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The objective of the assignment is to prepare tooling for an in-class casting mold workshop on Jan 27 and Jan 29. We will group the class into pairs. Each pair will be responsible for fabricating a mold pattern to use in class. On Monday we will prepare the pattern in a mold box and create a rubber mold. On Wednesday we will cast plaster into the rubber molds.

The pattern you create is a positive form in low relief. The resulting single-sided rubber mold will have the negative shape and include side walls to contain poured plaster. The plaster part will be upside-down in the rubber mold; the exposed back surface (on top during pouring) will be left open and be approximately flat.

Pattern Dimensions

Below you will find both sample CAD files and photos of the demonstration pattern and mold. Your pattern object should occupy a volume of no more than 150x150x24 mm (6x6x1 inch) and be mounted on a backing plate large enough to mount the mold box walls with clearance for the side walls of the rubber mold. A reasonable rubber side wall thickness is 9mm (3/8 in), and the mold box walls are 19mm (3/4 in) plywood, so the backing plate is typically at least 28mm (1 1/8 in) larger than the pattern on each side. The backing plate can be lightweight; we recommend 6 mm plywood.

The demonstration pattern below was created using eight layers of laser-cut 3mm acrylic bonded together, although you may choose to make a shallower profile. The pattern includes geometric low relief as well as etched linework. The rubber and plaster can both carry fine detail. The rubber is flexible enough to pull off corners and out of pockets, but please avoid undercuts, and recognize that deep vertical surfaces or narrow holes may make demolding a challenge. Please remember that the final part will be in plaster, so an overly thin pattern may be very fragile.

You may also use other materials in the pattern, e.g. hand-worked clay, wax, or small found objects. The example is image-based, but any sort of literal or abstract geometry or image composition which fits within the bounds is acceptable.

If assembling a pattern from parts, please be sure to bond any layers together firmly; if gluing acrylic, clamp firmly. The rubber has low viscosity and will seep into tiny openings or textures.

Sample Files

The zip includes a SolidWorks template for the pattern and mold. The PDF is the drawing showing critical dimensions.

Demonstration Mold

Materials

We will be using Poly 74-20 Liquid Rubber, a two-part liquid polyurethane.

Please bring a hot glue gun on Monday (if you have one) to use in assembling the mold box.

We will use Plasticine clay to seal openings in the mold box.

We will cast using USG Moulding Plaster, same as for the running mold.

Estimated Volumes and Mass

The bounding box for the pattern has a volume of 540 cc, so the maximum plaster mass is about 800 grams (1.8 lbs), but could be substantially less, depending on the specific contours.

The rubber is typically poured about 6 to 8 mm higher than the highest pattern feature. With 9 mm wall thickness, the absolute maximum block volume is about 850 cc, but of course some of that is displaced by the pattern. The absolute minimum would be about 300 cc (850-540, e.g. a solid pattern. The rubber has a specific gravity near 1.0, so a typical 450 cc mold weighs about 450 grams (1 lb). For reference, the rubber is about $8.50/lb.

Mix Ratios

The PolyTex 74-20 rubber specifies a mix ratio of 1A:2B by mass. In other terms, the Part A component is 33.3% of the total mass, and the Part B component is 66.6%. E.g. 450 grams of rubber requires 150 grams of Part A and 300 grams of Part B. Part B should be stirred before use. The pour time is 20 minutes, so we’ll need to work quickly after the parts are combined. The demold time is 16 hours.

The USG data sheet suggests a range of mix ratios, between 63 to 70 lbs of water per 100 lbs of plaster. In other terms, the water constitutes between 38.7% and 41.2% of the total mass of the mixed plaster. E.g. assuming a mid-range 40% water rate, 1 kg of plaster (2.2 lbs) would use 400 grams water and 600 grams dry plaster.

The objective of the assignment is to prepare tooling for an in-class running mold workshop on Jan 22. We will group the class into six groups. Each group will be responsible for fabricating a running mold sled to use in class. Each individual will be responsible for designing and fabricating a profile blade. During class, each group will produce a plaster part using one or more of the profiles.

As scaffolding, we are providing CAD models and drawings to use as a starting point.

Blade Profiles

We are basing the design of the profiling tool on the existing blades used with the robot. It is an arbitrary standard, but will keep your tools compatible with the robot end tooling for future use. The lab blades are made from milled steel, but it is acceptable to fabricate them from thin acrylic or plywood if additional support structure is provided by including a thicker backing plate.

Following is a mechanical drawing of the existing contours and mounting holes.

Sled

Each group is responsible for fabricating a running mold sled. We will provide a tabletop surface and an 80/20 rail against which to run the sled. The sled must rigidly support the blade and provide guide surfaces to ride along the table and the 1-inch high side rail.

A sketch of a sample design is available in the files below which looks like the following:

Sample Files

Following are the PDF drawing file, sample SolidWorks part, and several sample Rhino parts.

Spring 2020 Objectives

The core principle of the course is that students explore ‘hybrid’ design at the intersection of the physical and the digital by creating a novel design and production system. Students prototype a design and fabrication workflow, then apply it to producing a few sample artifacts. The physical component emphasizes the application of human dextrous skill.

The objective for the MuseumLab collaboration is to focus the projects on architectural interventions within the building.  The prompt for the students will be to develop a system which can produce site-specific 2D and 3D plaster artifacts for potential installation in the building.  A particular site of interest is the large arch in the central hall, and one or more project groups will be encouraged to use this location as a target site for plaster detailing.

In practice, the projects have generally been based on tools based we have in the dFAB lab:

1. computer-aided design software, notably Rhino w/Grasshopper
2. motion capture of gesture, both live and offline, using the dFAB Optitrack
3. robotic fabrication using custom end tools on the dFAB ABB 6640
4. CNC router fabrication
5. laser-cutting fabrication
6. linear and circular running molds
7. polyurethane mold patterning
8. robotic running mold paths

A secondary objective is developing a sample production system which could feasibly be replicated by the museum staff as an exhibit prototype.  This effort could be successful if the tools and techniques were constrained to those easily obtained or already available in the new on-site maker space.  This project would definitely not involve an industrial robot, but could involve basic standard machinery, Arduino-level electronic control, and mechanisms constructed using laser-cutter fabrication.

A final objective is to provide a final demonstration event toward the semester end at which the students show final artifacts, process documentation, and potentially process demonstrations.  Given that these are student projects, there is no guarantee the results will be suitable for permanent installation, but we plan to make artifacts available for temporary installation past the duration of the course.

Plaster

All projects for the spring semester will focus on fabrication in plaster. It is a versatile architectural material with a rich history and body of technique. It can be a challenging target due to the relatively short working time, messy handling, and thermal and expansion properties. The course will utilize traditional running and pattern mold techniques, but in the context of developing novel hybrid workflows which make best use of digital tools and analog processes.

Site Visits

A tentative plan for whole-class visits:

1. architectural survey visit early in the semester to see the building
2. a mid-semester test/demo trip to try out and critique early work-in-progress results
3. a late-semester demo trip to critique mature results
4. a final show

Exhibit Development

A secondary objective is directing at least one project toward means and materials which could be further developed by museum staff into an exhibit. As an example, consider the possibility of a plaster running mold incorporating mechanical or robotic elements.  The traditional running mold uses a metal blade in a carriage sled which is operated back and forth by hand to build up a linear form with a particular profile.  A mechanized variant could incorporate cams, gears, or cable drives to produce programmable variations in the detailing. A similar example would be a plaster-forming  ‘spirograph’ which would use gear-driven blades to create configurable geometric patterning in a medallion.