DE19534836A1 - Low cost, recyclable water-soluble mould core for injection moulding hollow plastic parts - Google Patents

Abstract

A water-soluble mould core for the injection molding of plastic parts with cavities and/or undercuts, which is placed in the mould and removed by dissolution after demoulding. The core (I) contains (A) at least 95 wt% sharp sand and/or glass particles and/or metal powder and (B) at least 1 wt% aq. binder soln. contg. 25-35 wt% polyvinyl alcohol (PVA). Also claimed is a process for the prodn. of core (I). Pref. metal powder is Al, bronze or Cu. Solid (A) has a particle size of 0.1-0.3 mm and a narrow particle size distribution, and consists of uniformly round particles. The core may be coated with at least one pore-sealing layer consisting of an anhydrous size with a thickness of 0.1-0.3 mm, pref. comprising sharp sand and/or graphite with a particle size of less than 0.07 mm and a solids content of at least 60 wt%. This may also be coated with a second layer based on PVA or polyacrylate.

Description

The invention relates to a water-soluble mold core for injection molding plastic share with undercuts and / or cavities and a manufacturing process of the mandrel.

The injection molding of plastic parts with undercuts and / or cavities is only possible using cores that are specially designed for this purpose. In practice, it will in processes with a reusable core with removable inner core, in processes with removable core and in processes with lost cores. Lost cores are molded cores, which together with the injection molded body ejected from the tool and then by disassembling it, Breaking, smashing, melting or removing by removal. Known are z. B. meltable mold cores made of low-melting metal alloys (DE-OS 29 22 303, EP 0 178 093) or water-soluble mold cores based on ceramic Mixtures (DE-OS 38 32 370).

The mold cores to be used must on the one hand be subjected to the thermal loads the injected plastic melt withstand and should on the other hand after the Ent shape the molded body from these can be easily and quickly removed. The latter practices have a major impact on the economics of the process. What is essential is further indicates that the mandrels have sufficient mechanical strength. In the DE-OS 37 36 574 describes a water-soluble mold core which consists of at least one partially water-soluble plastic based on acrylic acid or methacrylic acid, consists. Such a mandrel must have a modulus of elasticity of at least 5000 N / mm², one Elongation at break of less than 10% and short-term thermal stability in loading possess from 70 ° C to 150 ° C. The higher the mechanical property values of the The core is the more difficult and time-consuming it is remove material from the molded body. Due to the partially occurring high injection speeds during injection molding and the high ther mix loads, the mandrel must have high mechanical property values point, otherwise there is a risk that the mold core used during the  Injection molding breaks and rejects arise.

Even slight deformations of the mandrel do not result in quality Molded parts. The handling of the mold cores before and after the injection molding Process plays an essential role in the economics of the process. From to part is the high weight of the mold cores, e.g. B. weighs a metal melt core Intake pipe system for motor vehicles approx. 50 kg.

The object of the invention was to create a water-soluble mold core, which is inexpensive to manufacture, recyclable and can be made from art in a relatively short time removable fabric part. It was also an object of the invention to provide a suitable method to create the mold core.

According to the invention the object is achieved in that the mandrel consists of at least 95% by mass of quartz sand and / or glass particles and / or metal powder and at least 1% by mass of an aqueous binder solution based on polyvinyl alcohol with a poly vinyl alcohol content of 25 mass% to 35 mass%. This mold core enables one very economical method of injection molding with lost cores. The Metal powder can consist of aluminum, bronze or copper. It is advantageous if Quartz sand, glass and metal powder particles have a particle size of 0.1 to 0.3 mm and have a narrow grain size distribution and a uniform round shape. When using metal powder as a solid there is the additional advantage that the surface of the core faster when overmoulding the cores in the injection mold cools down and the binder is less thermally stressed. That has the result that molded parts with an almost smooth inner surface are obtained will. In all cases where the injection molding has a particularly good internal surface quality molded part is desired, it has proven to be useful if a mold core is used, which is covered with at least one pore-closing layer, which consists of a 0.1 mm to 0.3 mm thick water-free top coat, which consists of Quartz sand and / or graphite with a grain size of less than 0.07 mm is formed, and has a solids content of at least 60% by mass. In addition, the shape core with a second layer based on polyvinyl alcohol or polyacrylate be covered. The 0.1 to 0.3 mm thick top layer applied as the lower layer closes the pores between the molding material particles and thus improves the surface surface quality of the mandrel. It also protects the core binder from strong thermi shear stress during injection molding and prevents penetration of the Resin solution when applying a second layer on the mandrel. The solution and  and diluent of the top coat must be anhydrous in order to Exclude from loosening the water-soluble core binder. As a solution and Diluent is e.g. B. isopropanol suitable. The after drying the Second coat applied, consisting of a water-soluble Synthetic resin, gives the mold core with the top coat a high abrasion resistance and ensures the water solubility of the entire core when removing the Mold core from the plastic part after injection molding.

The following process steps are provided for producing the mold cores:

• a) Preparation of a storable binder solution based on polyvinyl alcohol with a polyvinyl alcohol content of 25% by mass in a heatable agitator by adding water and adding polyvinyl alcohol while stirring simultaneous heating up to a temperature of 80 ° C to the full the solution of polyvinyl alcohol,
• b) mixing the solids quartz sand and / or glass particles and / or metal powder with the binder solution prepared according to a) in a mixer in one quantity solid / binder solution ratio of 95: 5 to 99: 1, the solid being initially charged will, and
• c) feeding the free-flowing molding material into a core shooter and formation the desired mold cores at a pressure of 3.5 bar to 6.5 bar and a core box temperature of 150 ° C to 180 ° C.

The hardened mold cores can be retrofitted with a pore-closing one water-free size made of quartz sand and / or graphite with a grain size of smaller than 0.07 mm and a solids content of at least 60% by mass, for. B. by dipping into an isopropanol-based magnesium silicate coating. Finally can the mold cores with a synthetic resin layer, for. B. from polyvinyl alcohol be pulled, e.g. B. with an aqueous-alcoholic polyvinyl alcohol solution with a Polyvinyl alcohol content of 18% by mass. After applying the respective coating the mold cores are dried.

The flexural strength of the mold cores thus produced, coated or uncoated, be carries up to 10 MPa.

Depending on the machine setting and the location of the gate for the forth Plastic parts to be supplied range from the mechanical strength values of the invention  appropriate mold cores to ver a core break during injection molding prevent.

Another advantage of these mold cores is that after the Ent Form the plastic part completely disintegrate in water within a few minutes. The dissolution time is about 3 to 5 minutes for uncoated cores and for coated ones Cores approx. 7 to 10 min, at a water temperature of 20 ° C. At higher water temperatures can also be achieved with shorter dissolution times. For profitability However, it makes sense to do this in the process water circuit of an injection molding company to use available water that has a temperature of about 20 ° C. The solid used in each case and the aqueous polyvinyl alcohol solution can on be separated easily. Dispose of the approx. 0.1 to 0.3 mass% In practice, polyvinyl alcohol solution poses no problems. The resulting solid is cleaned and used in the cycle to produce new mold cores. The material composition for the mold cores is caused by the possible Reuse the solid at very low cost. Thus, the spray casting process with lost cores at much more economical conditions be performed.

The invention will be explained in more detail below using a few examples.

example 1

The water-soluble mandrel is produced using the following procedure steps.

a) Preparation of the binder solution

350 g of polyvinyl alcohol (Mowiol 4-88, Hoechst) are stirred in 650 g of water heated to a temperature of 80 ° C. After a complete solution will be avoided cooled from skin formation with constant stirring. The 35% by mass thus produced Polyvinyl alcohol solution is storable and must be stirred before reuse.  

b) mixing the solid with the binder solution (production of the molding material)

1900 g of Hohenbocka quartz sand (grain size approx. 0.2 mm) are in a wing mixer submitted and together with 100 g of a binder solution prepared according to a) for about 3 min touched. The molding material thus produced with a binder content of 1.75 mass% can stored for several hours in closed containers and therefore ver be working.

c) Production of the mold core (core shooting)

On a core shooter typical for foundries with a heatable mold the mold cores are at a pressure of 4.0 bar and a core box temperature of 170 ° C manufactured. After a dwell time of 90 s, the mandrel is removed from the mold tool removed by hand. After a 24- to 48-hour storage, it turns off Hardened mold core used directly during injection molding or first coated.

The coating is carried out as follows, with two different layers be applied:

A mold core produced in accordance with process steps a) to c) is immersed by dipping coated with a magnesium silicate-graphite coating based on isopropanol (cons TG-AK 140, KBO). After drying this, about 0.2 mm thick top coat, is the mandrel with an aqueous isopropanolic polyvinyl alcohol solution with a Polyvinyl alcohol content of 18.0 Ma .-% coated and then dried. The The flexural strength of the molded core coated in this way is 6.7 MPa.

The following is the use of the mold cores in the injection molding of a curved part Suction pipe made of polyamide 6.6 with 30 Ma .-% glass fibers for internal combustion engines explained the intake pipes each with an uncoated core and with a coat core can be produced on an injection molding machine of the Krauss-Maffei type 900-250 (clamping force 2500 kN) in a single tool.

A clamping device is used to insert the mandrel into the injection mold used, which has a tolerance-compensating effect due to its elastic deformability and serves the core recording. The location of the core in the injection mold is furthermore fixed with a core support. The time the core support is withdrawn is determined via the machine setting.  

After the end of the injection molding cycle after a cycle time of 25 s, the on Suction tube with the mandrel removed from the tool using a manipulator and transported on a conveyor belt into a 20 ° C water bath.

The dissolution time for the uncoated mold cores is approx. 3 to 5 min and for the be layered mold cores approx. 7 to 10 min. After the dissolving time of the mold cores, it will Intake pipe removed from the water bath. The quality of the intake pipes corresponded the required standards, taking the intake pipes with the coated form cores were characterized by a particularly good inner surface quality nen.

Example 2

The water-soluble mandrel was produced analogously to the process steps according to Example 1, but with the quantitative ratio of solid / binder solution was changed.

a) Preparation of the binder solution

The binder solution was prepared analogously to example 1.

b) mixing the solid with the binder solution (production of the molding material)

1940 g of Hohenbocka quartz sand (grain size approx. 0.2 mm) are mixed in a wing mixer submitted and ge together with 60 g of a binder solution prepared according to a) for about 3 min stirs. The molding material thus produced with a binder content of 1.05% by mass can be several Stored for hours in closed containers and therefore processed in batches will.

c) Production of the mold core (core shooting)

The mold core was produced under the same conditions as in Example 1. However, the mandrel thus produced was not coated. As a result of the relatively low proportion of binder in the mandrel, the bending strength of the uncoated mold core 4.8 MPa.  

Example 3

The water-soluble mandrel was produced analogously to the process steps according to Example 1, but using a different solid.

a) Preparation of the binder solution

The binder solution was prepared analogously to example 1.

b) mixing the solid with the binder solution (production of the molding material)

1900 g micro glass balls (Spheriglas 1922, Ballotini) with a defined grain size 0.15 to 0.25 mm are placed in a wing mixer and together with 100 g a binder solution prepared according to a) is stirred for about 3 min. The molding material thus produced with a binder content of 1.75 mass% can be used for several hours in closed containers kept in containers and therefore manufactured in batches.

c) Production of the mold core (core shooting)

The production and the coating of the mandrel were carried out under the same Be conditions as in example 1. The flexural strength of the coated mandrel is 7.8 MPa. The higher bending strength compared to example 1 is due to a more uniform and narrower grain size distribution of the solid consisting of micro glass spheres respectively.

The mold cores were used in injection molding under the same conditions as in Example 1. The dissolution time for the mandrel according to Example 2 was 1 to 3 minutes and for the mandrel according to Example 3 7 to 10 min.

Claims (11)

1. Water-soluble mold core for injection molding of plastic parts with cavities and / or undercuts, the core being inserted into the injection mold, then the melt being injected and, after the plastic part has been removed from the mold, the mold core being removed by loosening, characterized in that it consists of at least 95 Ma. -% quartz sand and / or glass particles and / or metal powder and at least 1 mass% of an aqueous binder solution based on polyvinyl alcohol with a polyvinyl alcohol content of 25 mass% to 35 mass%. 2. Mold core according to claim 1, characterized in that the metal powder made of aluminum minium, bronze or copper. 3. Mold core according to one of claims 1 or 2, characterized in that the Quartz sand, the glass particles and the metal powder components have a particle size of 0.1 to 0.3 mm and have a narrow grain size distribution. 4. Mold core according to one of claims 1 to 3, characterized in that the Quartz sand, glass and metal powder particles have a uniform round shape. 5. Mold core according to one of claims 1 to 4, characterized in that this is coated with at least one pore-closing layer consisting of a There is 0.1 mm to 0.3 mm thick water-free top coat. 6. Mold core according to claim 5, characterized in that the top coat from Quartz sand and / or graphite with a grain size of less than 0.07 mm, and has a solids content of at least 60% by mass. 7. Mold core according to one of claims 1 to 6, characterized in that this with a second layer, based on polyvinyl alcohol or polyacrylate is moved. 8. A method for producing a mandrel according to one of claims 1 to 7, by the following process steps:

• a) Preparation of a storable binder solution based on polyvinyl alcohol with a polyvinyl alcohol content of 25% by mass to 35% by mass in a heatable agitator by placing water and adding polyvinyl alcohol with stirring while heating to a temperature of 80 ° C until the polyvinyl alcohol has completely dissolved,
• b) mixing the solids quartz sand and / or glass particles and / or metal powder with the binder solution prepared according to a) in a mixer in a ratio of solid / binder solution of 95: 5 to 99: 1, the solid being initially charged, and
• c) Feeding the free-flowing molding material into a core shooter and forming the desired mold cores at a pressure of 3.5 bar to 6.5 bar and a core box temperature of 150 ° C to 180 ° C.

9. The method according to claim 8, characterized in that the mold cores with a pore-sealing water-free size made of quartz sand and / or graphite a grain size of less than 0.07 mm and a solids content of at least 60% by mass can be coated. 10. The method according to any one of claims 8 or 9, characterized in that the Mold cores are coated with a water-soluble synthetic resin layer. 11. The method according to claim 10, characterized in that the synthetic resin layer There is polyvinyl alcohol or polyacrylate.