Coordinate Measuring Machine Layout Protocols for Flexible Plastic Part Boundaries
Flexible plastic part boundary metrology requires ISO 10579 restrained datum targets and controlled clamping forces to yield repeatable CMM dimensional data.
Polyamide Moisture Expansion
Meaning
Engineering calculations for nylon assemblies must account for a dimensional change where the polymer expands as it absorbs water from the ambient environment. This specific occurrence of polyamide moisture expansion happens because the polar groups in the polymer chain attract water molecules, which force the chains apart and increase the overall volume. In a precision moulded housing, this increase in size can lead to parts binding in their guides or mechanical fasteners becoming overly tight.
Moulders use the term to distinguish between initial thermal shrinkage that happens inside the press and the secondary growth that occurs days or weeks after the part is shipped. Failure to predict this growth can lead to assemblies that fail in humid tropical climates after passing inspection in a dry facility.
Volume Fluctuation
Managing the effects of polyamide moisture expansion requires the simulation of the end environment to establish the final part size after reaching equilibrium. Parts leave the injection machine at their smallest dry as moulded dimensions before they slowly gain weight and size as they pull moisture from the air. This process continues until the internal humidity of the plastic matches the saturation level of the room, a state that can take months in thick wall sections.
Some manufacturers submerge their parts in warm water baths to force this expansion before they perform the final quality check. If this step is skipped, a part that fits into its mating unit perfectly on Monday might grow too large to remove by Friday. Using filled grades with minerals or glass slows the rate of expansion but does not stop the chemical mechanism itself.
Material Properties
Resistance to impact and flexibility increases simultaneously with polyamide moisture expansion as the water molecules act as an effective plasticizer for the hard resin. While the size increases, which is a negative for fit, the toughness of the part improves, which is a positive for performance in cold weather. Engineers have to balance these two outcomes when choosing between different variants like nylon six or nylon twelve.
Nylon twelve is generally preferred when dimensional stability is the priority because it absorbs roughly one third as much water as nylon six. If the humidity drops during a cold winter, the parts will contract as they lose moisture, potentially causing gaps or rattles in the product assembly. This dynamic nature means that a single measurement on a datasheet is rarely enough for a complete design.
Dimensional Compensation
Validation of tolerances involves calculating the difference between the manufactured target and the final stable size affected by polyamide moisture expansion. A moulder might intentionally make a part half a percent smaller than the final print size to allow for the inevitable growth during shipping. Verification protocols require that parts are stored in sealed foil bags if they must remain dry for subsequent welding or gluing operations.
Once the bag is opened, the clock starts on the growth cycle, which affects every critical feature from hole spacing to wall thickness. Quality reports must state the ambient humidity at the time of measurement to be truly useful for long term comparisons. Successful production depends on mastering these hidden changes in part volume.