Plastic Structural Design: plastic shrinkage compensation
author: Universe plastic
2024-09-09
Compensation In the design of plastic structures, plastic shrinkage compensation is an important consideration, because during the plastic molding process, after high temperature and cooling, plastic parts will shrink in volume to varying degrees due to the material.
In order to ensure the dimensional accuracy and shape stability of the final product, it is necessary to consider how to compensate for shrinkage during the design stage. The following is the knowledge shared by Universe Plastic Engineers about shrinkage compensation.
Effect of shrinkage
1. Affects the dimensional accuracy of plastic parts
During the cooling process of plastic, due to the contraction of molecular chains, the size of the product will become smaller. This change in volume is described by shrinkage. If the shrinkage of the material is not properly considered during design, the final product may not achieve the expected dimensional accuracy.
2. Shape and Warp
Uneven shrinkage can cause plastic parts to warp, deform, or develop internal stress. Factors such as cooling rates in different areas, changes in wall thickness, and mold temperature can all lead to inconsistent shrinkage.
3. Influence of material and flow direction
The shrinkage of plastics can differ in the direction of flow (MD) and transverse direction (TD), especially for fiber-reinforced plastics. This anisotropic shrinkage results in different dimensional changes in the product in different directions.
4. Surface quality
Shrinkage can affect the surface quality of the product, especially in areas with uneven thickness or complex geometries. Excessive shrinkage may cause defects such as depressions (shrink marks), ripples or lines on the surface.
5. Structural strength
Shrinkage will cause stress concentration inside the plastic parts and affect their mechanical properties, especially those with complex structures or sharp corners or thin-walled parts. If shrinkage is not properly compensated, it may cause the product to crack or break during use.
6. Assembly accuracy
In an assembly process involving multiple components, differences in shrinkage can cause gaps or interference between mating components, affecting the assembly accuracy and functionality of the product.
7. Product life
Internal stress caused by shrinkage may be released over time, causing the product to gradually deform, relax or crack during use, affecting the product's life and performance stability.
8. Production costs
Excessive shrinkage may complicate mold design and increase production costs and processing difficulty. Molds need to be finely tuned to handle different shrinkage characteristics, which can also extend product development cycles.
Shrinkage rates of different injection molded plastics
|
plastic material |
normal shrinkage |
Flow direction (MD) |
Transverse (TD) |
|
LDPE |
1.5% - 3.0% |
1.5% - 3.0% |
1.5% - 3.0% |
|
1.5% - 3.0% |
1.5% - 3.0% |
1.5% - 3.0% |
|
|
Homopolymer PP |
1.2% - 2.5% |
1.2% - 2.5% |
1.0% - 2.5% |
|
Copolymer PP |
1.0% - 2.5% |
1.0% - 2.5% |
1.0% - 2.5% |
|
GPPS |
0.3% - 0.6% |
0.3% - 0.6% |
0.3% - 0.6% |
|
HIPS |
0.4% - 0.7% |
0.4% - 0.7% |
0.4% - 0.7% |
|
0.4% - 0.7% |
0.4% - 0.7% |
0.4% - 0.7% |
|
|
PA6 |
0.7% - 2.0% |
0.7% - 1.5% |
1.0% - 2.0% |
|
PA66 |
1.0% - 2.5% |
1.0% - 1.5% |
1.5% - 2.5% |
|
POM |
1.8% - 2.5% |
1.8% - 2.5% |
2.0% - 3.5% |
|
PET |
1.2% - 2.5% |
1.2% - 2.5% |
1.5% - 2.5% |
|
PC |
0.5% - 0.7% |
0.5% - 0.7% |
0.5% - 0.7% |
|
RPVC |
0.2% - 0.5% |
0.2% - 0.5% |
0.2% - 0.5% |
|
SPVC |
1.5% - 2.5% |
1.5% - 2.5% |
1.5% - 2.5% |
|
PPS |
0.1% - 0.5% |
0.1% - 0.2% |
0.2% - 0.5% |
|
PBT |
1.2% - 2.5% |
1.2% - 2.0% |
1.5% - 2.5% |
|
GF-PA6/GF-PA66 |
0.2% - 1.2% |
0.2% - 0.5% |
0.8% - 1.2% |
|
GF-PP |
0.3% - 1.2% |
0.3% - 0.8% |
0.7% - 1.2% |
*Search above data sources
Plastic shrinkage compensation design
1. Shrinkage in different directions
The orientation of plastic molecular chains and fibers affects shrinkage behavior, and the shrinkage rate of plastic products may vary in different directions, especially for fiber-reinforced plastics.
2. Design thickness control D
esigners should try to keep wall thickness as uniform as possible to reduce the risk of uneven shrinkage. The wall thickness of plastic parts has a significant impact on shrinkage. Thick-walled areas will cause greater shrinkage and may cause defects such as warping and denting.
3. Design structural features
Features such as ribs, bosses, holes, etc. can affect the cooling and shrinkage behavior of the plastic. When designing these features, consider the following compensations measure:
①Increasing the root fillet of the ribs can help reduce stress concentration and prevent excessive local shrinkage.
②Adjust the ratio of ribs to wall thickness. It is usually recommended that the rib thickness does not exceed 60% of the wall thickness.
③Control wall thickness changes and avoid sudden wall thickness differences, as this will lead to uneven shrinkage.
4. Post-processing considerations
Sometimes, plastic parts require processing after molding, such as assembly, splicing, drilling or cutting. These processes may expose or amplify dimensional problems caused by shrinkage, so the impact of these processes on size needs to be considered during design and the design should be adjusted appropriately.
Simulation and tryout
In modern plastic design, simulation analysis using CAE (computer-aided engineering) software has become a routine step. These tools can simulate temperature changes and flow behavior during injection molding, predict shrinkage and deformation, and make corresponding adjustments during the design phase.
Summarize
By understanding and applying these shrinkage compensation strategies, designers can effectively deal with the dimensional changes and shape instability caused by shrinkage in the design of plastic parts, ensuring that the final product meets the design requirements.
Dongguan Universe Plastic Factory has more than 15 years of injection molding manufacturing experience, a professional design team and manufacturing equipment, and can provide high-quality and cost-effective injection molding services.
If you are choosing a high-quality plastic design manufacturer, please contact us for the latest service information.
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