DE102022116824A1 - Method for producing a structural element on an injection molded workpiece and injection molded tool - Google Patents

Abstract

For a method for producing a predetermined structural element on a surface of an injection-molded workpiece, an injection molding tool (10) is used, in which an injection molding cavity is provided which has a smooth surface. A material (20) with a plastic matrix is introduced into the injection molding cavity, the composition of the material (20) being varied within the injection molding cavity in such a way that the material shrinks locally differently during cooling, forming the workpiece with a predetermined structural element.

Description

The invention relates to a method for producing a predetermined structural element on a surface of an injection-molded workpiece and to an injection-molded workpiece obtained by this method.

Injection molding can produce workpieces with a wide range of geometries and dimensions. For this purpose, a plasticized material is dosed into a cavity of an injection molding tool, which is also referred to below as an injection molding cavity. The injection molding cavity represents a negative for the desired workpiece. After the injection molding cavity has been filled, the material is solidified so that the workpiece is formed within the injection molding cavity, for example through crosslinking and/or cooling processes in the material. The injection molding tool is then opened and the workpiece is removed.

However, further post-processing steps may be necessary in order to obtain the desired geometry, a specific surface quality and/or a desired surface structure of the workpiece after injection molding. Such additional post-processing steps are undesirable due to the associated costs and increased effort.

The object of the invention is to provide an adaptable and cost-effective method for producing injection-molded workpieces, in particular injection-molded workpieces with special requirements for the surface structure of the workpiece.

The object is achieved by a method for producing a predetermined structural element on a surface of an injection-molded workpiece, using an injection molding tool in which an injection molding cavity is provided which has a smooth surface. A material with a plastic matrix is introduced into the injection molding cavity, the composition of the material being varied within the injection molding cavity in such a way that the material shrinks locally differently during cooling, forming the workpiece with a predetermined structural element.

In conventional injection molding processes, one goal is to achieve the most even distribution possible of all components of the material filled into the injection molding cavity in order to avoid locally different behavior during cooling. According to the invention, it is now provided that such local differences in the composition of the material are specifically specified and thus exploited so that the material behaves differently in different areas of the injection molding cavity during cooling.

The term “shrinkage” here means a decrease in volume of the material in the injection molding cavity. In other words, the material shrinks to different extents in different areas of the injection molding cavity.

In particular, the volume decrease in at least two volume sections within the injection molding cavity differs from one another by at least 10%. At the same time, the dimensions of the specified structure can be determined via the difference in the volume decreases in different volume sections of the injection molding cavity.

In particular, the predetermined structural element has a height in the micrometer range, measured from the surface of the workpiece arranged adjacent to the structural element.

The locally different shrinkage behavior of the material can indicate a gradual change and/or a gradual change along the injection molding cavity.

A “smooth surface” of the injection molding cavity is understood here to mean that the injection molding cavity does not have any projections, depressions, grooves, grains or the like that create the specified structural element.

The material preferably comprises a shrinkage modifier, the material shrinking as it cools depending on the local concentration of the shrinkage modifier. In other words, the local shrinkage of the material can be controlled via the shrinkage modifier content. In particular, a higher concentration of the shrinkage modifier causes the material to shrink more.

It has been recognized that workpieces of a wide variety of shapes can be produced using the same injection molding tool by specifically adding a shrinkage modifier to the material that is introduced into the injection molding cavity of the injection molding tool in order to achieve a different shrinkage or shrinkage behavior of the material in predetermined areas of the injection molding cavity to cause material to be filled into the injection molding cavity. In this way, a large number of differently designed workpieces can be produced using the same injection molding tool and/or a large number of structures can be created on the surface of the workpiece.

According to the invention, the shrinkage modifier is not completely miscible with the other components of the material, in particular the plastic matrix. This means that when the shrinkage modifier is mixed with the other components of the material, inhomogeneities of the shrinkage modifier are formed.

The local dependence of the shrinkage behavior of the material is caused in particular by the fact that the shrinkage modifier has a different thermal expansion coefficient compared to the other components of the material, especially compared to the plastic matrix.

The shrinkage modifier can be uniformly distributed in the material within the volume of the plastic matrix or in locally different concentrations. It is also possible that the shrinkage modifier is present near the surface of the workpiece.

During the cooling process of the material, the surface quality of the resulting workpiece is then influenced by a hindered or promoted shrinkage of the material, caused by the shrinkage modifier. Microstructures can be created in this way.

The shrinkage modifier can be added to the material via dry mixing and/or compounding.

The shrinkage modifier can be introduced into the injection molding cavity simultaneously or sequentially in time to the other components of the material that have already been plasticized.

For this purpose, the same connection points to the injection molding cavity can be used as for the other components of the material. Alternatively or additionally, separate connection points can also be provided for the shrinkage modifier.

In one embodiment, the shrinkage modifier is mixed or mixed with the other components of the material in the same plasticizing unit. However, it is also possible for the shrinkage modifier to enter the injection molding cavity via an additional plasticizing unit.

The local concentration of the shrinkage modifier can be adjusted by varying the viscosity and/or the density of the plastic matrix across the injection molding cavity. It was found that a local increase in viscosity of the plastic matrix results in the shrinkage modifier depleting in the areas with locally higher viscosity, while the concentration of shrinkage modifier increases in areas of the injection molding cavity with locally lower viscosity, i.e. the shrinkage modifier accumulates.

It is also possible, alternatively or additionally, to vary the average polymer chain length of a polymer of the plastic matrix across the injection molding cavity in order to adjust the local concentration of the shrinkage modifier.

In one variant, the local concentration of the shrinkage modifier is adjusted by generating local temperature and/or pressure differences in the cavity. In this way, precise control over the local concentration of the shrinkage modifier and thus the shrinkage behavior of the material contained in the injection molding cavity is possible.

For example, the viscosity of the plastic matrix can be adjusted by temperature treatment of the mass. In other words, the plastic matrix can be specifically heated or cooled in selected local areas of the injection molding cavity in order to produce a different viscosity and thus a different concentration of the shrinkage modifier in these local areas. This effect is attributed to the fact that in local areas of the injection molding cavity that are exposed to a higher processing temperature, chain degradation in the polymer chains of the plastic matrix is increased, which in turn leads to a decrease in the viscosity of the plastic matrix in these local areas.

It is also possible for the temperature treatment to be carried out within a feed device, which then transfers the material into the injection molding tool, for example in a screw conveyor of the feed device.

Basically, the local concentration of the shrinkage modifier can generally be adjusted by varying the internal properties of the plastic matrix across the injection molding cavity.

The material in particular comprises 10 to 60 percent by weight of shrinkage modifier, preferably 10 to 40 percent by weight, based on the total weight of the material. Use less than 10 percent shrinkage modifier by weight set, it may happen that the material does not shrink locally to a sufficient extent. If the shrinkage modifier content is more than 60 percent by weight, the properties of the workpiece obtained can no longer be sufficiently determined by the plastic matrix.

According to the invention, the concentration of the shrinkage modifier in the material is varied locally within the injection molding cavity, in particular within the content limits specified above.

The shrinkage modifier is preferably selected from the group consisting of inorganic materials, fiber materials, thermoplastic polymers, reactive polymers and mixtures thereof.

The inorganic material preferably consists of a mineral powder or mineral particles.

Kaolin is particularly suitable as an inorganic material. Kaolin is available inexpensively worldwide and is compatible with plastics used in injection molding processes.

Glass fibers can be used as the fiber material.

Examples of suitable thermoplastic polymers are polyethylene (PE), polystyrene (PS), polyvinyl chloride (PVC), acrylonitrile butadiene styrene (ABS), polyamides (PA), polyphenylene sulfides (PPS) and polyethylene terephthalates (PBT).

If thermoplastic polymers are used as shrinkage modifiers, they are particularly different from other polymers in the plastic matrix.

Reactive polymers include, in particular, thermosets and elastomers.

The shrinkage modifier in particular has a predetermined microstructure. For example, the shrinkage modifier is in the form of particles, fibers and/or tubes (also referred to as “tubes”).

The composition of the plastic matrix is not further restricted as long as materials suitable for injection molding can be achieved based on this plastic matrix.

The plastic matrix preferably comprises a polyamide or consists of polyamide. Polyamides have been tested for use in injection molding processes, are available worldwide and enable the production of workpieces that have high mechanical and chemical resilience.

In addition, polyamide chains can be decomposed very precisely using the selected processing temperature in order to precisely adjust the viscosity and/or density of the material within the injection molding cavity.

The type and composition of the polyamide is fundamentally not restricted, but can be selected with regard to the desired mechanical and chemical properties of the workpiece to be produced.

The polyamide can be an aliphatic, partially aromatic and/or aromatic polyamide.

For example, the polyamide is selected from the group consisting of PA4, PA6, PA11, PA12, PA4.6, PA6.6, PA6.10, PA6.12, PA 12.12, PA6T, PA6/6.6, PA6T/6l, copolyamides and mixtures thereof, preferably from PA6, PA6.6, PA6.10, copolyamides and mixtures thereof.

The material comprises in particular 40 to 90 percent by weight of plastic matrix, preferably 60 to 90 percent by weight, based on the total weight of the material. At a content of less than 40 percent by weight, the workpiece obtained may have inadequate mechanical and chemical properties, while at a content of more than 90 percent by weight, sufficiently different local shrinkage behavior cannot be observed.

The plastic matrix can optionally include additives that do not affect the shrinkage behavior of the material, for example flash point modifiers, thermal stabilizers, processing aids, dyes, pigments and/or UV stabilizers.

The object of the invention is further achieved by an injection-molded workpiece with a structural element on a surface of the workpiece, obtained according to the method described above.

The properties and features of the method according to the invention apply analogously to the injection-molded workpiece according to the invention and vice versa and reference is made to the above statements.

The predetermined structural element is in particular a projection and/or a depression. In other words, the locally different shrinkage behavior of the material creates a projection and/or a depression on the surface che of the workpiece is generated, so that the injection molding tool itself or the surface of the injection molding tool facing the injection molding cavity does not have to have a corresponding, complementary recess and/or a correspondingly complementary projection.

It goes without saying that the workpiece can also have a plurality of predetermined structural elements, whereby the several predetermined structural elements can be designed the same or differently.

The workpiece is preferably a piston, in particular a hydraulic cylinder piston.

In this variant, the predetermined structural element can run radially circumferentially on the surface of the piston at least in sections, in particular completely. For example, the predetermined structural element is a projection designed as a circumferential ring.

For example, the predetermined structural element serves to center and/or fix the piston in an associated sleeve of a hydraulic actuator.

It is also possible that the specified structural element serves to change the tribological properties of the workpiece.

The injection-molded workpiece in particular has a reflective surface. In other words, the surface of the injection molded workpiece is mirror-smooth apart from the structural element.

• - 1 eine schematische Schnittansicht durch ein Spritzgusswerkzeug zum Durchführen eines erfindungsgemäßen Verfahrens,
• - 2 eine schematische Schnittansicht des Spitzgusswerkzeugs aus 1 nach Einfüllen einer Masse in eine Spritzgusskavität des Spritzgusswerkzeugs,
• - 3 eine erste Ausführungsform eines erfindungsgemäßen spritzgegossenen Werkstücks,
• - 4 eine zweite Ausführungsform eines erfindungsgemäßen spritzgegossenen Werkstücks,
• - 5 eine weitere schematische Schnittansicht des Spritzgusswerkzeugs aus 2, und
• - 6 eine dritte Ausführungsform eines erfindungsgemäßen spritzgegossenen Werkstücks.

Further features and properties of the invention result from the following detailed description of exemplary embodiments, the examples and the drawings. Show in these:

• - 1 a schematic sectional view through an injection molding tool for carrying out a method according to the invention,
• - 2 a schematic sectional view of the injection molding tool 1 after filling a mass into an injection molding cavity of the injection molding tool,
• - 3 a first embodiment of an injection molded workpiece according to the invention,
• - 4 a second embodiment of an injection molded workpiece according to the invention,
• - 5 another schematic sectional view of the injection molding tool 2 , and
• - 6 a third embodiment of an injection molded workpiece according to the invention.

1 shows a schematic sectional view of an injection molding tool 10 for producing an injection-molded workpiece 24 (cf. 3 , 4 and 6 ).

In the variant shown, the injection molding tool 10 has a first tool half 12 and a second tool half 14.

The first tool half 12 and the second tool half 14 are placed on one another in such a way that they form a cylindrical injection molding cavity 16, which extends axially along a length direction L.

It is understood that the injection molding tool 10 can also be constructed from more than two tool halves, for example in the case that workpieces with complex geometry are to be produced, and that the injection molding cavity 16 can also have a different geometric shape.

The surface 17 of the injection molding tool 10 facing the injection molding cavity 16, i.e. both the first tool half 12 and the second tool half 14, is designed to be smooth.

The injection molding cavity 16 can be supplied with a material 20 (cf.) via a feed device (not shown) and a feed channel 18. 2 ) which comprises a plastic matrix, can be filled as in 1 is indicated by the arrow P.

The first tool half 12 and the second tool half 14 also have temperature control devices 22 which are arranged along the injection molding cavity 16.

The temperature control devices 22 can be designed as a heating or cooling element. For example, the temperature control devices 22 can be cooling channels through which a cooling medium is guided and which run perpendicular to the axial length direction L of the injection molding cavity 16. The cooling medium can be a gaseous, liquid or supercritical cooling medium.

Of course, a different number and/or arrangement of temperature control devices 22 can be present than in 1 shown.

2 shows the injection molding tool 10 during the production of an injection molded workpiece 24 according to the invention (cf. 3 and 4 ).

For this purpose, as previously described, a material 20 is introduced into the injection molding cavity 16 via the feed channel 18 until the injection molding cavity 16 is filled with the material 20, as in 2 indicated by the slanted hatching.

The material 20 comprises a polyamide-based plastic matrix. However, other plastics can also be used, depending on the type of workpiece to be produced.

According to the invention it is provided that the composition of the plastic matrix within the injection molding cavity 16 is varied, the variation being carried out in such a way that the material 20 shrinks differently locally when it cools down.

In the in 2 In the method shown in the method shown, this is achieved in that the material 20 comprises a shrinkage modifier, the concentration of which in the material 20 is varied axially along the injection molding cavity 16.

Correspondingly, the material 20 has a different concentration of shrinkage modifier in a first volume section 26 than in a second volume section 28. In the illustrated embodiment, the concentration of the shrinkage modifier in the first volume section 26 is higher than in the second volume section 28.

Such a distribution can be achieved, for example, by varying the composition of an initial granulate, which is plasticized in the feed device (not shown) to the material 20 and fed to the injection molding tool 10, over time.

The different concentrations of shrinkage modifier result in the material 20, which is present in the first volume section 26, shrinking or shrinking more than the material 20, which is contained in the second volume section 28.

In the embodiment shown, the shrinkage of the material 20 takes place particularly symmetrically about the length direction L.

At the end of the cooling process, the workpiece 24 according to the invention is created from the material 20 (apart from a protrusion that results from the material 20 that was previously present in the feed channel 18), which can be removed from the injection molding tool 10 by the two tool halves 12 and 14 are separated from each other.

3 shows schematically the workpiece 24 according to the invention obtained in this way, which in the variant shown is a piston with a piston body 29, for example a piston for a hydraulic cylinder.

The workpiece 24 has a structural element 30 at one axial end, which is designed as a projection or elevation, that is to say has a larger diameter than the remaining piston body 29.

The projection results from the material 20, which was previously present in the second volume section 28 of the injection molding cavity 16, while the remaining part of the piston body 29 was created from the material 20, which was arranged in the first volume section 26 of the injection molding cavity 16 and in the embodiment shown had a higher concentration of shrinkage modifier.

It is understood that, depending on the influence of the shrinkage modifier used on the behavior of the material 20, a smaller decrease in volume of those volume sections in which a higher concentration of shrinkage modifier is present can also occur.

In the in 3 In the illustrated embodiment of the workpiece 24, the transition between the structural element 30 and the remaining piston body 29 runs in steps.

However, a gradual transition is also possible, as in the second embodiment of the injection-molded workpiece 24 in accordance with the invention 4 shown.

In 5 an alternative procedure is shown using a further schematic sectional view through the injection molding tool 10.

In this variant, the material 20 received in the injection molding cavity 16 is cooled by means of the temperature control devices 22 in the volume sections 32, which lie at the same height as the temperature control devices 22 along the axial length direction L.

However, the remaining volume sections of the injection molding cavity 16 are not cooled separately, so that the material 20 is exposed to a higher temperature in these areas. This leads to an increased breakdown of the polymer chains in the plastic matrix of the material 20 and thus to an increase in viscosity, which in turn leads to a depletion of the shrinkage modifier in these volume sections and to an enrichment of the Shrinkage modifier leads into the volume sections 32.

The composition of the first mass does not necessarily have to be determined by varying the composition of the starting granulate, but can be adjusted by the different temperature control.

In principle, the chain can also be broken down by temperature control in the feed device (not shown) and in this way a locally different composition can be achieved within the injection molding cavity 16.

In 6 the workpiece 24 is shown schematically, which is based on the method according to 5 can be obtained.

As can be seen, the workpiece 24 has a large number of structural elements 30, which are designed as projections and surround the workpiece 24 radially.

The injection molded workpieces 24 described above are merely examples. In practice, any other geometries can be created.

The method according to the invention makes it possible to produce different injection-molded workpieces 24 with the same injection molding tool 10 by merely specifically controlling the shrinkage behavior of the material 20 used.

Complex post-treatment steps for producing the corresponding structural elements 30 can be dispensed with.

The structural elements 30 produced can be used to fix and/or center the workpieces 24 in their intended installation position, for example by means of a press fit.

Claims (10)

Method for producing a predetermined structural element (30) on a surface of an injection-molded workpiece (24), wherein an injection molding tool (10) is used, in which an injection molding cavity (16) is provided which has a smooth surface (17), wherein a material (20) with a plastic matrix is introduced into the injection molding cavity (16), the composition of the material (20) being varied within the injection molding cavity (16) in such a way that the material (20) shrinks locally differently during cooling, forming the Workpiece (24) with a predetermined structural element (30). Procedure according to Claim 1 , wherein the material (20) comprises a shrinkage modifier, and wherein the material (20) shrinks upon cooling depending on the local concentration of the shrinkage modifier. Procedure according to Claim 2 , wherein the local concentration of the shrinkage modifier is adjusted by varying the viscosity and / or the density of the plastic matrix and / or the average polymer chain length of a polymer of the plastic matrix across the injection molding cavity (16). Procedure according to Claim 2 or Claim 3 , wherein the material (20) comprises 10 to 60 percent by weight of shrinkage modifier, preferably 10 to 40 percent by weight, based on the total weight of the material (20). Procedure according to one of the Claims 2 until 4 , wherein the shrinkage modifier is selected from the group consisting of inorganic materials, fiber materials, thermoplastic polymers, reactive polymers and mixtures thereof. Method according to one of the preceding claims, wherein the plastic matrix comprises a polyamide or consists of polyamide. Method according to one of the preceding claims, wherein the material (20) comprises 40 to 90 percent by weight of plastic matrix, preferably 60 to 90 percent by weight, based on the total weight of the material (20). Injection molded workpiece with a structural element (30) on a surface of the workpiece (24), obtained by a method according to one of Claims 1 until 8th . Injection molded workpiece Claim 9 , wherein the predetermined structural element (30) is a projection or a depression. Injection molded workpiece Claim 8 or 9 , wherein the workpiece (24) is a piston, in particular a hydraulic cylinder piston.

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