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Dip Molding Proceedures

Safety
Loading the Machine
Preheating
Time, Temperature and Speed
Dipping
Curing
Cooling
Ejecting (mandrels) or unloading and reloading (coatings)
Safety
  • Use protective clothing such as gloves (where needed) and long sleeves
  • Be aware of all emergency stop pushbuttons and procedures
  • Obtain training and read operators manual
  • Be aware of turret reverse procedures on rotary machines and conveyor reverse procedures on inline machines
  • Do not open electrical enclosures without authority
  • Ensure proper ventilation of PVC gases and smoke
  • Read and understand MSDS for raw material being processed
Loading the Machine

Single racks of mandrels are usually placed on the machine by operators or, if over 50-60 pounds, by overhead hoist. These racks must be held in place by alignment pins or other fasteners

Master rack systems are usually loaded by placing the master rack on the arms or conveyor first then dropping the adapter bars, with tooling affixed, into the master rack

When using the machine for coating parts, many different procedures have been successfully used to unload and reload the parts onto the machine for processing. The A.R.T.S. station is a popular automated means of adding operator, cooling, and primer stations. Gravity conveyors can also be used so that the operator can move the molded rack of parts onto it and then load racks that have been prepared for molding offline back onto the machine. A powered conveyer can be used with this system to transfer the unloaded racks to the gravity feed loading conveyor

Preheating

Tools or parts must reside in the preheat oven long enough to absorb adequate heat for the dip process. This is largely determined by trail and error and is effected by plastisol thickness requirements, tool material (alloy), oven heating characteristics, and number of preheat stations. On very heavy tools or parts, this part of the process can easily be the most time demanding of the entire process and would ultimately control overall cycle time. This is overcome by adding more preheat sections to the machine design when it is known that large, heavy parts will be processed

Most plastisol require a minimum of 320 degrees F or higher to gel. Also consider that some mold lubricants will burn off at temperatures over 500 degrees F

Although oven air temperatures can easily run over 550 degrees F, this does not guarantee mold temperatures will reach that point. Heat transfer characteristics of the oven, material and mass of the tooling, and resident time in the oven are all factors

Time, Temperature, and Speed

The instant the preheated parts or tools are indexed from the last preheat section they begin to cool. The rate that they cool depends on the tool or part alloy and ambient conditions such as temperature, air movement, and humidity

Rapid indexing is key to getting the preheated parts or tools into the dip station without excess heat loss. In the case of overhead dip systems, rapid traversing systems must be used to get the parts to the dip stations as quickly as possible

When double dipping parts on overhead dip systems, the latent heat of the tool must be sufficient to gel not only the first coating, but also penetrate the first coating and gel the second. That is why smaller parts that require multiple dipping must be processed on a machine designed with reheating capabilities between the dip stations (usually not an overhead dip system)

The speed in which the parts or tools are submerged into the plastisol should generally be accomplished as quickly as possible without trapping air in unwanted places or disrupting the plastisol surface excessively. Profile dipping is used to accelerate and decelerate dip speeds to help control these factors as well as control taper and drips

Dipping

Consistent moldings rely on precise motion control of the dip system although manual dipping systems can be less expensive when dip length and wall thickness tolerances are forgiving

Overhead dip system and moveable tank dip systems are available (standard on our machines) with servo driven, profile dipping that allows the operator to program various speeds of submersion and extraction based on position. This allows some control of tapered walls and drips

Steady motion at low speeds (as low as one inch per minute) is accomplished by precision servo and ball screw components. Vibration and slight motion disruptions can cause visual lines to appear in the molding (especially on transparent parts)

Plastisol level control, agitation, and temperature control is very important when close tolerance moldings are being processed

Curing (fusing)

Dipped tools or parts must reside in the cure oven long enough to properly cure the plastisol. Time and temperature requirements are sometimes determined by trial and error and are effected by plastisol thickness, tool material (alloy), oven heating characteristics, and number of cure stations. On very large tools or parts with heavy coatings, this part of the process can be the most time demanding of the entire process and would ultimately control overall cycle time. This is overcome by adding more cure sections to the machine design when it is known that large, heavily coated parts will be processed

Most plastisols require a minimum of 320-350 degrees F or higher to cure or fuse. This varies with different formulations. Even heating with turbulent air movement is key to evenly curing the entire rack of parts or tools

Unless proper air movement and heater element selection is critical too. It is possible to over cure the tips (closest to heat source) while under curing the top of the molding

Over curing parts can also cause the plastisol to become too liquid and drip. Severe over curing can cause the plastisol to drip onto heating sources and ignite

Proper curing is usually verified by visual inspection and tear testing

 

Cooling

There are basically 3 ways to cool parts that have been dip molded or coated; forced air, ambient air, and water quench

Water cooling is the most efficient means, but is restricted in most medical applications

Forced air cooling is usually accomplished by fans operating on a timed circuit. The operator programs the fan on time to turn the fans off before cooling the parts too much. Too much cooling can make a mandrel dipped part difficult to remove from the mandrel. Under cooling can cause the part to distort when being stripped and can be a safety hazard if the parts are to be handled immediately after being striped (ejected)

The cooling process can be the “bottleneck” of the operation when processing heavy parts, thick coatings, or double dipped parts. The plastisol coating actually insulated the tool or mandrel and holds the heat in the metal longer. Thicker parts or double dipped parts (especially when using foams) increase the insulation properties making it more difficult to cool the tooling. In this case, more than one cooling station can be incorporated in the machine design when it is known that this type process is needed

 

Ejecting (mandrels) or unloading and reloading (coatings)

On manual or semi automated systems, ejection can be done with concentrated compressed air, through a special air nozzle, directed at a key area of the molding (usually the dip line) after it has been partially cooled. The new FlexEject system automatically removes parts robotically, eliminating some labor and improving safety

Parts are usually ejected into a hopper then manual, pneumatically, or belt conveyor moved to a secondary cooling and packaging area

Machines designed for long parts (over 10-14” dip length) are offered with optional tilting devices that rotate the rack of tools up to 90 degrees in order to better access the tools for manual striping. In this case, j-shaped air nozzles can direct compressed air to the dip line area while the operator assist the removal of the part with the other hand

Completely automatic FlexEject systems can be programmed to remove dip molded parts without an operator

Q.C. measurements and inspections should be done after the parts are completely cooled as some dimensional changes can occur as the parts cool

Molded parts should be thoroughly cooled before packaging to prevent sticking together and marring other moldings

Coated parts are usually removed from the hangers on the rack, after being water cooled, by hand. New technology is now available to remove the parts (in most cases) robotically, eliminating some labor and improving safety.