🎯Getting Tolerances Right in Injection Molding: Why Standards Matter! (ISO 20457 & ISO 3302)

Hello and welcome to a new blog post. When it comes to plastic and rubber parts, achieving the right tolerances is key to ensuring quality, fit, and function. But did you know there are specific ISO standards that guide this process?

🔹 ISO 20457 for Plastics:

This standard provides clear guidelines for defining tolerances and acceptance conditions for dimensions of plastic parts produced by processes like injection molding, extrusion, or thermoforming.

Different tolerance groups (TG) are defined:

TG 6 for packaging parts

TG 5 for housing parts

TG 4 for precision parts (like gears)

Note: ISO 20457 applies only to unfilled thermoplastics.

🔹 ISO 3302 for Rubber:

For molded rubber parts, ISO 3302 is the go-to standard. It divides molded parts into four tolerance classes, from M1 (fine) to M4 (coarse). For most technical molded rubber parts, M3 (medium) is typically used.

Figure 1: Overview of ISO 20457 for thermoplastics and ISO 3302 for rubber injection molded parts.

In conclusion

Understanding and applying the right tolerance standards helps ensure your parts meet performance expectations and manufacturing requirements—saving time, reducing costs, and boosting customer satisfaction. 

As a rule of thumb, for precision injection molded parts such as gears, ISO 20457 TG 4can be applied. For housing parts TG 5 and packaging parts TG 3. For rubber molded parts,  ISO 3302 is key, and in particular M1 (fine) to M4 (coarse) (Figure 1).

Check out this post too, where tolerances are important to turn product requirements into plastic & plastic part specifications: read full post.

Thanks for reading & #findoutaboutplastics

Greetings, 

Herwig 

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Literature: 

[1] https://maxnext.io/de/blog/allgemeintoleranzen-im-spritzguss-nach-din-iso-20457-ehemals-din-16742-2/

[2] https://www.nh-technology.de/download/Design-Construction-Guide-Plastic-Parts.pdf

[3] https://www.zorge.com/gummi-lexikon/din-iso-3302-1/

[4] https://www.toleranzen-beratung.de/unternehmen/aktuelles/ansicht/iso-204572018-toleranzen-fuer-kunststoff-formteile-informationen-zur-neuen-norm/

Polymer Processing: Tolerance and Roughness Charts

Hello and welcome to this blog post on tolerance and roughness charts for different polymer processing techniques. This post supports you in the polymer shaping selection as part of the material selection process.

1. Tolerance chart: A part will not be exactly shaped to a specified dimension. There will be a deviation Δx from a desired dimension x which is allowed by the specifier. This is in general referred to as tolerance T which is defined as e.g. x =100 ± 0.1mm, or as 0.01. The bar chart allows selecting different polymer processing techniques to achieve a desired tolerance.

2. Roughness chart: A part will have different surface roughness R, which is measured by the root-mean square amplitude of the irregularities on the surface. A rough surface will have an R < 100 μm and a high polished surface has a R < 0.01 μm. The bar chart allows selecting different polymer processing techniques to achieve a desired surface roughness.

Thanks for reading and #findoutaboutplastics

Greetings

Herwig Juster

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Literature: 

[1] Granta - Material and Process Selection Charts, 2010