Sofia Martinez
Warpage is a common issue in the manufacturing of plastic parts, particularly those created through injection molding. It refers to the curvature, twisting, bending, or bowing of a plastic object due to various factors during the production process. These factors include differences in shrinkage, cooling rates, cavity pressure, fill rates, and mechanical stress. To measure warpage accurately, it is essential to employ specialized techniques and instruments such as dial gauges, surface profilers, coordinate measuring machines, and laser scanners. By understanding the causes and utilizing advanced measurement methods, manufacturers can minimize warpage and improve the quality of their plastic products.
| Characteristics | Values |
|---|---|
| Definition of warpage | Warpage is a term that describes how an object curves, bends, twists, or bows. |
| Warpage in plastic parts | Warpage in plastic parts refers to the distortion or deformation of a molded plastic part. |
| Causes of warpage in plastic parts | Differences in shrinkage, cooling temperature differences, material orientation, time and pressure of injection, pressure hold, mechanical stress, and storage temperature. |
| Measurement methods | Dial gauges, surface profilers, coordinate measuring machines (CMMs), height gauges, VR Series scanners, and laser scanners with supported software. |
| Solutions to warpage | Simulation software, mold adjustments, optimizing mold gate size and shape, consulting specialists in plastic injection molding, and using De-Warp apps. |
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What You'll Learn
Causes of warpage in plastic parts
Warpage is a common defect in injection-molded plastic parts. It occurs when the final product deviates from the shape of the tool cavity. This defect is preventable with help from an experienced moulder.
The underlying cause of warpage is variations in shrinkage. When a part shrinks uniformly in all directions, it becomes smaller but retains its shape. However, uneven shrinkage causes warpage. For example, if the plastic cools unevenly, it can lead to warpage. This can be due to differences in the rate of thermal expansion, with some parts cooling faster than others.
The cooling system is critical to preventing warpage. A precisely calculated cooling rate must be maintained for uniform cooling and shrinkage. If the cooling rate is uneven, it can lead to warpage. The gating system must also be properly designed, with sufficient gates in the right locations. The location of the gate is important, as it determines the flow pattern of the melt, which in turn affects the temperature distribution and which parts of the plastic cool first.
Other causes of warpage include differences in material orientation, time and pressure of injection and pressure hold, and mechanical stress during the production process. In some cases, warpage may be caused by the pins being too thin or too few, resulting in an uneven force being applied to the product and leading to bending and warpage.
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Measurement methods
Warpage is a term that describes how an object curves, bends, or bows. It is a three-dimensional characteristic that can be challenging to measure accurately with conventional measurement methods that only measure points and lines. Warpage in plastic parts is often caused by differences in shrinkage, cooling temperature differences, and material orientation during the solidification process.
Dial Gauges, Surface Profilers, and Coordinate Measuring Machines (CMMs)
These metrology instruments can be used to measure warpage and waviness on a surface. However, due to the three-dimensional nature of warpage, traditional methods may not always be suitable or accurate.
Height Gauge
A height gauge can be attached to an arm, and height measurements can be taken while tracing the gauge along the target surface. This method can help capture the changing heights and curvature of the plastic part.
VR Series Scanning
The VR Series by KEYENCE America offers a non-contact scanning method that captures the 3D shape of the target without the need for positioning. It displays the target height using colours and enables profile measurement at any part of the target. This method helps identify defects and their precise numerical values, allowing for easy control of warpage based on tolerance values.
Simulation Software
Simulation software, such as Moldflow and Moldex3D, aids engineers and designers in optimizing the mold design for injection-molded parts. These simulations help predict and minimize shrinkage and warpage issues by analyzing different mold designs and materials.
Digital Image Correlation (DIC)
This is an optical non-contact method that uses digital images to measure 3D coordinates, evaluate 3D surfaces, and analyze 3D motion and deformation. It provides a fast and accurate way to assess warpage in injection-molded plastic parts.
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Problems with conventional measurements
Warpage is a term that describes how an object curves or bows. It is a common defect in plastic products, often caused by uneven cooling and shrinkage during the manufacturing process. Conventional methods of measuring warpage include using metrology instruments such as dial gauges, surface profilers, and coordinate measuring machines (CMMs). However, there are several problems with these traditional measurement methods:
- Warpage is a three-dimensional characteristic, and conventional methods that only measure points and lines may struggle to accurately capture this.
- Conventional contact-type measuring instruments can find it extremely difficult to measure warpage across an entire object, especially after surface mounting. For example, it is challenging to measure warpage of the entire PCB (printed circuit board) with mounted components using traditional methods.
- Conventional methods may be unable to physically measure warpage, depending on the shape or size of the part.
- Traditional methods of mould design have involved trial and error, over-relying on the experience of designers. This can lead to warpage issues that require costly and time-intensive corrections.
- Conventional methods may not take into account the numerous variables that can lead to warpage, such as mould temperature, packaging pressure, cooling time, packing time, melting temperature, injection pressure, and machine parameters.
- Traditional methods may not be able to prevent issues such as sticking, deformation, and ejection problems, which can cause warpage during the manufacturing process.
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Latest solutions to warpage
Warpage is a common defect in plastic products, particularly those created through injection molding. It occurs when the shape of the injection product deviates from the shape of the mold cavity, resulting in a warped or deformed final product.
One way to address warpage is to ensure proper cooling during the production process. This can be achieved by increasing the size or number of vents, adjusting their location, or using conformal cooling methods to ensure uniform cooling throughout the part. Raised mold cores should also be cooled from the top to prevent local overheating and ensure proper cooling.
Another factor to consider is the material selection and compatibility when using molding techniques that involve multiple plastic materials. While the materials do not need to be the same, they must be chemically and/or mechanically compatible to avoid warpage and other defects. Accurately predicting the amount of shrinkage during the part design phase is crucial, and simulations can assist in determining if excessive shrinkage and warpage will occur.
Reinforcing ribs can be added to thin-walled or large flat areas of the product to enhance overall rigidity and stability, effectively reducing warpage caused by self-weight or internal stress. This not only improves the aesthetic appearance of the plastic parts but also helps maintain their performance and lifespan.
To accurately measure and assess warpage, advanced technologies such as the VR Series can be employed. This technology allows for the scanning and capture of the 3D shape of the target, enabling the visualization and identification of shape defects and their precise numerical values. This data can then be used to identify and correct the causes of defects, control warpage based on tolerance values, or perform trend analysis.
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Tools to measure warpage
Warpage is a three-dimensional characteristic, so conventional measurement methods that measure points and lines are inadequate. Therefore, various tools and methods are used to measure warpage. Here are some of the tools and methods used:
Dial Gauges
Dial gauges are fastened to an arm, and the changes in the measured value are read while the gauge is traced along the target surface. This method is useful for measuring height changes along the surface of the target.
Surface Profilers
Surface profilers are used to measure the profile or shape of a target surface. Traditional profilers use a stylus to trace the surface, while newer systems use lasers to measure complex shapes without making contact with the target.
Coordinate Measuring Machines (CMMs)
CMMs are used to measure several points across a part's surface to determine warpage. They are particularly useful for measuring large targets. However, they require physical contact with the target and can only measure a limited number of points, making it difficult to identify the entire shape of the target.
Laser Displacement Sensors
Laser displacement sensors calculate the height/distance changes between the sensor head and the target surface. 1D laser displacement sensors provide higher precision than 2D sensors and can measure transparent targets. Multiple sensors can be used to construct a 3D shape.
VR Series 3D Optical Profilometer
The VR Series by KEYENCE captures the 3D shape of an entire target surface without making contact with the target. It provides high accuracy and instantaneous results, making it a fast and effective tool for measuring warpage.
ZEISS INSPECT Optical 3D Software
This software includes the De-Warp app, which allows users to measure parts without a fixture and evaluate warpage correctly.
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Frequently asked questions
Warpage is a term that describes how an object curves, bends, or bows. It is a three-dimensional characteristic that can be caused by twisting, bending, or other deformations.
Warpage in plastic parts is often caused by differences in shrinkage, cooling temperature differences, and material orientation during the manufacturing process. It can also be caused by inadequate gate size, restricting the flow rate of molten resin, and resulting in pressure loss.
Warpage can impact the manufacturing process by causing contact failures, such as lead lifting of mounted electronic components. It can also result in parts that do not comply with the original drawing, requiring costly mold adjustments.
There are several methods to measure warpage, including dial gauges, surface profilers, coordinate measuring machines (CMMs), and laser scanners. More recently, digital image correlation (DIC) and virtual reality (VR) technologies have been introduced as non-contact methods for measuring warpage.
To prevent or reduce warpage, it is important to optimize mold parameters such as the runner system, cooling and ejection, injection pressure, and holding pressure. Additionally, consulting with specialists in plastic injection molding and utilizing simulation software can aid in designing molds that minimize warpage.
