Alexei Volkov
Voids in plastic, also known as vacuum voids, are empty spaces that form within a plastic part during the molding process. These voids are typically caused by shrinkage during cooling or air pockets that become trapped in the mold. Voids can affect the structural integrity of the plastic and lead to breakage. Therefore, it is important to detect and measure void content. This can be done through destructive methods, such as physically cutting or cross-sectioning the plastic, or through non-destructive testing methods like ultrasonic flaw detection and acoustic micro-imaging (AMI). Ultrasonic testing involves the use of high-frequency sound waves or ultrasound to detect internal defects, while AMI utilizes ultra-high-frequency pulses of ultrasound to inspect for voids, cracks, and delaminations. ASTM D2734 is another test method used to determine void content in compacted asphalt mixtures, which involves sample preparation and immersion in a solvent. By employing these testing methods, manufacturers can ensure the quality and performance of their plastic products.
| Characteristics | Values |
|---|---|
| Void Detection Method | Ultrasonic Flaw Detection |
| Ultrasonic Flaw Detection Process | Comparative process where the echo pattern of a good part is compared with that of a test piece |
| Interpretation | Changes in the echo pattern indicate changes in the internal structure |
| Presence of Voids | Detected by the presence of echoes within a marked gate or window representing the interior of the test piece |
| Inspection Time | A few seconds per test point |
| Equipment | EPOCH 6LT portable ultrasonic flaw detector, EPOCH 650 ultrasonic flaw detector, V110-RM transducer (5 MHz) |
| Void Causes | Shrinkage during cooling, air pockets trapped during molding, insufficient plastic in the mold |
| Void Appearance | Blisters, sinks, bubbles, bulges, vacuum vesicles, vacuum voids |
| Void Locations | Parts with wall thicknesses > 6 mm or 0.25 inch, areas where plastic is too thick or thin, areas where multiple channels come together to form a rib or wall |
| Void Impact | Reduced structural integrity, breakage, impact on appearance, reduced strength, increased susceptibility to moisture damage |
| Void Prevention | Increase mould temperature, increase injection pressure, increase injection hold time, increase runner/gate size, slow cooling rates, optimize plastic structure, control mold temperature and design |
| Void Testing | ASTM D2734 test method for compacted asphalt mixtures, acoustic micro-imaging (AMI), C-SAM (C-Mode Scanning Acoustic Microscope) |
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What You'll Learn
Ultrasonic flaw detection
In ultrasonic flaw detection, a transducer is used to generate high-frequency sound waves that are introduced into the test piece. These sound waves travel through the material and interact with its internal structure. When the sound waves encounter a flaw, such as a void, they reflect off the boundary and create an echo pattern.
The operator then compares the echo pattern from the test piece with the echo pattern from a known good part. Any changes or discrepancies in the echo pattern indicate the presence of flaws or discontinuities in the material. This comparison method allows for the identification of voids, bubbles, or other anomalies within the plastic without the need for cutting or X-raying the part.
This method of testing is widely applicable across industries, including plastics, aerospace, petrochemicals, and power generation. It is a valuable tool for ensuring the integrity and quality of manufactured parts, as it can identify flaws that may affect their performance or structural integrity.
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Non-destructive testing
NDT is used in a wide range of industries, including manufacturing, automotive, aerospace, and petroleum. The tests can be used for surface scanning, weld inspection, tube inspection, and condition assessment.
There are several different types of NDT used for testing plastics. One of the most common types of NDT is ultrasonic testing (UT), which uses high-frequency pulses of ultrasound to detect voids, cracks, and delaminations. Acoustic micro-imaging (AMI) is a type of ultrasonic testing that is used extensively in microelectronics to check the integrity of bonds between silicon chips and plastic substrates.
Another type of NDT used for testing plastics is radiographic testing (RT), which uses X-rays or gamma rays to create detailed images of the tested material and detect defects. A lower radiation intensity is typically required for plastics compared to metal.
Infrared thermography is another NDT method used to test plastics. It is based on the principle of thermal wave imaging and can identify energy loss and thermal-induced deformations.
Other NDT methods used for testing plastics include visual testing, leak testing, and laser testing.
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Vacuum voids and gas pockets
Vacuum voids are caused when there is insufficient molten plastic injected into the mold. As the plastic cools, it leaves empty spaces as it shrinks. Where the plastic touches the mold, it cools first, meaning the bubble will not be as visible: this is a void. When this shrinking occurs on the surface, it is called a sink defect.
Gas pockets, on the other hand, are usually the result of jetting, which is a front flow issue. They can also be caused by moisture, volatiles, and decomposition. To distinguish between a vacuum void and a gas pocket, simply warm the plastic part where the bubble is formed. If the bubble expands, it is a gas pocket; if it collapses, it is a vacuum void.
Ultrasonic flaw detection is a method used to locate internal voids without cutting or X-raying the plastic. High-frequency sound waves are coupled into the part by a small probe called a transducer, which reflects off the voids in a predictable way. This inspection takes only a few seconds per test point.
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Root cause analysis
Plastic injection moulding is a manufacturing process that can introduce defects in the final product. Voids are one such defect, which can be caused by shrinkage during cooling, air pockets trapped during moulding, or gas bubbles. These voids can affect the structural integrity of the plastic part and lead to breakage.
To perform a root cause analysis, the first step is to identify the failure mode. Plastic components can fail in several ways, including brittle fracture, ductile overload, creep rupture, and environmental stress cracking. Once the failure mode is determined, tests can be performed to identify the root cause.
In the case of voids in plastic, the root cause is often related to the injection moulding process. This could include issues with the mould design, such as inadequate venting or poor airflow, which can trap air and create voids. Another potential cause is not having enough plastic in the mould, which can leave empty spaces as the plastic shrinks during cooling.
To detect voids, non-destructive testing methods such as ultrasonic flaw detection or CT imaging can be used. These techniques allow for the identification and quantification of voids without damaging the plastic part. Once the voids are detected, the root cause can be addressed by altering the mould design, adjusting processing conditions, or using different materials.
It is important to note that there may not always be a single root cause, as multiple factors can contribute to the formation of voids. A comprehensive failure analysis report should be prepared, including recommendations to address the identified issues and prevent future occurrences.
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ASTM D2734 test method
ASTM D2734 is a standard test method for determining the void content of reinforced plastics or "composites". The test methods are applicable to composites for which the effects of ignition on the materials are known. Most plastics, glass, and asbestos reinforcements fall into this class. However, it is not applicable to composites where the effects of ignition on the plastics, reinforcement, and fillers are unknown. This includes materials such as silicone resins, metal reinforcements, organic materials, inorganic materials, and fillers consisting of oxides and carbonates.
The void content of a composite may significantly affect its mechanical properties. Higher void contents usually mean lower fatigue resistance, greater susceptibility to water penetration and weathering, and increased variation in strength properties. ASTM D2734 test methods specify procedures for determining the void content of reinforced plastic composite materials in testing labs. The test measures void content in reinforced plastics, revealing its quality and performance.
The standard test method is especially critical for carbon fibre, glass fibre, and aramid-reinforced polymers, ensuring structural integrity and performance in light, high-strength applications and aerospace-grade composites. Testing involves critical sample preparation, as the separation of fibres, resins, and voids without disturbing the properties of a sample is relatively difficult to achieve. Important tests relevant to ASTM D2734 include ASTM D618 (Practice for Conditioning Plastics for Testing), ASTM D792 (Test Methods for Density and Specific Gravity of Plastics by Displacement), and ASTM D1505 (Test Method for Density of Plastics by the Density Gradient Technique).
For semi-crystalline plastics, such as polyphenylene sulfide (PPS) and polyetheretherketone (PEEK), interference due to crystallinity can cause significant variation in the measurement of void content. In such cases, the density used in the calculation must be the actual density of the resin in the composite. Additionally, for composites with low and void contents, the value may be lowered from a true 0.2% to a calculated -0.1%, indicating an obvious error and the importance of accuracy in the tester's particular measurement.
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Frequently asked questions
Voids are air pockets or bubbles that form in plastic during the molding process. They are caused by shrinkage during cooling or air trapped during molding. Voids can affect the structural integrity of the plastic and lead to breakage.
There are a few ways to detect voids in plastic. One method is to use ultrasonic flaw detection, which involves sending high-frequency sound waves through the plastic. The sound waves will reflect off any voids present and create an echo pattern that can be analysed to identify discontinuities. Acoustic Micro-Imaging (AMI) is another non-destructive method that uses ultra-high-frequency pulses of ultrasound to detect internal defects in plastic.
In addition to AMI, Pixargus’ Shiny Detection module is a non-contact method that uses structured light to detect defects on high-gloss surfaces. Weighing the parts and comparing the weights before and after making changes to the moulding process is another non-destructive testing method.
To prevent voids in plastic, it is important to optimise the mould temperature and design. This includes controlling the cooling rate and ensuring the mould is not too thick or thin in certain areas. Increasing the injection pressure and hold time can also help ensure that enough plastic is injected into the mould.
