Understanding the Basics of Prepegs

Understanding the Basics of Prepegs Prepreg composite materials are becoming increasingly common in the composite industry due to their ease of use, consistent properties, and high-quality surface finish. However, there is much to understand about prepregs prior to committing to using this material. Prepreg The term prepreg is actually an abbreviation for the phrase pre-impregnated. A prepreg is an FRP reinforcement that is pre-impregnated with a resin. Most often, the resin is an epoxy resin, however other types of resins can be used, including the majority of thermoset and thermoplastic resins. Although both are technically prepregs, thermoset and thermoplastic prepregs are dramatically different. Thermoplastic Prepregs Thermoplastic prepregs are composite reinforcements (fiberglass, carbon fiber, aramid, etc.) that are pre-impregnated with thermoplastic resin. Common resins for thermoplastic prepregs include PP, PET, PE, PPS, and PEEK. Thermoplastic prepregs can be provided in unidirectional tape, or in fabrics that are woven or stitched. The primary difference between thermoset and thermoplastic prepreg is that thermoplastic prepregs are stable at room temperature, and generally, do not have a shelf life. This is a direct result of the differences between thermoset and thermoplastic resins. Thermoset Prepregs More commonly used in prepreg composite manufacturing is thermoset prepregs. The primary resin matrix used is epoxy. Other thermoset resins are made into prepregs including BMI and phenolic resins. With a thermoset prepreg, the thermosetting resin starts as a liquid and fully impregnates the fiber reinforcement. Excess resin is precisely removed from the reinforcement. Meanwhile, the epoxy resin undergoes partial curing, changing the state of the resin from a liquid to a solid. This is known as the B-stage. In the B-stage, the resin is partially cured, and usually tacky. When the resin is brought up to an elevated temperature, it often returns briefly to a liquid state prior to hardening completely. Once cured, the thermoset resin which was in the b-stage is now fully cross-linked. Advantages of Prepregs Perhaps the greatest advantage of using prepregs is their ease of use. For example, say one is interested in manufacturing a flat panel out of carbon fiber and epoxy resin. If they were to use liquid resin in a closed molding or open molding process, they would be required to obtain a fabric, the epoxy resin, and the hardener for the epoxy. Most epoxy hardeners are considered hazardous, and dealing with resins in a liquid state can be messy. With an epoxy prepreg, only one item needs to be ordered. An epoxy prepreg comes on a roll and has the desired amount of both resin and hardener already impregnated in the fabric. Most thermoset prepregs come with a backing film on both sides of the fabric to protect it during transit and preparations. The prepreg is then cut to the desired shape, the backing is peeled off, and the prepreg is then laid into the mold or tool. Both heat and pressure are then applied for the specified amount of time. Some of the most common types of prepregs take an hour to cure, at around 250 degrees F, but different systems are available at both lower and higher cure temperatures and times. Disadvantages of Prepregs Shelf Life: Since the epoxy is in a B-stage, it is required to be stored either refrigerated or frozen prior to use. Additionally, the overall shelf life can be low.Cost Prohibitive: When manufacturing composites through a process such as pultrusion or vacuum infusion, the raw fiber, and resin are combined on-site. When using prepregs, the raw material must first be prepregged. This is most often done off-site at a specialized company that focuses on prepregs. This added step in the manufacturing chain can add increased cost, and in some instances close to double the material cost.

The Common Application (Definition, Tips, and More)

The Common Application (Definition, Tips, and More) In the 2017-18 academic year, The Common Application is used for undergraduate admissions by nearly 700 colleges and universities. The Common Application is an electronic college application system that collects a wide range of information: personal data, educational data, standardized test scores, family information, academic honors, extracurricular activities, work experience, a personal essay, and criminal history. Financial aid information needs to be handled on the FAFSA. The Reasoning Behind the Common Application The Common Application had modest beginnings in the 1970s when a few colleges and universities decided to make the application process easier for applicants by allowing them to create one application, photocopy it, and then mail it to multiple schools. As the application process moved online, this basic idea of making the application process easier for students has remained. If you are applying to 10 schools, you will need to type in all of your personal information, test score data, family information, and even your application essay just once.   Other similar single-application options have emerged more recently, such as the Cappex Application and the Universal College Application, although these options are not as widely accepted yet.   The Reality of the Common Application The seeming ease of using one application to apply to multiple schools certainly sounds appealing if you are a college applicant. The reality, however, is that the Common Application isnt, in fact, common for all schools, especially the more selective member institutions. While, the Common Application will save you time entering all that personal information, test score data, and details of your extracurricular involvement, individual schools often want to get school-specific information from you. The Common Application has evolved to allow all member institutions to request supplemental essays and other materials from applicants. In the original ideal of the Common App, applicants would write just a single essay when applying to college. Today, if an applicant were to apply to all eight of the Ivy League schools, that student would need to write over thirty essays in addition to the common one in the main application. Moreover, applicants are now allowed to create more than one Comm on Application, so you can, in fact, send different applications to different schools. Like many businesses, the Common Application had to choose between its ideal of being common and its desire to be a widely used application. To achieve the latter, it had to bend to the whims of potential member colleges and universities, and this meant making the application customizable, an obvious move away from being common. What Types of Colleges Use the Common Application? Originally, only schools that evaluated applications holistically  were allowed to use the Common Application; that is, the original philosophy behind the Common Application was that students should be evaluated as whole individuals, not just as a collection of numerical data such as class rank, standardized test scores, and grades. Every member institution needed to take into consideration non-numerical information derived from things such as letters of recommendation,  an  application essay, and extracurricular activities. If a college based admission solely on GPA and test scores, they could not be a member of the Common Application. Today this is not the case. Here again, as the Common Application continues to try and grow its number of member institutions, it has abandoned those original ideals. More colleges and universities do not have holistic admissions than those that do (for the simple reason that a holistic admission process is much more labor intensive than a data-driven process). So in order to open the door to the majority of institutions in the country, the Common Application now allows schools that do not have holistic admissions to become members. This change quickly resulted in the membership of many public institutions that base admission  decisions largely on numerical criteria. Because the Common Application keeps shifting to be inclusive of a wide range of colleges and universities, the membership is quite diverse. It includes nearly all top colleges and top universities, but also some schools that are not selective at all. Both public and private institutions use the Common App, as do several historical black colleges and universities.    The Most Recent Common Application Starting in 2013 with CA4, the newest version of the Common Application, the paper version of the application has been phased out and all applications are now  submitted electronically through the Common Application website. The online application allows you to create different versions of the application for different schools, and the website will also keep track of the different application requirements for the different schools to which you are applying. The roll-out of the current version of the application was fraught with problems, but current applicants should have a relatively trouble-free application process. Many schools will ask for one or more  supplemental essays  to complement the essay you write on one of the seven personal essay options provided on the Common Application. Many colleges will also ask for a short answer essay on one of your extracurricular or work experiences. These supplements will be submitted through the Common Application website with the rest of your application. Issues Related to the Common Application The Common Application is most likely here to stay, and the benefits it provides applicants certainly outweigh the negatives. The application is, however, a bit of a challenge for many colleges. Because it is so easy to apply to multiple schools using the Common App, many colleges are finding that the number of applications they are receiving is going up, but the number of students they are matriculating is not. The Common Application makes it more challenging for colleges to predict the yield from their applicant pools, and as a result, many schools are forced to rely more heavily on waitlists. This uncertainly can come back to bite students who find themselves placed in waitlist limbo because colleges simply cant predict how many students will accept their offers of admission.