IPS Weld-on's Basic Principles of Solvent Cementing

To make consistently good joints, the following points should be clearly understood.

1. The joining surfaces must be softened and made semi-fluid.
2. Sufficient cement must be applied to fill the gap between the pipe and fitting.
3. Assembly of pipe and fittings must be made while the surfaces are wet and cement is still fluid.
4. Joint strength develops as the cement dries. In the tight part of the joint, the surfaces will tend to fuse; in the loose part, the cement will bond to both surfaces.

Penetration and softening can be achieved by the cement itself, by using a suitable primer or using both primer and cement. It is necessary to use a primer for certain materials and situations. A suitable primer will usually penetrate and soften the surfaces more quickly than cement alone. Additionally, using a primer can provide a safety factor for the installer, for he can know under various temperature conditions when he has achieved sufficient softening. For example, more time and additional applications may be required in cold weather.

More than sufficient cement to fill the loose part of the joint must be applied. Besides filling the gap, adequate cement layers will penetrate the surfaces and remain wet until the joint is assembled. Prove this for yourself. Apply on the top surface of a piece of pipe two separate layers of cement. First, apply a heavy layer of cement, then alongside it, apply a thin brushed-out layer. Test the layers every 15 seconds or so with a gentle tap with your finger. You will note that the thin layer becomes tacky and dries quickly (probably within 15 seconds). The heavy layer will remain wet much longer. A few minutes after applying these layers, check for penetration. Scrape the surface of both with a knife. The thin layer will have achieved little or no penetration. The heavy one will have achieved much more penetration.

If the cement coatings on the pipe and fittings are wet and fluid during the assembly, they will tend to flow together and become one cement layer. Also, if the cement is wet, the surfaces beneath them will still be soft, and these softened surfaces in the tight part of the joint will tend to fuse. As the solvent dissipates, the cement layer and the softened surfaces will harden with a corresponding increase in joint strength. A good joint will take the required working pressure long before the joint is fully dry and final joint strength is obtained. In th