Shrinkage
Laboratory > Physicals
| Analysis of Contractions and Deformations | |
The contraction is the contraction of the molded part as it cools after the injection. Most of the contraction of the part occurs in the mold while cooling. A small amount of shrinkage occurs after ejection as the piece continues to cool. After that, the part may continue to shrink very slightly for several hours or even days, until the temperature and moisture content stabilize. Thus, dimensional inspection must wait at least one day. The contraction (shrinkage / shrinkage) is the relation between the volume of the plastic in the melt and the volume of the plastic in its final usable state. When applied to the molding, it is expressed as the relationship between the dimension of the mold and the dimension of the piece that is molded. The contraction is expressed in inches per inch or as a percentage. The contraction also varies with the thickness of the wall. Some plastics contract differently in one direction than in the other. For example, polymers filled with long glass fibers will contract more in the transverse direction than in the longitudinal direction (flow). |  |
Shrinkage is inherent to the injection molding process. The shrinkage occurs because the polymer density varies from the processing temperature to the ambient temperature (see Specific volume (pvT diagram)). During injection molding, the variation in shrinkage both globally and through the cross section of a part creates internal stresses or residual stresses acting on a part with effects similar to externally applied stresses. If the residual stresses induced during molding are high enough to overcome the structural integrity of the part, the part will warp when the mold is ejected or cracked with an external service charge. |  |
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| Material Name | Shinkage % | Material Name | Shinkage % |
| ABS | 0.4 – 0.7 | PES | 0.6 – 0.8 |
| CA | 0.3 – 0.7 | PET | 1.8 – 2.1 |
| CAB | 0.2 – 0.5 | PMMA | 0.2 – 1.0 |
| CP | 0.2 – 0.5 | POM | 2.0 – 3.5 |
| EVA | 0.7 – 2.0 | PP | 1.0 – 3.0 |
| FEP | 3.0 – 6.0 | PPO | 0.5 – 0.7 |
| GPPS | 0.2 – 0.8 | PSU | 0.6 – 0.8 |
| HDPE | 1.5 – 4.0 | PTFE | 5.0 – 0.9 |
| HIPS | 0.2 – 0.8 | PVDF | 2.0 – 3.0 |
| LDPE | 1.5 – 4.0 | SAN | 0.2 – 0.6 |
| PA6 | 1.0 – 1.5 | PP/EPDM | 1.0 – 2.0 |
| PA66 | 1.0 – 2.0 | PUR/TRU | 0.5 – 2.0 |
| PBT | 1.5- 2.0 | SBS | 0.4 – 1.0 |
| PC | 0.6 – 0.8 | SEBS | 1.0 – 5.5 |
| MF | 0.6 – 1.0 | DMC | 0.5 – 0.2 |