---
title: "POM Injection Molding: Managing Shrinkage and Voids - JATERSON"
description: "Navigating POM injection molding requires precise control over shrinkage and venting to prevent defects. This guide analyzes material properties and processing strategies for high-volume production."
url: "https://www.jaterson.com/manufacturing/pom-injection-molding-shrinkage-voids.html"
language: "en"
type: "Article"
category: "Injection Molding Guide"
datePublished: "2026-08-29"
dateModified: "2026-08-29"
brand: "JATERSON"
manufacturer: "JATERSON"
image: "https://static.jaterson.com/uploads/industry/injection/bD8PojCnwlGGY.webp"
---

# POM Injection Molding: Managing Shrinkage and Voids

## The Project Failure That No One Saw Coming

In the procurement of precision plastic components,few scenarios are as frustrating as receiving a batch of parts that pass dimensional inspection during the first article phase but fail catastrophically during mass production.This occurs frequently with Polyoxymethylene (POM),commonly known as Acetal.The project usually begins with a promising prototype: the gears mesh smoothly,the sliding fittings move with low friction,and the surface finish is pristine.However,once the tooling is unqualified for full-scale production and the cycle times are minimized,the defects emerge.

![Precision POM Components: A Manufacturing Guide](https://static.jaterson.com/uploads/industry/injection/bD8PojCnwlGGY.webp)

The parts begin to show internal voids that are invisible to the naked eye but cause mechanical failure under load.Alternatively,the dimensions drift unpredictably,leading to assembly jams on the automated line.The root cause is rarely a flaw in the design itself,but rather a misunderstanding of how POM behaves thermally and chemically inside the mold.To avoid this failure stage,buyers and engineers must look beyond the material’s datasheet and understand the specific manufacturing constraints of injection molding POM.

## The Counterintuitive Nature of POM

When engineers select POM,they typically focus on its high stiffness,excellent dimensional stability,and low coefficient of friction.It is often chosen to replace metal in gears,bushings,and conveyor components.However,a counterintuitive property of POM is that while it is a rigid and stable solid in its final state,it behaves aggressively during the molding process.

Unlike amorphous plastics like ABS or Polystyrene,which cool and shrink relatively predictably,POM is a semi-crystalline thermoplastic.This crystallinity means that as the material cools from a melt to a solid,the polymer chains fold into tight,ordered structures.This transition releases significant latent heat and results in a volumetric shrinkage that is considerably higher than that of amorphous materials.If the [mold design](https://www.jaterson.com/capabilities/mold-design-tooling/) or the packing pressure does not compensate for this rapid and aggressive crystallization,the part will sink,warp,or develop internal voids.

## Material Properties and Manufacturing Implications

From a manufacturing perspective,understanding the following characteristics of POM is non-negotiable for project success.These properties dictate how the mold must be built and how the machine must be configured.

- **High Shrinkage Rate:** POM typically exhibits a shrinkage rate of roughly 1.8% to 2.5% in the flow direction and slightly higher transverse to the flow.This is significantly higher than materials like PC or ABS.Tooling must be oversized with precise correction factors to account for this contraction.
- **Low Melt Viscosity:** POM flows easily,which allows it to fill thin walls.However,this same fluidity makes it prone to flashing if the mold alignment is not perfect or if the clamping force is insufficient.
- **Thermal Sensitivity:** POM has a narrow processing window.If the melt temperature is too high,the material will thermally degrade,releasing formaldehyde gas.This gas not only creates voids and surface defects but can also corrode the mold steel over time.
- **Moisture Sensitivity:** While POM is hygroscopic,it absorbs less moisture than Nylon.However,even small amounts of surface moisture can cause splay marks (silver streaks) on the surface of the part.Pre-drying is standard practice to ensure cosmetic quality.

![POM Injection Molding: Managing Shrinkage and Voids](https://static.jaterson.com/uploads/industry/default/92HldCyjxMCmJ.webp)

## Mold Design Considerations for POM

The success of a POM project is often determined before the first shot is taken.The mold design must accommodate the specific thermal and mechanical demands of the material.

### Gate and Runner System Design

Because POM shrinks significantly and solidifies quickly,the gate design is critical.Small gates may freeze off prematurely,preventing adequate packing pressure from reaching the cavity to compensate for shrinkage.This leads to voids in thick sections.Conversely,gates that are too large can cause excessive shear stress or leave unsightly [gate marks](https://www.jaterson.com/knowledge/injection-molding-gate-marks/) that are difficult to trim.

For general components and tool accessories,full round runners are generally preferred over trapezoidal ones to minimize heat loss and pressure drop.Hot runner systems are effective but must be well-managed to prevent material drool or freeze-off,as POM’s low viscosity makes it prone to leaking if the valve gate control is not precise.

### Venting Strategy

This is a critical checkpoint that is often overlooked.As POM melts,it can produce volatile gases,particularly if it has been subjected to excessive heat history.If these gases are not allowed to escape the mold cavity rapidly,they will trap at the flow front,causing burning marks or short shots.POM molds require deeper and more frequent venting compared to molds for less volatile materials.In 2026,best practices suggest incorporating venting along the parting lines and,if necessary,using vented ejector pins to allow gas to escape from deep core areas.

### Steel Selection and Corrosion Resistance

For long-term production runs,the corrosive potential of the formaldehyde released during POM degradation cannot be ignored.While standard P20 steel may be acceptable for prototype or low-volume runs,high-volume production necessitates the use of corrosion-resistant steels or hardened stainless steel for the cavities and cores.This prevents the formation of pitting on the mold surface,which would eventually be reflected as surface defects on the molded parts.

## Processing Parameters and Quality Control

Once the tool is built,the process engineering phase begins.The goal is to find a stable processing window that maximizes [cycle time](https://www.jaterson.com/knowledge/cycle-time/) without inducing degradation.

### Temperature Control

Melt temperature for POM typically ranges between 190°C and 210°C for homopolymer grades,and slightly lower for copolymers.Exceeding 220°C risks degradation.The mold temperature is equally important.A hot mold (80°C to 120°C) promotes crystallization and improves surface finish,but it extends the cycle time.A cold mold freezes the material faster,reducing cycle time but potentially increasing internal stresses and decreasing crystallinity,which can impact the part’s mechanical properties.Finding the balance between cycle efficiency and part performance is a key engineering task.

### Packing and Holding Pressure

Due to the high shrinkage,POM requires significant packing pressure.This pressure must be applied immediately after the cavity is filled and maintained until the gate freezes.If the holding pressure is released too early,the material will sink into the part as it cools.If applied too long,the part may stick in the mold or be over-packed,leading to high internal stresses and warpage.

### Common Defects and Troubleshooting

When issues arise in production,they usually manifest in specific ways.Understanding the root cause allows for rapid corrective action without scrapping the production batch.

| Defect | Primary Cause | Corrective Action |
| --- | --- | --- |
| Sink Marks | Inadequate packing pressure or gate freeze-off | Increase holding pressure or time; enlarge gate size |
| Voids (Bubbles) | Trapped gas or shrinkage in thick sections | Improve venting; reduce melt temperature; increase packing pressure |
| Splay (Silver Streaks) | Moisture in material or overheating | Verify drying time and temperature; reduce barrel temperature |
| Burn Marks | Trapped air/gas igniting at end of fill | Clean or enlarge vent channels; reduce [injection speed](https://www.jaterson.com/knowledge/injection-speed/) |
| Warpage | Uneven cooling or high internal stress | Balance mold temperature; adjust cooling time; reduce packing pressure |

## Sourcing and Supplier Evaluation

For procurement managers and supply chain professionals,selecting a partner for POM molding involves verifying specific technical capabilities.A supplier’s experience with general plastic components does not automatically guarantee expertise with semi-crystalline materials like POM.

When evaluating potential manufacturers,such as JATERSON,it is important to discuss their approach to mold maintenance and venting.Ask specifically about their steel selection for POM molds and their protocol for drying material.Inquire about their process validation methods—do they perform a Design of Experiments (DOE) to map out the processing window before mass production begins?

Furthermore,consider the supplier’s ability to detect defects that are not visible to the naked eye.For critical safety components,does the facility have the capability to perform X-ray inspection or ultrasonic testing to ensure there are no internal voids in thick sections?These quality control measures are often the differentiator between a reliable supply chain and one plagued by intermittent failures.

## Conclusion

POM remains an excellent choice for high-performance,low-friction components,but it demands respect during the manufacturing process.The transition from a successful prototype to a stable mass production run hinges on managing the material’s crystallinity,shrinkage,and gas evolution.By prioritizing proper mold design,strict process control,and rigorous supplier evaluation,procurement teams can mitigate the risks of warpage,voids,and degradation.In the landscape of general injection molding and hardware manufacturing,success with POM is achieved not by treating it as a generic plastic,but by engineering the process around its specific and challenging properties.

## Related Resources

- [Injection Molding Guide](https://www.jaterson.com/manufacturing/injection-molding/)
- [Plastic Component Manufacturing Guide](https://www.jaterson.com/manufacturing/plastic-components/)
- [Hardware Manufacturing Guide](https://www.jaterson.com/manufacturing/hardware/)
- [Capabilities](https://www.jaterson.com/capabilities/)
- [Custom Manufacturing](https://www.jaterson.com/custom-manufacturing/)
- [Products](https://www.jaterson.com/products/)
- [Manufacturing Guides](https://www.jaterson.com/manufacturing/)
- [Buying Guides](https://www.jaterson.com/buying/)
- [Manufacturing Knowledge Base](https://www.jaterson.com/knowledge/)
- [Injection Molding](https://www.jaterson.com/knowledge/injection-molding/)
- [Injection Molding Q&A](https://www.jaterson.com/qa/injection-molding/)

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