Plastic shrinkage rate is the dimensional reduction percentage of a molded plastic part as it cools from its molten state to room temperature. If this shrinkage is not calculated in advance and compensated in the mold design, the molded plastic parts cannot meet the project requirements.
How To Calculate Plastic Shrinkage
Generally, the calculation method for plastic shrinkage rate is based on the German national standard DIN16901.
The shrinkage rate (S) is the relative difference between the mold size (D, at 23 °C ± 2 K) and the corresponding part size (M, 16-24 hours after molding, at 23 °C ± 2 K and 50 % ± 10 % air humidity). It is calculated according to Formula 1.
Formula 1: S=(D-M)/D×100%
If the size of the plastic part and the shrinkage rate of the material are known, then the mold size D=M/(1-S). In order to simplify the calculation, the formula 2 is generally used to calculate the mold size.
Formula 2: D=M+MS
If a more accurate calculation is required, use formula 3.
Formula 3: D=M+MS+MS²
However, when calculating the shrinkage rate, due to the influence of multiple factors, the actual shrinkage rate can only be approximated. Therefore, using formula 2 to calculate the mold cavity size also basically meets the requirements. When manufacturing molds, the cavity is processed according to the lower deviation, and the core is processed according to the upper deviation, so that appropriate adjustments can be made when necessary.
Why is it difficult to accurately calculate the shrinkage rate?
Firstly, the shrinkage rate of various plastics is not a fixed value, but a range. Because the shrinkage rate of the same material produced by different factories may be different. And, multiple batches of materials produced by a factory, these batches’ shrinkage rates may also be different. So, factories generally provide users with the shrinkage range.
Secondly, the actual shrinkage rate during the forming process is also influenced by factors such as the shape of the plastic part, mold structure, and molding conditions.
At last, we should consider factors first in mold design, such as the shrinkage range of the plastic, wall thickness of the part, shape, type, size and position of the gate, or other molding factors. Then, calculate the shrinkage rate for each section of the part.
When injecting molding for high-precision plastic parts, it is best to select plastics with a narrow shrinkage range, and leave the way open for mold modification. Finally, gradually adjust the mold through trial molding, and meet the requirement of dimension and precision.
The ending is a table of shrinkage rates for various plastics, for reference only. In actual injection molding projects, please check the shrinkage rate of the materials to be used.
Shrinkage Rates for Common Plastics
| Material | Density | Glass Fiber Content | Specific Heat Capacity | Processing Temperature | Mold Temperature | Shrinkage Rate |
| – | [g/cm³] | [%] | [kJ/(kg·K)] | [℃] | [℃] | [%] |
| PS | 1.05 | – | 1.3 | 180-280 | 10 | 0.3-0.6 |
| HI-PS | 1.05 | – | 1.21 | 170-260 | 5-75 | 0.5-0.6 |
| SAN | 1.08 | – | 1.3 | 180-270 | 50-80 | 0.5-0.7 |
| ABS | 1.06 | – | 1.4 | 210-275 | 50-90 | 0.5-0.7 |
| ASA | 1.07 | – | 1.3 | 230-260 | 40-90 | 0.4-0.6 |
| LDPE | 0.954 | – | 2.0-2.1 | 160-260 | 50-70 | 1.5-5.0 |
| HDPE | 0.92 | – | 2.3-2.5 | 260-300 | 30-70 | 1.5-3.0 |
| PP | 0.915 | – | 0.84-2.5 | 250-270 | 50-75 | 1.0-2.5 |
| PPGR | 1.15 | 30 | 1.1-1.35 | 260-280 | 50-80 | 0.5-1.2 |
| IB | – | – | – | 150-200 | – | – |
| PMP | 0.83 | – | – | 280-310 | 70 | 1.5-3.0 |
| PVC-soft | 1.38 | – | 0.85 | 170-200 | 15-50 | >0.5 |
| PVC-rigid | 1.38 | – | 0.83-0.92 | 180-210 | 30-50 | 0.5 |
| PVDF | 1.2 | – | – | 250-270 | 90-100 | 3.0-6.0 |
| PTFE | 2.12-2.17 | – | 0.12 | 320-360 | 200-230 | 3.5-6.0 |
| FEP | – | – | – | – | – | – |
| PMMA | 1.18 | – | 1.46 | 210-240 | 50-70 | 0.1-0.8 |
| POM | 1.42 | – | 1.47-1.5 | 200-210 | >90 | 1.9-2.3 |
| PPO | 1.06 | – | 1.45 | 250-300 | 80-100 | 0.5-0.7 |
| PPO-GR | 1.27 | 30 | 1.3 | 280-300 | 80-100 | – |
| CA | 1.27-1.3 | – | 1.3-1.7 | 180-320 | 50-80 | 0.5 |
| CAB | 1.17-1.22 | – | 1.3-1.7 | 180-230 | 50-80 | 0.5 |
| CP | 1.19-1.23 | – | 1.7 | 180-230 | 50-80 | 0.5 |
| PC | 1.2 | – | 1.3 | 280-320 | 80-100 | 0.8 |
| PC-GR | 1.42 | 10-32 | 1.1 | 300-330 | 100-120 | 0.15-0.55 |
| PET | 1.37 | – | – | 260-290 | 140 | 1.2-2.0 |
| PET-GR | 1.5-1.57 | 20-30 | – | 260-290 | 140 | 1.2-2.0 |
| PBT | 1.3 | – | – | 240-260 | 60-80 | 1.5-2.5 |
| PBT-GR | 1.52-1.57 | 30-50 | – | 250-270 | 60-80 | 0.3-1.2 |
| PA 6 | 1.14 | – | 1.8 | 240-260 | 70-120 | 0.5-2.2 |
| PA 6-GR | 1.36-1.65 | 30-50 | 1.26-1.7 | 270-290 | 70-120 | 0.3-1 |
| PA 66 | 1.15 | – | 1.7 | 260-290 | 70-120 | 0.5-2.5 |
| PA66-GR | 1.20-1.65 | 30-50 | 1.4 | 280-310 | 70-120 | 0.5-1.5 |
| PA 11 | 1.03-1.05 | – | 2.4 | 210-250 | 40-80 | 0.5-1.5 |
| PA 12 | 1.01-1.04 | – | 1.2 | 210-250 | 40-80 | 0.5-1.5 |
| PSO | 1.37 | – | – | 310-390 | 100-160 | 0.7 |
| PPS | 1.64 | 40 | – | 370 | >150 | 0.2 |
| PUR | 1.2 | – | 1.85 | 195-230 | 20-40 | 0.9 |
| PF | 1.4 | – | 1.3 | 60-80 | 170-190 | 1.2 |
| MF | 1.5 | – | 1.3 | 70-80 | 150-165 | 1.2-2 |
| MPF | 1.6 | – | 1.1 | 60-80 | 160-180 | 0.8-1.8 |
| UP | 2.0-2.1 | – | 0.9 | 40-60 | 150-170 | 0.5-0.8 |
| EP | 1.9 | 30-80 | 1.7-1.9 | ca.70 | 160-170 | 0.2 |
