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Molded Part Cracking


Chemical Stress Cracks

picture:chemical stress cracks

 Chemical stress cracks are typical brittle cracks which occur at or below the tensile strength of the plastic or material. Specifically, the name "chemical stress cracking" is given to the phenomenon where chemical agents adhere to, or come into contact with, areas where tensile stress is present (ie, where the part is being loaded), and where cracking occurs over time as a result of the combined effect of the chemical agent and the stress. The surfaces of this type of crack are smooth, and in certain cases, they may take on a mirrored appearance.

 In terms of growth mechanism, it is said that gaps open between molecules when the part is loaded or in some other condition of stress, that the chemical agent penetrates into these gaps, and that the cohesive power (or strong binding) between the molecules is reduced.

 The exact mechanism has not yet, however, been completely understood. Chemical stress cracks can be prevented from occurring by eliminating either the chemical agent or the stress.

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Synonyms for Chemical Stress Cracking

 Regularly used synonyms include the following: Environmental stress cracking (ESC) or environmental stress fracture Stress cracking or stress fracture Solvent cracking or solvent fracture.


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Crack Examples and Countermeasures

Contact with plasticized polyvinylchloride and flexible packing

Cause

 Plasticized polyvinylchloride and flexible packing contain added plasticizers, which are typical examples of chemicals that lead to chemical stress cracking. Products using plasticized polyvinylchloride include plastic hoses and electrical cords.

Countermeasure

 In electrical cords and the like, materials such as polyethylene, silicon rubber, and ethylene-propylene rubber are used to cover or shield the wires. Similarly, packing materials contain silicon rubber, ethylene-propylene rubber, or fluoroplastics.
 If contact with plasticized polyvinylchloride cannot be avoided, the part's material should be changed to a chemical-resistant grade.


Metal-insert moldings and press-fit metal components

Cause

 Metal components are often coated in press oil, cutting oil, anti-rust oil, and other similar types of oil used in the manufacturing process. Accordingly, if degreasing has not been carried out to a sufficient degree, these machining oils may act as a chemical agent that leads to chemical stress cracking.
 Plastic sections which come into contact with metal components often contain residual stresses from molding and compressive stresses caused by press-fitting components, and these can account for the stress factors leading to chemical stress cracking.

Countermeasure

 Thorough degreasing should be carried out for metal components; furthermore, these components should be preheated before being used.


Screwed assembly

Cause

Relatively large stresses exist around assembly screws, and particularly in the case of countersunk-head screws, these stresses can be excessive. Accordingly, they may constitute the stress factor leading to chemical stress cracking, and if chemical agents are also present in these areas, cracking will occur.

Countermeasure

 Component shapes should be designed in such a way that countersunk-head screws are not needed. Where usage is unavoidable, washers should also be used and screws should not be tightened beyond the appropriate torque.


Molded component thread shapes

Cause

 Molded components to be connected to plastic piping contain screw heads in certain areas for this purpose. It is highly probably that large stresses will result from over-tightening at the screw section, and this can lead to the generation of stress concentrations, particularly at the bottom of screw threads. Cracking will result if chemical agents come into contact with these areas of stress.

Countermeasure

 Components should be assembled with the correct fixing torques, and if cracking occurs even at the correct torque, the material should be changed to a chemical-resistant grade.


Usage of detergents to clean molded parts

Cause

 A wide range of different detergents are used to remove dirt and contamination from molded parts. Generally speaking, detergents contain a volatile chemical agent such as alcohol, and it is highly probable that these substances will constitute the chemical factor for the occurrence of chemical stress cracking.  Residual molding stress is present particularly in areas with thin walls or non-uniform cross sections, and if detergent should come into contact with these areas, cracking will result.

Countermeasure

 Detergents should be diluted with water before use.


Kitchen fittings

Cause

 It is highly probable that fittings in domestic kitchens, restaurants, and the like will come into contact with vegetable oil, and this substance is a notable cause of chemical stress cracking.

Countermeasure

 Change the material to a chemical-resistant grade.

* Please contact us about chemical-resistant grades.


Toilet and bath products

Cause

 By and large, the soap products used in toilets and baths often constitute chemical agents that result in chemical stress cracking.

Countermeasure

 Change the material to a chemical-resistant grade.

* Please contact us about chemical-resistant grades.


Oil and grease for sliding sections of metal components

Cause

 Certain types of oil and grease as used to promote sliding of sections of metal components can provide the chemical factors required for chemical stress cracking. Cracking will result whenever these substances come into contact with areas of plastic where stress is present.

Countermeasure

 Switch to the usage of fluoride-type oils and greases

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Chemical Resistance Testing (or testing to confirm resistance to chemical stress cracks)

 As shown in the photograph, the test pieces are mounted on a jig which maintains constant curvature, and the chemical agent is applied. After allowing the test pieces to stand in the prescribed environmental conditions for a specific period of time, the condition of physical degradation is checked. This method is referred to as the bending-form fixed strain test.


picture:ESCR test jig
Test conditions Strain range:
0.2 to 1.0% (in 0.1% steps) 
1.2 to 1.6% (in 0.2% steps)
  • Test environment :
    23 deg.C, 50 to 60% RH
  • Standing time:
    48 hours Test piece shape: 2 (t) x 10 (w) x 150 mm (l)

Note: Chemical resistance testing should be carried out in advance of usage for any chemicals which may make contact with plastic products, and it is crucial that chemical stress crack conditions be checked using these tests.

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!

 UMG ABS Ltd. accepts no responsibility for the quality or safety of any customer products which use our materials or which have made use of any type of data provided by this company.
 Customers are requested to independently determine the suitability of our materials for their products. We also request that sufficient attention also be paid to laws, regulations, and industrial rights.


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