Spin coating Versus Dip coating

How to successfully dip coat your lens

The process of dip-coating a pair (or many more) lenses can be a daunting prospect to labs after the great time and expense already gone into the production of lenses at that point, so the need to make sure you are working with the best solution for your needs together with reliable processes and equipment that removes as much risk as possible is second to none.

As opposed to other methods of applying a hard coat to lenses, the thorough cleaning or rather cleaning and lens preparation prior to coating being applied are a major part of successfully dip coating processes and long-term product reliability. Dip systems can be both separate stand-alone units but most of the main manufacturers now provide a complete cleaning and coating system. With an all-in-one system this emphasizes the importance of good lens preparation in a successful coating process which is often underestimated but is in fact vital. The surface preparation and cleaning solution can be created prior to the hard coatings that best produces results dependent on lens types and materials i.e., CR39, High Index, or Polycarbonate and single vision, bi-focal and progressive lenses can all have differing preparation methods before and during your coating process that can easily be achieved in a complete dip-coating process with uncompromising quality.

The lens preparation and cleaning are carried out in a series of stages before the coating is applied, these stages should have ultrasonic capability and controls followed by excellent rinsing of the lens surface. Final cleaning stages are most important and should harness high quality deionized water ‘polished’ and filtered to such degrees that with a controlled slow extraction of the lenses this leaves a stain-free finish surface that is then dried leaving your lens surfaces ready for their dip coating to next be applied.??

Arguably the ‘higher maintenance’ solution of applying a hard-coat over a countertop unit with spin-coating, dip-coating lenses is seen as the more ‘complete solution’ and preferential method to achieve the optimal hard coating performance. Encapsulating your lenses in an entire bath of coating solution leaving no area untouched over the entire geometry of the lenses and then being able to control the thickness of the coating through controlled coating extraction is technically the best solution. As either a hard-coated or additional A.R coated product by dip coating lenses this removes any visible or performance quality differences you could encounter between the front and back surfaces being applied, thereby providing the ultimate uniform base for additional treatments such as A.R or Mirror coatings you may further add.

Coating systems of today will have multiple coatings all on board one system, this system is general 3-5 coating stages comprising of primer and top hard coating solutions all independently controlled and maintained, chilled to manufacturers recommended temperatures and filtered constantly. Having a good team that can follow and ‘own’ the good housekeeping of your machine is second to none.

As a widely used method of coating in many different and increasing industries, while of course by lens manufacturers worldwide in larger optical labs. The approach to dip coating lenses is increasingly being adopted by smaller independent labs as they move to increase their throughput and coating capabilities and even becoming ‘licensed labs’ to larger lens suppliers coating processes. The equipment for the application of dip-coating is generally supplied available to achieve capacities of 20-1,000 lens in every hour of production offering a much less labour-intensive process in a batch production method.

Attributes of good hard-coating equipment is firstly that they are robustly constructed using very well manufactured clean stainless stain surfaces and importantly stainless-steel ultrasonic cleaning and coating tanks to provide a robust and continual operation for you for many years. The latest operational controls to ensure all cleaning and coating parameters are carried out consistently on each batch and increasingly higher levels of preventative maintenance and KPI’s are built into the systems. Precise servo-driven control of coating tanks is a must and especially good circulation, and filtration systems should come as standard to achieve extremely high yields and keep your coating material in the best condition for the longest period of its capable use.

While the preferred method of coating largely adopted worldwide, Europe and Asia are the main use cases of the dip coating method while a ever increasing adoption in the U.S sees many labs moving to the higher volumes of coating mix in their labs and are looking to the move to larger batch dip coating processes.

Courtesy of: MAFO Ophthalmic labs & Industry - issue 04/2021

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