Close
today i learned (til)

3-D printing

November 17, 2017

In my childhood, I always imagined the picture of me talking to myself. As I grow up, I first saw the introduction of 3-D printing on TV. 3-D printing makes me believe the possibility of creating another me.

3-D printing also called Additive Manufacturing(AM) refers to any process of printing 3-dimensional objects. Under the control of a computer, the raw material is piled on top of another.

To better understand the principle of 3-D printing, let’s first focus on the traditional way of printing—2-D printing. Traditional printing works on the pictures on a plane. Digital pictures consist of pixel plots with pure colors. It can be seen as every coordinate plot stores one color. Transmission is completed by synchronous orbits.

777

Instead of paper and paintings, the raw material for 3-D printing is normally thermal plastic which turns soft when heated. Nowadays the most common material for 3-D printing is ABS. PLA is also called Polylactic Acid. It has a very low melting point of below 185 Celsius. Therefore, it is used broadly in 3-D printing. It appears as wire rod just like wool, so it is not easy to stuck the painter.

Now that we know the material of 3-D printing, let’s see how the printer works. For x-axis and y-axis, we use a small steel plate on synchronous orbits. For z-axis, we use a thick rod appearing like a screw. And that is the basic construction of a 3-D printer. It is not as complicated as I originally thought. Basically, we just need to make a sprinkler head that can move in a three axis. Then, use silk thread to create planes and pile up the planes into a three-dimensional object.

We only need a JPG formed picture to print a picture. However, what kind of form do we need to complete a 3-D printing? In industrial design and architecture, rhino and solid work are really popular software. Almost all software in the world have the same basic function—squeeze. The form of a model for 3-D printing is STL..

图片 1

555

Now let’s discover the technology of 3-D printers.

  1. FDM(Fused Decomposition Modeling)

The key to FDM is to keep the temperature 1 Celsius above the melting point of the raw material. The sprinkler head is controlled by CAD tier data to make the raw material squeezed out and solidify as thin layers. The layers pile up to form a model.

name FDM
material ABS,PC,PLA
precision 0.025-0.762mm

The principle of construction of the system and operation are very easy. Maintenance cost is low. System operates safely. It can be used in lost wax casting and forming parts of any complexity. It does not need chemical cleaning.

444

  1. SLA(Stereo Lithography Apparatus)

SLA uses photosensitive resin as raw material and scans UV laser controlled by a computer so that the scanned area of the resin layser has photopolymerization reaction and form a layer of the component. Through continuously bonding the previous layer, the whole object is created.

name SLA
material Photosensitive resin
precision 0.1-0.2mm

This method has a high degree of molding process, a high precision of sizes and great quality. It uses CAD data model to lower the cost of repairing. It can work on objects with complicated structures.

333

  1. SLS(Selected Laser Sintering)

It uses CO2 laser as energy source and powder material as raw materials. First, put a thin layer of powder on the working table; Then, the laser beam is selectively sintered under the control of the computer according to the layered outline of the part. After the layer is completed, the next layer is sintered. Next, remove all excess powder after sintering, grind and dry the other treatments. Nowadays, wax, plastic powder and metal powder are broadly used.

name SLS
material Nylon, ABS

It can be used to make metal products with easy procedures. It does not need supporting structure. The utilization of material is high.

222

  1. Ployjet

Sliding back and forth along the x-axis, an ultrathin photosensitive resin is spread over the molding chamber. After each paving, the UV light bulb on the edge of the sprinkler head and immediately emits UV light to rapidly cure each layer of light-sensitive resin. After each print, the molded chassis inside the machine sinks, while the sprinkler head continues to work layer by layer until the prototype is completed.

name Ployjet
material Photosensitive resin polymeric material
precision 0.1mm

It can print 3-D model with high quality, great details and shorten the designing period. The range of choosing material is broad.

(https://www.zhihu.com/question/20382142)

Nowadays, 3-D printing has got some breakthroughs in many areas. Cornell creating machine lab states that Hydrocolloid Printing’s 3-D printing can custom food production. It is possible to print candy, chocolate and pasta now. Industrial 3D printers have existed since the early 1980s and have widely used for research purposes. Fashion designers also use the 3D printed bikini swimmer, shoed and dresses for generating ideas. Nike took advantage of 3-D printing technology in the modeling and production of Vapor Laser designed for American Players in 2012. 3-D printing has also been used in production of transplant organs, equipment and other medical fields. There are currently successful cases of a titanium pelvis transplanted by a British patient and a titanium mandibular transplanted by a Belgian patient.

( https://zh.wikipedia.org/wiki.E5.B7.A5.E4.B8.9A.E7.94.A8.E9.80.94)

 

Resources:

3D Printing.J.Maslin-Research Assoc.Dec-1976

3D Printing: the Ultimate guide to understanding and mastering 3d printing for life.Greg Norton-Createspace Independent-2015

Tags:
0Likes
Wendy Wei
About Author

Wendy is a second-year ISYE student in Georgia Tech. She loves traveling, listening to concerts and hanging out with her friends outside classes.

You May Also Like

today i learned (til)
2021’s Partial Lunar Eclipse (With Photos!)
Courtesy of Bella Beeco
Articles, Story Time, today i learned (til)
The Evolution from Fossil to Monster
today i learned (til)
Children of the Night
today i learned (til)
Artificial Intelligence Space Travel

Your journey starts here! Let’s talk!

ThemeRex © 2026. All rights reserved.
  Timothy Durham was convicted of rape when a test showed his DNA genotype matched the DNA recovered from the crime scene. Eye witnesses testified that Durham was in a different state at the time of the incident, but he wasn’t exonerated from his 3000 year sentence until follow up DNA analysis showed that the preliminary forensic analysis used in the trial was misinterpreted (Thomas et al.). DNA testing serves an invaluable part in our criminal justice system, but it comes with complications and bias that can lead to wrongful convictions and misinformed juries.  

Background:

Forensic genetics aims to identify the origin of a biological sample. It can be traced back to Karl Landsteiner who discovered human ABO blood groups. With blood groups, suspects could be eliminated based on their blood type. For example, if a suspect had type A blood and there was type B blood left at the scene by the perpetrator, the investigators could rule out the suspect. Unfortunately, the reverse didn’t work. If the suspect and perpetrator both had the same blood type, it can’t be concluded that the suspect was guilty. Blood identifiers evolved into 8 distinct categories which improved the ability to eliminate suspects, but could only conclude that (if the system matched) it might be the perpetrator or this person might be related to the perpetrator (Jobling and Gill). The field evolved in the 1980s with the discovery of Minisatellites, lines of repetitive DNA that don’t encode proteins. Minisatellite patterns in DNA create what we know today as the DNA fingerprint, and it has a much higher accuracy rate than previous methods (Augustyn et al.). Around the same time, a breakthrough for victims of sexual assault was discovered. In the case of assault, bodily fluids of both the victim and the assaulter can get mixed. A process called differential lysis was created which could selectively increase the concentration of sperm in a fluid sample thereby allowing for accurate DNA testing (Jobling and Gill). Forensics evolved to use single-locus probe testing after several years because of its ease of interpretation. It looked at specific minisatellites that had high variability between subjects. This was the first DNA-based method to be used in a criminal investigation. Two murder homicides from several years apart were connected using DNA evidence. The evidence was then used to exonerate an innocent man, and the guilty party was found when he tried to evade mandatory DNA testing for all men in the small town. His guilt was then reinforced when the DNA profiles matched, and he gave a confession (Jobling and Gill). Finding DNA at a crime scene doesn’t guarantee that you’ll find a murderer. There are issues with non-pure samples and limited samples that make DNA work difficult, and accreditation of facilities and new techniques and take time (Jobling and Gill).  

Subjectivity of DNA Interpretation:

In a 2009 publication, the National Academy of Science discusses the possibility for subjective influence in lab based forensics. The interpretation bias happens in complex cases where, for example, there could be a mixture of DNA. A study published in 2011 looked specifically at this issue. A gang rape happened in Georgia, and the mixture in DNA from the case was used to prosecute one suspect after another suspect took a plea bargain to testify against him. With DNA mixtures, there are three possible conclusions: suspect excluded, suspect cannot be excluded, and inconclusive. If the suspect is excluded, it means that there’s little to no possibility that his DNA is part of the sample. If the suspect cannot be excluded, it means that there’s a possibility that his DNA is part of the sample. In the trial, the DNA examiners who knew the context of the case concluded that the suspect being prosecuted could not be excluded. The 2011 study gave the DNA evidence from this case and the DNA profiles of the suspects to 17 forensic examiners. These examiners had no context of the case, and they all worked independently. Of the 17, only 1 came to the same conclusion (suspect cannot be excluded) as the examiners who knew the context of the case. Twelve of the 17 concluded that the suspect who was prosecuted and convicted had no relation to the DNA found in the sample. Not only does this show that subjectivity occurs due to the context of the case, but the 17 examiners came up with different findings based on their past experiences meaning that not all DNA analysis is objective. Some organizations like the Scientific Working Group on DNA Analysis Methods say that quantitative statistical evidence should be presented with every categorical conclusion, but the US doesn’t require this (Dror and Hampikian).

False Positives:

There are two categories that cause wrongful identification in a DNA report: coincidental matches and false positives. Coincidental matches are DNA matches between two unrelated people. The probability of these matches is about 1 to 500,000 for white individuals and about 1 to 350,000 for black individuals. This small probability of coincidental matches makes people feel confident in DNA evidence. False positives, on the other hand, are matches between DNA and test DNA that result from issues in the forensics lab. There have been multiple cases of forensics labs accidentally switching victim and suspected rapist DNA which lead to convictions that were later overturned when the lab realized the error. There are also cases of misreading or misunderstanding a sample. In a case from 1993, a man was convicted of rape because someone interpreted a mixed sample of victim and perpetrator DNA as an individual sample. The conviction was again overturned. To prevent false convictions, retesting DNA is allowed, but it’s not an efficient solution. Many people are pushing for statistical information on the probability of false positives, so that the jurors can make accurate judgements based on DNA evidence (Thomas et al.).  

Conclusion:

DNA analysis is a useful tool for the criminal justice system, but subjectivity can be an issue in forensic testing. To combat wrongful convictions, there are organizations like The Innocence Project who use DNA testing to free the innocent, but reformed forensic testing standards should be put in place to prevent subjectivity and to ensure juries are correctly informed during trials. https://www.innocenceproject.org/    

Citation:

Augustyn , Adam, et al. “DNA Finerprinting.” Encyclopedia Britannica , 26 Apr. 2018, www.britannica.com/science/DNA-fingerprinting#ref153186. Dror, Itiel E., and Greg Hampikian. “Subjectivity and Bias in Forensic DNA Mixture Interpretation.” Science & Justice, vol. 51, no. 4, 2011, pp. 204–208., doi:10.1016/j.scijus.2011.08.004. Jobling, Mark A., and Peter Gill. “Encoded Evidence: DNA in Forensic Analysis.” Nature Reviews: Genetics, vol. 5, no. 10, Oct. 2004, pp. 739–751., doi:10.1038/nrg1455. Thompson, William C, et al. “How the Probability of a False Positive Affects the Value of DNA Evidence.” Journal of Forensic Sciences, vol. 48, no. 1, Jan. 2003, pp. 1–8., doi:10.1520/jfs2001171.