber part fabricator to participate in the growth of TPEs.

Manufacture of hollow rubber parts: TSEs vs. TPEs

The manufacture of partially or entirely hollow parts from thermoset rubber has always been a challenge to the rubber molder. That they have succeeded in making such a variety of relatively complex hollow rubber parts at all is a tribute to the molder's ingenuity. The problem has always been the need for a core of some sort to occupy the "hollow" space, while the part is formed and vulcanized. After curing, some provision must be made to get the part off the core without damaging the part. Although there has been some work using collapsible, or destructible cores, most applications use a solid core, which must be withdrawn through an opening in the cured rubber part.

Thus the design of a hollow rubber part is at least partially dictated and restricted in complexity by the need to remove the core. Removal of the part from the core also limits the choice of rubber compound to one with high enough elongation to be stretched over the largest dimension of the core without tearing. This is especially difficult when the need for the shortest possible molding cycles requires that core removal be done while the rubber part is still hot. Compounds with borderline elongation will result in an unacceptable level of non-recoverable scrap, and even appropriate compounds will yield a certain percentage of parts that tear during demolding.

Blow molding eliminates all problems related to core removal, because there is no core to remove. All dimensions, convolutions, surface textures and embossed or inset printing is determined by the external mold. The core is simply compressed air. Therefore, part design does not have to allow for core removal and the elastomer compound can be selected for best cos/performance balance, without regard to its hot tear resistance.

Beyond the core problem, the transfer molding process, typically used to make small to medium-sized hollow rubber parts, is itself very restrictive, labor and energy intensive and slow. The molds are essentially multi-component, three-dimensional jig-saw puzzles. They must be assembled, preheated, loaded, compressed, disassembled, unloaded, stripped of cured runners and pad, and reassembled - all by hand - for every molding cycle. When complex hollow parts must be carefully removed from cores, this process is often so time consuming that the reassembled mold must be reheated before the blank for the next heat can be loaded and the cycled repeated. Non-recoverable scrap, in the form of cured pad, runners, sprues, flash and imperfect parts, sometimes rivals the weight of good finished parts produced in the heat.

Again, blow molding eliminates most of the problems associated with transfer molding. Molds are usually relatively simple two-piece aluminum shells, even for very complex parts. All preform sizing, handling and mold loading are automatic, as are part shaping and ejection from the mold. Molding cycles are timed in seconds, rather than the many minutes of transfer molding cycles. The preform is automatically preheated, metered to the mold to produce the desired part with essentially no flash. There is no cured pad or runner system and the trim is fully recyclable, as are any imperfect parts. The combination of simple, low-cost molds, fast cycle times, high productivity, and recyclable scrap all translates into finished parts at a lower unit cost than an equivalent thermoset rubber part.

In addition to cost-related advantages, blow molding is also a much more controllable and versatile process than transfer molding. Tighter dimensional control allows the molder to more easily meet the demanding specifications of high-performance applications. The parison wall thickness is programmable and can be deliberately varied to produce uniform wall thicknesses regardless of the part geometry, or differing wall thicknesses, to provide varying degrees of flexibility and durability. Far more complex shapes can be made by blow molding than by either transfer molding or injection molding. With a blow molding machine, a rubber parts fabricator can supply complex parts, which are now conceded to the plastics molders.

Blow molding

Basic blow molding concept Blow molding has been used routinely to make hollow parts by the plastics industry for many years. Most commonly to make the wide variety of shapes and sizes of polyethylene, and polypropylene bottles seen on the shelves of retail stores. However, this technique is also used to make numerous, functional hollow plastic parts, which are widely used in the automotive, appliance and industrial sectors. These parts range in size from a few grams to large drums and in complexity from a simple cylinder to highly convoluted shapes with strong undercuts and multiple wall thicknesses.

Conceptually, blow molding consists of extruding or injecting a tube of molten polymer as a perform (called a parison) and placing it inside a cooled mold, which pinches off the open end of the tube. The hot parison is then inflated with compressed air and forced against the interior walls of the mold cavity, which sets all of the exterior part dimensions, surface texture and printing. Working backward, from a rubber fabricator's point of view, the mold solidifies the finished part by taking heat away from it. The part cools enough to be demolded in seconds. At this point the mold opens and the part is ejected as the cycle automatically repeats. The "pinch" portion of the part is easily trimmed away, and the trim scrap is later reground for reuse.

Blow molding is commercially accomplished by three different major processes, one of which has three variations:

* Continuous extrusion (figure 1).

* Intermittent extrusion.

a. Accumulator head (figure 2).

b. Ram accumulator (figure 3).

c. Reciprocating screw (figure 4).

* Injection (figure 5).

Each of these processes will be discussed as it pertains to the blow molding of MPR.

Blow molding MPR

Rheological behavior of MPR MPR is being commercially molded by all three blow molding processes. However, the continuous extrusion and injection processes are preferred for MPR because of its rheological characteristics. The commercially available grades of MPR have no crystalline melting point and like most thermoset rubbers, are essentially amorphous. They will not flow with increasing temperature without also being subjected to shear. The application of shear at elevated temperatures produces a type of psuedoplastic flow known as shear thinning and is shown in figure 6. The viscosity of MPR is not very sensitive to melt temperature. This is shown in figure 7 where viscosity versus shear rate is plotted over the temperature range of 170 [degrees] C (338 [degrees] F) to 190 [degrees] C (374 [degrees] F).

The rheology of MPR makes it ideally suited to the blow molding process. All three processes involve hopper feeding TPE pellets to a heated extruder. The extruder softens MPR (melts other TPEs) and the screw provides the shear necessary to reach extrudable viscosity. The hollow tube parison must quickly develop enough strength to maintain its shape and withstand rapid expansion against the mold by compressed air inflation without rupturing. As soon as MPR is no longer under shear, its viscosity climbs rapidly to that of a partially cured part. Therefore, a MPR parison essentially "freezes" as shear is reduced or stops, regardless of its temperature. It has no tendency to sag under its own weight. Parison wall uniformity has been good and parisons up to thirty-six inches in length have been extruded without excessive sag. Blow-up ratios (the ratio of the finished part dimensions to that of the original uninflated parison) of greater than 3 to 1 have been achieved without blow outs.

Extrusion blow molding

Continuous extrusion As the name implies, continuous extrusion blow molding involves a conventional extruder, operating continuously, to extrude a parison vertically through a right-angle die, which is "captured" by a movable mold, when it reached the proper length for the part being made (figure 1). The mold "pinches off" the hot parison and it is immediately inflated with compressed air against the inside contours of the cool mold. Blow pressure utilized is a function of part size and parison wall thickness. Typical blow pressure range is from 30-100 psi. Then the cool part is ejected from the mold and the mold is ready to "capture" the next parison. The extruder screw speed and extrusion rate of the parison has to be timed to have the parison just reaching the right length when the mold has ejected the previous part and is ready to repeat the cycle.

Intermittent extrusion Intermittent extrusion involves using either a reciprocating screw machine or a conventional extruder to plasticate the TPE pellets and move the plasticated polymer into the front of the barrel of the retracting reciprocating screw or into an accumulator. In both cases, when the predetermined amount of plasticated polymer has been accumulated, it is injected through a die to form a parison. The injection is performed either by the forward motion of the reciprocating screw (figure 2) or by a ram located in the accumulator head (figure 3) or in a parallel ram extruder (figure 4). The remaining stages: closing the cooled mold around the parison; air inflation of the parison against the inside of the mold; part solidification mold opening and part ejection - are similar to those in the extrusion blow molding process.

In all three types of intermittent blow molding, the situation is less than ideal for MPR. The intermittent nature of the extrusion means shear is interrupted, which causes MPR to increase in viscosity. Careful design is required to eliminate dead spots in the system, where polymer could stagnate and possibly degrade. No problems are encountered with a fully streamlined system.

General conditions for extrusion blow molding MPR In all extrusion blow molding processes for MPR, the extruder must be capable of attaining a melt temperature of at least 160 [degrees] C (320 [degrees] F). Extruders with length to diameter ratios of between 15:1 and 30:1 can be used, with 20:1 preferred. Shear is necessary to produce a uniform, well-fluxed melt with MPR. To provide the required shear, a section of the screw should have several flights that are no more than 2.5 mm. (0.100 inches) deep. Any device that increases mixing and shear, such as pins, torpedoes, dams or barriers, will improve plastification and broaden the processing window. The screw compression ratio should be between 2.5 and 3.5. Only moderate working is necessary; high shear screws could cause undesirable overheating.

Screw speed can be adjusted to achieve output desired. Increasing screw speed with a screw suitable for MPR will cause an increase in melt temperature. Barrel temperature set points must be adjusted to maintain the desired melt temperature.

For extrusion blow molding, recommended process conditions for MPR are the same regardless of which blow molding process is used. A "reverse" barrel temperature profile is preferred for extruding parisons of MPR. Thus the temperature settings of the extruder barrel sections are slightly higher at the feed end and show a downward gradient as they approach the head of the extruder. Inflation air pressure should be in the typical range of 50 to 100 psi.

Suggested temperature settings for extrusion blow molding MPR are shown in table 1. [Tabular Data Omitted]

Parison die and accumulator head designs are also important for MPR. To maintain the well-fluxed melt developed in the extruder, the head should be of a small volume capacity as is reasonable, have a smooth, continuous contour and be individually temperature controlled. Large volume heads or heads without smooth contours can lead to holdup of material and subsequent deterioration of polymer performance.

Chilled [21 [degrees] C-49 [degrees] C (70 [degrees] F-120 [degrees] F)] molds are recommended when extrusion blow molding MPR. Parts develop strength rapidly in the absence of shear and have no tendency to stick to the mold. Thus, MPR parts with excellent surface finish and mold detail can quickly and easily be demolded.

Injection blow molding

The first step in injection blow molding is to injection mold a parison around a core pin or rod, which serves as a movable mandrel (figure 5). Then the heated, mandrel-supported parison is transferred to the blow mold. The blow mold is closed and air is introduced through the mandrel to expand the parison and form the part.

Successful injection blow molding of MPR requires a machine with capability to:

* Convert unheated pellets into a well-fluxed melt by heating, massing and shearing the MPR pellets.

* Achieve a melt temperature of 160 [degrees] C to 182 [degrees] C (320 [degrees] F to 360 [degrees] F), as measured at the nozzle with a needle pyrometer.

* Rapidly fill the mold through small gates to maximize shear, with generous venting to remove gases generated during injection, facilitating complete mold filing.

The barrel of the injection molding machine should be capable of attaining a melt temperature of at least 160 [degrees] C (320 [degrees] F). Plastics reciprocating scree machines should be equipped with at least three-zone heating control of the barrel for close temperature control and optimum output rates. General purpose, gradual transition screws with compression ratios between 2.5 and 3.5 and an L/D of 20:1 are suitable for molding MPR.

Screws equipped with full flow check rings (valves) or smear tips are recommended. Flow passages must be streamlined to eliminate melt stagnation and subsequent degradation. Ball type check valves are not recommended. Short reverse tapered nozzles (separately heated) with uninterrupted flow patterns are recommended.

The rheology of melt processible rubber makes it ideally suited to the high shear process of injection molding. The combination of barrel heat and shear is necessary to attain a properly fluxed, uniform melt prior to actual injection.

Suggested injection blow molding machine temperature settings and screw speeds for MPR are shown in table 2 [Tabular Data Ommitted]

Actual injection conditions are critical for MPR. Because MPR does not melt, it must flow through a combination of heat and shear. The objective is to generate shear as MPR enters the mold through a combination of restrictive gate design and fast fill rate (high ram speed).

Parts of MPR develop strength rapidly in the absence of shear. Thus, in injection blow molding the parison and mandrel can be transferred rapidly to the blow station. This characteristics of MPR makes it possible to produce quality blow molded parts in relatively short cycle times.

Potential applications for MPR

Blow molding is an excellent technique for producing complex rubber-like parts from melt processible rubber, enabling it to replace a variety of hollow rubber parts currently manufactured by the more costly transfer molding and injection molding processes.

The smallest commercial MPR parts produced by the extrusion blow molding process weighs only 30 grams (1.0 ounce), with dimensions as small as 15.24 centimeters (6 inches) long by 1.27 centimeters (1/2 inch) in diameter and a wall thickness of 0.51 millimeters (0.020 inches). One of the largest blow molded MPR parts, to date, weighed 2.27 kilograms (5 pounds), with dimensions as large as 1.23 meters (4 feet) in length by 0.66 meters (26 inches) in diameter and a wall thickness of 2.0 millimeters (0.080 inches).

Large parts being extrusion blow molded of MPR include well water pressure accumulators, chemical resistant tank liners, and flotation safety devices for pontoon boats. These parts in MPR are easy to collapse and insert into another container, e.g., a metal tank. Included in the smaller extrusion blow molded parts being made from MPR are winter windshield wiper covers and gasoline primer bulbs for outboard motors. Injection blow molded parts of MPR under development include automotive rack and pinion boots and bulbs for antifreeze testing devices.

Summary

Thermoplastic elastomers are capturing new and existing applications for rubber parts, primarily on the basis of more versatile and cost-efficient processing characteristics. Hollow parts have always been a challenge to the thermoset rubber molder. Existing transfer molding and injection molding techniques are time, labor and energy intensive, which translates to high part cost and low productivity. The blow molding of TPEs is clearly a superior process for molding hollow rubber parts, capable of producing high quality parts of excellent appearance and at a much lower unit cost.

By acquiring blow molding technology, the thermoset rubber molder could utilize his knowledge of rubber markets and part performance requirements to effectively compete with plastics molders. Existing business can be preserved and expanded into areas not possible with conventional thermoset rubber molding techniques.

MPR is the TPE most like thermoset rubbers in feel and appearance. The rheological characteristics of MPR make it ideally suited to blow molding. Suggested conditions for processing MPR by any of the commercial blow molding processes ensure that a well fluxed melt be achieved by the plasticating screw and that temperature and streamlining be controlled to ensure the formation of well defined parts.

MPR is being used commercially to make a wide variety of hollow rubber parts that are indistinguishable in appearance from those of a thermoset rubber. These parts are highly functional and resistant to exposure to severe conditions of weather and a wide range of fluids over a service temperature spectrum of from -40 [degrees] C (-40 [degrees] F) to 125 [degrees] C (257 [degrees] F).

PHOTO : Figure 1 - extrusion blow molding of MPR - continuous extrusion

PHOTO : Figure 2 - extrusion blow molding of MPR - intermittent extrusion ram accumulator head system

PHOTO : Figure 3 - extrusion blow molding of MPR - intermittent extrusion Ram accumulator system

PHOTO : Figure - 4 extrusion blow molding of MPR - intermittent extrusion reciprocating screw system

PHOTO : Figure 5 - injection blow molding of MPR

PHOTO : Figure 6 - flow curves for Newtonian and pseudoplastic materials

PHOTO : Figure 7 - effects of shear rate and temperature on the visocisty of MPR 70A molding grade