Student Research Projects

Fluoride Evacuation Device

An entrepreneur contacted the San Diego State University manufacturing department concerning an idea she had been working on for a fluoride tray that would evacuate saliva. She wanted to utilize the capabilities we had for three-dimensional modeling objects and then rapid prototyping them using steriolithography. The created SLA mold could then be used with a vacuum-forming machine to produce very accurate and detailed parts. With these operations, her device could be developed in an accelerated, cost effective process.

The saliva evacuation device that was desired received patent number 5,513,986. Previously existing fluoride application trays had to be used in conjunction with a saliva ejector, due to the fluoride causing increased salivation. A large quantity of the produced saliva pools beneath the tongue, however saliva from the parotid glands will flow over the bottom lip. The only way to remove this saliva with an ejector is to move it around the areas of the mouth where saliva gathers. The entrepreneur's idea would remedy this situation by providing a tray with passages for saliva to be evacuated through to one exit location in which the saliva ejector would be inserted.

A prototype was delivered to us by our sponsor which functioned properly, but was difficult to manufacture. It consisted of two separate vacuum-formed molds of low-density polyethylene sheeting attached together with hot glue. To attach the two molds required painstaking alignment and gluing. Neither of these operations were desired by our sponsor.

Our first idea resolved the two separate pieces of material as well as the gluing operation. It consisted of one mold which flipped over to create an evacuation tunnel for the saliva. The main section of the part was vacuum-formed over a male mold. The section which flipped over was created by a female mold. The "flap" was secured by placing 5/8" shrink-fit tubing over the two halves of the salivary exit where the saliva ejector was inserted.

To create the mold for this project, Pro/Engineer solid modeling software was used. The student spent approximately eighty hours learning the software before feeling confident enough to begin designing the part. Due to the complex geometry of the device, approximately forty hours were necessary to create a solid model, which consisted of both the male and female molds. A SLA file was then created for both molds to be transferred to the steriolithography machine where they were created.

The SLA molds were attached to a piece of hardwood. The female mold was positioned underneath the male mold by milling out part of the wood. The molds and their mount were placed in a vacuum-forming machine. Two-hundredths inch low-density polyethylene sheeting was chosen as the material to form with due to its flexibility and recyclable characteristics. Once the mold was formed, approximately eighty minutes of trimming was required to produce a finished part. Finally, the heat-shrink tubing was inserted and placed next to a heat source to shrink to a desired fit.

Upon inspection of our finished part, we noticed that it was going to be troublesome to secure the "flap" where it would work properly. Due to our choice of two-hundredths inch sheeting, the material was very flimsy, and especially thin at the flap because the material was stretched to form inside the female mold. After several unsuccessful attempts to alleviate this problem we realized to be able to remove saliva through a channel would require a design change.

After presenting the finished part and complications with the design to our sponsor, she brought up a design variant. Instead of having the "flap" flip underneath the male mold to cover the channel, the new idea would have it flip over the male mold. The channel would be placed on top of the male mold instead of inside. The flap would then be tightly pulled over the male mold, and thus would work effectively despite the thin material present. Saliva could enter the channel by placing strategic cuts in the flap on each side near the area where saliva generally pools under the tongue. The two halves of the "flap" and male mold which were connected with shrink-fit tubing had to be manipulated slightly for the new design, but worked just as effectively.

Our final design has been presented to our sponsor for testing. In both designs, the removal of the saliva which generally runs down the bottom lip from the parotid gland was desired, but the only way we could devise to evacuate this saliva was by means of holes drilled around the neck of the evacuation tube, as was done in the prototype presented to us by our sponsor. However, further brainstorming will be done to try to create a channel in the male mold to allow the saliva to run into the evacuation tunnel. Furthermore, due to trimming of the polyethylene, sharp edges are created which produce discomfort in the gums. Our sponsor has generated a lining material for us to try to form inside the part. This material or others that are currently being researched will hopefully alleviate the discomfort of the polyethylene on the dental patient.

To summarize the problem-solving process for this design project, and entrepreneur approached the SDSU manufacturing facility with an idea that needed improvement. The saliva evacuation device which she presented to us could perform its necessary function, however was difficult and time-consuming to manufacture. It was desired to create a vacuum-formed part out of one sheet of polyethylene which could be manipulated to evacuate saliva from a dental patient's mouth. Ease of assembly was pertinent to effectively and economically produce a competitive product. Creating the part out of one sheet required a male and female mold, with the female being folded over to attach to the male. Instead of painstakingly gluing the flap to the male mold, heat-shrink tubing was employed and was equally effective in its location. The design process could accommodate changes quickly by manipulating three-dimensional modeling designs and transferring the new models to SLA files. The new molds could be created in several hours using steriolithography.

There are a few potential design problems with this project. Testing must be done to indicate if the removal of saliva from the parotid glands is effective, or if a design variant is necessary. The sharp edges of the two-hundredths inch polyethylene either need to be smoothed out by some means or a foam liner may solve this problem. To ease the trimming of the part, a stamping apparatus could be implemented. This stamp would save time and money for a prospective manufacturer. It would also diminish the chances of human error when trimming the small features which are required. Finally, although the shrink-fit tubing does secure the flap to the male mold, there is the freedom to move forward and back between the two pieces of polyethylene. The stamping apparatus could also create cuts in the section of the adjoining surfaces that would allow an intertwining assembly and a loss of freedom of movement.

In summary, the design project provided a valuable learning experience for the involved student. An extensive amount of seat time was necessary to begin with the three-dimensional solid modeling. Over two-hundred fifty hours were spent learning the software and creating solid models. Advanced solid creating commands, which are not covered in introductory books on Pro/Engineer software, were implemented for the complex geometry. Upon successful creation of this project, further work on solid modeling design will be accelerated for the student due to the advanced experience. Also acquired was the ability to visualize how a mold will be necessarily designed to provide an acceptable vacuum-formed part. Introduction to steriolithography for the use of mold fabrication was valuable for future design projects involving plastics-forming processes. However, most importantly, the experience of communicating effectively with a customer provided the most benefit for the student. Without feedback from the customer and scheduled design meetings, the student would have had more difficulty developing the product to a point where it would carry out it's intended function. Moreover, due to feedback from the customer's expertise on saliva evacuation and fluoride application trays, product development was accelerated and the design intent was more clearly envisioned by the student.