Understanding tolerances of plastic parts

General tolerances in plastics processing

General tolerances in plastics processing The understanding of general tolerances of plastic parts is not always understood by mechanics used to CNC machining or tolerancing.

Some French plastics manufacturers also use a French standard (NFT58000), which has been obsolete for almost 20 years! DIN ISO 20547 dating from 2021 is much more suitable for technical materials!

Principle to understand:

Injected thermoplastic materials are first heated and then injected under pressure into a mould cavity. Between the plastic phase (soft material) and the solid phase, the material shrinks during cooling. The shrinkage is taken into account in the design and manufacture of the mould. (Overall, a cavity is made larger than the part...so that the right size is reached during cooling...quite easy isn't it?

except that ....

Not all materials have the same shrinkage rate
Shrinkage is not always the same in length and width of the material flow
The injection parameters used (pressure, temperatures) have an influence on the shrinkage
Mould temperature also varies the shrinkage (and also the potential deformation of the parts)
.... and the list can be much longer...if we also consider that the material supplier also has manufacturing tolerances...as does the mouldmaker...and the moulding company...

Since shrinkage is expressed as % of the length, it is immediately clear that the tolerance will also be proportional to the length of the product...(just as is the case for a geometric tolerance in mechanics, which is proportional to the dimension)..... This being said, we have already destroyed the concept of general tolerance to ± 0.2!

How ISO 20457 works

The first point to note is that the ISO20457 standard is not fixed like the previous one on existing resins since it takes into account intrinsic criteria of the materials and the types of transformation that can be used.

Are you ready? Let's get to the heart of the matter:

The tolerance grid (TG) to be used, will be based on 5 material / process and product criteria combined!

P1 : Which transformation process will we use?
Injection - Compression (1 point)
Extrusion - Blowing (2 points)
P2: What is the stiffness of the material used? (Elasticity module)
> 1200 MPA (1 point)
> 30 MPA (2 points)
> 3 MPA (3 points)
< 3 MPA (4 points)
P3: What is the shrinkage of the material used? (see above...)
<0.5% (0 Point)
< 1% (1 point)
< 2% (2 points)
> 2% (3 points)
P4: Shrinkage control (by process)
± 10% (1 point)
± 20% (2 points)
> 20% (3 points)
P5: What is the expected quality
Very accurate (-3 points)
Accurate (-2 points)
Neat (-1 point)
Normal (0 points)

TG = P1+P2+P3+P4+P5

Need an example?

Injected part (1 Point)
in ABS (modulus of elasticity: 1600 MPA) (1 Point)
Shrinkage: 0.7% (1 Point)
Control of shrinkage (do we have the data sheet?) - (1 Point)
Commonly used product (normal) - (0 Point)

TG = 1 + 1 + 1 + 1 + 0 = TG4

The standard ISO20457just like its ancestor NFT58000In addition to the above, a distinction is made between dimensions that do and do not include the parting line in the dimension chain.

FAQ plasturgie

Qu’est ce que la norme NFT 58000 ?

La norme NFT 58000 définit les tolérances applicables pour des pièces en plastique injecté, suivant le matériau plastique, la dimension et la classe de précision demandée par le cahier des charges du produit.

Quelle est la diférence entre la norme NFT58000 et la norme ISO20457 ?

La norme NFT 58000 a été éditée dans les années 80, la norme ISO20457 est beaucoup plus récente. D’autre part, la norme NFT58000 est utilisée uniquement en France, à l’international, la norme ISO20457 est plus courante !

Quel est l’impact des tolérances sur le cout de fabrication ?

Plus les tolérances sont serrées, plus la probabilité de réaliser une pièce non conforme est importante. Cela signifie que le process est plus délicat à maintenir dans une fenêtre acceptable. il est commun de dire que pour réaliser une pièce tolérancée à ±0.1 mm, il faut une machine capable de réaliser les pièces à ± 0.01 (soit 10 fois plus précise).

Tolérance et angle de dépouille…comment fait on ?

Effectivement, quand on parle de pièce injectée en plastique, les angles de dépouille entrent en jeu. il est commun de noter au plan un annotation de type : »dépouille en plus » ou « dépouille en moins », en fonction du jeu fonctionnel demandé.