ABS copper inserts – Plastic parts cracking problem analysis
Name: plastic parts
Material: GP-22 ABS + high concentration black masterbatch
Problems: Plastic parts out intact from the mold. Placed in the air for 2 to 3 days, some parts of the vertical cracks around the copper inserts, and some even completely split.
Analysis of the reasons and improvement programs: plastic parts in the molding process did not appear any problem. Indicating that the problem is not molding equipment and technology. According to our understanding and analysis, the problem is caused mainly by environmental stress cracking.
Plastic parts cracking problem analysis may have the following:
(1) Raw materials mixed with other impurities or doped with inappropriate. Or excess solvent, or other additives. Environmental stress cracking is a peculiar phenomenon of polyolefin plastics. It refers to the product when there are stress and some active medium contact. There will be brittle cracks, cracks eventually lead to the destruction of products. Such actives may be detergents, soaps, water, oils, acids, bases, salts and organic solvents that do not significantly swell the material. The necessary condition for environmental stress cracking is the stress in the pattern or part, and there are some stress concentration factors such as notches and scratches on the surface.
The ABS contains polyolefin components, and raw materials contained impurities or solvents indirectly provide these active substances. When the ABS plastic parts in the storage process due to various factors on the surface of the gap or scratches, then there will be cracks.
(2) Some plastics, such as ABS, etc., Will react with water vapor under humid conditions to cause a large strain of the workpiece to crack.
(3) Plastic in the barrel heating time is too long, it will also promote parts cracking.
Such as polystyrene and plastic containing this composition due to minimizing the use of metal inserts. Because of these plastic brittle hot and cold volume than capacity. If for assembly and strength requirements. An insert has to be added as the insert is made of metallic copper, and the dimensional change of the metallic copper insert when cooled is quite different from the thermal shrinkage of the plastic ABS. Resulting in a larger inner Stress, thus causing the product cracking. Specific improvements are as follows:
(1) Replace the insert material used, so that the expansion coefficient of inserts and ABS products should be as close as possible;
(2) Increase the fillet of the sharp parts of the insert, the fillet is too small may cause product stress concentration, resulting in product cracking;
(3) Increase the thickness of the plastic layer around the insert.
3，The preheating of the insert
In plastic products with inserts, there is a tendency for cracks to occur around the insert or to cause a reduction in the strength of the product, which is caused by the large difference in thermal properties and shrinkage between the metal insert and the plastic.
Therefore, in the design of parts. It should increase the thickness of the wall around the insert. In order to overcome this difficulty, pre-molding metal inserts pre-heating is also an effective measure. Because after preheating can reduce the temperature difference between melt and insert, in the molding can make the melt around the insert cooling slower, more uniform contraction, the occurrence of a certain hot material shrinkage effect, to prevent the occurrence around the insert Large internal stress.
Due to the uneven plasticization of plastic in the barrel or the different cooling rates in the mold cavity, uneven crystallization, orientation and shrinkage often occur, resulting in the internal stress of the product. Which is more prominent when producing thick-walled or metal insert products. There are the internal stress of the parts in the storage and use always happens during the degradation of mechanical properties and the optical properties of bad surface crazing, even cracking deformation.
The one method to solve these problems in production is to anneal the parts. The annealing process is to keep products in a heated liquid medium at constant temperature (such as hot water, hot mineral oil, glycerin, glycol, and liquid paraffin), or hot air circulating oven for a period of time.
For ABS plastic parts with embedded parts, reference is made as follows:
Annealing medium: water
Annealing temperature: lower than ABS thermal deformation temperature (85~100 degrees C) 10~20 degrees centigrade, or 60~70 degrees C
Annealing time: 2 hours