Introduction
In plastic injection molding, one of the most crucial factors to consider is plastic shrinkage. This natural phenomenon can significantly impact the quality and assembly of molded parts, making it essential for designers to account for it early in the design process. In this post, we’ll explain what plastic shrinkage is, its causes, and provide a helpful table outlining shrinkage percentages for various polymers. This knowledge will help designers ensure proper tolerances are maintained, preventing issues in part assembly.
What is Plastic Shrinkage in Injection Molding?
Plastic shrinkage refers to the dimensional reduction of a plastic part as it cools after being injected into a mold. The size difference between the mold cavity and the final part occurs due to the material contracting as it transitions from molten to solid form.
Causes of Plastic Shrinkage
Several factors contribute to plastic shrinkage:
- Material Properties: Different polymers have different shrinkage rates due to their molecular structure.
- Cooling Rate: Faster cooling leads to more shrinkage as the material contracts rapidly.
- Part Thickness: Thicker parts retain heat longer, potentially leading to more shrinkage in certain areas.
- Processing Conditions: Variables like injection pressure and temperature also influence shrinkage levels.
How Shrinkage Affects Part Assembly
When shrinkage is not properly accounted for in the design phase, it can affect part dimensions and ultimately the fit and function of assembled components. Parts that are too large or too small may not align properly, resulting in gaps, misalignments, or even mechanical failure. Designers need to consider tolerances when developing molds to accommodate for shrinkage and ensure that parts fit together as intended.
Shrinkage Rates for Different Polymers
Below is a table showing the typical shrinkage rates for various common polymers used in injection molding:
We’ve hand established this over years of trial and error with polymers.
If using a specialist material you may be able to request this value direct from your supplier.
| Polymer | Shrinkage Rate (%) |
|---|---|
| Acrylonitrile Butadiene Styrene (ABS) | 0.6% |
| ABS / PC ALLOY | 0.7% |
| Acetal / Delrin (POM) | 1.8% |
| Acrylic | 0.6% |
| CAB | 0.5% |
| HDPE | 3.0% |
| LDPE | 3.0% |
| NYLON 6 | 1.2% |
| NYLON 6 – 30% GF | 0.4% |
| NYLON 6 / 6 | 1.5% |
| NYLON 6 / 6 – 33% GF | 0.5% |
| NYLON 11 | 1.0% |
| NYLON 12 | 1.4% |
| PEEK | 1.1% |
| POLYCARBONATE (PC) | 0.6% |
| POLYESTER PBT | 2.0% |
| PET (SEMI-CRYSTALLINE) | 0.4% |
| PET (AMORPHOUS) | 0.4% |
| POLYPROPYLENE (COPOLYMER) | 2.0% |
| POLYPROPYLENE (HOMOPOLYMER) | 1.5% |
| POLYPROPYLENE (TALC FILLED) | 1.0% |
| POLYPROPYLENE (30% GF) | 0.5% |
| POLYSTYRENE | 0.5% |
| POLYSTYRENE (30% GF) | 0.1% |
| PVC P | 1.5% |
| PVC U | 0.4% |
| SAN | 0.6% |
| SAN (30% GF) | 0.1% |
| TPE | 2.5% |
Conclusion
Plastic shrinkage is a fundamental aspect of injection molding that designers must account for to ensure the dimensional accuracy and functionality of their parts. By understanding the causes of shrinkage and the shrinkage rates of various polymers, you can make informed design decisions and prevent costly assembly issues.
Visit Newark Tools Ltd for Professional Toolmaking Services
At Newark Tools Ltd, we specialize in precision toolmaking, including plastic injection molds and die casting molds. If you need help with mold design or manufacturing, contact us today to see how we can assist in turning your project into reality!