Mesh Pretreatment & Degreasing

Question

What is mesh pretreatment?

Solution

Mesh pretreatment is the process of cleaning and preparing the screen mesh surface to improve stencil adhesion and eliminate coating defects, with the ultimate goal being optimum stencil performance and durability.

With virgin monofilament synthetic screen mesh, we recommend using an abrasive degreaser product, followed by a degreaser/pretreatment between subsequent stencils. For all other mesh, we recommend using a degreaser/pretreatment prior to stencil production and reuse.

Today there are carefully formulated mesh preparations available that go a step beyond conventional degreasers. For example, SaatiChem's Direct-Prep™ 2 actually treats the mesh surface. Direct-Prep 2 leaves a microscopic layer of an adhesion promoter on the surface of the mesh threads that improves film lamination and the coating and bonding of direct photoemulsions.

Question

Why should I degrease my screen fabric? Isn't it clean when I receive it?

Solution

Yes, it is. However, handling during stretching can contaminate the mesh (skin oils, dust, etc.). These invisible contaminants can then impair stencil adhesion. Also, as we've described, good mesh preparation is more than just cleaning.

A properly degreased/treated screen surface should display a thin, even film of water with no beading. Inadequate mesh preparation can lead to:

splits, spots and fisheyes in the wet emulsion coating
pinholes
blotchiness due to uneven stencil thickness
dry spots or air pockets trapped under the capillary film
premature stencil breakdown due to poor adhesion

Question

Isn't degreasing necessary only with capillary films?

While it's true that special degreaser/mesh pretreatment products were designed originally for use with capillary film, research has shown that benefits apply to an equal or greater degree to direct photoemulsions. The products were sought for capillary films because of their ability to raise the surface energy of the mesh, enabling it to hold an even film of water on the surface.

The extra ingredient in these products that makes all the difference, was actually used in the past as a pretreatment to enable dye to adhere to polyester. As an illustrative point, consider the diazo sensitizer used in emulsions to be a form of dye. The special mesh preparation provides a link to attach the emulsion polymers firmly to the otherwise inert polyester surface. It is also the "wetting" action of the thin layer of adhesion promoter, when combined with an effective cleaner, that enables this type of product to improve the coating quality of your emulsion.

Question

Why not use solvents or household cleaners to degrease my screen?

Solution

Screen printing solvents will dissolve grease and oil (except silicone oils) on the fabric surface, but during evaporation these contaminants will redeposit themselves. Caustic degreasers should also be avoided because they increase the alkalinity of your waste water. If used in sufficient quantity, they may even raise the pH above the limit allowed by your local water treatment authority.

Household detergents are also unsuitable; they contain ingredients like lanolin and perfumes that leave a film on the mesh.

Question

Why is fabric roughening or abrading necessary?

Solution

Due to the nature of virgin synthetic monofilament mesh, it is necessary to gently abrade it before use to make it more receptive to good stencil adhesion. If you have ever seen polyester mesh under high magnification, you can attest to the surface of the threads being as smooth and featureless as polished glass. This, combined with the fact that polyester is a difficult substrate to stick to, means your emulsion coating needs all the help it can get to maximize adhesion. (The inherent surface traits of multifilament synthetic or stainless steel mesh already promote adequate adhesion.)

This becomes more and more important as the mesh count decreases; fewer threads mean less surface area, and therefore less to grip. A slight roughening of the surface increases the physical adhesion of the stencil to the fabric and enables it to better tolerate adverse conditions. For instance, it is better able to withstand the flexing and stretching that occurs if you ever have to print with a high off-contact. Particularly when the humidity starts to rise and your stencil is susceptible to losing some abrasion resistance.

Your stencil on a virgin polyester screen that hasn't been abraded can be compared to a coat of paint on an unprimed surface. Subjected to the battering of wind and rain, it begins to flake.

Note: household scouring powders should not be used. Their grain particles are too large, uneven and aggressive, which can cause clogging of the mesh openings or excessive thread damage.

Properly abraded fabric

Virgin monofilament fabric

Over-abraded fabric

Question

Why is good wetting action important to stencil adhesion?

Solution

All stencil systems contain a percentage of water, so the more receptive a screen is to water, the better the stencil adhesion. Equally important is that the screen be receptive uniformly.

Question

What are the special mesh preparation considerations when using a pure photopolymer emulsion?

Solution

Correct mesh preparation is essential when using a pure photopolymer emulsion in order to maximize adhesion during stencil processing, and durability on press. Prior to coating, a mesh prep/degreaser containing a wetting agent should be used. This leaves the smooth, inert surface of monofilament polyester primed to adhere much better to pure photopolymer stencils. This is illustrated in the photograph where one half of the screen was degreased only with a detergent cleaner and the other side was degreased with SaatiChem's Direct-Prep 2.

Degreased with a detergent

Degreased with SaatiChem Direct-Prep 2