Transfer molding

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Transfer molding (BrE: transfer moulding) is a manufacturing process in which casting material is forced into a mold. Transfer molding is different from compression molding in that the mold is enclosed [1] rather than open to the fill plunger resulting in higher dimensional tolerances and less environmental impact. [2] Compared to injection molding, transfer molding uses higher pressures to uniformly fill the mold cavity. This allows thicker reinforcing fiber matrices to be more completely saturated by resin. [2] Furthermore, unlike injection molding the transfer mold casting material may start the process as a solid. This can reduce equipment costs and time dependency. The transfer process may have a slower fill rate than an equivalent injection molding process. [2]

Contents

Process

Figure 1: Transfer Molding basic process Transfer molding.svg
Figure 1: Transfer Molding basic process

The mold interior surfaces may be gel-coated. If desired, the mold is first pre-loaded with a reinforcing fiber matrix or preform. [2] Fiber content of a transfer molded composite can be as high as 60% by volume. The fill material may be a preheated solid or a liquid. It is loaded into a chamber known as the pot. A ram or plunger forces material from the pot into the heated mold cavity. If feed-stock is initially solid, the forcing pressure and mold temperature melt it. Standard mold features such as sprue channels, a flow gate and ejector pins may be used. The heated mold ensures that the flow remains liquid for complete filling. Once filled the mold can be cooled at a controlled rate for optimal thermoset curing.

Variations

The industry identifies a variety of processes within the transfer molding category. There are areas of overlap and the distinctions between each method may not be clearly defined.

Resin transfer molding

Figure 2: Resin Transfer Molding Cope Drag Clamp Mixing chamber Fiber preform Heated mold Resin Curative RTM process.png
Figure 2: Resin Transfer Molding
  1. Cope
  2. Drag
  3. Clamp
  4. Mixing chamber
  5. Fiber preform
  6. Heated mold
  7. Resin
  8. Curative

Resin transfer molding (RTM) uses a liquid thermoset resin to saturate a fiber preform placed in a closed mold. The process is versatile and can fabricate products with embedded objects such as foam cores or other components in addition to the fiber preform. [3]

Vacuum assisted resin transfer molding

Vacuum assisted transfer molding (VARTM) uses a partial vacuum on one side of a fiber mat to pull the resin in for complete saturation. VARTM uses lower plunger forces which allows molding to be carried out with cheaper equipment. The use of a vacuum may allow the resin to adequately flow and or cure without heating. [4] This temperature independence allows thicker fiber preforms and larger product geometries to be economical. VARTM can produce parts with less porosity than regular transfer molding with a proportional increase in casting strength. [1]

Micro transfer molding

Also called transfer micromolding, micro transfer molding is a process that uses a mold to form then transfer structures as small as 30 nm onto thin films and microcircuitry. [5] Unlike normal scale transfer molding, the micro form can and is used with metals as well as non metals. [6]

Defects

Limiting defects is key when commercially producing any sort of material. Transfer molding is no exception. For example, voids in a transfer molded parts significantly reduce strength and modulus. [7] There can also be defects when fibers are used around sharp corners. The resin flow can create resin rich zones on the outside of these corners. [8]

Pressure distribution

There are several contributing factors to voids in the final product of transfer molding. One is a non uniform pressure distribution among the material being pressed into the mold. In this case the material folds in on itself and generates voids. Another is voids in the resin being forced into the mold beforehand. This may be obvious, but it is a main contributor. Things to be done to limit these molds include pressing the resin in at a high pressure, keeping the fiber distribution uniform, and using a high quality properly degassed base resin.

Sharp corners

Figure 3: Sharp corner generates voids in transfer molding Defect picture.png
Figure 3: Sharp corner generates voids in transfer molding

Sharp corners are the problems with all mold based manufacturing, including casting. Specifically in transfer molding corners can break fibers that have been placed in the mold and can create voids on the inside of corners. This effect is demonstrated in Figure 3 on the right. The limiting factor in these designs is the inner corner radius. [8] This inner radius limit varies depending on resin and fiber selection, but a rule of thumbs is the radius though be 3 to 5 times the laminate thickness. [8]

Materials

The material most commonly used for transfer molding is a thermoset polymer. This type of polymer is easy to mold and manipulate, but upon curing, hardens into a permanent form. [9] For simple homogeneous transfer molded parts, the part is simply made of this plastic substrate. On the other hand, resin transfer molding allows for a composite material to be made by placing a fiber within the mold and subsequently injecting the thermosetting polymer. [10]

Defects known as voids and dry resin (in the case of resin transfer molding) are possible in transfer molding and often are exacerbated by high viscosity materials. This is because a high viscosity plastic flowing through a thin mold may miss entire vacated areas, leaving air pockets. When air pockets are left in the presence of fiber, this creates a “dry” area, which prevents load from being transferred through the fibers in the dry area.

Materials used for the plastic are often polyurethanes or epoxy resins. Both of these are soft and malleable before curing, becoming much harder after setting. Materials used for fibers vary extensively, although common choices are carbon or Kevlar fibers, as well as organic fibers, such as hemp. [11]

Related Research Articles

Composite material Material made from a combination of two or more unlike substances

A composite material is a material which is produced from two or more constituent materials. These constituent materials have notably dissimilar chemical or physical properties and are merged to create a material with properties unlike the individual elements. Within the finished structure, the individual elements remain separate and distinct, distinguishing composites from mixtures and solid solutions.

Fiberglass, or fibreglass, is a common type of fiber-reinforced plastic using glass fiber. The fibers may be randomly arranged, flattened into a sheet, or woven into glass cloth. The plastic matrix may be a thermoset polymer matrix—most often based on thermosetting polymers such as epoxy, polyester resin, or vinyl ester resin—or a thermoplastic.

Thermosetting polymer Polymer obtained by irreversibly hardening (curing) a resin

In materials science, a thermosetting polymer, often called a thermoset, is a polymer that is obtained by irreversibly hardening ("curing") a soft solid or viscous liquid prepolymer (resin). Curing is induced by heat or suitable radiation and may be promoted by high pressure, or mixing with a catalyst. Heat is not necessarily applied externally, but is often generated by the reaction of the resin with a curing agent. Curing results in chemical reactions that create extensive cross-linking between polymer chains to produce an infusible and insoluble polymer network.

Injection moulding Manufacturing process for producing parts by injecting molten material into a mould, or mold

Injection moulding is a manufacturing process for producing parts by injecting molten material into a mould, or mold. Injection moulding can be performed with a host of materials mainly including metals, glasses, elastomers, confections, and most commonly thermoplastic and thermosetting polymers. Material for the part is fed into a heated barrel, mixed, and injected into a mould cavity, where it cools and hardens to the configuration of the cavity. After a product is designed, usually by an industrial designer or an engineer, moulds are made by a mould-maker from metal, usually either steel or aluminium, and precision-machined to form the features of the desired part. Injection moulding is widely used for manufacturing a variety of parts, from the smallest components to entire body panels of cars. Advances in 3D printing technology, using photopolymers that do not melt during the injection moulding of some lower-temperature thermoplastics, can be used for some simple injection moulds.

Fibre-reinforced plastic is a composite material made of a polymer matrix reinforced with fibres. The fibres are usually glass, carbon, aramid, or basalt. Rarely, other fibres such as paper, wood, or asbestos have been used. The polymer is usually an epoxy, vinyl ester, or polyester thermosetting plastic, though phenol formaldehyde resins are still in use.

Pre-preg is a composite material made from "pre-impregnated" fibers and a partially cured polymer matrix, such as epoxy or phenolic resin, or even thermoplastic mixed with liquid rubbers or resins. The fibers often take the form of a weave and the matrix is used to bond them together and to other components during manufacture. The thermoset matrix is only partially cured to allow easy handling; this B-Stage material requires cold storage to prevent complete curing. B-Stage pre-preg is always stored in cooled areas since heat accelerates complete polymerization. Hence, composite structures built of pre-pregs will mostly require an oven or autoclave to cure. The main idea behind a pre-preg material is the use of anisotropic mechanical properties along the fibers, while the polymer matrix provides filling properties, keeping the fibers in a single system.

Compression molding Method of molding

Compression molding is a method of molding in which the molding material, generally preheated, is first placed in an open, heated mold cavity. The mold is closed with a top force or plug member, pressure is applied to force the material into contact with all mold areas, while heat and pressure are maintained until the molding material has cured; this process is known as compression molding method and in case of rubber it is also known as 'Vulcanisation'. The process employs thermosetting resins in a partially cured stage, either in the form of granules, putty-like masses, or preforms.

Blow molding Manufacturing process for forming and joining together hollow plastic parts

Blow molding is a manufacturing process for forming and joining together hollow plastic parts. It is also used for forming glass bottles or other hollow shapes.

Polyester resins are synthetic resins formed by the reaction of dibasic organic acids and polyhydric alcohols. Maleic Anhydride is a commonly used raw material with diacid functionality in unsaturated polyester resins. Unsaturated polyester resins are used in sheet moulding compound, bulk moulding compound and the toner of laser printers. Wall panels fabricated from polyester resins reinforced with fiberglass—so-called fiberglass reinforced plastic (FRP)—are typically used in restaurants, kitchens, restrooms and other areas that require washable low-maintenance walls. They are also used extensively in cured-in-place pipe applications. Departments of Transportation in the USA also specify them for use as overlays on roads and bridges. In this application they are known as PCO Polyester Concrete Overlays. These are usually based on isophthalic acid and cut with styrene at high levels—usually up to 50%. Polyesters are also used in anchor bolt adhesives though epoxy based materials are also used. Many companies have and continue to introduce styrene free systems mainly due to odor issues, but also over concerns that styrene is a potential carcinogen. Most polyester resins are viscous, pale coloured liquids consisting of a solution of a polyester in a reactive diluent which is usually styrene, but can also include vinyl toluene and various acrylates.

Bulk moulding compound (BMC), bulk moulding composite, or dough moulding compound (DMC), is a ready-to-mold, glass-fiber reinforced thermoset polymer material primarily used in compression moulding, as well as in injection moulding and transfer moulding. Typical applications include demanding electrical applications, corrosion resistant needs, appliance, automotive, and transit.

Sheet moulding compound (SMC) or sheet moulding composite is a ready to mould glass-fibre reinforced polyester material primarily used in compression moulding. The sheet is provided in rolls weighing up to 1000 kg. Alternatively the resin and related materials may be mixed on site when a producer wants greater control over the chemistry and filler.

A thermoset polymer matrix is a synthetic polymer reinforcement where polymers act as binder or matrix to secure in place incorporated particulates, fibres or other reinforcements. They were first developed for structural applications, such as glass-reinforced plastic radar domes on aircraft and graphite-epoxy payload bay doors on the space shuttle.

Resin casting is a method of plastic casting where a mold is filled with a liquid synthetic resin, which then hardens. It is primarily used for small-scale production like industrial prototypes and dentistry. It can be done by amateur hobbyists with little initial investment, and is used in the production of collectible toys, models and figures, as well as small-scale jewellery production.

Three-dimensional composites use fiber preforms constructed from yarns or tows arranged into complex three-dimensional structures. These can be created from a 3D weaving process, a 3D knitting process, a 3D braiding process, or a 3D lay of short fibers. A resin is applied to the 3D preform to create the composite material. Three-dimensional composites are used in highly engineered and highly technical applications in order to achieve complex mechanical properties. Three-dimensional composites are engineered to react to stresses and strains in ways that are not possible with traditional composite materials composed of single direction tows, or 2D woven composites, sandwich composites or stacked laminate materials.

Out of autoclave composite manufacturing is an alternative to the traditional high pressure autoclave (industrial) curing process commonly used by the aerospace manufacturers for manufacturing composite material. Out of autoclave (OOA) is a process that achieves the same quality as an autoclave but through a different process. OOA curing achieves the desired fiber content and elimination of voids by placing the layup within a closed mold and applying vacuum, pressure, and heat by means other than an autoclave. An RTM press is the typical method of applying heat and pressure to the closed mold. There are several out of autoclave technologies in current use including resin transfer molding (RTM), Same Qualified Resin Transfer Molding (SQRTM), vacuum-assisted resin transfer molding (VARTM), and balanced pressure fluid molding. The most advanced of these processes can produce high-tech net shape aircraft components.

Tailored fiber placement

Tailored fiber placement (TFP) is a textile manufacturing technique based on the principle of sewing for a continuous placement of fibrous material for composite components. The fibrous material is fixed with an upper and lower stitching thread on a base material. Compared to other textile manufacturing processes fiber material can be placed near net-shape in curvilinear patterns upon a base material in order to create stress adapted composite parts.

Vacuum Assisted Resin Transfer Molding (VARTM) or Vacuum Injected Molding (VIM) is a closed mold, out of autoclave (OOA) composite manufacturing process. VARTM is a variation of Resin Transfer Molding (RTM) with its distinguishing characteristic being the replacement of the top portion of a mold tool with a vacuum bag and the use of a vacuum to assist in resin flow. The process involves the use of a vacuum to facilitate resin flow into a fiber layup contained within a mold tool covered by a vacuum bag. After the impregnation occurs the composite part is allowed to cure at room temperature with an optional post cure sometimes carried out.

A void is a pore that remains unfilled with polymer and fibers in a composite material. Voids are typically the result of poor manufacturing of the material and are generally deemed undesirable. Voids can affect the mechanical properties and lifespan of the composite. They degrade mainly the matrix-dominated properties such as interlaminar shear strength, longitudinal compressive strength, and transverse tensile strength. Voids can act as crack initiation sites as well as allow moisture to penetrate the composite and contribute to the anisotropy of the composite. For aerospace applications, a void content of approximately 1% is still acceptable, while for less sensitive applications, the allowance limit is 3-5%. Although a small increase in void content may not seem to cause significant issues, a 1-3% increase in void content of carbon fiber reinforced composite can reduce the mechanical properties by up to 20% Void content in composites is represented as a ratio, also called void ratio, where the volume of voids, solid material, and bulk volume are taken into account. Void ratio can be calculated by the formula below where e is the void ratio of the composite, Vv is the volume of the voids, and Vt is the volume of the bulk material.

Thermoplastics containing short fiber reinforcements were first introduced commercially in the 1960s. The most common type of fibers used in short fiber thermoplastics are glass fiber and carbon fiber . Adding short fibers to thermoplastic resins improves the composite performance for lightweight applications. In addition, short fiber thermoplastic composites are easier and cheaper to produce than continuous fiber reinforced composites. This compromise between cost and performance allows short fiber reinforced thermoplastics to be used in myriad applications.

Light resin transfer moulding is a process by which products of Composite materials are manufactured using a closed mold system.

References

  1. 1 2 Hayward, J. S.; Harris, B. (September 1, 1990). "The effect of vacuum assistance in resin transfer moulding". Composites Manufacturing. 1 (3): 161–166. doi:10.1016/0956-7143(90)90163-Q.
  2. 1 2 3 4 Ornaghi, Heitor Luiz; Bolner, Alexandre Sonaglio; Fiorio, Rudinei; Zattera, Ademir Jose; Amico, Sandro Campos (October 15, 2010). "Mechanical and dynamic mechanical analysis of hybrid composites molded by resin transfer molding". Journal of Applied Polymer Science. 118 (2): 887–896. doi:10.1002/app.32388. ISSN   1097-4628.
  3. ↑ Kendall, K. N.; Rudd, C. D.; Owen, M. J.; Middleton, V. (January 1, 1992). "Characterization of the resin transfer moulding process". Composites Manufacturing. 3 (4): 235–249. doi:10.1016/0956-7143(92)90111-7.
  4. ↑ Heider, Dirk; Graf, A.; Fink, Bruce K.; Gillespie, Jr., John W. (January 1, 1999). "Feedback control of the vacuum-assisted resin transfer molding (VARTM) process". Process Control and Sensors for Manufacturing II. 3589: 133–141. doi:10.1117/12.339956.{{cite journal}}: Cite journal requires |journal= (help)
  5. ↑ Cavallini, Massimiliano; Murgia, Mauro; Biscarini, Fabio (January 2, 2002). "Direct patterning of tris-(8-hydroxyquinoline)-aluminum (III) thin film at submicron scale by modified micro-transfer molding". Materials Science and Engineering: C. Current Trends in Nanotechnologies: From Materials to Systems, Proceedings of Symposium S, EMRS Spring Meeting 2001, Strasbourg France. 19 (1–2): 275–278. doi:10.1016/S0928-4931(01)00398-8.
  6. ↑ Choi, Seong-O; Rajaraman, Swaminathan; Yoon, Yong-Kyu; Wu, Xiaosong; Allen, Mark G. (2006). "3-D patterned microstructures using inclined UV exposure and metal transfer micromolding" (PDF). Proc. Solid State Sensors, Actuators and Microsystems Workshop (Hilton Head, SC). Retrieved March 8, 2016.
  7. ↑ Kang, Moon Koo; Lee, Woo Il; Hahn, H. Thomas (September 1, 2000). "Formation of microvoids during resin-transfer molding process". Composites Science and Technology. 60 (12–13): 2427–2434. doi:10.1016/S0266-3538(00)00036-1.
  8. 1 2 3 Holmberg, J. A.; Berglund, L. A. (January 1, 1997). "Manufacturing and performance of RTM U-beams". Composites Part A: Applied Science and Manufacturing. 28 (6): 513–521. doi:10.1016/S1359-835X(97)00001-8.
  9. ↑ Pascault, Jean-Pierre; Sautereau, Henry; Verdu, Jacques; Williams, Roberto J. J. (February 20, 2002). Thermosetting Polymers. CRC Press. ISBN   9780824744052.
  10. ↑ III, William H. Seemann (February 20, 1990), Plastic transfer molding techniques for the production of fiber reinforced plastic structures , retrieved March 8, 2016
  11. ↑ Rouison, David; Sain, Mohini; Couturier, M. (June 1, 2006). "Resin transfer molding of hemp fiber composites: optimization of the process and mechanical properties of the materials". Composites Science and Technology. 66 (7–8): 895–906. doi:10.1016/j.compscitech.2005.07.040.
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