Splay in Injection Molding: Causes, Prevention and Solutions

Plastic injection molding enables us to manufacture a vast number of objects in modern life. It’s an unparalleled, versatile manufacturing process that produces everyday plastic components. These range from complex medical devices to heavy-duty automotive components. 

Yet, it requires great attention to maintain consistent quality. Various defects may occur during the injection molding process. It’s important to understand them in order to produce quality plastic components according to specifications.

In this post, we’ll discuss one such defect – splay. Also known as surface delamination. Splay appears as ugly streaks or blemishes on the surface of injection molded parts and is usually silver in color. 

It affects the appearance and sometimes the performance of injection molded parts. Let’s explore what causes splay. Along with that we can talk about how to prevent it from happening and troubleshooting tips to produce flawless plastic components.

What is Splay?

Splay, also called “silver streaks,” is a cosmetic blemish that can affect plastic injection molding. Splay shows up as unwanted streaks or marks on molded parts’ surfaces. These marks often have a silvery or white appearance and are unsightly.

Splay can be an important defect to note for quality control. Here is how it is seen:

• Visual Appearance: Splay shows up as streaks, lines, or undulating formations on the plastic parts. These formations are often parallel to the direction of flow when molten plastic is injected.
• Typical Indicators: Splay often manifests as a shiny or iridescent appearance in the affected areas. It may have a mild change in surface texture from the surrounding smooth surface. The splay marks may also show a minor bump or irregularity in extreme cases.

Splay should not be confused with other surface imperfections. Flow lines, for instance, are quite common and also show up as light lines on the surface of the plastic. 

However, flow lines are usually inconsequential and won’t impact the performance of the molded part.  Splay, while sometimes just a surface issue, can, on occasion, point to an underlying problem. The issue could impact the strength or performance of the molded component.

Causes of Splay

Splay doesn’t just happen. It is caused by a variety of conditions, which can be loosely grouped into material, process, and tooling/equipment issues.

Material

• Wet Raw Materials: All plastic resins absorb moisture from the air, especially hygroscopic materials such as nylon or polycarbonate. Trapped moisture can vaporize during the injection phase and leave behind tiny gas bubbles. These bubbles are carried along with the molten plastic to the surface, causing an erratic flow that appears as a splay.
• Impurities and Contaminants: Foreign particles or degraded plastic (regrind) in the resin can also upset the molten flow and create splay.

Process Parameters

• Temperature Settings – Too High: The plastic resin will degrade with excessive overheating, and this can lead to moisture and gas, which will contribute to splay. 

Temperature Settings – Too Low: The melt will not completely fuse and may not fill evenly, resulting in higher propensity for splay.

• Injection Pressure and Speed: Too High: High injection pressure forces any entrapped gas bubble to the surface and causes splay. 

Pressure Settings – Too Low: The mold may not be completely filled, resulting in air pockets that can potentially cause splay. Injection Speed may also affect – Very high injection speed can create turbulence in the melt and trap air bubbles within.

• Mold Venting – Insufficient: Vents need to be placed strategically on the mold to allow any air to escape during the injection phase. Lack of sufficient venting may allow air pockets to get trapped in the solidifying plastic, which will result in splay.

Equipment and Tooling:

• Injection Molding Machine Issues: The machine itself or the equipment may be worn out or broken, causing problems with the process. For example, if valves leak or there is inconsistent injection pressure, the flow will not be consistent, and air will be trapped in the process.
• Mold Design Deficiencies: There is a chance that the gating is not optimal or there are no adequate venting channels inside the mold. It will create areas where the air cannot escape and will create the conditions for splay to occur.

Splay Effects Production

Splay is not just a surface issue. Splay can affect the entire molding process.

Appearance

The most noticeable effect splay has on the molded part is appearance. These defects can make otherwise usable parts unusable for aesthetic applications. This may result in scrap and rework.

Mechanical Properties

Splay can lead to reduced mechanical properties of the molded part in extreme cases. Microbubbles of trapped gas in the plastic matrix can become stress concentrators. 

This reduces the part’s ability to withstand stress before cracking or breaking. This should be considered if the part is structural in nature.

Financial Effects

Splay means extra costs in molding parts. Scrap due to splay means rework, which means using materials and labor to mold a new part. Troubleshooting and taking corrective action can also lead to schedule disruptions.

Prevention and Control Measures

Luckily, there are a few things you can do to prevent and reduce splay.

Prepare Your Materials Properly

• Dry the raw materials, especially hygroscopic resins, before processing. Resins and other additives can contain a significant amount of moisture, which must be removed by proper drying before use.
• Use clean and contaminant-free resin to reduce the chance of foreign particles in the material that can disrupt flow and cause splay. Dirty materials are a primary cause of splay.

Molding Processing Conditions

• Temperature control set points are correct and consistent during the injection phase of the process. Excessive temperatures during injection will cause degradation, which may generate gas and contribute to splay.
• Injection speed and pressure are optimized for the material’s flow characteristics. This will ensure proper cavity fill with minimum air pocket entrapment.
• Mold venting and cooling are optimized to allow effective air escape during injection, with no possibility for any bubbles to become entrapped in the solid plastic.

Maintenance

• The injection molding machine components are in good order and operating as designed. Worn or malfunctioning machine parts can cause improper flow patterns and contribute to splay.

Design Changes

• Examining the molds can help reduce splay. Molds should be designed to allow smooth flow of the plastic. Vents placed in strategic locations can allow air to escape more effectively.

Summing Up

In summary, splay is a troublesome defect, and can be quite annoying to have it appear in your injection molded parts. However, understanding the causes and taking the necessary steps to prevent it is key to overcoming it. By following the steps mentioned above, manufacturers can ensure consistent and high-quality parts.

If you’re looking for a trusted injection molding partner, you can always turn to HITOP Industrial.