Mold coatings boost tool performance.

Synergistic ceramic coatings improve aluminum molds for mission-critical composite structures.

Ordinarily, the molding and curing of composite plastic components is considered a standard manufacturing procedure by the engineering staff of the McDonnell Douglas Helicopter Co, Mesa, AZ. However, the molding requirements encountered in a more recent project demanded new and unusual approaches. The part involved is a mission-critical structure with demanding performance requirements and tight tolerances.

Mission-critical translates into "If the part were to fail, the missile mission would fail also." This specialized component consists of six deep, zero-draft compartments segregated by internal walls and bulkheads.

The basic material used is a 0.1" thick hybrid carbon/Kevlar laminate. Two of the walls are rigid foam sandwich structures 0.4" thick). On the ends of the component are highly structural mounting flanges 0.25" thick) fabricated primarily from carbon fabric. The assembly's function is that of a payload dispenser module-a structural container for housing the missile ordnance. The problems faced by the project engineers involved the cure tooling for the hybrid composite used in fabricating the container.

The problems

The tight tolerances and complex configuration of this payload module challenged tool designers, and they had to employ several nonconventional tooling methods. Three molding problems faced the engineers when working with the hybrid composite: (1) difficulty in achieving quick and nonbinding release from the mold, (2) destructive wear on the base plate and the aluminum molds caused by friction generated by the composite's reinforcing materials, and (3) high labor and time costs involved in frequent cleaning and preparation of mold surfaces to eliminate contaminating residues.

A processing-cost disadvantage in composite reinforcing materials in use today (carbon, Kevlar, glass, and quartz) is the accelerated frequency of platen mold aluminum surface may be considered as "file hard," comparable to that of hard chrome-plated steel. In accordance with Federal Test Method Standard 141, Method 6192, Cs/17, the wheel-weight loss (as measured under repeated tests) is 4 mg or less after 10,000 cycles. This is ten times better than test parameters require for hard-coat anodize on aluminum.

Even more important, the polymer impregnation that results from the new synergistic coating provides a smooth, slippery surface with permanent lubricity. Tests indicate that static friction of the HCR coating decreases with increase in load. The static coefficient of friction (0.05) is also lower than the dynamic coefficient. This characteristic eliminates the problem of stick-slip, in which higher breakaway friction causes undesirable vibrations.

Very few solid substances will permanently adhere to the polymer-impregnated surface of an HCR-coated part. And while some tacky materials may exhibit some temporary adhesion, almost all substances release easily. It is this attribute of synergistic coatings that made them particularly effective in solving this hybrid composite's precision metal mold problem.

How the coating process works

In this HCR coating and impregnation process, the aluminum in the surface of the mold is converted to aluminum oxide. An "H20" portion of the newly formed ceramic surface is replaced with General Magnaplate's inert polymeric materials that provide a self-lubricating surface. In the process, the aluminum crystals expand and form porous anchor crystals that remain hygroscopic for a short period of time.

Colloidal bimetallics and polymers are then introduced under controlled conditions of properly balanced suspension, time, and temperature to permanently interlock with the newly formed crystals. This results in a harder-than-steel continuous lubricating plastic/ceramic surface of which the polymeric as well as the colloidal bimetallic particles become an integral part.

How well does it work? It is the permanent-release attribute of the synergistic coating that makes it well-suited to solving McDonnell Douglas' problems in maintaining its aluminum molds for curing advanced composite laminates. Release of the composite from the mold is swift and free of hang-ups, even in tight areas with small radii. According to Project Manager Kevin L Obrachta, who authored a paper on this project's cure tooling, "You could sheet it off with your hands."