Cooling is the key part of the plastic molding process, and it is where something interesting happens. While releasing the heat, the parts tend to change size, and while it is often insignificant, it may sometimes lead to great size and/or shape inconsistencies for parties of thousands of details.
Oftentimes, even the smallest of variations in size or shape drastically affect plastic’s performance, so avoiding them during cooling can help you save up on waste.
In this article, we will learn what shrinkage in injection molding is, how you need to address it, and why it should not be neglected.
The Impact of Shrinkage on Plastic Parts
Plastic shrinkage affects molded parts in several connected ways. The first is dimensional accuracy. If the contraction is not predicted correctly, the finished part may fall outside design tolerances and fail to fit with mating components. A small difference in contraction can lead to gaps, misalignment, or higher assembly force.
The second effect is visual quality. Uneven contraction can pull the surface inward and create sinks waviness, or distortion. Even when a part still works mechanically, these defects can lower the perceived quality of the product. This is one reason why mold shrinkage is discussed early in both industrial and consumer product programs.
The third effect is cost. When contraction causes dimensional drift, the factory may need to adjust tooling, revise processing conditions, or add secondary operations. Those corrections raise labor and machine time, while scrap also increases. For that reason, understanding shrinkage injection molding behavior is part of controlling both quality and cost in long-term production.
Why Product Designers Need to Know Injection Molding Shrinkage
Although molders solve many dimensional issues on the press, product designers still need a working understanding of injection molding shrinkage. Early design choices have a significant impact on the resin’s behavior during tooling and production.
- Optimizing designs: With a better understanding of contraction, designers may more precisely size features and lower the likelihood of mismatch after cooling.
- Material selection: Tolerance geometry and use conditions determine the optimal choice because different resins exhibit significantly varying plastic shrink rates.
- Iterative development: Because fewer adjustments are required following sampling, early awareness reduces the trial-and-error cycle.
- Cost efficiency: More accurate forecasts cut down on waste, tool adjustments, and postponed launch schedules.
This knowledge helps designers work more effectively with tooling teams and molders. It also improves the chance that the first sampling round will be close to the target dimensions.
Factors Influencing Injection Molding Shrinkage
The amount of contraction in a molded part is shaped by the process, the part itself, the mold, and the resin. A thin, evenly packed part will behave very differently from a thick, complex part made from a high-crystallinity polymer. Wall balance matters a great deal because thicker sections cool more slowly and often show greater contraction. At the same time, local features such as ribs, reinforcement inserts, or engraved details can resist movement and create smaller local changes.
Molding Processes Factor
The molding cycle directly affects final dimensions. Changes in temperature, pressure filling, and packing alter how the polymer contracts during cooling, so shrinkage in injection molding should be treated as a process result rather than a fixed value.
Stable melt and mold temperature improves consistency, while higher packing pressure helps compensate for contraction and reduce the final shrinkage rate. Melt temperature influences molecular behavior during solidification, and mold temperature can increase contraction, especially in semi-crystalline materials.
Cooling time is equally important. Parts continue stabilizing in the tool, and early ejection can lead to further deformation outside the cavity. Flow speed also affects molecular orientation, which may cause directional variation. As a result, the same resin can show different plastic material shrinkage under different settings.
Plastic Properties Factor
One of the main causes of the differences in shrinkage between resins is material qualities. Molecular weight, additives, crystallinity, and polymer structure all affect the outcome. For this reason, the plastic material shrinkage value found in datasheets ought to be regarded as a general guideline.
Because they produce denser structures during cooling, semi-crystalline materials often shrink more than amorphous ones. Although they may cause directional variance, fillers can lessen shrinking. Even comparable materials may behave differently in practice due to pigments and batch variations.
Mold Structure Factor
Mold design controls how resin fills packs and cools, which directly affects the plastic shrinkage behavior. Larger gates improve packing while gate placement determines how pressure is transmitted across the part.
Areas farther from the gate may receive less packing and show higher mold shrinkage. Internal restrictions can limit movement in specific regions, creating uneven contraction. Thermal balance is also important, as uneven cooling can cause distortion after ejection.
For this reason, injection molding shrinkage should be addressed during tooling design rather than corrected later. A well-designed mold improves dimensional consistency from the start.
Plastic Structure Factor
Part geometry controls heat transfer and contraction behavior. Thick walls cool more slowly and tend to increase plastic shrinkage, while thin sections cool faster and remain more stable. Uneven wall thickness often leads to local variation and a higher risk of warpage.
Structural features such as ribs, bosses, and reinforced areas can resist contraction, resulting in lower shrinkage locally. Inserts may also restrict movement. Because of this, the shrinkage of plastic material is rarely uniform across the entire part.
Flow direction also affects results. Parts may shrink less along the flow and more across it. A balanced design with consistent wall thickness helps maintain predictable shrinkage and better dimensional stability.
Effects of Uneven Shrinkage in Molded Parts
Uneven contraction in molded parts is commonly called warpage. It happens when some areas cool or pack differently than others, which creates internal stress and changes the final shape.
- Part deformation
- Dimensional variation
- Visible defects
- Long-term damage
- Appearance and fit issues
- Higher production cost
Methods to Determine Shrinkage
The most common methods used to determine mold shrinkage and dimensional contraction in molded resins include:
- ASTM D955: Standard test method for measuring shrinkage from mold dimensions of thermoplastics. It is widely used for injection and compression-molded specimens.
- ISO 294-4: Standard for determining molding contraction of thermoplastic test specimens made by injection molding.
- ISO 2577: Standard for determining contraction in thermosetting molding compounds.
These methods provide a controlled way to compare materials, though actual part behavior may still differ in real tools. Even standardized values should be validated against the true geometry process window and end-use tolerances of the part.
Shrinkage Values (%) of Plastics
A practical shrinkage chart of plastic materials helps designers estimate tooling compensation and compare common resins before sampling begins. The values below are typical ranges rather than absolute constants because geometry, gate layout, packing, and cooling all affect the final result.
PA6 Plastic Shrinkage
PA6 is widely used for structural parts because it balances toughness, wear, and processability, though moisture and crystallinity can affect the final result.
| Material | Typical Shrinkage (%) | Notes |
| PA6 unfilled | 0.7 to 1.6 | Higher contraction is common in thicker sections |
| PA6 mineral-filled | 0.4 to 1.0 | Filler helps reduce movement |
| PA6 GF reinforced | 0.2 to 0.8 | Directional effects may appear with reinforcement |
PA66 Plastic Shrinkage
PA66 is often chosen for technical parts that need heat resistance and good stiffness, though its contraction can still vary with grade and reinforcement.
| Material | Typical Shrinkage (%) | Notes |
| PA66 unfilled | 0.8 to 1.5 | Typical for general-purpose grades |
| PA66 mineral-filled | 0.4 to 0.9 | Reduced movement compared with unfilled grades |
| PA66 GF reinforced | 0.2 to 0.7 | Lower average contraction with possible directional variation |
PP Plastic Shrinkage
PP is common in high-volume production because it flows well and offers a good cost-to-performance balance, though plastic shrinkage is often higher than in more rigid engineering resins.
| Material | Typical Shrinkage (%) | Notes |
| PP homopolymer | 1.5 to 2.5 | Higher values are common in thicker walls |
| PP copolymer | 1.0 to 2.0 | Slightly lower and often more forgiving |
| PP filled | 0.8 to 1.5 | Filler helps improve dimensional control |
PC Plastic Shrinkage
PC is a low-contraction amorphous polymer that is widely used when optical quality, toughness, or dimensional control matters.
| Material | Typical Shrinkage (%) | Notes |
| PC unfilled | 0.5 to 0.7 | Stable for many precision applications |
| PC filled | 0.3 to 0.6 | Lower movement with improved rigidity |
PC/ABS Plastic Shrinkage
PC/ABS blends are often used in housings and appearance-sensitive parts because they balance impact performance with easier processing.
| Material | Typical Shrinkage (%) | Notes |
| PC/ABS standard grade | 0.4 to 0.7 | Common range for general housings |
| PC/ABS reinforced grade | 0.3 to 0.6 | Better dimensional stability |
This blend is often selected when both toughness and controlled contraction are important.
PC/PBT Plastic Shrinkage
PC/PBT blends combine the dimensional stability of PC with improved chemical resistance and flow behavior, though the PBT portion can influence final contraction.
| Material | Typical Shrinkage (%) | Notes |
| PC/PBT standard grade | 0.5 to 1.0 | Depends on blend ratio and tool conditions |
| PC/PBT reinforced grade | 0.3 to 0.7 | Reinforcement reduces movement |
PC/PBT should be validated in the real tool rather than estimated only from generic data.
ABS Plastic Shrinkage
ABS is one of the most widely used materials for cosmetic and structural housings because it offers balanced toughness, processability, and moderate contraction.
| Material | Typical Shrinkage (%) | Notes |
| ABS standard grade | 0.4 to 0.7 | Common range for many molded housings |
| ABS high-flow grade | 0.3 to 0.6 | May help with thin-wall filling |
| ABS reinforced grade | 0.2 to 0.5 | Lower movement with added rigidity |
These values make ABS a familiar choice for parts that need a reliable balance between appearance and dimensional control.
To reduce shrinkage and improve part quality, apply the following proven methods.
How to Prevent Fluctuations in Plastic Shrinkage
A stable molding process is the best way to avoid shrinkage. Here are the main steps to take and why they work:
- Balance flow and gates: Balanced runners and gates help maintain even pressure and reduce sinks or warpage.
- Optimize cavity arrangement: A layout with even heat distribution supports more consistent dimensions.
- Control part deformation through gating: Parts with round or complex geometry may need centered or multi-point gates to improve packing balance and reduce distortion.
- Use uniform wall design: Thick sections cool more slowly and tend to shrink more, while ribs, bosses and engraved areas can restrict movement locally.
- Maintain stable packing pressure: Changes in hold pressure or hold time can shift the final shrinkage rate even when the same resin and tool are used.
- Allow proper cooling before ejection: Early ejection can lead to continued movement outside the mold. Controlled cooling improves solidification and dimensional stability.
A stable process usually gives better results than trying to correct shrinkage after defects appear.
FAQ
When does shrinkage occur?
Injection molding shrinkage takes place during cooling inside the mold. A smaller amount continues after ejection until the part reaches room temperature.
What are the units of shrinkage?
Shrinkage is usually expressed as a percentage %.
What is the significance of shrinkage?
It affects part dimensions, fit, appearance, and tooling accuracy.
How deep are plastic shrinkage cracks?
They are usually shallow at first but can grow deeper under stress or repeated load.
What is plastic shrinkage, and how can it be avoided?
It is the size reduction that happens as the molded polymer cools. It can be reduced through good part design, balanced tooling, and stable processing.
What is the shrinkage of plastic injection molding?
It is the percentage difference between mold size and cooled part size.
What is a good shrinkage percentage?
For many molded parts, about 0.4% to 0.7% is considered good and manageable, though the ideal value depends on the resin.
How to calculate plastic shrinkage?
Use the formula: (Original size – Cooled size) / Original size × 100%.
How much shrinkage is acceptable?
For many commercial molded parts, 0.1% to 2.5% injection molding shrinkage is a common acceptable range, depending on the material and tolerance requirements.
