TEMPERATURE CONTROL DEVICE FOR A PLASTIFIING SCREW OF A PLASTIFIING UNIT

Description

Technical area

[0001] The present invention relates to a plasticizing screw with a tempering device according to claim 1. State of the art

[0002] According to the current state of the art, a plasticizing cylinder of an injection molding or extrusion machine consists of a single, one-piece component. This is a steel cylinder tube, which can be made of various grades depending on the material being processed and its wear resistance. Versions range from hardened steels and nitrided steels to centrifugally cast bimetallic cylinders for processing particularly abrasive plastic materials. Conventional heating of the plasticizing cylinder is achieved using ceramic heating bands that are clamped force-fit around the plasticizing cylinder (see also DE102012112747A1).

[0003] Another technical solution uses so-called heating coils instead of ceramic heating bands. For this purpose, the plasticizing cylinder has labyrinth-like grooves along its length into which square heating coils are inserted. The advantage of this solution is that the heater is positioned closer to the melt channel and thus reaches the process temperature more quickly. The larger contact surface of the heating coil is advantageous here, as it not only has a contact surface on the cylinder diameter, but also has two lateral contact surfaces with the plasticizing cylinder due to its integrated installation in the plasticizing cylinder (see also DE102018112939A1).

[0004] WO2010056281A1 describes another solution for heating plasticizing cylinders. Here, the plasticizing cylinder is coated with a plasma-sprayed, metallized ceramic surface, allowing it to be electrically heated.

[0005] Solutions are also known from Internet reports that use gas heating using natural gas ring elements instead of electrical heating (see, for example, Krauss-Maffei Kunststofftechnik GmbH, Munich) - a patent for this could not be found.

[0006] For cross-linkable plastics, liquid temperature control (bores in the plasticizing cylinder and, if necessary, the use of heat-conducting cartridges or pipe coils around the plasticizing cylinder) is used - these are described, for example, in DE3139024A1 or DE202014005708U1.

[0007] While plasticizing screws in injection molding have traditionally been constructed as a single-piece unit made of solid steel and without heating options, application-specific internally heated plasticizing screws are used in extrusion. The plasticizing screws are heated by internal heat pipes or spiral elements, as described, for example, in patent DE10013474A1.

[0008] The main disadvantage of the known solutions is the poor efficiency of the system. The external heating and the thick-walled plasticizing cylinder result in a high heat transfer torque. Temperature control is very sluggish and reacts too slowly to system drifts. This sluggishness also arises when heating the plasticizing cylinder, as the mass (wall thickness) of the cylinder must first be heated through. Furthermore, due to the external mounting of the heating elements, a significant amount of waste heat is radiated into the environment and does not reach the interior of the plasticizing cylinder as desired. The segmented arrangement of the external heating elements leads to thermal bridges and thus to inhomogeneous heat distribution – due to the design, seamless contact between two heating elements cannot be achieved.

[0009] In the event of an accident, leaks occur between the nozzle of the plasticizing unit and the mold nozzle. These leaks lead to molten plastic overmolding the external heating bands – in these cases, the only option is to replace the heating bands.

[0010] Processing disadvantages arise from the fact that the plastic granules only melt from the outside (the plastic granules melt on the inside wall of the cylinder), which accordingly requires longer melting times. To reduce this melting time, higher back pressure and faster screw speeds are often used - however, these two parameters also result in greater wear on the screw drive unit and a higher required drive power = power consumption. In addition, temperatures above the permissible processing temperature are selected to achieve the shortest possible melting times. However, the higher melt temperature due to the above points also means a higher article temperature, which rarely allows for damage-free demolding. For this reason, the finished part must undergo a longer cooling period before demolding, thus lengthening the cycle time.

[0011] When changing materials and colors, deposits often form on the non-tempered plasticizing screw, which then requires a complex cleaning process using chemical or physical cleaners to remove the deposits on the colder plasticizing screw.

[0012] From CN 111844611 A, an apparatus for injection molding is known which comprises a tempering device for a plasticizing cylinder of a plasticizing unit, wherein the plasticizing cylinder is formed from a hollow cylinder, in the central cavity of which the tempering device for tempering the plasticizing cylinder is accommodated, wherein the tempering device is formed from an inner cylinder for accommodation in the hollow cylinder, on the outer circumferential surface of which at least one flat heating element is arranged or applied.

[0013] DE 36 03 500 A1, DE 601 27 679 T2, and DE 38 89 410 T2 describe heating devices for a plasticizing cylinder. The plasticizing cylinder consists of an inner cylinder, on at least one of whose outer surfaces a flat heating element is arranged. The temperature control device is accommodated in the plasticizing cylinder, which is designed as a hollow cylinder.

[0014] KR 2005 0095728 A discloses an injection molding machine with a plasticizing screw designed as a hollow screw, with heating elements arranged on the inside of the hollow screw. A cable runs axially within the heating elements and is connected to a power source via a sliding contact. CN 107 053 624 B discloses an injection molding machine with a plasticizing screw designed as a hollow screw, with an electric heating tube arranged axially therein. A cavity filled with a heat transfer medium is formed between the inner wall of the hollow screw and the outer surface of the electric heating tube.

[0015] CN 207 901 523 U discloses an injection molding machine with a heating device in which a heating element is embedded in the interior of the plasticizing screw and connected to it. JP 2004 188601 A also describes an injection molding machine with a plasticizing screw, which can be heated from the inside using heating cartridges. JP 2004 219032 A shows a conveying and heating device for powdered or granular material, e.g., rice bran, in which a thin-film heater can be attached to the inside of a screw shaft. Description of the invention

[0016] The present invention is based on the object of eliminating the aforementioned disadvantages.

[0017] According to the invention, the above object is achieved according to the preamble of claim 1 in conjunction with the characterizing features. Advantageous embodiments and further developments of the plasticizing screw according to the invention are specified in the dependent subclaims. Brief description of the drawings

[0018] Further objects, features, advantages and possible applications of the plasticizing screw according to the invention will become apparent from the following description of embodiments with reference to the drawings.

[0019] The drawings show Fig. 1 the plasticizing screw according to the invention in an advantageous embodiment in view; Fig. 2 the plasticizing screw according to the invention in a further advantageous embodiment in side view. Implementation of the invention

[0020] As from Fig. 1 and from Fig. 2 As can be seen, the plasticizing screw 1' according to the invention is formed from a hollow cylinder 11, in the central cavity 110 of which the tempering device 10 for tempering the plasticizing screw 1' is accommodated.

[0021] The temperature control device 10 is formed from an inner cylinder 100 for receiving in the hollow cylinder 11, on the outer surface 1000 of which at least one flat heating element 101 is applied, wherein the flat heating element 101 is formed from a sleeve 1017 which is applied to the outer surface 1000 of the inner cylinder.

[0022] In a particularly advantageous embodiment of the invention, the at least one flat heating element 101 comprises at least one electrical resistance heating conductor for converting electrical energy into heat with at least two electrical connections 1011 which can be contacted in a connection area.

[0023] The at least one planar heating element 101 is preferably designed as a thick-film heater.

[0024] The plasticizing screw 1' according to Fig. 1is hollow inside and thus offers the possibility of accommodating the cylindrical temperature control device 10. This temperature control device 10 preferably consists of a thermally conductive inner cylinder 100 and a sleeve 1017 made of steel or ceramic. If the sleeve 1017 or the inner cylinder 100 is made of an electrically conductive material, a full-surface dielectric layer is first applied to the outer surface of the sleeve 1017 or the inner cylinder 100. Subsequently, a preferably meander-shaped heating structure of the heating element 101 is screen-printed on the sleeve 1017 or the inner cylinder 100. Contacting of the heating element 101 is effected via the start and end points of the printed resistance heater at one end of the sleeve 1017 or the inner cylinder 100.

[0025] Finally, a second full-surface dielectric or cover layer is printed on for further insulation and protection against mechanical influences. To ensure the precise positioning of the individual printing layers, the sleeve 1017 or the inner cylinder 100 has a small registration on one end to make it easier to adjust it in the rotary screen printing machine. The inner diameter of the sleeve 1017 with thick-film heating corresponds precisely to the outer diameter of the inner cylinder 100, so that it can be pushed on with a form-fitting fit at room temperature. The inner cylinder 100 has a collar on one side that corresponds to the inner diameter of the plasticizing screw 1'. This ensures the correct position of the sleeve 1017 and acts as a stop for it. To prevent the sleeve 1017 from slipping, it is secured at the other end of the inner cylinder with a locking pin 1013.The sleeve 1017 is thus positively positioned between a collar of the inner cylinder, a locking pin, and the outer diameter of the inner cylinder 100. The sleeve 1017 mounted on the inner cylinder 100 is then positively inserted into the plasticizing screw 1' at room temperature up to a defined stop. The correct and secure position of the mounted sleeve 1017 with thick-film heater on the inner cylinder material is secured within the plasticizing screw by a grub screw or screwed-in screw tip. In order to achieve a full-surface and force-fitting contact of the sleeve 1017 in the heated state, the inner cylinder 100 and the plasticizing screw 1' as well as the sleeve 1017 are made of materials with different thermal expansion coefficients. The inner cylinder 100 preferably has a higher thermal expansion coefficient than the plasticizing screw 1'.The sleeve 1017 preferably has a higher coefficient of thermal expansion than the inner cylinder 100. A longitudinal recess or cutout 1010 (preferably a slot) preferably extends over at least 75% of the length of the sleeve 1017 and supports the thermal expansion behavior. The opening 1012 (e.g., a bore) at the end of the longitudinal slot prevents the sleeve 1017 from completely tearing through or opening under thermomechanical stress. The electrical connection of the heating element 101 is preferably made via two bores in the vortex outlet 1014 of the plasticizing screw 1', with a spring contact with electrical insulation (to the plasticizing screw) being screwed into each of these two bores. These bores or screwed-in spring contacts are positioned correctly over the contact points of the heating element and thus represent an electrical connection via the spring-loaded contact.At the drive-side end of the plasticizing screw is a sliding or rotating contact for supplying electrical power to the heating element. The connection between the rotating part of the slip ring 1016 and the spring contacts in the vortex outlet of the plasticizing screw is established by means of heat-resistant electrical cables 1015. The heat-resistant electrical cables 1015 are preferably located in a groove in the longitudinal direction of the plasticizing screw 1' that protects it from external influences. The area of ​​the contacting solution described above is protected by a dust-, water-, and dirt-repellent housing and / or potting compound and also provides external electrical protection.

[0026] The key advantage of the invention lies in the increased heating efficiency. Due to the positioning of the heating elements in the plasticizing screw close to the melt channel, the system can be heated up significantly faster, thus resulting in a shorter downtime when restarting after a production interruption. Furthermore, the position of the heating elements allows for a rapid response to system drift, e.g., in the form of minor temperature fluctuations due to batch fluctuations in the plastic granulate.

[0027] Furthermore, the full-surface application of the thick-film heating eliminates thermal bridges between individual heating zones, preventing potential process fluctuations caused by inhomogeneous heat distribution. Homogeneous melting is achieved through the proven external heating system via the plasticizing cylinder and the innovative internal heating system via the plasticizing screw 1'.

[0028] Thanks to the technical solution of the internally heated plasticizing screw 1', almost 100% of the applied heat energy reaches the plastic granules. Thermal equilibrium in the cross-section of the melt channel is achieved by melting the plastic granules from the outside and inside. This reduces the risk of unmelted plastic granules and subsequent surface defects in the finished part. Since the area of ​​heat energy input is increased by the invention, the plastic granules no longer need to be melted at the highest processing temperature and possibly above it. To achieve the shortest plasticizing times and thus cycle times, this has been common practice to date and is detrimental to the material properties – thermal degradation of the material may occur.

[0029] Since the processing temperatures can be reduced due to the faster and more homogeneous heat input, a short cooling time and faster reaching of the demoulding temperature of the finished part is possible, which in turn leads to faster cycle times and increased output.

[0030] The homogeneous heat input allows for a reduction in the screw speed and the back pressure of the machine. The value of these two parameters influences, among other things, the frictional heat input into the plastic melt.

[0031] To compensate for the lack of heat output from the barrel heater and to achieve a more homogeneous plastic melt, these parameters are generally set higher in conventionally heated systems. However, higher screw speeds and higher back pressure negatively impact wear on the plasticizing screw drive unit and energy consumption.

[0032] If color and / or material changes are required on the machine, chemical or physical screw cleaners are used to clean the system. These achieve a cleaning effect on the screw through chemical reactions or abrasive action, dissolving existing deposits from previous production. By using the heated plasticizing screw 1', such cleaners can be eliminated in the future, as the deposits can be dissolved by increasing the temperature of the plasticizing screw.

[0033] Aerogel insulation keeps the introduced energy / temperature within the system of the plasticizing unit, reducing not only the energy input but, more importantly, the previously known waste heat losses. This may also eliminate the need for air conditioning in the production hall or the production environment for process-stable production.

[0034] An encapsulated structure using stainless steel sheet sheathing protects the underlying insulation from possible overspray and makes replacement of the insulation and other functional elements (e.g. temperature sensors and contacts) unnecessary in the event of an accident.

[0035] If the heating elements become defective, they can be replaced separately from the plasticizing screw 1' - existing plasticizing systems can be retrofitted by reworking.

[0036] Due to the more effective melting of the plastic granulate, the plasticizing unit can be shortened and thus requires less material (steel) and production space for setting up the machine. List of reference numbers

[0037] 1'Plasticizing screw 10Temperature control device 11Hollow cylinder 100Inner cylinder of the temperature control device 101Heating element 110Central cavity of the hollow cylinder 1000Shell surface of the inner cylinder 1010Recess or cutout 1011Electrical connections 1012Opening 1013Securing pin 1014Swirl outlet 1015Electrical cables 1016Slip ring 1017Sleeve LLongitudinal direction

Claims

1. Plasticising screw (1') with a temperature control device (10), wherein the plasticising screw (1') comprises a hollow cylinder (11), in whose central cavity (110) the temperature control device (10) for controlling the temperature of the plasticising screw (1') is accommodated, and wherein the temperature control device (10) is formed from an inner cylinder (100) for accommodation in the hollow cylinder (11), characterised in that at least one flat heating element (101) is applied to its outer surface (1000), wherein the flat heating element (101) is formed from a sleeve (1017) which is applied to the outer surface (1000) of the inner cylinder.

2. Plasticising screw (1') according to claim 1, dadurch gekennzeichnet, dass the at least one flat heating element (101) comprises at least one electrical resistance heating conductor for converting electrical energy into heat with at least two electrical connections (1011) which can be contacted in a connection area.

3. Plasticising screw (1') according to claim 1 or 2, characterised in that the at least one flat heating element (101) is designed as a thickfilm heater.

4. Plasticising screw (1') according to one of the preceding claims, characterised in that the resistance heating conductor is printed onto the inner cylinder (100) or the sleeve (1017) of the heating element (101) by means of an electrical resistance paste.

5. Plasticising screw (1') according to one of the preceding claims, characterised in that a dielectric layer is applied to the outer surface (1000) of the inner cylinder (100) or the sleeve (1017).

6. Plasticising screw (1') according to one of the preceding claims, characterised in that at least one recess or cut-out (1010) extending in the longitudinal direction (L) of the heating element (101) is formed on the flat heating element (101).

7. Plasticising screw (1') according to claim 6, characterised in that the recess or cut-out (1010) extends over at least approx. 75% of the length of the heating element (101).

8. Plasticising screw (1') according to claim 6 or 7, characterised in that the recess or cut-out (1010) is slotted.

9. Plasticising screw (1') according to one of the preceding claims 6 to 8, characterised in that an opening (1012) is formed at the end of the recess or cut-out (1010) on the heating element (101), the diameter of which is greater than the width of the recess or cut-out (1010).