Overmolding of Metal Inserts
Shown below are two components with integrated overmolded metal inserts: on the left, a water filter for home use; on the right, a pump lever for a lawn mower. At first glance, both parts appear well-executed.
On closer inspection, however, it's clear that the component on the left has an issue: a crack in the overmolded plastic. Under the microscope, the crack is clearly visible – but it's barely noticeable to the naked eye.
This actually caused problems in the component mentioned above, as the filter could no longer ensure proper sealing. However, such product failures—often only noticeable in the later stages of a project—can be avoided through the following strategies:
Factors affecting overmolded parts:
Tool and Part Design
Avoid sharp edges or corners: stress concentrations can lead to cracking. All sharp transitions should therefore be rounded.
Optimized wall thicknesses: large variations in wall thickness lead to uneven cooling and internal stresses within the part.
Insert position: it should be as centered and symmetrical as possible to allow for stress distribution.
Clearance and tolerances: the insert should not fit too tightly in the cavity. It is better to allow for slight clearance.
Processing and Temperature Control
Mold temperature: if too low, it can lead to excessive shrinkage and stress cracking.
Melt temperature: if too high, it can cause degradation reactions; if too low, it can result in poor adhesion.
Cooling: uneven or overly rapid cooling leads to internal stresses. This increases the risk of cracking during insert placement or cooling.
Insert Technology
Preheating the insert: cold metal inserts create high temperature differences, which can lead to stress cracks. → Preheat to approx. 80–120 °C.
Surface treatment: a clean and possibly textured insert surface improves adhesion and reduces localized stresses.
Decoupling through design: for particularly critical material combinations, rubber buffers, sealing lips or similar elements can serve as a buffer zone between plastic and metal.
Post-Processing Stresses
Assembly forces / Screws: if the component is later subjected to mechanical stress (e.g. screws passing through the insert), additional stresses may promote cracking.
Temperature fluctuations during operation: cyclic loading caused by differing thermal expansion between metal and plastic can lead to cyclic stresses and crack formation.
Selection of Suitable Combinations
Suitable combination partners: for example, glass fiber reinforced materials often exhibit reduced shrinkage and behave more similarly to metal.
Adhesion promoters: in cases of significant material differences, chemical bonding via adhesion promoters can be beneficial.
Simulation: using filling simulation (Moldflow) to identify potential risk areas in advance.
In our examples, material selection was the decisive factor: the water filter was made of ABS, while the pump lever used a glass fiber reinforced PA46-GF30. Although the reinforced material has a higher breaking strength, it also has a lower elongation at break. However, the key difference between the two materials lies in their coefficient of thermal expansion:
- For ABS, the coefficient of linear thermal expansion is 0.088 mm/(m·K)
- For PA46-GF30, the coefficient of linear thermal expansion is 0.025 mm/(m·K) (parallel) or 0.060 mm/(m·K) (perpendicular).
The reinforced material exhibits lower shrinkage and is therefore subjected to less stress than the unreinforced ABS. For comparison: steel has a coefficient of linear thermal expansion of approximately 0.012 mm/(m·K). This means that steel shrinks less than plastics and is closer to the behavior of the reinforced plastic.
In the component affected by stress cracking, a material change ultimately led to the desired result.