The invention resides in the field of molded products and particularly relates to a removable core for use in an injection molding process. The invention further relates to a method for manufacturing hollow-walled injection molded products, at least injection molded products provided with hollow spaces.

To create hollow spaces or cavities in injection molded products, cores are generally used, which must be removed after injection molding as completely as possible.

Such cores can be divided into two main categories, namely cores for repeated use and cores for single use.

Cores for repeated use may either be fixed in a mold or die or be detachable or removable so as to enable easier removal of the product from the mold or die. This type of core is generally made of die steel and will normally last the whole service life of the die.

In a large number of embodiments, especially when complicatedly shaped cavities, e. g. highly undercut shapes, are to be provided in the product or when the cavity is not properly accessible for a core for repeated use, such cores cannot be used. For these embodiments the cores must be deformed or broken down to enable their removal. This type of core is designated by the term"core for single use".

The present invention relates to this type of core.

Thus, when manufacturing hollow-walled products from metal, so-called sand cores are generally used. These cores are formed out of sand-like material using water and additives and can be broken down after being circumjected with metal in a mold. European patent application 0 466 419 describes sand cores in which starch is used as binder.

In the course of years a shift has increasingly been observed from hollow-walled cast products cast metal to hollow-walled injection molded products manufactured from a

thermoplastic. The main advantages of using the injection molding technology as compared with the casting of products from metals is a weight reduction of 30-60%, an improved quality of the surface of the product and a more accurately and better controllable design without the need for carrying out mechanical treatments or posttreatments. Via this technology, more complicated product forms can be obtained and more functions can be integrated into one product. Especially the advantages regarding the quality of the product surface and the design are connected with the fact that the injection molding process involves considerably lower temperatures, so that other core materials can be used. A method for manufacturing hollow products by means of injection molding is the so-called "hollow gas injection molding"technique. Thick-walled products are thereby made hollow by blowing in a gas. This technique does not render complex internal geometries possible.

Especially for the manufacture of hollow-walled injection molded products in one piece, which products may have a complex smooth internal geometry with a high degree of dimensional stability, techniques have been proposed using e. g. melting cores, salt cores and dissolving cores.

Melting cores are chiefly manufactured from metal alloys melting at low temperatures. Such cores which preferably have a melting point between about 90 and 200°C, may be composed of, e. g., combinations of tin, bismuth, lead, antimony and cadmium. In this connection reference is made to European patent application 0 433 000.

The alloys, which are relatively expensive, are shaped into cores in a separate device. These cores are then placed or fixed in a die and circumjected with the desired material. As last step the cores are removed by melting, after which the core material can be reused. This melting can be carried out, inter alia, by immersing the molded product in a hot medium, such as hot oil, polyhydric

aliphatic alcohols, modified polyglycol ethers, silicones or alkaline solutions. Such a process unavoidably leads to waste streams of products, which are environmentally harmful, while, moreover, metal components and/or components from the hot media, often toxic, always remain in or on the hollow-walled products.

For the manufacture of salt cores similar molding devices are required as necessary for the known sand cores.

The salt cores must be dried, and steps must be taken to limit as much as possible the corrosive and hygroscopic character of the employed salts. After circumjection of this core material, the salt is removed by dissolving in a suitable medium. The aim is to recycle the employed materials as completely as possible, but also when salt cores are used, compounds are released that are environmentally harmful. An example of salt cores is described in French patent application 2 080 725.

Cores on the basis of polymeric materials have often been proposed in the literature. Thus, a core of polyacrylate filled with a mineral is described by Kalivoda in Kunststofftechnik, Neue Werkstoffe und Verfahren beim Spritzgießen, VDI Verlag GmbH, Düsseldorf (1990), 59-80.

Hollow bodies are manufactured from this water-dispersible polymeric material, usually by welding together two shell- shaped products. These hollow bodies are required to enable a rapid removal of this core material. After circumjection with a water-insoluble thermoplastic, a probe is inserted which squirts the polyacrylate material with jets of water.

The polyacrylate is discharged as a slurry from the shaped hollow injection molded product. The slurry is dried, and the recovered polymer product can be reused, at an efficiency of about 80%.

Furthermore, the so-called"lost-foam"process is known and described in, e. g., European patent application 0 438 352 and by Van Niftrik in Metaal en Kunststof 13/14 (1996), 30-33. A plastic foam, in particular polystyrene

foam, is used herein as filler, which foam substantially decreases in volume after a heating step.

French patent 1 100 359 describes cores for manufacturing hollow-walled products, which cores are based on the"lost wax"principle. The term"wax"is very broadly defined in this publication and comprises all kinds of low- melting compounds, including gelatin, starch, alginate and methyl cellulose. The cores are removed by means of a mild heating step.

Japanese publication 62-152713 teaches the use of a soluble core, such as a core of a neutral polysaccharide consisting of a-16 bonded maltotriose units.

In Japanese publication 05-337594 a core for metal products is manufactured by subjecting a mixture of water- insoluble cellulose, water-soluble cellulose, oil or fat and water to a number of drying and heating steps. The core is removed by burning out.

German"Offenlegungsschrift"18 07 193 relates to hollow-walled products and proposes a shaped core of a plastic material which is soluble or biodegradable or which can be removed by the action of heat. Suitable plastic materials are not specified.

Finally, reference is made to the international patent application WO-95/07170. Reference is made herein to a large group of water-soluble materials proposed for the manufacture of cores. It is concluded that only hydroxyl group-containing compounds having a high molecular weight are satisfactory. The examples describe polyvinyl alcohol and acrylate resin cores.

All the above plastic materials for use as core continue to give problems in the removal step. The applicant has carried out a large number of tests in which these problems stood out clearly.

Thus, it was found for water-soluble cores of hydroxyl groups-containing macromolecules that they in fact only dissolve completely if they can be wetted completely with

warm water. This generally causes problems when injection molding material is circumjected. In practice, therefore, the core is not dissolved to 100%, and the dissolving rate is too low for industrial scale-up. Besides, a wet waste product fraction remains, which must be dried for further processing.

Attempts have been made to accelerate the dissolving of a core of water-soluble starch by adding the enzyme amylase to the water. Although a somewhat faster dissolving step was thus created, the adaptations necessary for the use of enzymes, and in particular the adaptations for a temperature optimal for the enzyme, render this option hardly practical.

Also the use of a brittle core in combination with a removal technique based on ultrasonic pulverization gave no good results. Instead of pulverization, melting and burning occurred.

It is an object of the present invention to provide a material for cores for use in injection molding processes, which material should be easily removable as completely as possible from an injection molded product, and which after removal from the injection molded product can be processed or discharged without problems relating to health or environment.

Besides, the material must preferably be injection moldable, so that it better fits in with the technology, comprising process and devices, used for the manufacture of the desired hollow-walled end product. In this connection, it is observed that the manufacture of, e. g., melting and salt cores requires special devices which are usually not present in an injection molding factory. This renders the use of these cores less attractive. An important advantage of being able to use an injection molded core in combination with an injection molded end product is that conventional multi-component injection molding devices can

be used. In such devices different materials can be processed into one product, which has obvious advantages.

Thus the object of the invention is to provide a moldable, removable material, which does not cause problems to environment and health and is preferably rapidly biodegradable for the manufacture of cores for creating cavities in the products to be injection molded. Besides, this material is preferably inexpensive. It is a further object of the invention to provide a method by which theses cores can be removed from the injection molded product as completely as possible.

These objects can be achieved with the removable core according to the invention which is manufactured from a biodegradable, moldable material and contains a blowing agent. Thus the invention relates to a removable core of a biodegradable, moldable material containing a blowing agent, for use in an injection molding process for manufacturing a hollow-walled injection molded product.

In a second aspect the invention relates to a method for manufacturing a hollow-walled injection molded product, comprising injecting an injection molding material into a mold in which the core according to the invention, which core contains a blowing agent, is placed, and removing the core using the blowing agent present in the core.

The invention further relates to the use of a biodegradable, moldable material comprising a blowing agent as removable core in an injection molding process.

The core according to the invention, after being circumjected with an injection moldable material, can be removed completely by activating the blowing agent in a known manner. Through this activation gases are formed, which expel the cores from the circumjected material and may or may not fragmentize the core.

The blowing agent must be compatible with the moldable material from which the core is manufactured and, if in a less degree, with the material provided around the core by

means of injection molding. Furthermore, the blowing agent must be activated neither during the formation of the core nor during the step of circumjecting the core with an injection moldable material. On the basis of this information, it is obvious to those skilled in the art te select suitable blowing agents for different core materials and injection molding processes.

Thus, blowing agents, such as sodium bicarbonate, can be used, which blowing agents disintegrate in a heating step. However, this blowing agent is known to disintegrate at temperatures exceeding about 60°C, so that it is not suitable for being included in an injection moldable or extrudable material, for which injection molding or extrusion conditions are necessary which require a temperature of at least 60°C. On the other hand, it can be suitably included in materials which are shaped into cores, e. g., by molding at lower temperatures. When a shaped core is used in an injection molding process, the blowing agent present in the core is generally not activated, because the operating conditions (residence time, prevailing pressure, temperature transfer etc.) in the injection molding process do not initiate the decomposition reaction in the core.

In a very much preferred embodiment the blowing agent is water. This blowing agent can be activated by heating the injection molded product to the extent that steam arises from water. Preferably, this is done by heating the core with microwaves, e. g. generated with a microwave oven.

Under the influence of the generated microwaves the water is heated and steam is formed. This causes the core to foam more or less out of the injection molded product, after which it can be easily removed. When using a core material containing water and an injection molding material not containing water, the material for the intended end product is not impaired. When using water and bicarbonate as blowing agent, an intensified blowing action is found.

Examples of moldable materials which, according to the invention, contain blowing agents or in which blowing agents can be included, are preferably biodegradable macromolecules, such as biodegradable polysaccharides, synthetically or fermentatively prepared biodegradable polyesters, such as polyhydroxyalkanoates and polyvinyl alcohols. Especially the above-cited publication WO-A- 95/07170 refers to suitable classes of compounds.

The above materials are shaped into a core by known per se methods. This can be done, e. g., by molding, extruding and injection molding. As stated above, the moldable materials are preferably shaped into a core by injection molding.

To obtain good results, at least 3 and preferably at least 5 wt. % of a blowing agent are generally present in a core, although, depending on the type of blowing agent and the nature of the material of which the core mainly consists, lower amounts are sometimes already satisfactory.

In a preferred embodiment the core is shaped from a moldable material, and preferably an injection moldable material, selected from the group consisting of starches, e. g. potato, wheat, maize or tapioca starch, celluloses, inulin, proteins, such as gluten and caseinates, as well as derivatives and/or mixtures thereof. A very important advantage of this class of materials is that the raw materials therefor can be easily supplemented by nature.

Moreover, these polymers already. naturally contain a certain amount of water, so that no separate steps need to be taken to distribute a blowing agent uniformly through the core. Thus, potato starch contains between 5 and 30 wt. %, and more usually between 10 and 20 wt. % water; tapioca starch between about 5 and 15 wt. % water; cellulose between about 7 and 13 wt. % water, and inulin about 10 wt. % water.

Suitable injection molding formulations of the above materials are known to those skilled in the art and are

described, inter alia, in Dutch patent 1001036, in the brochure by Avebe on the product PARAGON, in Kunststoff Magazine 8/9 (1996), 28-29, and in Kunststof en Rubber 4 (1996) 21-23. The starch and/or cellulose formulations may contain up to 25% additives, such as fillers, flow- improving agents etc., which are preferably also biodegradable.

The above injection moldable materials on the basis of starch, cellulose, inulin and proteins, which are used to form the above cores according to the invention, can be injection molded into products without collapse or shrinkage cavities. The shaped cores can be solid without any problem, which positively influences the dimensional stability of the core. Furthermore, no thermal deformation of the core takes place during circumjection, because for a short period of time the thus shaped cores can excellently resist temperatures up to about 300°C, as may occur in the injection molding process. Moreover, this preferred core does not adhere to the material from which the final hollow-walled product is manufactured, which is particularly important to remove the filler from the injection molded product as completely as possible. Because of the nature of the core material, no waste streams involving a risk to the environment can be released.

Besides, this type of cores can be excellently circumjected with materials known for injection molding uses, such as polyamides, polyesters and polyolefins, which may be fiber-reinforced, if required. These polymers are not limited to the water-insoluble polymers.

When a core is manufactured from injection moldable starch, it is advantageous if the injection moldable composition partly, e. g. between 10 and 30 wt. %, contains a starch hydrolysate. Such hydrolysates are obtained by partially decomposing native or modified starch by chemical or enzymatic hydrolysis to obtain a shorter-chain starch (Dextrose Equivalents of about 5-30). Through the presence

of such hydrolyzed products the surface quality of the core, and thus the quality of the surface of circumjected material that comes into contact with the core, increases.

In the method according to the invention for manufacturing a hollow injection molded product, an injection molding material is injected into a mold in which a core according to the invention is placed, in fact in an analogous manner as for the known processes using melting cores, salt cores and dissolving cores. Besides, when using those embodiments of the invention in which an injection moldable core material is employed, it is possible to use injection molding technologies, such as multi-component injection molding.

Many of the cores used according to the invention can in principle also be removed by means of (hot) water.

However, this removal step proceeds slowly, while, moreover, not all the injection molded plastic materials have an equally good water resistance, for which polyamides, and particularly nylons, are used by way of example.

The invention will now be illustrated with reference to the following non-limitative examples.

Example 1 From thermoplastic starch which optionally contains sodium bicarbonate molded bodies. were manufactured by injection molding. The compositions from which the molded bodies are shaped are specified in Table 1.

Table 1 Thermoplastic starch sodium bicarbonate (wt. 1 Paragon IM 10122 - Paragon IM 1010 5 Paragon IM 10002- Paragon IM 1000 5 Paragon IM 1000 10 1 based on the weight of the thermoplastic starch.

2 commercial product of Avebe, The Netherlands.

The thermoplastic starch had a glass transition temperature of about 80°C, which temperature is far above the mold temperature during injection molding.

The cores were circumjected with pre-dried polyamide (Durethan BKV 30H) on a DEMAG-NC-IIIK (60 tons) injection molding machine. In spite of high temperatures (polyamide: 260°C ; mold: 60°C) no deformations or burning effects were observed. The core did not adhere to the polyamide product.

The core was removed by heating the polyamide/ thermoplastic starch product in a 1500 W microwave oven.

Both types of Paragon showed a similar foam-out behavior.

After about 30-60 seconds (depending on the amount of core), foaming out of a dry, hard foam was observed.

According as more sodium bicarbonate was present at the blowing agent, water, a more homogeneous foaming behavior was observed. It is assumed that with the water/bicarbonate combination locally accelerated foaming out occurs less quickly.

Mas PCT 0624 Example 2 comparative Example 1 was repeated using a core of polyhydroxyalkanoate (Biopol ; Monsanto). This material contains neither water nor any other blowing agent. Injection molding of more thick-walled cores was found to be difficult, because Biopol slowly crystallizes. The cycle times during injection molding were longer as compared to the thermoplastic starch according to Example 1 The core could hardly be removed using microwaves. No foaming occurred; the material became soft and week.

Example 3 comparative Example 1 was repeated using polyvinyl alcohol (Mowiol 28-99; Hoechst). This material had to be dried before it could be processed by injection molding; it contained neither water nor any other blowing agent. Removal of the core using micro waves with water functioning as blowing agent gave a soft, sticky foam. The removal of this foam proceeded with a little more difficulty than with the dry, hard starch foam.

Example4 Cores were manufactured in conformity with the procedure as described in Example 1 from the following compositions.

Composition 1: Paragon IM 1010 (Avebe, The ; Composition 2: Paragon IM 1010 (82.4 wt%) Paselli (17.6%; cold-soluble starch, which is enzymatically partially hydrolyzed; Avebe, the Netherlands); Composition 3: Paragon IM 1010 (76.1 wt %) Paselli (23.9 wt %) Composition 4: Paragon IM 1010 (63.4 wt %) Paselli (36.6 wt %).

The resulting cores were circumjected with three injections molding plastic materials which require different

processing temperatures: Styrene-acrylonitrile (LURAN 368R; processing temperature 220°C ; glass transition temperature 105°C ; optically transparent) Polycarbonate (Bayer Makrolon 2805; processing temperature 300°C ; glass transition temperature 148°C ; optically transparent) Polyamide, glass-filled 30% (BKV-30; processing temperature 260°C ; glass transition temperature 50°C ; optically not transparent).

Example 4a By injection molding of composition 4 at a temperature of 145°C and a mold temperature of 25°C a core was obtained. This core was circumjected with styrene- acrylonitrile at a temperature of 220°C and a mold temperature of 30°C. The core was foamed out by heating the product in a microwave oven for 1 minute at a power of 700 W. The core could be manually withdrawn from the product.

Example 4 By injection molding of composition 4 at a temperature of 145°C and a mold temperature of 25°C a core was obtained. This core was circumjected with polycarbonate at a temperature of 300°C and a mold temperature of 80°C. The core was foamed out by heating the product in a microwave oven for 1 minute at a power of. 700 W. The core could be manually withdrawn from the product.

Example 4c By injection molding of composition 4 at a temperature of 145°C and a mold temperature of 25°C a core was obtained. This core was circumjected with glass-filled polyamide at a temperature of 260°C and a mold temperature of 20°C. The core was foamed out by heating the product in

a microwave oven for 1 minute at a power of 700 W. The core could be manually withdrawn from the product.

Example4d By injection molding of composition 1 at a temperature of 145°C and a mold temperature of 25°C a core was obtained.

This core was circumjected with styrene-acrylonitrile at a temperature of 220°C and a mold temperature of 30°C. The core was foamed out by heating the product in a microwave oven for 1 minute at a power of 700 W. The core could be manually withdrawn from the product.

When comparing Examples 4a-4d, the surface of the cores was found to be smoothest according as more Paselli was present.

Example 4e By injection molding of composition 2 at a temperature of 145°C and a mold temperature of 25°C a core was obtained.

This core was circumjected with polycarbonate at a temperature of 300°C and a mold temperature of 80°C. The core was foamed out by heating the product in a microwave oven for 1 minute at a power of 700 W. The core could be manually withdrawn from the product.

Example 4f By injection molding of composition 3 at a temperature of 145°C and a mold temperature of 25°C a core was obtained.

This core was circumjected with glass-filled polyamide at a temperature of 260°C and a mold temperature of 20°C. The core was foamed out by heating the product in a microwave oven for 1 minute at a power of 700 W. The core could be manually withdrawn from the product Example 4g comparative

By injection molding of composition 4 at a temperature of 145°C and a mold temperature of 25°C a core was obtained.

This core was circumjected with styrene-acrylonitrile at a temperature of 220°C and a mold temperature of 30°C. The circumjected core was placed in hot water. The core slowly dissolved; not all the core was removed.

Example 4h comparative By injection molding of composition 4 at a temperature of 145°C and a mold temperature of 25°C a core was obtained.

This core was circumjected with styrene-acrylonitrile at a temperature of 220°C and a mold temperature of 30°C. The circumjected core was placed in an aqueous a-amylase solution of 70°C. Although dissolving proceeded a little more rapidly than in Example 4g, not all the starch was dissolved yet after 24 hours.

Example 4i comparative By injection molding of composition 4 at a temperature of 145°C and a mold temperature of 25°C a core was obtained.

This core was circumjected with styrene-acrylonitrile at a temperature of 220°C and a mold temperature of 30°C. The circumjected core was subjected to an ultrasonic pulverization treatment. The core melted and burned locally.