Recycling for 3D Printing

Campus Climate Lab · BU Sustainability · RASTIC

Campus-based system to wash, shred, and dehydrate post-consumer and lab-generated plastics into uniform, extrusion-ready feedstock for closed-loop additive manufacturing.

Waste Diversion Goal

Mean Moisture Loss

0.00%

Std Dev (10 reps)

±0.00 g

Dehydration Setpoint

0°C

PLA Decomp. Onset

~0°C

Team

Nik Polyakov · Reuben Danyali · Declan Lacey (ECE)

System Pipeline

Collect, Shred, Wash, Separate, Dry, Extrude

The system transforms heterogeneous campus plastic waste through six staged subsystems into pellets or filament-ready feedstock. Each stage was selected through test-driven decision logic.

Collect

Mixed campus streams PLA, PP, PE, PET

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Shred

Dual-shaft rotary shear Controlled particle size

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Wash

Ultrasonic cavitation Shred-first principle

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Separate

Density separation PP/PE float, PET sinks

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Dry

Thermal + desiccant Silica gel chamber

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Extrude

Extrusion-ready feedstock output

1. ID Risks

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2. Run Tests

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3. Pick Arch.

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4. Instrument

Dehydration Experiments

10 reps

Series B · 0.71% mean loss · σ = 0.191 g

Residual moisture is a primary cause of extrusion defects. Absorbed water vaporizes during melt processing and forms internal porosity. Two cook series were run at 60°C, 1 ATM for 15.5 to 17.6 hours. Testing led by Reuben Danyali.

Hypotheses Tested

H1Vacuum lowers water boiling point and accelerates dehydration by increasing volatilization driving force.

H2Vacuum vents trapped gas from irregular fragment cavities and reduces bubble formation during downstream extrusion.

H360°C is far below PLA thermal decomposition onset (~250°C), so degradation risk is negligible at dehydration setpoints.

Series A · Baseline

-0.800 g

Mean mass change (-0.29%). Negligible moisture in as-received PLA confirmed.

Series B · Pre-moisturized (72 hr)

+0.702 g

Mean mass loss (+0.71%). σ = 0.191 g across 10 replicates. True moisture removal confirmed.

Series B Raw Data · 60°C · 17:59 hr · 10 Replicates

Sample	Init. Mass (g)	Final Mass (g)	Loss (g)	Loss %	Note
B1	91.405	90.659	0.746	0.82%	Pre-moist.
B2	105.666	105.004	0.662	0.63%
B3	99.149	98.147	1.002	1.01%
B4	97.329	96.844	0.485	0.50%
B5	97.583	96.786	0.797	0.82%
B6	102.117	101.263	0.854	0.84%
B7	107.839	107.173	0.666	0.62%
B8	101.213	100.751	0.462	0.46%	Shifted
B9	107.303	106.411	0.892	0.83%
B10	76.339	75.885	0.454	0.59%	±10mL

Mass Loss Distribution

Next: vacuum operation at -50 to -80 kPa gauge to assess cycle time reduction. Pressure-sealing vessel with temperature, pressure, and time logging plus safety interlocks to be designed.

Washer Architecture Evaluation

1 of 4

Ultrasonic selected · shred-first

Shred-first, wash-second: shredding exposes internal cavities otherwise inaccessible to cleaning. Four architectures were evaluated before selecting ultrasonic cavitation for its ability to reach irregular fragment geometries without mechanical abrasion.

+ Simple, low cost

- Misses internal cavities

+ Gravity-based flow

- Limited for small fragments

+ Continuous throughput

- High wear risk

Selected

+ Cavitation reaches internal surfaces

- Higher unit cost

Cleaning Mechanisms

Cavitation: Bubbles collapse asymmetrically near surfaces, generating microjets at hundreds of m/s. Mechanical impact removes adhered contaminants from deep within fragment geometry.

Acoustic Streaming: Fluid motion carries detached material away, contributing to cleaning efficiency across diverse fragment shapes.

Validation Test Plan

01Before vs. after contamination and odor comparison (qualitative).

02Residue check: rinse stage added if surfactant residue is observed after draining and drying.

03Controlled variables: wash time, solution concentration, bath temperature.

04Assess whether cleaning solution affects feedstock surface chemistry.

Slurry Transport and Density Separation

PP PEPET PLA

Peristaltic pump · STM32 · NEMA 17

Custom peristaltic pump automates removal of floating plastic fragments from the wash bath, exploiting density differences for a first-pass polymer separation. ECE integration by Declan Lacey.

Microcontroller

STM32

Motor

NEMA 17

Pump Type

Peristaltic

Fallback

Flip Bucket

Polymer Density Separation

PP0.90 to 0.91Floats

PE0.91 to 0.97Floats

PET1.38 to 1.40Sinks

PLA1.21 to 1.25Sinks

Water Line (ρ = 1.00)

PP 0.90

PE 0.94

PLA 1.23

PET 1.39

The STM32 breakout board will serve as primary controller for all subsystems as further I/O is integrated into the final design.

Shredder Subsystem

SinglevsDual ✓

Dual-shaft rotary shear selected

Shredding is central to the system: it increases surface area for washing and drying, improves process consistency via controlled particle size, and reduces bridging in downstream handling.

Single-Shaft + Screen

+ Controlled output size

- Sensitive to film-like plastics

- Blade geometry critical for PET

- Higher wear sensitivity

Dual-Shaft Rotary ShearSelected

+ High-torque, slow-speed, shear-dominant

+ Robust for mixed and irregular streams

+ Reduced gumming risk

Procuring ready-built unit

Why Shredding Enables the System

Exposes internal cavities so ultrasonic cleaning reaches all surfaces.

Uniform fragment size yields consistent drying residence time.

Controlled particle distribution improves extrusion feedstock packing density.

Reduced bridging enables reliable conveyance through downstream stages.

Desiccant Drying Chamber

v1 Threaded

v2 Sliding

Silica gel · threaded seals · PTC heating

Current Build

Threaded-seal prototype. Functional for testing, but threading requires significant torque on the joint and limits ease of access.

Next Revision

Sliding top and bottom doors replace threaded seals. Lower actuation force on the joint enables robotic handling and faster manual access for material loading and unloading.

The current desiccant chamber is a prototype used to validate the drying process. It uses silica gel with a threaded-seal enclosure to maintain low-moisture conditions and prevent reabsorption. Airflow routes through the silica gel bed before contacting plastic, using top ingress and bottom egress. A redesigned version is in development featuring sliding door interfaces on both ends to reduce joint stress and support robotic integration.

Desiccant

Silica Gel

Current Seal

Threaded

Next Seal

Sliding Door

Heating

PTC Integrated

Next Pressure

-50 to -80 kPa

Parameters to be Tuned from Ongoing Tests

01Target moisture content thresholds.

02Required residence time for complete drying.

03Optimal airflow rates through desiccant bed.

04Silica gel regeneration frequency at saturation.