Potting system
A potting system is understood to be a process engineering device for pouring or covering components, often from the electrical and electronics industry , with cast resin . The dosing, mixing and preparation of the resin and its components take place in it. It is therefore a subspecies of the dosing systems .
Structure and functionality of a potting system
Design criteria
Depending on the application, potting systems differ in structure, size and degree of automation.
There are one-component casting resins in which the resin and hardener are already mixed in the delivery condition, and two-component systems in which the resin and hardener must be mixed in a fixed ratio in the potting system. Other components such as fillers , accelerators , plasticizers or additives may be added. The potting system must therefore be adapted to the number and proportion of the components.
The viscosity , reactivity and abrasiveness of the casting resin and its components also play a role. Some resins only have a sufficiently low viscosity for processing at elevated temperatures, so that the entire potting process must take place at a specific temperature using appropriately heated system components. Preparation and casting often take place under vacuum (see below).
The volume of cast resin required for a component can range from less than 1 ml in the electronics sector (e.g. in the manufacture of LEDs ) to 100 l or more in the encapsulation of coils for energy technology . The respective potting systems are therefore dimensioned very differently.
The degree of automation ranges from manually operated systems through semi-automatic to fully automatic systems for series production that are completely integrated in a production line.
One-line and two-line systems
Potting systems can be roughly divided into single-line and two-line systems. In the single-line system, all components are metered into a mixer, mixed and prepared so that the finished casting compound can be removed from the container. A two-line system basically consists of two single-line systems in which two premixes, one resin with fillers and possibly other components and one hardener with fillers and possibly other components, are made up. The premixes are then conveyed to a static mixer by metering pumps , where they are mixed and poured.
While single-line systems represent the simpler and more cost-effective structure, they have the disadvantage compared to two-line systems that the hardening reaction usually begins with the mixing of resin and hardener and the mixture then has to be processed within the pot life . If the container is empty, new compound must first be added, as long as no potting is possible. Any remaining quantities must be disposed of.
In the case of two-line systems, the reactive components only come into contact with one another immediately before potting. The premixes can be stored practically indefinitely. This makes production more flexible, and continuous work is possible through sufficient storage of the premixes. Furthermore, the potting always takes place without any significant increase in reactivity, which guarantees constant viscosities. Only as much reactive material is produced as is necessary for potting, so there is practically no waste.
In addition to the mixers and conveying and metering devices, the system can also have containers for the preparation and pre-treatment of the components and storage containers for premixes.
Process steps
Material conveyance
In the system, the liquid components such as resin and hardener and possibly solid fillers must be conveyed out of the container and within the system. The finished casting resin mixture or the premixes must also be potted.
Which conveying method can be used depends mainly on the viscosity of the material and the abrasiveness of the fillers.
For resins of low to medium viscosity are
suitable. High viscosity resins can be conveyed with drum follower plate pumps .
The bulk material-like fillers are usually moved by means of vacuum or screw conveyors.
By arranging the system components appropriately on top of one another, material movement can also be achieved by gravity alone .
In the case of mixtures with fillers, the material usually has to be moved constantly or at least at regular intervals in order to avoid sedimentation . When material is transported in pipes, circulation lines are therefore often available in order to be able to pump the mass in the circuit.
dosage
The individual components or premixes must be dosed according to the recipe in the appropriate ratio to one another. After all, the potting system has to eject the exact amount of casting resin required. Various methods are available for dosing.
Gravimetric dosing
The material is dosed according to weight, i.e. weighed . Weighing extends the cycle times, even if it allows a very precise quantity determination. Usually the container with the material to be dosed is on a scale, the weight loss after being conveyed out corresponds to the dosed amount.
This method is mainly used for fillers.
Volumetric dosing
Dosing devices that eject a constant volume are particularly simple, less prone to failure and work reliably.
One example is piston dispensers. The ratio of resin to hardener in a two-component system can be precisely determined by the ratio of the cross-sectional areas of two dosing pistons that are pushed out at the same time. The amount dispensed is determined by the piston stroke.
Dosing via time measurement
This dosing method means that a constant outflow rate must be guaranteed by using appropriate pumps. The material flow is released by a valve and interrupted again by the valve after a predetermined time.
This method is particularly error-prone, since every fluctuation in the flow rate results in different dosing quantities. Creating and monitoring an absolutely constant flow requires a relatively high amount of electronics.
Dosing with flow sensors
With flow sensors can be flow rates measured. These sensors can also be used for dosing.
Mix
After dosing, the components must be homogenized . This is not a completely simple task, especially when mixing the fillers with the liquid components resin and hardener. The powdery or fibrous fillers are sometimes very fine-grained and have a correspondingly large specific surface area that has to be wetted by the liquid components . The particles can form lumps which have to be dissolved in order to achieve a low viscosity of the casting compound. Perfect dispersion primarily depends on the mixing intensity and time.
The following mixing processes can be distinguished:
Dynamic mixing
The components meet in a stirred tank (mixing chamber) and are mixed by a rotating component. The chamber is also a storage container.
The disadvantage is that here only batchwise , i.e. H. Continuous operation is not possible: The container is first filled, then mixing takes place. Only then is the mixture available for further processing. If a reactive compound is used as in the single-line system, the entire contents of the mixer must be used up within the pot life. If this does not happen, there is a risk that the material will harden in the mixer, which entails costly cleaning or, in the worst case, changing the mixer. Even with proper use, depending on the material, regular, sometimes automated cleaning is usually necessary. Waste or hazardous waste is generated.
The moving parts are subject to wear.
Static mixing
The components are mixed by pumping them into a pipe made of metal or plastic, where they are divided several times by built-in rigid separating blades and then brought together again. The mixing tube is either cleaned, which can be achieved by flushing with just one component, or disposed of when it hardens.
With static mixers, a continuous process without interruption is possible. The components are mixed within a short period of time and immediately proceed to the next process step. This predestines them z. B. for the final mixture of resin-filler and hardener-filler premixes for reactive potting compound in a two-line system. Since there are no moving parts, these mixers require little maintenance and wear, and are also cost-effective and space-saving.
The disadvantage is that mixing time and intensity can hardly be influenced. Static mixers are also not suitable for incorporating fillers.
Static-dynamic mixing
With this principle, a plastic mixing tube contains a helix that is driven by an external motor. This method is rarely used.
Material preparation
Temperature control
Many casting resins are processed at elevated temperatures to lower their viscosity. The corresponding potting systems are equipped with heated containers, pipes and mixers. Conversely, the heating medium such as water or oil can also be passed through a cooling unit in order to cool the material during production interruptions. The progress of the reaction is slowed down and sedimentation is prevented by the increased viscosity.
Drying and degassing
Moisture can have undesirable effects on the material properties. In particular, the powdery and often porous fillers can absorb moisture on their large surface, which then makes wetting by resin and hardener difficult. Air and other gases can also be present in the liquids as bubbles or in dissolved form and negatively influence the viscosity or later lead to cavities in the potting.
That is why the fillers are often dried before processing , while liquid components and mixtures are usually degassed . In many cases, degassing takes place in vacuum mixers in parallel with homogenization. Continuous material throughput is possible with continuous degassers .
Ideally, the resin, hardener, filler, etc. are heated separately and vacuum-dried and degassed. The further process steps such as dosing and mixing then also take place under vacuum to prevent renewed entry of air and moisture.
The actual potting
Design options for the potting
The simplest option is to pour a certain amount of casting resin into a stationary workpiece by pouring it into a point. It is only necessary to ensure that the filling is not too fast in order to avoid the inclusion of air bubbles. Ideally, the mold is therefore filled from below.
During the potting, however, the application unit, i.e. the part of the system from which the resin exits, or the workpiece can also move. Many variants of the potting process are possible through appropriate control. For example, adhesive beads or dams of different shapes can be poured. Dams cast with highly viscous, thixotropic material can be filled with a thin resin in a second step ( dam & fill ). The exit speed of the resin can be varied during the casting process or the casting process can be carried out in several portions. The workpiece or the application unit can perform complex movements, which means that even difficult potting tasks can be solved.
For series production, potting systems with multiple application units can be used for up to approx. 30 potting systems at the same time.
Vacuum potting
Many workpieces such as B. Transformer windings have strong undercuts . Air can be trapped there when potting. For many components, however, an absolutely bubble-free encapsulation is necessary, especially in high-voltage applications in order to ensure freedom from partial discharge .
Such components are usually cast under vacuum. Since it is not technically possible to achieve an absolute vacuum, but there is still a residual pressure of a few mbar , bubbles can still form, but are then almost completely compressed when the vacuum is broken. If the potting takes place, for example, at 5 mbar, any bubbles that may occur are compressed to a volume 200 times smaller when venting to atmospheric pressure (approx. 1000 mbar) according to the above-mentioned relationship .
The vacuum encapsulation takes place in vacuum chambers or vessels, which can be provided with an entrance and an exit lock in order to shorten the cycle times. As a rule, only solutions in which the workpiece is moved and the application unit is rigidly mounted are technically feasible there.
Automatic pressure gelling (ADG)
See also: curing in the article casting resin
The casting resin is subject to a chemically induced shrinkage which to shrinkage during curing cavities can lead and cracks. With the automatic pressure gelation process (ADG), the components are therefore cured under pressure. Cast resin is continuously injected in order to compensate for the shrinkage. This enables very short curing times to be achieved.
In this process, the potting system must press the material into the mold with pressure and maintain this pressure during the hardening process. The casting mold must be very stable here and is usually opened and closed automatically by a so-called closing machine. It is also possible to supply several locking machines from a central mixing and processing unit via ring lines. Each clamping machine then has its own dosing unit with static mixer.
application areas
The potting of electrical and electronic components is primarily intended to provide reliable electrical insulation and prevent the ingress of moisture and dirt. On the other hand, cast components are practically no longer repairable, difficult or impossible to recycle and require special attention to the cooling inside. Typical areas of application are:
Potting of electronic components and assemblies
Assemblies in which individual components are plugged into a circuit board are usually encapsulated in order to protect them from environmental influences and mechanical damage. In these cases there is usually only one form to fill in, which is a relatively simple task.
Manufacture of LEDs
Light emitting diodes are manufactured in fully automatic systems. This also includes pouring it into transparent plastic. Short cycle times are particularly important here in order to lower the price of the lamps. This is an application example for potting with multiple dosing heads.
Potting of power engineering components
The windings of electric motors , transformers (especially of cast resin transformers ), chokes and instrument transformers are often cast with cast resin for insulation and protection against environmental influences. The components are surrounded with a mold, which is then filled with resin. Due to the fine structure of the gaps with their strong undercuts and the component size, the encapsulation of windings places particularly high demands on the encapsulation process. Also insulators are often made from cast resin. In the series production the automatic pressure gelation is often used. During a rapid resin start-up reaction in the closed, preheated mold, additional resin is refilled under pressure in order to prevent cracks and cavities. The still soft raw part can soon be removed and slowly hardens.
See also
literature
- W. Knappe, O. Heul: Plastics processing and tool making. In: Bodo Carlowitz (Ed.): Die Kunststoffe: Chemistry, Physics, Technology (= Plastics Handbook. Volume 1). Hanser, Munich / Vienna 1990, ISBN 978-3-446-1441-6-3 , pp. 477–479 ( limited preview in Google book search)
- R. Stierli: Epoxy casting and impregnating resins for the electrical industry . In: Wilbrand Woebcken (ed.): Duroplaste (= plastic manual. Volume 10). 2nd Edition. Hanser, Munich / Vienna 1988, ISBN 3-446-14418-8 , pp. 513-518, ( limited preview in Google book search)