Reinforcements
The role of the reinforcement in a composite material is fundamentally one of increasing the mechanical properties of the neat resin system. All of the different fibres used in composites have different properties and so affect the properties of the composite in different ways. However, individual fibres or fibre bundles can only be used on their own in a few processes such as filament winding. For most other applications, the fibres need to be arranged into some form of sheet, known as a fabric, to make handling possible. Different ways for assembling fibres into sheets and the variety of fibre orientations possible lead to there being many different types of fabrics, each of which has its own characteristics.
Table 3.1: Basic Properties of Fibres and Other Engineering Materials
Material Type
|
Tensile Strength
(MPa)
|
Tensile Modulus
(GPa)
|
Typical Density
(g/cc)
|
Specific Modulus
| |
Carbon HS
|
3500
|
160 - 270
|
1.8
|
90 - 150
| |
Carbon IM
|
5300
|
270 - 325
|
1.8
|
150 - 180
| |
Carbon HM
|
3500
|
325 - 440
|
1.8
|
180 - 240
| |
Carbon UHM
|
2000
|
440+
|
2.0
|
200+
| |
Aramid LM
|
3600
|
60
|
1.45
|
40
| |
Aramid HM
|
3100
|
120
|
1.45
|
80
| |
Aramid UHM
|
3400
|
180
|
1.47
|
120
| |
Glass - E glass
|
2400
|
69
|
2.5
|
27
| |
Glass - S2 glass
|
3450
|
86
|
2.5
|
34
| |
Glass - quartz
|
3700
|
69
|
2.2
|
31
| |
Aluminium Alloy (7020)
|
400
|
1069
|
2.7
|
26
| |
Titanium
|
950
|
110
|
4.5
|
24
| |
Mild Steel
(55 Grade)
|
450
|
205
|
7.8
|
26
| |
Stainless Steel
(A5-80)
|
800
|
196
|
7.8
|
25
| |
HS Steel
(17/4 H900)
|
1241
|
197
|
7.8
|
25
| |
Table 3.1 show a variety of fiber used as the reinforcement in composite. In our case, we choose glass-E glass as our reinforcement since it is low cost, impact resistance, good chemical resistance, and high tensile strength. It also most common compare to other material.
Matrix
In this part, we have to determine what type of matrix we going to used. For the product we decided to use organic types of matrix. However, there are two choices we’ve came across. There are thermoplastic or thermoset.
Thermoset polymers are the matrix of choice for most structural composite materials. The single biggest advantage of thermoset polymers is that they have a very low viscosity and can thus be introduced into fibres at low pressures. Impregnation of the fibres is followed by chemical curing to give a solid structure, which can usually be carried out isothermally. An advantage of thermoplastics is that the moulding can be carried out non-isothermally, i.e. a hot melt into a cold mould, in order to achieve fast cycle times. However, polymerised thermoplastics tend to have melt viscosities between 500 and 1000 times that of thermosets, which necessitates higher pressures, causes processing difficulties and adds expense.
Thermoplastic composite polymers can, however, be readily recycled, an increasingly important issue in many markets, but especially in the automotive sector. For instance, an advanced thermoplastic composite component can be chopped to pellet-size and injection-moulded to yield long-fibre reinforced mouldings, which can in turn be recycled at the end of their life. Thermoset composite materials, on the other hand, can only be ground and used as filler, a process which decreases the value of the composite enormously.
Another advantage of thermoplastic composites are their superior impact and damage resistance properties. Over 90% of polymers used in composites are thermosets, with thermoplastic composites still a niche market, mainly due to the difficulties in processing.
In this project we use thermoset matrix Phenolics, the oldest of the thermoset plastics, have excellent insulating properties and resistance to moisture. Chemical resistance is good, except to strong acids and alkalies. This is an extremely versatile, fairly inexpensive, polymer. Phenolic was the first truly synthetic polymer and it is often referred to as the “workhorse of thermosetting (T.S.) resins”. A heat-cured T.S. resin has overall excellent properties, particularly dimensional stability, elevated temperature, creep resistance and applications requiring heat resistance.
Resin Transfer Molding (RTM)
Resin Transfer Molding (RTM) is a low pressure, closed molding process which offers a dimensionally accurate and high quality surface finish composite molding, using liquid thermoset polymers reinforced with various forms of fiber reinforcements. Typically polymers of Epoxy, Vinyl Ester, Methyl Methacrylate, Polyester or Phenolic are used with fiberglass reinforcement. Other reinforcements are offered for more demanding applications such as Arimid, Carbon and Synthetic fibers either individually or in combination with each other.
The matrix selection of polymer and reinforcement dictates both molding material cost, as well as molding mechanical and surface finish performance. Along with the polymer and reinforcement the addition of mineral fillers may be added to enhance fire retardancy, flex modulus and surface finish.
Reinforcements are presented in their dry form to the mold in either binder-bound chopped mat, random-continuous strand mat or woven cloth format. The fiber has been either "preformed" to the exact shape of the molding tool in a previous operation or is hand-tailored during the loading process in the molding tool. After the fiber is installed into the mold, a premixed catalyst and resin is injected into the closed mold cavity encapsulating the fiber within. The primary surface of the molding may be gel-coated, a process of spraying the mold surface before installing the fiber. If a gel coat is not required, the exterior finish would be the same from the front to back of the molded part. The RTM process has the inherent advantage of low-pressure injection; it usually does not exceed 100 psi of resin injection pressure during the mold-fill process.
The Benefits of using RTM
Closed Molding process is cleaner and healthier which attracts higher skilled employees.
· Closed molding operator turn-over is dramatically reduced through improved working conditions.
· Closed Molding area has organized process flow to maximize throughput efficiency.
· Moldings can be manufactured to close dimensional tolerances.
· Components will have good surface finish on both sides.
· Selective reinforcement and accurate fiber management is achievable.
· Ability to build-in fiber volume fraction loadings up to 65%.
· Consistency in thickness and fiber loading, resulting in uniform shrinkage.
· Inserts may be incorporated into moldings.
· Tooling costs comparatively low compared to other manufacturing processes.
· Uses only low pressure injection.
· Low volatile emission during processing.
· Ability to produce near net shape moldings, reducing material wastage.
· Process can be automated, resulting in higher production rates with less scrap.
· Ability to mold complex structural and hollow shapes.
· Low resultant voidage in molded components.
· Ability to achieve from 0.1mm to 90mm laminate thickness.
The Traditional Way
The traditional RTM process begins with loading the mold cavity with a fiber, typically fiberglass or similar reinforcement material. Once the mold is closed and clamped, the resin pump begins pumping catalyst and resin, as two separate components to the mix head, where the components mix and are fed into the mold as a single component material. The mold then fills, and as you see in this illustration, the mold fills excessively leaking resin around the parameter and out of the vents. That has been a historical problem with the process, one that has been corrected today and is unnecessary use of materials and VOC's as it has been in the past
The RTM Process
Resin Transfer Molding (RTM) is a low pressure closed molding process, where a mixed resin and catalyst are injected into a closed mold containing a fiber pack or preform. After the resin has cured, the mold can be opened and the finished component removed.
A wide range of resin systems can be used including: polyester, vinylester, epoxy, phenolic and methyl methacrylates, combined with pigments and fillers including aluminum trihydrates and calcium carbonates if required. The fiber pack can be glass, carbon, arimid, or a combination of these.
Figure 3.1: Simple process of RTM
Figure 3.1 shows a simple RTM process in a making of a product. First, the resin and catalyst are injected to a mixing head before transfer to mold.
RTM light process
Today the process begins the same way using dry fiber reinforcement, typically fiberglass or similar reinforcing material. The resin and catalyst pump simultaneously the moment the mold is closed and clamped under force. The catalyst and resin meet precisely at the same time at the mix head and are completely mixed together as a single component as they're pumped into the mold. The mold is controlling its internal pressures by MPG valve, which is unique to the process, and sends signals back to the pump head as we communicate with what's going on inside the cavity and preventing. Any resin from leaking around the parameter or out of the vents. The table 3.1 shows the steps in making the RTM light Process.
Table 3.1: Light RTM Process
 |
The first step is mold creation. We make the lower mold and the top mold. When the rtm light mold complete we going to start with initiation of proceedings. See figure
Figure 3.3: lower mold
First is the gel coat application. The gel is applied to the mold before we put the reinforcement. After some time, install the reinforcement to the mold. Then, cut the excess fiber. After that, close the mold. Check the vacuum connection for close the mold. Then, continue with vacuum system testing. Next step, inject the resin into the mold. When the resin exceed through the catch pot, stop the resin injection. Next remove excess resin. Then, do the exothermic reaction control. Remove the mold by using air injection. Lastly, the process is completed.
In order to get a better end product we asserted the process by using the vacuum assisted. This process is called vacuum assisted resin transfer molding (VARTM)
Vacuum assisted resin transfer molding (VARTM)
Vacuum assisted resin transfer molding (VARTM) is likely the most common acronym of all used in the discussion of low pressure closed mold reinforced composite molding. The often used VARTM acronym is most accurately applied to the process of vacuum infusion, that is where the composite is molded using a rigid mold to provide part geometry and a thin flexible membrane over the fiber, with outer atmospheric pressure compressing the fiber tight against the rigid mold surface. It should be noted however that VARTM simply means to add vacuum at the exit vent of the molding tool, thus any form of resin transfer molding in which vacuum is applied to the vent would qualify for the VARTM association. Often, the aerospace industry will have very high fiber volume composites molded within a rigid matched mold set in which the fixed cavity mold is clamped closed using a press and the resin injection pressures typically range less than 100 psi, yet could be as high as 600psi as used to fill the mold encapsulating the dry fiber within
The process started with masking the product edge. This can be made using a simple masking tape by taping around the product edge. Next, apply gel coat on the mold. After that, remove the masking tape. Wait for a couple minutes for the gel to cure.
After the gel is cured, load the fiber reinforcement. Then, trim excess fiber. Make sure the process is done carefully. To get a good finishing product, place the additional fiber in thicker area. Next, clean the mould flanges before closing the mold. Fit the injection valve then connect the valve to the injection machine. Next step, connect the waste tank then fit the vacuum catch pot. After that, connect the vacuum to the flange. Then, connecting vacuum to the catch pot. Turn the vacuum pump ON
Next process will be starting the injection. The resin flows from peripheral channel. When the resin reaches the catch pot, the injection will stop. The machine will flush the injection valve. Next process, disconnect the machine. Take a few minutes foe resin to cure.
The final process, disconnect the vacuum supply and remove the catch pot. Open the mould and remove the molding by using air ejector.
Reference
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