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Achieving dimensional consistency in injection molding is often more challenging than it seems, and plastic shrinkage is one of the problems encountered during the process. Shrinkage during cooling can cause parts to exceed tolerances, deform, and sometimes require costly adjustments after mold making. For manufacturers and product developers, understanding the causes and solutions to shrinkage can lead to more stable mold production.
What Is Plastic Shrinkage in Injection Molding
Plastic shrinkage in injection molding refers to the reduction in the volume and dimensions of a part as molten plastic cools and solidifies in the mold. As the temperature decreases, the polymer chains become more tightly packed, leading to shrinkage. This shrinkage is generally classified into volume shrinkage and linear shrinkage, with linear shrinkage being crucial for mold design and tolerance control.
Shrinkage is unavoidable, but it is predictable and controllable. By adjusting material selection, mold design, and processing conditions such as holding pressure and cooling, the injection molding manufacturers can effectively control shrinkage and maintain dimensional accuracy.
Main Causes of Plastic Shrinkage
Plastic shrinkage in injection molding is influenced by a variety of interacting factors, including material properties, part geometry, mold design, and processing conditions. Understanding these variables is crucial for predicting shrinkage rates and minimizing manufacturing defects.
Material Type
The type of polymer has the greatest impact on shrinkage rate. Semi-crystalline plastics (such as PP, POM, and HDPE) have higher shrinkage rates due to the ordered molecular structure formed during cooling. For example, the shrinkage rate of HDPE injection molding is typically between 1.5% and 4.0%, depending on the processing conditions. In contrast, amorphous materials such as ABS and PC have more disordered molecular structures, resulting in lower and more uniform shrinkage rates (typically 0.4%–0.7%). Materials such as PVC also have relatively low shrinkage rates; typical PVC shrinkage is around 0.2%–0.6%, making it suitable for parts with high tolerance requirements.
| Material Type | Structure | Typical Shrinkage |
| ABS, PC | Amorphous | 0.4% – 0.7% |
| PVC | Amorphous | 0.2% – 0.6% |
| PP | Semi-crystalline | 1.0% – 2.5% |
| HDPE | Semi-crystalline | 1.5% – 4.0% |
| POM | Semi-crystalline | 2.0% – 3.5% |
Part Design
The geometry of a part directly affects the uniformity of material cooling and shrinkage.
Wall thickness: Variations in injection molding wall thickness affect cooling rate and local material mass, making thicker sections more susceptible to shrinkage differences and sink marks.
Uneven wall thickness: Variations in wall thickness result in uneven cooling rates, causing differences in shrinkage and warping.
Mold Design
Mold design determines the heat dissipation method and the material filling and compaction method.
Cooling system design: Poor cooling-channel design can create uneven thermal conditions, so adequate cooling time in injection molding and balanced heat removal should be evaluated together.
Gate location and number: Improper gate location can cause an imbalance between flow and compaction, resulting in inconsistent shrinkage rates.
Ventilation: Insufficient ventilation can cause air stagnation, affecting compaction pressure and leading to localized shrinkage defects.
Processing Parameters
Injection molding conditions play a crucial role in controlling shrinkage.
Melt temperature: Higher temperatures increase material expansion, leading to greater shrinkage upon cooling.
Mold temperature: Mold temperature in injection molding affects cooling rate, crystallization behavior, shrinkage, and dimensional stability, especially for semi-crystalline materials.
Holding pressure and time: Sufficient holding pressure compensates for material shrinkage; insufficient holding pressure leads to voids and shrinkage marks.
Cooling time: Insufficient cooling time may cause post-molding shrinkage, affecting dimensional stability.
Fillers and Additives
Material modification can significantly alter shrinkage behavior.
Glass fiber reinforcement: Reduces total shrinkage, but introduces anisotropy (directional shrinkage differences).
Nucleating agents: Affect crystallization rate, helping to control shrinkage uniformity.
Mineral fillers: Improve dimensional stability by reducing total shrinkage, especially suitable for engineering plastics.
Typical Shrinkage Rates of Common Plastics
The table below lists typical shrinkage rates for commonly used thermoplastics in injection molding. The data is based on factual information. The values in the table represent the linear mold shrinkage range under standard processing conditions; actual shrinkage rates will vary depending on part design, mold temperature, and processing parameters.
| Material | Type | Typical Shrinkage (%) |
| ABS | Amorphous | 0.4 – 0.7 |
| PS (Polystyrene) | Amorphous | 0.3 – 0.6 |
| PC (Polycarbonate) | Amorphous | 0.5 – 0.7 |
| PVC (Rigid) | Amorphous | 0.2 – 0.5 |
| PMMA (Acrylic) | Amorphous | 0.3 – 0.6 |
| PP (Polypropylene) | Semi-crystalline | 1.0 – 2.5 |
| PE (LDPE) | Semi-crystalline | 1.5 – 3.0 |
| PE (HDPE) | Semi-crystalline | 1.5 – 4.0 |
| POM (Acetal) | Semi-crystalline | 2.0 – 3.5 |
| PA6 (Nylon 6) | Semi-crystalline | 1.0 – 2.0 |
| PA66 (Nylon 66) | Semi-crystalline | 1.0 – 2.5 |
| PET | Semi-crystalline | 1.5 – 2.5 |
Semi-crystalline materials (e.g., PP, POM, HDPE) show higher and less uniform shrinkage due to crystallization during cooling. For example, hdpe shrinkage in injection molding can reach up to 4.0%, requiring careful mold compensation.
Amorphous materials (e.g., ABS, PC, PVC) exhibit lower and more predictable shrinkage, making them suitable for tight-tolerance parts. For instance, PVC shrinkage in injection molding is relatively low, typically in the range of 0.2–0.5%.
Reinforced grades (e.g., glass-filled plastics) typically show reduced shrinkage (by 30–50%), but may introduce directional shrinkage.
How to Reduce and Control Plastic Shrinkage
Optimize Part Design
Maintain uniform wall thickness to promote even cooling and reduce warping, internal stress, and sink marks. Avoid excessively thick sections and use reinforcing ribs where possible, especially when working with semi-crystalline materials such as HDPE.
Improve Mold Design
Use evenly distributed cooling channels to maintain consistent mold temperatures. Proper gate placement and effective venting also help ensure balanced filling and packing, reducing localized shrinkage and internal voids.
Optimize Material Selection
Material properties have a direct impact on shrinkage. Amorphous plastics such as ABS, PC, and PVC generally have lower shrinkage, while semi-crystalline materials such as PP and HDPE tend to shrink more. Fillers can reduce shrinkage but may cause anisotropy. Injection molding material selection should consider shrinkage, mechanical requirements, tolerances, and dimensional stability.
Adjust Processing Parameters
Stable packing pressure, holding time, mold temperature, and cooling conditions are essential for consistent shrinkage control. Increasing packing pressure or holding time can compensate for material contraction, while proper mold temperature helps balance crystallinity, cooling, and dimensional accuracy. These parameters should be optimized within a stable injection molding process window.
Common Shrinkage Defects and Solutions
Plastic shrinkage not only affects part dimensions but also often leads to visible or structural defects, thus impacting product quality and assembly performance.
Sink Marks
Localized shrinkage in thicker sections, ribs, or bosses can appear as sink marks in injection molding when packing cannot fully compensate for material contraction.This is caused by insufficient material compensation during cooling, where the shrinkage of the inner core is greater than that of the solidified outer layer.
Main causes: Excessive wall thickness, insufficient holding pressure, insufficient holding time, or excessively high mold temperature.
Solutions: Increase holding pressure and holding time to compensate for volume shrinkage; redesign thicker sections, using reinforcing ribs instead of solid material; optimize gate location to improve material delivery efficiency; reduce melt and mold temperatures where appropriate.
Warpage
Uneven or directional shrinkage is one of the main contributors to warpage in injection molded parts, especially when cooling, geometry, or material orientation is unbalanced.This is especially common in semi-crystalline materials because variations in cooling rates lead to differences in crystallinity and internal stress distribution. In HDPE shrinkage injection molding, higher shrinkage rates and crystallinity make parts more prone to bending or twisting under uneven cooling conditions.
Main causes: Uneven wall thickness, uneven cooling, unbalanced filling, or improper gate design.
Solutions: Ensure uniform wall thickness and balanced part geometry; design efficient and symmetrical cooling channels; optimize gate location for uniform flow and filling; control mold temperature to minimize cooling differences.
Dimensional Deviation
Dimensional deviations refer to the inability of molded parts to meet specified tolerances due to inconsistent or excessive shrinkage. This issue is particularly critical for precision parts with stringent assembly tolerance requirements. While amorphous materials generally have more stable shrinkage properties, even relatively low shrinkage rates (e.g., PVC typically shrinks by around 0.2% to 0.5%) can still lead to deviations if process control is inadequate.
Main causes: Inaccurate shrinkage compensation in mold design, unstable processing conditions, or inherent material variations.
Solutions: Apply accurate shrinkage rates during mold design and verify them through mold testing; maintain consistency in processing parameters (temperature, pressure, cooling time); select materials with more predictable shrinkage rates for high-precision applications.
By addressing these defects through a combination of design optimization, process control, and proper material selection, manufacturers can significantly improve part quality and reduce production costs.
Conclusion
Plastic shrinkage affects dimensional accuracy, mold design, and production stability. Understanding how material, part geometry, and processing conditions influence shrinkage helps reduce warpage, sink marks, and dimensional deviations.
For tight-tolerance or complex parts, HingTung provides material evaluation, DFM analysis, and mold design optimization to improve consistency and reduce trial runs. Contact HingTung injection molding factory to discuss your project.