Chipped glass plate, image via richrap.blogspot.com
Most materials expand on heating and shrink on cooling. However not every material shrinks by the same amount on cooling for a constant temperature drop. This property is measured by the coefficient of thermal expansion (CTE). The CTE describes the rate of shrinkage per unit temperature. Glass has a linear CTE of around 1-17 x 10-6 K while that of 3D printing thermoplastics lies in the range of 40 to 200 x 10-6 K. The linear CTE of glass is thus about an order of magnitude lower than that of the printed thermoplastic. As parts cool, a tensile force is exerted on the surface of the glass while the parts are still adhered to the glass. If the parts are adhered too strongly to the build-plate they will continue to exert a tensile force as they cool down further (figure 1a, b).
Figure 1a: Printed part which has just finished printing, the part and the build-plate are roughly at the same temperature. The part is already exerting some force as a result of shrinkage during the solidification of the deposited FDM thermoplastic
Figure 1b: part and glass build-plate cool down, since the part experiences a larger rate of shrinkage than the glass build-plate the net result is a tensile force on the glass surface which leads to crack initiation and propagation.
To the naked eye a sheet of glass appears to have a perfect and smooth surface. However at the microscopic level glass has defects. These defect scan either result during the manufacturing process, or else can be introduced during use in the form of scratches. These can act as points of stress concentration and thus increase the likelihood of microscopic cracks to form. Since glass is a brittle material, tensile forces exerted on the surface will cause the cracks to propagate, eventually leading to failure. This can either lead to small piece of glass chipping off the build plate or to a catastrophic failure of the whole glass sheet in some cases.
A more severe case of glass chipping resulting from the use of improper adhesives, in this case using Magigoo PA with ABS filament
Thus the combined action of thermal shrinkage and the adhesion of the part to the glass can cause enough force to damage the glass surface. Generally this is not a problem, especially with consumer grade filaments and non-demanding printing conditions. Nonetheless several factors can increase the probability of chipped glass.
Most users experience problems with chipped glass on switching to PET-G after working ‘chip free’ with PLA. Unlike PLA, PET-G sticks aggressively to glass, and also PEI, increasing the chance that the build-plate will be damaged. As with PET-G some materials can be more prone to danging glass build-plates due to over adhesion.
A PET-G print causing a catastrophic failure on a glass build-plate, image via richrap.blogspot.com
These include:
Glass cracking can also be result from the printing conditions which can promote over adhesion or accelerate wear on the glass build plate. These include:
Printing with the nozzle too close to the build-plate
With every print, the surface of glass (and other materials) will deteriorate. Every heating and cooling cycle and part removed will introduce further defects and exacerbate existing ones. This is inevitable and some materials as mentioned above will wear the surface more quickly.
The wear on glass build surface can also be worsened when using sharp tipped tools to remove parts. These might introduce scratches to the surface of the glass which can act as points of stress concentration and eventually leading to cracks and chips.
Sharped tipped metal tools similar to the one pictured above can scratch the glass thus providing with points of stress concentration
Join us in the next post to see how you can reduce the chances of damaging your build-plate by modifying slicer settings and printing conditions. Until then happy and safe printing!
We hope you found this post interesting feel free to drop us any questions.
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As mentioned last week, some 3D printing filaments are modified to improve their adhesion and also reduce warping. This can cause the printed part to over-adhere and can lead to build-plate damage. If a specific brand of filament is giving you a tough time during part removal, it is probably a result of over-adhesion. You can slightly reduce the adhesion by tweaking your slicer or printer settings.
By either reducing the first layer flow or slightly increasing the Z-offset, your printer will not flatten the first layer on the build-plate as much as it usually does. This reduces the adhesion and makes part removal less of a chore. Through testing we have found that reducing the first layer flow by about 85% significantly improves the ease of part removal. This value also ensures that the adhesion still adequate to prevent warping.
Ideal nozzle height, image via matterhackers.com
Nonetheless, since each printer and material varies we recommend performing some adhesion tests to find the optimum settings. This blog post goes into more detail on how to perform adhesion tests.
With most 3D printing materials it is always advisable to wait for the part to cool before attempting to remove it. In most cases it becomes easier to take the part off once it cools. Furthermore you run less risk of deforming the part on removal.
One should also always leave the part to slowly cool inside the printer rather than taking the build-plate out to cool it more quickly. Cooling the build-plate too quickly will increase the chances of damaging the build-plate. Some manufacturers also recommend that the end g-code is modified so as to cool the build plate more slowly.
Wait for your build-plate to cool down, slowly!
If you follow these steps and you find that the part is still firmly attached to the build plate, do not despair. It is not a good idea to try and wedge the part up using a scraper, as this can damage the build-plate further and also promote glass chipping. If using Magigoo, you can wet around the perimeter of the part with water. After waiting for the water to seep in under the part it should become much easier to remove. Alternatively you can submerge the part and build-plate in warm water and wait for it to self detach.
Incidentally this method is also good for removing fragile parts or parts made out of soft materials such as TPUs which tend to stick too well. In this case Magigoo is acting as a releasing agent!
The Magigoo range comes in a variety of different flavors for different types of materials. This includes the original Magigoo for ABS,PLA, PET-G, HIPS and TPUs. Magigoo PA for Nylon, Magigoo PC for Polycarbonate and Magigoo PP and PPGF for polypropylene and glass filled PP materials respectively. More recently we introduced Magigoo HT for high temperature materials and Magigoo Flex for certain classes of TPEs. It is important to use the correct Magigoo stick according to the material you are printing. For more information you can visit the tested materials section on our website.
The Magigoo range
As mentioned in the previous blog article, after repeated use build surfaces wear down and will become more susceptible to damage. By following these precautions you can extend the life of you build-plate and prevent premature failure:
The use of sharp-tipped tools can scratch your build-plate surface
We hope you found this post interesting feel free to drop us any questions.
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Polymers are made of large molecules with a repeating chemical structure, otherwise known as a monomer. The mechanical and chemical properties of a polymer are affected not only by the chemical make-up of the repeat unit but also the molecular arrangement of the repeat units in the polymer chains, and, the interactions between these polymer chains. In thermoplastic materials, the polymer chains are held together with weak intermolecular forces. As a result, thermoplastic materials can be repeatedly softened from a solid to a viscous fluid on application of heat.
Currently most of the thermoplastic material types available for injection moulding are also available for 3D printing, with more materials being adapted for FDM AM each day. Figure 1 below shows the common material classes available for 3D printing. The materials are split into three different classes; commodity, engineering and high temperature, with some materials overlapping the commodity and engineering class.
Figure 1: Classification of thermoplastic materials via: specialchem.com
Generally, engineering materials tend to require higher nozzle and chamber temperatures than commodity plastics during printing, since these materials tend to have higher melting temperatures. Printing at lower than recommended temperatures will potentially lead to print failure or parts with inferior mechanical properties. Similarly, high temperature thermoplastics require even higher nozzle and working temperatures for printing. These materials often require an actively heated build chamber during printing for reliable results.
Figure 2: Diagrammatic view of layer by layer process in FDM printing
Due to the layer upon layer nature of FDM parts (figure 2) tend to have voids or empty spaces between the layers which are detrimental to the mechanical properties of the final part. As a result, the bond strength between layers of extruded material is often cited as being the weakest and most critical parameter affecting the mechanical properties of FDM parts. In other words, 3D printed parts are often much weaker in the Z-direction than in the X- and Y- direction.
The adhesion between printed layers is achieved through thermally-driven diffusion welding. Thus, different fabrication parameters including nozzle temperature, environmental temperature, print speed, nozzle width, extrusion width, layer height and others will have an effect on the resulting mechanical properties of a printed object. Since different materials have different temperature requirements, the material type will also affect the z-strength of a printed part. In fact higher temperature materials are more prone to poor z-strength and delamination if the printing parameters are not adequate.
One of the misconceptions I’ve held when I started working in the 3D printing sphere was that PLA is only suited for aesthetic prints and that functional prints using PLA were not feasible. This is due to the fact that often parts printed with PLA tend to break easily especially in areas with thin sections. This would lead one to believe that PLA is not a very strong material. However when looking at the typical mechanical properties for this material in the table below, one can easily see that it is stronger than most common ‘engineering’ materials in terms of Ultimate tensile strength and also in terms of stiffness. CNCkitchen also did some interesting tests with these materials.