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Joining 3D printed parts

Joining 3D printed parts


In the previous articles we covered many scenarios where 3D printing can successfully produce parts with increasingly complex requirements. All these parts, however, were relatively small (max 300 mm for the longest side) due to the restricted size of most 3D printers. The time has come to (literally) start thinking out of the box. Using the so-called “plastic welding gun” we can join multiple small parts into a bigger one and create plastic objects that are impossible to print at once. Although there are conditions and restrictions to this method, it can surely expand the possibilities of a 3D printer.


How it works


The welding gun is a rather simple, yet ingenious tool. It uses electric current to heat up specially shaped metal wire bits. Once they are hot enough, they are pressed into the plastic which melts around the wire and holds it in place. Usually the wire bit melts both of two plastic parts, positioned next to each other and hence creates a strong metal link between them. A single wire bit may be insufficient to support the required shape so many bits are used, creating a metal seam along the two original parts. Despite the bits small size, a seam created that way is surprisingly strong – the wire bits are securely incorporated into the plastic and the wire itself (about 1 mm diameter) is impossible to tear with bare hands. There are wires with different shapes and sizes – depending on the application, geometry and load requirements can be used wider or narrower bits, almost straight or with a right angle profile.



Despite heating the wire bits up to several hundred degrees Celsius, the tool is quite safe. It uses the wire resistance to generate the heat and the actual gun itself remains cool.




As systems of all kinds become more complex, one of the best coping strategies is the modular design. Smaller components and subsystems are easier to manage, maintain, produce and replace when damaged. That approach can be implemented in the 3D printing tasks as well – multiple subsections of a large part can be printed separately and then joined with the welding gun. Having multiple printers at hand means that many of the modules can be printed at once, which effectively reduces the total manufacturing time.

A suitable object for the method, described in the article, are all kinds of machine enclosures. As modern workshops develop more and more multi axis machines and robot arms, complex geometry enclosures will be seen more often. With the ability to join multiple parts in one final object, even a small 3D printer can be used to create plastic covers for a 2 meter tall robot.

Another important note – when using the welding gun, one can join parts from different plastics. As the wire bits melt the two parts simultaneously but independently, as long as the plastic’s melting temperature can be matched by the gun, a secure joint can be made. That is clearly an advantage over other joining methods such as glueing or actual welding.



We hope this article was helpful and will let you reach new heights in your 3D printing journey. 

If you ever need advice or a trusted partner, don’t hesitate to contact us on social networks or through our website!

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Case study 3: Strengthen the Z-axis in 3D printed parts

Case study 3: Strengthen the Z-axis in 3D printed parts

Figure 1 – the same 3D printed detail without (left) and with (right) enhancement

Technology of 3D printing

“3D printing” is the general term for several technologies that use a layer-by-layer deposition of material to produce a complete part. At DHR Engineering, we utilize the method of Fused Deposition Modeling (FDM), which relies on the deposition of molten plastic filament in layers to build the desired part from them (read more about it in our first article). Once a layer is complete, the printer moves vertically to the next layer, where it prints the corresponding geometry. To achieve a successful print, it is important to find the right settings for the filament heating temperature and the vertical distance between the layers. Ideally, the current layer will heart up the printed one and the distance between the two will be small enough, so that the new filament will adhere tightly without compromising the existing geometry.


Figure 2 – FDM working principles. Source:


Anisotropy and what to do about it

Despite numerous attempts and improvements in the technology, 3D-printed parts are anisotropic. The strength and tensile properties in the vertical Z direction are typically 4-5 times lower than in the other two planes (source). Similarly, parallel to the printed layers (in the X and Y directions), the part may have significantly lower resistance to shear loading. Due to these characteristics of 3D printing, additional methods are sought to enhance the mechanical properties of functional parts. Here are some of the most popular solutions:

  • Changing the geometry – The best way to avoid layer separation of the part is to eliminate the forces acting there; this solution is applicable primarily in the initial planning phase, where there is sufficient design freedom.
  • Changing the printing orientation – For simpler parts, rotating around the X or Y axes may solve the problem by moving the part’s weak point instead of the forces acting on it. However, this method does not work for parts with multiple perpendicular loadings.
  • Thicker parts – If the applied forces are relatively small, thickening certain elements of the part can improve its strength enough to withstand loading even at its weakest point.
  • Metal inserts – Using metal inserts in 3D printed parts is indeed worth a discussion in its own right. In this article we will focus on the use of standard bolts. Incorporated in the printed part, they bear the load instead of the plastic, significantly improving the part’s mechanical properties. Despite the  drawbacks of this approach, which include more complex design and printing, as well as the need for additional processing and assembly, the added work is totally worth it.


Figure 3 – using metal inserts to strengthen the 3D printed parts in the vertical axis


Benefits of using metal inserts

Including metal components in the design of 3D printed parts helps in several ways:

  • The forces created by tightened bolts compress the layers of the part, increasing the friction between them and thus increasing the maximum shear load.
  • The bolts distribute the load over a larger area/more layers, effectively reducing the forces acting on each individual layer.
  • The bolts themselves bear a significant portion of the loads acting on the part. For example, in shear loading perpendicular to the Z-axis, a reinforced part can withstand much higher loads compared to a purely plastic part because the force would need to break the bolt itself.

Figure 3 depicts a 3D printed bearing housing used at DHR Engineering workshop. The part is supported on one side only (cantilever), and the combined static and dynamic load is approximately 1200 N. The cylindrical portion of the part, where the metal shaft resides, is printed parallel to the Z-axis of the 3D printer.

During operation, various forces, acting in different planes, are generated. Perpendicular to the printing axis, the part exhibits excellent mechanical properties under tensile lading because the forces act within the plane of the plastic filaments. However, parallel to the Z-axis, tensile forces attempt to separate the layers, which is the weakest point of any 3D printed part.

To prevent layer separation under the current loading conditions, we decided to add two M4 bolts along the Z-axis and increase the load-bearing capacity. After successfully testing the part at its nominal load, we gradually increased the load. During one of the strength tests, to our surprise, the part failed through the layers (Figure 1, Figure 4). This demonstrates that the bolts have increased the part’s resistance beyond that of the plastic and have transformed the weakest plane into the most reliable one.


Figure 4 – the enhanced part broke through the layers where typically is the weakest point

See you soon!

In this article, we have once again shown that 3D printing offers much more than it may seem on the surface. The demand for complex, yet reliable parts will only increase but at DHR Engineering we are always up for a challenge!

We would be glad to help you as well!


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Case study 2: Pneumatic tool for cleaning the CNC table

Case study 2: Pneumatic tool for cleaning the CNC table


Figure 1 – pneumatic tool demonstration


Perhaps the biggest advantage of six-axis robots is their versatility. Being programmable, they can be set up for countless operations, using a variety of different end-effectors. There are companies specialised in developing end-effectors for gripping, inspecting, scanning, welding or glueing. However, because of availability or cost reasons, sometimes it is more convenient for the users to create their own tools to perform a specific task. An example for such a case is a simple 3D printed nozzle for cleaning the CNC mill table that DHR Engineering developed.


Figure 2 – 3D printed nozzle mounted on the Schunk gripper


Why are metal chips such a problem?

During every milling operation, metal waste accumulates in the form of chips of various sizes. Given the tight tolerances that modern machines work with, it is crucial that all surfaces in the mill are well cleaned. The presence of chips can lead to displacement or distortion of the workpiece, resulting in unacceptable inaccuracies in the final product. The accumulation of a large volume of chips interferes with the normal operation of the machine, making movements difficult and hindering automation. It is common practice for the operator to monitor the cleaning of the tools and work surface. This task can be automated with a six-axis robot and a 3D printed nozzle, allowing for a continuous process from loading the blank to the finished part.


Design specifics

In designing the tool, we tried not to make it more complicated than necessary (you can see the 3D model in Figure 3). We planned to use the robot’s controllable pneumatic lines and only needed to send the air in the right direction. The nozzle we created is a plastic cube with suitable mounting and pneumatic holes. The main specifications we considered are listed below.

  • The nozzle had to fit the gripper geometry and the existing mounting holes. We 3D printed the threaded mounting hole, and the overhanging detail is designed to rest on the wall of the gripper for easier positioning.
  • The part required an interface for connecting to the pneumatic system of the robot. With this particular robot, the pressure is 6 bars, and the connection to the gripper is made through a standard 1/8-inch connector. This thread is also 3D printed.
  • The nozzle exit hole required an optimal size and position to provide a strong stream towards the milling table. In Figure 2, it can be seen that the nozzle is directed perpendicular to the jaws, allowing the gripper to be positioned very close to the milling table during cleaning.
  • The detail had to be sturdy and easy to print; a simple design, small size, and light load – the ideal moment to use the services of 3DHR, which would cost you only 2 BGN.



The entire process of creating the nozzle takes a total of 3 hours. This includes conceptual and technical design, creating the 3D model, printing, installation, and programming the robot. We can reuse the finished 3D model again if we need to change the dimensions, create a second nozzle for another robot, or replace a damaged or worn part.



Whether it’s small details like our #Doosan robot nozzle or large, complex prints, 3D printing will certainly be part of the future for many industries. If you’ve never worked with plastic parts, now is the perfect time to start and stay ahead of the competition!

On our website, you can upload a 3D model and automatically get a price and time estimate for printing your parts!

We would be happy to answer any questions – please contact us!

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Case study 1: Plastic jaws for a pneumatic gripper

Case study 1: Plastic jaws for a pneumatic gripper


Automated CNC machines have long been the standard equipment in the metal working industry but connecting all the different operations in a fully automated flow still requires human intervention. Companies like DoosanRobotics have developed six-axis robots that, with the help of various end-tools (grippers, suction cups, etc), can complete many complex tasks, reducing the need for human labour in repetitive tasks. However, can these robots cope with the most common operation – loading and unloading parts from the CNC mill?

What are the barriers ahead of a fully automated workshop?

For most parts the manufacturing process is divided into two steps – processing the upper and side surfaces (1) and processing the bottom surface (2). For a robot to be able to bridge the gap between the two operations, the robot needs to be equipped with a suitable tool which can handle the blank and the processed component precisely and securely (one such tool is the Schunk gripper). Usually two sets of gripper jaws are used – standard parallel jaws to hold the blank and custom made jaws that fit onto the partly processed part (for example, a cylindrical detail on one side of the part created with the first operation in the mill). 

The custom jaws are often made of aluminum blocks with the exact shape that needs to fit in cut inside. The preparation of those jaws requires investment of time and money on its own. Once manufactured, the aluminum jaws allow almost no changes to the design of the part they hold, and if changes are made, the jaw set is no longer usable. Also, in some cases, the process cannot be automated because the standard and custom jaws need to be manually swapped between operations. The modern solution to all of these problems is 3D-printing.

Figure 1 – forces acting on the jaws during use


Quick, easy, cheap

In the first article of the series we described the requirements that a metal part should fulfil in order to be replaced with a 3D printed one. In the case of the gripper jaws, all of the conditions are met:

  • Gripping and moving small to medium sized parts exerts forces less than 1500 N and the plastic jaws will have no problem withstanding those. That includes cyclic loading as well – the set of jaws we currently have at the workshop have been used for more than 20 000 cycles.
  • The ambient temperature in the workshop is about 25 degrees Celsius.
  • We need just a few sets of jaws so no large batches are produced.

But why would we want plastic parts after all? Here are some good reasons: 

  • 3D printing allows for very complex designs without manufacturing overhead. This gives us the capacity to create jaws with multiple interfaces that will be able to grip the processed part in all stages of its production. As can be seen on Figure 2, interfaces can be located on both sides of the jaws (flat interface on one side, curved interface on the other side), maximising the number of shapes that can be manipulated with the set.
  • The manufacturing time for a set of jaws with dimensions of 100 х 40 х 15 mm is only 2 hours, with zero additional processing required. According to the calculator on our website, the price is 35 BGN as of March 2023.
  • In case the design of the processed part is altered in any way, the plastic jaws can be easily modified in the 3D model and reprinted in just a few hours. Similarly, a new set can be created if the jaws are damaged or worn out
  • The plastics are way softer than metal and the printed jaws can never scratch or damage the processed part. The part appearance is the first sign of a good (or bad) quality work

Figure 2 – interfaces on both sides of the jaws – flat (inside) and curved (outside)


The secrets of good design

Making a good tool requires careful design in various areas – manufacturing, use, reliability, maintenance. Here is what we took into account during the creation of the plastic jaws:

  • To reduce manufacturing time, warehouse management complexity and downtime lost in replacing jaws between different operations and parts, we wanted to include as many interfaces as possible in the jaw set. On Figures 2 and 3 you can see the three interfaces we managed to combine – cylindrical grip for the shape after the first operation; flat surface on the inside for the blank; a second flat gripper with different size available if the outside interface is used. The combined jaw set lets us automate the whole manufacturing process for the part on Figure 3 – loading the blank, swapping orientation after the first operation and unloading the finished product after the second operation.

Figure 3 – the same jaw set can be used for multiple operations


  • A very important design consideration is the calculation of cyclic and peak forces that act on the jaws. Both the geometric design and 3D printer parameters are based on that calculation. On Figure 1 you can see the main forces acting on the jaws – vertical forces that appear during lifting (red) and horizontal forces due to gripping the part (blue). Taking into account the force calculations and following the best design practices, we developed the appropriate geometry. Regarding the print itself, we used 80 % infill density and the reliable PETG plastic. Also, the filament layers are parallel to the horizontal forces, where the most strength is required.
  • Doosan collaborative robots can achieve repeatability of 0.1 mm. We needed a secure and precise way to mount the custom printed jaws to the robot in order to maximise its capabilities. As the 3D printing technology still cannot support such small tolerances, we used a well known trick to get the job done – metal inserts (Figure 4). A dedicated slot was left at the back of each jaw for tight tolerance (h7) inserts – they guarantee that the relative position between the robot and the jaws is always the same.

Figure 4 – precise jaws mounting thanks to the locating inserts



The 3D printed gripper jaws are definitely a success – we have been using them for months in our workshop with zero problems.

Next week’s topic is related to a seemingly insignificant activity – cleaning the mill table. Even though many operators overlook that part of the job, the presence of metal chips restricts automation and may cause misalignments and scraped parts. We have a simple yet effective solution to offer. 

See you next week!