Rubber molding tolerances are allowable variations in the dimensions of a rubber product. They are expressed as plus or minus values, divided into drawing designations, and have both fixed and closure dimensions. The Rubber Group provides rubber molding services and helps customers to specify molding tolerances for part drawings.
Keep reading for answers to frequently asked questions (FAQs) about rubber molding tolerances, and contact us to request a quote or discuss your application.
Why do rubber parts have tolerances?
All manufacturing processes and machines have some degree of variability. Tolerances provide an acceptable range of variations for manufactured part dimensions. Designers and engineers assign tolerances to part drawings to communicate acceptable limits. These tolerances indicate what’s needed for parts to fit together properly, function as intended, and meet quality standards.
How do rubber tolerances differ from metal tolerances?
Rubber products have tolerances to account for dimensional variations that occur during injection, transfer, or compression molding. These tolerances are not as “tight” or exacting as the tolerances for machined metal parts. That’s because rubber is more sensitive to temperature and humidity. It’s also because rubber is more flexible than metal and uses different tooling.
What are the drawing designations for rubber molding tolerances?
There are three drawing designations for rubber molding tolerances.
- A1 – High Precision
- A2 – Precision
- A3 – Commercial
The Association for Rubber Products Manufacturers (APRM) maintains these drawing designations and arranges them into tables. The Rubber Group provides a provides a downloadable PDF with ARPM molding tolerances. As a rule, tolerances with a higher degree of precision are more challenging to achieve and are therefore more expensive.
What’s the difference between fixed and closure dimensions?
Fixed dimensions are features that are machined into one half of a mold. They are determined by the precision machining of mold plates and are not affected by flash thickness. By contrast, closure dimensions are created across the parting line where the two mold plates meet. They are dependent on the final closure of the mold and affected by flash thickness.
What else do designers need to consider when applying tolerances?
Consider the following when applying tolerances
- Fixed dimension tolerances apply to each fixed dimension.
- Closure dimension tolerances are determined by the largest closure dimension.
- Very soft rubber (15 to 30 durometer) shrinks by 3% to 4%
- Firm to almost rigid rubber (65 to 85 durometer) shrinks 1.5% to 2%, which allows for tighter tolerances.
Who can help designers with rubber molding tolerances?
The Rubber Group can help customer to specify the right rubber molding toleranccs for their part drawings. If you need injection molding, compression molding, or transfer molding services, we invite you to contact us.