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Challenges in the production of filament for 3D printers

31.12.2017

In 3D printers working in FDM technology (Fused Deposition Modeling), products are formed by melting the polymer and laying it layer by layer.

The raw material in this production are filaments, which are produced by melt extrusion on single-screw extruders (3D printing filament production line is described here) from plastic materials like PLA, ABS, PET, PMMA, PC or TPU.

Picture 1. 3D Printing filament production line.

The plastic material in forms of granules is dryed in specialized dryers in order to eliminate humidity, afterwards it is served in a closed system of feeders to the hopper of the extruder. Simultaneously with the main material, a dye is given, usually in the form of masterbatch. In the plasticizing system, the polymer melts and blends with the dye. The material passes through the extruder reaching the appropriate pressure on the head, which causes the material to „swell” after exiting the extruder.

The die (in the extruder tool) regulates the thickness and shape of the obtained extrudate, which is still deformable and undergoes stretching by the tension of the line. The hot material enters through the inlet to the hot tub, in which for most plastics an elevated temperature is used to reduce profile shrinkage.

Another bathtub with cold water is to ensure dimensional stability at a later stage. Before the filament passes through the roceslis haul-off ,the cooling water is blown out of the strand by compressed air, and the diameter is checked by a biaxial laser measurement. Then the roceslis wound on reels.

Always new materials for 3D printing

3D printing is used in many industries and area of life, the spectrum of potential applications is constantly growing, which leads to a strong trend in the search for new, innovative materials for the production of filaments. New products appear on the market: original colors, unused polymers, blends with the properties desired by customers, high-filled composites and even an eco-friendly filament from recycled plastic.

The first filaments were made of PLA and ABS and they are still the most popular materials, but they have limited usability and unchanging properties – they are hard and brittle. A few years ago, elastic filaments from thermoplastic elastomers became the revolution. The properties of filaments were also changed by additives such as metallic powders or processed wood.

Some polymer manufacturers are taking the next step towards 3D printing and synthesizing special plastics with properties optimized for the FDM method. Such examples may be copolyesters that do not contain styrene, so they do not give off an unpleasant odor during printing. Research roces being carried out on a specialized polymer with high mechanical properties that can be used for 3D printing using the FDM method.

One of the main challenges in development activities in the filament industry is the selection of an appropriate production technology on the extrusion line. Often solutions for the processing of new materials must not only affect the parameters of extrusion, but also changes in the devices. Thanks to the knowledge of our engineers and excellent technical facilities, such studies at Zamak Mercator are very popular and lead to effective solutions.

Wymagania jakościowe filamentu 3D

Filament for 3D printers must meet very stringent quality requirements – a strictly defined diameter and homogeneous core of the line. Any deviation causes deterioration of the print quality or even clogging of the plastic in the printer head. Usually, the accuracy of the filament diameter within +/- 0.02 mm is required, and the ovality (the difference between the largest and the smallest diameter in the cross section) should be below 0.01 mm.

Tworzywo ma znaczenie

For the extrusion roces, the type of plastic and its parameters are very important, in particular MFR (melt flow rate), i.e. the melt flow rate. For extrusion, it is recommended to use plastic grades with MFR below 4 [g / 10 min] ((200 °C / 5 kg) according to ISO 1133). The higher the MFR, the faster and under less load the molten polymer flows. The MFR depends on the temperature and structure of the polymer – its average molar mass.

Extrusion of materials with a higher melt flow roces possible, but may cause problems with the dimensions of the filament. The most common problem is the ovalisation of the strain under the influence of gravity, and the easy flow of the material can even prevent the line from setting. For this reason, high MFR plastics are extruded at temperatures lower than those recommended by the manufacturer.

Problemy i rozwiązania

Obtaining the required quality is often problematic, due to the very large roce of factors affecting the extrusion roces. The strict requirements of the accuracy of the product dimensions and often the non-standard properties of the material are the reason why filament production for 3D printers is one of the most difficult technologies for extruding free plastics.

The most common problems that filament manufacturers must meet are:

  • instability of filament diameter (pulsation);
  • ovalization;
  • air bubbles in the core of the filament;
  • surface defects.

Pulsation of the diameter is associated with sudden pressure changes in the head, which cause uneven flow of the material from the tool, and when pulling out the line can even break it. Pressure pulsations can be prevented by using a plastic melt pump that feeds the molten polymer at an even speed.

The melt pump is installed between the plasticizing system and the head. The pump provides a stable dosing of the material and allows to increase the pressure achieved. The change of the polymer flow direction occurring in the pump reduces the stress in the material, which has a positive effect on the quality of the extrudate.

 

Picture 2. The melt pump

 

The ovalization manifests itself in various cross-sectional dimensions of the filament in the OX and OY axes. It often occurs in profiles of material with a high MFR and for larger filament diameters (2.85 and 3.00 mm). Ovalization of the filament may cause clogging of the 3D printer head, which makes the printing process much more difficult. The reduction of the ovalization in the Zamak Mercator lines is accomplished through the use of an innovative vacuum system mounted on the inlet to the hot tub.

Picture 3. The vacuum system used on the inlet to the hot tub

Air bubbles in the extrudate core appear when the material contains moisture or when gas is trapped in the pellets. The occurrence of such inhomogeneities in the material may also indicate the mixing of granules of various materials that are not compatible with each other. In this case, it is necessary to ensure proper quality control of the raw material.

Picture 4. Cross-section of the filament that contains air bubbles.

Prevention of the air bubbles can be carried out in several ways. The simplest of them is the change of temperatures in the first zone of the plasticizing system and the increase of head pressure. The temperature change of the screw and cylinder in the feed zone allows for a more rigid packing of the solid material. Often, increasing the temperature in the first zone of the extruder barrel, reduces the amount of air, but in some cases may also help to lower the temperature. If these activities are not enough, sometimes it must be necessary to change the machine structure:

  • the use of a grooved hopper zone;
  • increasing the screw compression ratio.

Proposed solutions to problems in the production of filament for 3D printing, have a source in literature and confirmation in our experience gained during the testing of various materials.

Zamak Mercator Sp. z o.o. offers the opportunity to carry out research in the field of plastics, rubber and other non-standard materials.

We can offer to use test lines for extrusion as well as the qualified employees with knowledge, both theoretical and practical in this area.

You can find out more about the Zamak Mercator research offer here.

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