When designing a plastic mold, once the mold structure is determined, detailed design of each part of the mold can be carried out, including the dimensions of each template and component, as well as the dimensions of the cavity and core. At this stage, it will involve key design parameters such as material shrinkage rate. Therefore, only by accurately understanding the shrinkage rate of the formed plastic can the dimensions of various parts of the cavity be determined. Even if the selected mold structure is correct, improper parameters will result in the inability to produce qualified plastic parts.
The characteristic of thermoplastic plastics is expansion after heating and contraction after cooling, with a reduction in volume under pressure. During the injection molding process, molten plastic is first injected into the mold cavity, and after filling, it cools and solidifies. Upon removal from the mold, the plastic part undergoes shrinkage, known as molding shrinkage. During the period from removal to stabilization, the dimensions may still undergo minor changes, with continued shrinkage referred to as post-shrinkage.
Another change occurs in certain hygroscopic plastics due to moisture absorption, resulting in expansion. For example, nylon 610 increases by 2% in size when its moisture content is 3%, while glass fiber-reinforced nylon 66 increases by 0.3% when its moisture content is 40%. However, molding shrinkage primarily influences this. Currently, the method for determining the shrinkage rate of various plastics (molding shrinkage + post-shrinkage) generally follows the provisions of DIN16901 of the German national standard. This involves measuring the difference between the mold cavity size at 23°C ± 0.1°C and the corresponding plastic part size measured under conditions of 23°C temperature and 50±5% relative humidity after being left for 24 hours after molding.
The shrinkage rate S is represented by the following equation:
S=（D-M）/ D X 100% (1)
Where: S - shrinkage rate; D- mold dimension; M - part dimension.If the mold cavity is calculated based on known part dimensions and material shrinkage rate, then in mold design:
D=M/(1-S)
For simplifying calculations, the following equation is generally used to determine mold dimensions:
D=M+MS (2)
For more accurate calculations, the following equation is applied:
D=M+MS+MS2 (3)
However, when determining the shrinkage rate, as the actual shrinkage rate is influenced by numerous factors and only approximate values can be used, using equation (2) to calculate the cavity size essentially meets the requirements. In mold manufacturing, the cavity is processed with a negative deviation, while the core is processed with a positive deviation, facilitating necessary adjustments if required.
The main challenge in accurately determining shrinkage rates is the variability among different types of plastics. Shrinkage rates aren't fixed values but rather a range. Even within the same factory, the shrinkage rates of the same material can vary between different production batches. Consequently, factories can only provide users with a range of shrinkage rates for their manufactured plastics. Additionally, the actual shrinkage rate during the molding process is influenced by factors such as the shape of the part, mold structure, and molding conditions. Let's delve into the impact of these factors in next article.