Meet Wall Lessons Learned: Cast Vs Extruded Acrylic

We here at AMLGM learned some important lessons while building our MeetWall for the Market Street Prototyping Festival. In this instructable we will discuss the best strategies for working with cast and extruded acrylic for laser cut mechanical fabrications.

First things first, here is a description of the overall project for which laser cut acrylic was an integral part:

video link: http://www.amlgmlabs.com

“MeetWall is an interactive intervention that facilitates the serendipitous social interaction between passerby’s on Market Street. MeetWall is composed of an array of lightweight tiles that rotate about their center in response to sensor input. MeetWall’s Kinect sensors monitor proximity and location of those within range of the wall. As one gets closer to the wall, the tiles are activated, and rotate to 45 degrees to allow some transparency. When two people are aligned at either side of the wall, the panels in the zone of said people rotate to 90 degrees, allowing full visual connection. This sudden and unexpected face to face with a stranger provides a unique experience that strips away the anonymity of the urban street. During phases where no individual is within immediate range, the wall flutters and flirts, inviting people to investigate.

AMLGM is a San Francisco based practice engaged in the interdisciplinary blend of modernism, and the logic of design optimization. AMLGM seeks out complex urban problems that can be solved architecturally via sector disruption and leveraging of the opportunities provided by new technology. The practice continues to develop concepts for the evolving urban condition.

We at AMLGM recently completed a kinetic wall installation for the Market Street Prototyping Festival (see project description) and here are some lessons learned and best practices for fabricating the tiles”

Let's break out those Calipers!

Step 1: Things to Have

To do this right you will need the following:

1 calipers

2 sheet of cast or extruded acrylic

3 plastic scorer for cutting down larger sheets

4 access to a laser cutter

5 CAD software

Step 2: Determine Design Tolerances

Your tolerances depend on your project. If you are designing something that involves gears and snap fittings, and precise coordinated moving parts, you will probably want to work with a tight tolerance. For the gears and and snap fittings in our project, we found we were operating with tolerances down to the .01 mm level.

When thinking about your tolerance, the material you specify will have an inherent tolerance as well that you have to take into account. Basically your tolerance can only be as precise as the tolerance of the material you are working with (in this case the thickness of the acrylic).

So - that brings us to extruded vs. cast acrylic (this is something you want to pay very particular attention to):

First things first, lets address the differences in fabrication between cast and extruded acrylic: cast acrylic is made by pouring the resin into big flat trays where is cures into one big sheet extruded acrylic is made by extruding, or pushing, the resin through a die to form a long flat sheet. The result is that cast acrylic has a wavier and more uneven surface (which results in a changing thickness across its length) and extruded has a much more consistent thickness.

These variances in the standard thickness range in difference from 0.1 - 0.01 mm in the cast acrylic which is a huge deal if you are trying to fit parts with a tight tolerance. Cast acrylic usually comes with a brown paper covering and is usually opaque colors. Extruded is typically clear and comes with a clear plastic cover. Cast acryllic tends also to be harder, but a bit more brittle whereas extruded is a bit softer, but more ductile. Both laser cut just fine. In short, if tight tolerances are of a concern for you, we recommened that you use an extruded product. Here is a link to a more in-depth article: http://www.accessplastics.com/cast-vsextruded-acry...

Step 3: Measure Your Material...and Design to It!

Now that we know the differences between cast vs. extruded, lets get into measuring the actual material you have. Although each material is called out as a nominal dimension, almost certainly the actual dimension will vary from one production batch to the next. Thus, it is important to measure your specific material, and use that as your basis of design.

This is a lot easier and more precise with a trusty pair of calipers.

As you can see in the photo, the white cast acrylic varies a significant amount in thickness over its length. Not good if you need to mass produce consistent parts.

Extruded Acrylic, while more consistent in thickness, can vary from one production batch to the next. We recommend estimating the amount of acrylic you will need, and buying in bulk all from one production batch. This will give you sheets of acrylic with the most consistent thickness. This also means that you won't be able to finalize your cut templates till you have purchased and calipered your material. Knowing this ahead of time will save you a significant amount of time. There is no point in doing step 4 without having purchased the material first.

Step 4: Test and Refine

Once you have ascertained your true material thickness, adjusted your drawings, and set your initial tolerance, its time to test on the laser cutter. This is important to do before you start your production!

Each laser cutter will cut slightly different based on series of factors (i.e. from product to product, power, clean lens, etc) So designing a concise test swatch to test the key connections and key tolerances will save material and time. See image above.

We laid out what we thought our initial tolerances may be based on the caliper measurements. Then we created a small test swatch with the same cut geometry, increased and decreased the tolerance by .1mm initially and later by smaller and smaller tolerance to refine the connections. In some of our last tests before production, we were stepping our tests by as little as .01mm to achieve great fits.

Once you have found the perfect fit, its time to update the cut files to reflect the ideal tolerance and then time to start production!