The Guide to Rapid Prototyping Techniques: 3D printing, CNC-machining and casting

Posted By on Dec 16, 2015 | 0 comments


If there is something that just comes with experiences, it’s to pick the right manufacturing methods for the state of your project. To be even more honest, our sales team gets a lot of calls from companies or individuals ready to choose 3D printing over other production methods because they lack the knowledge on what other techniques can do and for what prices. We figure out that making a list of the different ways to quickly get your parts to life might be useful to help you choose the best way to build your prototype. 3D printing can do a lot, but it’s not the solution for everything. Let’s get started.

First thing first, prototyping is not a linear process. There is no perfect path to the perfect prototype nor there is a way to know for sure what you should use to create your prototype. However, it’s certain that depending on where you are in your prototyping phase (and for that we recommend that you take a look at another post dedicated to prototyping), you will have to use more or less complex methods and they will also cost you more or less money. These are the big questions you should be able to answer at each step of your prototyping phase:

  • What’s the overall quality of the prototype and the complexity of the part?
  • Do I need any specific material?
  • What quantity do I need?
  • What are my resources (time, money)?

Depending on the answer, you’ll have the choice to do it the dirtiest way (with glue, cardboard and scotch tape) or to use the latest prototyping methods. There are basically two kinds of prototyping and small-scale production techniques that are very different. One the one hand, you have additive manufacturing, commonly known as 3D printing. And on the other hand, you find subtractive manufacturing, directly related to CNC machining. A third technique exists though, and it uses “low quality” molds to make a low volume of units. It’s probably not the most efficient way to make just one part, but for making multiple similar parts on your prototype or your first run of production, it can be useful and a viable alternative to both additive and subtractive manufacturing.

How does it work?

On a blog dedicated to 3D printing, additive manufacturing has been covered extensively. To sum it up, 3D printing refers to a large choice of different manufacturing technologies that have in common to create objects by fusing together layers of material to build a part from a CAD files (or 3D file). Those different technologies are widely covered on our page dedicated to 3D printing or in our glossary.

Subtractive manufacturing, however, uses a fundamentally different approach. Instead of adding layer of material on top of each other, it uses a block of material and removes part of it with a very high precision until only the shape is ready. As it’s the case with 3D printing, CNC machining is more than just one technologies and it includes:

  • Turning: this is a process in which the workpiece is put into high-speed rotation while a moving cutting tool is removes the material.
  • Milling:  this is a process in which the cutting tool itself is put into rotation to bring cutting edges to bear against the workpiece.
  • Drilling: this a process in which holes are drilled by rotating a cutter on the workpiece.

Casting  is mostly used to create multiple models  from a first master model that will be used as a reference. The master model is typically built using an additive manufacturing technology. Both stereolithography and Polyjet process are good candidates. The master is the perfect replica of the finished product. Once it has been 3D printed, silicone rubber is pourred over it. After being cured, the master is removed. The silicon mold is ready with a cavity that can be filled with a liquid urethane resin. Depending on the urethane resin, the different “form, fit and function” criterias can be tested. The mold itself can often be used around 20 times before being too degraded by filling it with hot resin.  Instructables provide lots of useful guides to create this kind of mold.

Which one is better for your need?

Part quality and complexity

The most remarkable difference  between 3D printing and CNC machining is the complexity of the objects that it can produce. Without any doubt additive manufacturing is a clear winner when you’re looking at making very complex parts with intricate geometries or moving parts. Particularly, it’s very useful when you’re creating parts that are partially hollow or with integrated assemblies. It can do it in a matter of hours and for a reasonable cost.

In terms of quality, additive manufacturing is both bad and good as it really depends on what you’re looking for and the cost you can assume to finish the part. Additive manufacturing delivers a fair quality every time with almost all technologies. It means that if you’re at the first steps of your prototyping phase, the tolerances and surface finishes (right outside of the machine) will be enough. However, the closer you get from the finished product (or if you’re using 3D printing for the finished product), the more intensive the finishing part will be. This means mostly sandblasting,  polishing and dying or painted the part. If correctly done, this steps will ensure that you have very high-quality parts, but it has a price. Online 3D printing service, like Sculpteo, offers a range of available finishes to get good quality part.

However, if you’re using metal 3D printing (DMLS, EBM, etc). It’s a necessity to use extensive finishing on the part such as polishing and even in most cases CNC machining. It means that when 3D printing in metal, you might also have to combine it with other prototyping technologies. The main upside in this case is to be able to deliver very complex parts (often hollow or partially hollowed) that are impossible to achieve otherwise. Metal 3D printing is not necessarily meant for prototype and is also widely used for end-use parts.

In contrast, CNC machining can not achieve hollow parts in a single go, and it’s fairly difficult to create very complex design. Though, it is generally more precise than additive manufacturing and offers better tolerances. The main upside of using CNC machining is the good quality of surface finishes. Thanks to the precision of the cutting tools, the machines are able to create parts that looks as good as any on-the-shelf product.

This is why subtractive processes are generally preferred when you’re moving closer to the industrialization phase. It can be a great help to create your tooling as it includes lots of the “design limits” of injection molding. All in  all, the choice between additive manufacturing and subtractive processes under quality/complexity factors will be made regarding the function the part has. Additive manufacturing are a great help for fit check, to determine the right form factor and make sure function can be assured by replicating an action a limited amount of time. Though it sometimes remains the only possibility to build a part (to know more about this, check out our guide to the 4 areas where 3D printing is unstoppable).

Finally, casting through urethane resins offers a good solution to create parts with a good quality and fair complexity. However, the many steps between the creation of the mold (3d printing and finishing) to the de-molding and finishing of the castings can largely impact the accuracy of the tested part.

Subtractive processes can be a better choice for production closer to the final product because it provides a better surface finish and better tolerances. However, it generally more expensive making it only interesting for production of parts with high-stress uses or high-quality finishes.

Materials

There are today just a few limitations in the domain of material for both 3D printing and CNC machining. But the basic remains the same:

  • Additive manufacturing includes lots of different technologies which usually works with different materials. When you can into account the whole spectrum of technologies and materials that each of them can do, it’s possible to prototype with almost all engineering plastic, or at least plastic and resins that simulates them (like it’s the case for the Stratasys Connex printers), and a large choice of metal alloys.
  • CNC machining allows to use blocks of most engineering plastic and metal. It offers these technologies the possibility to prototype almost every properties that the final part will have as it uses the same materials.  For any “natural” material like woods or stones, CNC stays the way to go. If 3D printing offers some kind have charged resin, it won’t be as good as a protytype made from the raw material.
  • Urethane Casting offers a wide range of polyurethanes with which you can test different hardnesses. It’s also possible to incorporate custom colors and textures, while testing basic mechanical properties. However there is one major downside to using this kind of materials: urethane resins don’t have the same properties then molded thermoplastics that will be used during the industrialization phase.

If you need to create a part or prototype out of a particular material, your choice of manufacturing method depends on what your needs are. Additive manufacturing methods are almost always used for manufacturing parts from plastics, though some new technologies can print parts in other materials such as metal. However, for most metal manufacturing, as well as wood, foam, or other materials, CNC routing and milling is likely the only option.

Part Quantity

Part quantity is a key factor from determining which prototyping technique you’ll use. In some cases, a single part is all you need. While others will want a first run of 10, 20 or 100 parts. The overall rule is that part quantity shouldn’t affect your prototyping too much. If in the past it was widely admitted that additive manufacturing means just a few units, it’s not the case anymore and you can get sweat deals when ordering high-quality parts in batch. (if you’re not familiar with it, I suggest that you take a look at Sculpteo Batch Control, which allow you to manufacture many units with a full control on the printing parameters)

Additive manufacturing strong suits is that the 3D printers can run starting from one unit, but is still efficient when running small to medium batches of parts. The specificity of some 3D printing technologies (such as SLS or SLA) means that the 3D files don’t need support and can be produced in the shortest lead-time.  On the opposite, CNC-machining ask for a number of preparatory steps to make a single part. Programming a CNC router’s tooling path requires time and skills. That’s why the ‘starting fee” of CNC is higher than the one for 3D printing, even though they are both well under industrial grade injection molding.

That being said, subtractive manufacturing offers a range of material, surface finishes and tolerances difficult to achieve otherwise. In some cases, this might be the only option to consider.

As mentionned earlier, the casing solution means you’ll have to make a first 3D printed model. As such, it’s only useful if you’re looking at making lots of units or parts with material that can’t be done with 3D printing.

Resources (time and money)

Ultimately, the resources will decide for you what kind of prototyping you’ll go for. If time is not a problem, then money might dictates the term. The other way around is also true. Basically, those rules applies:

– 3D printing: A reduced setup is required resulting in a short lead-time. If you’re looking for specific finishes, it will be more costly and will take time. Still it can remain faster and cheaper than other techniques for low volumes.

– CNC-machining: There is a non-negligeable time of setup which translates to cost. After that you’re good to make many units and spend minimal time on finishes.

– Casing: Combines the speed of 3D printing for the master and the possibility to make small runs. It take some setup and skills to make accurate parts, which inevitably transforms to costs. However, it can also be a good solution for small budget and DIY experts.

 

 

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