Guide to Diagnosing and Minimizing Injection Molding Shrinkage

All plastics shrink and lose volume during injection molding as they transition from molten to solid state. Excessive shrinkage leads to warpage and parts smaller than intended. Addressing the root causes through material selection, process optimization, simulation, and design minimizes shrink defects. This guide covers diagnosing, predicting, and controlling shrinkage.

What is Shrinkage?

Shrinkage refers to the volumetric contraction of plastics as they cool and solidify after injection molding. As the temperature drops, polymers chains become more ordered and dense, reducing volume. Parts reduce in size in all three dimensions.

Some key characteristics of shrinkage:

• It increases as wall thickness rises. Thicker areas see more shrink.
• Directionality varies with flow patterns; usually greatest in flow direction.
• Holds dimensional tolerances only if uniform and predictable.

While unavoidable, excessive shrinkage causes defects if not properly accounted for in design and processing. A controlled shrink is desired.

Identifying Shrinkage Issues

Trouble signs of excessive or variable shrinkage include:

• Parts smaller than design dimensions and tolerances.
• Warpage from uneven shrinkage stress.
• Sink marks as material pulls inward.
• Deformed features that warp or curve.
• Cracks from stresses exceeding strength.
• Fill problems from early freeze-off.
• Shrunken exterior surface but intact core.

Tracking measured dimensions identifies problematic shrinkage. Predictive analysis also anticipates issues.

What Causes Shrinkage Variation?

Many variables influence shrinkage in injection molding, including:

• Material – Amorphous vs semi-crystalline; fillers affect shrinkage.
• Geometry – Shrinkage increases with wall thickness.
• Process settings – Pack pressure, times impact shrinkage.
• Tolerances – Tighter specs allow less margin.
• Mold temperature – Colder molds increase shrinkage.
• Flow length – Longer flow paths see more shrinkage.

Avoiding drastic changes between these factors ensures uniform shrinkage.

Material Selection for Low Shrinkage

Choosing the right resin is key. Material factors impacting shrinkage:

• Amorphous vs semi-crystalline – Amorphous materials like ABS and PC shrink more uniformly. Crystalline materials like nylon have higher more anisotropic shrinkage.
• Fillers – Glass fiber, minerals reduce shrinkage versus neat resin.
• Molecular weight – Higher MW resins shrink more with greater chain entanglement.
• Viscosity – Lower melt viscosity resins shrink more under cooling.

Select a material that balances mechanical needs with minimal, predictable shrinkage.

How Process Settings Impact Shrinkage

Settings must balance packing the mold while limiting material degradation:

• Pack pressure – Higher pressure packs material to compensate for shrinkage. But too high degrades material.
• Pack time – Longer times supply more material to account for shrinkage.
• Melt temperature – Higher temps lower viscosity for better flow but may degrade material.
• Mold temperature – Colder molds increase shrinkage upon ejection but ensure solidification.

Optimized settings deliver the right amount of low-degradation melt.

Predicting Shrinkage with Moldflow Simulation

Simulation software like Moldflow analyzes shrinkage:

• Simulate shrinkage distribution and directionality.
• Input actual resin rheology and shrinkage data.
• Identify problem areas for sink marks, warpage, etc.
• Test different materials to compare shrinkage behavior.
• Optimize process settings for minimal shrink variation.

These insights fine tune the process before ever molding a part.

Design Strategies to Minimize Uneven Shrinkage

Smart part design also minimizes shrink variation:

• Uniform wall stock – Eliminates drastic thickness changes causing uneven cooling.
• Ribs and gussets – Manage material distribution to avoid localized shrink stresses.
• Generous radii – Reduces stress concentrations prone to warping.
• Draft angles – Facilitate even part ejection; avoid binding.
• Minimal sinks/voids – These can warp and crack after ejection.
• Short flow lengths – Long flows increase variation along length.

Sophisticated part geometry balances shrinkage tendencies.

Mold Gate Locations Matter

Gate placement influences directional shrinkage:

• Side gating – Induces longitudinal shrinkage parallel to melt flow upon entry.
• End gating – Creates transverse shrinkage across the path of flow.
• Multiple gates – Balances shrink stresses from different directions.

Gate strategy should match the shrink characteristics needed for that part geometry.

Allowing for Shrinkage in Tolerances

Dimensional specifications must account for shrink variation:

• Increase overall part size envelope to allow shrinkage leeway.
• Tighten critical feature tolerances only where required.
• Apply broader bilateral tolerances for stable processing window.
• Adjust tolerance with location based on flow distance.

Building in shrinkage margins prevents out-of-spec parts.

Incorporating Shrinkage Compensation

Directly counteracting known shrink values prevents defects:

• Oversize mold dimensions – Incrementally enlarge mold to account for shrinkage %.
• Post-mold adjustment – Additional sizing, drilling, or finishing processes.
• In-mold expansion – Gas assist injection molding forces plastic outward.

Deliberate compensation steps help achieve final accurate dimensions.

Conclusion

Plastic shrinkage in precise injection molding is inevitable, but its effects can be minimized through a concerted approach. Analyzing the interactive effects of chosen material, part geometry, mold design, process settings, and tolerance specifications allows holistically addressing root causes of excessive shrinkage. Incorporating the right combination of engineering, simulation, designed-in features, and compensation strategies enables controlling shrinkage variability. The result is molded components that consistently meet dimensional and functional requirements without warp-induced defects. Anticipating and proactively adjusting for shrinkage is key to successful injection molding.

In summary, plastic fabrication minimizing shrinkage defects involves:

• Selecting low shrinkage materials
• Optimizing process settings for material and design
• Using Moldflow analysis to predict shrink behavior
• Incorporating shrink-reducing features into the part design
• Compensating with oversized tools and post-molding steps

With science-based understanding and control of material shrinkage mechanics, manufacturers can confidently injection mold parts within specifications.