Control of the average molecular size of the finished resin is essential for correct flow in plywood and particleboard applications while in the hot press prior to curing. Too low a level of condensation (i.e., low-molecular-weight resins) may give too much flow; the resin
‘‘runs away’’ from the wood or sinks into it rapidly under pressure, leaving ‘‘starved’’ glue lines. This can be corrected by lowering the pH by adding an acid or acid-producing substance, usually a curing agent, hardening catalyst, or simply, hardener. If a resin of too high a condensation stage (i.e., high-molecular-weight resins) is on hand, its flow under normal pressure and temperature may be too low to produce good results. This can usually be corrected by adding flow agents to it, provided that at least some flow is left in the resin. It is generally an advantage to produce resins with ample flow in the factory. Their storage life is longer and finishing can be done at any time, at short notice, to specification, particularly by adjusting the flow and speed of cure.