DIY Vacuum Chamber
Project Origin:
Some hobbyist's projects benefit from casting parts made of resins or similar material. In order to properly cast these parts, it is often beneficial to have access to a vacuum pot or something capable of removing air bubbles trapped within the liquids used for making the molds or the casts themselves. Often, the vacuum equipment required can be costly due to the mechanics of the machines and equipment used. Often, it's not cost effective for a hobbyist to invest a significant amount of money on equipment they may only use a handful of times. Many of them opt out of this step of vacuum treating of their liquids and risk unsightly bubbles which then require time consuming touch up.
Recently, I was faced with a similar dilemma when I began planning a project that required a handful of small custom parts which could be made from resin. I knew detail would be important, so I researched what this type of treatment would require in the way of equipment. Although prices for vacuum equipment ranged a bit, based on some factors about my project, I was looking at a price tag of close to $200. This certainly wasn't in my budget for a project that hadn't even been proven in concept yet. I continued to search around for alternatives such as other DIY's but many of those still seemed clumsy, bulky, pricy still, or just not very practical. For example, one DIY I found required you to run what seemed to be a couple of hundred gallons through a Venturi, expelling all that water onto the ground, in order to obtain a vacuum! Seemed fairly wasteful to me, not to mention the hassle of running a garden hose into the living room spewing water on the carpet, just to remove the bubbles from my molds. Vacuum pumps were another hang up for me. I happened to have a vacuum test gauge from some auto repairs I did, which was capable of providing a suction. However, trying to use it to draw a vacuum on a bell that was 37 times larger than the molds and castings I was trying to produce would have most likely taken about 3 hours to accomplish.
So during this process of figuring out which approach might be most practical, I found myself making several sketches of my own design, of what I felt I was needing. I drew smaller or modified versions of some of the vacuum contraptions I had seen, but the strength of some materials was questionable. After rummaging around through some scrap materials I had in my garage, I came across some PVC pipe. The specifications of the PVC seemed promising since the pressure rating was printed right on the side. I had also worked with forming some other things out of PVC before, so it seemed like a viable material for creating a vessel capable of holding a vacuum. Now that I had something to work with, I began to refine some of my sketches based on PVC components. What I came up with was a modular type of chamber which could be used for both creating the molds and pouring the castings.
Scientific Concept:
Vacuum, stemming from the Latin word vacuus for "vacant" or "void", is space void of matter. Typically in applied physics and engineering, the term vacuum refers to any space in which the pressure is lower than atmospheric pressure and is measured in inches of mercury (Hg).
Purpose:
This project creates a container capable of sustaining a vacuum which can extract air bubbles from liquid silicone and resins before curing into molds and castings.
Materials / Tools (for 2" prototype):
Approximately 24 inches of 2 inch diameter schedule 40 PVC pipe
Two 2 inch PVC end caps
1/8 in. MPT to 1/8 in. barb fitting
Small tube of clear silicone
Hacksaw
Utility knife
Step drill bit
Drill motor
Adjustable wrench
Pencil
Scrap piece of angle iron***
Sheet of card stock
Sheet of graph paper
Chop saw (recommended)
Measuring tape
Automotive vacuum test kit
Safety:
**WARNING**: It is recommended that tools and equipment described in this article should only be used and operated by competent professionals. Appropriate Personal Protective Equipment should be worn at all times.
Build time:
Fabrication of PVC components: 30 to 45 minutes
Application of silicone: 20 to 30 minutes
Silicone cure time: 24+ hours
Fabrication & Assembly:
After deciding to keep my initial prototype compact, I cut a section of the 2" PVC pipe to a length of 2". This would become the chamber body. Next, I want to cut the pipe exactly in half down the length of what will be the chamber body. To do this, I use a piece of graph paper to find and mark at the exact opposite sides of the pipe sidewall. Then I use the scrap angle iron as a straight edge to mark the lines running up the pipe. Before cutting the pipe in half, I put reference marks on the pipe which will help realign the two halves. With the two lines as a guide, I use the hacksaw to carefully cut along the length of the PVC pipe sidewall. I stop just short of cutting through the first sidewall to help keep the pipe stable while cutting the opposite side. Once both cuts are complete, I mark a centerline along each of the four edges created by the cuts. Then, with the hacksaw, I carefully use the blade along each edge on the centerline to create a shallow channel which runs the whole length of each. This channel will help provide a bonding surface for a bead of silicone that will be applied.
Next, using the silicone caulking, I run a small bead along the edges, working the silicone into the channel. Then with a small piece of card stock, I hold it slightly rounded and use it to shape the bead into a slight crown approximately 1/16" above the surface. It's critical that this remains consistent across the entire edge to provide an air tight mating surface. I repeat this process for all four edges of the two halves of the chamber body. I set these two parts aside to cure overnight.
Since the chamber is designed so that both of the 2 inch end caps are to fit onto the body without glue, and since I didn't want them to bind on the pipe when attached, I cut about an inch off the open end of each end cap. To keep the caps steady as I cut and to assure the cuts would be square, I placed each end cap on a spare piece of the pipe. I also cut some scraps about 1/2" from some of the pipe that I slipped over the spare pipe which then served as a spacer to keep the pipe parallel with the saw fence as each cut was made. In retrospect, It may have been sufficient to place each cap on opposite sides of the scrap piece to keep it parallel for the cuts. The caps will represent a top and a bottom to the chamber, so I found it convenient to mark them as such. Taking the top cap, I use a step bit to drill a hole just large enough to thread in the barb fitting into the end, slightly to one side.
My next step is to create a sealing surface on the inside of the two end caps. To achieve this, I cut a notch in a piece of card stock the same width of the end cap sidewall, and measuring the depth of the notch approximately 3/8". Holding the first cap open end up, I apply a bead of silicone along the lower inside edge of the end cap, making two passes to provide enough thickness needed. Then, using the card stock as a screed, I work It around the edge applying light pressure downward and toward the outer wall. My goal is to make an even smooth contact surface perpendicular to the sidewall, so I make an additional pass with the screed to remove any uneven areas. After the silicone is smooth, I repeat the same process for the other end cap as well. Then I carefully run a bead of silicone around where the barb fitting meets the cap to assure an airtight seal, and let the silicone cure overnight.
After all the silicone has cured, I remove any excess silicone from the edges of the chamber body and from the inside edges of both end caps. Using the reference marks to align the two chamber body halves, I put them together and place them into the bottom end cap. To help hold things together, I use a leftover piece from one of the end caps and place it over the body as well. Then I place the top end cap onto the body.
Use:
At this point, I assemble my vacuum test kit and attach it's hose to the barb fitting. Then while holding the end caps firmly to the chamber body, I squeeze the vacuum pump handle and begin to draw a vacuum on the chamber. The silicone provides the positive seal required during evacuation. During this process, I use the vacuum gauge to verify that my new vacuum chamber is airtight. This configuration has proven to be capable of sustaining about 29 Hg. I release the pressure through the purge valve on the pump, and once the pressure within the chamber has equalized, the chamber is easily disassembled.
The way the chamber works is that the PVC pipe was originally designed to fit very tight into the cap using a bonding agent such as PVC glue. However, once I cut the pipe with the hacksaw, I removed approximately 3/32" of total material from the circumference of the pipe. This allowed the two halves to fit easily into the caps without binding. By adding the silicone to the adjoining edges, it replaced the cut material with a flexible airtight seal. The silicone placed within the end caps also provided a positive seal against the rim of the pipe also. After testing this prototype, I duplicated the chamber body using a 6" length of PVC and was able to use the same end caps. This test verified that the device was modular. That meant that I could virtually create almost any length of a chamber and still use the same end caps I believe the concept would also apply to a larger diameter PVC pipe, however, there may be a limitation on the maximum diameter due to a possible risk of implosion. As I scale the size of the chamber up, I may find it necessary to design an inner support ring to minimize this risk. I plan to make prototypes using 6" and 8" diameter pipes and will likely limit the length of the chambers to 12".
I realize there are a few issues that may need to be addressed before I can use the chamber efficiently for my casting projects. For one, the bottom cap should be smoothed across the entire surface before pouring a mold to keep it from sticking to the cap. Also having a base to hold the chamber upright and stationary would be helpful. Once I work out those issues, I may even use a clear tube with similar properties for the chamber body in order to see the effects of the vacuum in use.
Project Tips:
Using a new hacksaw blade will minimize distortions in fabricating the channels in the body.
It's important not to overwork the silicone. Smoothing the silicone over with no more than a couple of passes early prevents uneven gelling and works better than several passes or waiting too long.
It's recommended that when extracting air from liquids using vacuum, that the container body should be about 4 times the volume of the amount of liquid to allow for expansion during the process.
Factoids:
A standard vacuum cleaner produces enough suction to reduce air by around 20%.
Even outer space is not considered a perfect vacuum due to an equivalent of a few hydrogen atoms present per cubic meter on average.
The first laboratory vacuum was produced by Evangelista Torricelli in 1643.
The use of vacuum in incandescent light bulbs protects the hot filament from oxidation.
Tags:
vacuum, chamber, pressure, vessel, container, mold, casting, hobbyist, DIY, prototype, invention, fabrication, testing, cool science