MANUFACTURING PROCESS FOR A MOLDED PART

Description

[0001] The invention relates to a method for producing a molded part comprising a prefabricated insert part and a molded part section molded onto the insert part, wherein the prefabricated insert part has a connection region comprising a first polyamide material, wherein the molded part is produced by providing a polymerizable plastic precursor mixture containing at least a first lactam monomer for producing a second polyamide material, by filling the plastic precursor mixture into a mold, by heating the plastic precursor mixture to a starting temperature T s and by polymerizing the plastic precursor mixture at or above the starting temperature T s in the mold to form the second polyamide material and molding the molded part section, wherein an inner contour of the mold determines an outer contour of the molded part section.

[0002] EP 2 818 297 B1 discloses a method for producing molded parts with an insert element. This insert element is a polyamide-based reinforcement structure that provides local stabilization within the corresponding molded part. The manufacturing process is based on injection molding.

[0003] DE 10 2011 009 748 A1 also describes a manufacturing process for multilayer plastic components, which can be implemented as a rotational molding process. The process is based on a two-stage procedure: First, a polymer layer is created by anionic polymerization of a first polymer starting mixture under protective gas. Subsequently, a second polymer starting mixture is applied to the interface of this polymer layer together with a copolymerizable impact modifier and polymerized. However, this process does not involve the integration of a prefabricated insert.

[0004] DE 10 2014 106 998 A1 discloses a rotational molding process in which a different shape of an insert element is introduced into the rotational mold. This insert element is not a mechanically defined component like a strut, but rather an impregnable or absorbent material. This material locally absorbs the molten plastic compound, resulting in a targeted and locally defined increase in the wall thickness of the molded part.

[0005] The publication DE 10 2017 130 747 A1 describes a rotational molding process for producing a molded part into which a prefabricated strut is incorporated. The strut is inserted into a rotational mold and secured, with its first and second ends resting or nearly resting against the inner wall of the mold. The special aspect of D1 is that the rotational mold has a projection in a connecting region between the two ends of the strut. During the subsequent rotational molding of the starting material, a molded part is produced that has a firm and positive connection to the strut at three positions, namely at both ends and at the indentation in the molded part wall formed by the projection. This third position leads to a mechanical reinforcement of the strut's connection.

[0006] DE 10 2009 040 930 A1 discloses a generic method in which an insert in the form of a support element is connected to the wall of a container during its manufacture. To manufacture the container, the support element, prefabricated from a thermoplastic material, for example, polyethylene, is mounted in a rotational mold, which is subsequently filled with the same plastic, for example, polyethylene. The rotational mold is set in biaxial rotation and heated, whereby the wall of the container is formed by thermoplastic primary forming and the support element is sintered around the plastic material, at least in some areas.

[0007] Such a process allows a special functional area to be provided on a molded part, even though this functional area cannot be realized at all in the manufacturing process under consideration (primary molding process), or at least not with the desired design features or tolerances, or in the desired position or orientation. The special functional area can nevertheless be realized by manufacturing it at least separately and, in particular, by another process (such as an injection molding process or additive manufacturing) and then using it as a prefabricated insert during the production of the molded part, to which the remaining sections or regions of the molded part are molded "in situ," so that the prefabricated insert forms an integral part of the final molded part.

[0008] A disadvantage, however, is that the advantages of such a process cannot yet be transferred to the production of molded parts where the plastic material intended for the molded part section cannot be processed thermoplastically at all or only with the acceptance of technically and / or economically suboptimal results. Such cases can arise, for example, when the molded part section is to be made of a cross-linked plastic, in particular a highly cross-linked plastic (thermosets), or when the intended plastic is a thermoplastic but has a very high melting temperature, so that the processing temperatures required for thermoplastic primary forming are high and thermoplastic processing is disadvantageous, for example because it would damage the plastic itself or have a detrimental effect on the service life of the mold.

[0009] The invention is therefore based on the object of providing an alternative method for producing a molded part comprising a prefabricated insert and a molded part section molded onto the insert. In particular, it appears essential to no longer rely on a method in which the primary forming of the molded part section to be molded is based on thermoplastic processing.

[0010] This object is achieved according to the invention by a method comprising the features of claim 1. In this case, the following method steps are carried out in detail: (a) Providing the prefabricated insert part comprising a bonding region comprising a first polyamide material, (b) Providing a polymerizable plastic precursor mixture containing at least one first lactam monomer for producing a second polyamide material, (c) Heating the bonding region to an activation temperature TA , (d) Filling the plastic precursor mixture into a mold, (e) Heating the plastic precursor mixture to a starting temperature TS , (f) Polymerizing the plastic precursor mixture at or above the starting temperature TS in the mold to form the second polyamide material and molding the molded part section, wherein an inner contour of the mold determines an outer contour of the molded part section, (g) Arranging the insert part on or in the mold such that the bonding region comes into contact with the plastic precursor mixture at least temporarily during polymerization,(h) forming a connection between the molded part section and the connection area of ​​the insert part.

[0011] The features of the method according to the invention are also referred to as "method steps" or "steps" within the scope of this invention and are labeled alphabetically for ease of reference. However, this is expressly not intended to imply a specific order of the method steps. Rather, each of the method steps mentioned above or explained below can, as far as technically possible and expedient, be carried out before and / or during and / or simultaneously with and / or after another method step. The mention of a component in the singular also includes several similar components, as far as technically possible and expedient.

[0012] It is crucial that, in addition to the aforementioned process steps, the activation temperature TA is at most 50°C lower than a first melting temperature (polymer melting temperature) T P1 of the first polyamide material. Furthermore, the activation temperature TA is at most 50°C, preferably at most 30°C, and very preferably at most 15°C higher than the first melting temperature T P1 . Particularly preferably, the activation temperature TA is at most as high as the first melting temperature T P1 . This minimizes or avoids adverse effects, such as thermal or thermo-oxidative degradation of the first polyamide material and / or deformation of the bonding region or the entire insert.Preferably, the activation temperature TA, while maintaining one of the aforementioned maximum values, is at most 40°C, preferably at most 30°C, particularly preferably at most 20°C, and most particularly preferably at most 10°C lower than the first melting temperature T P1 of the first polyamide material. Surprisingly, activation temperatures TA below the first melting temperature T P1 already provide the prerequisite for a stable, but above all, materially bonded, connection between the connection region and the molded part section (see steps (f) and (h).This can be attributed to the fact that polymer chains at the heated surface of the bonding area already acquire a certain degree of mobility, leading to the formation of a (semi-)interpenetrating network and / or an actual chemical bond between the polymer chains of the two polyamide materials of the prefabricated insert on the one hand and the molded part on the other. The first melting temperature T P1 can be determined according to ISO 11357-3.

[0013] The prefabricated insert provided in process step (a) can have any technically feasible shape and can be designed, for example, as a grommet, nozzle, fastening tab, filter bowl, quick-release fastener, coupling, thread, bayonet, flange ring, or contoured part. The insert can be made at least partially or even predominantly of another plastic or another material (e.g., metal, ceramic), as long as at least the connection area is made of the first polyamide material.

[0014] The prefabricated insert part and / or the connection area is / are preferably not manufactured in the mold used to produce the molded part. In this case, production takes place in a mold other than that mentioned in steps (d), (f), and (g) or in a manufacturing process without a shaping mold, for example in an additive manufacturing process or by mechanical processing of a semi-finished product. Rather, the production of the connection area or the complete insert part can in principle be carried out by any of the primary forming or forming processes known in plastics processing, for example by an injection molding process, a 3D printing process, a gravity casting process, a rotational process, a blow molding process, an extrusion process, or a thermoforming process. Production by mechanical processing of a semi-finished product is also conceivable.This enables the use of insert parts that are not accessible using the mold and expands the technical possibilities in the design of the molded part.

[0015] The first polyamide material comprises at least one plastic with polyamide bonds in a polymer main chain and / or in polymer side chains. For example, the first polyamide material can comprise polyamide 6, polyamide 11, polyamide 12, polyamide 66, or polyamide 69, but its definition also includes a copolymer or blend comprising two or more of the aforementioned polyamides.

[0016] The first polyamide material preferably has the lowest possible moisture content so that the polymerization according to step (f) is not disrupted. It is therefore preferred if the insert or the connection area is pre-dried.

[0017] The first polyamide material may contain additives such as impact modifiers, reinforcing fillers (glass fibers, carbon fibers and polymer fibers), non-reinforcing fillers (for example calcium sulfate, calcium carbonate, barium sulfate, silicates such as wollastonite, kaolin, mica, hornblende, quartz, glass spheres and PTFE), flame retardants (e.g. phosphorus compounds such as organic phosphates, phosphates, ammonium polysulfates and halogenated organic compounds), mold release agents (e.g.Silicone oils and high-melting waxes), lubricants and slip agents (e.g. paraffinic oils), antistatic agents (e.g. quaternary ammonium salts), agents for increasing thermal and / or electrical conductivity (carbon black, metals, metal oxides and carbon nanotubes), dyes, pigments, stabilizers (e.g. UV stabilizers such as benzophenone derivatives, hydrolysis inhibitors such as carbodiimides, polycarbodiimides, antioxidants such as sterically hindered amines and phenols), plasticizers (e.g. adipates and phthalates), anti-blocking agents (e.g. paraffin, polyethylene and montan waxes), thickeners (e.g. vinyl aromatic diene copolymers) and processing additives such as reactive retarders and nucleating agents such as molybdenum sulfide.

[0018] The polymerizable plastic precursor mixture provided in process step (b) may optionally contain, in addition to the first lactam monomer, further components, for example one or more of the additives mentioned above with reference to the first polyamide material, wherein in the case of the plastic precursor mixture only those additives are suitable which do not inhibit the polymerization.

[0019] The heating of the connection region in process step (c) preferably takes place before the connection region comes into contact with the plastic precursor mixture. The inventors have found that heating activates the first polyamide material for forming a bond to the molded part section. The heating of the connection region can take place outside the mold before step (g) or inside the mold after step (g), with a heating device in the form of a radiant heating element, for example a conventional infrared radiator or a suitable laser source, or a contact heating element being suitable. The connection region can also be designed to be heatable itself, for example by embedding a resistively or inductively heatable heating element in the first polyamide material of the connection region.

[0020] During heating, energy is preferably introduced, at least predominantly, directly into a surface section of the connection area that comes into contact with the plastic precursor mixture in step (g). By directly heating the surface relevant for the connection to the molded part section, the activation temperature TA is reached there quickly and in a way that protects the material. In contrast, introducing energy elsewhere on the insert part is disadvantageous, since the low thermal conductivity of the first polyamide material, which is typical for plastics, requires long heating times and / or strong local overheating, which can lead to material damage.

[0021] The plastic precursor mixture can be introduced into the mold in step (d) before, simultaneously with, or after heating the bonding area (step (c)). For this purpose, a plastic precursor mixture already provided outside the mold in step (b) can be introduced. In this case, step (d) takes place after step (b). Alternatively, the components of the plastic precursor mixture can be introduced into the mold either individually and / or in the form of two or more premixes, each in parallel or one after the other. In these cases, where the components are only combined in the mold, the plastic precursor mixture is also provided in step (b) at the same time as it is introduced in step (d).

[0022] The heating of the plastic precursor mixture according to step (e) can take place after step (d) within the mold. For this purpose, the mold can be designed to be heatable or, for example, arranged in an oven. Alternatively, the heating of the plastic precursor mixture can take place outside the mold before the plastic precursor mixture, heated to the starting temperature Ts, is introduced into the mold. In this case, too, the mold is preferably designed to be directly or indirectly heatable and is preferably preheated to at least the starting temperature Ts before the plastic precursor mixture is introduced.

[0023] The mold can be, for example, a die casting mold or a reaction injection molding mold. The mold can also be a rotary mold, which rotates at least temporarily about a main axis during polymerization and preferably additionally about a second secondary axis intersecting the main axis. In the latter case, a molded part in the form of a hollow body can also be produced using the method according to the invention.

[0024] During the polymerization of the plastic precursor mixture according to process step (f), the first lactam monomer in the mold in situ the second polyamide material, with the molded part section being formed simultaneously. The plastic precursor mixture can be heated to the starting temperature Ts either before being poured into the mold or only in the mold. This temperature is required for polymerization to proceed at a sufficient rate.

[0025] To arrange the insert part according to process step (g), the insert part can be attached directly or indirectly to the mold, or at least fixed relative to the mold, in such a position and orientation that the connection area of ​​the insert part is exposed in an interior of the mold. The arrangement of the insert part in the mold can take place before, during, or after the plastic precursor mixture is poured into the mold.

[0026] In process step (h), a bond is formed between the molded part section formed in step (f) and the connection area of ​​the insert. This bond is at least partially cohesive. Within the meaning of the invention, a non-detachable bond is formed through fusion and / or the establishment of chemical bonds and / or intermolecular or chemical bonding forces, which is held together by atomic and / or molecular forces.

[0027] A material bond within the meaning of the invention arises in the manufacturing process of the molded part, while the molded part section is initially formed and in situis formed onto the connection area of ​​the insert part. This material bond must therefore be distinguished from other material bonding methods in which two (each pre-produced) joining parts are joined together, namely adhesive bonding and thermoplastic welding. This can also be seen in the finished product. In contrast to adhesive bonding, no additional material (adhesive) is used between the connection area of ​​the insert part and the connected connection area of ​​the molded part section. In contrast to welding, no seam (weld seam) in the sense of bead formation occurs. While in heated element welding, for example, bead formation occurs due to mushrooming (deformation) of the joining surfaces of the joining parts pressed against the heating elements and / or against one another, products can be manufactured without such bead formation using the process according to the invention.

[0028] Some limitations that limit the combinations of possible joining partners, for example, in laser welding, can be circumvented by the method according to the invention. For example, in the case of direct laser welding, at least one joining partner must be made of a material with a very high absorption capacity for the infrared laser light of the commonly used diode or Nd:YAG lasers. In particular, two amorphous and transparent, but also two semicrystalline and translucent, yet uncolored / natural-colored plastics cannot be easily joined by direct welding.At least one of the two joining partners must then be provided with a suitable IR or NIR absorber (IR: infrared; NIR: near infrared), either in the form of a surface coating in the joining area or in the form of an additive, whereby the (N)IR absorber must also exhibit as little absorption as possible in the visible spectral range (i.e. be essentially colorless). In the case of laser transmission welding, one joining partner must consist of a material with a very high absorption capacity for the laser light used, while at the same time the material of the second joining partner, which is irradiated during transmission welding, must be as transparent as possible for the (N)IR laser light. Two colored plastics cannot therefore simply be joined together using transmission welding. This applies in particular to pigment-colored plastics, especially plastics colored black by the addition of carbon black, for example.In this case, a special, laser-transparent colorant must be used for the material of the second joining partner to be irradiated. The above-mentioned (color) combinations are accessible using the process according to the invention without the need for special (N)IR-absorbing additives or (N)IR-transparent colorants.

[0029] While in thermoplastic welding processes, the respective material of the joining parts involved must be locally heated well above their respective melting temperature, the first polyamide material of the insert part can be activated even below its melting temperature, so that the material-to-material bond in the process according to the invention can be achieved without local melting of the first polyamide material in the connection area. The differences or similarities in the thermal history of the material of a joining part in the immediate joining area, on the one hand, and in a region of the joining part spaced apart from the joining area, on the other hand, which can be determined, for example, by DSC examinations, allow conclusions to be drawn about the joining process used.Microscopic examinations of cut surfaces or microtome sections through the joining area, for example using optical microscopy, fluorescence microscopes, atomic force microscopy (AFM) or scanning electron microscopy (SEM), also allow a differentiation between the different joining processes.

[0030] The product of the process according to the invention is the molded part, which comprises the insert part and the in situ molded-on molded part section. By providing a bonding region made of a (first) polyamide material in step (a) in conjunction with the use of a first lactam monomer in step (b), which polymerizes to form a (second) polyamide material, a basic compatibility is achieved between the material of the bonding region and the material of the molded part section, which enables a good bond.

[0031] The method can, for example, be carried out in such a way that the prefabricated insert part is first provided and its connection region is brought to the activation temperature TA (process steps (a) and (c)). The insert part is then arranged in or on the mold (process step (g)). In parallel or subsequently, the plastic precursor mixture is provided, brought to the starting temperature Ts and poured into the mold, which is preferably preheated to at least the starting temperature Ts (process steps (b), (e), (d)). The plastic precursor mixture polymerizes in the mold, whereby the molded part section is formed (process step (f)). At the same time, the connection between the connection region and the molded part section is formed (process step (h)). The molded part can then be removed from the mold, in particular after prior cooling.

[0032] Preferably, the plastic precursor mixture contains an anionic catalyst and an activator. The polymerization is then an activated anionic polymerization, which allows high polymerization rates and short cycle times to be achieved, making the process efficient overall.

[0033] In principle, strong bases can be used as anionic catalysts for the polymerization of lactam monomers. For example, bases based on organometallic compounds, such as sodium alkoxides and sodium amides, can be used as catalysts. The use of alkali metals, alkali metal salts of lactams (e.g., sodium caprolactamate), alkali metal alkoxides, alkali metal hydroxides, and, particularly preferably, alkali metal hydrides is also particularly advantageous. Also suitable are alanates according to the formula M[AlH x (OR) 4-x ] n , where: M = metal of group Ia or IIa of the periodic table, R = CH 2 CH 2 OR' with R' = C1-C4 alkyl, x = 1 to 4, n = valence of M.

[0034] Examples are Na-bis-(2-methoxy-ethoxy)dihydridalanate or Na-tetra-(ε-caprolactam)alanate.

[0035] In principle, all acylating compounds can be used as activators, either directly or after reaction with the respective lactam monomer. These include, for example, acid anhydrides and acid halides (especially acid chlorides), organic peroxides, esters, lactones, ureas and thioureas, as well as linear or cyclic isocyanates, such as hexamethylene-1,6-bis-carbamido-caprolactam, and allophanates or mixtures of these substances. Low-molecular-weight polymers, often referred to as prepolymers, with N-acyllactam end groups can also be used. The use of acylcaprolactam, polycarbodiimides, and mono- and polyisocyanates and their derivatives is particularly advantageous.

[0036] Preferably, the starting temperature TS (see process step (e)) is at least as high as a monomer melting temperature T M1 of the first lactam monomer and at least 10 °C lower than a second melting temperature (polymer melting temperature) T P2 of the second polyamide material. While maintaining the above-mentioned upper limit, the starting temperature Ts can also be in particular at least 10 °C, preferably at least 20 °C, more preferably at least 30 °C, and especially preferably at least 40 °C higher than the monomer melting temperature T M1 . The first lactam monomer is then in the molten state, the plastic precursor mixture is flowable overall, and polymerization starts quickly. The monomer melting temperature T M1 of the first lactam monomer and the second melting temperature T P2 can be determined, for example, by means of differential scanning calorimetry (DSC) according to ISO 11357-3.

[0037] In a preferred embodiment, the provision of the plastic precursor mixture according to process step (b) comprises the process steps: Providing a first monomer mixture comprising the first lactam monomer and an alternative component which is selected as a component from a group consisting of the two alternatives catalyst and the activator, wherein the first monomer mixture is preferably provided in the form of a first melt, Providing a second monomer mixture comprising the first lactam monomer and / or a second lactam monomer and comprising the other alternative component which is the respective other component from the group consisting of the catalyst and the activator, wherein the second monomer mixture is preferably provided in the form of a second melt as an alternative or in addition to the first monomer mixture, Combining the first monomer mixture and the second monomer mixture.

[0038] By providing monomer mixtures, particularly in the form of melts, the components of the plastic precursor mixture can be easily handled before their final preparation, while the separation of catalyst and activator, which are thus never contained together in a monomer mixture, prevents premature onset of polymerization.

[0039] The first monomer mixture and the second monomer mixture can be combined before the plastic precursor mixture is poured into the mold, for example, in a mixing container upstream of the mold. Alternatively, the first monomer mixture and the second monomer mixture can also be combined in the mold itself.

[0040] In a further embodiment of the method, the plastic precursor mixture is held at or above a waiting temperature TW for a time (waiting time) tW before the bonding region of the insert part first comes into contact with the plastic precursor mixture. The waiting temperature TW is at least as high as the monomer melting temperature T M1 , in particular at least as high as the starting temperature TS . In other words, the waiting time tW begins when the provided plastic precursor mixture first meets the temperature criterion of reaching the waiting temperature TW, and it ends when the plastic precursor mixture and the bonding region first come into contact with each other.The plastic precursor mixture can have a constant temperature during the waiting time or a temperature that varies over time, with a temperature profile having at least one minimum and at least one maximum, whereby the waiting temperature TW is not undercut during the waiting time tW. From a chemical point of view, the start of the waiting time tW essentially marks the point in time at which all the prerequisites for the onset of polymerization are met, so that the plastic precursor mixture is already polymerizing during the waiting time tW. The waiting time tW can therefore also be referred to as the pre-polymerization time in the sense of polymerization before the first contact between the plastic precursor mixture and the bonding area. The term "plastic precursor mixture" therefore also includes the polymerizing or partially polymerized plastic precursor mixture, including those formed or created during the polymerization.forming portions of oligomers and / or polymers.

[0041] Since the polymerization of lactams is exothermic, the temperature of the polymerizing plastic precursor mixture rises during the waiting time t W , at least when these monomers are used. If the plastic precursor mixture spends at least part of the waiting time t W in the mold preheated to a temperature above the starting temperature TS, it is additionally heated by the mold during the waiting time t W . By providing a waiting time t W , the plastic precursor mixture hits the bonding area of ​​the insert at a higher temperature. This supports its heating to the activation temperature TA or, if the starting temperature TS is below the activation temperature TA, the bonding area is cooled at least to a lesser extent on first contact than would be the case without the waiting time (pre-polymerization time) t W .

[0042] The waiting time t W can be at least 10 seconds, preferably at least 30 seconds, in particular at least 90 seconds, more preferably at least 120 seconds. It is advantageous if the waiting time t W is at most as long as the time required by the plastic precursor mixture (from the start of the waiting time t W ) to reach a viscosity of 5000 mPa s, preferably to reach a viscosity of 2000 mPa s, particularly preferably to reach a viscosity of 1000 mPa s. The viscosity can be determined according to ISO 2555:2018. At such viscosities, embedding the bonding region in the polymerizing plastic precursor mixture is well possible from a rheological point of view. At the same time, the polymerization has then not progressed too far, so that a bond can still form between the bonding region and the molded part section.

[0043] The first polyamide material can be cross-linked. Cross-linking the polymer chains improves the heat resistance of the bonding area. This allows for the use of a slightly higher activation temperature (TA) without causing excessive deformation of the bonding area.

[0044] In a preferred embodiment of the process, the first lactam monomer is a caprolactam or a laurolactam. Caprolactam (melting temperature approx. 70°C) is the monomer of polyamide 6 (melting temperature approx. 218-222°C), and laurolactam (melting temperature approx. 150-153°C) is the monomer of polyamide 12 (melting temperature approx. 178-182°C).

[0045] When using the lactams mentioned, the second polyamide material accordingly comprises or is a polyamide 6 or a copolymer or a blend comprising polyamide 6 or a polyamide 12 or a copolymer or a blend comprising polyamide 12. These polyamides have good performance properties and, compared with other thermoplastics, have relatively high application temperatures.

[0046] In a preferred embodiment of the process, the second lactam monomer is a caprolactam or a laurolactam.

[0047] In a further embodiment of the method, the first melting temperature T P1 of the first polyamide material is lower than a second or the second melting temperature T P2 of the second polyamide material. In particular, the first melting temperature T P1 is at least 15°C lower, preferably at least 20°C lower, more preferably at least 25°C lower, more preferably at least 30°C lower, more preferably at least 40°C lower than the second melting temperature T P2 . The melting temperatures T P1 and T P2 can be determined according to ISO 11357-3.

[0048] The inventive relationships between the first and second melting temperatures T P1 and T P2 lead to the advantages explained below. As the first melting temperature T P1 decreases, the absolute activation temperature TA (specified above relative to the first melting temperature T P1) to which the connection region of the insert part must be heated in step (c) also decreases to the same extent. This makes it easier to activate the connection region for forming a connection to the molded part section. Further advantages arise in connection with the embodiment described below, which, however, is also advantageous independently thereof.

[0049] Temperature relationships according to the invention can be achieved by suitable selection of the first polyamide material and the first lactam monomer. Starting, for example, with ε-caprolactam as the first (and only) lactam monomer, polyamide 6 (melting temperature approx. 220°C) is formed as the second polyamide material. Polyamide 11 (melting temperature approx. 190°C) or polyamide 12 (melting temperature approx. 180°C) can then advantageously be used as the first polyamide material. Copolyamides, such as PA6 / 66, PA6 / 12, or PA6 / 69, are also particularly suitable as the first polyamide material in this regard. The melting temperatures of copolymers can be adjusted via their monomer mixing ratio, whereby values ​​below the two melting temperatures of the corresponding homopolymers can also be achieved. For example, the melting temperature of a PA6 / 12 copolymer with a molar monomer mixing ratio of caprolactam:laurolactam of 50:50 is approximately 138°C.

[0050] In a further embodiment of the process, a maximum polymerization temperature T Pmax is lower than a second or the second melting temperature T P2 of the second polyamide material. The term "polymerization temperature" refers to the temperature of the plastic precursor mixture or of the second polyamide material forming or formed from the plastic precursor mixture during the process.

[0051] Such temperature control avoids or reduces adverse effects on the second polyamide material (e.g., degradation) and / or the mold (e.g., reduced service life due to repeated large temperature changes). In particular, melting of the second polyamide material formed during polymerization is minimized or avoided, so that the molded part can be removed from the mold after steps (f) to (h) quickly and without an unnecessarily long cooling phase, because the second polyamide material does not reach a melt state and therefore, at least for solidification, no cooling is required. This has a positive effect on the cycle time and the energy efficiency of the process.

[0052] Particularly in connection with the embodiment described above, it is advantageous if the first and second melting temperatures T P1 and T P2 have the preferred temperature relationships explained above. This opens a temperature window between the first melting temperature T P1 and the second melting temperature T P2 , which makes it easier, on the one hand, to sufficiently activate the first polyamide material, whose minimum activation temperature TA depends on the first melting temperature T P1 , and, on the other hand, to simultaneously conduct the process at sufficiently low polymerization temperatures, in particular below the second melting temperature T P2 , which is accompanied by the advantages described above.

[0053] In a preferred embodiment of the method, the connection area is heated by a heating device, preferably arranged on or in the mold. The use of the heating device allows the heating of the connection area to be controlled. Furthermore, this enables reliable heating of the connection area to a specific temperature over a selectable heating time.

[0054] In a further embodiment of the method, the heating device is in contact with the connection region by means of a contact region during a contact time t K and is moved relative to the connection region at the end of the contact time t K and at least partially releases it for contact with the plastic precursor mixture.

[0055] The contact area can be part of a heating element or heating element of the heating device. Contact heating is particularly efficient and easy to control. The contact time (heating time) depends on the size and mass or heat capacity of the contact area on the one hand, and the (target) temperature Ttarget, heat output, size and mass or heat capacity of the heating element on the other, and can be determined for the respective conditions through preliminary tests.

[0056] In a further embodiment of the method, a target temperature T Soll of the heating device during heating of the bonding region is at most 50°C lower than the first melting temperature T P1 . This allows the advantages explained above with regard to the (minimum) activation temperature TA to be achieved.

[0057] Preferably, the target temperature T Soll is at most 40°C, more preferably at most 30°C, more preferably at most 20°C, and more preferably at most 10°C lower than the first melting temperature T P1 . Very preferably, the target temperature T Soll is at least as high as the first melting temperature T P1 , in particular at least 10°C higher than the first melting temperature T P1 . Higher target temperatures T Soll generally enable faster heating of the bonding region. However, it is advantageous if the target temperature T Soll is at most 100°C higher than the first melting temperature T P1 , preferably at most 80°C higher, more preferably at most 65°C higher.

[0058] Favourable ranges for the target temperature T Soll are therefore limited by any combination of one of the minimum values ​​specified above relative to the first melting temperature T P1 with one of the maximum values ​​specified above relative to the first melting temperature T P1.

[0059] Compliance with the previously specified minimum values ​​of the target temperature T target enables the formation of the connection between the connection area of ​​the insert and the molded part section within the scope of the inventive method. Compliance with the previously specified maximum values ​​of the target temperature T target prevents damage to the first polyamide material.

[0060] The method according to the invention is described below using several non-limiting embodiments, whereby the materials and components used are first explained: Examples

[0061] The insert was a round rod made of uncolored polyamide 6 (the first polyamide material) with a diameter of approximately 7 mm. A flat end of the round rod formed the connection area. A centered hole was provided in the round rod, terminating at a distance of 0.5 mm from the flat end at the connection area of ​​the round rod, for accommodating a temperature sensor connected to a data logger. The temperature sensor could thus be positioned directly at the connection area.

[0062] To produce the plastic precursor mixture, two monomer mixtures were initially prepared. The first monomer mixture contained 500 ml of liquid (i.e., molten) ε-caprolactam (moisture content ≤ 0.01 wt.%), 14 g of an activator preparation (hexamethylene-1,6-bis-carbamido-caprolactam in caprolactam, activator C20P from Brüggemann, Heilbronn), 0.3 g of wax (Licowax E from Clariant Plastics & Coatings GmbH, Frankfurt), and 4 g of lamp black, particle size < 50 µm. The wax served as a demolding aid. The carbon black was added solely to facilitate examination of the transition region on the finished molded part between the (uncolored) first polyamide material of the bonding area and the (colored) second polyamide material of the molded part section formed from the plastic precursor mixture.The second monomer mixture contained 500 ml of liquid ε-caprolactam and 20 g of a catalyst preparation (sodium caprolactamate in caprolactam, Addonyl Kat NL catalyst from Lanxess, Cologne). The monomer melts were kept at 100°C under exclusion of air and moisture until use.

[0063] A controlled, electrically heated aluminum casting mold was used as the molding tool. The interior space for the plastic precursor mixture was shaped like an upright cylinder with an inner diameter of 22 mm and a height of 20 mm. The molded parts or molded part sections formed in this tool were therefore cylinders with a diameter of approximately 22 mm and a maximum height of approximately 20 mm, depending on the amount of material added.

[0064] A controlled electrically heated heating stamp was used to heat the connection area.

[0065] In each of the exemplary embodiments, the process described below was followed. The column numbers refer to Table 1 below, which provides an overview of the process parameters and results.

[0066] The mold was heated to 165°C, see column (F). The round rod, equipped with a temperature sensor, was placed upright with its flat end on the heating die, which had been heated to the target temperature T target , see column (B). The round rod was fixed relative to the heating die, and the heating die and the round rod were surrounded by a shielding element in the contact area to minimize convection losses. Subsequently (timed in conjunction with the previous step and the following step), measured amounts of the first and second monomer melts, heated to 100°C, were added to the preheated mold in a ratio of 1:1 and mixed rapidly, thus combining them to form a plastic precursor mixture with a starting temperature TS of 100°C, column (E).After the heating time (column (C)) of the connection area had elapsed (timed in conjunction with the previous step), the round rod was removed from the heating die and secured in a holder such that it was positioned concentrically in the die and immersed 3 mm deep in the plastic precursor mixture. The molded part was removed from the die 10 minutes after the monomer melts had been added. At this point, solid polyamide 6 (as the second polyamide material) had formed from the liquid plastic precursor mixture in the form of the second molded part section, into which the connection area of ​​the round rod was embedded.

[0067] The strength of the connection between the round rod and the molded part section formed from the plastic precursor mixture and molded onto the end of the round rod was then determined using a tensile testing machine. For this purpose, the molded part produced using the process was fixed in appropriate holding elements at the free end of the round rod on the one hand and at the cylindrical molded part section on the other. An increasing tensile force was applied until either a necking or a fracture occurred in the area of ​​the round rod, or the connection area separated from the molded part section. The maximum tensile force achieved is shown in column (G). Table 1: (A) (B) (C) (D) (E) (F) (G) Example Target temperature T Target heating stamp Heating time connection area Temperature connection area at the end of the heating time (=activation temperature TA ) Starting temperature TS plastic precursor mixture Temperature of the mold when adding the plastic precursor mixture Maximum achieved pulling force Nr. °C S °C °C °C kN 1 200 210 175 ± 2 100 165 2,2 ± 0,3 2 210 210 185 ± 2 100 165 2,4 ± 0,2 3 220 210 195 ± 2 100 165 2,6 ± 0,05 4 240 210 220 ± 3 100 165 2,6 ± 0,06 5 260 210 225 ± 3 100 165 2,6 ± 0,04 6 280 210 235 ± 3 100 165 2,6 ± 0,04 R1 -- -- Room temperature 100 165 no connection R2 100 210 78 ± 2 100 165 < 0,2...0,4

[0068] The waiting time t W (pre-polymerization time) between the mixing of the monomer mixtures in the mold and the first contact of the round rod with the plastic precursor mixture was approximately 10 seconds in each case.

[0069] The polymerization temperature, ie the temperature of the plastic precursor mixture or of the second polyamide material forming or formed therefrom, remained below approximately 190°C (T Pmax ≈ 190 °C) throughout the entire process and thus significantly below the melting temperature T P2 of the second polyamide material (polyamide 6).

[0070] For Examples 1 to 6, as well as R1 and R2, shown in Table 1, five process runs were each carried out with identical parameters. The activation temperature TA, column (D), and the results relating to the strength of the connection between the insert part and the molded part section, column (G), are each shown as mean values ​​in Table 1. Reference Examples R1 and R2 relate to processes not according to the invention in which no stable connection was achieved between the connection area (flat end of the round rod) and the molded part section produced in the die casting tool. The embodiments of the process according to the invention enable the production of stable molded parts, see Examples 1 to 6, column (G).

[0071] A molded part 10 produced according to the method described above at a heating stamp temperature T of 220°C (ie according to Example No. 3) after carrying out the tensile test described above is in Figure 1 shown in a substantially axial sectional view. The molded part 10 comprises the (uncolored) prefabricated insert part 12 in the form of a round bar and the (black-colored) molded part section 14. The connection area 16 of the insert part 12 is embedded in the material of the molded part section 14 and connected to the molded part section 14. The round bar 12 has a constriction 18 caused by the loading during the tensile test.

Claims

1. A method for producing a molded part (10) including a prefabricated insert part (12) and a molded-part portion (14) molded onto the insert part (12); the prefabricated insert part (12) having a connection region (16) including a first polyamide material; the molded part (10) being produced by providing a polymerizable plastic precursor mixture containing at least a first lactam monomer for producing a second polyamide material, introducing the plastic precursor mixture into a mold, heating the plastic precursor mixture to a start temperature (TS), and polymerizing the plastic precursor mixture in the mold, at or above the start temperature (TS), such that the second polyamide material is formed and the molded-part portion (14) is molded; an inner contour of the mold determining an outer contour of the molded-part portion (14), characterized in that the insert part (12) is positioned on or in the mold in such a way that the connection region (16) heated to an activation temperature (TA) comes in contact with the plastic precursor mixture at least temporarily during the polymerization, the activation temperature (TA) being no more than 50 °C lower and no more than 50 °C higher than a first polymer melting temperature (TP1) of the first polyamide material, whereby a bond forms between the molded-part portion (14) and the connection region (16) of the insert part (12).

2. The method as recited in claim 1, characterized in that the plastic precursor mixture contains a catalyst and an activator.

3. The method as recited in claim 1 or 2, characterized in that the start temperature (TS) is at least as high as a monomer melting temperature (TM1) of the first lactam monomer, preferably at least 10 °C higher than the monomer melting temperature (TM1), and at least 10 °C lower than a second polymer melting temperature (TP2) of the second polyamide material.

4. The method as recited in any of claims 2 or 3, characterized in that the providing of the plastic precursor mixture is accomplished using the following steps: i. providing a first monomer mixture including the first lactam monomer and an additive composed of one of the two alternative components of catalyst or activator, ii. providing a second monomer mixture including the first lactam monomer and / or a second lactam monomer and an additive composed of the other of the alternative components of catalyst or activator, iii. combining the first monomer mixture and the second monomer mixture.

5. The method as recited in any of claims 3 or 4, characterized in that the plastic precursor mixture is maintained at or above a waiting temperature (TW) for a time (tW), in particular a time (tW) of at least 10 seconds, before the connection region (16) of the insert part (12) first comes into contact with the plastic precursor mixture, and in that the waiting temperature (Tw) is at least as high as the monomer melting temperature (TM1), and / or in particular at least as high as the start temperature (TS).

6. The method as recited in any of the preceding claims, characterized in that the first lactam monomer is a caprolactam or a laurolactam.

7. The method as recited in any of claims 4 through 6, characterized in that the second lactam monomer is a caprolactam or a laurolactam.

8. The method as recited in any of the preceding claims, characterized in that the first polymer melting temperature (TP1) is lower than a second / the second polymer melting temperature (TP2) of the second polyamide material.

9. The method as recited in any of the preceding claims, characterized in that, during the polymerization, a maximum occurring temperature (TPmax) of the plastic precursor mixture and of the second polyamide material forming and formed from the plastic precursor mixture is lower than a second / the second polymer melting temperature (TP2) of the second polyamide material.

10. The method as recited in any of the preceding claims, characterized in that the heating of the connection region (16) is carried out using a heating device, which is preferably disposed on or in the mold.

11. The method as recited in claim 10, characterized in that the heating device is in contact with the connection region (16) via a contact region during a contact time (tK) and, at the end of the contact time (tK), is moved relative to the connection region (16), clearing it at least partially for contact with the plastic precursor mixture.

12. The method as recited in any of claims 10 or 11, characterized in that a setpoint temperature (Tsetpoint) of the heating device during the heating of the connection region (16) is no more than 50 °C lower than a second / the second polymer melting temperature (TP2) of the second polyamide material.