Thermoforming Design Guidelines
Thermoforming Design Guidelines (DFM): Rules, Tips & Cost Optimization
Thermoforming is an affordable technology for creating packaging, trays, panels, housings, and more. These parts are durable, inexpensive, and relatively quick to produce. Thermoformed parts can be rigid or flexible; opaque or translucent; and come in a variety of materials with different mechanical, thermal, and chemical properties.
Although thermoforming has some design limitations and cannot achieve the fine detail of injection molding, machining, or 3D printing, thermoforming offers lower startup costs than injection molding or machining and a lower cost per part than 3D printing, making it a good solution for small- to medium-sized orders.
Because of these design restrictions, it is essential to follow certain guidelines when creating thermoformed parts. Doing so will help create more accurate and durable parts in addition to prolonging the life of the molds for each part. We put together this guide to cover the general principles of thermoforming design for manufacturing as a place to start thinking about any thermoforming project.
What is Thermoforming DFM?
Thermoforming DFM (Design for Manufacturing) is the process of designing plastic parts in a way that makes them easier, faster, and more cost-effective to produce using thermoforming. It focuses on optimizing factors like material usage, mold design, wall thickness, and draft angles to ensure smooth production and consistent part quality.
Key Thermoforming Design Guidelines
Fundamentally, thermoforming works by drawing a hot sheet of thermoplastic down onto a mold. Each draw uses a single sheet. These sheets can be formed into one or multiple parts, depending on the size of the part. Increasing the number of parts created per draw is one of the best ways to decrease the cost per part of thermoformed products.
After the hot thermoplastic has been drawn down over the mold, a vacuum may be used to suck out any air and accurately reproduce the details of the mold. This process is referred to as vacuum forming. Once the part has cooled, any excess plastic from the sheet is trimmed away, leaving a finished part. These cuts can occur at the base of the part, along the wall, or in any number of custom variations to create features like holes and slots.
Before thermoforming can begin, tooling needs to be made. These molds and trim jigs can be 3D printed, cast, or machined. If the final product is going to be translucent or transparent, the mold may need custom finishing like sanding or polishing in order to achieve optimal surface finish as the finish will pick up on the tool side of theparts. Depending upon the technology and material used, the lifetimes of these molds can range from hundreds of forms to permanent tooling for unlimited forms.
How to Design Parts for Thermoforming
Because thermoformed parts are formed from a single sheet and need to be pulled off of a mold once formed, there are some specific design factors that must be considered when developing thermoformed parts. These include draw ratio, material thickness, draft angles and detail. When designing for thermoforming, it is also important to allow for shrinkage as the hot plastic cools. This will not only ensure that the finished parts have the desired dimensions, it will also allow the part to be lifted off of the mold without damaging the part or the tool.
Draft Angles in Thermoforming
Draft refers to the slight taper or angle in the mold design that facilitates the easy removal of formed plastic parts. The base of the mold creates a “parting line”, similar to injection molds, and must be where the form is at its widest.
Because sheet plastic shrinks when it cools, parts will pinch and stick to a mold if they have completely vertical walls. Therefore, all thermoformed parts require some draft to ensure that they can be removed from the mold.
Wall Thickness
Wall thickness is a critical factor in thermoforming design, as it directly affects part strength, material distribution, and manufacturability. Uniform wall thickness is preferred to avoid weak spots, warping, or thinning during the forming process. Designers should account for material stretching, especially in deep-drawn parts, and adjust initial sheet thickness accordingly to maintain structural integrity.
Male vs Female Molds
Most forming tools are male molds, or positive molds, meaning the tool side, or mold surface that produces the contours in your part, is on the inside of the form. These molds typically require 3-5 degrees of draft to release.
If the tool side is on the outside of the part, also known as a female mold, or negative mold, 1-2 degrees of draft is usually sufficient.
Tool Side Selection
Choosing which surface, the inside or outside, is the tool side is an important consideration because it acts as the controlled surface of your part. Male tooling will typically have less thinning due to better draw ratios (see the draw ratio section below) and cost less to make. If your plastic is textured, the mold will not affect the texture. Female molds provide better feature detail and higher external accuracy. Controlling your critical surface can also give you flexibility to change the sheet gauge later without needing new tooling.
Male Mold with 3 Degrees Draft
Female Mold with 1 Degree Draft
Draft Analysis
Undercuts
Undercuts, or negative draft, also prevent the demolding of a part. Minor undercuts on thin-gauge films are very doable thanks to the inherent flexibility in most plastics. They are very common in HIPS, PP, HDPE, and PETG packaging. A snap fit is an example of this.
For thicker gauges, there are two ways to deal with undercuts: collapsible molds or overcompensating on the opposite side.
Collapsible molds are engineered molds where pieces slide or break apart during demolding and can be reassembled. Sliding joints or magnets are a common way to do this. Separable pieces that interfere with the demolding come off the mold during each cycle. This adds a lot of cost to a project due to increased engineering and mold costs, as well as increased cycle time between forms to reassemble the mold.
Overcompensation of draft means having extra draft on the opposite side of your undercut to allow for the part to be de-molded at an angle. For instance, if you have a -5 degree undercut on a mold, you must have an additional 5 degrees on the opposite side so a part can be removed.
Warning! Undercuts cause excessive thinning due to the sheet plastic stretching more than normal and are very risky. Please consult an engineer. They are best avoided if possible.
Negative 12 Degree Draft Profile
Part and Bed Size
Machine Sizes
Our thermoformers range in size to accommodate various forming areas and draw depths. The specifications are as follows:
- Small Thermoformers: 19″ x 17″ forming area, up to 11″ draw.
- Medium Thermoformers: 44″ x 33″ forming area, up to 24″ draw.
- Large Thermoformers: 92″ x 44″ forming area, up to 40″ draw.
- Extra-Large Thermoformer: 120″ x 60″ forming area.
The largest commonly available sheet plastic measures 96″ x 48″, with a 4″ loss from clamping, equating to the maximum size for our large machines. For larger sheets, a minimum purchase of 500 to 1000 lbs of material is required, sometimes up to 2500 to 5000 lbs. Larger machines increase manufacturing costs.
Multiple Forming Tools
We might quote a job as a 1-up, 2-up, 4-up, etc. We are explicitly calling out the number of forming tools you are purchasing. This defines the number of forming tools we will run in your forming window. Unless parts are very large we typically try to run multi-up tooling for larger volume runs. Even though your tooling cost is slightly higher your unit cost will dramatically improve because we are cutting down on the labor time required to form your parts.
If we can form 5 sheets of 1/4″ ABS in 1 hour, we can only make 5 parts with a 1-up tool, but we can make 20 parts per hour with a 4-up tool. This cuts the forming labor by 75%!
Medium Thermoformer with 2-up Mold and Plug Assist
A production setup with a complicated mold design. Two forming tools are used to get twice as many parts per cycle. Without a plug assist the boss features on this part would web. Multiple molds and plug assists add up front cost but enable consistent and cost effective at scale.
Material Considerations
Custom runs are cost-effective for large orders or when standard sheets yield high waste. For example, producing 28″ x 28″ pieces from a 48″ x 96″ sheet results in significant waste. For large volume runs customers also benefit from additional savings in price per pound with custom runs.
Accounting for Draw in Forming Area Calculations
The Z height (draw) of a part reduces the forming area by its Z dimension. Additionally, 3-4 inches are needed for clamping.
Adding space to accommodate draw will ensure that a part does not overly thin during the forming process, as described below under the Draw Ratio section.
For instance, a part 24″ x 12″ x 6″ tall requires a sheet size of 34″ x 22″ (24″ + 6″ + 4″ by 12″ + 6″ + 4″). This rule is conservative; parts with high draft walls may need less Z accommodation. For example cylindrical parts must adhere more strictly to this rule compared to conical ones. Consult an engineer when in doubt.
Adjustable Forming Windows
On our small machines forming windows can be adjusted for smaller sizes, such as 5″ x 5″, 11″ x 11″, and 15″ x 15″, reducing material costs for smaller parts.
For larger machines, custom vacuum boxes are built to match part size, optimizing material use. These boxes are part of the tooling cost.
Material Price Calculation
The required sheet footprint (X+Y) determined from your forming area calculation above, combined with the selected plastic and gauge thickness, determines the material cost. For short runs we are trying to maximize the yield on stock 4’x8′ sheets – orders under one sheet must buy a full sheet. For orders over 1000 lbs custom runs are available and are made to size with no waste.
The total material price has an additional 10 – 20% scrap factor, depending on part difficulty and order size (smaller orders have more scrap due to setup.)
1mm Thick PETG Bath Bomb Mold - 6" Forming Window
Draw Ratio
When designing a part to be thermoformed, draw ratio, or the proportion of depth to the width of the part, is one of the most important considerations. The depth of any cavity cannot exceed a 4:3 ratio of depth to width. For optimal results, we recommend keeping as close to a 1:1 ratio as possible. The part’s Z height is compared to it’s X and Y individually.
To calculate the overall draw ratio, divide the total surface area of the 3D surfaces by the forming area.
For male molds that is the entire forming area up to the clamps. For female molds it is measured from the mold opening the part is drawn into.
Plug Assists
Excessive draw ratios in thermoforming require careful execution due to their associated risks. When thinning is moderate, form quality can be improved by implementing a plug assist. This method uses a mechanical force to press the material against the mold, rather than relying solely on vacuum pressure.
In cases of large draws, the plastic may lack sufficient pressure to adhere fully to the mold surface. The plug assist provides the necessary additional force to ensure proper mold surface sealing. Also the plastic may bunch together or form webs. A plug assist constrains the material, preventing such issues.
However, if the excessive draw leads to significant thinning, a plug assist may not suffice. In such scenarios, it may be necessary to increase the gauge of the starting sheet or redesign the part to address the problem effectively.
Example of Webbing
Thickness
Thermoplastic sheets come in a wide variety of thicknesses, ranging from 0.015” to 0.250”. When thinking about how thick of a sheet to use for your part, it’s often helpful to understand how much your part will thin out during forming. To do this, divide the sheet’s thickness by the draw ratio of the mold. This will give you an idea of the average thinning, though, in reality, it will vary above and below this. The only way to understand the exact local thinning on a part is to make one and measure it.
The thinnest points will usually be at the corners of internal cavities, followed by the corners of ridges. To minimize thinning, we recommend keeping radii on corners and edges as large as possible, especially at the bottom of cavities and channels. As a general rule of thumb, parts should have a minimum fillet radius of ½ the material’s original thickness on the inside of the bend.
Common Thermoforming Design Mistakes
- Excessive thinning from high draw ratios and sharp corners. Try reducing draw or adding fillets to your design. 1:1 draw ratio or less
- Webbing or bunching of material during draw. Spread features out more to allow plastic to stretch into recesses.
- Plastic Shrinkage. Every plastic has a unique shrink rate. Calculate that shrink and adjust the mold size to accommodate. Amorphous plastics like HDPE and PP shrink more than crystalines line ABS or PETG.
- Collapsible Tooling and Undercuts. These can be very tricky and require a great deal of experience. Please consult our expert.
- Non-Uniform Thicknesses. Features that are not formable can be achieved via 3D printing / machining and assembly with glue or hardware
Thermoforming Tolerance Guidelines | ||
|---|---|---|
| ACTIVITY | TOLERANCE | GUIDELINE |
| FORMED MEASUREMENTS < 6″ 6″ – 12″ 12″ – 18″ > 18″ | +/-0.015″ +/-0.025″ +/-0.030″ +/-0.030″ +/-0.002″ per inch | Tolerance and size increase proportionally |
| DRILLED HOLE DIAMETERS | +/-0.005” | Holes Equal or Lesser than 1” |
| DRILLED HOLE DIAMETERS | +/-0.010” | Holes from 1” to 5” |
| SLOTS | +/-0.010” | Slot Equal to or Lesser than 1” Any Direction |
| SLOTS | +/-0.020” | Slots Greater than 1” |
| 5 AXIS TRIM | +/-0.015” | Trim Features under 5” |
| 5 AXIS TRIM | +/-0.020” | Trim Features Greater than 5” |
Thermoforming Materials
Thermoforming materials like HIPS, PETG and ABS offer a range of mechanical, chemical and aesthetic properties. Thermoformed parts can rigid or flexible; transparent or opaque; and food-safe, heat-resistant, chemical-resistant, or UV-resistant. Below are our most commonly used materials for thermoforming. To get more information about any material, check out the included data sheet for all the specifications.
HIPS (Polystyrene)
Our most commonly-used material. Inexpensive, functional material that can be brittle at low temperatures and can off-gas at higher temperatures. Used for packaging trays, covers and light-duty structural pieces. Food-safe versions available.
PETG (Polyethylene Terephthalate) (Polyester)
Moderately inexpensive material with good water and oxygen barriers. Able to stand up to substantially lower temperatures than HIPS. Often used for food-safe applications, freezer packaging and water bottles.
ABS (Acrylonitrile Butadiene Styrene)
Medium-cost impact-resistant engineering plastic which can be flame retardant or UV resistant when blended with other materials. Used for high-end packaging and moderate-load structural components.
Kydex T (ABS/PVC) or Kydex 100 (Acrylic/PVC)
Expensive flame-retardant engineering plastic with high impact resistance. Used for moderate-load structures, covers and enclosures that require fire resistance. Kydex 100 is our go-to material for radomes.
PC (Polycarbonate)
Medium- to high-cost engineering plastic with high stiffness, impact strength and temperature resistance, plus options for UV and scratch resistance. Often used for glass replacements on phones, TVs, lights or glasses, as well as high-temp applications. Harder to form than most thermoplastics, especially for fine details.
PE, HDPE or LDPE (Polyethylene)
Moderately hard, inexpensive plastic with high chemical resistance. Does not off-gas at high temperatures. Chemical and thermal durability makes it well-suited for chemical-resistant containers. Higher shrink rate than other materials, which lowers tool life and increases variability between parts.
PP (Polypropylene)
Moderately-priced alternative to PE which improves thermal and mechanical properties. Higher level of chemical resistance than most plastics. Can be used as an engineering plastic. Used for chemical-resistant applications, including food contact.
PVC (Polyvinyl Chloride)
Hard engineering plastic with strong mechanical properties as well as high chemical and electrical resistance. Can be made rigid or flexible. Used for certain chemical-resistant containers.
Acrylic
An inexpensive, rigid and brittle plastic with relatively high UV resistance. More difficult to form than other plastics. Not intended for tight bends or details. UV resistance makes it well-suited to outdoor applications.
Molds for Thermoforming
Traditionally, molds for thermoforming have been machined from urethane or aluminum. Now, 3D printing has become an excellent choice to produce tooling for parts smaller than 11” x 15”. 3D printed molds, especially those produced with Multi Jet Fusion printing, significantly lower the cost per part and speed up production time when compared to machined molds. Moreover, they can easily achieve complex shapes like undercuts that would be expensive or impossible to machine.
Ultimately, each technology offers different strengths for the tooling process. Molded tooling, for example, is good for creating multiple long-lasting molds. Machined tooling is a better option for making large molds, and, depending on the material used, can be permanent or semi-permanent. Even among 3D printed molds, there are different ideal applications for each type of 3D printer.
Material Selection
Material selection determines the performance, durability, and cost of thermoformed parts. Different plastics behave differently under heat and pressure, so choosing the right material depends on factors like strength requirements, flexibility, temperature resistance, and end-use application. Common thermoforming materials include ABS, PET, and polycarbonate, each suited for specific use cases.
Types of Molds
Multi Jet Fusion (MJF)
The overall best tooling option for parts smaller than 15”x11”
The cheapest print option
High accuracy
Permeant tooling
Generally does not require additional finishing for opaque plastics
Aluminum-Filled Urethane
Create long-lasting tooling or multiples of a tool at a lower price than printing
Requires a 3D printed or machined master mold
Permanent tooling
Cheaper to reproduce in multiples than 3D printing, easily get high level of finish for polished parts
Machined Wood/MDF
Absolute cheapest option for medium to large tools for prototyping
Life is less than 50 forms
Poor to average accuracy
Machined Urethane Foam
The best option for larger, permanent tools where matte surface finish on tool face is acceptable
Average to good accuracy
Machined Aluminum
Permeant tooling
Used for automated, high-volume production with fast heating and cooling cycles
Good to excellent accuracy
Best surface finish of any mold, needed for high polish on mold side or clear parts
Trimming
Once the thermoplastic sheet is formed around the mold, it must be trimmed of excess material to create the final part. The type of cut used at this stage will impact the cost and speed of the finishing process. Trimming in generally done on a 3 Axis or 5 Axis CNC mill or if simple enough can be done with a hand router or on a table saw. Most capable forming shops will employ many different CNC machines and other tools to give great versatility in the trims they can accomplish.
In addition to creating the final part shape via trimming the outline from the formed mold trimming is vital to creating other features in a model like mounting holes, openings, vents and much more.
Vertical Cut
Most common cut
Creates a lip around the part a specified distance from the wall
Recommended minimum lip of 1/16”
Can be done on 3 Axis CNC or hand trim with end mill or router bit
Horizontal Cut
Second most common cut
Cuts the wall at a specified height
Primarily used for aesthetics or for fitting parts together
Can be done on 3 Axis CNC or hand trim with saw bit
Hole Cutout
Holes of most shapes can be cut out of a flat surface
Generally requires a small radius when used in corners (1/8” recommended, 1/16” achievable)
Can be done on 3 or 5 Axis CNC
Custom Cut
Varies from a multi-height horizontal cut to a complex, organic cut across multiple surfaces
Requires 5 Axis CNC milling
Finishing
Custom finishes are a great way to add improved aesthetics and functionality to thermoformed products. Like all of our products, we offer a range of finishes for our thermoforming services.
Custom Painting
Custom paint can be applied to formed parts to achieve a variety of colors and lusters. Custom pantone and RAL color matches available as well as lusters from flat to high gloss.
ESD Coating
A clear ESD coating to give your plastics electrical discharge that can be applied to plastics including PETG, ABS and PVC
UV Clear Coat
A clear coat (matte or gloss) that extends the life of plastics in outdoor applications
Sandblast
Adds light diffusion to clear part or high-quality matte texture to opaque parts
Get a Quote on Your Thermoforming Project Today
Thermoforming is a great plastic manufacturing technology with a wide range of applications. When paired with 3D printed tooling, it gains even more utility.
If you want to get started with a thermoforming project, RapidMade can help. Get started today by getting a free quote and design analysis in under 24 hours using our quote tool, or learn more by contacting us or one of our regional sales representatives.
When to Use Thermoforming vs Injection Molding
Factor | Thermoforming | Injection Molding |
Best for Volume | Low to medium production | High-volume production |
Tooling Cost | Low (aluminum molds) | High (steel molds) |
Per-Part Cost | Higher at scale | Lower at scale |
Lead Time | Faster setup and production | Longer due to tooling complexity |
Design Complexity | Moderate | High (complex geometries possible) |
Wall Thickness Control | Less precise (material stretching) | Highly precise and consistent |
Material Waste | More trimming waste | Minimal waste |
Surface Finish | Good, but limited detailing | Excellent, high-detail finishes |
Part Strength | Lower structural consistency | Higher strength and uniformity |
Ideal Use Cases | Large parts, prototypes, custom enclosures | Small, complex, high-precision parts |
Time to Market | Quick turnaround | Slower initial setup |
FAQs
What is draft angle in thermoforming?
Draft angle in thermoforming refers to the slight taper added to the vertical walls of a part to allow easy removal from the mold. Without proper draft, parts can stick to the mold, causing defects or damage. Typically, a draft angle of 3° to 5° is recommended, depending on the material and part depth.
What is the minimum wall thickness in thermoforming?
The minimum wall thickness in thermoforming depends on the material and part design, but it generally ranges from 0.5 mm to 1.5 mm. Designers must account for material stretching during forming, which can reduce thickness in deeper areas. Maintaining uniform thickness helps ensure strength and consistency.
Is thermoforming cheaper than injection molding?
Yes, thermoforming is usually cheaper than injection molding for low to medium production volumes due to lower tooling costs and faster setup. However, for high-volume production, injection molding becomes more cost-effective because of its lower per-part cost and higher efficiency.
What is thermoforming design for manufacturing?
Thermoforming design for manufacturing (DFM) is the process of designing plastic parts to ensure efficient production, cost reduction, and structural integrity during thermoforming.