Experiment 1 (Lab Report)

October 3, 2017 | Author: Rabiatul Adawiah | Category: Polyethylene, Polymer Chemistry, Polymers, Amorphous Solid, Chemistry
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MARKS Title Objectives Introduction List of equipment and materials Procedure Results & Discussion Conclusion References TOTAL

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1. OBJECTIVE 1.1 To develop practical skill in compression molding technique for polymer materials. 1.2 To investigate the effect of cure time and cure temperature to the compressed polymer product.

2. INTRODUCTION Compression molding is one of the earliest technique in which the material is restricted between a male and female mold surfaces [1]. It is also called matched die molding is a process used almost for main classes of polymer which are thermoplastics, thermosets and rubbers. Compression molding is a processing technique that combines forming the product along with simultaneous curing of the resin by applying pressure and usually heat [2]. Matched-die mold are design and fabricated to withstand pressure and frictional abrasion of repeated moulding cycles. They are usually made of a hard metal such as tool steel and are highly polished or chrome plate and contain a smooth surface finish on the consolidated part [3]. Sometimes, this molding technique called the compression press. After starting material is charged into the mold, the mold in hydraulically closed to make the material conform to the desired configuration and duplicate the finish mold. The curing is completed at elevated temperature under intermediate pressure. The principles of compression molding are quite simple. It undergoes four principles which are charging, closing, heating, cooling and opening. For charging, it need to charge a precise amount of molding compound into the bottom half of a heated mold. Closing the mold halves together to compress the charge, forcing it to flow and conform to the shape of the cavity. Heating the compound by means of the hot mold and let it flow completely fulfill the mold. Cooling is to stabilize the products. Opening the mold halves and removing the part from the cavity [4]. Polymer material is placed in the mold cavity that is in the open position. While under pressure, the material is heated, which causes it to cross-link and to harden. When the material is hard, the mold is opened and the part is removed.

Figure 1 : Compression molding processing technique Figure 2 is a diagram of a compression molding machine. This diagram shows the most important elements of the compression molding process. The molding machine consists of a heavy metal base onto which slide rods are attached and a compression unit that slides up and down on the slide rods. These slide rods guide the movement of the compression assembly from its open to closed positions. The movement of the compression assembly and the force to clamp the compression assembly against the base are supplied by a hydraulic unit mounted above the compression assembly.

Figure 2 : Compression molding machine

3. COMPONENT AND EQUIPMENT 3.1 Two platens hot press 3.2 Rectangular mould 3.3 Digital weight balance 3.4 Ultra high molecular weight polyethylene 3.5 Over-Head projection 3.6 Glove 3.7 Scrapper

4. PROCEDURE 4.1 The ultra high molecular weight polyethylene was weighted with digital weight

balance. 4.2 The compression moulding machine, as shown in Figure 3, was switch on. 4.3 The temperature was set to 165°C, as for the preheat time is set to 10 minutes and the

cooling time was set to 10 minutes. 4.4 The rectangular mold was sandwiched with two platens and placed at the upper heavy

metal press for preheat until it reached the hot press temperature. 4.5 After reached the required temperature, the heated mold is removed carefully with

glove and scrapper. 4.6 The over-head projection (OHP) was lay below the cavity. As the ultra high

molecular weight polyethylene was placed at the middle of the mold, or known as the cavity. A layer of OHP is placed above the cavity. 4.7 Placed the mold back in the machine. The compression moulding machine will

preheat for 10 minutes, then the mold was been pressured for 10 minutes. 4.8 After the 10 minutes, the mold required cooling. The mold is removed from the upper

heavy metal with glove and scrapper, and been placed in the lower heavy metal for curing for 10 minutes. 4.9 After the mold been cure for 8 minutes, the mold was removed from the machine.

The polymer product was removed by hand and scrapper. The final product then been trimmed into required shape and sizes.


Figure 3 : The product of compression molding before trim

Figure 4 : The product of compression molding after trim

6. DISCUSSION Based on experiment, we used ultra high molecular weight polyethylene (UHMWPE) as the sample. It has long chain that can transfer load more effectively to the polymer backbone by strengthening intermolecular interactions. Besides, it has tendency to expand when exposed to heat [5]. In this experiment, the mold is sandwiched by overhead projection (OHP) to produce a smooth surface product. The OHP is been selected due to it had a high resistance to heat. OHP is classified as the polyester type 1, which known as the polyethylene terephthalate (PET). OHP had a much higher melting point, which is 254°C, compared to the ultra high molecular weight polyethylene. Thus it will not contaminate the final product. Referring to the experiment, it used compression molding machine to develop skills for the polymeric material. Compression molding machine have two zones which are hot zone and cold zone. There are four main factors for successful compression mold which are amount of material, heating time and technique, force applied to the mold and cooling time and technique [6]. Amount of sample need to be extra compared to the required amount to make sure it fulfill all the area in the cavity to avoid the uncured part. Besides, the uncured part present because of the powdered sample used. It shows the disadvantage using powdered sample. The product of the compression molding experienced flash type. The flash forms the seal as it spews and rapidly cures, at the mold parting line. The disadvantage of flash type is that there is no restriction to the flow of material and no backpressure is built up in the molded part. Hot zone used to preheat and heat the sample. Preheat functioning to melt the sample and fulfill the sample within the mold. While cold zone to cooled the sample slowly to avoid quenching due to rapid cooling [7]. For addition information, there are advantages and disadvantages of compression molding. The advantages are low initial setup cost, fast setup time, capable of large size parts beyond capacity of extrusion techniques, good surface finish and waste relatively little material. In contrast, the disadvantages are production speed is not up to injection molding standards, limited largely to flat or moderately curved parts with no undercuts and less than ideal product consistency [8].

Properties of ultra high molecular weight polyethylene are highest impact strength, odorless, tasteless and non-toxic. UHMWPE highly resistant to corrosive chemical except oxidizing acid, has extremely low moisture absorption and a very low coefficient of friction is self-lubricating and is highly resistant to abrasion. In contrast with high density polyethylene (HDPE), it is low in cost, impact resistance is from -400C to 900C, moisture resistance, good chemical resistance and it is readily processed by all thermoplastic methods. HDPE exposed to several disadvantages which are high thermal expansion, poor weathering resistance, subject to stress cracking, difficult to bond, flammable and poor temperature capability [9]. The parameter needs to be considered in hot press are temperature and pressure. Temperature should be suit with the environmental condition to avoid over heating, avoid quenching due to rapid cooling after heating and avoid the product from expand due to heat apply. Besides, the pressure apply need to be concern. If too much pressure applied, it will cause shock and crack to the product.

7. CONCLUSION As the conclusion, the experiment conducted able to develop practical skill in compression molding technique for polymeric material. The mechanical and chemical properties of compressed polymer product is highly depending on the cure time and cure temperature. If the cure time is too fast, the final product will experience short shot. If the cure temperature is too high, the final product experienced degradation.

8. REFERENCES [1] F.W. Billmeyer, Textbook of Polymer Science, JohnWiley & Son.Inc, 1984. [2] Callister,W.D Jr. Material Science and Engineering :An introduction, 5th Ed.New York: John Wiley, 2000. [3]Anil Kumar & Gupta, R.K Fundamentals of Polymers, McGraw Hill,Singapore, 1998.

[4]Gottfried ,W.D.Jr. Material Science –Structure –Properties Application, HanserGaeder, 2001. [5] J.A. Bryson, Plastics Material, 7th ed., 1999. [6] A. Brent Strong, Plastics Materials and Processing, 2nd Ed., Prentice Hall. [7] Ward, I.M., Mechanical Properties of Solid Polymers., 2nd Ed., John Wiley & Sons, Chichester. [8] Martin Moeller, Krzysztof Matyjaszewski, Polymer Science : A Comprehensive Reference, 2012. [9] Focatiis, D. S. A. D. (2012). "Sample Preparation Tooling for Near-Shape Compression Moulding of Polymer Specimens." Polymer Testing 31: 550-556.

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