The coefficient of thermal expansion differs from that of metal by 5 to 10 times. Therefore, when the plastic products with metal inserts are cooled, the degree of shrinkage formed by the two is different. Because the shrinkage of the plastic is relatively large, it tightly hugs the metal insert and surrounds the insert. The inner layer of the plastic is under compressive stress, while the outer layer is under tensile stress, resulting in stress concentration.
When setting up the juice insert, the following points should be noted:
(1) The insert should be firmly fixed in the plastic part
In order to prevent the insert from rotating or falling out of the plastic part under force, the surface of the insert must be designed with appropriate convex and concave shapes. It is the most commonly used diamond knurling, which has higher tensile and torsional strength; it is straight knurling, which allows the plastic part to elongate a little in the axial direction when the insert is long, to reduce this direction However, there must be an annular groove on this insert to prevent it from being pulled out when force is applied. It is a hexagonal insert, which is less used because of stress concentration at its sharp corners; Sheet-shaped inserts fixed by holes, cuts or partial bending; thin-walled tubular inserts can also be fixed by edge bending; needle-shaped inserts can be fixed by flattening or bending one of the sections.
(2) The inserts in the mold should be positioned reliably
The insert in the mold must be reliably positioned during forming, and the height of the insert must not exceed 2 times the diameter of the positioning part. The fixing method of the external thread insert in the mold. Use the polished rod part on the insert to cooperate with the mold; adopt the form of a shoulder to match, which can increase the stability of the insert after insertion, but also prevent the plastic from flowing into the thread; there is a protruding ring on the insert to form The ring is pressed against the mold to form a sealing ring to prevent the inflow of plastic.
Under normal circumstances, the fit between the insert and the mounting hole of the template during injection molding is; the fit between the insert and the mounting hole of the template during compression molding is. When the insert is too long or has a slender rod shape, a support should be provided in the mold to prevent the insert from bending, but this will leave holes in the plastic part.
(3) The wall thickness around the insert should be large enough
Due to the large difference in shrinkage between the metal insert and the plastic part, there is a large internal stress in the plastic around the insert. If the design is improper, it will cause the plastic part to crack. Maintaining a proper thickness of the plastic layer around the insert can Reduce the cracking tendency of plastic parts.
For metal inserts of phenolic plastic and similar thermosetting plastics, the thickness of the surrounding plastic layer can be seen in the table. In addition, the insert should not have sharp corners to reduce stress concentration.
When thermoplastic injection molding, large inserts should be preheated to close to the material temperature.
For plastics whose stress is difficult to eliminate, a layer of polymer elastomer can be covered around the insert or annealed after forming. There is also enough plastic layer thickness on the top of the insert, otherwise blisters or cracks will appear.