Lecuture 8 Flashcards
How are big blocks of polystyrene made from casting-shaped objects (nvm this is an example of foaming oppies come back to it when you get foaming :))?
The polystyrene comes in tiny pellets of about 1mm. The first step is to expand the pellets inside a stea, chamber, the 200C heat causes the pentane gas in the pellets to escape allowing air to enter the pellets and inflate them up to 40 times their original size. The factory then bags the expanded pellets into huge sacks.
To make expanded polystyrene products they start by dumbing sacks of expanded pellets into what is called the block mold machine, the more pellets they put per cubic meter the denser the product will be. They then steam fuse the pellets together molding them into a block. This molding process takes 5 to 20 minutes depending on the density. Now that the block is ready it can be cut into whatever the client ordered!
What is solution casting?
The solution casting method is a widely accepted method for developing composite films. It involves rigorous stirring or ultrasonication of the polymer composites and nanomaterials in the organic solvent or water to achieve desired dispersion and homogeneity in the prepared film-forming solution. Then the film-forming solution can be cast on the Petri plates or glass sheets. The plates with solution are kept for drying in a hot air oven for at least 12 to 24 h, depending upon the solution constituents. When the solvent evaporates, it is left with thin films of composite materials composed of polymer matrix and nanomaterials
What is Film-calendering?
In the calendering process, polymers are first melted in the barrel of an extruder. This occurs under controlled pressures and temperatures to prevent degradation of the thermoplastic, a problem that can affect both appearance and physical properties. The molten thermoplastic is then extruded onto one of a series of highly polished metallic rolls in a roll “stack.” The rolls have a gap between them that determines the thickness of the film. As the molten polymer passes between the rolls it gradually cools. At the same time, the pressure of the rolls spreads the material evenly to form a continuous length of high-quality thin film with a uniform width, thickness and surface.
This phase of production requires maintaining process conditions that are specific to each thermoplastic. The metal rolls must be heated and cooled according to the polymer’s thermal stability characteristics to ensure the resulting film has the required appearance, low stress, uniform finish and physical properties.
What is the procedure for extrusion and its two types discussed?
Insulation thermoplastic feed stack is fed to the feed barrel from the hopper. Hoppers are actually vacuum-fed automatically but maybe filled manually. As the feedstock enters the feed barrel is driven forward by the rotation of the screw. The feed stock plasticizes or melts as the rotating screw drags it forward; this is referred to as fry flow. Dry flow causes the thermoplastic molecules to slide over each other, creating frictional heat heat which melts the material.
External heating bands provide additional heat to the feed barrel bringing the material to its final temp.
There are two principal plastic extrusion processes:
- Profile extrusion (basically direct extrusion): found in the next flash card
- Flim blowing extrusion (what you came for): Find it in flash card 6
What is profile extrusion?
Profile extersion is a horizontal process used to make long continuous shapes having a constant cross-section or profile. The profile is extended into a long horizontal cooling tank or tunnel filled with water. This water cools and solidifies the extruded plastic as it exits the die. At the end of the cooling tank, Haul-off roles pull the profile away from the die at a uniform controlled speed. As the profile is pulled through the haol-off rolls a cutter shears it into the required length for further processing.
What is the film-blowing extrusion process?
The blow film extrusion process uses an extruder to pump plastic vertically through a die that has a 360-degree annual opening on its top. This produces a tubular film extrusion which is subsequently filled with air, as a result, the tube expands out into a bubble having a diameter larger than the diameter of the annular opening of the die. The tubular bubble cools as it is pulled. When sufficiently cool the bubble is flattened within a collapsing frame, the flattened plastic is then pulled through a series of rollers which help monitor tension on the flattened plastic film as it is guided to a winder and wound to a core for later use.
What is the process of blow molding?
Molten tube if plastic is extruded from the machine and cut with a hot blade. A water-cooled mold then captures it and a blowing nozzle is instered. A blast of pressurized air is blown forcing the hot plastic to take the shape of the mold. The mold is then cooled and the plastic is solidified.
What is the process of thermoforming?
The product is made out of polycarbonate cut sheets of about 8mm in thickness, this raw material has high transparency just like glass.
The cut sheet is heated on both sides then with a blow the flexible cut is vacuumed into the mold. after cooling for about 180 seconds, the material stabilizes
What is melt spinning?
Melt spinning is the standard manufacturing technique for conventional degradable absorbable synthetic biopolymers. Melt-spun biopolymer fibers, especially PLA, PGA, PCL, and their derivatives, are widely used since they have FDA approval and are commercially available. In brief, molten thermoplastic polymers are fed in a constant flow rate into a spinning head, where the polymer is pressed through a spinneret equipped with holes of defined geometry. The resulting continuous filaments are cooled, drawn, and collected.
The melt spinning process is distinguished by defined filament cross-section geometries and a huge variety regarding fineness and filament count. The spinneret may contain a large number of holes enabling a high spinning capacity unmatched by other spinning processes. No solvent is needed in the spinning process ensuring a high purity of the spun polymers. However, ultra-fine fibers may be fabricated by two-component spinning or the melt-blown process. By using a defined ratio of copolymers, fibers with adjustable absorbance kinetics may be produced.
Due to the process characteristics only polymers with high decomposition temperatures and low melt viscosities can be used in melt spinning. Thus, the range of biopolymers is limited due to denaturation or decomposition of those sensitive materials.
What is dry spinning process?
- Solution Preparation: Polymer is dissolved in a solvent (20-30% concentration). Higher concentration makes better fibers but is harder to handle.
- Heating: The solution is heated to reduce thickness.
- Filtration: It is filtered and sent to the spin block.
- Spinning: The solution is pushed through small holes (spinneret) into warm air, where the solvent evaporates, turning the liquid into solid fibers.
- Collection: The fibers are wound onto bobbins or treated for specific purposes. A finish is applied for lubrication and to reduce static.
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What is wet spinning?
Wet spinning of fiber is a form of solution spinning where polymer powder is dissolved in a suitable solvent and the polymer solution is extruded through spinneret into a solvent-non solvent mixture (coagulant). Due to mutual diffusion of solvent and non-solvent, polymer solution coagulates to form fibers
What is fused deposition moldding?
An FDM 3D printer works by depositing melted filament material over a build platform layer by layer until you have a completed part. FDM uses digital design files that are uploaded to the machine itself and translates them into physical dimensions. Materials for FDM include polymers such as ABS, PLA, PETG and PEI, which the machine feeds as threads through a heated nozzle.
To operate an FDM machine, you first load a spool of this thermoplastic filament into the printer. Once the nozzle hits the desired temperature, the printer feeds the filament through an extrusion head and nozzle.
This extrusion head is attached to a three-axis system that allows it to move across the X, Y and Z axes. The printer extrudes melted material in thin strands and deposits them layer by layer along a path determined by the design. Once deposited, the material cools and solidifies. You can attach fans to the extrusion head to accelerate cooling in some cases.
To fill an area, multiple passes are required, similar to coloring in a shape with a marker. When the printer finishes a layer, the build platform descends and the machine begins work on the next layer. In some machine setups, the extrusion head moves up. This process repeats until the part is finished.
What is selective laser sintering?
Selective laser sintering is an additive manufacturing (AM) technology that uses a high-power laser to sinter small particles of polymer powder into a solid structure based on a 3D model.
SLS 3D printing has been a popular choice for engineers and manufacturers for decades. Low cost per part, high productivity, and established materials make the technology ideal for a range of applications from rapid prototyping to small-batch, bridge, or custom manufacturing.
Recent advances in machinery, materials, and software have made SLS printing accessible to a wider range of businesses, enabling more and more companies to use these tools that were previously limited to a few high-tech industries.
Process:
- Printing: A thin layer of powder is spread on a platform. A laser heats and fuses specific areas to create solid layers, while unfused powder supports the structure. The platform lowers after each layer, and the process repeats.
- Cooling: The part cools inside the printer, then outside, to prevent warping.
- Post-Processing: The part is removed, cleaned of excess powder, and can be further refined. Unused powder is recycled.
What is stereolithography?
SLA 3D printing, or Stereolithography, is an advanced and widely used 3D printing technology that produces exceptionally impressive results. This innovative technique allows designers, engineers and makers to create complex three-dimensional objects with unparalleled precision and stunning detail.
SLA 3D printing starts with a digital 3D model of the desired object. This model is then converted into thin, horizontal layers by specialized slicer software. These layers act as a kind of blueprint for the 3D printer, the so-called G-code.
During printing, a vat is filled with liquid photopolymer resin, a material that reacts to ultraviolet (UV) light. A build platform is slowly lowered into the resin and a UV laser scans the first layer of the 3D model, curing the resin in the correct places.
Once the first layer is complete, the build platform is lowered slightly, and the process is repeated for each layer until the entire object is built. After SLA 3D printing, the object is removed from the build chamber and cleaned to remove excess resin.
