The injection molding (often called injection molding or injection molding process referred to) is a molding method , which is mainly in the plastic processing is used. The respective material is liquefied ( plasticized ) with an injection molding machine and injected under pressure into a mold, the injection molding tool . In the tool, the material returns to its solid state through cooling or a crosslinking reaction and is removed as a finished part after the tool is opened. The cavity, the cavity , of the tool determines the shape and the surface structure of the finished part. Today, parts in the weight range of a few tenths of a gram up to a size of 150 kg can be produced.
With this process, molded parts that can be used directly can be produced inexpensively in large numbers. The cost of the tool makes up a large part of the investment required. As a result, even with simple tools, the economic threshold is only reached with a few thousand parts. On the other hand, the tools can be used to manufacture up to a few million parts.
Injection molding, in particular advanced special processes, allows an almost free choice of shape and surface structure such as B. smooth surfaces, grains for touch-friendly areas, patterns, engravings and color effects. Together with its economic efficiency, this makes injection molding the most widespread process for the mass production of plastic parts in practically all areas.
The molded parts that are manufactured by injection molding can be categorized as follows: A parts - precision parts with the highest requirements, B parts - technical parts with high demands and C parts - geometrically simple molded parts made from standard plastics with low requirements. All categories have the following essential quality features: molded part dimensions and weight, strength, warpage and surface properties.
Subdivision of the injection molding process
In injection molding, plastics are almost exclusively processed. These can be divided into thermoplastics , thermosets and elastomers . All three types of material can be used in the injection molding process, with thermoplastic injection molding having the greatest economic importance. It is the most commonly used plastics processing method.
Therefore, in the following, the process will first be explained for thermoplastics. In principle, the injection molding of thermosets and elastomers works in the same way and differs primarily only in the operating parameters (e.g. temperatures).
The special procedures described below represent extensions or modifications of the basic process for certain applications.
Thermoplastic injection molding
Basic structure of an injection molding machine
The screw- piston injection molding machine commonly used today consists of two units: the injection unit or plasticizing unit , which plasticizes, processes and doses the plastic, and the closing unit , which closes, holds and reopens the mold.
The injection unit essentially consists of a horizontal cylinder, the plasticizing cylinder and a screw located in it . The screw rotates and can also move axially in the cylinder. At one end of the plasticizing cylinder is the funnel for loading the raw material , at the other is the nozzle , which can either be closed or not, it represents the transition to the closing unit.
The clamping unit consists of the mold itself, which can be separated into two halves (mold parting plane). The halves are mounted on two clamping plates , one of which, the nozzle side , is rigid and faces the nozzle of the injection unit. The other, the ejector side , is movable. It can be moved away from the nozzle side hydraulically or electromechanically (toggle mechanism) or pressed onto it with force. It contains the eponymous mechanism, which demolds the injection molded part (ejects).
Injection and clamping units must be tempered according to the material, component and process (the injection mold may be temperature-controlled). Since the two units have different temperatures, they can be moved away from each other for thermal separation , except for hot runner systems.
Piston injection molding machines were used until 1956 .
Plasticizing and dosing
The thermoplastic material trickles into the threads of the rotating screw in the form of granules . The granulate is conveyed toward the screw tip, and by the heat of the cylinder and the frictional heat , which in the cutting and shearing formed of the material, heated and melted . The melt collects in front of the screw tip because the nozzle is closed at this point. Since the screw can move axially, it retreats from the pressure and it also unscrews itself from the mass like a corkscrew . The backward movement is braked by a hydraulic cylinder or electrically so that dynamic pressure builds up in the melt . This dynamic pressure in connection with the screw rotation compresses and homogenizes the material.
The screw position is measured and as soon as an amount of material sufficient for the workpiece volume has accumulated, the dosing process is ended and the screw rotation is stopped. The screw is also actively or passively relieved, so that the melt is decompressed.
In the injection phase, the injection unit is moved to the clamping unit, pressed down with the nozzle and the screw is pressurized on the back. The melt is pressed under high pressure (usually between 500 and 2000 bar ) through the open nozzle and the sprue or the sprue system of the injection molding tool into the forming cavity. The non-return valve prevents the melt from flowing back towards the feed hopper.
During the injection, attempts are made to achieve the most laminar flow behavior possible for the melt. This means that the melt is immediately cooled down in the mold where it touches the cooled mold wall and remains solidified and “stuck”. The advancing melt is pushed through the melt channel, which is narrowed as a result, at an even higher speed and with even more shear deformation and is deformed towards the edge at the front of the melt. Heat dissipation via the mold wall and heat supply through shear heating are superimposed. The high injection speed creates a shear rate in the melt that allows the melt to flow more easily. However, rapid injection is not always desirable, because the high shear rate also increases the breakdown of molecules. The surface, appearance and orientation are also influenced by the injection phase.
Press down and cool down
Since the tool (typically 20 to 120 ° C) is colder than the plastic compound (typically 200 to 300 ° C), the melt cools down in the mold and solidifies when it reaches freezing point . The cooling goes hand in hand with a volume shrinkage , which has a negative effect on the dimensional accuracy and surface quality of the workpiece. In order to partially compensate for this shrinkage, a reduced pressure is maintained even after the mold has been filled so that material can flow in and compensate for the shrinkage. This pressing can continue until the sealing point is reached, i.e. the sprue has solidified.
After the pressing has been completed, the nozzle can be closed and the plasticizing and dosing process for the next molded part can already begin in the injection unit. The material in the mold continues to cool during the remaining cooling time until the core , the liquid core of the workpiece, has solidified and a rigidity sufficient for removal from the mold is achieved.
The injection unit is then moved away from the clamping unit (lifted off), since no more plastic can escape from the sprue. This serves to prevent excessive heat transfer from the warmer nozzle to the colder tool and thus prevents the nozzle from cooling down too much (freezing).
For demoulding , the ejector side of the clamping unit opens and the workpiece is ejected by pins penetrating into the cavity. It then either falls down ( bulk material ) or is removed from the tool by handling devices and stored in an orderly manner or immediately sent to further processing.
The sprue must either be removed by separate processing or is automatically cut off when demolding. Injection molding without sprue is also possible with hot runner systems , in which the sprue system constantly remains above the solidification temperature and the material contained can thus be used for the next shot.
After demolding, the tool closes again and the cycle starts again.
|Injection unit in front
|Injection unit back
|Physical cooling time
Types of plastic and products used
Injection molded parts can be produced with weights between a few milligrams and approx. 150 kg. The processed thermoplastics are modified by additives and fillers for injection molding processing and later use. These also include very hard inorganic fillers such as glass spheres or, very often, glass fibers .
In vehicle construction, polyolefins , mainly polypropylene , are used for dashboards and bumpers in injection molding. Plexiglas (PMMA) and polycarbonate (PC) are used for transparent objects (car headlights, taillights). Polystyrene (PS) and its copolymers (ABS = acrylonitrile-butadiene-styrene ) are mainly used for toys (e.g. Lego ) and household appliances. Polyamide (PA), polyoxymethylene (POM) and many other technical plastics are also used in mechanical engineering and electrical engineering, but in much smaller quantities for mostly high-priced parts.
In natural fiber injection molding , thermoplastics filled with natural fibers are used.
Thermoset injection molding
Thermoset injection molding differs from thermoplastic injection molding in its operating parameters. Duroplasts usually harden when exposed to heat. After hardening, re-melting is no longer possible, only recycling . The not yet hardened mass must therefore be injected into the mold at a comparatively low temperature (approx. 30 to 110 ° C depending on the material) and hardened there at a higher temperature (between 130 ° C and 250 ° C depending on the thermoset).
The prepared molding compound flows u. a. worse than thermoplastic melts because of the high filler content. The injection molding process must be tailored to these particularities.
The injection molding machine works with a screw conveyor that generates little shear heat. The achievable pressures are up to 2500 bar . The molded part is demolded while hot. Sprue-free injection molding is possible with cold runner systems , the counterpart to the hot runner system, in which the sprue is cooled and crosslinking is prevented.
With thermoset injection molding, very large wall thicknesses of the parts of up to 50 mm can be achieved.
A typical area of application for thermosets ( BMC ) are vehicle headlights , more precisely their reflectors , in which the good dimensional accuracy and temperature resistance of thermosets comes into play. With wall thicknesses of up to approx. 4 mm, however, the cycle times for thermoset processing are usually longer than those for thermoplastics, so that thermosets, if their special electrical and mechanical qualities can be dispensed with, usually lose out in comparison to thermoplastics despite their generally lower material price, despite their generally lower material price.
Elastomer injection molding
Elastomers - such as natural rubber - vulcanize in the same way as thermosets when exposed to heat. The tool has to be hotter than the molding compound during the injection process. Thermoplastic elastomers are an exception.
Injection molding of elastomers takes place on screw injection molding machines. Elastomers can be drawn in in the form of free-flowing powders or in the form of a band by a special screw conveyor that introduces little shear into the plasticized mass. The cylinder is usually heated to approx. 80 ° C with a liquid (water temperature control) in order to avoid overheating, which would result in premature vulcanization of the elastomer. Furthermore, the burr-free injection molding of elastomers is a particular challenge, since elastomers are very thin in the flow area (just like thermosets), so the effort involved in tool design is somewhat higher than with thermoplastic tools.
Apart from the special features mentioned, the injection molding process is basically similar to that of thermoplastic injection molding. The screw kneads and mixes the molding compound, which is then prepared homogeneously. This allows z. B. compared to pressing, in which an inhomogeneous temperature distribution results from the supply of heat from the outside, achieve an improvement in the quality of the molded parts produced. Some mechanical properties can be up to 30% higher than those of pressed elastomers.
When flowing in the nozzle and in the runners, frictional heat is generated. This shortens the vulcanization time. This makes the injection molding process particularly economical. The high material viscosity makes relatively large gate cross-sections necessary. Cold runner systems can also be used with elastomers.
In elastomer injection molding, injection molding machines with several clamping units for several tools and different parts are often used, as the vulcanization time is considerably longer than the preparation time in the injection unit.
The basic method described above can be modified or expanded for special application purposes.
Multi-component injection molding enables parts to be made from different plastics in a single operation. In the in-mold process , materials inserted into the injection mold are back- injected, e.g. B. the rigid foam of a sports helmet or labeled foils ( in-mold labeling , IML ). Alternatively, with in-mold decoration ( IMD ) only a printed film, which is later removed, can be inserted so that only the color remains on the part. The metal film insert molding is used to produce plastic parts with metal foil coating.
If the material is injected into a mold that is not completely closed and the mold is only then completely closed, this is called injection compression molding . This can be extended to swelling flow stamping by back -injecting materials, analogous to the in-mold process .
In internal pressure injection molding (also called fluid injection technology ( FIT ), can be divided into gas internal pressure injection molding ( GID ) and water internal pressure injection molding ( WID )), a gas or water is additionally injected, which escapes after solidification or hardening, so that a cavity is created. In contrast, in the gas back pressure method ( GHD ), the gas introduced between the mold wall and the molded part is used to press firmly against the opposite wall in order to a. to achieve a high surface quality. Gases can also be generated with propellants distributed in the plastic, which results in a foam ( thermoplastic foam casting , TSG ).
The powder injection molding ( Powder Injection Molding ( PIM )) allows the production of metal and ceramic parts by metal and ceramic powders with a binder injection molded, the binder is then removed and the remaining powder finally sintered is.
The Exjection or extrusion molding is a combination of injection molding and extrusion represents. When injection stretch blow molding is an injection-molded blank z. B. subjected a PET blank for bottles to a subsequent stretching and blow molding process.
The tandem tools are similar to a classic tool. There is one cavity on each of the two parting lines. The sprues can be made hot or cold, as desired. The doubling makes it possible to inject the first cavity; while this cools down, the other cavity is filled. So it is possible to use the unused time of the first cooling productively.
Due to the wide range of options for choosing the component geometry, the precise injection molding process and the material, various, mostly undesirable, effects can occur during the injection molding process or the finished component. Sprue areas in which the surface structure and shape deviates from the rest of the component are created at the interface between the sprue and the actual component. Reasons for this can be a local reorientation of the molecules of the material, too high an injection speed or too low process temperatures. Air inclusions occur when the component geometry is poorly selected, the process temperatures are too low, the injection speed is too high or the ventilation is inadequate. If such air inclusions are strongly compressed by the process, local very high temperatures can occur and, as a result, charring in the component. This is also referred to as the diesel effect . Sink marks and general warpage, i.e. the deviation of the component shape from the actually desired one, arise after molding, during cooling. Various factors have to be taken into account for this. When choosing the component geometry, for example, mass accumulations due to locally different wall thicknesses of the component can lead to the individual areas shrinking to different degrees during cooling or also cooling at different speeds. A non-optimal process temperature or, more rarely, an insufficiently homogeneous starting material can also cause these problems.
Plastic injection molding machines are encapsulated systems on which exposure to hazardous substances is usually only possible during certain activities, in particular during retrofitting and maintenance work and, if necessary, when taking samples and when leaks occur . Nonetheless, decomposition products from the processing of thermoplastics in injection molding machines in the air can pose a risk to employees.
The risk assessment recommendations of the accident insurance institutions (EGU) are based on measurements of possible decomposition products of thermoplastics in injection molding machines in the air at workplaces . They represent a suitable assessment procedure for exposure when processing plastics in injection molding machines and offer practical support in carrying out the risk assessment - including protective measures - in accordance with the Hazardous Substances Ordinance . Dermal hazards, e.g. B. from hot surfaces and heated plastics must be considered separately in the risk assessment.
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- S. Stitz, W. Keller: Injection molding technology. Hanser, 2001, ISBN 3-446-21401-1 .
- Topic page " Basics of injection molding " of the magazine Kunststoffe
- Whitepaper "Increase Quality, Reduce Energy Consumption - A Guide" by the Institute for Integrated Production Hanover
- Formula collection on the subject of injection molding for the practitioner (comes from a lecture at a university with a renowned plastics institute)
- IPH Whitepaper Energy Saving. Retrieved January 15, 2018 .
- Christian Bonten: Plastics Technology Introduction and Basics , Hanser Verlag, 2014.
- http://kunststoff.bearx.de/ausbildung/spring Fehler / Fehler.htm accessed on July 9, 2015.
- German Statutory Accident Insurance e. V. (DGUV): DGUV Information 213-728 - Recommendations for risk assessment of the accident insurance carriers (EGU) according to the Hazardous Substances Ordinance - processing of thermoplastics in injection molding machines. Retrieved November 11, 2019 .