Plastic mold size and shrinkage of the relationship(1)
Design of plastic mold, determine the structure of the mold can be a detailed design of each part of the mold, that is, determine the size of each template and parts, cavity and core size. Major design parameters, such as shrinkage of the material, will be involved. Therefore, it is necessary to know the shrinkage rate of forming plastics to determine the size of each part of the cavity. Even if the selected mold structure is correct, but the use of improper parameters, it is impossible to produce quality plastic parts.
Thermoplastics are designed to expand when heated, contract when cooled, and shrink when pressed.In the process of injection molding, first melt plastic injection into the mold cavity, after the completion of filling melt cooling and solidification, from the mold to take out the plastic parts appear shrinkage, this shrinkage is called forming shrinkage. Plastic parts from the mold to take out the stability of this period of time, the size will still appear small changes, a change is to continue to shrink, this contraction is called after the contraction.
Another change is that some hygroscopic plastics expand as a result of moisture absorption. For example, when the water content of nylon 610 was 3%, the size increased by 2%. When the water content of glass fiber reinforced nylon 66 was 40%, the size was increased by 0.3%. But the main one is forming shrinkage.
Shrinkage rate S is expressed by the following formula:
Where: S- shrinkage rate; D- mold size; M- plastic part size.
If the mold cavity is calculated according to the known plastic part size and material shrinkage rate, it is D=M/(1-s). In the mold design, in order to simplify the calculation, the following formula is generally used to calculate the mold size:
D = M + MS (2)
If a more accurate calculation is required, the following equation should be applied:
When making the mold, the cavity is processed according to the lower deviation, and the core is processed according to the upper deviation, so as to make proper dressing when necessary.
Because the shrinkage rate of the same material produced by different factories is not the same, even the shrinkage rate of the same material produced by a factory with different batch. Numbers is not the same. The actual shrinkage rate in the forming process is also affected by the shape of plastic parts, mold structure and forming conditions. The effects of these factors are described below.
Shape of plastic mold parts
For the wall thickness of forming parts, generally due to the longer cooling time of the thick wall, the shrinkage rate is also larger. For general plastic parts, when the molten material flow direction L size and perpendicular to the molten material flow direction W size difference, then the shrinkage rate difference is also large. From the molten material flow distance, the pressure loss away from the gate part is large, so the shrinkage rate is larger than the gate part. Due to the shape of reinforcing rib, hole, boss and carving, the shrinkage rate of these parts is small.