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February 03, 2020 6 min read

After finishing the design of your carbon fiber part via CAD or by modifying an existing part, you need to create either a plug or a mold. The choice between these two options is critical and will be dependent on your manufacturing method as well as your end goal. First, it may be important to define the difference between a plug and a mold.

Plug Vs Mold

The terms “plug” and “mold” are often confused or used interchangeably along with the term “tooling”. However, a plug and a mold are very different and must be differentiated. A plug is the “male” replica of the final part you are planning to make. It should be a mostly “positive” shape, like a hill. A mold is a mostly “negative” shape, like a valley. To make a carbon fiber part, a mold is always required, but you do not always need to make a plug. This choice is dependent on your manufacturing process. For most DIY applications, you will need to start by making a plug. Because a plug is a positive shape, it is easy to sand and form to the correct shape. Sanding an external radius is far easier than sanding an internal radius. Whether you are using a CNC to machine foam or hand shaping Bondo or clay, the step of making a plug will be needed. You will use this plug to then pull a fiberglass mold.

Visualize a plug as a hill and a mold as a valley.

It is possible to go directly to a mold under a few circumstances. For high end tooling made from metal, there is no need to make a plug when you can simply CNC your final shape. You may be limited by internal features that a CNC bit cannot reach within a mold cavity, so you may still be forced to machine a plug first. Another reason to go straight to a mold is if you are prototyping and want to make changes quickly or you are making a one-off part. You could choose to 3D print a mold or machine a foam cavity. Both options work great for making a single use mold. You could get lucky and use the mold a few times, but it is best to bet that the mold will only last for making one part. In either case, you will still need to finish the mold surface so that it is smooth and will release properly. You can follow ourPlug Finishing Guide to finish these temporary molds.

Go straight to mold for prototypes and one-offs.


Depending on how complex you designed your part, you will need to determine if you can make your part in a single mold, a multi-part mold, or multiple molds and then post bond the part together. If your part has undercut or negative geometry features, you will need to split the mold or part into multiple pieces. This is far more advanced and goes beyond the scope of this post. Get in touch with a CF expert to discuss this in more detail. We will proceed with the design of a single cavity mold with no undercut. This process does cover 90% of the automotive parts we make, with the exception of spoilers and parts with internal structures.

If your part has undercut or negative geometry features, you will need to split the mold.

Designing a Plug or Mold in CAD

The rest of this blog will assume you are designing your plug or Mold in a CAD program. Fusion 360is a great cheap (or free) option with plenty of online resources to help you get started in CAD. Once designed, you can send the files out to be 3D Printed or machined from a company like3D Hubsfor very reasonable costs! If you are intending to make your plug by hand, continue reading this post as it will relate, but then skip ahead to ourSplash Plug Fabrication blog.

Whether you are designing a plug or a mold, the process will be the same until the last step. Following is a general list of operations within a CAD program that will convert your part into a plug or mold. Some programs have built in Mold tools that can be helpful, but they may not be worth the added cost if you can follow the procedure below:

  1. Surface Offset:
    It is best to design a plug/mold as a surface and then convert it to a solid. If your part is designed as a surface, then you can skip to step 2. If not, you will need to select all external surfaces of your model and offset them by zero (or 0.00001”). Do not select any internal surfaces, including holes or cutouts. The surfaces you select here will be your final aesthetic carbon surfaces.
  2. Surface Fills:
    All holes and cutouts that will need to by drilled or removed later must be filled. Use a tangent surface fill or something similar to make these features disappear.
  3. Extend Surfaces:
    Extend all external surfaces by 0.25” tangentially. This adds extra material to your plug/mold so that you ensure your part’s features will be captured and not lost in a corner or tight radius.
  4. Draft Analysis:
    Perform a draft analysis to confirm one last time that your part has no negative geometry. You may need to adjust your pull direction to ensure there are no undercuts. Create a plane perpendicular to your pull direction.
  5. Flange Design:
    As a rule of thumb, it is good to design a plug with 6 inches of flange. Often you will lose and inch or two of flange in the molding process. If you are going straight to mold, you can get away with just 4 inches of flange, or less, depending on how you will layup your part. For resin infusion and vacuum layups, you need to make sure you have enough room to run tacky tape and resin lines, plus have some extra material sticking out of the mold to help release it with a wedge. If you intend to do a wet layup or use a press to create pressure, you can get away with as little as 1 inch of flange.
  6. Flange Surfaces:
    These flanges should be made as parallel to you pull direction as possible. In CAD, use a Ruled Surface or Swept Surface feature that is constrained perpendicular to the pull direction. Extend them as far as desired.
  7. Boundary Surfaces and Fills:
    It would be a lie to say that the above steps will go easily and smoothly. In most cases you will run into areas of a file that simply do not want to extend or merge. You will be required to troubleshoot and fill in sections using other surfacing features.
  8. Convert to a Solid:
    You now must make the decision of whether this is a mold or a plug. If it is a plug, form a flat surface an inch below your flange and extrude a surface upward to your flange. The result should be a “hill” on a 1 inch base. If you are making a mold, create the flat surface 1 inch below the deepest portion of your part and extrude up to your flange. The result should be a “cavity”.

Design Features

Our motto at CF is “Do the Hard Work Upfront.” This is abundantly important when designing a plug that will dictate the shape of your mold and finally the shape of your part. When possible, include indexing features or scribe lines in your plug that will translate to trim lines of your final part. Design your offsets for the tools you will using to trim you part. We extend our flanges by 0.25” because we have a cutting tool that cuts at exactly 0.25” from a flat surface. Make design decisions that will save you time and headache down the line. It is hard to cover all of the nuances to designing a mold in CAD in a few paragraphs. Check out our SolidWorks walkthrough tutorial below for a more in-depth walkthrough.

Make design decisions that will save you time and headache down the line.

Feel free to reach out toYour Carbon Fiber Expertswith specific questions about this process and read our other blogs in this series to learn more about fabricating strong, light, and beautiful carbon fiber parts! Common Fibers is here to help!