Din 16901
DIN 16901 was adopted in 1973 to provide plastic parts with tolerances. The standard was replaced by the modern DIN 16742 in 2013.
Inexpensive but problematic plastic material
The main manufacturing processes for plastic molded parts are injection molding and thermoforming. In injection molding, plastic granulate is heated to a plastic molding compound and injected into a separable mold at high pressure.
In the case of thermoforming, a plastic sheet is placed over a hot tool and pressed. In both processes, post-processing is only required to a limited extent.
The workpieces are deburred and unavoidable production residues – such as webs from risers – are removed. In addition, plastic molded parts are provided with the necessary holes or punched holes at best.
Further in-depth machining, such as milling, grinding or turning of metal parts, does not normally take place in this manufacturing process. Reworking would not only reduce productivity.
A major challenge in the manufacture of plastic products continues to be the difficulty of recycling the raw material.
While metals can be melted down again and again without any problems, the recycling of plastics is still only possible to a very limited extent. This also rules out costly, ablative reworking of the plastic molded parts in the production process.
The challenge of tolerance specifications
The shape of the plastic molded parts depends on various factors:
- Homogeneity of the molding compound
- Process temperature
- Contact pressure of the mold halves
- Condition of the injection mold
- Ambient temperature
- Position deviations of moving parts in the mold
Furthermore, even under ideal conditions, plastic has properties that distinguish it from metals, which are comparatively easy to calculate.
For example, plastic molded parts have a strong shrinkage behavior after solidification, which must be taken into account during production.
In practice, this is achieved by “re-pressing” the molding compound in the cavity. Nevertheless, shrinkage in plastics is a serious factor for deviations from the standard.
DIN 16091 and its deficits
This multitude of factors can have a negative impact on the tolerance of the molded parts. DIN 16901 was created in 1973 to get a handle on this.
This standard was valid for almost 40 years. After 20 years, however, major gaps have become apparent which have made it necessary to revise DIN 16901. The shortcomings of the standard were as follows:
- No consideration of shrinkage behavior
- lack of CAD processing
- Molding compound based on only 43 components taken into account
- No consideration of modern measurement methods
Innovations in DIN 16742
These serious shortcomings of DIN 16901 have led to the automotive industry no longer taking it into account for over 20 years and instead working with its own regulations.
The successor standard since 2013 has been DIN 16742, which is once again recognized across all industries. A significant difference to DIN 16901 is that the new standard no longer works with a “molding material table”.
Instead, it provides the user with general influencing factors of processes and materials. This gives the user an indication of the technically feasible tolerance for each type of plastic.
The subsequent manufacturing and testing costs are also specified for smaller tolerances. Shrinkage is also taken into account in the new standard. This means that DIN 16901 is no longer a recommended basis for determining the tolerances of plastic molded parts.
New challenges for the tolerance of molded plastic parts
The new standard is also based on an original end product. However, this shows that DIN 16742 will also need to be revised in the near future.
This is because the production of plastic molded parts still refers to the classic manufacturing processes such as thermoforming or plastic injection molding. The new additive processes are not included in the standard.
In 2013, industrial 3D printing was still a marginal phenomenon. However, the possibilities of rapid prototyping and rapid molding show just how powerful modern plastics are.
In some cases, these 3D measurement technology processes are not only used to produce end products in the 3D printer. Modern machines and materials even enable the production of tools for plastic injection molding.
These are nowhere near as durable as traditionally milled or eroded steel injection molds. However, these quick tools are perfectly adequate for small batches and can produce plastic molded parts using traditional methods for a fraction of the cost. Control of position and dimensions is made possible by optical 3D measuring methods.
Changing standards
Hardly any other branch of production is as dynamic as the processing of plastics. It is therefore a good thing that the old DIN 16901 has now been updated.
However, the rapid development in this area shows that the standard setters cannot afford to wait another 40 years until the next revision. Otherwise, DIN 16742 will suffer the same fate as DIN 16901 – it will simply be ignored by the industry.
