ABS Trouble Shooting - Molded Part Cracking / Plated part cracking

Typical cracking of plated parts

Plated layer cracking (at high temperatures)	Plastic layer cracking (at low temperatures)

Causes:

The principal cause of this type of cracking is thermal stress - i.e., tensile stress in excess of the corresponding material characteristic (i.e., strength) which is caused as a result of differences in the degree to which the plated and plastic layers expand or contract in response to changes in the ambient temperature. Specifically, cracking occurs in the plated layer at temperatures higher than the plating temperature; and in the plastic layer, at lower temperatures.

Material	Young's modulus (MPa)	Coefficient of linear expansion (1/deg.C)
Copper	1.23 x 10**5	16.6 x 10**-6
Nickel	2.01 x 10**5	12.8 x 10**-6
Chromium	2.48 x 10**5	8.2 x 10**-6
ABS	1,900 to 4,000	60 to 95 x 10**-6

Occasionally, the plastic layer may be subjected to chemical attack as a result of contact with remnants of the plating fluid.

Countermeasures

The simplest countermeasure for this type of failure is to eliminate all edges from the plastic component to be plated, and this works by preventing the concentration of thermal stress at the corresponding sections when changes occur in the ambient temperature.
- Buffering of flash at parting sections and other similar treatments are used to smoothen the surface.
- Gating surfaces are made as smooth as possible.
- Corners (and particularly those at acute angles) are rounded.

Cracking in high-temperature environments
Insufficient ductility of the metal layer and large tensile stresses during electroplating lead to this type of failure; accordingly, the composition of the plating fluid is adjusted to increase the ductility of the separation layer and reduce electroplating stresses.

Cracking in low-temperature environments
This type of fracture occurs when contraction of the plastic is prevented by stiffness of the metal, and as a countermeasure, the metal layer is made thinner to reduce the force of constraint. As can be expected from the table of material characteristics presented above, it is important that the material thickness for copper - a buffer material - be made relatively thick, and that the thickness of materials such as nickel and chromium be made thin.

A grade of material with a low coefficient of linear expansion should be selected in order to reduce the discrepancy in expansion.

Care should be given to residual plating fluid on the plastic component (and in particular, on bosses and other similar structures), and in certain cases, design changes or the like should be implemented to prevent chemical attack. Furthermore, the reduction of residual strain caused by molding is also effective in this regard. If designing plated products, please refer to ABS Plating. For special grades,

please refer to Type-Specific Product List.

Reference

The metal and plastic layers of a cylindrical component which still contains flash and which has been subjected to plating are considered in the following example.

Section	External diameter (mm)	Internal diameter (mm)	Young's modulus (MPa)	Coefficient of linear expansion (1/deg.C)
Plastic	100	60	2,300	8.0 x 10**-5
Metal layer	110	100	200,000	1.0 x 10**-5

Flash: Height = 5 mm, Apex angle = 60deg

At high temperatures		At low temperatures
A concentration of tensile stress (red) which leads to cracking can be seen on the inside of the metal layer on the flash. If cracking were to occur, its origin could be traced back to this point.		The generation of a high degree of stress can be seen at the inside wall of a plastic section which is not directly under the flash. Whereas a reduction of stress at the inside radius would be expected, this material is constrained by the metal layer, and it can be seen that the stress level actually increases. Accordingly, tensile stress develops in the plastic layer as a result of the constraining force.

Outcome

In the metal layer, a concentration of stress (smaller than that at high temperatures) can be seen at the outer surface of the flash, and there is a danger that this will lead to cracking at that point.