During injection molding process, it is crucial to ensure product dimensional accuracy. Ensuring efficient processes and minimizing scrap while ensuring customer satisfaction. After dimensional inspection, probability that product meets preset specifications is greatly improved, especially for products with high quality requirements such as medical equipment. This is particularly critical.
In this article, we’ll share why dimensional inspection is important, as well as key steps and best methods for dimensional inspection of injection molded parts.

Dimensional issues in plastic injection molding can have a significant impact on subsequent mold operations, especially during assembly.
Here are possible impacts of sizing issues:
Fit and Tolerance Issues: Products with incorrect dimensions may not fit together as intended.
Seal and gasket issues: In applications with high sealing requirements, such as automotive or electronic component industries, dimensional changes may affect effectiveness of seals and gaskets.
Cosmetic Issues: Dimensional inconsistencies can also affect visual appearance of an assembled product.
Increased scrap rates: If parts do not meet specified dimensions, higher scrap rates may occur during assembly process.
Assembly time and cost: Correcting dimensional issues during assembly requires additional time and labor.
Testing and quality assurance challenges: Dimensional changes may require adjustments to testing and quality inspection processes.
Customer Satisfaction and Product Performance: Ultimately, if sizing issues are not resolved, final product may not meet customer expectations.
To address these challenges, it is crucial to closely monitor and control injection molding process to achieve precise product dimensions. Implementing quality control measures and utilizing proper molding techniques helps ensure that product meets specified tolerances and contributes to a smooth mold assembly operation.

Before you begin dimensional inspection, you must have a thorough understanding of design specifications. Reference product drawings, CAD models, and other relevant documents to determine critical dimensions and tolerances that must be met.

Select appropriate measuring tool based on specific requirements for dimensional inspection. Common tools include calipers, micrometers, height meters and coordinate measuring machines (CMM). Make sure to calibrate these tools regularly to maintain their accuracy.

Dimensional inspections are performed in a stable and controlled environment to minimize external influences. Maintain stable temperature and humidity levels to prevent dimensional changes. Use a dedicated inspection area with appropriate lighting and cleanliness to ensure accurate measurements.

It is recommended that inspection begin by focusing on critical dimensions listed in design specifications. These dimensions have a significant impact on functionality and performance of final product. Solving hidden dangers in this area first is crucial to ensuring product quality.

Consider automated inspection systems such as visual inspection and laser scanning to increase efficiency and accuracy. These systems can quickly scan and measure multiple dimensions simultaneously, providing comprehensive analysis of formed parts.

You can also use in-chamber sensors and process control systems such as CoPilot to ensure your process is running properly. Cavity pressure data is a key indicator of product quality and consistency. Maintaining consistent cavity pressure helps produce products with uniform dimensions and characteristics. However, fluctuations in cavity pressure may cause changes in product dimensions.

SPC tools help monitor injection molding process by collecting and analyzing data in real time. Implementing SPC ensures that dimensional variations are within acceptable limits, helping to maintain consistent product quality.

Keep detailed records of inspection results, including measurements, tolerances, and any deviations from design specifications. Documentation is critical for traceability, quality control and continuous improvement.

If size differences are found, root cause analysis needs to be performed. Resolve and correct them to prevent similar issues from occurring in future production runs. Using a process control system, such as CoPilot, and a data networking system, such as The Hub, not only help determine root cause of an issue, but also establish an audit trail for later review. And CoPilot has a built-in artificial intelligence assistant called MAX, which can tell you what's wrong and how to fix it.
Dimensional inspection of molded products in plastic injection molding is a meticulous process that requires great attention to detail. By following these guidelines, you can ensure dimensional accuracy of your molded products, resulting in high-quality products that meet or exceed customer expectations.

Axis of main runner is generally located on the center line of mold and coincides with axis of injection machine nozzle. In injection mold of horizontal and vertical injection machines, axis of main runner is perpendicular to parting surface (see Figure 1), and cross-sectional shape of main runner is circular. In injection mold for a right-angle injection machine, axis of main runner is parallel to parting surface (see Figure 2). Cross-section of main runner is generally generally a cylindrical shape with equal cross-section, and cross-section can be circular, semicircular, elliptical or trapezoidal, with elliptical shape being the most widely used. Main points of main runner design are as follows:

1) In order to facilitate pulling of condensate from sprue, main runner is designed to be conical (see Figure 5-4). Cone angle a=2 degrees to 4 degrees. Usually, diameter of inlet end of main runner should be determined according to nozzle diameter of injection machine. When designing cross-sectional diameter of main runner, attention should be paid to alignment of nozzle axis and main runner axis. Diameter of inlet end of main runner should be 0.5~lmm larger than nozzle diameter. Contact form between inlet end of main runner and head of nozzle is generally an arc surface(See Figure 5-5). Usually concave spherical radius R2 at inlet end of main runner is 1~2mm larger than nozzle spherical radius R1, and concave depth is about 3~5mm.

 

Figure 1 Shape and size of main runner

 

Figure 2 Injection machine nozzle is in spherical contact with main runner bushing (R2>R1)
1 fixed mold base plate 2 main runner bushing 3 nozzle
2) Junction between main runner and sub-runner adopts a rounded transition with a radius R of 1~3mm to reduce resistance when material flow turns to transition.
3) Under premise of ensuring good molding of plastic parts, length L of main runner should be as short as possible to reduce pressure loss and waste. Generally, length of main runner depends on thickness of mold plate, opening of runner and other specific conditions.
4) Set up main runner bushing. Since main runner has to repeatedly contact and collide with high-temperature plastics and nozzles, it is easily damaged. Therefore, main runner is generally not opened directly on mold plate, but is set separately in a main runner bushing, as shown in Figure 3.

 

Figure 3 Form of main runner village set