DE102014010271A1 - Injection molded foamed thermoplastic lightweight construction - Google Patents

Abstract

The invention relates to an injection-molded lightweight construction structure made of thermoplastic material, which consists of a non-foamed compact skin layer and a foamed core layer and having a curved bead.

Description

The invention relates to the technical field of lightweight construction. In the plastics processing industry, there is a significant demand for lightweight construction in the use of structure-optimized components with simultaneous weight savings, high flexural rigidity and optimum mechanical strength.

This object is achieved by an injection-molded foamed reinforcing body made of plastic according to claim 1. Advantageous embodiments will become apparent with the features of the subclaims. In particular, this lightweight component according to the invention forms a structure with an outer compact skin layer and an inner foamed core layer which, due to their special geometry, generates an increased weight-related bending stiffness.

In order to realize lightweight solutions, it is common practice in the plastics processing industry to stiffen surface load-bearing components by ribbing while maintaining or reducing the component weight.

• – Differentialbauweise (Einzelteile punktuell verbunden); beispielweise Haut + Stringer + Rippen in klassischem Blechbau, verbunden durch Nieten, Bolzen oder Punkten.
• – Integralbauweise (aus einem Stück geformtes Bauteil); beispielweise Platte oder Schale mit herausgefrästen Stegen und Rippen zur Kräfteeinleitung oder zur Biegesteifigkeitserhöhung.
• – Integrierende Bauweise (zu organischer Einheit verbundene Einzelelemente); beispielweise Haut mit aufgeklebten Stringern oder Verstärkungen, aus Blechlamellen aufgebaute Trägerprofile.
• – Verbundbauweise (verschiedene Materialien nach ihren spezifischen Eigenschaften zweckvoll kombiniert); beispielweise Sandwichplatte mit steifen Blechhäuten und Schaumstoff- oder Wabenkern mit hohem spezifischen Volumen; oder durch Glasfasern, Kohle- oder Aramidfasern verstärkte Kunststoffe.

According to the state of the art, it is known that lightweight construction solutions can be achieved by the following means: lightweight construction of materials, constructive lightweight construction, lightweight construction and manufacturing processes. As can be seen from the reference work "Leichtbau: Elemente und Konstruktion" by Johannes Wiedermann, components are characterized by their external shape and their component geometry:

• - Differential construction (individual parts connected punctually); For example, skin + stringer + ribs in classic sheet metal construction, connected by rivets, bolts or points.
• - Integral construction (one-piece molded component); For example, plate or shell with milled ridges and ribs for force introduction or bending stiffness increase.
• - Integrative design (individual elements connected to organic unity); For example, skin with glued-on stringers or reinforcements, carrier profiles constructed from laminations.
• - composite construction (various materials purposefully combined according to their specific properties); for example sandwich panel with rigid metal skins and foam or honeycomb core with high specific volume; or plastics reinforced by glass fibers, carbon or aramid fibers.

As the most common geometry variant for load-bearing components, the cross-ribbing has prevailed in the plastics processing industry. This conventional cross ribbing is designed for injection molding of compact thermoplastic molding compounds. It has the ability to record different load cases that frequently occur in practice, as well as the load case bending, but also train and torsion.

From the patent EP 0 850 113 B1 For example, a method for producing a reinforced molded article having a hollow outer shape and a filling having an open or closed cell metal foam having high resistance to deformation is known. The component may have different longitudinal sections and cross sections. As a hollow outer molded part known hollow moldings made of metals, plastics, ceramics or glass can be used. In the manufacture of these moldings is a two-step process, wherein the hollow outer mold is prepared by drawing, casting, extrusion, hydroforming and then foamed with the foam starting material.

From the patent DE 199 44 352 C2 is the production of a lightweight molded part as a cover and / or housing by sandwich injection molding (two-component injection molding) known. The molding comprises an outer layer and an inner layer of a foam-like plastic material, wherein the outer layer comprises thermoplastic hard plastic and the inner layer polypropylene foam. By merging the two components in a spray nozzle, the desired geometries can be achieved.

In these variants of the prior art, there are significant shortcomings. In particular, the known foamed thermoplastic molding compositions do not constitute a special geometry that meets the requirements of the bending load. Thermoplastic foam injection molding (TSG) can cause typical surface problems caused by the process. In fact, the lower the foam density, the higher the likelihood that the gas bubbles will penetrate the surface of the component and become visible as streaks. Another common cause of failure is the re-blowing of the propellant, which can lead to deviations in the dimensional accuracy of the component.

It is the object of the present invention to solve these problems as well as to improve the weight-related bending stiffness of lightweight plastic components. The present invention provides The task of proposing an alternative design with increased mechanical properties. The aim of the invention is the design of lightweight-optimized rigid moldings, in particular of flat load-bearing ribbed molded parts with foam based on thermoplastic, which can be produced by an injection molding process.

• – bei Leichtbauteilen mit tragender Funktion orientiert sich der Leichtbaueffekt an der Zielgröße einer maximalen spezifischen gewichtsbezogenen Biegesteifigkeit. Für die Aufnahme der Biegekräfte eines Sandwichaufbaus ist die Steifigkeit der am weitesten von der neutralen Achse entfernten Schichten vorrangig. Je weiter die Schichten sich der neutralen Achse nähern, desto weniger Festigkeit ist erforderlich. Im Sinne des Leichtbaus (d. h. gleiche Biegesteifigkeit bei weniger Gewicht) muss nahe der neutralen Achse vorrangig an Gewicht eingespart werden. Dies wird durch möglichst leichte Schäume im Inneren der Leichtbaustruktur erreicht. Die erfindungsgemäße Gestaltung fördert durch eine höhere Wanddicke das optimale Verschäumen eines Thermoplasts, was eine minimal erreichbare Dichte im Formteilinneren ermöglicht;
• – günstig im Sinne einer leichtbauoptimierten Gestaltung ist die Einbringung von hochfestem Material möglichst weit entfernt von der neutralen Achse des Formteils. Das hochfeste Material sollte die äußere Schicht des Formteils bilden. Zu diesem Zweck ist im Gegensatz zur konventionellen Kreuzverrippung die Geometrie der Schaumverrippung rechteckig ausgebildet. Erfindungsgemäß wird die äußere Schicht von Sicken bzw. Noppen durchbrochen, damit sich – bedingt durch die größere Oberfläche – noch mehr hochfestes Material fern der neutralen Faser anlagert. Die Dicke der thermoplastischen Hautschicht, die sich an der Kavitätswand ausbildet, steigt durch diese geometrische Gestaltung an.

In order to size the lightweight structure according to the invention, the following general design guidelines have been considered:

• - For lightweight components with load-bearing function, the lightweight effect is based on the target size of a maximum specific weight-related bending stiffness. In order to absorb the bending forces of a sandwich construction, the rigidity of the layers farthest from the neutral axis is paramount. The further the layers approach the neutral axis, the less strength is required. In terms of lightweight construction (ie equal flexural rigidity with less weight), weight must be prioritized near the neutral axis. This is achieved by the lightest possible foams inside the lightweight structure. The inventive design promotes the optimal foaming of a thermoplastic by a higher wall thickness, which allows a minimum achievable density in the molding interior;
• - Favorable in terms of lightweight design, the introduction of high-strength material as far as possible from the neutral axis of the molding. The high strength material should form the outer layer of the molding. For this purpose, in contrast to the conventional cross ribbing, the geometry of the foam ribbing is rectangular. According to the invention, the outer layer of beads or nubs is broken, so that - due to the larger surface - even more high-strength material attaches far away from the neutral fiber. The thickness of the thermoplastic skin layer, which forms on the cavity wall, increases by this geometric design.

In view of these design rules, the present invention provides a lightweight structure of thermoplastic material consisting of a non-foamed compact skin layer and a foamed core region. In the following, this core area is called "core layer". The foamed thermoplastic of the core layer is compatible with the skin layer. The lightweight construction according to the invention is produced by injection molding, which may be a one-component as well as a two-component sandwich injection molding process. The lightweight structure is a rib that is rectilinear or curved and whose cross-section is a convex quadrilateral, preferably an isosceles trapezoid. In the middle of the short baseline of the trapezoidal cross-section and perpendicular to it, located over the length of the rib in the longitudinal direction of a groove-like depression, hereinafter referred to as "bead". The shape of this bead is curved, preferably U-shaped. In comparison with the short baseline of the trapezoidal cross section, the width of the bead is 10 to 50%, preferably 20%. The depth of the bead is between half and twice the bead width, preferably equal to the bead width. The ratio of the cross-sectional area of the unfoamed skin layer to the cross-sectional area of the foamed core layer is limited to between 0.20 and 0.66, and is preferably 0.4. The density of the foamed core layer in comparison with the density of the skin layer is lower by less than a third, preferably less than half. Consequently, depending on the thermoplastic used, the density of the core layer is 0.01 to 0.7 g / cm ³ , preferably 0.5 g / cm ^3, and the density of the skin layer is 10 to 0.5 g / cm ³ .

In one embodiment of the invention, the lightweight structure is provided with a reinforcing layer of unidirectionally or bidirectionally reinforced preconsolidated semifinished product. Hereinafter, this reinforcing layer will be called "cover layer". In this case, this cover layer lies on the surface of the rib which is opposite to the bead. The material of the cover layer may be a semifinished product made of organic sheet, unidirectional tapes, woven fabric or a film based glass, carbon, textile fiber or metal, preferably based on glass fiber, which is pre-impregnated with a polymer material compatible to the skin material matrix. The thickness of this cover layer is from 0.05 to 5 mm. Several lightweight structures can be interconnected by a common flat cover layer.

The resulting advantageous effects of the invention are that with the same mechanical performance, the lightweight construction according to the invention is lighter than the conventional lightweight structures of the prior art and can be mass produced suitable for injection molding. The weight-related bending stiffness of the lightweight construction according to the invention is higher in comparison with the conventional cross ribbing for compact injection molding. This causes an increased lightweight effect: with the same bending stiffness, the component thickness can be reduced and thus weight can be saved. Foam-compatible design of the lightweight structure according to the invention a clear lightweight effect is achieved. A high-strength surface layer in combination with an inner lightweight foamed core region produces an increased weight-related bending stiffness. The density of the entire lightweight structure, depending on the thermoplastic used is at least 20% reduced compared to the initial density of the thermoplastic. The Cost-effective production in one- or two-component injection molding enables short cycle times and mass production suitability. The inventive solution reduces penetration of the foam bubbles to the molding surface by accelerated solidification on the enlarged cavity surface. The increased accumulation of skin material outside the area of the bead advantageously prevents the typical TSG swelling of thicker molding walls. The scope of the lightweight molding according to the invention is wide. In the automotive field, uses may include, for example, battery carriers, front end supports, footwell covers, damping troughs, spare wheel wells, or tailgate covers. In general engineering, there are many applications for machine elements that seek to reduce moving masses. Further applications are mechanically highly loaded housings or areas of housings, which should also be light at the same time.

The present invention will be explained in more detail with reference to embodiments and drawings. In this case, further features and advantages are disclosed according to the invention. Various features of different embodiments can be combined.

By two embodiments of the invention, lightweight structures according to the invention are shown. In the lightweight structure features of lightweight materials, lightweight construction and lightweight construction are advantageously combined with each other by manufacturing processes. The lightweight structures according to the invention without and with a cover layer are compared with a conventional cross ribbing.

For both one-component and two-component sandwich injection molding, the dimensions of the lightweight structures are calculated so that the diameter of the bead is 0.1 to 10 mm, preferably 1 to 5 mm and that the wall thickness of the skin layer is outside the range of the bead 0 , 1 to 5 mm, preferably 1 mm.

As a first embodiment of the invention, a lightweight structure is shown, which was produced in one-component injection molding. The one-component process produces an integral foam structure with a compact skin and a foamed core, whereby the transition from foamed to compact structure takes place gradually. The foaming can be done chemically (blowing agent) or physically (by introducing propellant into the melt). It forms a skin layer of compact thermoplastic at the Kavitätsoberfläche. The ratio between skin and core contents is determined mainly by the materials used and by the process of foaming. The local distribution of the layer thicknesses depends on the flow and cooling processes.

• – PP ein ungefülltes unverstärktes Polypropylen-Produkt ist;
• – Organoblech ein kommerziell erhältliches endlosfaserverstärktes vorkonsolidiertes Halbzeug ist.

In the one-component injection molding process lightweight structures according to the invention are compared with each other and with a conventional cross ribbing of the prior art (demonstrator). The lightweight structure (A) was calculated using the following mixture of materials (Table 1), where:

• PP is an unfilled polypropylene unreinforced product;
• - Organic sheet is a commercially available continuous fiber reinforced pre-consolidated semi-finished product.

Tabelle 1 All core layers of the foamed lightweight structures consist of 2% chemical endothermic blowing agent produced foam of unreinforced polypropylene with a density of about 0.5 g / cm ³ . In order to produce a finer-celled foam, an addition of nucleating agent is optionally possible. The variant of the lightweight structure with beading shows the highest weight-specific flexural rigidity:

Table 1

Compared to the conventional cross ribbing (D) made of PP, which has no covering layer and whose density is 0.90 g / cm ³ , the density of the entire hybrid component (A) is 0.68 g / cm ³ . By a reinforcing cover layer of organic sheet, the weight-specific bending stiffness of the hybrid component (A) can be further increased.

As a second embodiment of the invention, a lightweight structure is shown, which was produced in the two-component sandwich injection molding. In the two-component sandwich injection molding process, two plastic components are injected one behind the other into a tool, which in principle is constructed like a one-component mold, via a common gate. First, a partial filling with the first component, which forms the skin layer. The second component, injected immediately thereafter, completes the filling in such a way that the first component always remains outside due to the laminar flow process. Thus, the second component inevitably forms the core layer. The skin and core layers are not mixed and can be recognized by a sharp phase boundary. The ratio between skin and core proportions is determined by the dosage of the first and second components. The local distribution of the layer thicknesses - as with the one-component method - depends on the flow and cooling processes. For lightweight applications, the core layer may be physically or chemically foamed. For lightweight construction structures subjected to bending, the task is to optimize the weight-related stiffness. That is, the load-bearing structures (in this case, the compact but heavy skin layer) are placed in the outer areas away from the neutral bending plane in order to absorb the tensile stresses there, ie in areas of maximum deformation. In the interior, however, should be a light core area with foamed structure. This increases in tendency of the position close to the neutral bending plane of the component ever smaller tensile stresses in the bending direction. The core layer also has the function to absorb the compressive stresses perpendicular to the neutral bending plane and thus to keep the skin layer in the desired position. This is particularly relevant for the considered planar components, in order to avoid a collapse of the core layer and thus a weakening of the component strength. In the two-component sandwich injection molding process, the ratio of the skin and core portions to one another can be controlled globally within certain limits via the viscosity ratio and dosing ratio of the two components as well as via the injection speed. This global influence is also limited in single-component foam injection molding via process control. To locally control the ratio of skin and core, the cooling conditions must be locally different; Higher edge layer thicknesses require higher cooling rates. The high thermal conductivity of the steels greatly limits the possibilities of local temperature control. By means of the method, the cooling conditions and thus the skin layer thickness are influenced by the design of the molding surface both for the one-component and two-component injection molding process described above. A large surface facilitates cooling and generates an increased edge layer thickness. In the following, the cooling behavior of a thermoplastic melt with decreasing wall thicknesses of a molded part in the TSG is explained in general. A 60 × 60 mm board was produced in thicknesses of 1, 2, 3, and 4 mm by two-component sandwich skin compacting and foamed core layer sandwich injection molding. The ratio of the molding surface to the molding cross-section increases exponentially at. The result is a faster solidification of the melt at the cavity surface of the mold with decreasing wall thicknesses, accompanied by an increased accumulation of compact thermoplastic in the skin layer. According to the invention, this effect is utilized by introducing a bead into the cavity surface. As wall thicknesses increase, so does the core layer content. This effect is advantageously used by the inventive compared to the conventional cross ribbing thicker rib wall thickness in terms of lightweight construction. At a wall thickness of 4 mm in the middle of the molding, a swelling of the wall cross-section can be seen. This undesirable effect typical of TSG is successfully prevented by the increased accumulation of compact thermoplastic in the skin layer.

In the two-component sandwich injection molding process lightweight structures according to the invention are compared with each other and with a conventional cross ribbing of the prior art (demonstrator) (Table 2a and 2b).

• – PP ein ungefülltes unverstärktes Polypropylen-Produkt ist;
• – PP GF 30 ein Polypropylen-Produkt, gefüllt mit 30% Glasfaser ist,
• – PP H-GK ein Polypropylen-Produkt, verstärkt mit Hohlglaskugel ist,
• – Organoblech ein kommerziell erhältliches endlosfaserverstärktes vorkonsolidiertes Halbzeug ist.

The lightweight structures (A 'and B') were calculated with different material mixtures based on the material matrix polypropylene (Table 2a), where:

• PP is an unfilled polypropylene unreinforced product;
• - PP GF 30 is a polypropylene product filled with 30% glass fiber,
• - PP H-GK is a polypropylene product reinforced with hollow glass sphere,
• - Organic sheet is a commercially available continuous fiber reinforced pre-consolidated semi-finished product.

Tabelle 2a

Table 2a

Compared to the conventional cross ribbing (D ') made of PP GF 30 which has no covering layer and whose density is 1.12 g / cm ³ , the density of the entire hybrid component (A') is 0.64 g / cm ³ and the density of the entire hybrid component (B ') 0.54 g / cm ³ . By a reinforcing cover layer of organic sheet, the weight-specific bending stiffness of the hybrid component (A ') can be further increased.

• – PA6 ein ungefülltes unverstärktes Polyamid 6 – Produkt ist,
• – PA6 GF 30 ein Polyamid 6 – Produkt, gefüllt mit 30% Glasfaser ist,
• – Organoblech ein kommerziell erhältliches endlosfaserverstärktes vorkonsolidiertes Halbzeug ist.

The lightweight structure (A ") was calculated using a material mixture based on the material matrix polyamide 6 (Table 2b), where:

• PA6 is an unfilled unreinforced polyamide 6 product,
• PA6 GF 30 a polyamide 6 product filled with 30% glass fiber
• - Organic sheet is a commercially available continuous fiber reinforced pre-consolidated semi-finished product.

Tabelle 2b

Table 2b

Compared to the conventional cross ribbing (D '') of PA GF 30, which has no top layer and whose density is 1.34 g / cm ³ , the density of the entire hybrid component (A ") is 0.99 g / cm ³ , By a reinforcing cover layer of organic sheet, the weight-specific bending stiffness of the hybrid component (A '') can be further increased.

Overall, the comparison of the foamed lightweight structures according to the invention with beading with the conventional compact cross ribs shows consistently higher values of the weight-related bending stiffness for the foamed lightweight structures. The molded parts produced in foam-rib geometry in the two-component sandwich injection molding achieve higher weight-specific bending stiffnesses than the conventional cross-ribbed shaped parts produced by one-component injection molding.

Advantageous embodiments will be explained below with reference to FIGS.

1a and 1b show a compact Kreuzverrippung (demonstrator) according to the prior art in sectional view and in plan view.

2a and 2 B show the lightweight construction according to the invention in a sectional view or in plan view.

3 shows the lightweight construction according to the invention with a cover layer according to an embodiment in side view.

4a . 4b . 4c and 4d show the cooling behavior of a thermoplastic melt with decreasing wall thicknesses of a molded part.

LIST OF REFERENCE NUMBERS

1

lightweight structure

2

skin layer

3

core layer

4

Beading

5

topcoat

6

Conventional cross ribbing (demonstrator)

In 1a and 1b is a conventional compact cross ribbing ( 6 ) according to the prior art. In 2a and 2 B is the lightweight structure according to the invention ( 1 ), which consist of a non-foamed, compact skin layer ( 2 ) and a foamed core layer ( 3 ) is shown. The lightweight structure ( 1 ) is a rib whose cross-section is an isosceles trapezoid. In the middle of the short baseline of the trapezoidal cross-section and perpendicular to it, located over the length of the rib in the longitudinal direction of a channel-like U-shaped bead ( 4 ). In 3 is the lightweight structure according to the invention ( 1 ) with a cover layer ( 5 ). In 4a . 4b . 4c and 4d is to clarify the Cooling behavior of a thermoplastic melt in the cavity of a plate-shaped injection molding tool with the dimensions 60 × 60 mm in sectional view. The plate thickness is 1 mm ( 4a ), 2 mm ( 4b ), 3 mm ( 4c ), and 4 mm ( 4d ). The panels were made by two-component sandwich injection molding with a compact skin layer ( 2 ) and foamed core layer ( 3 ) produced. On the left, in the sectional view, the outer area of the plates, on the right, the central area of the plates shown.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• EP 0850113 B1 [0006]
• DE 19944352 C2 [0007]

Claims (10)

Lightweight structure ( 1 ) made of thermoplastic material, which consists of a non-foamed, compact skin layer ( 2 ) and a foamed core layer ( 3 ) and, which is produced in the one- or two-component sandwich injection molding process, characterized in that - the lightweight structure is a rib whose cross-section is an isosceles trapezoid; That in the middle of the short baseline of the trapezoidal cross-section and perpendicular to it over the length of the rib in the longitudinal direction of a groove-like curved bead ( 4 ) is located; - that the width of the bead ( 4 ) Is 10 to 50% of the short baseline of the trapezoidal cross section; - that the depth of the bead ( 4 ) is between half and twice the bead width; That the ratio of the cross-sectional area of the skin layer ( 2 ) to the cross-sectional area of the core layer ( 3 ) is limited to between 0.20 and 0.66; - that the density of the core layer ( 3 ) by less than one third of the density of the skin layer ( 2 ) is lower. Lightweight structure according to claim 1, characterized in that the bead ( 4 ) Is U-shaped. Lightweight structure according to one of claims 1 or 2, characterized in that the width of the bead ( 4 ) Is 20% of the short baseline of the trapezoidal cross section. Lightweight structure according to one of claims 1 to 3, characterized in that the depth of the bead ( 4 ) is equal to the bead width. Lightweight structure according to one of claims 1 to 4, characterized in that the ratio of the cross-sectional area of the skin layer ( 2 ) to the cross-sectional area of the core layer ( 3 ) Is 0.4. Lightweight structure according to one of claims 1 to 5, characterized in that the density of the core layer ( 3 ) less than half the density of the skin layer ( 2 ). Lightweight structure according to one of claims 1 to 6, characterized in that the density of the core layer ( 3 ) 0.01 to 0.7 g / cm ³ and the density of the skin layer ( 2 ) Be 10 to 0.5 g / cm ³ . Lightweight structure according to one of claims 1 to 7, characterized in that the lightweight structure ( 1 ) with a cover layer ( 5 ) is provided of unidirectionally or bidirectionally reinforced preconsolidated semi-finished product, the outer layer ( 5 ) may consist of organic sheet, unidirectional tapes, fabric or film based on glass, carbon, textile fiber or metal. Lightweight structure according to claim 8, characterized in that the thickness of the cover layer ( 5 ) from 0.05 to 5 mm. Lightweight structure according to one of claims 8 or 9, characterized in that several lightweight structures ( 1 ) by a common flat cover layer ( 5 ) are interconnected.

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