PVC dip coating vs heat shrink for busbars

Two ways to insulate a busbar

A coated busbar is a mechanical part with an electrical job. The coating has to keep a live bar away from a person, a cable or an enclosure, and it has to survive the temperature, the cleaning and the movement of the application. PVC dip coating and heat shrink are the two common ways to do that on a rigid or formed bar.

Dip coating is a plastisol process. The bar is cleaned, primed where needed, heated and dipped into a PVC dispersion, then cured in an oven. The plastisol fuses into a continuous film that follows the shape of the bar. Heat shrink starts with an extruded sleeve that is expanded at manufacture. The sleeve is slid over the bar and warmed, and it shrinks back onto the surface.

The processes look similar in a photograph and behave differently in a drawing. Dip coating builds thickness from the outside. Heat shrink follows the shape it is given, so the bar has to be designed for the sleeve, not the other way round.

Coverage at bends and cut ends

A dip coating wraps the outside and the inside of a bend, and it seals a cut end because the plastisol covers the edge. That matters on a bar with a tight bend or a sheared end, where a sleeve would bridge the bend or leave the end open. The film thickness at a bend is lower than on a flat face, because the plastisol drains away from a convex surface, so the process needs a check at the bend.

Heat shrink covers a gentle bend if the sleeve can follow the radius without wrinkling. A tight bend is harder. The sleeve may bridge the inside of the bend, leaving an air gap, or wrinkle on the outside, leaving a thin spot. Cut ends stay open unless a cap or an adhesive-lined sleeve is used.

Edge coverage is often the deciding factor. A busbar has sheared edges and punched holes, and those edges are where a coating is most likely to be thin. Dip coating flows over an edge and builds a fillet. Heat shrink has to stretch over the edge, which thins the wall at exactly the point that needs protection.

Temperature and fault behaviour

PVC is a thermoplastic. Its useful service temperature is typically around 105 degrees C, and it softens as it approaches that limit. Crosslinked polyolefin heat shrink is usually rated for 125 to 135 degrees C, with special grades above that. The difference matters on a busbar near a heat source, in a sealed enclosure or on a bar that carries a fault current for a short time.

The fault case is worth a separate look. A short circuit raises the bar temperature quickly, and the coating has to stay in place long enough for the protection to clear the fault. A thermoplastic can soften and flow; a crosslinked sleeve is less likely to melt, but it can still char or split. The protection settings, the bar size and the coating all belong in the same thermal check.

Do not choose a coating by its continuous rating alone. Ask for the rating basis, the test standard and the fault duration. A coating that passes a continuous test may behave differently in a short-time fault, and the enclosure and the mounting change the result again.

Thickness and dielectric performance

Dip coating builds thickness over several dips. A single dip might give 0.3 to 0.5 mm on a flat face, and a second dip builds towards 1 mm. The thickness varies with the shape, the withdrawal rate and the viscosity, so the drawing should call a minimum on the surfaces that matter, not a single figure for the whole part. The dielectric strength of PVC is high enough that a thin film can hold a useful voltage, but a thin spot at an edge is the weak point.

Heat shrink thickness is set by the sleeve wall and the shrink ratio. A 2:1 sleeve on a bar that is close to its expanded size gives a wall close to the nominal. A 3:1 sleeve on the same bar has to shrink further, and the wall thins as it stretches around a corner. The installed wall is what matters, and it should be measured on a sample, not taken from the catalogue.

Both coatings add to the bar dimensions. A 1 mm coating adds 2 mm to the width and the height, which changes the clearance to another phase or to the enclosure. The coating also changes the bend radius that the bar presents to a cable. Those effects belong in the layout, not in a note added after the busbar drawing is released.

Cost, tooling and volume

Dip coating has a higher setup cost. The line needs a bath, a cure oven, racks and masks, and the first parts need a trial to set the thickness. Once the line is running, the unit cost falls with volume, and complex shapes cost little more than simple ones because the process covers the geometry without extra labour. Masking the joint areas is the main variable cost.

Heat shrink has a low setup cost. The sleeve is bought in a range of sizes, cut to length and shrunk with a heat gun or a tunnel. The labour is per part, and a long bar with several sleeves takes longer than a short one. At low volume, that makes heat shrink the cheaper route. At high volume on a complex bar, the labour can exceed the cost of a dip line.

Repair and rework

Heat shrink is easier to repair. A damaged sleeve is cut away, the bar is cleaned and a new sleeve is fitted. That can be done at the assembly line or in the field with a heat gun, provided the bar can be accessed. The repair is visible, but it restores the insulation without stripping the whole part.

Dip coating is harder to repair. A damaged area can be patched with a liquid coating or a sleeve, but the patch is not the same as the original film and the joint between the patch and the coating is a weak point. A full recoat means stripping the old film and going through the line again, so replacement is often cheaper than repair.

Choosing between them

Start with the temperature. If the continuous or fault temperature is above the PVC rating, heat shrink is the safer route unless a special PVC grade is qualified. Then look at the geometry. A tight bend, a cut end that must be sealed or a complex shape points to dip coating. A straight bar with a moderate bend and a need for field repair points to heat shrink.

Then look at the volume and the rework model. A high-volume programme with a complex bar usually justifies a dip line. A low-volume or prototype programme rarely does. In between, the labour content and the packing cost decide.

Whichever route is chosen, put the coating on the drawing with a minimum wall, a temperature rating, a colour and a test. State which areas stay bare for the joints, and say how the coating is inspected. A drawing that says only that the bar is coated leaves the key variables to the supplier.