Understanding PPS Plastic Shrinkage: A Crucial Guide for Manufacturers and Engineers

Understanding PPS Plastic Shrinkage: A Crucial Guide for Manufacturers and Engineers

Engineers and manufacturers working with high-performance PPS plastic face immense pressure to produce dimensionally stable parts. Even minute amounts of shrinkage or warpage can detrimentally impact performance and part fit in precision assemblies.

As such, developing comprehensive insight into the shrinkage properties and behavior of PPS is a mandatory competency for any organization seeking to leverage the many benefits of this unique polymer while avoiding the headaches of dimensional variation.

This guide pulls the curtain back on the key factors that drive shrinkage in PPS, essential measurement and testing considerations, proven methods to minimize shrinkage, and proactive troubleshooting to nip issues in the bud.

custom PPS plastic product

Why PPS Plastic Shrinkage Presents Big Challenges

PPS (polyphenylene sulfide) offers an exceptional blend of temperature resistance, chemical resistance, and dimensional stability that makes it a popular choice for parts with tight tolerances for industries like automotive, aerospace, electronics, and industrial equipment.

However, like most thermoplastics, PPS still undergoes shrinkage during processing from the combined effects of thermal contraction as the polymer cools as well as crystallization:

  • As hot molten PPS cools, it contracts based on its coefficient of thermal expansion values.
  • Additionally, the PPS molecules rearrange and pack into an organized crystalline structure. This process squeezes out free volume, leading to additional shrinkage.

If uncontrolled, this shrinkage can lead to:

  • Warpage that causes parts to be out of specification
  • Dimensional variation between parts in the same run
  • Fit issues when assembling shrinkage-sensitive components
  • Scrap and rework costs to address out of tolerance parts

Clearly, having concrete working knowledge of the primary drivers influencing PPS shrinkage along with methods to predict, control, and troubleshoot issues is invaluable protection from preventable but painful manufacturing disruptions.

PPS plastic

Key Factors That Influence PPS Shrinkage Rates

While all amorphous and semi-crystalline thermoplastics display some volumetric shrinkage, the exact levels vary based on material composition and process conditions.

Engineers can forecast shrinkage amounts in PPS by understanding relationships between:

Crystallinity Level

PPS is a semi-crystalline plastic, meaning polymer chains align into ordered crystalline regions interspersed with amorphous areas.

The overall degree of crystallinity depends on:

  • Cooling rate - Faster cooling allows less time for organized crystalline arrangements to form
  • Presence of nucleating agents - These substances seed and facilitate crystal growth

Parts with higher crystalline content generally show increased shrinkage compared to largely amorphous materials. This occurs because the ordered packing of polymer chains in crystalline areas ejects free volume.

However, crystalline regions also restrict polymer chain mobility. At very high levels of crystallinity, shrinkage can actually decrease because the limited chain motion prevents further densification.

Molding Process Parameters

Key variables during molding, including:

  • Melt temperature
  • Mold temperature
  • Fill rate
  • Packing pressure
  • Cooling rate

...all significantly influence crystallinity development rates and ultimate shrinkage levels.

For example, high melt and mold temperatures followed by rapid cooling can reduce crystallinity and shrinkage. On the other hand, a cooler mold and slower cooling enables greater crystalline content and enhanced shrinkage.

Dialing in process settings correctly is crucial to achieve uniform shrinkage behavior across a production run.

Modifiers and Fillers

Introducing additives like glass fibers, minerals, impact modifiers induces physical restrictions that impede polymer chain mobility during crystallization. This lowers maximum potential shrinkage.

Similarly, nucleating agents promote formation of many small crystal domains instead of fewer large crystals. This reduced crystallite size limits shrinkage as well.

In filled compounds, shrinkage may also become anisotropic - displaying orientation-dependent values due to aligned fibers or filler particles.

Modified PPS sheet material

Part Geometry

Differences in wall thickness or overall part size can create uneven cooling patterns. This produces variability in crystalline content and shrinkage.

Additionally, some geometries tend to warp or distort more readily from imbalanced shrinkage strains.

Measuring and Predicting Shrinkage Rates in PPS

Quantifying volume change during processing is crucial for selecting optimal tooling dimensions and forecasting end-product dimensions.

Common methods for characterizing and predicting shrinkage in PPS plastic include:

Tracking Mass Density Changes

Monitoring subtle density fluctuations at various process stages estimates voids and volume loss due to crystallization. This requires precision measurement equipment and methods.

Following Dimensional Changes

Dimensional analysis uses metrology on molded specimens under controlled conditions to directly characterize shrinkage amounts. Values can be determined uniquely along flow and transverse directions to capture anisotropy.

Common standards like ASTM D955 provide testing procedures to promote consistency.

Analyzing Crystallinity

Advanced techniques like DSC (differential scanning calorimetry) identify thermal transitions associated with crystalline melting that estimate overall crystallinity values.

Since higher crystallinity drives more shrinkage, these measurements infer shrinkage behavior.

Computer Simulations

Mold filling simulations apply established shrinkage models and predicted crystallinity results to forecast final part dimensions.

They provide directional shrinkage estimates and even predict warp and twist deformation.

Validation builds confidence in using simulations for production predictions.

Shrinkage Rule of Thumb Values

General shrinkage guidelines for unfilled PPS range from:

  • 0.004-0.008 in/in (0.4-0.8%) along flow direction
  • 0.008-0.012 in/in (0.8-1.2%) across flow

However, properties differ between specific polymer grades and manufacturers. Always request shrinkage documentation from material suppliers when selecting PPS grades.

PPS CNC Processing

Proven Ways Manufacturers Minimize PPS Shrinkage

Armed with shrinkage rate information, engineers can then make adjustments during material selection, tool design, and process configuration to control shrinkage to acceptable levels:

Material Selection

  • Compare shrinkage values in material supplier datasheets for various grades and compounds. Opt for grades with suitable crystallization kinetics and filler loadings to minimize shrinkage for a given part geometry and process.
  • Leverage modified PPS grades using nucleating agents or impact modifiers that restrain crystallization and shrinkage.

Part Design Strategies

  • Where possible, specify uniform wall thicknesses to enable consistent cooling and shrinkage. Differences in wall thickness or cross-sectional area lead to non-uniform shrinkage strain.
  • Include proper mold offset tolerances by accounting for predicted shrinkage amounts during tool design. Insufficient offsets crash parts or introduce flash.
  • Add reinforcing ribs or gussets to minimize risk of warpage from shrinkage strains.
PPS plastic CNC machining workshop

Process Optimization

  • Start molding trials by matching supplier recommended processing conditions**. This establishes an appropriate baseline before further optimization.
  • Increase melt and mold temperatures in tandem to promote faster cooling while avoiding excessively high or low viscosities. Faster cooling restricts crystallinity.
  • Adjust packing pressure, hold times, and cooling rates to limit crystallinity based on PPS grade specifics and part thickness.
  • Verify stable dimensions early in a production run before continuing with longer cycles.

Troubleshooting PPS Shrinkage When Issues Arise

Despite best efforts in design and processing, dimensional stability issues still emerge. Here is an action plan to diagnose root causes and restore control:

Detecting Shrinkage Issues

Typical indicators of excess shrinkage include:

  • Short shots from insufficient material due to extra volume loss
  • Sinks, voids, and warpage from differential shrinkage
  • Dimensional variation between parts and cavities
  • Parts outside specified tolerances

Diagnosing Underlying Factors

Methodically assess known influence factors:

  • Material - Wrong grade or batch difference?
  • Mold - Uneven temperatures or worn tooling?
  • Machine - Fluctuating conditions?
  • Process - Deviations from proven recipe?
  • Environment - Ambient changes during run?

Identify prime suspects behind dimensional instability.

Taking Corrective Action

Address root causes:

  • Adjust material dried properly? Change grade?
  • Improve temperature control and tool monitoring
  • Tune filling, packing, and cooling steps
  • Enforce process disciplines
  • Compensate tooling dimensions

Implementing Robust Countermeasures

Prevent repeats by:

  • Improving process monitoring and control
  • Optimizing tool cooling designs
  • Validating any material or parameter changes
  • Strengthening dimensional measurement practices
  • Capturing prior shrinkage rates for next tool iteration
    High temperature resistant PPS screws

Get back on track to dependable production.

Best Practices for PPS Plastic Shrinkage Control

Specifying the right material composition using accurate shrinkage data, designing parts that minimize sensitivity, and consistently applying optimized molding processes culminates in dimensionally stable components ready for precision applications.

Some key guidelines that manufacturers and engineers should follow when leveraging PPS plastic include:

Know Your Material

  • Insist on shrinkage values from your material supplier rather than guesses or assumptions during grade selection
  • For modified PPS, understand impact of fillers/nucleants on crystallization behavior

Design Using Realistic Allowances

  • Use measured shrinkage rates from prior programs or supplier values for tool offset targets
  • Perform molding simulations on the part design using validated shrinkage assumptions early when changes are easier

Control Processing Conditions

  • Start process development using recommended parameters from material suppliers as an initial baseline
  • Adjust melt temperature, mold temperature, fill rate, packing pressure,cooling rate in a methodical way to find the optimum combination that produces complete fills while avoiding excessive shrinkage
  • Faster cooling generally restrict shrinkage but increases chance of warp; slower cooling allows lower molded-in stresses but permits higher crystallinity and shrinkage - careful tuning is needed
  • Monitor key characteristics like weight consistency, dimensions from multiple locations, presence of visual defects after demolding as process tuning indicators
  • Increase sampling rate and measurement frequency, particularly when adjusting process settings or starting a new production batch, to swiftly flag dimensional deviations
  • Apply additional cooling time for thicker wall sections prone to latent shrinkage not detected immediately after molding
  • Employ real-time monitoring on parameters like melt/mold temperatures,fill times,pressures etc and abort production if significant deviations occur until root cause is addressed

Controlling shrinkage is among the most persistent challenges with precision polymer processing. PPS's semi-crystalline nature enables tailored dimensional stability capabilities but also necessitates heightened process rigor. Learning shrinkage root causes and risk factors empowers engineers and manufacturers to unleash PPS’s full potential. This guide outlined key considerations starting from material selection all the way through troubleshooting production issues for anyone seeking to adopt PPS. Master its shrinkage properties and enjoy the benefits of high heat resistance without geometric uncertainty!

PPS processed parts

Ready to stop fighting shrinkage and start leveraging PPS for your demanding applications? Get in touch with us to learn about available PPS material grades, get shrinkage measurements on your part design, or discuss requirements. Or browse our selection of stock shapes and finished components. We aim to make adopting PPS’s benefits seamless and pain-free. Contact us today to get started!

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