A pocket device for cutting and joining filament offcuts
At the end of every 3D-printing spool there is a length of filament too short for a complete part. This project designed and built a portable device that cuts the ends of two lengths of PLA and fuses them together into one continuous filament. The cutter gave a clean cut in 18 of 20 attempts. The joint kept an acceptable diameter in 15 of 18 trials, but only 8 of those 18 joints had high or medium mechanical strength.
Context
A filament joint must meet two conditions to pass through a printer. Its diameter cannot exceed the usual tolerance of 1.75 mm filament, which is ±0.05 mm, because a bulge jams the nozzle. And it must withstand the pulling and bending of the feed system without breaking.
The commercial devices and home-made methods reviewed in the thesis solve one of the two, or require manual skill. The goal was to combine end preparation and joining in a single box.
Cutter
If the two ends are not flat, air is trapped and the joint is weak. The cutter uses a scalpel blade mounted on a button that slides on two shafts with springs. The blade's path is fixed, the cut comes out straight and the button returns by itself. The blade stays inside the housing.
The first button required too much force and the cuts were not repeatable. It was lengthened, given more thumb contact area, and the springs were added.
Button with blade, shafts and springs in the aluminum lid
Fuser
Two halves of an aluminum block form a channel with the diameter of the filament. Each half has a 40 W cartridge heater and an NTC thermistor, controlled by a RAMPS 1.4 board with a program that heats to the set temperature and then switches off. When the box is closed, the channel acts as a mold and the joint keeps the filament's cross-section.
Schematic section: aluminum box, insulating silicone, hot block and heaters
The hot block is surrounded by silicone so that the outer box does not heat up. Using Fourier's equation for a plane wall and the conductivity of the silicone used, the minimum thickness was estimated at between 0.83 and 2.22 mm, and 2.77 mm with a 25% margin. The prototype ended up with walls between 5 and 8 mm because of manufacturing constraints.
Machined aluminum box with the silicone mold
Blocks with heaters and thermistors installed
The box was first printed in PLA to tune the hinge and the cutter, and the final version was machined in aluminum. The prototype weighs about 235 g, measures 3.24 × 5.95 × 4.5 cm and cost about 742,000 Colombian pesos as a one-off.
Results
Filament end before and after the cutter, under the microscope
Twenty filaments were cut, from two brands and from unidentified offcuts. In 18 the surface was flat and free of burrs, and each cut took about two seconds.
Eighteen joints were made with different combinations of filament, temperature and time.
| Criterion | Joints | Percentage |
|---|---|---|
| Diameter ≤ 1.80 mm | 15 of 18 | 83.3% |
| High or medium strength | 8 of 18 | 44.4% |
| No strength | 10 of 18 | 55.6% |
Two joints between filaments of different origin
Joint zone seen under the microscope
The joints that worked were those between filaments of the same class and hardness, at 195–200 °C for 8 to 10 seconds. When the two lengths were of different brands or colors, one began to degrade before the other melted. The 190 to 210 °C range given by manufacturers is not enough as a single setting: brand, pigment and storage conditions shift the right temperature.
An offcut joined to the end of a new spool
What is missing
More than half of the joints do not hold, and the main use case, joining offcuts of mixed origin, is precisely the one that fails. Strength was classified in qualitative categories, without an instrumented test. The document does not report prints made with joined filament, so passage through the extruder and nozzle is untested.
Temperature control is on-off, and the author recommends a PID. There is no active cooling, so the user must wait before removing the filament and there is a risk of burns. The small parts were hard to manufacture and required reprints and manual adjustment. Only PLA was tested.
How it fits in Robiolab
The project belongs to the digital fabrication line that supports the lab's experimental work, together with the filament extruder and the DLP printer. It has no bioinspired component. Its contribution is a documented design, with drawings and code, and a result that is useful for whoever continues it: the mold geometry solves the diameter, and what remains to be solved is thermal control according to the material.