Inspection of Injection Molded Parts: Real Examples of Quality Control

Quality inspection of injection-molded parts is crucial for ensuring the performance and reliability of plastic products. Plastic products are used in various fields such as automobiles, electronic devices, and household appliances. Consequently, their quality directly affects the performance and durability of these products. Quality inspection is not only an examination of the product’s appearance but more importantly, a means to ensure its functionality and safety. This process reflects the manufacturer’s production level and quality management capabilities.

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For injection-molded products, meeting stringent appearance and functional requirements is essential, particularly in terms of matching dimensions with corresponding parts. Even minor defects can lead to a decline in the overall performance of the product or cause malfunctions. Therefore, precise and systematic quality inspection is key to guaranteeing high-quality products. This ensures their stability and reliability in practical applications.

Typically, we implement these quality control procedures during various stages of the production:

1. DFM and Mold Flow Analysis: Before the production of molds, it is essential to study and confirm the maturity of product design and mold design. It is crucial to identify potential injection molding defects and strategize ways to minimize their risks.

2. Material testing: Sometimes, before producing plastic products, performance testing of materials is needed to ensure they meet the requirements and that their calibration parameters are accurate.

3. In-Process Inspection: During the production process of plastic products, the primary areas of inspection are appearance and dimensional accuracy.

Appearance Inspection focuses primarily on the surface quality of the product. This includes checking whether the color of the product is consistent, whether the glossiness meets the required standards, and whether the surface is smooth and free from flaws.

Dimensional Inspection involves precise measurements of dimensions that are vital for the assembly or use of the product. Tools such as calipers and go-no-go gauges are often used in this process. The accuracy of these dimensions is critical to ensure the product can correctly integrate with other components and maintain its overall functionality.

Of course, some products may require additional testing, including destructive testing. However, through these two types of inspections, the quality of plastic products can be effectively guaranteed to meet the expected standards during production, thus ensuring the performance and reliability of the final product.

Adequate natural lighting is certainly the most ideal. When artificial lighting is necessary, it needs to meet the following conditions:

The lighting conditions in the inspection environment must meet specific standards, which include:

Light Source Color Temperature: The light source should conform to customer or standard requirements, typically choosing a color temperature of K or K to simulate natural light. This aids in accurately observing the product’s color and texture.

Color Rendering: The color rendering index of the light source should be higher than Ra>90, to correctly and clearly display the product’s true color.

Brightness of the Light Source: Appropriate brightness is key. Generally, the illuminance of reflected light should be around Lux (+/-500Lux), and the brightness for transmitted light should be about cd/m², facilitating detailed observation.

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Uniformity of Illumination: The lighting should be uniform, ensuring that the brightness at any point on the illuminated surface is not less than the central brightness, to evenly observe the entire product surface.

Specific aspects of appearance inspection focus on the following:

Color and Gloss: The color and gloss of the product are checked for consistency with the standard sample (master sample). This can be done through manual visual inspection or using a colorimeter, noting that a flat area of the product is required for colorimeter testing.

Surface Quality of the Product: Careful inspection is conducted to ensure the product’s surface is even and uniform, and free from scratches, flow marks, weld lines, splay, warping, bubbles, deformation, or injection molding defects. These issues can affect both the aesthetics and functionality of the product.

Through these detailed inspections, the appearance quality of products can be effectively ensured, meeting production standards and customer demands.

Dimensional inspection of products requires more time, deeper professional knowledge, and greater analytical and technical skills compared to appearance inspection. Dimensional inspection can be categorized into two types:

Contact Measurement involves tools such as calipers, micrometers, and coordinate measuring machines (CMMs). The first two methods demand considerable experience and carefulness from the operator to ensure accuracy, while CMMs require programming and time-consuming setup. Additionally, due to physical contact, these methods might slightly deform the product, affecting the accuracy of the measurements.

Non-Contact Measurement utilizes tools like projectors, image measuring instruments, and scanners, measuring products optically or electronically. Although this method has some margin of error, it is more suitable for tiny parts or very small details that traditional methods cannot accurately measure.

Utilizing inspection gauges to offset part deformation

Given the softness and elasticity of plastic, precisely measuring its dimensions can be particularly challenging, especially for larger products or areas requiring precise fitting. For example, when measuring a circular fitting surface, the slight deformation in injection-molded products may cause the circle to appear slightly elliptical. Contact measuring tools like calipers might show slight variations in diameter at different points, but the average diameter may still be within acceptable limits.

Metal Inspection Gauges

In such cases, specially fabricated CNC machined metal gauges can be used for measurement. These gauges, crafted according to the exact contours of the plastic parts, can more accurately reflect the actual dimensions of the product as they eliminate the influence of bending or deformation. This method allows for a more precise grasp of product dimensions, ensuring they meet design and functional requirements.

Example 1 provides a great practical instance. By using this inspection gauge, we can check the tightness or looseness of its fit with corresponding parts. Its fitting surface follows the contour along the entire circumference.

Due to such a wide span, coupled with the product’s potential moderate deformation within reasonable limits, it becomes challenging to precisely measure its contour dimensions using calipers or a coordinate measuring machine.

When we use the inspection gauge shown in the picture for measurement, we can feel the tightness of the fit and visually assess if the contour aligns perfectly with the gauge. This method helps us to detect the presence of any local uneven shrinkage.

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I am looking for information on best practices of measuring internal and external molded threads.

I currently have a impact modified nylon (as well as some additional proprietary modifiers) injection molded (USA made) part with a nominal wall of .100" and a external thread of 1.660-16 UN-2A RH. The pellets are properly dried and the majority of the 200+ dimensions are in tolerance. The pack pressure is high (I do not know specific parameter). The part is about 8.3" long x 1.8 diameter and hollow. It is about .007 out of round. It is an A class tool and I would say that it was properly molded.

The accompanying face cap is Zytel 101 which is over-molded on to lexan.

Both the body and the face cap thread together very well.

Unfortunately, metal go/no-go thread gauges were purchased before the parts were tooled. The parts do not fit the purchased gauges. The body will not fit into either ring gauge, and the face cap fits on to both male gauges. Regardless, the parts form, fit and function are excellent.

Upon further investigation it seems that my company often purchase gauges that do not fit the parts and tosses them into a drawer and forgets about them. No one has an answer as to how to QC inspect threads, but they insist that my product must be inspected. This has been hot button issue as of late, but no one want to commit to a solution. This is not made easier by the fact that the nylon changes dimensionally based on humidity.

What have people here found to be a good method of QC inspecting plastic threads? Custom thread gauges? Optical comparator? Push on plug and ring gauges accompanied by thread pitch gauges? Any suggestions would be helpful. Thanks! First let me preface this with a warning that I am a materials guy, not a machine / mechanical person.

In your shoes I'd probably be interested in the force required to tighten the screw and the strength of the joint once made. That's what I'd test, i.e. torque to turn it and then how much force is needed to rip the screw out of the socket.

After all, once can measure a lot but who cares what the thread looks like as long as it works?

Chris DeArmitt - PhD FRSC CChem
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On my planet, if the parts fit together well enough, and do so consistently, we call it a day.

If you must inspect the threads, then the logical thing to do is measure a decent sample of them as well as you can, change the drawing so the tolerances cover the normal size range, and have special gages made to inspect to that bastard size, whatever it is.

Try to ensure that the cost of the special gages comes out of the budget of whoever is insisting on inspection.


Mike Halloran
Pembroke Pines, FL, USA I should have also specified that the user will be screwing and un-screwing the face cap on a daily basis. The amount of force to removed the face cap laterally would be extreme(tons), not something I need to test. The face cap is hand tightened. We have had problems in the past of receiving parts after a few years that required pipe wrench installation. This is very undesirable.

Mike, I agree. Unfortunately others don't. And they have a good point. How to you ensure that the parts coming in 5 years from now are the same ones arriving today? Apparently retained samples are not good enough. How do you reject a vendor's parts when they are out of spec and don't work when they didn't meet the spec in the first place? We use China tooling and molding a lot and they often try to see what they can sneak past our QC department. Not my choice.

If you look at the thread call out you'll see we already purchased special gauges, and two sets of them at that! This was my exact suggestion, measure the threads to define a new call out and make new gauges, and don't jump the gun next time.

Any other thoughts out there? 1) If making precise measurements on nylon, you need to accurately moisture condition.
2) If it is modified nylon, you need to ensure the compound is consistent as variations can influence shrinkage and moisture uptake.
3) Investigate what clearance between the threads functions correctly and draw your spec to that. The drawing and gauges and QC specs/methods should always reflect what works, not the other way round.

It's pretty simple at the end of the day. What is the basic purpose of QC. It is to ensure parts are suitable for purpose at minimum cost, not that parts fit a gauge or match a drawing.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &
for site rules >>> How to you ensure that the parts coming in 5 years from now are the same ones arriving today? Apparently retained samples are not good enough. <<<

Okay, where were these standards proponents when the current parts failed to meet the chosen standards? Will they have grown a backbone by the time you change vendors again?

In order for inspection to be effective in getting what you want, you need to be prepared to send back an entire lot, or dump it in a landfill.

Once you have done that, word gets around, and first article inspections go a lot smoother.



Mike Halloran
Pembroke Pines, FL, USA Gauges made for metallic threads will usually fail threads designed for metal made in plastic. Nylon is especially difficult due to the ambient conditions. (e.g. what temp to measure? what moisture content?, etc)

If you have a shadow graph, you will see that the threads shrink more at the root than at the tips due to to differential cooling rates (varying sections), so a full thread form inspection will result in rejection anyway. (So called "A" class tool* notwithstanding!)

In a previous life, working for multi-nationals, plastic threads were always fit and function, as MikeHalloran suggested. By all means keep "Master Samples", but you will need to know material moisture content, blah blah blah before checking.

Note to designers: Never specify a thread developed for metal to be made in plastic.

Cheers

H

* Whats this then?




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