Thursday, September 15, 2016

Plastics

This post is a basic overview of plastics, some common types, and their properties. There's a particular emphasis on some of the plastics used in digital fabrication processes (3D printing and vacuum forming).

Etymology

The word plastic is derived from a Greek word (plastikos) meaning "capable of being shaped or molded". Plasticity is the general property of all materials that are able to irreversibly deform without breaking. This occurs to such a degree with plastics that their name is an emphasis on this ability.

Types

There are two basic types of plastics:

  • Themoplastic
  • Thermoset

The primary difference between these two is that thermoplastics can be remelted back into a liquid and then reshaped and reused. Thermoset plastics always remain in a permanent solid state. 

Thermoplastics Plastics

Thermoplastics pellets soften when heated and become more fluid as additional heat is applied. The curing process is completely reversible as no chemical bonding takes place. This characteristic allows thermoplastics to be remolded and recycled without adversely affecting the material’s physical properties.

Most materials commonly offer high strength, shrink-resistance and easy bendability. Depending on the resin, thermoplastics can serve low-stress applications such as plastic bags or high-stress mechanical parts.

Thermoplastics Pros

  • Highly recyclable
  • Aesthetically-superior finishes
  • High-impact resistance
  • Remolding/reshaping capabilities
  • Chemical resistant
  • More environmentally friendly manufacturing

Thermoplastics Cons

  • Generally more expensive than thermoset plastics
  • May melt if heated


Thermoset Plastics

Thermoset plastics contain polymers that cross-link together during the curing process to form an irreversible chemical bond. The cross-linking process eliminates the risk of the product remelting when heat is applied, making thermoset plastics ideal for high-heat applications such as electronics and appliances.

Thermoset plastics significantly improve the material’s mechanical properties, providing enhances chemical resistance, heat resistance and structural integrity. Thermoset plastics are often used for sealed products due to their resistance to deformation.

Thermoset Pros

  • More resistant to high temperatures than thermoplastics
  • Can be used for both thick to thin wall products
  • Higher levels of dimensional stability
  • Cost-effective


Thermoset Cons

  • Cannot be recycled
  • More difficult to surface finish
  • Cannot be remolded or reshaped


Common Plastic Types and Uses

A brief list of some commonly used plastics: 

Polyurethanes (PU) – Used in cushioning foams, thermal insulation foams, surface coatings.
Polycarbonate (PC) – Used in compact discs, eyeglasses, riot shields, security windows, traffic lights, lenses.
Polylactic acid (PLA) – A biodegradable, thermoplastic derived from lactic acid commonly used in 3D printing.
Acrylonitrile butadiene styrene (ABS) – Used in 3D printing, electronic equipment cases (computer monitors, printers, keyboards).
Polycarbonate/Acrylonitrile Butadiene Styrene (PC/ABS) – This is a blend of PC and ABS that creates a stronger plastic. Used in car interior and exterior parts, and mobile phone bodies.
Polyester (PES) – Used in fibers, textiles.
Polyethylene terephthalate (PET) – Used in carbonated drinks bottles, peanut butter jars, plastic film, microwavable packaging.
Polyethylene terephthlate Glycol-Modified (PETG) – Used in vacuum formed products.
Polyethylene (PE) – Used in a wide range of inexpensive uses including supermarket bags, plastic bottles.
High-density polyethylene (HDPE) – Used in detergent bottles, milk jugs, and molded plastic cases.
Low-density polyethylene (LDPE) – Used in outdoor furniture, siding, floor tiles, shower curtains, clamshell packaging.
Polyvinyl chloride (PVC) – Used in plumbing pipes, window frames, flooring.
Polypropylene (PP) – Used in bottle caps, drinking straws, yogurt containers, appliances, car fenders (bumpers), plastic pressure pipe systems.
Polystyrene (PS) – Used in packaging foam ("peanuts"), food containers, plastic tableware, disposable cups, plates, cutlery, CD cases.
High impact polystyrene (HIPS) - Used in refrigerator liners, food packaging, vending cups.
Polyamides (PA) (Nylons) – Used in fibers, toothbrush bristles, fishing line.


3D Printing Plastics

A comparison of the properties of some common 3D printing plastics: PLA versus ABS.

PLA - The wide range of available colors as well as translucent material is often desirable. PLA can also have a glossy feel to it. The plant based origins are appealing as is the semi-sweet smell as compared to ABS. When properly cooled, PLA generally has a higher maximum printing speeds, lower layer heights, and sharper printed corners. Combining this with low warping on parts make it a popular plastic for home printers, hobbyists, and schools.

ABS - Its strength, flexibility, machinability, and higher temperature resistance make it often a preferred plastic for engineers, and professional applications. ABS has a petroleum base which is less applealing to some. The hot plastic smell deters some from its use. The requirement of a heated print bed means there are some printers simply incapable of printing ABS with any reliability.


Recycling

The greatly expanding use of plastics in the early 20th century resulted in environmental concerns due to its slow decomposition rate after being discarded. Thermoplastics can be remelted and reused. Thermoset plastics can only be ground up and used as filler material (although the purity degrades with each reuse).



  1. PET (PETE), polyethylene terephthalate
  2. HDPE, high-density polyethylene
  3. PVC, polyvinyl chloride
  4. LDPE, low-density polyethylene,
  5. PP, polypropylene
  6. PS, polystyrene
  7. Other types

Wednesday, August 31, 2016

CNC Router Rotary Axis

This post documents adding a rotary axis to my CNC Router Parts router.

My router was a 3-axis machine. Adding a 4th rotary axis allows considerably more flexibility as the part to be machined can be rotated to expose different faces for cutting. I'd previously done some 4-axis router work at Stamps School of Art & Design - here's that post. I really wanted to add this capability to my router.

After exploring a few options - most of which were very expensive - I chose to go with an inexpensive kit from China. It was only $358 including shipping. I'd have to figure out how to get it hooked in to the electronics then build my own mount.

Rotary Axis Hardware

I ordered it and it shipped from Hong Kong the next day. It was ordered on Thursday June 30th and arrived in Michigan, USA on Tuesday July 5th. Not bad!

It includes a tail stock, a self-centering 4 jaw chuck, a Nema 34 stepper motor, and a 4:1 ratio belt drive. It includes both inside and outside jaws.

Electronics Hookup

To connect it to my setup I had to solder on a 4 pin XLR male connector.

Then using one of CNC Router Parts standard cables I could plug directly into my controller:

I had to set the dip switches in the electronics cabinet to match the 4.8 amps of the motor (the other motors I'm using are 7 amps). Then set the steps per revolution in Mach3 and it was ready to run.

Under Table Mount

I went through several iterations when designing how to mount the motor/chuck and tailstock. The requirements were:
  • It had to be always hooked up - never removed from the machine. I wanted to be able to use it without much hassle. 
  • It had to be moved out of the way when I wanted to 3-axis route using the entire 4'x4' bed. 
  • It had to be rigid when locked in place. 
  • And it obviously had to support a variety of part sizes including wide ones. 
I messed around with a lot of ideas. One was cantilevering the parts from one of the horizontal rails in the frame. But I thought this would be too much torque on the frame - the motor/chuck weighs 19 pounds.

In the end I decided to make it out of aluminum extrusions. I  choose components from 80 / 20 Inc. They have quite a few parts including sliding ones. And a great CAD library of their parts. So I could fully 3D model the design.

This is the final design, as seen from below the table. The existing frame is light blue. The new components are gray. The handles shown lock movable parts of the assembly.

Here's a front view of the frame in the lowered, beneath the table position, so 3-axis routing can happen over the full table:

Here's the frame raised up, slid up those verticals, ready to route. The center line of the axis is right at the top of the table. That's the lowest reach of the spindle:

Here's the tail stock moved forward for a smaller part. It slides along on the horizontal rail:

This configuration allows for a part that's 22" long and 14" in diameter.

After having 80/20 review the design they gave me a quote. The parts arrived at my house in just under two weeks.

Assembly

The parts were easy and enjoyable to put together. The linear bearings as 80/20 refers to them work quite well. Things slide smoothly and lock solidly,

Loosen the four corner yellow handles and you can lower the entire assembly beneath the table.

In the up position the centerline of the axis is right at the top of the frame which is close to the lower limit of the reach of the tool.

Chuck Setup

The 4th axis has a four jaw self-centering chuck which has to be assembled in a particular sequence. If this isn't followed the jaws will not all line up correctly. This process is outlined below:

Identify the numbered jaws and the numbered slots. The jaws are labelled on their side. The slot numbers are stamped into the back face of the slot (you'll need a flashlight to see them).

  • Using the key, turn the scroll observing the number one slot until the outermost end of the scroll appears. You should see the edge of the thread just appearing to enter the slot. 
  • Next turn it back a little just enough to allow the #1 jaw to enter the slot. 
  • Then push the jaw down as far as it will go (which isn't very far). 
  • Next, using the key turn the scroll so that it engages the first tooth of the jaw and starts to pull it inwards. You can give it a light tug to make sure it's engaged. 
  • Rotate the chuck until the next slot is up then turn the scroll until the start of the thread appears in the next slot. 

Repeat the above with the #2 jaw and continue with jaws 3 and 4. This procedure ensures that all the jaws are synchronized with the scroll. Here you can see them coming together properly, and aligned with the point of the tail stock:

Toolpath Programming

There are several possibilities for how to program the cutting. A powerful, but very simple method is to simply rotate the part to a new face, then 3-axis mill it from that position.

I'm using my standard 3-axis router profile. I manually rotate the chuck using the MDI interface of Mach3. I just type in G90 G0 A90.0 and it rotates to an absolute rotation of 90 degrees. Then I run the toolpath from the side. Doing G90 G0 A-90.0 and it rotates to the other side. It's really easy, and of course keeps the simple 3-axis toolpath programming.

An alternative is to rotate the part as it is cutting much like a slow speed lathe.

Summary

I'm excited to finally have this running. First work will be some portrait sculptures much larger than I've been able to do in the past.

Saturday, August 27, 2016

New Profile Linear Rails for CNC Router

CNC Router Parts, the company which makes the parts kit for my router, came out with a new version of the system. The most significant change was using profiled linear rails and bearings. These happen to be the same bearing type used on the Onsrud routers at the Taubman College FABLab I'm used to using. So I decided to give these a try and ordered an upgrade kit. The parts shipped about one and a half weeks after ordering.

Disassembly

The first step was to strip the router down to the frame. It started here:

The current rail system looks like this. The rails are easiest to see at the top of the picture - hardened steel rails held at a 45 degree angle by brackets.

It was easy to disassemble and I was able to leave all the electronics intact except for simply disconnecting the motors. I removed the spoilboard and took the opportunity to re-tune the frame: making sure everything was square, level and aligned flat.

To be replaced are the hardened steel rails, the carriages for gantry, and the entire Z-axis assembly. I'll be reusing some of the plates as reinforcement for the frame and base. But I now have a LOT of spare parts!

Installation

With the frame cleared the new system could be installed. The rails are polished steel. They are fastened to the extrusions with T-nuts every few inches:

The linear bearings slide along the rails and are attached to other parts with four screws. The black plastic plate in the middle of the bearing keeps the ball bearings in place until the unit is slid onto the rail.  There is a grease fitting on one end and they periodically need to be re-filled.

The new rails get aligned parallel to the top using the provided fixture which keeps them 10mm from the top of the extrusion:

Once the rails are installed the new gantry supports are put in place. These are about 50% thicker than those they replace. This design seems more refined than what was there before. Attached to these supports are the two bearings which grip the rails.

New vertical extrusions are attached to the gantry supports.

The horizontal member of the gantry is now supported on 1" thick aluminum plates. These are secured through the plate into the end of the vertical extrusion. Then the gantry is secured from below, and using a face plate from behind. This system feels a lot more secure than what was used before. 

There are new linear rails on the gantry for the Y axis travel. 

Four linear bearings are attached to a plate which supports the Z axis. There's no detectable play in it whatsoever. There's a new Z axis assembly. It is better in that the motor no longer moves up and down. Instead just he plate the spindle is attached to moves. This means you don't need a cable carrier now. The Z axis assembly is on the table still to be installed.


Here's the Z axis and spindle installed:

I removed the spoilboard to check if the Y axis (across the front of the machine) was perfectly parallel to the front rail. If not I'd adjust the proximity sensors at the back to further tune the square. Things are looking good!

Final Tuning

With everything up and running it was time to check how accurate things are. The first step was to check for runout on the spindle. I put a 1/4" endmill in the spindle and ran it at 8,000 RPM. Then I moved a dial gage onto an area of the shank with no flutes to see if the needle moves. No motion I can see in the needle.

Next is to bore some holes into a sheet of melamine to test for square. If it's not square you can compensate by adjusting the number of steps per inch in the motor setup. I reset both X and Y to 2038 steps. Then milled a pattern of 3/8" holes which measure outside to outside at 8" square.

The holes are milled to 0.379". The dowel pins are 0.374". It's a tight fit. It measures 8.000" exactly in both directions. So far so good.

Next to check the diagonals. They each measure 11.159". Nice!
 

Summary

This new rail system has a much more solid feel. There is no play at all in any axis of motion. It's also much simpler to put together. This is a real step forward for their kits. I see they also improved the frame (no more corner brackets which can slip). And a much better cross brace setup for the base. CNCRouterParts is really moving things forward.

Wednesday, May 25, 2016

Slip Casting Update

This post is an update to my earlier post on slip casting 3D printed forms. It contains some of the new designs I'm working on and some pictures of the earlier finished pieces.

I've gotten some of the first pieces glazed and fired. The 3D print is on the left. The first glazed/fired sample is on the right. They shrink (about 11-12%) during the first firing. The top gets sanded flat making them smaller still.



Glazing

The first step after firing is to rinse them off. The first firing leaves some residue and you want a clean surface for the glaze to adhere well.

The next stage is to put some wax on the bottom of the pieces so they don't get any glaze there which would stick them to the kiln shelf:

I've come to learn that it's better to put a "foot" on the form. In this way you can glaze all the way to the bottom of the visible form - the foot will hold it up keeping the glaze off the kiln shelf.

I really wanted to experiment with the glazing. At the Ann Arbor Art Center, this is how the glazes are stored - 5 gallon buckets with a brush to stir them up. The solids settle out and need to be mixed back in - it takes a few minutes for some that haven't been used in a while:

The vases get dipped using tongs. The vase in the rear has two dips which overlap. So there will be three different colors. The vase in the front is a solid color from a single dip. Once the glaze has dried it's easy to smooth out the bumps and drips you see. You can also fill in the areas where the tongs hit by just rubbing adjacent glaze into the holes.

Waxing, dipping, brushing, drizzling...

Here are some of the finished pieces:



New Designs

I've also been working on some new designs. The first ones are simple two part molds. This one is shown in the 3D printer at Taubman College Fab Lab.

Here's the printer (on the right):

This is going to be a mug. Quite large in anticipation of the shrinking.

Molds in progress... starting to add the clay to support the form. Note the extra clay around the lip of the cup. This allows the slip cast form to be higher than the final part. It can then be sanded down flat to get it to touch the points at the top of the design. That's necessary because the slip cast form is often uneven at the very top. This also seals the cup against the cottle board so no plaster can flow in.

Fully embedded in the clay. Need to seal the vertical edges and it's ready to pour.

First half done - very crisp detail:

Here's the first one just out of the mold. After it hardens a bit I need to fix the seams, sand the top edge down, and clean it up.

Here are a few of the originals and the new forms ready to bisque fire. I've found a card scraper is really good at shaving the dry clay into a flat plane - perfect for cleaning up the seams:

Here are three after glazing:

Here's another design - about 7.5" tall. It was printed at Shapeways. The Shapeways PLA prints are a little crisper than the ABS prints at Taubman. I actually prefer the ABS plastic for slip casting though. They are harder and clean up easier.

I added a foot after printing (a scrap piece of cherry glued on). Doing this after the printing is better because the print won't require support material at the bottom - the foot would require a large cantilever of the form outside the foot:

This one needs a three part mold. Here are the set-up for the three mold parts and the result:

So the form is buried more than half way - keeping about 120 degrees of the form exposed to the plaster:

Next another third of the form is blocked off. This is ready to pour:

Set up for the final pour which completes it:

The three resulting parts:

Some more pieces getting fixed up for bisque firing - the three piece mold ones take a fair amount of work:

And after bisque firing:

One in bright blue and another in a green/blue:
 

Belle Kogan Slip Cast Forms

I went to a small exhibition at the University of Michigan of the slip casting work of industrial designer Belle Kogan (1902-2000). Here are some pictures of the work. These pieces are from the collection of Bernard and Barbara Banet of Ann Arbor, Michigan.

I love the geometric aspect to some of these pieces, in particular her Prismatique line. These are all two or three piece molds. I find these pieces very inspiring.
Vase, 1962

Vase, 1962

Compote, 1962

Compote, 1962

Vase, 1938

Pitcher Vase, 1938

Vases, 1938

 Gladiolus Vase, 1952

Double Vase, 1953

I was so fond of Belle's work I had to pick one up for myself - off eBay. They are plentiful and cheap as they were mass produced.