WO2006032732A1

WO2006032732A1 - A method and device for injection moulding

Info

Publication number: WO2006032732A1
Application number: PCT/FI2005/050324
Authority: WO
Inventors: Risto Sandell; Pentti JÄRVELÄ
Original assignee: Risto Sandell; Jaervelae Pentti
Priority date: 2004-09-21
Filing date: 2005-09-21
Publication date: 2006-03-30
Also published as: FI20041215A; EP1799422A1; FI20041215A0

Abstract

In a method of injection moulding, a solid thermoplastic material (R) is subjected to a stroke whose energy brings it to a molten or flowable state, and is led into a mould cavity (12), in which the material is solidified and is given a shape defined by the mould cavity. The thermoplastic material (R) is pressed in the impact space against a solid counter surface, i.e. an anvil, from which it is led along a feed channel or channels (11) into the mould cavity (12).

Description

A method and a device for injection moulding

The invention relates to method for injection moulding which is of the type presented in the preamble of the appended claim 1. The invention also relates to a device for injection moulding.

For manufacturing plastic pieces of a given shape, injection moulding is generally applied, in which a thermoplastic polymer together with addi¬ tives are melted thermally, injected into a mould cavity defining the shape of the piece, the molten material is cooled down, and the solidi¬ fied piece is removed from the mould. A problem with these known methods is that it is particularly difficult to manufacture small pieces with a high dimensional accuracy (the feeding of the molten material and the even filling of the mould). Furthermore, the manufacturing speed is not the best possible either (cf. the above-listed successive work stages). There are numerous examples of small plastic parts with a high dimensional accuracy. Various devices of precision mechanics require plastic parts with a high dimensional accuracy. Another exam-, pie is quite ordinary household utensils, such as plastic spoons.

Attempts to solve the problems involved in the manufacturing of small parts have been primarily based on the modification of conventional injection moulding techniques, relating to the pretreatment and feeding of the molten raw material, and the properties of the mould (so-called micro injection moulding techniques).

US patent 3,198,569 discloses a method for manufacturing cup-shaped articles by using two plungers or impact hammers moving against each other and pressing the plastic material supplied between them into a mould cavity at the same time when the kinetic energy of their impact is converted to heat, melting the plastic. The force by which this impact is generated is not disclosed in more detail in the publication.

Although it is possible, in principle, to use this method to melt the plas- tic and to feed it into the moulding cavity in one single impact stage, the method involves some disadvantages. A high kinetic energy can be achieved by the cooperation of the impact hammers moving towards each other, but they must be made to move simultaneously, that is, they must be synchronized.

It is an aim of the invention to present an improved method for injection moulding by which it is possible to make small pieces (typically having a weight of less than 1 g) with accurate dimensions, the shape of the pieces possibly being very complex, in short manufacturing cycles and with a relatively simple device. To attain this purpose, the method according to the invention is primarily characterized in what will be pre¬ sented in the characterizing part of the appended claim 1.

In the invention, the plastic material is pressed and melted by an impact hammer against a solid counter surface which is used as a kind of anvil. The material is led from the counter, surface along a feed channel . or channels into :the mould cavity. According to an advanta¬ geous embodiment, the counter surface is on the surface of a backing piece which also forms one wall of said feed' channel or channels.

The impact of the impact hammer which makes the solid plastic mate¬ rial melt quickly, can be generated by explosion energy ("firing" of the plunger) or by using an adaptive material operating through quick response to effect a quick movement by means of an external control variable affecting the dimensions of the material (for example a mag- netic field).

In the following, the invention will be described in more detail with ref¬ erence to the appended drawings, in which

Fig. 1 is a general view on the device,

Fig. 2 shows a detail A of the device, and

Fig. 3 shows another embodiment of the device. Figure 1 shows a moulding device comprising an impact channel 6 formed in a frame piece 3. An impact plunger 2 is arranged to move axially in the impact channel. The impact channel opens to the lower surface of the frame piece 3, against which a backing piece 7 is placed, its surface forming a solid counter surface used as a kind of anvil to limit the impact space formed at the end of the impact channel while the space is limited on the opposite side by the plunger 2. The impact channel 6 consists of a sleeve fixed in a bore in the frame piece, the plunger 2 being, in turn, fitted inside the collar to be movable with a tight clearance. The backing piece 7 is placed on a mould table 8 arranged to be movable up and down by means of an actuator 9 to detach the backing piece 7 from the frame piece 3 and to bring it into contact with the same (opening and closing of the mould). An impact generator which is functionally connected to the plunger 2 is marked with reference numeral 1. A frame plate 4, to which the frame piece 3 is fixed, is supported by several connecting bars 5, which are fixed at their lower ends to a base plate 10 and which are simultaneously used as guides for the mould table 8 moving up and down.

Figure 2 illustrates a detail A in Fig. 1. It shows the lower end of the impact plunger 2 and the^' upper part of the backing piece 7. The impact channel 6 opens to the lower surface of the frame piece 3 which is against the upper surface of the backing piece 7 when the mould is closed before the moulding operation. The backing piece 7 and the end of the impact plunger 2 together limit an impact space, where solid granular polymer-based raw material R has been supplied, which can be a synthetic polymer used in conventional injection moulding and which can be melted, supplied in the molten form into the mould defin¬ ing the shape of the piece to be moulded of the polymer, and solidified in this shape. The upper surface of the backing piece 7 is provided with a feed channel 11 extending away from the counter surface and from the impact space (in the radial direction with respect to the impact channel) and ending in the mould cavity 12 which is also formed in the upper surface of the backing piece 7, for example, by some working method. The backing piece 7 and the frame piece 3 constitute the halves of the mould which are separated from each other to open the mould and to remove the moulded piece. Like the impact plunger, they are made of a suitable metal, such as steel.

When the impact generator 1 is used to effect a quick impact on the impact plunger 2, it presses the solid thermoplastic material against the surface of the backing piece 7 at a high linear speed (for example, higher than 100 m/s) and converts the plastic material to a molten state and simultaneously presses it along the feed channel 11 into the mould cavity 12. After the molten material has flown into the mould cavity and filled the same, it is solidified. It is now possible to open the mould and remove the solidified moulded piece by lowering down the backing piece 7.

The melting of the thermoplastic polymer is based on the conversion of the kinetic energy of the impact plunger into thermal energy by the effect of the internal friction of the material to be worked. The method can be called "adiabatic injection moulding", as far as the conversion of the polymer from the solid state to a molten or liquid flowable state is concerned (no heat needs to be introduced from the outside).

In the impact channel, the plunger plasticizes the material. Because the flow cross-sectional area of the feed channel 11 is significantly smaller than the cross-sectional area of the impact channel 6 in the direction of the impact, the material to be moulded can enter the narrow channel 6 first when its viscosity is sufficiently low as a result of the plasticizing. As the material to be moulded flows in the feed channel 6, the viscosity may be reduced further by the effect of shearing forces caused by the walls of the channel, which reduces the inhomogeneity of the material to be moulded.

The melting, cooling and solidification of the material can be influenced by controlling the temperature at different points of the device. The raw material itself may be preheated to a temperature that is higher than the room temperature but is still below the melting point of the plastic so that the state of the material is not changed. The preheating can be used to accelerate the conversion of the polymer to the molten or flow- able state by the effect of the energy of the impact. The impact channel 6 may also be heated to a temperature which does not change the raw material. Similarly, the temperature of the mould itself (the backing piece 7) may be adjusted to control the solidification of the material in the mould cavity.

The invention is well suited to the moulding of such plastic materials which are thermally sensitive in the sense that they do not last long times at temperatures higher than their melting temperature, for exam¬ ple the biodegradable polymer PLA. Also other materials which are dif¬ ficult to be processed in the molten state, such as inherently electro- conductive polymers, may be used. The temperature-sensitive polymer which can be moulded by the method according to the invention, may be, for example, such that disintegrates at high temperatures by the effect of entrained water. In the method according to the invention, the _* material only needs to be kept for a short time at the temperature higher than the melting point, required for the moulding. However, the invention is also suitable for conventional thermoplastic polymers, such as polyolefins (PE and PP). -«

As the source of the impact energy, it is possible to use the pressure of an expanding gas which may be produced by an explosion process. For example, it is possible to use a pistol operated by an explosive and providing a quick stroke. It is possible to use, for example, known commercially available pistols operated by cartridges used in the con¬ struction industry, HILTI DX 460 being one but not the sole example. Such pistols comprise an impact plunger having a stroke of a prede- termined length and a desired impact velocity which depends on the cartridge.

Another alternative is to use energy that effects a very quick dimen¬ sional change in an adaptive material as the energy for effecting the stroke. Actuators made of adaptive materials can be defined by acronym ISA (induced-strain actuators), which are characterized by an induced strain by the effect of an electric field or a magnetic field. These include, for example, magnetostrictive materials, which have been subjected to extensive research and development and which the actuator effective on the impact plunger can be made of. It may also be necessary to provide the actuator with a motion amplifier by which the deformation of the material is converted to a stroke of sufficient length.

The invention is not limited to the structures shown in the appended figures. For example, in Fig. 2, the feed channel 11 may extend, in the lateral direction between the frame piece 3 and the backing piece 7, away from the impact space in only one direction, ending in the mould cavity 12. Similarly, it is possible that the feed channel and the mould cavity are, different from the illustration of Fig. 2, worked on the surface of the frame piece 3, and the surface of the backing piece 7 limiting them is flat. Also in this case, the interface of the mould halves consists of the abutting surfaces of the pieces 3 and 7. It is also possible that the feed channel 11 extends in the mould cavity 12 in another direction than the direction of the surface of the backing piece, for example, more or less in the direction of the impact, which requires different working of the mould than described above. The cross-sectional view of Fig. 2 shows two different parts of the same mould cavity 12 on the right and left hand sides, but it is also thinkable that the material to be moulded is led from the same impact channel simultaneously into two or more separate mould cavities to mould two or more separate pieces, respectively.

Figure 3 shows, as a representation of Fig. 2, yet another embodiment which can be used to combine two different batches of material to be moulded in the same moulded piece. The device comprises two impact channels 6, each provided with a separate impact plunger 2. A feed channel 11 for material to be moulded extends from each impact channel 6 into the same mould cavity 12. The possibilities to arrange the feed channels 11 and the mould cavity 12 in the device are the same as those described above. The first impact channel 6 contains the first raw material R1 (batch 1 of material to be moulded), and the second impact channel 6 contains the second raw material R2 (batch 2 of material to be moulded). The plungers 2 strike and plasticize the materials to be moulded simultaneously, wherein the material batches in molten or flowable form are mixed with each other, ending up via their own channels 11 in the same mould cavity 12. The channels and the mould cavity can be arranged in relation to each other in such a way that the polymer materials are mixed as well as possible. Figure 3 is only intended for illustrating the device and the method, and the placement of the impact channels 6 and the feed channels 11 may also be different; for example, the channels 11 may join before the mould cavity 12. In Fig. 3, the impact channels 6 are formed in the same frame piece 3, and the counter surfaces are formed in the same backing piece 7. The feed channels 11 and their common mould cavity 12 are formed by working the same backing piece, but they can also be formed in the frame piece 3 by the same principle as that described above with reference to Figs. 1 and 2..

The raw materials R1 , R2 may be different polymer materials with respect to either their physical properties or their chemical structure, but they may also be exactly the same material. The same material may be used, for example, if because of the shape of the moulded piece and the mould cavity, respectively, it is appropriate to feed the material from different directions from different impact channels and feed channels. In the embodiment of Fig. 3, the plungers 2 can be easily synchronized to operate simultaneously such that they are coupled to the same generator of impact energy.

The number of impact channels 6 and feed channels 11 extending from them may also be more than two if more than two material batches are to be combined to the same moulded piece, respectively.

The plastic particles may be fed into the impact space along a supply channel that opens into the impact channel 6. In Figs. 1 and 2, broken lines are used to illustrate one possible location of the supply channel of the raw material. In Fig. 2, the impact plunger 2 is shown in the impact stage after it has moved forward in the impact channel, past the mouth of the raw material supply channel. When a new batch of raw material is fed into the impact space, the impact plunger 2 is withdrawn sufficiently far back to the initial position, opening the connection from the supply channel to the impact channel 6.

The raw material batch to be plasticized in the impact channel 6 does not necessarily consist of particles, but it may be a single solid piece, for example a tablet compressed of particles. The advantage is thus that no air is entrapped in the raw material batch. Also more tablets than one such tablet can be dosed into the impact channel 6 at a time. For the dosing, a separate feeding device may be provided, which inserts a tablet/tablets at a suitable stage into the impact channel 6. The solid piece of fixed size, instead of granular raw material, is illustrated by broken lines in Fig. 3. The raw material batch, whether in powder form or one or more pieces of fixed size, contains not only thermoplastic polymer but possibly also other additives of the plastic to be included in the moulded piece.

The invention can also be used for the manufacture of such composite pieces which do not consist wholly of plastic material. For example, another material may be placed in the mould cavity 12, or its wall may consist partly of such a material (wood, metal, ceramics, etc.)

In both of the above-presented embodiments it is also possible to apply two-step operation of the plunger 2. By the first stroke, the plunger melts the plastic in the impact channel, in which step the connection from the impact space 6 to the mould cavity 12 is closed (the feed channel/channels 11 closed). By the second stroke, whose energy may be lower than in the preceding impact, the plunger 2 pushes the molten plastic material from the impact channel 6 via the feed channel 11 into the mould cavity 12, prior to which the feed channel 11 has been opened. The second stroke may be a continuation of the first stroke of the plunger, directed towards the counter surface. The movements of the means opening and closing the feed channel can be easily syn¬ chronized with the different stroke steps of the plunger by control automatics. In tests carried out with the invention (a pistol operated by catridges used as the actuator), the same tensile strength values of the manu¬ factured pieces have been achieved as with conventional methods of injection moulding, when PP and PE (polypropylene and polyethylene) have been used as the raw materials.

Claims

Claims:

1. A method for injection moulding, in which a solid thermoplastic mate¬ rial (R) is subjected to an impact, by whose energy it is brought into a molten or flowable state, and is led into a mould cavity (12) in which the material is solidified and is given a shape defined by the mould cavity, after which the moulded piece is removed from the mould, character¬ ized in that the thermoplastic material (R) in solid state is pressed in an impact space against a solid counter surface, i.e. an anvil, from which it is led along a feed channel or channels (11) to the mould cav¬ ity (12).

2. The method according to claim 1 , characterized in that the impact energy is generated by a gas pressure, for example by means of an explosion.

3. The method according to claim 1 , characterized in that the impact energy is generated by means of a solid material capable of deforming by a quick response, for example by means of an adaptive material.

4. The method according to any of the preceding claims, character¬ ized in that more than one batch (R1 , R2) of thermoplastic material are pressed in their separate impact spaces (6) and are led via their own feed channels (11) into the common mould cavity (12).

5. A device for injection moulding, comprising an impact plunger (2) which is arranged to be movable in an impact space and to effect an impact on a solid thermoplastic material (R) in the impact space, to bring it into a molten or flowable state, and a mould cavity (12), in which the thermoplastic material brought into a molten or liquid state is solidified and is given a shape defined by the mould cavity (12), char¬ acterized in that the impact space is limited, in the direction of the stroke of the plunger (2), by a solid counter surface, i.e. an anvil, one or more feed channels (11) extending from the anvil and ending in the mould cavity (12).

6. The device according to claim 5, characterized in that the solid counter surface is on the surface of a backing piece (7) which also forms one wall of said feed channel or channels (11).

7. The device according to claim 6, characterized in that the backing piece (7) and the piece (3) comprising the impact channel (6) and the impact plunger (2) constitute mould halves which are detachable from each other.

8. The device according to any of the preceding claims 5 to 7, charac¬ terized in that the impact generator (1), which generates the impact movement of the plunger (2), is operated by the pressure of an expanding gas, for example by an explosive.

9. The device according to any of the preceding claims 5 to 7, charac¬ terized in that the impact generator (1), which generates the impact movement of the plunger (2), is operated by an adaptive material capable of acting by a quick response, for example by means of a magnetostrictive material.

10.The device according to any of the preceding claims, characterized in that the device comprises more than one impact space (6), one or more feed channels (11) extending from each space and ending in the same mould cavity (12).