Multi-component injection-molding of rigid-flexible combinations.

Multi-component injection molding offers ecological and economical advantages. Products manufactured in this highly automated way are often of much higher value (ref. 1) than, for example, parts that are welded or glued afterwards. Moreover, a lavish pretreatment necessary for metals is not needed; the same goes for the use of coupling agents and the considerable effort of handling (ref. 2). A substitution of metals by thermoplastics suggests itself wherever the profitable properties of metal are not fully exploited. Due to this, the two-component injection molding technique has made considerable progress in recent years. Especially combinations of rigid and flexible materials are of major interest to the industry (ref. 3). Up to now, two-component injection molding is mainly used for the production of thermoplastic-thermoplastic combinations and combinations of thermoplastic elastomers (TPE) and thermoplastics. Thermoplastic elastomers have a lot of advantages for the processor, for example, easy, fast and automated processing. Due to the various TPE-types available, these materials can be found in many applications like household appliances, toys, power tools, automotive parts, etc. Concerning high temperatures, chemical resistance, physiological safety and long term mechanical properties (creeping, dynamic load, etc.), the application of thermoplastic elastomers is limited. In this respect, curing rubbers and liquid silicone rubbers (LSR) offer new possibilities for two-component injection molding. Due to their properties, these classes of materials allow extended applications of rigid-flexible combinations (ref. 2) and can substitute rubberr-metal parts.

Within the rigid-flexible combinations, the rubber-metal combinations are a huge field that has been well researched and documented (refs. 4-7). Most of the publications focus on the adhesion of elastomer on metal, for example by optimizing bonding agents or processing steps like degreasing, sandblasting and primering.

Since the processing of TPE and thermoplastics using the two-component injection molding technique is well known and wide spread in the plastics processing industry, and because of the new possibilities curing elastomers are offering, the investigation described deals with thermoplastics combined with rubber, and focuses on thermoplastics combined with LSR.

Adhesion between the materials

A substantial property of injection molded combination parts is the adhesion between both components. Concerning the adhesion of two polymer partners, there are several findings in hand. More detailed information concerning the individual mechanisms of adhesion in combined formations can be found (refs. 3 and 8). Regarding this topic, technical, chemical and electrostatical processes are involved. Besides the material combination itself, there are many effects and influences on the adhesion regarding adhesive strength of two-component parts. For example, the process parameters have to be taken into account, also the properties of each material, the geometry of the parts and the resulting flow process, as well as the surface quality of the interface between the two components. The aim is to increase adhesion in the interface between the two components to such a level that the failure of the parts under service will no longer happen in the interface, but within the flexible component.

Rubber-thermoplastic combinations

Up to now there is only little specialized literature available concerning rubber-thermoplastics combinations (refs. 4-7 and 9). Nitzsche (ref. 5) examines the adhesion of different thermoplastics (among others POM, PA 6, PA 6.6) and a NR compound by using external bonding agents. Each thermoplastic component underwent a different pretreatment (e.g., sandblasting, chemical pretreatment). In order to avoid uncontrolled deformations within the elastomer component, the thermoplastic rigid component is introduced into the mold and subsequently overmolded by the elastomer.

Very few examinations have been made concerning the so-called "fusion bonding," a preliminary stage of two-component injection molding (ref. 10). Here, the thermoplastic component is overmolded onto the cured elastomer; because of diffusion processes, the two materials join without using external bonding agents. During injection molding, the elastomer insert is heated up and, in addition, is exposed to injection pressures of about several hundred bar. As these influences have to be resisted without any alterations of the part-dimensions, fusion bonding bears a problem with pressure-resistance (refs. 7 and 10). Only Richter (refs. 7 and 9) published research results in the field of two-component injection molding and rubber-thermoplastics combinations, especially combinations using PPE and polyamide 6.12. The flexible component was overmolded onto the rigid component using a rotating mold system similar to the molds used in multi-component injection molding. As the trials show, self adhesive combinations without external bonding agents can be produced in that way.

Our own research (refs. 11 and 12) concerning rubber-thermoplastics combinations suitable for two-component injection molding shows a good adhesion of PA 12 and PA 6.12 on different X-NBRs. The adhesion was tested with the aid of peel tests and tensile tests. In order to find suitable mold concepts for this special material combination, it will be demonstrated how to use computer simulation for the analysis of temperature distribution and curing rate.

Molding and testing rubber/thermoplastics combinations

All thermoplastics used in rubber-thermoplastics combinations should be temperature-resistant. This is important because the process is different in comparison with conventional two-component injection molding of thermoplastics combinations (thermoplastic material is cooled down in the mold; elastomeric material is heated up in the mold). So the production of rubber-thermoplastic combinations by means of the two-component injection molding technique requires more effort than a combination of thermoplastics and thermoplastic elastomers (both materials are cooled down in the mold). Because of the thermal separation necessary within the mold, elastomer compounds with low curing temperatures are best suitable. The two materials combined should be expected to join, as for example, polyamide 6.12 and X-NBR-rubber (carboxylated NBR-rubber)(refs. 7 and 9). Figure 1 shows the test-specimen manufactured in a two-step process. First the polyamide strips were molded, then the strips were cut to fit, inserted into the mold and overmolded with elastomer. Finally, the adhesion strength of the molded specimen was tested with the aid of a peel test.

[Figure 1 ILLUSTRATION OMITTED]

Testing and simulation results of rubber/thermoplastics combinations

Within the scope of these trials (ref. 11), only the compound recipe of the X-NBR-rubber component has been varied. During overmolding, the mold temperature was kept constantly at 160 [degrees] C in all trials. The results of the peel tests and the necessary injection times are shown in figure 2. It can be noticed that the adhesion strength, as well as the necessary injection time, vary according to different compound recipes. Especially different carbon blacks and light fillers have a striking effect, whereas the influence of the other components is not that big. In order to detach the rubber layer of the compounds 4, 8 and 11, peeling forces of 230 N are necessary; compounds 2, 9 and 10 (compounds without light fillers) are already torn apart at 60 N. During peel tests, two distinct properties of the test-specimen have been noticed to be a problem - first their relative shortness and second their thick and stiff elastomer part. As a certain amount of the general length has to be divided by hand in order to fix them into the testing machine, the test specimens are too short to offer sufficient peel length. Moreover, the thickness of the elastomer requires an increased effort in testing because the whole test specimen will deform with comparatively rigid compounds (80 Shore A).

[Figure 2 ILLUSTRATION OMITTED]

Apart from mold design, mold optimization and calculation of local temperatures in mold and part simulations can be of great use calculating the scorch index and the curing rate in dependence of heating time and mold temperatures. As such calculations show, heat transfer within the mold - from the hot side of the elastomers to the cold side of the thermoplastics - cannot be totally prevented, especially as the elastomer generally requires long heating times; even isolation plates would have no satisfying effect. Therefore, it is advisable to use thermoplastics that are temperature resistant. Results are presented in figure 3, where the curing ratio as well as the temperature are depicted as a function of time for different layers over the specimen's thickness. Starting conditions are 100m [degrees] C for the inlaying thermoplastics strip, 120 [degrees] C for the elastomer and 160 [degrees] C as mold temperature. As shown, the thermoplastics reach the temperature of the mold wall in comparatively short time (curves A, B and C). The thermoplastic insert acts as an isolator which reduces the heat flow from the mold wall to the elastomer component to a large extent. This becomes clear by taking a closer look at the asymmetrical curing of the elastomer (punctured curves E, C, D); in layer E the part is cured sufficiently, which cannot be said of layers C and D.

[Figure 3 ILLUSTRATION OMITTED]

The long heating times that are typical for rubber processing can be optimized by the design of the elastomer component; its walls should be as thin as possible. Also, the heating time can be reduced by choosing thermoplastics that are temperature resistant, so that the mold temperature can be raised. Computer simulations have proved to be very helpful. The examinations have shown a good adhesion of polyamide 6.12 and X-NBR-rubbers.

LSR and thermoplastic molded to rigid-flexible combinations

Here too, the first aim is to get suitable self-adhesive LSR-thermoplastic material combinations, and the second one is to determine the optimum set of process-parameters. TPEs have a lot of advantages for the processor, for example an easy, fast and automated processing. The materials are supplied ready for processing, with properties defined and guaranteed by the manufacturer. Liquid silicone rubbers have the same advantages; the only difference is that these materials cure thermally. The polyaddition-reaction happens without the emission of volatile contents, preventing blistering and mold fouling. After having reacted to a material-dependent temperature limit, this reaction takes a very fast course (3 - 7 s/mm wall thickness) compared to conventional rubbers, which in turn leads to shorter cycle times. The raw materials are delivered uncured and ready for processing in two standardized hobbocks, separated as component A and B, storable at room temperature. The material is taken from the hobbocks by a fully automated dosing system, then mixed in a ratio of 1:1 and fed into the screw aggregate. The pot life of the mixed LSR in an injection unit at 25 [degrees] C amounts to several days. The curing reaction takes place at temperatures ranging from 120 to 200 [degrees] C in the injection mold.

LSR and its properties

The LSR standard materials for injection molding are characterized by good thermomechanical properties. They rarely show any modification of their mechanical properties over a wide range of temperature. At -50 [degrees] C they are flexible; at the same time they can bear mechanical loading up to temperatures of 180 [degrees] C, even if the hardness is low. They show good creep resistance, good attenuation properties and a high elastical transfer to reserve. Rigid-flexible parts made of LSR are applicable for constant load at temperatures up to 180 [degrees] C (ref. 3). At this point, an application of TPE-combinations is no longer possible because the processing range of conventional thermoplastic elastomers is already reached here. Because LSR has a high heat resistance, it is possible to use multi-component parts in the automotive industry. At the moment, for example, the whole air intake of the new engine of General Motors' Northstar consists of a multi-component combination part (thermoplastics/LSR) produced in one step (ref. 3). Because they are transparent, tasteless and unscented, as well as free from softeners and physiologically safe, LSR materials can also be used in sanitary facilities, toys, sports articles, food industry and medical technology; applications are to be found among respirators, cannulas, catheters and tubes. Especially in the field of medical technology, LSR competes more and more with latex products, because a growing amount of patients is allergic to latex. Further fields of application are seals for scuba diving, babies' dummies or high voltage insulators (refs. 8 and 14).

Characterizing the adhesive strength of LSR/ thermoplastics combinations

Using test specimens, it will be presented how LSR-thermoplastics combinations are manufactured in two-component injection molding (ref. 15), and which adhesion strengths can be achieved. Furthermore, possibilities to optimize cycle time and adhesive strength will be demonstrated. Because of the high mold temperatures that are necessary for the curing of LSR, the applied thermoplastics have to be temperature-resistant. With regard to the necessary hot-cold-separation in the mold, LSRs that can be cured at low temperatures are ideal for such applications. To be suitable for injection molded parts, the selected material combinations are expected to be adhesive and to complement each other in their material properties. This fundamental aptitude is given in the tested material combinations of polyamide 12 and partially aromatic polyamides, respectively, and self adhesive LSRs (ref. 8). The test specimens (figure 4) are manufactured out of these material combinations in a one-step two-component injection molding process without external adhesion mediators. The utilized injection molding machine has a horizontal thermoplastics aggregate and a vertical silicone aggregate with a hydraulically operated shut-off nozzle. The LSR is supplied with help of a conventional, automatic dosing and conveying system.

[Figure 4 ILLUSTRATION OMITTED]

The utilized injection molding mold system is a core back mold. First the polyamide is injected; a slide (core or sucker pin) closes the cavity section for the LSR. After the slide is pulled out, the liquid silicone rubber is injected onto the polyamide (by means of a separate injection gate) and thermally cured. The disadvantage of this mold concept becomes clear immediately: There is no thermal separation possible. As proved by tests, the optimal mold temperature concerning the tested material combination is 145 [degrees] C. At this level of mold temperature, the tests had been reproducible at a cycle time of 70 s; at the same time, this temperature proved to be not suitable at all for any of the two materials. Regarding the polyamides, the temperature was too high; regarding the LSR, it was too low to get short cycle times for an efficient production.

Mold techniques

Concerning cycle times, the application of rotating mold systems (rotary platforms, rotary molds, rotary crosses and index plate molds) would be an advantage. To start with, the first cavity of the rotating mold system is filled with thermoplastic material. As soon as the thermoplastic has sufficiently cooled down, the mold opens and the rotating mechanism transfers the part into the second cavity; here the elastomer material is injected onto the thermoplastic part. While the curing goes on, the next thermoplastic sample is produced and transferred into the second cavity just after the removal of the finished rubber-thermoplastic part. The transplanting technique is a variation of the rotating systems. The sample is produced in the first cavity and then, with help of a handling device, transplanted into the second cavity, where the second material is injected.

Regarding the thermal separation in rotating mold systems, as for example index plate molds, it is important to know which improvements of quality and cycle times can be achieved and which temperatures or differences in temperature have to be obtained. The tests have already had good results at a difference in temperature of 60 [degrees] C (ref. 15). Therefore, the cycle time can be clearly reduced in thermally separated molds; as the LSR cures very fast, the time needed for the two partial steps can be adapted all the more if one meets the design guidelines (refs. 11 and 16). In order to guarantee a heat flow as high as possible from the mold into the part, the LSR component should have a thin wall and surround the outside of the thermoplastic part. By contrast, internal and deep LSR components where the heat flow can only pass via a small front side should be avoided.

The rotating mold systems have an advantage over the core back technique as far as both cavities can be filled simultaneously; whereas the core back technique allows sequential injection only. Moreover, the rotating technique, in principle, guarantees a better thermal separation of the thermoplastics pre-injection cavity and the elastomer finish-injection cavity. By contrast, in the core back technique, an intermediate opening of the mold is not necessary because of the movable elements within the mold. Which method is suitable and best for which part has to be decided each time and case in terms of technical and economical factors.

Advanced mold design using simulation

With help of computer software for injection molding of thermoplastics and elastomers, a rheological and thermal layout of the mold can be done. Cadmold\Mefisto (ref. 17) can be used to calculate the filling and holding pressure phase of the thermoplastic component; Cadmold\Mefigum (ref. 18) lends itself to calculate the filling and heating phase of the elastomer component. The important thermal calculation can be done by Cadmold\Mex (ref. 19). This program allows a two dimensional calculation of temperatures for a cross-section of mold and part. In addition, variations in geometry and in material can be simulated, the resulting distribution of temperatures can be analyzed and the curing reaction of the elastomer component can be calculated. In other words, the computer software gives evidence of the distribution of temperature in both components, the thermoplastics and the elastomer, which can be correlated with the adhesion of the two components (refs. 3 and 8) in turn.

Examinations of test specimen and testing results (LSR)

Different polyamides and polyesters, as well as several types of liquid silicone rubber, were molded to test specimen. Heating and cooling times were considered as the most important process parameters to be varied. In addition to that, different roughnesses of the rigid component interface have been examined; they had been caused by different roughnesses of the slide. Afterwards, the molded parts were submitted to a standard tensile test (refs. 3, 20 and 21) in order to check the adhesion strength. Before, some of the parts had been post-cured (100 [degrees] C, 0,5 h), so that post-cured and not post-cured parts could be compared with each other.

The results of the tensile tests are shown in figure 5. The measured adhesive strengths of the two component test specimen are drawn in dependence on the different interface roughnesses. It is to be seen that there is no consistent dependence of the adhesive strength from the surface roughness. Concerning the two partially aromatic polyamide types, the adhesion strength rises up to a roughness of 1,2 [micro]m. The two polyamide 12, in turn, reach their highest adhesion strengths at smooth, polished interfaces. This is already known from combinations of thermoplastics and thermoplastic elastomers (ref. 3). Because LSR is a curing system with internal coupling agents, the following tests analyze the influence of post-curing on the adhesion strength. As pre-tests and earlier examinations have shown, post-curing supports both the driving out of volatile contents and a postreaction of the coupling agents. Figure 6 compares post-cured and not post-cured samples of different polyamide-LSR combinations. A highly increasing adhesion strength of the post-cured parts is clearly visible. Without exception, post-curing leads to a high improvement of adhesive strength and so to cohesive fracture. In elastomer processing, post-curing or tempering is often used for post-curing and driving out volatile contents. The longer a part is post-cured with higher temperatures, the more its adhesion strength rises. These results are similar for combinations of similar surface qualities; they show a rising adhesion strength for all combinations by means of post-curing. During the tensile tests it became evident that there is no difference in the fracture-behavior of the two material classes; in every single combination the interface started to tear at a certain point and then slowly peeled off. Therefore, regarding the partially aromatic polyamide pairs, a failing of material is more predictable.

[Figures 5-6 ILLUSTRATION OMITTED]

Good adhesion strength is the condition for non-positive joint combinations; it is to be found in the adhesion of modified LSR types (of Wacker Chemie, Burghausen) to different thermoplastics (for example polyamides and polybutyleneterephthalates - PBTs). An antiadhesive coating of the mold walls is not necessary; the production works with conventional molds. Thermal and rheological simulations prove to be a suitable help calculating the right course of the process as well as the right layout of the mold and part. In cases of flanging with injected seal lips, a weak adhesion will do if it ensures that the seal is not lost. The seal lip only has to stick to the flange until it is installed. Another possibility to avoid adhesion problems is to use undercuts and openings; the resulting positive joint anchorage brings about the cohesion of the part. Examples are shower heads made of PBT-LSR combinations, switch elements (ref. 22), pneumatic parts in central lockings of cars, etc. Disadvantages are, as a rule, higher efforts in mold design, resulting in higher costs for the mold, and at the same time a restricted freedom during the construction.

Conclusions

The examinations show a good adhesion strength between thermoplastics and LSR, respectively, between polyamide 6.12 and X-NBR rubber. Both of these rubber-thermoplastics combination parts can replace rubber-metal combinations and extend the application fields of thermoplastic rigid-flexible combinations (TP-TPE). In the first instance, thermoplastics can replace metal if its properties, for example a high stiffness, are not fully exploited - as it is the matter with assembly helps. In the second instance (TP-TPE), the thermally curing rubber offers advantages if the mechanical and thermomechanical properties and the chemical resistance of the TPEs are not sufficient. The use of LSRs is profitable in many respects. They cure very fast, which on the one hand makes the whole process more economical, and on the other hand makes the thermal separation in multi-component injection molding molds easier because there is no need to level out different temperatures. Furthermore, the problem of mixing and batch variations can be dropped with LSR. In summary, rubber-thermoplastics combinations carry great potential and could be manufactured economically, especially combinations of LSR and polyamide, as well as LSR and polybutylenterephthalate.

References

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(12.) D. Hasenberg, Haftungsuntersuchungen von NBR/ther

Christoph Ronnewinkel and Edmund Haberstroh, Institut fur Kunststoffverarbeitung an der RWTH Aachen