In time, even the strongest of metals will rust. From aluminum to steel, no metal is truly resistant to the passage of time and the ravages of the elements. Wood can weaken and be destroyed in any number of ways: it can rot, it can break, it can become too moist and succumb to mold or mildew, it can even be devoured entirely by termites or other insects. Simple plastics can warp or crack or simply melt away when exposed to heat. It’s true, no material can retain its shape and strength forever – but fiber reinforced plastics (FRP) come pretty close.
Fiber reinforced plastics are a composite material consisting of a matrix, usually a thermoset resin, and a reinforcement of fibers. Thermoset polymers used as resins in the manufacture of FRP include polyurethane, polyester, vinyl ester, epoxy, and occasionally phenol formaldehyde. Fibers chosen to serve as reinforcement are typically glass, basalt, carbon and aramids such as Kevlar, Nomex or Technora. In the past, paper, wood, and asbestos fibers were also used, but they have become significantly less common in recent years due to their low durability when compared to glass and carbon.
Like all composite materials, FRP exhibits beneficial properties exceeding those of either of its components. The reinforcing fiber adds strength and elasticity to the tough but weak matrix, creating a tough, long lasting material with the ability to produce sturdy, complex shapes in a variety of sizes.
Fiber reinforced plastics are strong, durable, and resistant to impact, extreme weathers and temperatures, and corrosion. FRP even boasts a strength to weight ratio higher than that of metals such as steel and aluminum, thermoplastics, and even concrete. Today, various FRP products are used in a number of industries including aerospace, automotive, marine vehicle, construction, and the manufacture of ballistic armor.
Despite the fact that it has so rapidly become the material of choice in so many industries, in truth fiber reinforced plastics are just barely over one hundred years old. The first fiber reinforced plastic was Bakelite, invited by Leo Baekeland in 1905. It was made from a combination of phenol formaldehyde resin and mixed asbestos and wood fibers. Within a few years, Bakelite saw wide use in the manufacture of everything from children’s toys and kitchen appliances to jewelry and firearms.
Baekeland’s invention inspired the creation of ever stronger and more efficient fiber reinforced plastics as science and industry progressed. The glass and polyester composite, most commonly referred to as “fiberglass” was developed in the 1930s, and soon saw use in the military aircraft and commercial watercraft industrials during the following decades. The 1950s saw the rise of carbon fiber, and the 1960s that of aramids, including the now world famous Kevlar.
Today, FRP is available in a wide variety of compositions utilizing different resins and fibers and featuring fiber volume anywhere between 20% and 70%. There are also a large number of different processes which can be utilized in the manufacture of FRP parts. At Romeo RIM, our engineers can help you determine which process is best for your manufacturing needs.
The Fiber Reinforced Plastic Molding Process
High-quality fiberglass reinforced plastic parts can be produced in any number of ways. However, the two most common processes utilized for making strong, flexible parts are known as the hand lay-up and the spray-up. Both are low cost, labor- and time-efficient, and create consistent, durable parts for use in any number of industries.
Crafting Fiber Preforms
However, first, the reinforcing fiber must be made into mats or meshes called preforms. Preforms can be created using one of four methods: weaving, knitting, braiding or stitching. These methods produce preforms in a variety of widths and strengths. It is important to consider factors such as the size and shape of the mold and the necessary strength of the finished product when choosing your method of preform manufacture.
In addition, preforms can be either two-dimensional (fibers are aligned only along the x- and y-axes) or three-dimensional (fibers aligned along the x-, y- and z-axes). Recently, three dimensional preforms have been increasing in popularity due to their strength and cost-efficiency. Three-dimensional orientation also decreases the risk of creating weak spots with low fiber content in the finished product.
While traditionally the fiber is placed into the mold and then saturated with resin, sometimes preforms are soaked in some amount of liquid resin, usually epoxy, before the molding and saturation process. These are known as pre-impregnated or pre-preg preforms, and are known for creating finished products with a high stiffness.
The Hand Lay-Up
Once the preforms are complete, they can be placed into the mold in one of two ways. A hand lay-up, as the name indicates, involves the preform being laid in the mold by hand. Once the preform has been placed, it is saturated with resin until the reinforcing fibers are thoroughly wetted. The composite then cures in the mold via the application of heat and some pressure until it obtains solid form.
The hand lay-up method is usually used in conjunction with the process of resin transfer molding (RTM) or vacuum assisted resin transfer molding (VARTM). It is highly recommended for large parts and parts featuring a geometrically complex mold shapes.
In contrast to a hand lay-up, a spray-up process involves both the resin and the reinforcing fiber being inserted into the mold via mechanical processes rather than human labor. Short strands of fiber are sprayed into the mold via a pneumatic gun. The saturating resin can then either be inserted via a separate but similar gun or co-injected along with the fibers from a single spray head.
Spray-up is usually associated with the process of reaction injection molding (RIM), specifically structural reaction injection molding (SRIM). While the spray-up process usually comes with higher equipment cost due to the necessity of using specialized machinery, it simultaneously saves on labor costs as human workers are not needed to place the fiber preforms into the mold. SRIM using a spray-up technique is recommended for parts which require both high overall strength and stiffness. It is also the recommended process if encapsulation is required, especially encapsulation of complex materials such as textiles.
The Mold & Beyond
Both the hand lay-up and spray-up techniques are frequently carried out as an open-mold process. Typically, concave female molds are used, but convex male molds may also be utilized. Molds are usually made from either fiberglass or metals such as aluminum.
Mold release is applied to prevent the finished product from sticking to the mold upon the completion of the curing process. A gel coat may also be added to provide color to the finished part. FRP adheres excellently with the gel coat and may even be used to produce parts which require a variety of colors.
Although resin transfer molding and structural reaction injection molding using the hand lay-up or spray-up processes are the most common methods of molding FRP parts, it is important to note that they are far from the only ones. Alternate molding processes include bladder molding, compression molding, autoclave and vacuum bag, and mandrel wrapping. While these methods are not considered as time- cost- or labor-efficient as RTM or SRIM, they may still be useful in specific circumstances or for specifically shaped parts.
Choosing the Right Reinforcing Fiber
In addition to deciding on the manufacture method of your preform and the ideal molding process for your part, it is also important to consider what type of reinforcing fiber will be used to create your FRP material. Different types of fiber offer different advantages to suit your manufacturing needs.
Glass is the most widely and commonly used material in the manufacture of fiber reinforced plastics. It is most frequently seen in conjunction with thermoset polyester or polyurethane – this combination is so ubiquitous that it bears its own special name, fiberglass.
Glass is extremely easy to work with, allowing for the creation of specific fiber alignment to best suit specific part designs. It results in the highest strength, greatest elasticity and most deformation resistance of all the available fibers. It also features excellent resistance to both extreme heat and extreme cold. Glass-reinforced FRP is commonly used in the manufacture of automobile gas and clutch pedals, insulation for doors and windows, and load-bearing products such as elevator cables.
Both carbon fibers and aramid fibers such as Kevlar significantly boost the elasticity of the completed product. They also simultaneously increase both the compression strength and the electrical strength of the FRP material. Carbon- or aramid-enhanced FRP is lightweight, corrosion resistant, and X-ray transparent, making it an excellent choice for the manufacture of medical equipment. Carbon fiber reinforcement is also a common selection in the aerospace industry – recently, carbon-based FRP has been used in the manufacture of rudders for airplanes such as the Airbus 310.
Basalt fibers are highly resistant to a number of both environmental and inorganic factors. Basalt-enhanced FRP boasts the highest heat and chemical resistances of the available fibers. Its specifically high resistance to salt has led to its wide usage in the manufacture of boats, bridges and piers.
While it is significantly less common due to the advent of glass, carbon and other alternatives, wood fibers do continue to provide some advantages in certain forms of FRP. The use of wood fibers results in a product with a high tensile modulus and strength, although less environmentally resistant than inorganic alternatives.
While parts made using fiber reinforced plastics are long-lasting and highly resistant to wear and tear, and their molding process has been lauded as energy-saving, they have prevented some environmental concerns in regards to disposal, reusability and recyclability.
It is extremely difficult to reduce FRP to its component parts for reuse or recycling – once the resin matrix and the reinforcing fibers have combined in the mold, separating them again is nearly impossible. In addition, once a thermoset polymer has cured into solid form, returning it to a liquid is a similarly impossible task. Because of these properties, it is difficult to reuse or recycle FRP products which have reached the end of their usefulness.
However, merely disposing of FRP products in a landfill is not the perfect solution either. Due to the inorganic nature of many of the resins and fibers used, fiber reinforced plastics decompose slowly – or in some cases, not at all.
The problem of disposing of or reusing FRP materials in an environmentally conscious manner has not yet been completely solved. However, strides have been made via the development of biodegradable or UV-degradable resins called “bioplastics”. Products reinforced with basalt fiber also decompose significantly quicker due to the organic nature of the chosen fibers.
Structural Benefits of Fiber Reinforced Plastics
Despite the existence of some environmental concerns, fiber reinforced plastics are overall a highly advantageous material providing a number of benefits in a wide variety of areas from physical properties to resistances to aesthetics.
Fiber reinforced plastics first gained prominence as a manufacturing material due to their increased strength when compared to non-reinforced polymers, thermoplastics and even metals such as steal and aluminum. FRP products pair that strength with increased elasticity, durability, light weight and flexibility. The strength and elasticity of the finished product can be adjusted, as it depends on the mechanical properties of the chosen fiber as well as its relative volume, length and orientation within the resin matrix.
FRP can be used to create large, complex structures in any number of geometric shapes, including contoured and rounded parts. Material thicknesses of between 1/16 inch and ½ inch can be attained. In addition, molded FRP products boast tight tolerances – approximately 1/100 (0.01) inch on the tool side an 3/100 (0.03) inch on the non tool side. A precise and uniform thickness and tolerance can be obtained by carefully controlling the fiber ratio across all surfaces and areas of the part.
Fiber reinforced plastics also possess an excellent resistance to any number of factors including impact, deformation, extreme heat, extreme cold, mold, mildew, insects, and chemical corrosion. In addition, the majority of FRP products are waterproof, non-porous, non-sparking and feature extremely low thermal conductivity. For this reason, FRP is often used in the manufacture of high-performance parts for demanding industries such as aerospace, construction and waterproof. Its non-magnetic properties and transparency to radio waves and EFI / RFI transmissions has also led FRP to be chosen as the material used for medical machinery such as MRI and CAT scanners.
Aesthetic Benefits of Fiber Reinforced Plastics
The beneficial properties of FRP can be enhanced via the use of particular resins and special coatings. Two common examples are the addition of coatings to make the part flame-retardant or fillers to promote electrical conductivity. However, even without any additions, FRP is a durable material which can last years in extreme weathers and temperatures with minimal wear.
In addition to their physical toughness and long lifespan, FRP materials also allow for the production of aesthetically pleasing products. It experiences almost no shrinkage coming out of the mold, meaning that results will be highly consistent across an entire production run.
FRP can be painted in-mold using a gel coating, which produces a glossy Class A finish directly out of the mold. The material adheres extremely well to the gel coat, removing the risk of paint chipping, cracking, or flaking. In addition, the paint will remain shiny and bright over a long period of time with minimal wear or fading.
Lastly, choosing FRP as your material can have a large number of benefits for you as a manufacturer. It is cost-, time- and labor- efficient, boasting quick cycles, minimal need for skilled workers, and affordable materials. The manufacturing process is easy and highly repeatable even for product runs in the thousands. The standard open mold process, the possibility of in-mold rather than post-mold painting, and of course the long lifespan of the finished product itself provide even further opportunities for saving money.
There are so many reasons why choosing FRP is the best option for your next manufacturing project. From geometrically complex shapes to large, high-performance parts which can resist the wear of the elements, FRP’s benefits are practically uncountable. Contact Romeo RIM today and our engineers will help you select the correct resin, fiber, and molding process to help you produce FRP parts of unequaled quality!