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Understanding Plastic Fabrication Methods

November 13, 2017

Plastic fabrication is defined as any process which is used to design, manufacture, or assemble products made from plastic or composites containing plastic. There are an extremely wide variety of plastic fabrication methods available, each with their own unique advantages and disadvantages. Additionally, certain methods are better suited for certain parts – fabricating small plastic kitchen containers will likely not utilize the same method as fabricating large roofs or body panels for heavy trucks. Whatever you wish to make from plastic, there is a fabrication method out there for you!

Plastic fabrication methods can be widely divided into a number of categories depending on the techniques used and the distinctive characteristics of the resulting products. Some of the most common categories include:

Compounding, which is also referred to as blending, refers to any process in which two or more plastics are combined into one. Lamination is any method in which a plastic is used to form a barrier along the surface of another material. It is frequently utilized to strengthen a product or increase its resistance to heat, electricity, chemicals or other natural and artificial risks. Plastic welding, similar to the more common metal welding, uses heat to melt pieces together. While welding is still a process primarily associated with metal, it can be an effective process when working with plastics that react poorly to other forms of adhesion.

Lastly, molding refers to any method which forms plastic into a shape by allowing it to harden within a pre-formed mold. Molding is the most versatile of the plastic fabrication varieties, allowing for the creation of everything from extremely large parts to extremely geometrically complex ones. Molding is used to produce everything from household appliances to heavy machinery, toys to airplane parts. It has become one of the most popular ways of fabricating plastic due to the time- and cost-efficiency of the process and the strength, durability and stiffness of the completed parts.

There are a variety of sub-types of molding, which are traditionally separated according to:

  • Whether heat, pressure or a combination of both (such as an autoclave, a machine which applies both heat and pressure simultaneously is used to cure the finished product
  • The method by which the plastic enters the mold
  • The presence or absence of reinforcing fibers such as glass, carbon or aramid (Kevlar)

The following guide briefly describes several of the most commonly used plastic molding processes, their benefits and some of their applications. It also contains links to a number of more in-depth articles discussing the processes available here at Romeo RIM.

Hand Lay-Up

The hand lay-up is one of the most basic variants on the plastic molding process. It is cost-efficient due being open-mold (using a single concave female or convex male mold) and requiring minimal tools, but rather labor-intensive as human workers are required to assist in filling the mold.

As the name suggests, hand lay-ups involve sheets of material (usually a mesh, mat or roving of fiber such as glass or carbon) called plies, being placed into the mold one by one by hand, forming a laminate stack. Then, the plies are saturated with liquid resin, usually a polymer, and cured via the application of pressure. Human workers are usually also used to apply pressure via tools called hand rollers.

Usually, dry plies are used, but it is also possible to utilize pre-impregnated or pre-preg plies, which already contain a small amount of resin. A variation, the wet lay-up, involves applying resin after each ply is laid into the mold, rather than all at once at the end of the process. This process is used to ensure the equal distribution of resin across all surfaces of the product.

The hand lay-up technique is used to produce parts that are smooth, strong (due to the presence of reinforcing fibers) and possess simple geometries. An example would be the sides or bottom of a plastic swimming pool.

For more information about the use of the hand lay-up in the manufacture of fiber reinforced plastic (FRP), see our article here.

Spray-Up

The spray-up is in many ways a mechanical, automated version of the more labor-intense hand lay-up. It is also open mold and features reinforcing fibers which are saturated by a liquid resin which is usually a polymer. However, in a spray-up, chopped fibers are sprayed into the mold via a pneumatic gun. Sometimes, hand rollers are used to provide additional compression.

The hand lay-up and mechanical spray-up processes can be used in tandem on a single mold to further increase the strength of the finished product. First, a mesh or fabric is laid into the mold; additional chopped fibers (usually glass) are then added via the spray gun.

It is important to note that the spray-up process is occasionally associated with health risks caused by the emission of volatile organic compounds (VOCs) such as styrene into the surrounding air. Workers assisting with the spray-up process should be careful to take proper safety precautions at all times.

Although it is more time- and labor-efficient than the hand lay-up, the two processes are used to manufacture rather similar parts: smooth, strong and often not geometrically complex. Spray-up molding, for example, can produce hulls for personal watercraft or various components for cars and trucks.

For more information about the use of the spray-up in the manufacture of fiber reinforced plastic (FRP), see our article here.

Resin Transfer Molding

Like the hand lay-up and the spray-up, the resin transfer molding process is used to create strong parts which have been reinforced via the addition of glass, carbon or aramid fibers. It differs from the previous two methods in that it is a closed mold process, utilizing two halves of a mold which are closed so that pressure can be exerted mechanically, rather than by hand rollers, during the curing period.

Like with a hand lay-up, resin transfer molding begins with meshes, mats or preforms of fiber inserted into the mold. Once the mold has been closed and secured, liquid resin (usually a polymer such as polyurethane) can be either injected into the mold using pressure or pulled in via a vacuum. The second process is specifically known as vacuum-assisted resin transfer molding.

Resin transfer molding produces large, strong, lightweight parts in more complex shapes than can be achieved via hand lay-up or spray-up. It is commonly used in the manufacture of wind turbine blades, bathtubs, and complex automobile parts.

For more information on resin transfer molding, see our article here.

Resin Film Infusion

During resin film infusion, layers of dry fibers are alternated with layers of epoxy resin in a semi-solid form. A vacuum bag is then used to remove all trapped air from between the layers of material. Heat is applied to melt and cure the resin, saturating the layered fibers as it does so.

Resin film infusion produces parts with a higher fiber volume than is possible with other processes. It primarily sees use in the aerospace industry.

Compression Molding

Compression molding is one of the oldest and most traditional of the various plastic molding processes. While it has fallen out of favor due to the expensive equipment and necessity of skilled human labor, it still sees use in products with extremely large production runs, such as automobile fenders or bumpers or clothing fasteners like buckles or buttons.

It does provide the benefit of using a highly affordable material: sheet molding compound (SMC). SMC is a composite which consists of fibers of approximately 1” in length, usually glass, suspended in a bath of resin. SMC is loaded into a hydraulic press where extreme heat (often up to 700 degrees Fahrenheit / 371 degrees Celsius) and pressure are applied in order to cure the part.

For more information on compression molding, including a comparison with injection molding, see our article here.

Filament Winding

Filament winding is a highly automated, highly repeatable process which is usually used to create hollow, cylindrical objects such as pipes or storage tanks. Long fibers are pulled through a bath of polyester, vinyl ester or epoxy resin before being wound around a rotating cylindrical tool called a mandrel. The movement of the mandrel forms the resin-saturated fiber into the desired shape.

Filament winding is a fairly expensive process, due to the very specific tools required. However, it is possible to use pre-impregnated or pre-preg fibers in order to save on the cost of the resin bath.

Recently, automated fiber placement machines have been used in some industries to place materials onto mandrels during the process; however, these machines remain fairly expensive.

Centrifugal Casting

An alternative to filament winding, centrifugal casting features woven mats placed around the edges of a rotating mold. Resin is then injected to the mold; the rotational forces cause the resin to saturate the reinforcement as the part cures. Like filament winding, centrifugal casting is used to produce cylindrical items such as pipes and storage tanks.

Pultrusion

The word “pultrusion” combines pull and extrusion and refers to the forces exerted on fibers in this continuous, mechanical manufacturing process. Woven or braided fibers are first pulled through a resin bath before being further pulled between two heated metal dies in order to cure.

While an expensive process due to tool requirements, pultrusion is notable for the high smoothness and strength of the parts it produces. Depending on the composition of the resin bath, products can also be made resistant to flame, heat, electricity, chemicals, or environmental factors. As a result, it is often used in the construction of furniture and machinery for chemical plants or agricultural facilities.

Extrusion

Extrusion is another continuous process which is considered the opposite of pultrusion due to the use of pushing forces rather than pulling. Resin is pulled into a heated barrel, allowing it to melt, before curing in a shaped metal die. Extrusion is more commonly used with metals, especially aluminum, for the manufacture of support beams. However, recently, extrusion using plastic as a material has become possible. Like filament winding, it is often used to create pipes.

Injection Molding

By and large, the manufacturing method which boasts the greatest versatility is injection molding, along with its various sub-types (discussed below). Injection molding features extremely quick cycle times and, frequently, less expensive machinery than similar processes.

Standard injection molding involves the injection (usually via machinery) of liquid thermoplastic resin into a closed mold. Many kinds of resin can be used during this process, though some of the most common include polyurethane, polyester, vinyl ester and epoxy.

While high heat and pressure must be applied in order to successfully cure the part, the quick cycle time (often as low as one minute) is often considered to make up for these drawbacks. Injection molding is used in a number of industries from automotive and heavy machinery to the manufacture of personal pools and spas.

Reaction Injection Molding

A sub-category of injection molding, reaction injection molding (RIM) relies on the use of chemical reactions to decrease the heat and pressure required to cure the finished part. Because of this, it is considered an improvement in time-, cost- and labor-saving when compared with standard injection molding.

Reaction injection molding utilizes thermoset polymers rather than thermoplastic, with the most common being polyurethane. The two components (for example, polyol and isocyanate) are stored in separate containers before being pumped into a mix-head. There, some pressure is applied as they are injected into the mold. In the mold, a chemical reaction and low application of heat cause the polymer to cure into a solid.

This more advanced process can be used to produce any of the parts commonly made via injection molding – but even stronger, stiffer and more lightweight. Parts can also be painted in-mold for high class, glossy finishes in a veritable rainbow of colors. Geometrically complex parts are also a good choice for reaction injection molding due to the low risk of damage in-mold or during the removal process.

For more information on reaction injection molding, see our article here.

Reinforced Reaction Injection Molding

Reinforced reaction injection molding (RRIM) is a variety of RIM which adds milled or chopped glass or carbon fibers to the thermoset polymer in order to strengthen and stiffen the finished part. It provides all the benefits of standard RIM, including in-mold painting, the possibility for complex geometries, and aesthetically pleasing Class A finishes right out of the mold.

For more information on reinforced reaction injection molding, see our article here.

Structural Reaction Injection Molding

Structural reaction injection molding (SRIM) is the second common variety of RIM. Like RRIM, it also adds glass or carbon fibers to strengthen, stiffen and reinforce the thermoset polymer. However, instead of chopped fibers, fiber mats, meshes or preform are used. SRIM has sometimes been considered a more advanced version of the hand lay-up, spray up or resin transfer molding processes.

SRIM produces the stiffest products when compared to RIM and RRIM, and is therefore frequently used for parts which require high impact resistance, such as the bumpers of automobiles.

For more information on structural reaction injection molding, see our article here.

Long Fiber Injection Molding

Romeo RIM’s signature technology, long fiber injection (LFI) takes the advanced technologies present in RIM, RRIM and SRIM to the next level. The traditional two step molding process is reduced to one as a robot sprays long glass fibers and resin into the mold simultaneously. The cure time is a few minutes, allowing for rapid, efficient production of high quality parts.

The length of the fibers (anywhere from ½ inch to 4 inches) results in a product which is even stronger, more durable and more lightweight than anything possible using any of the numerous other plastic molding techniques. In addition, the reduction of two steps to one decreases the time required, while the automation of the process decreases both labor and costs.

LFI also possesses all of the aesthetic advantages associated with RIM and its variants. In-mold painting is possible, creating high quality high or low gloss Class A finishes right out of the mold. LFI-molded products are also capable of mimicking a number of textures, including detailed ones such as wood and stone.

For more information on long fiber injection molding, see our article here.

The amount of plastic fabrication methods available today is truly astounding. Each process has its own advantages and disadvantages, and products parts with unique strengths and benefits. Contact Romeo RIM today and our engineers will help you figure out just which plastic fabrication method is right for your next manufacturing project!

 

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