How Much Does PETG Shrink? A Scientific Answer

During 3D printing, quickly heating and then cooling the filament leads to an overall shrinkage of the product. Because of this, it’s critical to keep shrinkage in mind when designing your 3D model. 

PETG shrinks 0.3% to 0.8% after printing and cooling. The exact shrinkage rate will vary depending on the additives in your PETG filament, your printed object’s design, and how quickly you cool the print. 

In the rest of this article, I’ll continue to scientifically explain how and why PETG shrinks during the 3D printing process. I’ll also give you some tips on compensating for PETG shrinkage. So, let’s get into it!

Why Does PETG Shrink?

PETG shrinks because it undergoes thermal contraction and expansion as you print it. Thermal contraction is when the volume of a material decreases as the temperature decreases. Thermal expansion is essentially the opposite of thermal contraction. When a material is heated, it expands. 

Both of these processes are at play during 3D printing. When the filament you use is heated, the volume of the plastic will expand, making it larger than its original size. Then, as it cools, the filament’s volume decreases, causing your end product to shrink. 

The decrease in volume is caused by a reduction in the kinetic energy of the material. At lower temperatures, there’s less energy because particles are moving slower and are squeezing closer together. The boundaries of the material gradually shrink as the particles move closer together. 

This thermal contraction that occurs as the filament cools gives PETG its strong, rigid structure. As PETG cools, those molecules settle in closer together, both shrinking and hardening. 

Expansion and contraction affect all forms of physical matter, including gasses, liquids, and solids. However, it’s most noticeable in gasses and less prominent in solids. 

You may be unknowingly familiar with thermal expansion and thermal contraction. For example, when a jar is difficult to open, many people will run the cover under hot water to get the particles moving faster, thus expanding the lid and making it easier to remove. This process is an example of thermal expansion. 

PETG Compared to Other Filaments

PETG stands for Polyethylene Terephthalate glycol and has quickly become a popular filament material in the world of 3D printing. 

PETG improves some of the downfalls of other filaments such as ABS and PLA. However, it also has all the best qualities of both of these filaments. For example, PETG has the strength of ABS and the nontoxicity and easy-to-use qualities of PLA.

The material is a perfect blend of chemicals that make it strong, durable, and versatile. The particular chemicals also cause it to differ from other filaments since it can withstand thermal expansion and thermal contraction. 

Does PETG Shrink More Than PLA and ABS?

You may be looking for the optimal filament to create your prints with. PETG might be what you’re looking for if a low shrinkage rate is essential to you.

PETG shrinks about as much as PLA and ABS. However, that’s not always the case. PLA and ABS both have extremely variable contraction rates. PLA can shrink from 0.2% to 3%, while ABS can shrink from 0.8% to 8%. 

Still, PETG usually shrinks less than ABS, making PETG the optimal material choice when considering shrinkage rates. The lower rate lets you print larger objects than you could with PLA and ABS while the filament maintains its other superior qualities.

However, while the shrinkage rate may be superior to other filaments, it’s certainly not negligible. Read on to learn how you can best prepare for PETG shrinkage. 

How Do You Compensate For PETG Shrinkage?

To compensate for PETG shrinkage, you’ll need to scale your design so that it takes this into account. That means you should scale the size of your object up by the shrinkage rate of the filament.

As I mentioned before, PETG expansion and contraction can vary depending on the brand you use. Lower quality brands have less pure PETG and will therefore shrink more. The best way to determine the overall shrinkage rate is to experiment yourself.

Run a test print on your original design. Determine the difference in measurements of your model versus the end product. Then, you can do the calculations to figure out the exact shrinkage rate.

Here are the steps to do the calculation for the shrinkage rate:

1. Divide the end product’s measurements by the original model’s measurement.
2. Multiply that number by 100.
3. Subtract that number from 100.

Let’s take an example. Say you have a cube model that is 5x5x5 inches (12.7x.12.7×12.7 cm). 

After printing these dimensions, your cube comes out to be 4.98×4.98×4.98 inches (12.65×12.65×12.65 cm).

First, calculate 4.98 divided by 5 to get 0.996. Multiply by 100 to get 99.6. Then, 100 minus 99.6 equals 0.4. Therefore, the shrinkage rate is 0.4%.

Now, you must scale your model up by 0.04%. 

Here are the steps to do the calculations to rescale your model:

1. Multiply your original model’s measurements by the shrinkage rate.
2. Add that value to the dimensions of the original model.

So, going back to our example, we should multiply 5 times 0.4% and get 0.2. 5 plus 0.2 equals 5.2.

Therefore, instead of printing 5x5x5, we would print 5.2×5.2×5.2. This size will compensate for the shrinkage rate so that your end product comes out to the original model’s measurements. 

The Ideal Printer Settings To Minimize PETG Shrinkage

To minimize the amount of shrinkage when using PETG filament, you should also make sure the environmental conditions are the most optimal.

First, there’ll be an ideal printing temperature depending on your brand. PETG filament will melt anywhere between 220 to 270°C to (428 to 518°F). The heated bed temperature must also be ideal for your specific filament brand and formula. So, be sure to check the spool or the website where you got your PETG for the best settings. 

Second, there’s an ideal printing speed when using PETG filament. The faster the printing is done, the lower quality your end product will be. 

Ideal printing speed can be anywhere between 30 mm/s (1.18 in/s) to 100 mm/s (3.94 in/s). Still, the optimal print speed can vary based on your brand and the object you’re printing. The brand of the filament you use should have a recommendation, but sometimes you can only determine the ideal printing speed through trial and error.

Conclusion

The shrinkage rate of PETG is between 0.3 to 0.8%, depending on the brand you use and your product’s design.

While this may be less significant than other filaments, you should consider it when designing your model. You can easily compensate for the shrinkage rate through a test experiment and simple calculations.