Plastic Extrusion Tolerances
Plastic Extrusion Tolerances
Plastic resin expands as it’s heated and shrinks as it cools. This causes slight dimensional variations during the extrusion process as the melted plastic passes through the extrusion die and cools into a solid part. Without further processing, plastic extrusions will always deviate from the ideal dimensions specified in the engineering design.
The key to a successful part is to determine the maximum range of dimensional variation, or tolerance, that’s acceptable for the proper fit, form, and function of your part.
Standard Extrusion Tolerances
Every feature of an extruded part needs a specified tolerance to facilitate quality control. Fortunately, you don’t have to scrutinize every single dimension. For non-critical parts and features, performance requirements can be met under a standard range of tolerances.
| Measurement (MM) | Profile Dimensions | Wall Thickness | Length, No Fab | Length, Fab |
| 0.00 - 2.00 | +/- 0.25 | +/- 0.25 | +/- 1.50 | +/- 1.00 |
| 2.01 - 3.18 | +/- 0.25 | +/- 0.30 | +/- 1.50 | +/- 1.00 |
| 3.19 - 5.00 | +/- 0.25 | +/- 0.38 | +/- 1.50 | +/- 1.00 |
| 5.10 - 12.70 | +/- 0.25 | n/a | +/- 1.50 | +/- 1.00 |
| 12.71 - 25.40 | +/- 0.38 | n/a | +/- 1.50 | +/- 1.00 |
| 25.41 - 50.00 | +/- 0.50 | n/a | +/- 2.00 | +/- 1.00 |
| 50.01 - 63.50 | +/- 0.50 | n/a | +/- 3.00 | +/- 1.00 |
| 63.51 - 101.60 | +/- 0.76 | n/a | +/- 3.00 | +/- 1.00 |
| 101.61 - 500.00 | n/a | n/a | +/- 3.00 | +/- 2.00 |
| 500.01 - 1000.00 | n/a | n/a | +/- 4.00 | +/- 3.00 |
| 1000.01 - > 1000.01 | n/a | n/a | +/- 5.00 | +/- 4.00 |
Hole Size +/- 0.30
Tighter tolerances can be held for critical features, but doing so may require specialized tooling that can be cost prohibitive. That being said, carefully consider what needs to be maintained as “critical” and consult with your extruder to see if modifications can be made to the material, design, or process to improve tolerances without additional tooling.
Achieving Optimal Tolerances
If you need to specify tighter tolerances but don’t have the budget for specialized tooling and fixtures, there are a few things you can do to ensure your dimensions are held as close to ideal as possible.
The major factors that influence the level of tolerance that can be maintained in plastic extrusion are the shrink rate of the material, the complexity of the design, and the process conditions under which it’s manufactured. Any change in these factors will affect the tolerances of the finished part.
To achieve the best tolerances possible without the use of secondary processes, look at each factor for opportunities for improvement.
Material
All thermoplastics are subject to thermal expansion as they’re melted into a viscous state and thermal contraction as they’re cooled into a solid. The degree of contraction, or the shrink rate, has the greatest impact on tolerance control.
There are methods to improve shrink rates throughout the extrusion process, but as a general rule, rigid materials such as PVC will have a lower shrink rate and maintain tighter tolerances than flexible materials such as TPE.
Profile Design
The more complex the profile design, the harder it will be to maintain tolerances. For the best results, keep the wall thickness of your profile uniform and avoid the use of hollow areas.
Wall Thickness
The flow of plastic through the extrusion die needs to be consistent so that it cools at the same rate along the length of the profile. Variations in wall thickness make it difficult to regulate the flow and can result in inconsistencies in shrinkage and distortions such as bowing and twisting. For tighter tolerances, make the wall thickness as uniform as possible.
Hollows
Hollows are created by specialized tooling within the profile die and maintained during cooling with vacuum sizing. The resulting inner dimension is much more difficult to cool than the outer dimensions, resulting in uneven cooling of the hollow wall and increased warpage. Projections within the hollow are even more difficult to control. For tighter tolerances, avoid hollows whenever possible.
Hollows that exist to maintain uniform wall thickness are still preferable to unbalanced walls.
Process Conditions
Since plastic isn’t 100% contained by a metal mold in the extrusion process, the condition of the surrounding environment has a significant influence on the final result. Process conditions should be anticipated and controlled for consistency throughout the life of the program.
A change as small as ambient temperature or humidity can impact warpage. A change in machine operators or fabrication personnel can result in variations. Imagine the significant consequences of even a small change in the material from batch to batch!
With so many variables in process conditions, it’s critical that your extruder knows what conditions might affect your part and has sufficient quality control measures in place to mitigate against them.
Resources for Engineers
If you’d like an expert opinion on specifying or maintaining tolerances for your extruded plastic part, call the extrusion team at Gemini Group. We’ve spent over 40 years strengthening our ability to hold tolerances and we’re eager to help you succeed.
To learn more about plastic extrusion tolerances, materials, and design, download your free copy of the Plastic Extrusion Design Guide. Simply fill out the form below to get access today!
Contact Gemini
Gemini Group’s extrusion team is comprised of Gemini Plastics, Sierra Plastics, and Gemini Plastics de Mexico. Our sales engineering team would love to hear from you. Feel free to contact them at 248-435-7271 or plasticsales@geminigroup.net.