Dimensional control is a central concern in injection molding. Even when the mold is precisely machined, the final dimensions of a plastic component will not exactly match the cavity size. The reason lies in injection molding shrinkage, an inherent material behavior that occurs as molten polymer cools and solidifies. If shrinkage is not properly anticipated, parts may exhibit dimensional deviation, assembly interference, excessive clearance, or warpage.
What Is Injection Molding Shrinkage?
Injection molding shrinkage, also called plastic material shrinkage or molding shrinkage, is the reduction in size of a plastic part as it cools from molten to solid. It results from the density difference between the molten and solid polymer, with polymer chains packing more tightly during cooling. Thermoplastics also expand when heated and contract when cooled, and molten polymers compress under pressure. Shrinkage is usually expressed as a percentage, showing the difference between the mold cavity and the final part.
It is also important to note that shrinkage is not a fixed value. Even for the same polymer grade, the actual shrinkage depends on part geometry and processing conditions.
When Does Shrinkage Occur?
Most injection molding shrinkage happens during the cooling phase while the part is still in the mold. As the polymer solidifies, it contracts and pulls away from the cavity surface.
After demolding, most heat has dissipated, and major shrinkage has occurred, but a small amount may continue for hours or even days until the part reaches thermal and moisture equilibrium. For consistent measurements, inspections should not be done immediately after ejection.
At our company, Erye’s engineers follow a standardized procedure before dimensional inspection. Dimensional inspections must be delayed for at least 24 hours after demolding to reduce false rejection caused by post-mold shrinkage.
How to Calculate Injection Molding Shrinkage Rate?
Shrinkage is expressed as in percent %. The calculation relies on the difference between the mold cavity dimensions and the measured dimensions of the cooled plastic part. In general terms, the forming shrinkage rate represents the proportional reduction in material volume or length after the polymer melt cools and solidifies.
According to the German standard DIN 16901, the shrinkage rate S is determined using mold cavity dimensions measured at 23°C ± 0.1°C and part dimensions measured after 24 hours at 23°C and 50 ± 5 percent relative humidity.
The formula is: S={(D－M)/D}×100%
Where:
- S is the shrinkage rate (%).
- D is the mold cavity dimension
- M is the molded part dimension after conditioning
For example, if the mold length is 100 mm and the resulting part measures 98 mm, the shrinkage rate equals 2 percent. This value directly influences part dimensional accuracy, clearance fits, and potential interference during assembly.
The same plastic material exhibits higher shrinkage rates when the injection-molded part has greater wall thickness.
Plastic material shrinkage is not a single fixed value but exists within a range. In addition to wall thickness, multiple other factors contribute to this variation.
Causes of Variation in Molded Parts or Shrinkage
Variations in injection molding shrinkage arise from several interrelated factors that affect molecular packing, cooling behavior, and internal stresses within the part.
1. Degree of Crystallinity
Higher crystallinity generally increases shrinkage. During crystallization, polymer chains pack more densely, resulting in greater volume reduction.
2. Molecular Chain Flexibility
Materials with more flexible molecular chains may orient and stretch more easily during flow. Depending on processing conditions, this can increase shrinkage or create anisotropic shrinkage behavior.
3. Mold Temperature
Elevated mold temperature slows the cooling rate, allowing higher crystallinity to develop and thereby increasing shrinkage.
4. Injection and Packing Pressure
Higher injection and holding pressure compress the melt more densely into the cavity. Improved packing reduces internal voids and compensates for volumetric contraction, thereby decreasing molding shrinkage.
5. Runner and Gate Design
Poor runner or gate design leads to uneven filling and residual stresses, resulting in larger fluctuations in shrinkage across different sections of the part.
6. Filler Content
Adding fillers such as glass fiber or mineral powder restricts polymer contraction. Higher filler content typically reduces shrinkage because rigid particles limit volumetric reduction. However, fillers may also introduce directional shrinkage differences, especially in glass-fiber-reinforced materials.
7. Molding Process Type
Different forming methods exhibit different shrinkage behaviors. Blow molding generally shows higher shrinkage compared with injection molding or extrusion due to stretching and non-uniform cooling effects.
How to Control Injection Molding Shrinkage Rate
Control of injection molding shrinkage involves adjustments in material selection, mold design, and processing parameters.
Selection of plastics with inherently lower or more predictable shrinkage ranges, or incorporation of suitable fillers, helps reduce overall contraction.
In mold design, apply shrinkage compensation by scaling the cavity dimensions upward according to the calculated rate.
Optimization of gate placement and maintenance of uniform wall thickness minimizes differential shrinkage and associated warpage.
Adjust the processing parameters, increasing injection and holding pressure, extending holding and cooling times, and selecting appropriate melt and mold temperatures reduce voids and promote uniform solidification. Avoidance of excessively high melt temperatures prevents unnecessary expansion followed by contraction.
Mold flow analysis software provides simulations that predict shrinkage behavior before tool construction, allowing early parameter adjustments during trial runs. Prototype tooling that replicates production conditions further verifies shrinkage values and enables modifications before full-scale manufacturing.
Typical Shrinkage Values of Common Plastics
The following table presents typical molding shrinkage ranges for selected common plastics under standard injection molding conditions. Actual values depend on grade, filler content, and processing conditions.
| Material | Typical Shrinkage (%) |
|---|---|
| ABS | 0.4 – 0.7 |
| Polyethylene (PE) | 1.5 – 3.5 |
| Polypropylene (PP) | 1.0 – 2.5 |
| Polycarbonate (PC) | 0.5 – 0.7 |
| Nylon (PA) | 1.0 – 2.0 |
| POM (Acetal) | 1.5 – 2.2 |
| PMMA | 0.3 – 0.6 |
| Glass-Filled Nylon | 0.2 – 0.8 |
| Polystyrene (PS) | 0.40–0.70 |
| PEEK | 1.20–1.50 |
Conclusion
Injection molding shrinkage is an inherent and unavoidable characteristic of thermoplastic processing. It is the essential consideration in the production of accurate plastic components. Proper management from the design stage through processing reduces defects such as warpage and fit issues, supports consistent quality, and contributes to efficient manufacturing.
Erye provides professional injection molding service for precision custom plastic parts. Welcome to contact us and get a custom solution for your project.
