- Resin casting
A resin casting system is a technical installation to process
casting resinfor the purpose of filling, sealing, covering or soaking technical parts, especially in the field of electricity and electronics like transformers, LCDs and other small and big parts.
Due to progressing miniaturization and introduction of electronics into new areas, quality requirements for the parts are rising. So the quality of casting has to be increased as well. To obtain the required quality, on one hand, the resin system has to be developed and optimized accordingly. On the other hand, the resin casting system has to work more and more precisely to obtain best casting. Because of continuously increasing cost pressure, casting devices must be capable of increased quality, while also becoming faster and more reliable.
Requirements for good casting
First of all, by casting, electrical and electronic parts have to be insulated reliably and penetration of moisture has to be excluded totally. Very often heat has to be conducted out from the part properly, an attribute that can be improved by the choice of an appropriate casting resin.
Process steps in a resin casting system
In a resin casting system the following processes have to be performed:
*Conditioning of the resin mix
A good resin casting system provides high quality casting of the same high standard even during long series in mass production.
Several properties of the resin mix, with or without filler material, one component or two components (resin + hardener), are crucial for the quality of the product:
*Even distribution of the fillers (no settling)
*Elimination of air and moisture
*Constant, often elevated temperature to maintain low viscosity
Even distribution of the fillers without setting is maintained by ongoing stirring.
Air and moisture are eliminated by evacuating the device.
Elevated temperature is reached and maintained by thoroughly controlled heating of the vessel, the material feeding lines, the pumps and metering heads.In filled, complex resin mixes conditioning is especially crucial for the quality of the product.
Which kind of feeding pump has to be used depends mainly on the viscosity of the material and the abrasiveness of the fillers.
For low to medium viscous material:
Gear pumps are not fit for abrasive material
For highly viscous material
follower plate pumps are connected with an eccentric screw pump or a scooping piston pump. Metering is controlled for mass, time, and volume to determine the amount of resin dispensed.
Metering by mass
Weighing provides very exact determination of amount, but it lengthens the cycle time. Also, a scale within a production line can be quite sensitive to malfunction and hard to use on boards populated with many parts. Because of these issues, this method is rarely used.
Metering by using Volume
Getting constant volume is, technically, relatively simple. Metering systems relying on constant volume dispensation therefore are especially simple and reliable.A very good way is the use of piston metering heads. The ratio of resin to hardener can be determined exactly by the ratio of the width of two separate pistons, one for resin, one for hardener. Both pistons have to push simultaneously. The amount is determined by one common stopper, limiting the lift of both pistons equally.
Metering by using time
This method demands that the appropriate pumps provide precisely constant flow of the material. Flow of material is started by a controlled valve and stopped after a certain time.
This method is especially susceptible to metering flaws, because the slightest change in flow speed causes different amounts of dispensed resin and/or hardener. Providing for an absolutely constant flow calls for relatively high electronic complexity, but provides much greater flexibility in adjusting the proportions of hardener to resin.
In two component resins, thorough mixing is crucial to obtain equal reaction between resin and hardener throughout all the material. There are three possible ways to mix:
*static mixing tube
*dynamic static mixing tube
Static mixing tube
The compounds meet in a mixing tube made of plastic. The tube contains immobile walls to divide and bring together the material several times, mixing resin and hardener by this process. The mixing tube is not cleaned after use, but discarded.
moving parts, no cleaning process, no residue of cleaning liquid:disadvantages: mixing tubes as litter, intensity of mixing can hardly be varied
The compounds meet in a mixing chamber, usually made of stainless steel and there they are mixed homogeneously by a rotating blender. To optimize the mixing effect the rotational speed can be controlled electronically. Mixing chamber and blender have to be cleaned with a special cleaning fluid to be used again. Usually this happens automatically.:advantage: the mixing intensity can be controlled exactly.:disadvantages: more due to failure because of moving parts, cleaning required, possibly residue of cleaning fluid, used cleaning fluid is waste or even hazardous waste, expensive replacement or cleaning of the chamber in case of encrustation because of hardened resin within the mixing chamber.
A mixing tube made from plastic contains a helix driven by an external motor. This method is hardly ever used.
To provide for best casting, the part and the dispensing unit have to be moved relatively to each other. In principle there are two ways:
*Movement of the dispensing unit
*Movement of the part
For many applications, casting can only be done successfully in vacuum. This is true especially for parts with much undercut, i.e. soaking of transformation coils. In such cases bubble free casting can only be obtained in vacuum. For this purpose casting systems are equipped with vacuum chambers. To shorten cycle times there can be an airlock at the entrance and one at the exit. In vacuum casting only systems where the parts are moving and the dispenser remains in place can be built reasonably.
For the production of series, casting systems with multiple dispensing units can be used. At this time (2006) there are systems running capable to do up to thirty dispensing acts simultaneously.
Variability of casting
The most simple way of casting is, to cast a certain amount of resin into one spot of a not moving part. Such simple systems sometimes are called metering systems.Centrifugal casting tables are today available. The mold is fixed on this rotary table and while the resin mix is dispensed into the mold, centrifugal force ensures a solid, clean bubble free fill. The part is also stronger (due to stress hardening. In some cases, the parts are equal to pressure injection castings.
By using adequate controls, many variations of the casting process are available. For instance, dams can be cast in different forms. Cast from highly viscous, thixotropic material, dams can be filled with resin of low viscosity (dam & fill). The deposition speed of the resin can be varied during casting or casting can be done in several portions. At the same time the part can execute complex movements. Using additional options of that kind make it possible to solve difficult casting problems.
A casting system can be combined with many different production steps within a production line.
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