The Relationship Between Plastic Mold Dimensions and Shrinkage Rate
When designing a plastic mold, once the mold structure is determined, detailed designs of various parts of the mold can be carried out. This includes determining the dimensions of each template and component, as well as the dimensions of the cavity and core. At this stage, it involves important design parameters related to material shrinkage rates.
Therefore, it is essential to have a precise understanding of the shrinkage rate of the forming plastic in order to determine the dimensions of various parts of the cavity. Even if the selected mold structure is correct, improper parameter choices can result in the production of subpar plastic parts.
Thermoplastic materials tend to expand when heated and contract when cooled, and their volume may also shrink under pressure. During the injection molding process, molten plastic is first injected into the mold cavity, and after filling, the material cools and solidifies. When the part is removed from the mold, it undergoes shrinkage, known as forming shrinkage.
During the time it takes for the part to stabilize after being removed from the mold, there may still be minor dimensional changes, one of which is continued shrinkage, referred to as post-shrinkage.
Another change is the expansion of certain moisture-absorbing plastics due to moisture absorption.
For example, when nylon 610 has a moisture content of 3%, its dimensional increase is 2%. And for glass fiber-reinforced nylon 66 with a moisture content of 40%, the dimensional increase is 0.3%. However, the primary factor at play is still forming shrinkage.
How to Determine the Shrinkage Rate?
Currently, the method for determining the shrinkage rates (forming shrinkage + post-shrinkage) of various plastics is generally recommended in accordance with the German national standard DIN16901. This involves measuring the difference in dimensions of the mold cavity at 23°C ± 0.1°C and the corresponding plastic part placed for 24 hours after forming, under conditions of 23°C temperature and 50 ± 5% relative humidity.
The shrinkage rate S is represented by the following equation: S = ((D – M) / D) × 100% (1)
Where: S – shrinkage rate; D – mold size; M – part size.
To calculate the mold cavity size based on the known part size and material shrinkage rate, it can be expressed as D = M / (1 – S). In mold design, for the sake of simplifying calculations, the following formula is commonly used:
D = M + MS (2)
If a more precise calculation is required, the following formula can be applied:
D = M + MS + MS2 (3)
However, when determining the shrinkage rate, due to the influence of numerous factors on the actual shrinkage rate, only approximate values can be used. Therefore, using formula (2) to calculate the mold cavity size generally meets the requirements. When manufacturing molds, the mold cavity is processed with a lower deviation, while the core is processed with an upper deviation, making it possible for necessary adjustments when needed.
The main reasons for the difficulty in precisely determining the shrinkage rate are:
(1) The shrinkage rate of various plastics is not a fixed value but rather a range. Different factories produce the same material with different shrinkage rates. Even for the same material produced by a single factory, different batches may have varying shrinkage rates.
Therefore, each factory can only provide users with a range of shrinkage rates for the plastics they produce.
(2) The actual shrinkage rate during the molding process is also influenced by factors such as the shape of the plastic part, mold structure, and molding conditions.
The following provides an introduction to the influence of these factors.
Factors Affecting the Actual Shrinkage Rate
1. Plastic Part Shape
For molded parts with different wall thicknesses, thicker walls generally have longer cooling times, resulting in a larger shrinkage rate.
For typical plastic parts, when there is a significant difference between the dimensions in the melt flow direction (L) and the dimensions perpendicular to the melt flow direction (W), the shrinkage rate differences are also significant.
In terms of the distance from the melt flow, there is greater pressure loss further away from the gate, leading to higher shrinkage rates in those areas. Reinforcements, holes, protrusions, and engravings, among other shapes, exhibit resistance to shrinkage, resulting in smaller shrinkage rates in these areas.
2. Mold Structure
The gate type also affects the shrinkage rate. Using a small gate can lead to an increased shrinkage rate because the gate solidifies before the packing phase is completed.
The cooling circuit structure within the injection mold is also a crucial aspect of mold design. If the cooling circuit is improperly designed, it can result in temperature imbalances throughout the plastic part, leading to differences in shrinkage. This can result in dimensional deviations or deformations in the plastic part.
In thin-wall sections, the impact of mold temperature distribution on the shrinkage rate is even more pronounced.
3. Mold Dimensions and Manufacturing Tolerances
In addition to calculating the basic dimensions of mold cavities and cores using the formula D = M(1 + S), there is also a matter of machining tolerances for molds.
Conventionally, the machining tolerance for molds is set at one-third of the part tolerance.
However, due to variations in plastic shrinkage rate ranges and stability, it is essential to rationalize the determination of size tolerances for parts molded from different plastics.
In other words, parts molded from plastics with a larger shrinkage rate range or less stable shrinkage rates should have larger size tolerances. Otherwise, there may be a significant amount of oversized parts that become scrap.
To address this issue, various countries have established national or industry standards specifically for size tolerances of plastic parts. China has also issued ministerial professional standards in the past. However, most of these standards do not include corresponding size tolerances for mold cavities. The German national standard, DIN16901, specifies size tolerances for plastic parts, and the corresponding size tolerances for mold cavities are covered by the DIN16749 standard. This standard has a significant influence worldwide and can serve as a reference for the plastic mold industry.
Regarding Size Tolerances and Permissible Deviations for Plastic Parts
In order to reasonably determine the size tolerances for parts molded from materials with different shrinkage characteristics, standards have introduced the concept of molding shrinkage difference
△VS. △VS = VSR – VST (4)
Where:
- VS – molding shrinkage difference
- VSR – molding shrinkage rate in the direction of molten flow
- VST – molding shrinkage rate perpendicular to the direction of molten flow.
Based on the △VS value of plastics, the shrinkage characteristics of various plastics are divided into four groups. The group with the smallest △VS value is the high-precision group, and so on, with the group having the largest △VS value being the low-precision group. Precision technology, 110, 120, 130, 140, 150, and 160 tolerance groups are compiled based on the basic dimensions. It is stipulated that for parts molded from plastics with the most stable shrinkage characteristics, size tolerances can be selected from the 110, 120, and 130 groups.
When molding parts using plastics with moderately stable shrinkage characteristics, size tolerances should be selected from the 120, 130, and 140 groups. If size tolerances from the 110 group are used for parts molded from such plastics, there may be a large number of parts with size deviations.
For parts molded using plastics with poorer shrinkage characteristics, size tolerances should be selected from the 130, 140, and 150 groups.
For parts molded using plastics with the worst shrinkage characteristics, size tolerances should be selected from the 140, 150, and 160 groups. When using this tolerance table, the following points should also be noted: The general tolerances in the table are used for dimensions without specified tolerances.
The direct annotation of tolerance for deviations is used for specifying the tolerance band of dimensions on plastic parts. The upper and lower deviations can be determined by design personnel. For example, with a tolerance band of 0.8mm, various upper and lower deviations can be chosen, such as 0.0; -0.8; ±0.4; -0.2; -0.5, etc.
In each tolerance group, there are two sets of tolerance values, A and B. A is the dimension formed by the combination of mold components, adding the deviation caused by the non-interference of mold components.
This added value is 0.2mm. B is a dimension directly determined by mold components.
Precision technology is a set of tolerance values specifically established for plastic parts with high precision requirements. Before using plastic part tolerances, it is necessary to determine which tolerance groups are applicable for the plastic material in use.
