We all know that defects in plastic products include: broken ejector pins, whitening, elevated ejector pins, deformation, dimensional tolerances, shrinkage, misaligned glue, gate markings, color mixing, specks, glue sticking, whitening, cracking, needle deviation, ejector cracks, mold sticking, air entrapment, air marks, drag marks, flashing, and other defects caused by human factors, such as batch scratches, abrasions, pressure deformation, fingerprints, oil stains from missed wipes, residual glue wire, and dust.
Among these, the most common factors that can lead to misjudgment are: broken insert pins, whitening, elevated ejector pins, deformation, dimensional tolerances, shrinkage, misaligned glue, gate markings, color mixing, color difference, specks, glue sticking, mold sticking, sand holes, whitening, cracking, needle deviation, air entrapment, air marks, drag marks, elevated ejector pins, flashing, batch scratches, scratches, and abrasions. Let's analyze them one by one.

Broken pins refer to excessive glue or insufficient depth in post hole of a semi-finished plastic product, preventing screws from being fully seated or impossible to drive during assembly. In this case, we can safely declare part defective and unacceptable. However, we must pay attention to whether screw can actually be fully seated. Of course, completely blocking post hole is definitely scrapped. If length of an M3 or M4 screw is shorter than post's full breakage depth, then our quality control assessment will certainly accept it. Alternatively, if part has a specific depth requirement and customer accepts it within that depth, and assembly is not affected, our on-site response is to issue an exception document, immediately isolate and identify offending part, and then notify quality engineer, QA engineer, or sales engineer of issue. We will seek to minimize product scrap and make final decision on whether to accept or reject part, or whether to require immediate mold repair, based on opinions of relevant engineers.

Colleagues who have worked with plastic product quality will undoubtedly be familiar with this issue. This is simply a white ring left on the surface of a plastic part after ejector pins eject part during demolding. We can immediately determine it as NG or scrap. However, when this occurs, we first need to determine whether semi-finished part requires oil spraying, what type of oil should be used, and whether it can effectively cover surface. Secondly, we need to determine level of surface finish and how part is assembled, and whether it will be visible in a conspicuous area on finished product after assembly. This is where we need to analyze these issues before making a final judgment.

 

This refers to appearance of a white ring on the surface of a plastic part that protrudes above surface of part and feels raised to touch. While technically NG and scrapped, we still need to determine whether product with this elevation is truly scrapped. This is determined comprehensively based on several factors: 1. Is semi-finished plastic part an internal component? Does assembly affect product's usability and functionality? 2. Generally, elevation should be within a certain range, but if it occurs on primary surface, it is unacceptable. As long as it does not affect the overall assembly, function, appearance, or dimensions of product, we will request improvements from production department during on-site quality control before proceeding with production. Confirm before shipping!

Deformation can be categorized as deformation due to pre-packaging pressure, post-packaging extrusion, and deformation occurring during production after product is removed from mold. Product deformation has both positive and negative tolerances. If present, determination is based on positive and negative tolerances. If not, determination is based on whether it affects assembly process and final product's appearance. During process quality control, efforts should be made to minimize product overstocking; packaging materials must also comply with packaging material specifications specified by PE engineering project. However, if deformation during production is caused by improper mold design or deformation during mold trials, production should proceed according to sample. Since every plastic product undergoes mold trials, sample approval, and trial production before production, it's not a major concern. Of course, our company or customer's engineering R&D department may overlook this issue, so our quality management personnel are welcome to raise it during production and provide suggestions.

 

Dimensional deviations are generally caused by several factors:
1. Have we accurately located measurement reference points indicated on engineering control drawings?
2. Is measuring tool expired, lost in precision, or unclear in scale? Or was it because we didn't zero tool?
3. Are there errors in measuring technique and reading? For example, when using calipers to measure length of a semi-finished plastic casing, are we ensuring that part being measured is perpendicular and parallel to caliper? Are we ensuring that outer measurement point is maximum value, and inner measurement point is minimum value? Are we applying too much force when measuring? If it's hardware, is clamp too loose? Did we carefully check minimum scale mark on measuring tool before reading?
4. Are measured position and measuring tool used accurate?
For example, we typically use a knife-edge micrometer to measure slot tolerance of a column, and we cut it with a hacksaw. Should we use a regular micrometer?
When measuring depth of a blind hole, is outer diameter of pin gauge significantly different from inner diameter of blind hole? Are pin gauges and blind hole gauges properly positioned at their lowest points before reading and taking measurements? Of course, when performing precise in-process measurements, if measurement techniques, controlled engineering drawings, and measuring tools are all correct, then issue lies with molding parameters and mold. For example, if length and width dimensions of a semi-finished plastic product meet requirements of controlled engineering drawings during molding, but its secondary surface (i.e., side of product) exhibits severe drag, this will affect the overall appearance of finished product and is unacceptable. While this dimensional deviation may be acceptable under engineering regulations, we will need to issue an exception notification and notify relevant engineering personnel for confirmation before making a final decision. If depth of blind hole is out of tolerance, this is a problem with mold pin, but it does not affect assembly. We will also require confirmation from relevant engineering personnel before making a final decision!

Shrinkage of semi-finished plastic products refers to appearance of depressions on the surface of product, which is actually unacceptable. However, we make a comprehensive assessment based on following situations:
1. Engineering Considerations: If surface of semi-finished plastic product does not shrink, its dimensions will still be out of tolerance and unacceptable.
2. Molding: Molding parameters may not be set correctly during shrinkage, or injection molding machine's performance may not be fully controlled.
3. Mold: Glue may be too thick somewhere on semi-finished part. It could also be that part is pulled and dented during ejection, or if it doesn't shrink, secondary surface scratches may occur.

 

If any of these three situations occur, our on-site quality control personnel will immediately notify relevant production staff, such as production foreman, production machine technician, or senior production technician. Then, an exception message will be generated and fed back to relevant engineering staff for comprehensive evaluation before acceptance or rejection.

We in industry all know that uneven glue refers to glue shortages in semi-finished parts. But does this mean that all glue shortages mean scrapping? Does all semi-finished parts require adjustment of molding parameters? Answer: Not all cases of uneven glue require adjustment of molding parameters or scrapping! To determine whether uneven glue shortages require adjustment of molding parameters, we need to understand underlying issues.

 

First, is uneven glue flow caused by improper mold venting?
Second, is glue line on semi-finished product too thin?
Third, does uneven glue flow affect assembly, functionality, and appearance of finished product?
Fourth, is machine performance uncontrollable?
For the first three situations, we need to refer to customer-signed sample, limit sample, and AOD board for confirmation. In last situation, we directly declare it NG, but it is necessary to combine previous three situations for a comprehensive assessment. Reason is simple: same mold can exhibit different quality conditions when produced on different machines, or similar quality conditions have occurred in the past. We also use defective quality record to confirm this. If necessary, we can notify relevant engineering personnel for technical assistance. As we in industry know, this kind of situation is inevitable when using same mold, machine, and different materials. Avoid it!

Gate is location of glue inlet. Once puncture marks appear, they will appear on product surface. These marks can be noticeable or subtle. Similarly, when it comes to process quality, we shouldn't simply dismiss them as 'Fail'. We must make accurate assessments. Of course, if such a situation occurs, production must be discontinued. Molding parameters must be adjusted immediately or mold must be checked for issues. Furthermore, if unsure, process quality control personnel should contact relevant quality control assistant or engineering manager for confirmation.

 

Color mixing and color variation are familiar issues in process quality control. They are also a significant factor in increasing production costs. Semi-finished plastic parts most prone to color mixing are those made with toner mixed with raw materials. First, we need to determine whether color mixing is 'Fail'. We must first understand customer's acceptance standards, whether raw materials are sourced from same supplier, and ratio of toner to raw materials. We must also consider whether equipment is suitable for processing this type of plastic semi-finished product.

 

We also understand that color variation is inevitable when raw materials and toners are sourced from different suppliers. In the past, customers often complained about inconsistent colors from batch to batch. Root cause was our over-sensitivity to production costs. We believed that using pelletized materials would be more expensive than using toner mixed with raw materials. However, pelletized materials offer a lower scrap rate for color mixing and color differences, resulting in more stable colors. However, using toner mixed with raw materials has a higher scrap rate (according to incomplete statistics, scrap rate is over 20%). This clearly indicates which costs are higher in long term. Regarding process quality control, we can enlist help of quality engineering and business engineering for technical support. For example, we can use color charts, color difference values, colorimeters, or color calibration booths. I believe that for process quality control, it's best to maintain long-term storage of limit plates, AOD plates, and NG samples signed by relevant engineering department for mixed-color semi-finished plastic products. Any discrepancies must be sealed and not used for reference!

During manufacturing process, our peers often complain about foreign matter or black spots on primary surface of semi-finished plastic parts. Here, I'll refer to these as "noise"!

 

Depending on customer's requirements, some imperfections may be acceptable, while others must be scrapped. Depending on actual production conditions, customer may also provide technical assistance, such as providing film of imperfections. This isn't enough, as we all know that some imperfections may appear to exceed standard. For example, Toshiba and Apple customers require a 0.1mm diameter imperfection, but actual imperfection is 0.15mm. However, it appears blurry within inner plastic layer of primary exterior surface of semi-finished plastic product at a depth of 0.2mm. Even at a distance of 60cm to 45cm, even blinking five times under a 40W or 60W standard fluorescent light source may not be clearly visible. This is acceptable. Another consideration is distance between two imperfections. If distance between two imperfections is 50 to 45mm, two imperfections ≤0.15mm are acceptable. However, what if two very blurry imperfections are less than 50 or 45mm apart? Should this result in a NG? This is something that engineering department needs to answer. I believe that even blinking five times under a 40W or 60W standard fluorescent light source might not be enough to see clearly, so it's still acceptable.

Gluing and mold sticking in plastic semi-finished products are two different concepts. Gluing refers to irregular, scar-like indentations on primary and secondary surfaces of semi-finished plastic part; mold sticking refers to complete adhesion of screws, positioning posts, and locating bones within mold cavity of semi-finished plastic part. However, extent of gluing varies, but generally speaking, an NG is unacceptable, including mold sticking. If we don't detect such issues during our process quality control, it simply means we made a basic mistake.

In plastic semi-finished products, trachoma can be unrelated to mold, while others are closely related to mold itself. It can also appear on the surface or sides of semi-finished plastic part. If it's not a problem with mold material itself, issue will be determined based on size or whether it should be NG. Why do we say this? First, we need to determine whether customers have complained about this issue with semi-finished plastic products. Second, trachoma in semi-finished plastic products affects appearance of finished products. The most important consideration is customer's perspective. If foreign matter is very small (≤0.1mm or ≤0.15mm in size, and less than 0.1mm in depth), even a small amount should be acceptable. However, improvements should be made during production. If problem is caused by mold material itself, we will immediately report issue to relevant engineering staff for follow-up improvements and the best solution before making a final decision.

Some of our colleagues in plastics quality control have been doing this for years and have likely received customer complaints regarding whitening and cracking. Let's first understand where whitening and cracking typically occur in semi-finished plastic products. Generally speaking, they can be found in following areas:
1. Product's ribs
2. Product's inserts (i.e., sides)
3. Base of column
4. Buckle
In fact, comparing the two, whitening is less serious than cracking. Whitening has varying degrees of severity. Major clients like Kyocera demand that our suppliers tolerate any degree of whitening, as it will affect lifespan of finished product. As everyone knows, Toshiba and Kyocera are major Japanese-owned Fortune 500 multinational corporations, so they consider any whitening in semi-finished plastic parts defective. However, we also observe whitening in semi-finished plastic parts due to mold alignment issues. As long as plastic frame is not deformed, we will process and ship parts. This will not affect lifespan of finished product, but cracking will result in scrapping. As long as there is whitening at insert, column base, or buckle, but no cracking, no impact on assembly and screwing, and no impact on product's lifespan, we can also process and ship parts. Specific situation depends on customer's requirements. During production, we will also issue an exception report and notify relevant engineering personnel for follow-up action.

Here, needle deviation refers to misalignment of pins within mold, resulting in an oversized screw hole in plastic semi-finished product. Larger holes can cause screw thread slippage during assembly, seriously impacting product functionality and usability. Quality control colleagues responsible for product dimensions should take note: hole diameters must be kept within tolerance. If hole diameter is excessive, it should be reported to senior management at level of Assistant Quality Engineer or above for assessment and follow-up. Only after optimal quality assurance is achieved can shipment be completed.

Air trapping in plastic semi-finished products occurs when a poorly vented system is installed within mold cavity. There are several possible causes of air entrapment in plastic semi-finished products:
1. Glue missing along edge of joint
2. Burnt appearance along edge of joint
3. Incomplete glue flow along buckle joint
4. Bifurcated lines along secondary surface
5. Burnt white or black appearance along semicircular joint
6. Incomplete glue flow along positioning post
In summary, we can come up with following problems: bone position lack of glue, bone position burnt, bone position burnt white and black, buckle position lack of glue, clamping line, positioning column lack of glue. Should all products be deemed NG? Answer depends on circumstances. First, we need to consider whether there have been complaints regarding these types of defects, and whether their severity is comparable. Furthermore, we need to check for similar defects in our sample management: limit samples for missing glue at frame, burnt glue at frame, white or blackened glue at frame, missing glue at buckle, clamping lines, missing glue at locating post, AOD samples, engineering confirmation samples, and customer-signed samples. If such defects are found, our process quality control will strictly adhere to these samples when determining acceptance or rejection. If not, our process quality control personnel will immediately issue a quality defect document and contact relevant engineering management personnel for a comprehensive assessment before making a final decision.

Air marks are small, hazy, sub-circular or oval-shaped areas on primary or secondary surfaces of a semi-finished plastic product, corresponding to inlet area. This condition is closely related to mold inlet size and molding parameters. If defect is noticeable on both primary and secondary surfaces, we should carefully check customer-signed samples before making a NG or OK determination. Generally speaking, production is required to improve before production resumes. However, if cause is an improper mold design, rather than a problem with molding parameters, requiring a production adjustment can be challenging.
Those in this industry are well aware of concept of "gains and losses": there are gains and losses, there are gains and losses. When you want to correct for air marks, production technicians will increase injection speed to remove them. However, this may cause pinch line on semi-finished plastic part to be significantly misaligned with mold plate. I recommend that process quality control personnel align mold plate before deciding whether to have production technician adjust molding parameters. Specifically, issue a quality error feedback form to senior production technician to seek the best solution and assist with quality control. At the same time, we should also notify relevant engineering management personnel to provide technical support to quality department!

Let's first analyze where drag and raise on plastic products are most common.
1. Secondary surface or side surface texture
2. Insert and pin placement
3. Punch-through hole
4. Beveled ejector pin placement
5. Core extraction point
6. Mold runner
7. Raised identification mark on primary surface
Depending on product and customer requirements, if a plastic part exhibits scratches but not height, we typically request that production department make corrections before production begins. However, there is an exception: if the entire secondary surface or side surface texture is scratched, then if our quality control personnel instruct production technician to make corrections, this will affect the overall length and appearance of part, resulting in shrinkage. I recommend that our quality control personnel carefully check sample, scratch limit plate, or AOD sample before recommending improvements to production technician. Otherwise, new issues will arise. Quality department's sample management office generally keeps samples of plastic parts with scratches. We should review these samples before deciding whether improvements are necessary. Of course, there are also cases where a single, deep drag mark appears, which necessitates a mold reduction. Resulting part's shippability must be considered. However, I believe that parts that cannot be shipped should be immediately classified as NG. However, we still notify quality assistant or quality engineer for a review. If minor drag marks occur on inserts, pins, through-holes, beveled ejectors, core pulls, mold guides, or raised markings on primary exterior surface, as long as excess glue powder is wiped clean with a white cloth, they are acceptable. However, production department must be instructed to immediately address issue before production can resume. If improvements are not achieved, an exception ticket will be issued and relevant engineering personnel will be notified to follow up on quality issue.

Speaking of flash on plastic parts, I'd like to discuss your understanding of flash requirements.
At my company, some clients have specific requirements for plastic part flash. For example, Toshiba and Kyocera, two Fortune 500 companies, require a flash of ≤0.15mm around perimeter of semi-finished plastic parts from our suppliers. In today's mature plastics industry, we also need to consider production costs. If all plastic parts were to be processed due to flash, our company would be spending a significant amount of manpower on flash processing, and flash processing would also introduce many uncontrollable factors into our on-site quality control. For example, flash damage or deformation can be introduced into good parts, leading to secondary complaints! Although this only accounts for a defect rate of around 5%, if calculated on a PPM basis, it's a very high defect rate. So, where does flash occur on a plastic part? It's most likely to occur at parting point between front and back molds. For example, at hole, puncture point, slide, buckle, bevel pin, and spool pin positions, among other areas. Following factors can cause flash on semi-finished plastic parts:
1. Unstable molding parameters on machine
2. Oversized mold vents
3. Loose bobbin needles
4. Loose ejector pins
5. Damage to inserts
If any of above five abnormalities result in flash on plastic parts, they are unacceptable. The first abnormality is particularly severe, as it can lead to dimensional defects and render semi-finished part unusable. Flash caused by abnormalities in categories 2, 3, 4, and 5 still requires processing. After completing order, on-site quality control personnel should promptly notify senior production technicians for follow-up and optimization of this flash.

If flash is present during processing on plastic parts, then flash damage is an anomaly. Generally, if flash occurs during secondary surface processing but does not damage part's surface texture or affect assembly clearance, it is acceptable. However, if part is a fully internal component, then minor flash damage that does not affect assembly is acceptable. However, some customers have specific requirements regarding concept of scratches on semi-finished plastic products, and shipment is based on a comprehensive assessment based on customer's acceptance criteria.

Scratches and abrasions are also unacceptable during process quality control, especially for products produced by polishing molds. Workers are also required to avoid excessive accumulation of parts on their work surfaces and to place anti-scratch and abrasion pads on their work surfaces to prevent scratches and abrasions on plastic parts. Another situation our process quality control personnel should be aware of is when such products require rework due to other factors. I recommend that for such products, unless there are serious defects among good parts, workers should not be required to rework to reduce risk of scratched and abraded products. It is best to increase frequency of inspections on machine surfaces to prevent other defects from flowing into good parts, thereby reducing rework.
Finally, we also know that when it comes to process quality control, we shouldn't easily say "OK" or "NG," nor should we easily ask production department to arbitrarily adjust molding parameters to improve existing quality. We should consult with relevant engineering management personnel, frequently check samples, AOD boards, engineering boards, and customer-signed engineering samples, we should avoid making judgments and instead seek the best answers from engineering. This will reduce our own misjudgment rate and improve our work efficiency and professional skills.