DE2318685A1 - Foamed plastics with defined texture - made by injecting to defined temp programme - Google Patents

Abstract

Structural (i.e. having a defined skin and core structure) foamed plastics with a smooth, hard outer skin can be produced by following temp. programme: the smelt with expanding agent is introduced first at low, then high, then low temp. (the high being at least sufficient to cause foaming); the control may be in the collector zone of the plasticising barrel and/or in the injection sequence, esp. by varying the shear deformation, i.e. by slow-fast-slow screw speeds.

Description

Method and device for the production of structural foam parts made of plastic The invention relates to a method for injection molding plastic structural foam parts, in which an injection molding material provided with a propellant is melted, simultaneously but the melting temperature is kept below the boiling temperature of the blowing agent and only the material forming the foamed molding core during injection the temperature required for foaming is reached in the mold cavity, as well as a device for performing the method, comprising a tool and a Injection molding machine with a program-controlled nozzle opening or with a program-controlled one Screw feed speed and an adjustable nozzle opening.

In principle, thermoplastic foam parts can be used on any injection molding machine The disadvantage is often the poor surface and the uneven foam structure thermoplastic foam molding machines with very high Injection speeds overcome these disadvantages by a considerable amount To diminish wet. Nevertheless, newer developments are reserved to use foam parts smooth surface, compact outer skin and evenly foamed core lower To achieve density.

In the meantime, there are some ways to achieve this goal become known: a) temperature-controlled tool b) counter pressure in the tool c) coated Tool d) two plasticizing units, one with a compact and one with a propellant-containing ' Sehmelze (British Patent No. 1.156.217) e) subsequent addition of a propellant (P 2161 742.5) f) Text temperature increase of the entire molding compound only during injection at or above the foaming temperature with an expandable tool (P 2204 157.2) The The procedures carried out are either very time-consuming or not functionally reliable.

The invention is therefore based on the object of providing a method for Injection molding of structural foam parts with a smooth surface, compact outer skin and to create foamed core that eliminates the disadvantages mentioned.

This object is achieved according to the invention in that the propellant-containing Injection molding compound plasticized below the foaming temperature of the propellant and only during the injection of the mixture that the foamed molding core brought forming material to at least the boiling temperature of the mixture and the material which forms the compact outer skin and which compactly seals the sprue below this temperature is kept that the mixture in a cooled, already forming the final dimensions of the molded part or if necessary (for certain geometries) Only after the required mass has been injected into the mold cavity that can be expanded to its final dimensions with is injected at the appropriate speed.

To carry out the method according to the invention, a device (i) used on an injection molding machine (Fig.i) between tool (2) and cylinder (3) is attached. By means of this device the effect exploited the fact that plastic melts spontaneously at high shear rates heat | 1, 21. By changing the shear rate during injection Melt containing blowing agent in the mold cavity can be used for production of a structural foam part with the above-mentioned properties, desired course of the melt temperature (Fig.2). It is possible to change the shear rate: a) by changing it of the flow channel (nozzle opening) (4) with constant flow speed of the Melt b) by changing the flow rate while the nozzle opening remains the same c) by a combination of a) and 'b).

to a) One possibility of the temperature curve sketched in Fig. 2 is to achieve, the nozzle opening (4) by moving the needle accordingly (5) to be positively controlled (7) via the feed of the screw (6). To comply with defined Adjustable stops (8) can be provided for gap widths (4). Needle point and nozzle bores can be conical or cylindrical. To an inadmissible Appropriate temperature control can prevent the nozzle and needle from heating up (9) can be provided.

to In order to achieve the temperature profile sketched in Fig with a constant nozzle opening (4) the screw advance speed accordingly controlled (e.g. slow-fast-slow).

Claims (2)

Claims 1. Process for the production of structural foam parts made of plastic with a smooth surface, compact outer skin and foamed core through influence the temperature profile of the blowing agent-containing plastic melt entering the tool, d a d u r c h e k e n n n z e i c h n e t that the temperature of the successive in The proportion of melt entering the mold cavity is low, high (at least equal to Foaming temperature of the mixture), is low, this temperature profile can be achieved by a) a corresponding mass temperature curve in the melt collecting space of the plasticizing cylinder b) by measures taken during injection, preferably by changing the shear deformation of the melt. 2. Apparatus for performing the method according to claim 1, d it is noted that the flow channel is during the injection is changed from the melt collection chamber to the mold cavity (an example is shown in Fig.i given).