Recycling 3D Prints and Waste Plastic into Filament (PET & PLA)

Main Ideas, Concepts, and Lessons

• Goal of the video (Part 2): Demonstrate how waste plastics—especially unwanted 3D prints made of PLA—can be mechanically recycled into usable 3D printer filament by shredding/grinding and re-extruding material.
• Core theme: Recycling plastics into filament is more difficult than it looks because real-world inputs are inconsistent, especially in:
  • Cleanliness and sorting
  • Particle size and shape (affects flow and extrusion stability)
  • Moisture removal needs
  • Thermal history / degradation
  • Hydrolysis risk (PLA especially)
  • Economics and throughput constraints

Key Process Stages Introduced

1. Reclaiming (shredding large plastic parts)
2. Granulating (secondary shredding to suitable fragment size)
3. Drying (removing moisture)
4. Extruding (testing filament viability; handling bulk density/thermal history issues)
5. Pelletizing and re-extruding (workaround to homogenize material properties and blend ratios)
6. Economics and why it’s usually not profitable versus cheap virgin pellets

Second Major Transition: PET Bottles

• The video also explains that PET bottles can be recycled into filament.
• However, the same desktop-style recycling approach has major limitations with thin-film bottle plastic, unless supported by better shredding and material-handling strategies.

Final Takeaway

Maker-grade equipment is best at converting unwanted 3D prints into filament. Recycling household plastics (bottles/containers) typically requires more specialized industrial equipment to be reliable and economical.

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Methodology / Step-by-Step Instructions

A) Recycling Unwanted 3D Prints (PLA Parts) into Filament (Mechanical Workflow)

1) Source & Sorting

• Use resin/filament-type-specific sorting bins so students can sort unwanted prints.
• Only recycle parts that are:
  • Not painted
  • Not glued
  • Free of screws and heat-set inserts
  • Made from the base plastic (e.g., PLA for the PLA stream)
• If the plastic is dirty, wash it.
• Expect color inheritance:
  • Output trends brown/black depending on input.
  • Recoloring to a lighter color is not possible.
  • Therefore, sort by color where possible.

2) Feed Preparation for the Reclaimer (Size Constraints)

• The filament extruder needs input around 3–4 mm diameter (small enough for screw flights).
• Large parts must be shredded first because they’re too big to feed the screw.

3) Shredding with the Reclaimer (Two-Stage Blade Sets)

• Place large parts into the hopper(s).
• Reclaimer internal operation:
  • Right side “shredder”: large teeth grab parts and crush them against chamber walls.
  • Pieces too large continue getting recut across repeated passes.
  • Smaller pieces exit into a collection bin.
• Example timing: ~30 minutes to shred about 1 kg of plastic parts.
• Output characteristics:
  • Typical fragment size after shredder: 8–12 mm (too large for direct extrusion).

4) Secondary Shredding / Granulation

• Run shredder output through a granulator with thinner/smaller blades.
• Goal: finer, more uniform regrind particle size suitable for extrusion.
• Example timing: ~15 minutes to granulate about 1 kg.
• Result: regrind becomes ready for extrusion.

5) Desktop-Machine Limitation (Practical Constraint)

• The reclaimer can’t accept very large pieces.
  • Example: a 3-inch diameter sphere is near the maximum hopper-friendly part size.
• Shape also matters:
  • Intricate geometry shreds better (more edges/texture).
  • Round/flat objects can bounce and resist shredding.
• Because of these limits, household recyclables (like common containers/bottles) are harder to process efficiently in this setup.

6) Moisture Drying

• Dry the regrind in an oven after granulation to reduce moisture.
• Why: shredding increases surface area, increasing water absorption.
• Moisture can cause bubbles/voids in extruded filament.

7) Extrusion Test and Observed Physical Behavior

• Compare pellets vs regrind melting:
  • Regrind has lower bulk density (melts less densely for the same volume).
  • During melting, regrind forms clumps and flows less uniformly than pellets.
• Explanation: thermal history
  • Regrind already experienced prior heating:
    • extruded into filament
    • printed into parts
    • then remelted/granulated (extra thermal cycle effects)

8) Thermal History & Mechanical Performance Limits

• Concepts:
  • Plastics degrade with repeated heating (thermal history concept).
  • PLA degrades significantly under thermal stress (including hydrolysis when heated with water).
  • Real-world PLA recyclability is limited; industrial systems blend recycled material with virgin pellets to preserve mechanical properties.
• Key operational issue:
  • Mixing recycled regrind and pellets is hard due to variable particle size distribution and bulk density differences, leading to inconsistent extrusion.

9) Attempted Single-Pass Regrind + Pellet Extrusion (and Why It Fails)

• Conceptual model: each screw flight can fill differently with pellets vs lower-density regrind.
• Experiment outcomes:
  • With lower regrind percentage (~25% by weight):
    • Filament diameter initially within tolerance
    • Output becomes too variable to control reliably
  • With higher regrind percentage (~50% by weight):
    • Diameter instability increases
    • Overheating increases; viscosity drops
    • Soft filament piles near die
    • Spooling becomes impossible due to inconsistent output/overheating

10) Solution: “Re-Pelletize” Regrind to Standardize It

• Workaround: create uniform-sized recycled pellets before blending again.
• Steps described:
  • Use a 3 mm die (replace extruder die)
  • Clean out fines (small overly-ground particles) using an air tool (e.g., “Civ” mentioned)
  • Extrude 100% regrind into temporary 3 mm filament at slower speed to reduce overheating
  • Pelletize the 3 mm filament into ~3 mm pellets for uniform mixing
  • Then blend recycled pellets with virgin pellets for final filament production

11) Pelletizing Setup (3 mm Pellet Production)

• Use a pelletizer with:
  • Cutter motor (example: 1/2 horsepower)
  • 6-flute cutter (using an HSS cutter mentioned)
• Pellet length control via:
  • Cutter RPM
  • Feed rate
• Suggested pellet size:
  • ~3 mm length for mixing/conveying consistency

12) Final Blending Ratio for Producing Filament

• Rationale (study-backed idea): each recycling pass reduces mechanical properties due to polymer chain degradation and viscosity changes.
• Maker constraints:
  • Limited availability of recycled feedstock
  • Theoretical target: at most 50% recycled to preserve properties
  • Typical practice: ~25% recycled due to capacity limits
• For the video’s demonstration:
  • Uses a 50/50 blend
  • Adds half the normal colorant, because recycled material already contributes color (red in the demo)

13) Final Extrusion

• Blend in the hopper and extrude to produce final 3D printer filament (referenced contextually as 1.75 mm after die swaps earlier in the narrative).

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B) Recycling PET Bottles into Filament (Constrained Approach vs Better Shredding)

1) Eligible Plastics (Resin Identification Codes)

• Use resin ID symbols (numbers in triangle):
  • #1 PET (water/soda bottles): best candidate
  • #2 HDPE, #4 LDPE, #5 PP: avoided
• Reason given:
  • Higher crystallinity makes #2/#4/#5 harder to print (high warp).

2) Avoid Mixing PET with PETG

• PET and PETG have different processing temperatures and should not be mixed.
• PETG container designations were also affected by California law, making PETG less economical for single-use recycling.

3) Bottle Processing Plan (DIY Approach Fails)

• Start with PET bottles.
• Attempted approach: consumer DIY “bottle peeler + extruder”
• Problems:
  • Very low throughput (long time per bottle)
  • Need to inflate/remove creases and heat treat to flatten
  • Output behaves more like tube than solid fiber
    • leads to underextrusion on printers
• Conclusion: unsuitable for meaningful-scale recycling.

4) Use a Higher-Throughput PET Bottle Shredder (“Shreddy”)

• Use ActionBox’s PET bottle shredder (mentioned as “Shreddy” v5.0).
• Requirements:
  • Remove bottle cap/seal (often not PET)
  • Remove labels (other plastics/materials)
  • Rinse bottles holding other chemicals
• Advantage: strong motors + aggressive teeth suited to thin-wall bottle plastic
• Example claim: only seconds per bottle

5) Continue the Same General Pathway

• Shred → granulate/prepare → extrude
• Bottle shred output is too large to feed directly into the screw extruder.
• The material is granulated to achieve suitable particle size.

6) Flowability Problem (Thin Film PET)

• PET flakes often have poor flowability.
• Measured via angle of repose:
  • PET pellets flow well (angle < ~45°)
  • PET flakes behave near ~90°, meaning they won’t spread/flow into the hopper.
• Consequences:
  • Inconsistent feed rate
  • Inconsistent filament diameter or die plugging
• Workaround:
  • Manually “cram” material into the feed port if flow is too poor.

7) Conveyance Inside the Screw (Friction & Plugging)

• Simplified mechanical example:
  • Low friction between smooth/low-density flake and barrel can cause spinning with the screw rather than advancing axially.
  • Barrel heat can prematurely melt spinning material → plugging risk.
• Industrial approach:
  • Grooved barrels and conical compression designs to increase friction and bulk density.
• Maker workaround:
  • Mix in some pelleted PET from testing to improve friction and bulk density
  • Run extrusion nice and slow to reduce premature melting (spooling may still be difficult)

8) Re-Extrude After Pelletizing

• Staged process:
  • bottle flake → 3 mm filament → pelletize → re-extrude into 100% recycled spool
• Example outcome:
  • ~4 hours of on/off work produced about 1 kg of PET filament

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Economics / Logistics Emphasized

• Waste-to-filament economics are usually unfavorable at maker scale because:
  • Virgin PLA pellets are cheap (example: ~$6/kg bulk)
  • Equipment cost is high:
    • Filabot sells reclaimer + pelletizer as a pair for ~$12,000 (as stated)
  • Break-even requires recycling extremely large quantities (example: ~2,000 kg to return investment)
• Time estimate for 1 kg recycled material:
  • Reclaimer shredding: 30 min
  • Granulating: 15 min
  • Extruding regrind (slower): 1 hr
  • Pelletizing: 15 min
  • Re-extruding final product: 30 min
  • Total: ~2.5 hours per kg
• Conclusion:
  • With labor included, it’s unlikely to make money; the main value is educational and process-learning.
• Policy note:
  • If taxes/policies raise virgin resin prices, home/maker recycling could become more economical.

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Speakers / Sources Featured

• Dr. D (Dr D Flow) — primary presenter; also narrator explaining experiments and economics.
• Filabot (brand/company) — referenced as the source for reclaimer/extruder/pelletizer equipment; also mentioned for pricing/model context.
• ActionBox (YouTube channel) — source of the PET bottle shredding machine mentioned as “Shreddy” (v5.0).
• Dave and Allen — referenced as designers/creators associated with “Shreddy”.
• “Researchers” / studies — referenced generally (no specific author names) regarding:
  • PLA recyclability limitations
  • mechanical property loss
  • molecular weight reduction
  • thermal degradation effects