Injection molding machine
An injection molding machine (completely plastic injection molding machine , often called SGM for short ) is a machine that produces plastic parts from plastic in granulate or string form. For this purpose, the required molding compound is prepared in the injection unit and injected into a die that represents a negative mold ( cavity ) of the desired plastic part. Depending on the process used ( thermoplastic injection molding , thermoset injection molding or elastomer injection molding ), various machine components are heated or tempered.
Injection molding machines basically consist of two assemblies: the injection unit or plasticizing unit , which prepares the raw material and injects it into the tool under pressure, and the clamping unit , which takes the tool (also form ) and opens and closes it.
In 1872, John Wesley Hyatt invented the first injection molding machine.
Injection unit
construction
The heart of the injection unit is a screw shaft , also known as a screw , which is inserted in a cylinder. The inside diameter of the cylinder is equal to the outside diameter of the screw. The cylinder is called a screw cylinder or a plasticizing cylinder . In the rear of the screw cylinder there is either a funnel into which the granulate is filled and trickles through an opening (the filling block ) into the cylinder, or a suction eye through which the cord blanks are drawn. Turned by a drive, the screw rotates in the screw cylinder and transports the raw material towards the screw tip.
In thermoplastic injection molding , the screw cylinder is heated from the outside by means of electrical heating bands . This heat and the special geometry of the screw not only convey the granulate, but also shear it ; the plastic melts and is plasticized and homogenized. At the top of the screw cylinder there is a nozzle that forms the transition to the tool.
In the case of thermoset injection molding and elastomer injection molding , on the other hand, the cylinder is tempered in order to prevent an excessively high melt temperature, which is caused by internal friction, as otherwise the molding compound would already react in the cylinder.
In the course of the metering process, the molding compound is now usually transported through a non-return valve to the nozzle and then backed up in front of it. In order to provide enough storage space for the molding compound, the screw is only subjected to a low pressure (back pressure) axially so that it can move towards the filling funnel and the so-called screw antechamber is formed between the non-return valve and the nozzle, in which the volume of the material is located.
The dynamic pressure acts against the melt, so that the melt is compressed. The pressure exerted by the melt moves the screw back.
During the injection process, the screw is pushed axially towards the nozzle, the non-return valve closes and the mass volume is injected through the nozzle into the tool.
After the volumetric filling of the mold, there is a switch to holding pressure. The mass must remain in the cylinder (residual mass cushion), otherwise the pressure cannot act on the mass. The holding pressure is required to compensate for the volume shrinkage.
slug
The three-zone screw is often used in thermoplastic processing. The plastic granulate is drawn in in the so-called feed zone and conveyed to the next zone, the compression zone, where the plastic is plasticized and compressed ( degassed ). The melt is then homogenized in the metering zone and finally pressed by the non-return valve in front of the screw, which moves axially backwards as a result of the increasing dynamic pressure in the cylinder.
Propeller screws differ in terms of geometry (compression ratio, flight depths, pitch and zone division) and the material design. Nitrided steel screws with σB = 1000 N / mm² are used when there are no abrasive or corrosive loads. Through- hardened cold-work steel is used up to a screw diameter of around 80 mm, and above this screw armored screws are often used. In the case of very high abrasive loads, powder metallurgy screws are also used. Very corrosive fluoropolymers (e.g. PVDF , PFA ) are also processed with Ni-based alloys .
There are barrier screws, with a second screw flight with a barrier web separating the melt from the residual granulate. As a result, higher plasticizing rates are achieved, especially with polyolefins. Mixing and shearing parts are added to achieve a better homogenization of the melt.
Special coatings also serve to improve the properties of the screw. Among other things, the multilayer chrome or PVD coatings (e.g. titanium nitride ), which are used for transparent plastics such as PC or PMMA. Above all, the adhesion of the plastic to the screw surface is reduced. Due to their high hardness, PVD or carbide spray coatings applied by means of HVOF (e.g. vanadium carbide ) are also used as wear protection.
The screw is affected by the torsion during dosing, the wear and tear caused by fillers such as glass fibers and rock dust, temperature gradients (cold intake, warm nozzle), temperature changes, especially in the intake area due to screw movement during injection and dosing, and corrosion (e.g. flame retardants or corrosive decomposition products ) loaded and limited in their service life.
Like the clamping unit, the screw can be driven electromechanically or hydraulically .
The axial screw movement during injection is force and position regulated and therefore highly dynamic, precise and reproducible. In the production of technical parts, high-quality hydraulic injection units have similar precision values as electric injection units. In thin-walled applications, however, the electric injection units have a precision advantage.
The dosing movement is speed and position controlled. An electric metering drive has a primarily positive effect on energy consumption in electric injection molding machines. In the case of hydraulic machines, however, an electric metering drive can certainly increase the energy consumption (parallel idling energy of the hydraulic pumps ).
Plasticizing cylinder
The surface of the cylinder has a greater friction than the screw surface, otherwise the molding compound would turn on the spot. To increase the friction on the cylinder, in addition, is also used grooved cylinder. But this is much less common than with extrusion cylinders, since the axially movable screw can unscrew like a corkscrew if the friction is too high. The molding compound must not melt in the feed zone, otherwise the friction on the cylinder is reduced and bridges are created. Therefore the support body is cooled with water.
The loads are the same as with the worm, but there is almost no torsion. For uncritical applications, the plasticizing cylinders are made of nitriding steel, but centrifugal bimetal cylinders are often used. The plasticizing cylinder is harder than the screw, since the screw is easier to change, the plasticizing cylinder is the more expensive component and unevenly hard materials are less prone to cold welding (seizing).
Non-return valve
The non-return valve (RSP) prevents the volume of mass in front of the screw from flowing back into the screw threads during injection and repressing, so that the screw functions as a piston. The non-return valve sits at the end of the screw and usually consists of three parts: the tip or also called the screw tip, the locking ring and the pressure ring. The locking ring sits on the screw tip and in front of the pressure ring and both act as a stop. If the dosage is carried out, the melt presses the locking ring against the screw tip, so that a gap is created between the locking ring and the pressure ring (locking ring stroke) through which the melt can flow. When the injection is made, the locking ring is pressed against the pressure ring and thus closes the screw threads. The distance between the locking ring and the cylinder is called the screw play. There are different versions of the RSP. The locking ring or pressure ring is supported by wings (3 or 4 feet), or the closure of the melt flow is released via one (one-ball RSP) or several balls (multi-ball RSP).
Filling funnel
The filling funnel can be shut off and emptied with a slide and has a level indicator. There may also be metal separators ( magnets ) with sieves, holders for paint mixing and dosing devices and a device for discharging static charges.
For powdery, poorly flowing molding compounds, funnels with electromagnetic vibrators or funnels with agitators are used.
For ( hygroscopic ) plastics (e.g. PC, PA, PET, PBT), heatable funnels with a conveyance directly from the Thermolift into the plasticizing unit are suitable. The dryer is mounted directly on the machine.
Funnels with a tamping device are used to process non-free-flowing molding compounds.
Temperature control
The plasticizing cylinder is heated with heating bands to support the melting of thermoplastics. A liquid temperature control with a lower temperature is used for crosslinkable plastics.
In the catchment area, cooling with water can take place in order to avoid the formation of plugs or bridges. Even with poorly flowing fitting materials, especially PVC, cooling is used to avoid overheating. As the cylinder size increases, so does the need for cooling.
Piston injection molding machine
A simple piston can be used instead of a screw . This method has many disadvantages, e.g. B. a slower and more uneven heating, as well as a separate dosing of the granulate required here, but advantages in micro injection molding of particularly heat-sensitive plastics. Marbled parts are also easier to produce with piston engines. Piston injection molding machines tend to be niche products because of the rarity of the tasks they can do better, which were practically unavailable for decades. But they were the standard injection molding machines until 1956. You can now buy small devices again. With these, however, plasticizing is occasionally taken over by a screw, while the piston is only responsible for injecting and pressing down.
Clamping unit
construction
The clamping unit usually consists of three plates that are arranged vertically on a machine frame in an axis. The fixed platen carries one half of the mold (nozzle side) and is the platen closest to the injection unit. The fixed clamping plate has a hole in the middle through which the nozzle can pass and dock onto the tool. Air is forced out. The second platen is the movable platen. The second half of the tool (ejector side) is mounted on it. It is movable and is pushed mechanically or hydraulically towards the fixed platen. Since the two mold halves are pushed together, this is called closing . Finally, the third plate is the face plate. It has a supporting function because the toggle lever or the hydraulic cylinder for closing the clamping plates is located between the face plate and the movable clamping plate. The front plate is therefore necessary to build up strength. The exception is the two-plate lock, in which both the travel movement and the closing pressure are carried out via the bars between the two plates. The ejector , which is hydraulic cylinders, is also located on the movable platen. When the mold is open, they actuate small metal pins in the mold, which then push the molded part out of the mold. There are also connections for core pulls here . This allows slides in the tool to be operated in order to be able to make undercuts.
As a rule, the clamping units are arranged horizontally and vertical machines are found especially where parts have to be inserted and overmoulded, such as for electrical connectors.
The force values acting on the clamping unit are differentiated according to DIN 24450 into:
- Closing force
- Locking force
- Buoyancy
- (maximum) mold opening force
- Ejector force
Force generation
There are different ways of building up the force in the clamping unit ( clamping force ). A distinction is made between hydraulic machines, in which the mold is built up and held by large hydraulic cylinders, and toggle-lever machines, in which a large, double-acting toggle lever compresses the tool. The toggle lever itself is again moved either by a hydraulic cylinder or, in the case of an electric machine, by an electric motor. Due to the mechanics of the lever, however, significantly less force is required to hold the mold together than with exclusively hydraulic systems.
Knee lever machines usually work faster and more energy-efficiently, but become inefficient on larger machines due to the huge toggle mechanism. That is why one works more hydraulically in larger systems.
Hydraulic systems consist of 2 units, the form-locking hydraulics that travel a long way with little force, and the locking hydraulics that only have to cover a short distance with great force.
The advantages of hydraulics in comparison to electric drives are the larger and more precisely adjustable closing and locking forces and that the hydraulics for core pulls are already available. If the machine does not have a controllable servo pump , operation is associated with a high noise level and high energy and cooling water requirements. Oil leaks are also possible
Another design are the electric injection molding machines, in which all axes are driven electrically. They work with servomotors that z. B. drive the toggle lever, the screw, the ejector, etc. via recirculating ball spindles or roller screws. There are also electrical series in which the clamping force is generated directly by electric motors. However, this is only possible with machines with low clamping forces, since electric motors cannot provide a good compromise between force and speed. The drive technology for the clamping unit in large machines (> 5000 kN) is also disproportionately complex. In hybrid machines, a hydraulic clamping unit is combined with an electric injection unit.
Electric machines are generally quieter, more energy-efficient and cleaner, since no hydraulic oil is required. They allow faster and temporally parallel movements that can only be achieved with hydraulic machines using multi-pump technology. Disadvantages are the higher investment costs and the high demands on servomotors with long holding pressure times.
There are also some special forms, such as the two-platen clamping unit. Here the tool is not compressed from behind, but rather pulled together hydraulically. Due to the special design, special, large tools can be used and the installation space of the machine is less.
The great advantages of the tie-bar-less machine, in which the face plate and fixed clamping plate are connected via an H-frame, are the flexibility for large and bulky tools, the quick and easy tool change and the optimal conditions for clean room applications. Both the static and dynamic platen parallelism of tie-bar-less machines is unsurpassed. On the one hand, the massive frame construction prevents the clamping unit from bending, and on the other hand, the 3-point plate guide with a precision guide on both sides and the central locking cylinder prevents any deviation in parallelism of the movable platen. Tilting of the plate is prevented by appropriate pretensioning of the so-called FlexLink. The fixed clamping plate is also massively supported by the machine frame, so that the plate cannot "wobble" with the resulting dynamic parallelism problems even during faster travel movements. When the clamping force is applied, the machine maintains the platen parallelism or it adapts exactly to the parallelism of the mold.
Interlock types
With the knee lever system, the tool is closed when the knee lever is slightly bent. The remaining travel of the toggle lever generates the locking force and a positive locking.
In the case of a direct hydraulic locking system, the locking cylinder is used to lock the lock, so it represents a force- locking lock.
With the form-fitting hydromechanical locking, the clamping unit is locked mechanically (e.g. via a rotatable pressure plate on which the support bolts can be supported) after the locking force has been applied. The tool driving force is then absorbed by the lock.
Clamping
The tools are usually clamped onto the machine by screwing the clamping plates of the tool directly onto the clamping plates of the machine or by means of clamping claws.
Another method is to fix the tools with electric magnets. However, this method is not widely used and can only be used to a limited extent due to the lower holding forces.
A quick mold change is also achieved through the use of hydraulic quick clamping systems, here the clamping plates of the mold are wedged by small hydraulic cylinders. The disadvantage here is that all shapes require special, identical system clamping plates for this purpose.
Tool
Ejector unit
The ejector has the task of ejecting a finished plastic part from the mold after the mold plates have been opened. The ejector usually consists of a hydraulic cylinder attached behind the movable mold plate, the piston rod of which extends through the movable mold plate. Ejector pins in the mold can be coupled to this piston rod so that when the ejector moves forward, the ejector pins in the tool also move forward and eject the part. Often one encounters the central ejector or the ejector plate. A central ejector is attached in the middle of the movable mold plate and extends through the plate with its piston rod. There are also ejectors that move a plate behind the movable mold plate. Several further rods can be attached to this plate, which can also be moved through the movable mold plate. This means that parts of molds that are not arranged centrally or for which several ejector pins have to advance separately can also be ejected.
With the advent of the all-electric injection molding machine (i.e. no more hydraulics), the ejectors are no longer operated by a hydraulic cylinder, but by an electric motor. Often, ejectors are also installed on the fixed mold plate or the cylinder is built into the mold and not on the injection molding machine. The position of the ejector is usually monitored, for example by limit switches or distance measurement. This u. a. prevents the mold from being closed with the ejector advanced. Monitoring is also required when the ejector and core pull interact.
It is also possible to remove the workpiece with chain devices or robot arms.
Sequence of the injection molding process
The process depends on many factors. However, there is always a basic scheme in which further steps can be integrated as required.
At the beginning of the cycle, the mass volume is added and the mold is opened.
First, the mold is closed and the injection unit with the conveyor unit is brought up to the mold. Then the molding compound is injected and sealed with a hold (reduction of shrinkage during cooling). During a certain waiting time for the molding compound to cool down or react, metering is carried out and the conveyor unit is moved back to its starting position. If the molded part has cooled down sufficiently or has reacted completely to be removed from the mold, the mold is opened and the molded part is usually ejected - a new cycle can begin.
Orders of magnitude and variants
Injection molding machines are available in a wide variety of sizes. They differ not only in the amount of molding compound processed, but also in the pressure with which the plastic is injected, in the area of closing, and consequently in the force with which the tool is compressed.
The information with which one can evaluate and assess an injection molding machine in terms of its size is specified in the Euromap 1 standard. Then an injection molding machine is characterized by
- the maximum clamping force with which the tool is compressed, given in kN (kilo- newtons )
- the position of the clamping unit: with a horizontal clamping unit marked with an H , with a vertical clamping unit by a V , which is added to the specification of the maximum clamping force
- the calculated stroke volume of the injection unit in cm³, based on an injection pressure of 1000 bar . In other words, the amount of molding compound that the injection unit can inject into the clamping unit at a pressure of 1000 bar (see below).
Example: 2100 H / 1330 is an injection molding machine with a maximum clamping force of 2100 kN, a horizontally mounted clamping unit and a calculated stroke volume of 1330 cm³.
Special designs such as B. Multi-component injection molding machines offer the possibility of producing the often required complex molded parts from different colors or types of plastic in one process.
Service life, service life and utilization
The service life of an injection molding machine depends largely on its use. Frequent tool changes as well as fast injection cycles over long periods of time can negatively affect the service life of the machines. Often, however, it is not the machine but the injection molding tool that limits the real utilization of the system.
Web links
literature
- Injection molding machines . In: Friedrich Johannaber (Ed.): Plastic machine guides . 4th edition. Hanser Verlag, Munich / Vienna 2004, ISBN 3-446-22042-9 ( limited preview in the Google book search).
Individual evidence
- ^ Friedrich Johannaber, Walter Michaeli : Injection molding manual . 2nd Edition. Hanser Verlag, Munich 2005, ISBN 978-3-446-22966-2 , chap. 8.1 Piston injection molding machines , p. 1001-1021 ( beck-shop.de [PDF]).
- ↑ EUROMAP 1: Description of injection molding machines , PDF