The major characteristic of direct liquid emulsion to take note of is the fact that they always react to ultraviolet light. This reaction causes the emulsion to become hard which can also be referred to as ‘cured.’ If exposed to a normal ambient light over a longer period, you will also get the same reaction. This is the reason behind the use of direct liquid emulsion under red or yellow safe-light conditions. In other words, an average amount of exposure from the UV source is necessary to expose your screens within a reasonable time. A lower amount of light from the UV light source would require more time.
It is important to note that excessive exposure to heat or longer time under the UV source can prove to be problematic. A light source such as the sun can cause the emulsion with heat alone to solidify. Depending on their photosensitivity, the reaction time may vary. Also, the intensity of the light source has a role to play in the reaction of the emulsion, which is why it is important to always consider your light source when choosing an emulsion.
Commonly, there are two types of photosensitizers which are namely: Diazo and Styryl Basolium Quaternary (SBQ). The Diazo Photo sensitizer has been around longer than the SBQ sensitizer which is a new type of photosensitizer. Any of this sensitizer can be added to a resin base of polyvinyl alcohol or polyvinyl acetate to make an emulsion.
Based on the two above mentioned photosensitizer, the emulsion can be categorized into a group of three which are: Diazo emulsions, Dual-core emulsions, and photopolymer emulsions. The Dual-core emulsions is a combination of the better parts of the Diazo emulsions and photopolymer emulsions, while also compensating for the individual flaws in each of them.
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- The Diazo emulsions are the cheapest of the three, also with the lowest light sensitivity. This will be the wrong type of emulsion to use with a weak light source. Although they have the widest exposure latitude of the three. They make thick stencils and may not be the best choice for fine details. They are either water-resistant or solvent resistant so it is best to have the two types at your disposal. To achieve an optimum result, they should be mixed with sensitizers, although this reduces their shelf life to about two to three months.
- Photopolymers, also known as SBQ photopolymers, also known as a one-pot emulsion, have the lightest sensitivity of the three, therefore it is the fastest emulsion of the three. Although they are quite expensive, they have a longer shelf life of up to one year. They are also a good choice for fine detail. Their latitude exposure is smaller than that of the Diazo emulsion so they require a good light source. While some SBQ are water-resistant and are not affected by ambient humidity.
- The Dual cure emulsions have a combination of the qualities of both Diazo and SBQ emulsions. Their exposure time is between the range of that of the SBQ and Diazo emulsions. They can produce fine detail and their latitude exposure is wider than that of Diazo. They are less expensive than SBQ but with a shorter shelf life
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