Process description and process selection information for Fiber Reinforced Plastics

Introduction
Fiber-reinforced plastics (FRP Composite) can be fabricated using several processes – hand layup and spray-up lamination, continuous lamination, spin casting, resin transfer molding and its variations, injection molding, etc. However, let’s now focus not on individual processes, but on 3 important categories to which these processes belong
• Open molding
• Low volume closed molding
• Compression molding

Process Descriptions
A. Open Molding – Open molding is the simplest and most widely used process to produce FRP parts. It is done in ambient shop conditions. The mold is generally fabricated from FRP. The cosmetic surface of the part is fabricated next to the mold. The back of the mold is open. While most fabrication processes involve the application of the exterior coating after the main structure of the part has been built, open mold parts are built from the exterior to the interior. The first step in open molding is to apply the gel coat (the exterior coating of the part) to the mold. The remaining layers of the laminate design, which will include some but not all of the following, back the gel coat :
1. Barrier Coat – this is applied behind the gel coat. A barrier coat improves part cosmetics, reduces cracking, and improves osmotic blister resistance in marine parts.
2. Skin Laminate – a relatively thin glass fiber reinforced laminate fabricated behind the gel coat. Skin laminates improve cosmetics and osmotic blister resistance.
3. Print Blocker – a sprayable syntactic foam material used behind a skin coat to improve laminate cosmetics.
4. Coring Materials – lightweight materials used to build part thickness and stiffness without adding weight.
5. Bulk Laminate – the main portion of the laminate that provides most of the structural properties.

Glass fiber reinforcement used in skin and bulk laminates can be applied by hand layup or spray-up techniques. Emissions from open mold processes are significant and are regulated by Federal NESHAP standards and, in some cases, State and Local regulations.

B. Low volume closed Molding –
The category of low-volume closed molding processes includes processes in which liquid resin is transferred into a closed cavity mold containing reinforcing materials. Over time, many variations of low-volume closed molding processes have evolved such as
• Vacuum infusion
• Seamann Composites Resin Infusion Manufacturing Process (SCRIMP®)
• Conventional RTM
• Light RTM (shell laminate RTM)
• Silicone bag RTM
• Closed Cavity Bag Molding (CCBM®)
• Multiple Insert Tooling (MIT®) RTM
• Zero Injection Pressure (ZIP®) RTM.

Parts fabricated using these processes may or may not have a gel coat on the exterior surface. The part size for these processes is limited by mold and part handling considerations. Part-to-part consistency is better than with open molding due to less dependence on operator skill. Also, two-sided cosmetic parts can be produced. Emissions from these processes are still regulated; however, they are much lower than with open molding due to the closed portion of the process. Emissions from the gel coat application, if used, are the same as for open molding.

C. Compression molding
Compression molding is another closed molding process. It uses clamping force during mold closure to flow a pre-manufactured compound through a mold cavity. A hydraulic press generally provides the clamping force. Compression molds are generally made from chrome-plated tool steel. Sheet molding compound, bulk molding compound, and wet molding compound are examples of pre-manufactured compounds.

If an external coating is needed on a compression molded part, it is generally post-applied; however, in-mold coatings are available. Part size is limited by press platen size. Part-to-part consistency is excellent. Emissions from compression molding are still regulated; however, they are much lower than with open molding due to the closed nature of the process.

The following table gives you the gist of the differences between the processes we just discussed

1. Numbers taken from unified emissions factors; 35 percent styrene content resin; mechanical non-atomized application; and 30 percent fiberglass.

2. Numbers are taken from EPA AP-42 emission factor; 35 percent styrene content resin; compound paste (25 to 100 percent resin); and 30 percent fiberglass.

Process Selection
When the best process to use for the fabrication of a specific part is not obvious, process selection should be accomplished through a process trade study. A process trade study involves comparing the part fabrication costs and part performance factors for a specific part fabricated by various processes. Part fabrication costs include but are not limited to equipment costs, tooling costs, material costs, and labor costs. Part performance factors are dependent on the specific part being studied but can include, weight, strength requirements, and appearance requirements. Emissions of Hazardous Air Pollutants (HAP) or other regulated materials vary by process and may also factor into process selection. An example trade study follows.

The subject part is from the deck of a run-about boat. It is a hinged hatch cover that provides access to an under-deck storage compartment or cooler. The step face features a non-skid profile on the external surface. The step face comprises glass skins over a foam-filled honeycomb core. The part measures 11 inches by 25 inches with a 1.5-inch tall perimeter flange. The design criteria include an impact of 300 pounds from a three-foot elevation.

The part is shown in Figure 1 alongside.
Processes considered in the trade study were open molding and several low-volume closed   

molding processes including vacuum infusion, silicone bag RTM, light RTM, and conventional RTM. Equipment costs, tooling costs, material costs, and labor costs were calculated for each process on a per-part basis.

Costs are based on typical values in the year 2005 and are presented as relative costs with open molding at 100 parts produced as the baseline. The number of parts to be produced varied from 10 to 9,000. Parts were to be produced over a three-year time frame with an equal number of parts per year. Process trade study cost results are shown in Table 1 below. The cost per part decreases as the number of parts produced increases. However, the amount of decrease depends on the process, meaning that Figure 1 Trade Study Hatch Cover different processes are the most cost-effective at different production rates.

Table 1- Process Trade Study Cost Results

Conventional RTM is not a cost-effective option for hatch cover production at production run sizes of less than 1000 parts. For production run sizes greater than 1,000 parts this process becomes competitive with light RTM, but does not become cheaper than light RTM even at production run sizes of 9,000 parts due to the need for a gel-coated surface.

The cost comparison could be different for large production runs of a non-gel
coated part. While competitive with light RTM, silicone bag RTM is never the cost-effective process for hatch cover production. This is due to the cost of the materials needed to fabricate the silicone bags. However, silicone bag RTM can be an excellent process selection for parts with closed contours that are not easily fabricated by other processes.

Light RTM is the low-cost process for hatch cover production at production run sizes greater than 100 parts.

Vacuum infusion is competitive with open molding as the low-cost process for hatch cover production runs of less than 100 parts. At higher production rates vacuum infusion is not cost-effective due to the cost of the consumable materials (vacuum bag film, sealant tape, etc.) needed for each part. Overall process trade study results including part appearance, strength, cost, and emissions, for the hatch cover, are shown in Table 2.

The use of closed a closed molding process reduces emissions by 50 percent in comparison to open molding with low VOC materials. The emissions differences as well differences in part appearance could influence process selection for hatch cover production.

This trade study is provided as an example of the type of evaluation that can be done to make an informed decision on process selection. The conclusions reached are not valid for all part types, sizes, complexity, and specific combinations of labor, material, and capital costs

Open molding is the low-cost process for hatch cover production runs of less than 100 parts. But it is not cost-effective for production runs greater than 100 parts.

More information
This information on FRP composites is an extract from a book titled ‘Composites Application Guide’.

We hope you found this informative. Please feel free to share your articles in our future newsletters.

by D. Selwyn Jebadurai and A. Suresh Babu

Glass fibre reinforced polyester composites have been utilized in many applications including automotive, transportation, structural, piping, chemical storage tanks and windmill components. Unsaturated polyesters are commonly used as matrix for GFRP composite parts as they have the advantages of low viscosity, fast cure time and low cost. Glass fibre is the most widely used reinforcement because of its features like high strength, corrosion resistance, easy availability and low cost.

In general, fibre reinforced polymers are known to have high in-plane tensile strength and stiffness properties and are much weaker in through thickness properties, such as the resistance to inter-laminar fibre-matrix cracking and delamination. The inter-laminar region is devoid of fibre reinforcement and fails via various modes, primarily delamination and matrix cracking which may develop through the service life of the structure. In recent years micro and nano-scaled particles have been considered as filler material for matrix resin to produce high performance composites with enhanced mechanical properties. Carbon Nanotubes (CNTs) have been the focus of research since their discovery by Sumio Iijima in 1991. CNTs have a combination of outstanding mechanical, electrical and thermal properties that make them suitable for numerous applications. The unique mechanical properties of CNTs as well as their low density and high aspect ratio make them ideal candidates to act as reinforcement for polymer composites. Functionalized MWCNTs lead to a more homogeneous distribution in the matrix and a reduced risk of agglomerates when compared to non-functionalized MWCNTs.

A study was conducted to analyse the influence of Carboxyl functionalized Multi-Walled Carbon Nanotubes (COOH – MWCNTs) on glass fibre reinforced polyester composites. CNTs were obtained from Quantum Materials Corporation, Bangalore. The average outer and inner diameters of nanotubes were 12 nm and 8 nm respectively.

The length of nanotubes were between 4 and 5 microns and their specific surface area were between 250 m2/g and 290 m2/g. Tensile strength of CNTs was found to be > 55 GPa. The Scanning Electron Microscope (SEM) and Transmission Electron Microscope (TEM) images of MWCNTs are shown in Figures 1 and 2 respectively.

Figure 1 SEM image of MWCNTs

Figure 2 TEM image of MWCNTs

Glass fibre reinforced unsaturated isophthalic polyester laminates were made without MWCNTs and with varying MWCNTs content (containing 0.2 and 0.5 wt% MWCNTs) by Vacuum Resin Infusion process. The composite laminates consisted of five layers of glass fibre mats. These five layers comprised of alternate layers of chopped strand mat (450 gsm) and woven roving mat (600 gsm). The MWCNTs were dispersed in the resin by ultrasonication for 1 hour and then the resin infusion process was carried out as shown in Figure 3. Specimens were characterized to determine tensile strength, tensile modulus, flexural strength, flexural modulus and inter-laminar shear strength (ILSS). The test results are shown in figures 4-8.

 Figure 3 Fabrication of Composite

  Figure 4 Comparison of Tensile Strength

Strength of Laminates

Composite laminate with 0.2 wt% MWCNT exhibited better mechanical properties than composite with 0.5 wt% MWCNT. It could be due to the homogenous dispersion of CNTs in the matrix at lower concentration. These homogeneously dispersed CNTs in the matrix act as interfaces for stress transfer and hence serve as additional nano-reinforcement to the matrix. Laminate with 0.2 wt% exhibited 58% increase in tensile strength and 235% increase in tensile modulus. There was a 23% increase in flexural strength and 13% increase in flexural modulus of the laminates with 0.2 wt% MWCNTs compared to laminates without MWCNTs. The inter-laminar shear strength was found to increase by 12% with 0.2 wt% MWCNTs. This study also shows that higher concentration of CNTs (0.5 wt%) leads to drop in mechanical properties which could be due to agglomeration of CNTs.
Further improvements in laminate properties can be achieved by optimizing the carbon nanotube content in the matrix. Thus there is huge potential to use carbon nanotubes in conventional fibre reinforced polymer composites for many structural applications. The safety aspects of handling virgin CNTs need to be understood clearly, although it is safe once they are incorporated in the matrix.

Figure 5 Comparison of Tensile Modulus
of Laminates

       Figure 6 Comparison of Flexural Strength
of Laminates

Figure 7 Comparison of Flexural Modulus
of Laminates

Figure 8 Comparison of Flexural Modulus
of Laminates
                                                                                                       

We at Link Composites were enthused by your response to our first newsletter. We are encouraged by your support, and look forward to being in regular touch with you from now on. If you would like us to cover a particular topic or vendor, please write in. We value your suggestions, and look forward to a better interaction with you through these newsletters.

Info-Tutorial: Carbon Fibers (Contd. from previous issue): Key Markets, End Products Key Markets and End Products Wind Energy India is now home to over 12 international and domestic wind turbine manufacturers including the likes of Suzlon, Vestas, WinWind, RRB Energy, etc. Plans are ongoing to manufacture blades with rotor diameters of 100M+ in India. Carbon fibers are the enabling material for blades above 45M in length. Zoltek has partnered with GBT (Global Blade Technology) from Netherlands to provide the wind energy industry with all the support tools its needs to move towards Carbon fibers. CNG pressure Vessels With the introduction of high tech buses in India that run on CNG there is a growing need to effectively manage the weight of the CNG cylinders that these buses are carrying. Type 2 (Metal Liner + Hoop wrap composite) and Type 3(Aluminum liner + Full wrap composite) have tremendous potential here. Currently all such cylinders continue to be imported and it’s only a matter of time before we see the commercial production of these cylinders domestically.

Infrastructure Most of the bridges, building, piers etc., in India are now beginning to age. The Fiber wrapping of beams and columns for seismic retrofitting and structural rehabilitation represents an easy and cost effective solution to this problem. CFRP plates and Uni Directional Carbon fiber wraps are today widely being used to enhance the life of bridges and decks and to retrofit beams and columns. Automotive Europe and the Americas have now begun to look at carbon as a strategic material and there is a lot of activity surrounding the Carbon fiber/weight reduction and fuel efficiency programs of many automotive majors. There is a great potential for fabrics weavers, preform manufacturers, intermediate product form developers (SMC, BMC, LFT) and Tier 1 Automotive suppliers in India to be a part of this growth. Oil & Gas The Oil & Gas industry has a huge need for tethers, drill risers etc that need to be strong, stiff and corrosion resistant. Pultruded Carbon fibers work well in these environments. Buoyancy modulus utilizing milled carbon fibers are also in demand. Electricity Transmission Cables The High throughput HT transmission cable utilizes a carbon core conductor. The negative CTE (Coefficient of Thermal expansion) of Carbon offers the ability to build longer spans of cable.

Other products such as Aircraft secondary structures, Boat hulls, Propeller Shafts, Roller Blades, PC casings, X – ray tables, Orthotools, Carbon – Carbon brakes for Aircrafts and cars and sporting equipment offer tremendous scope for Composite fabricators to develop end products in Carbon with a minimal capital investment. To summarize – The future of carbon fiber usage will be driven by the world’s need to Generate new forms of energy or reduce fossil fuel consumption to ensure a cleaner and greener tomorrow. – Akhil Hebbar, Zoltek We thank Mr. Hebbar for sharing this enlightening article with us. In our Next Issue: In our next newsletter our Info-Tutorial will focus on MoldWiz sacraficial release and its uses. Do send in your case-studies and articles for publishing in our YouSpeak column. Write to us at response@elink.co.in Contact us Plot No: 3B+3, Unit No: 6, D-1 Block, MIDC, Chinchwad, Opp. ADOR Welding Ltd, Chinchwad, Pune – 411019 Telefax: 020 – 30724590                            020 – 30727824 www.elink.co.in