Semi-crystalline materials such as POM (acetal), PA (nylon), PBT, and PET (polyester) are easier than amorphous materials. This is especially taken into account when designing molds and molding. If this problem is not taken into account in the design, it cannot be remedied in the next steps. This section will discuss the causes and ways to prevent or reduce them.
The main reason for the occurrence
For semi-crystalline materials, their shrinkage is quite high and is affected by many factors. For unreinforced materials, it is mainly affected by the wall thickness and the surface temperature of the mold, that is, the change of wall thickness and the unsuitable mold temperature will cause the mold to occur. Due to the orientation of glass fibers, the difference in shrinkage characteristics of glass fiber reinforced materials is usually extremely obvious. The effect of different wall thicknesses on shrinkage is quite small. The main cause is the difference between the longitudinal orientation of the fibers and the orientation in the vertical flow direction. Essentially, this occurs due to wall thickness distribution, gate location, throttling and bypass flow, and the inherent rigidity of the molded part itself.
The causes of these occurrences vary depending on whether the material is glass fiber reinforced or not, and usually completely different phenomena occur on the same part.
How can this be prevented?
The wall thickness of the unreinforced material should be the same. Melt accumulation is avoided as much as possible. Multi-point gates can be used to obtain high-pressure gradients that minimize shrinkage differences. The mold heating system should be designed so that the heat dissipation is as uniform as possible. (See section VI of this section)
For glass fiber reinforced materials, the symmetry of the molded part is just as important as the same wall thickness. The asymmetry of the molded part impedes the melt flow, orienting the fibers and eventually causing it. If the molded part is asymmetrical, the blind cores are mixed to achieve balance during the design phase. Gate location is also very important. Every bypass and every weld is a potential factor.
What should producers do?
Assuming that the part to be molded has the correct gate and mold design, the manufacturer can control the holding pressure and mold temperature to a point. In general, several thermal cycles are used to achieve the best heat dispersion.
For reinforcement materials, changing the injection speed or lowering the mold temperature can be of some help. If the possibility of what will happen later is not taken into account during the mold and part design stage, it is hoped that the molding conditions will not be corrected later by improving the molding conditions.
What if it appears?
The most important measure is to carry out a mold filling study, i.e. to partially fill the mold in several stages. This is especially true for glass fiber reinforced materials. By studying the melted front end profile, the fiber orientation can be re-reoriented. For reinforcing materials, you can refer to their shrinkage curves and take measures such as flow aid or throttling to reduce. These will alter the front end profile of the melt, thus affecting.
This approach requires a wealth of practical experience and a continuous accumulation of knowledge so that effective preventive measures can be taken in the future. There are still limitations due to the properties and physical state of the raw materials. For example, for crystalline polymers, it is not possible to obtain the same flatness as amorphous polymers. In this case, it is necessary to mix low and semi-crystalline polymers. Due to chemical modification, or mixing between different reinforcing materials, there is some compromise between material properties and occurrence. The last and most expensive method is to change the mold. For critical parts, if the molding process is similar, it is best to use modifiable inserts.
