SafeSource

Joanna went to the hobby shop to cut 1" spaced channels for the lid mold, while meryl sanded the wooden pieces created last time. Robin further epoxied the mold together further, and Paul assembled the pieces of the new wooden lid mold.

Another thermoforming was done of the Lexan. The pyrometer read 380F at time of thermoforming. This attempt didn't form around the edges and corners well, so more pieces of plastic were used in an attempt to form a tighter vacuum. A second form was done, and this one formed tightly.

The new mold was epoxied together after sanding down the parts to roughly fit together. Much epoxy was used as the 3D printing was far from accurate.

Paul and Joanna went to the hobby shop to cut the fill port, while Robin and Meryl prepared the bucket mold bas and lid mold, and also met with Prof. Powell, who helped them to develop a model for modeling the heat and energy transfer

3D Printing

3D printing was done of the bucket mold center pieces and were depowdered by Robin and Joanna. Powder stuck to both pieces in chunks on the right most yz-plane faces.

Lexan (Polycarbonate) Forming

We narrowed the thermoforming range of the Lexan to 320-400F and UHMW to 260F-430F. On thermoforming, it was discovered that the material sagged after about 1min 30sec, giving forming temps at 3min 15sec. The polycarbonate stuck to the mold however, and it broke the mold's top.

Thermoforming UHMW PE

UHMW PE with the minimold was used for thermoforming. The 19"x19" sheet was heated for ~14min to the point that it reached 370F in the center and 320F at the corners. It was prestretched, during which it decreased in temperature by ~40F

Final Mold

Because of the dimensions of the thermoformer, it was decided that a 2/3 size bucket would be made rather then making a full size bucket. This will allow us to add webbing blocks to the mold's base and hopefully remove all of the webbing that occured on the previous mold. The base of the mold will now be 10"x10"

Two pieces were printed the night before for this new large mold. The pieces appear to be much softer then the previous blocks. It is possible to dent the sides of the mold using your hands. They will be infused with the cyanoacrylate to see if that helps their strength.

The batch also has a very rough surface. There are no undercuts, but the surface is uneven and has a stripe at one point indicating that the 3d printer didn't work quite right.

The last thing done today, a minimold was cut in half, and one half was heated with a heat gun. The UHMW PE became transparent at about 255F. The plastic also "snapped" back into a flat sheet when heated, indicating shape memorty

Mini-mold Thermoforming

This morning Robin and Meryl finished preparing the mini-mold for thermoforming. Robin drilled vacuum holes into the mold base between the webbing blocks and the bucket mold. Meryl continued to sand the bucket mold to get a smooth surface without undercuts. Sanding of the fiberglass resin was also done on Friday after it had fully hardened. We also measured the dimensions of the bucket mold.

We went to the Pappalardo Lab at about 11:15 and were able to complete one iteration of the bucket before lunchtime at noon. We used the 1/16" UHMWPE and cut it to size (~16" square). The mold was covered with Butcher's wax and placed in the thermoformer while the furnace was heating up. We clamped the plastic in the mask (curled side down) and began heating. We checked the temperature intermittantly using the optical pyrometer.

Initially, as it softened, the "waves" in the plastic looked somewhat irregular. The sheet quickly became translucent due to loss of crystallinity. The corners took longer to become translucent. It took approximately 5-7 minutes for the entire sheet to reach temperatures within the thermoforming range (260F-430F). At 7.5 min., the temperature was 300-320F. We prestretched the sheet a little bit before raising the mold, but didn't measure the temperature of the sheet after doing so. Once the plastic touched the top of the mold, the temperature dropped to ~250F. The plastic on the sides of the mold dropped to ~220F.

It seemed that the sheet was too cold to thermoform. It didn't fully conform to the mold.--nearly an inch of the bottom of the bucket mold never contacted the plastic. The webbing blocks seemed to work pretty well, but we can't really tell until we form at the proper temperature when webbing will be more of a concern. The L-shaped block didn't work very well, but it was also positioned fairly close to the mold. The blocks located 1.75" from the mold seemed to work best.

Upon lifting the mask, the sheet immediately warped, considerably (the warping hadn't lessened after one hour). It was not too difficult to remove the sheet from the webbing blocks, but it took a bit of effort to separate the bucket mold. Luckily, the mold wasn't damaged.

We wanted to try one more time before lunch without checking on the plastic too often. We would also have made sure that the sheet was still hot enough after prestretching before raising the mold. The power, unfortunately, was shut down after 3 minutes of heating the plastic. We left the plastic and mold there to cool down, before picking it up an hour later.

Presentation and more work

Joanna and Meryl presented a progress report to the class and it went over well. Upon returning to lab, Robin and Joanna went to ask Ken Stone (kenstone@mit.edu, MIT Hobby Shop Supervisor, x3-4343) for pieces fo scrap wood and advice on how to better draft the fill port. They were able to obtain more than enough particle board for our needs (16"x16"x3/4" for base) and plywood for feet and filler port. Ken also offered to help us cut the fill port with a bandsaw.

We need to fit our entire bucket, lip, and web blocks into a 16"x16" area. This is probably not possible with our current mold because the rim alone will take up 1/2" of our 1" excess on each side. Consequently, we need to determine the maxiimum desired size of our bucket (less than 14") and web blocks (can they be 1/4" thick and still effective?). Joanna borrowed several thermoforming manuals from the library to help us answer these questions.

Robin and Paul continued to sand down the 3DP pieces of our mold to ensure that they fit together well. We will probably remove the 3DP rim and replace it with a lip made of wood attached to the base.

Meryl placed an oder for an Omega Pyrometer today also, that will be shipped to Toby Bashaw. We should have it by early next week.

Getting back into swing

Today brought back work from break. Our progress was discussed in detail amongst the group members and the week's plans were made. A 1/3rd scale model of our mold was printed and began to be put together. It had some difficulties in that it printed with streaks, and pieces were too thin owing to the 1/3rd nature of the mold. We obtained a new therocouple to test the surface of the plastic during thermoforming This thermocouple reacted much faster then the last one, giving us hope that it would work without having to hold it in place long enough to make the plastic cool.

The second thermoformed lid (theremoformed two weeks ago Friday) was cut and trimmed. This lid did snap onto the bucket and stayed there. The lid also has a fill port, which included a course filter to keep hands from being able to reach into the bucket.

Back of the Envelope

Today was all about fact finding. The samples for beam bending were removed from the 3d printing machine, only to be found to be extremely weak and fragile. The samples that we did have were taken to Dr. Yin Lin Xie and it was determined they would work, but were too fragile and different from normal 3d printed materials to bother with tests.

Also, using an industrial heat gun, ABS stips were warmed and shaped into snaps to test different snap designs. These designs were attached to the bucket and pulled on and off.

Solidworks was used to design the basis of what will become our 3d printed mold. The initial mockup went very well initially. Pictures should be available at the presentation on Thursday.

Zcorp returned our call. They told us that they had no thermal data on their materials. They are going to send us what mechanical data they can. Also, they suggested that we try a hollow mold, with a honeycomb structure on the inside. The air gaps could then be filled with epoxy. This could be done, although we see it as being more difficult to design and print.

We have put in for a PO for UHMW and VHMW PE. Hopefully that will be ordered and shipped soon. We 3d printed another set of tests, fixing the binder concentration so that it hopefully printed correctly.

Refining

Today's tasks... We're in the process of finding more data on the materials and final product requirements. Information on ASTM methods of 3 pt beam bending were discerned in order to test the ceramic printing material and plastics. Back of the envelope calculations were used to determine the mechanical and physical requirements of the bucket. Calculations predict roughly 4 ft-lb/in of force (200N)

We are also working on obtaining the properties of the 3d printing material via ZCorp in an attempt to avoid having to investigate the thermal and mechanical properties ourselves further then what we have. ANSYS is being used to try to model the properties of different lid locking mechanisms for bucket lid design.

We've done some work to model the bucket mold. A very basic design was built in clay so that we could all visualize what was wanted. Work was then done to determine the dimensions necessary, and what was possible in the 3d printer. The dimensions possible are roughly 11"x8"x8".

We are planning on ordering sheets of Lexan at 1/16" and 3/16" from GE Plastics. The exact specifications of the plastic can be found here in pdf format.

Issues about the thermal and residual stress in thermoformed plastics have been discussed. The fact that the lid went from warped at room temperature to flat was particularly disturbing, as the exact cause was not immediatly obvious. It was theorized that perhaps residual stress release over the course of a few days was the cause, or possibly that it wasn't completely cooled yet.

Formalizing the design process

Today the SafeSource team has been working on getting our project ready for the formal design review. The main issues we have been working on today:

1. Obtaining information about the initial plastic suggestions, including availability, cost, and standards
2. Modeling the lid-bucket interface in 2d using ANSYS
3. Modifing our previous bucket lid mold to improve the undercut in the snap and include a pour hole
4. Creation and updating of our online laboratory notebook

Updating the top of the lid, it no longer is warped. Aparently at some point over the weekend the material relaxed, and now lies flat. We don't currently think that cooling was a large factor in this as the temperature during Friday's cutting appeared to be room temperature, but the actual cause is still up for debate

Pappalardo Lab

Volumetric measurements were made of the bucket we inherited from last year. It turned out to hold 17.5L of water. There was some bulging in the sides of the bucket, about 3/32" to 5/32".

Work continued on making the male lid mold. A small, ~3mm, lip on the bottom was added by routing the bottom of the mold to try to facilitate a better undercut.

The actual process involved taking an ~24"x24"x1/8" sheet of ABS, attaching it to the thermoforming holder, and using radient heat to warm it to a workable temperature. The manner of determing if the plastic was at working temperature was to touch it and see if it felt "right" where right was a trampoline sort of feeling. The mold, which had been covered in baker's wax, was then raised into the plastic, and wiped down to cool and form the part. Finally, the mold was removed using impact force (hitting it against the side of a table).

After removal, the mold was observed to be warped slightly, such that it wouldn't lie flat. Excess material was removed with a jigsaw, and the lid was placed onto the bucket, and found to *nearly* snap onto the mold.

Instron Testing and Lid Mold Design

Our first review presentation was given today. Primarily the design of the bucket and need to select a material design were discussed. The necessasisity of having as few as possible number of pieces was discussed, as well as the necessity of certain chemical compatabilities, such as potash, or NaOH

Compressive Instron testing was done by Meryl and Dr. Yin Lin. For this a 2" cube of 3d printing material infused with zr10 infiltrant resin (something close to super glue). The cubes were oriented by the manner in which they were printed. The z plane was the printing plane, x was the front to back plane, and y was the side to side plane. The printer printed on the z plane, parallel to the y plane, and perpindicular to the x. Cubes 1-4 were infused with a new bottle of resin, cubes 5-9 were infused with an old bottle of resin

Also created was a male mold for a bucket lid. The basic design was a wooden rectangle cut into four triangular sections, measured to be the size of the bucket, plus a small (1/8") addition to account for size differences in the bucket and lid. The edges were routed with a 1/4" radius router on the top and 3/8" radius cut on the bottom. The idea behind the four triagonal pieces was to create a hinging mechanism that would allow the mold to snap out of the undercut in the lid.