What We Do
Powder Coating – Electrostatic Spray
Electrostatic spray powder coating is the most common form of powder coating. Powder is pumped into a spray gun, past an electrode where it becomes charged and onto a grounded part. As the powder builds up on the grounded part the attractive force decreases until excess powder falls to the floor of the spray booth. This helps the coating to be of uniform thickness and minimizes or eliminates drips and runs. The dry powder covered part is then heated up until the powder melts and flows out to a smooth film. Some plastics chemically change (“cure”) as they melt. Others simply melt and flow out to a smooth finish.
- Because the parts are not immersed in a tank, there is flexibility to coat large items. The size of the oven is the limiting factor here.
- A small fluidizing hopper can be used to hold the powder, so for testing or small volume orders only a small amount of powder is needed.
- Most plastics in one form or another can be processed into a spray able powder: nylon, polyester, epoxy, urethane, etc., whereas fewer powders work effectively for fluidized bed coating.
- Thicker is not always better. Many plastic coatings have ideal performance characteristics with thicknesses of 2-4 mils (.002-.004”). Fluid bed coatings, however, are 6 mils and up.
- Available in many off-the-shelf colors and chemistries.
- Edges, angles, and cavities in parts are more difficult to coat because of a phenomenon known as the Faraday Cage Effect. A powder coated part develops electrical fields around edges and the openings of cavities which tend to block the powder from adhering to those surfaces. Adjustments to the spray equipment can minimize the difficulties.
- There are many factors that need to be controlled or compensated for: ambient temperature and humidity, moisture content of the powder, electrical properties of the gun and controller, proper electrical ground in the spray booth, etc.
- Powder collects in the pleats and pores of the filter and not all of it is dislodged by the back pulse of the purge cycle. If you want to ‘reclaim’ the powder from the floor of the spray booth and re-use it you must have dedicated cartridge filters that you change out whenever you change colors or chemistries. That requires extra set-up/tear down time and extra filters that must be bought, organized and maintained. The alternative is to ‘spray to waste’ and dispose of the powder whenever you clean out the booth. If the job uses a large amount of powder and/or the powder is relatively expensive, then it may pay to reclaim.
- Problem: Moveable walls attached to each other with locking cams. If the panels were not locked they could fall. In the unlocked position the locking pin was visible, but was tucked away in the locked position. However, engineers wanted to increase visibility so it would be seen if the panel came unlocked.
- Solution: We powder coated the wire with safety yellow polyester powder that had high visibility.
Powder Coating – Fluid Bed
In fluid bed coating, air is pumped into a layered tank of air and powder. A special porous membrane separates the bottom section of the tank into which air is pumped from the top section of the tank that holds the powder. As the air is pumped into the bottom section it moves through the membrane and into and out of the powder. This aerates the powder and makes it possible to dip a part into it. The process involves preheating a part, dipping it into a fluidizing bed where powder adheres to the hot part, then sending the powder covered part into an oven where it flows out to a smooth, cured finish. It is, in many ways, a simpler coating than electrostatic since the primary factor is the heat resident in the part at dip. That heat together with the dwell (or time spent sitting in the powder) determines the “build” of the coating. Fluid bed coating is generally thicker than an electrostatic powder coat and is, in many ways, a simpler more consistent process. Because the surface of the bed is ‘simmering,’ if a precise dip line is required, then the parts will need to be masked.
- Fluid bed powder coat can give a thicker coating than electrostatic spray. We find that the range is from about 6-30 mils (.006-.030”).
- Because the powder is contained in a dip tank, almost all the powder that does not coat the part is reclaimed.
- There is no Faraday cage effect so edges and holes do not typically present an issue.
- Fluid bed coating is usually more easily controlled than electrostatic spray coating. The heat and fluidizing factors are easier to control than the electrical charge, good ground, properly dry powder, and electrical fields that must be considered in spray powder coating.
- Many parts have areas where a coating is not desired. Since there is a dip line often those parts can be hung so the no-coat area is out of the dip tank. This effectively “masks” that area and can save considerable expense compared with expensive tooling or manual masking that would be required in a spray application.
- Because the powder is contained in a dip tank, the tank must be emptied and carefully cleaned whenever production requires a change of powder.
- As a part hangs on the line at dip the coating will be thinner at the top and thicker at the bottom. That is because the bottom is first into the powder and last out. Hanging long parts in a horizontal orientation can compensate for this.
- As the part is dipped into the tank of powder, ledges and cavities in the part may fill up with powder which may be too thick or may not cure properly. If a part cavity is facing down, the rising air may form a pocket in the part and prevent the powder from coating it.
- Occasional spurts of powder shoot ¼” or more above the simmering surface of the fluid bed. These are called “geysers” and make it difficult to hold tight tolerances on the dip line. Things that affect the frequency and extent of these geysers:
- The specific gravity of the powder. The denser the plastic the more air pressure is needed to keep it moving.
- The shape of the powder particles. Most powders are cryogenically ground. Plastic polymers are frozen and then smashed to break them into small pieces. Those pieces are then sieved to get an ideal particle size. Some powders are mechanically ground which produces a more spherical powder particle. Those powders will simmer together with less jumping/geysering.
- The quality and condition of the membrane. The size and consistency of the pores of the membrane can affect the uniformity of the air bubbles that move up through the powder and, thus, the uniformity of the boil. Membranes may have inherent inconsistencies and can be damaged by hot parts that fall off processing hooks and into the tank. Membrane pores may also be blocked by wedged-in powder particles or by oil/contaminants in the air supply.
- Where geysers cannot be sufficiently controlled for a given requirement then parts may need to be masked prior to dipping. With smaller parts this is usually not a cost-effective solution.
- Problem: A torsion spring is used in a moveable headrest, however, there is a twang noise as the spring moves back and forth. A PVC coating would ‘dampen’ that twang. Since the torsion spring has space between the coils it can be coated without ‘welding’ the coils together and changing the force of the spring. A liquid plastisol dip would have drips and runs as the coils came out of the liquid. A spray powder coat would not provide sufficient thickness to really dampen the sound.
- Solution: Fluid bed coating is ideal. We were able to apply a nice coating of each of the coils with very little webbing of the PVC from coil to coil.
In Dip Coating, a part is heated and then dipped into a vinyl liquid. It is then sent through a cure oven where the coating becomes smooth and strong. The vinyl (or plastisol) is available in various colors, textures and hardness. Thickness varies between .010" to .250" depending on factors of time and temperature.
- Ability to make thick coatings.
- Absorb impact-to protect hands and/or equipment
- Vibration damping to protect from carpal tunnel syndrome.
- Vibration damping to inhibit noise transmission (buzz, rattle, squeak).
- A straight dip line for aesthetics or where there is very little clearance between coat and non-coat areas.
- Resistant to most chemicals
- Plastisol is very strong. For instance, one of our standard plastisols has a minimum tensile strength of 1575 PSI, meaning that it takes at least 1575 lbs of pulling per square inch to tear it!
- Plastisol is self-extinguishing. Plastisol will burn when exposed to a flame, but when the flame is removed the plastisol will stop burning.
- Electrical insulator
- Relatively low expense for custom colors. Custom colors can be formulated by the 5 gallon pail, whereas custom colors in powder usually require a minimum order of 250 lbs. or more.
- The material formulator can add various ingredients to get style and performance characteristics.
- Style characteristics include color, texture (ranging from a rough sand-like finish to high gloss), glow-in-the-dark, etc.
- Performance characteristics include Ultraviolent light stability, various hardnesses, resistance to chemicals, mold and mildew inhibitors, cold temperature flexibility, adhesion to a substrate, etc.
- Plastisol does not break down quickly. If just thrown into a landfill it can last for many years. There are, however, many uses for recycled plastisol.
- Though self-extinguishing, PVC gives off hydrogen chloride gas when it burns which can irritate eyes and lungs.
- Problem: A metal crush bracket was located inside the dashboard of an automobile. Its purpose was to provide some structural support and yet collapse under crash stresses. However, wiring harnesses were also routed through that space. The wires could be cut against the sharp edges of the bracket. Engineers proposed to coat the sharp edge with plastisol to protect the wires.
- Solution: We designed a machine to coat the edge except for two weld areas that needed to be free of coating. No primer was necessary because slots were stamped near the edge of the part and the plastisol flowed through those areas to lock it in place.
In Dip Molding, a mold (mandrel) is heated and then dipped into vinyl. The amount of resident heat in the mandrel at the time of dip combined with the time it is submerged in the vinyl determines the thickness of the coating. The operator will speed up and slow down the speed of withdrawal in order to avoid air pockets and to minimize drips and runs. The coated mandrel is then sent through a cure oven where the coating cures, becoming smooth and strong. As it conveys out of the cure oven and past cooling fans, the plastisol coating is separated from the mandrel with compressed air and placed into a cooling water bath. The cold water fixes the shape of the part. If desired, the part can then be cut to length and windows or holes stamped into it with special trim dies. Because the coating is removed from the metal mold, the final product is closer to a molded part than a coating, and can be compared to blow-molding and injection molding.
- Dip molding often works very well for sleeves or tubes where interior clearances are critical. In other molding processes, variation in the thickness of the plastic can restrict interior spaces and create problems for tubes or straps that need to fit inside them. Since the inside of a dip molded part is fixed by the mandrel, variation in the thickness of the plastic does not affect the interior space.
- The finished look of a blow molded or injection molded part is fixed by the imprint of the mold. In dip molding the finished look can be varied with different types of plastisol. Plastisol finishes include gloss, matte, and texture.
- Dip molding is often less expensive than other types of molding. Mandrels can be CNC cut or even 3-D printed relatively easily to dip parts.
- With blow mold and injection mold, the outside of the finished product is an imprint (or negative) of the inside of the mold. In order to remove the plastic product from the mold, the mold has to be able to open and consequently parting lines are also molded into the part (e.g., the line seen on a one-gallon milk jug). Dip mold does not have this limitation since the mold imprint is inside the finished part.
- Wall stock thickness is dependent on the physics of the dip. The first part of the mandrel into the material during the dip is also the last out. Therefore, from a vertical perspective, the top of the part will be thinner than the bottom.
- The last portion of the mandrel to leave the plastisol will have a parting line or drip. The drip may be unnoticeable or the parting line unobjectionable, but it is a factor to consider. If the part is a sleeve that is cut to length, then usually the drip is trimmed off anyway. If the part has a more critical “A-surface,” then often the mandrel can be manipulated so that the parting line is not on that surface.
- From a production perspective, though fairly straightforward, there are a number of steps to the process: filling the dip tank and adjusting the leveling bars to the correct height; setting the dwell, cycle time, and heats; preheating and dipping with variable speeds depending on the product; cure, cool, and blow-off; put into cooling bath; set up trim die and trim; inspect and pack.
- Problem: A customer who supplied seat belt accessories for the aerospace industry was having problems with interior clearance on blow molded sheath covers.
- Solution: S&C eliminated those problems by dip molding the sheath covers - because the interior space of a dip molded part is fixed by the mandrel.
- Problem: A metal display used in retail sales has an exposed projecting edge, which poses a safety hazard to customers.
- Solution: Using a dip molding process, S&C developed a protective sleeve to fit on the projecting edge.
- Problem: Drivers often reach under the seat "blindly" to locate the release bar in order to move the seat back and forth.
- Solution: S&C provides a cushioned grip by dip molding a textured sleeve that is inserted onto the bar. The sleeve has a friction fit that does not require any kind of adhesive to stay in place.