Chloe Zhang
Splay is a common defect in plastic injection molding, appearing as off-colour, often silvery, streaks on the surface of the molded part. This cosmetic defect is usually caused by moisture, heat, and shear. While heat is a primary culprit, moisture is the base issue of this defect, as the plastic resin pellets can absorb or be coated by moisture during the manufacturing process. This moisture turns to steam, creating tiny gas bubbles that cause streaking when the mold cavity is filled. To prevent splay, it is important to keep moisture out of the plastics and machines, and to understand the effects of moisture, heat, and shear in processing conditions.
| Characteristics | Values |
|---|---|
| Type of defect | Cosmetic |
| Appearance | Silver or white streaks |
| Cause | Heat, moisture, and shear |
| Other causes | Lubricants, metal protectants, rust preventatives, screw or gate shear, poor dryer heat performance, excessive back pressure, high screw speed, gate size |
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What You'll Learn
Heat
Splay is a common defect in plastic injection molding, appearing as off-colour streaks, often silver, on the surface of the molded part. It is primarily a cosmetic issue, but it can also weaken the molded part. The three main causes of splay are heat, moisture, and shear, with heat being a primary culprit.
Excessive back pressure can also lead to an overheat condition. If back pressure is set too high, it can overheat the plastic as it passes through the compression zone. It can also cause material degradation, leading to poor part qualities. Changes made to back pressure require a minimum of 20 minutes to allow for shear changes to take effect and for the barrel temperature to settle into a new heat-soaked condition. Materials that are overprocessed in the compression zone have shorter molecule chains, which can weaken part strength and change shrink rates.
To correct heat-related splay defects, it is important to review all conditions that might be leading to the issue. The best practice is to remove the heat from the process until the overheating condition is corrected. Splay conditions can be corrected through a deliberate analysis of current molding conditions, and process corrections should be based on facts, not guesses.
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Moisture
Splay is a common defect in plastic injection moulding, appearing as off-colour or silvery streaks across the surface of the moulded part. It is primarily a cosmetic defect, but it can also affect the strength of the finished product. Splay is caused by tiny gas bubbles that are dragged across the surface of the moulded part when the mould cavity is filled.
In addition to moisture in the plastic granules, excessive steam moisture can also taint the injection moulding procedure. Poor dryer heat performance is a leading contributor to splay conditions. If back pressure is set too high, it can cause material degradation, leading to poor part qualities. Changes to back pressure require a minimum of 20 minutes to allow for shear changes to take effect and for barrel temperature to settle into a new heat-soaked condition.
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Shear
Splay is a common defect in plastic injection moulding. It appears as off-colour, usually silvery, streaks on the surface of the moulded part. Splay is primarily a cosmetic defect, but it can also weaken the finished part.
Another cause of shear is a gate that is improperly sized. Smaller gates might require a slower beginning-of-fill speed. Splay conditions that recur during the middle of the fill are a clear sign of shearing conditions, and slower injection speeds in the affected fill area will often correct the condition.
It is important to note that changes made to back pressure require a minimum of 20 minutes to allow for shear changes to take effect.
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Rust preventatives
Splay, sometimes called "silver streaks," is a common defect in plastic injection molding. It appears as off-colour streaks, often silvery in colour, on the surface of the molded part. Splay is usually caused by moisture in the plastic granules, which turns to steam when heated, creating tiny gas bubbles that are dragged across the surface of the molded part. Other causes of splay include excessive heat and shear.
To prevent rust on plastic injection molds, which can be costly and time-consuming to fix, several rust preventive products are available. Here are some detailed descriptions of a few rust preventatives:
Cortec's EcoAir Tool & Die Rust Preventative
Cortec's EcoAir Tool & Die Rust Preventative is a partially bio-based, non-traditional aerosol product that protects the internal surface of the mold that forms the injected plastic. It is packaged in bag-on-valve spray cans powered by compressed air, making it non-flammable and safer for transportation and worker safety. This product also offers a degree of cleaning action, displacing water, grime, and process fluids from the metal surface.
Slide Products
Slide Products offers a range of rust preventatives, including Mold Shield, No-Rust, and Quick RP Rust Preventive with Indicator Dye. These products are designed to protect injection molds from rust during storage and shipping. Mold Shield is a dry formula that protects against fingerprint acids and seals out condensation. No-Rust provides up to five years of protection and is effective for long-term storage and heavy coatings. Quick RP's unique feature is its red tracing dye, ensuring complete and uniform coverage.
White Rhino
White Rhino is another rust preventive that provides up to two years of protection for injection mold assemblies used in food packaging applications. It performs similarly to Mold Shield in terms of protection.
When selecting a rust preventive, it is important to consider factors such as ease of application and removal, protection duration, and environmental and safety concerns. Proper mold maintenance also includes daily cleaning at the end of each use, using solvent cleaners, and applying rust preventatives to maintain optimal mold performance.
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Gate size
Splay is a defect in plastic injection molding that appears as off-colour streaks, often silvery in hue, along the surface of the molded part. It is primarily a cosmetic defect, although it can weaken the molded part. The three main causes of splay are heat, moisture, and shear, with heat being the primary culprit.
Regarding gate size, a smaller gate size might require a slower beginning-of-fill speed. If the screw that melts the plastic is running at too high a speed, or the gate is improperly sized, it can create splay effects. It is important to ensure that the parts manufacturer is capable of producing defect-free plastic injection molding parts.
To correct splay conditions, it is necessary to identify the root cause. If the root cause is heat, it is important to review all conditions that might be leading to heat splay and make the necessary adjustments. For example, if the barrel temperatures are too high, the melt temperature should be verified to be within the window established by the material manufacturer. If the root cause is moisture, it is important to keep moisture out of the plastics and machines, as even a tiny amount of water in the material can cause streaking. If the root cause is shear, slower injection speeds in the affected fill area will often correct the condition.
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Frequently asked questions
Splay is a cosmetic defect that occurs in plastic injection molding when silver or white streaks appear on the surface of the molded part. It is caused by tiny gas bubbles that are dragged across the surface when the mold cavity is filled.
The three main causes of splay are heat, moisture, and shear. Moisture and shear are considered the primary causes.
Moisture can get into the plastic at various points in the manufacturing process. It can be present in the plastic resin pellets that are melted to form parts, or the plastic can absorb moisture during transportation, storage, or preparation for manufacturing.
Heat-related splay defects are usually caused by high barrel temperatures. Mold temperatures and barrel/mold temperature transitions can also cause splay defects. Poor dryer heat performance and excessive back pressure can also lead to overheating.
Shear can cause splay if the screw is running at too high a speed or if the gate is improperly sized. Splay due to shear can also come from a mixing section that has too tight of a clearance.
