Manufacturing techniques

In order to meet the market need, researchers and industries are developing a great number of manufacturing techniques. The costs of production, as a considerable part of total costs, usually determine which will be followed for a product. Different processes usually take either low capital investment with high labor, or high capital investment with low labor. The most common techniques are:

 

Most widely used, it is a simple but effective process which takes relatively low capital investment but high labor cost. “Open” molds are tools that reproduce (or duplicate) only one side of a product, or a component. For the other side another mold has to be used, and another component has to be fabricated. The two components are glued back to back and the outcome is a product with two finished “faces” (and a seem between them). Lay-up is performed in the following steps: Pigmented gel coat is first applied by brush or spray. After gel coating, a thin coat of resin (usually polyester) and a thin layer of reinforcement are placed on, and worked by hand with brushes and rollers, so the resin fully impregnates the fabric. Other layers (usually chopped strand mat) follow, until the desired thickness and strength are achieved. After cure, the component is pulled out of the mold (or released) and trimmed. Post-curing at elevated temperatures in or out of the mold may also take place. The mold is cleaned, re-released (if no multiple release agent is used) and returned to use. Quality is relatively poor, mainly because high resin/reinforcement ratio is incorporated in the finished product (higher resin/reinforcement ratio implies lower strength/weigh ratio).

 

The previous process can be greatly improved by vacuum bagging, with a small increase in capital investment. The gel coat and impregnating procedure is the same, but before cure, the component is sealed on the mold under a vacuum bag. The air is drawn and the component is compressed by the atmospheric pressure against the mold surface by the vacuum bag (serving as the “upper tool”.) This pressurization drives the excess resin and most of the entrapped air out of the component. The improvement in strength/weight ratio is so great, that the product can be classified even as “aerospace quality”. The process is particularly useful for small production runs and prototyping.

 

Similar to vacuum bagging, with the difference that reinforcement is laid on the mold dry. The mold and the reinforcement are sealed and vacuum is drawn form one side. Once air-tightness is assured resin and hardener are mixed and introduced in the dry reinforcement by the sucking power of vacuum. A special “flow fabric” and network of “spiral tube” facilitate the procedure and make sure that resin travels fast everywhere in the mold cavity, and fully impregnates all the layers or dry reinforcement. The outcome is aerospace quality, repetitive, featuring very low resin content and high strength.

 

This relatively low capital investment process is developed for high volume production. Chopped fiber reinforcement together with resin (usually polyester) in the form of a spray are deposited simultaneously on to the released and gelcoated open-mould surface. The resulting outcome closely resembles that of chopped strand mat. Quality is poor, mainly because the component incorporates a high resin ratio, but it is a very economic way of manufacturing low priced parts.

 

Some times called injection molding, this capital intensive process employs a coupling (male and a female) metal mold that is heated. The reinforcement is cut with precision and placed in the mold cavity. Usually instead of laying the pieces of reinforcement fabric one by one, a preform is used (many different layers of reinforcement are pre-cut and held together in particular pattern, according to the shape of the mold, with the help of a “binder”. This way loading the reinforcement in the mold can be done with one move.) After loading the reinforcement the two matching molds are closed tightly and catalyzed resin is pressed inside through the carefully positioned openings or injection “gates”. The air is expelled through other carefully positioned openings, the “vents”, and the reinforcement is saturated. The whole process can be assisted by vacuum (Vacuum Assisted RTM.) When full cure is reached, the component is ejected from the mold cavity. RTM is used for high production volumes. Quality is very good and highly repetitive.

 

This process is a conjunction of RTM and resin infusion. Like RTM we have two matching molds, but here they are made out of composites. Usually upper one is thinner and more flexible than the lower. The two molds are closed (or sealed) air tightly by vacuum pressure alone or vacuum and other mechanical clamping. Catalyzed resin is pressed inside the mold. Injection pressure is much lower that in RTM, where metal molds are used. Heat may be applied, but usually, again, much lower than in RTM. However, much bigger components can be manufactured with RTM light, mainly because composite molds can be constructed much bigger. Also, it is less capital intensive that RTM.

 

Like RTM, two matching metal molds are heated. Instead of dry reinforcement, prepregs or pre-impregnated preforms are used. Prepregs are fabrics that are pre-impregnated with resin (sometimes resin and fillers) and treated with temperature in such a way that are partially cured (the so called “B stage” of cure.) When reheated in hot press molding, the resin becomes liquid again, and finally cures.

 

The same as hot press, with the difference that reinforcement is placed in the mold dry and impregnated by hand, with brush and rollers. The mold closes with lower pressure. After partial cure, pressure and temperature can be raised. Raising pressure or temperature too early will cause too much resin to escape from the cavity, leaving the reinforcement too dry or “resin starving”. For lower production runs, it is a low capital intensive method, as in this process the mold can be made by composites.

 

Is a capital-intensive process used mainly to manufacture small and large diameter tubing and pressure tanks in medium to high production volume. As the name implies, it involves the winding of continuous, pre-saturated reinforcing filaments around a rotating mandrel, until the whole surface is covered at the desired depth. The filaments are saturated as they pass through a resin bath just before they meet the mandrel. The winding, depending on the complexity of the machine, can be performed in two or more angles. Also, towpreg or pre-impregnated, B stage filament can be used (prepreg winding). After cure, at the final stage of production the mandrel has to be removed, usually with the help of a hydraulic extractor. New, computer controlled machines and the use of new, innovative mandrels (like inflatable or sectional) and mandrel shapes have made possible the manufacture of surprisingly more complex components. Quality of the finished products is usually very good, as the filament reinforcement is continuous and price relatively low. However, most of the wound products are somewhat resin rich, and lack longtitudinal reinforcement (mandrel rotation prohibits reinforcement to be placed along the longitudinal axis of the component).

 

Is a sophisticated, continuous, high capital and material intensive process for the manufacture of composite profile. Unlike filament winding which mounts reinforcement in the transverse (or circumferential) direction of the mandrel, pultrusion places the primary reinforcement in the longitudinal direction. As a principle, it is similar to the production of aluminum profile (although aluminum extrusion is a pushing action.) It is performed by pulling continuous filaments together with chopped strand mat tapes through a resin bath to a heated metal die cavity of the desired cross section and shape. This die serves as the mold and the curing oven at the same time. The higher the temperature of the die, the larger the speed of pulling. It is also possible, instead of using a resin bath for saturation, to inject the resin directly into the die cavity. As with aluminum, the profiles produced can be very complex. As the profile is pulled out already cured, a saw at the end of the production line cuts it to the desired length. Of course, length of the product is unlimited. Quality of pultruted parts is very good and price very low. They are used mainly in the construction industry. Their main disadvantage is the lack of transverse reinforcement as the pulling mechanism makes its placement very difficult.

 

In this capital intensive process, metal or composite mold is treated with multiple release agent (this substance requires a particular baking cycle before it can be put to use,) and loaded with layers of prepreg. The whole mold is sealed in a vacuum bag. The air is drawn out of the bag. The vacuum pressed mold and bag are placed in an airtight heating chamber (or an autoclave). The door of the autoclave closes and heat is applied (usually 120 -180 oC). As a result of heating the resin starts to liquefy. This is when pressure is introduced (usually 3 – 8 atm.) This way, the air and excess resin are expelled out of the component. After a few minutes in these conditions the component is cured. Air is released from the machine and the cooling cycle starts. In a fraction of an hour the machine cools and the door can open. Autoclave unmatched quality manufacturing is an exotic production method that increases the spectrum of composite processing. Autoclave products are used in industries like aerospace, military and F1 and everywhere quality and performance are very important.