As saying goes: "No one is perfect, no gold is pure, no one is a sage, no one can make mistakes!" Now, what we want to talk about is that this small ideological mistake has brought immeasurable losses to our work, our company, our department, and even ourselves.

 

Plastics industry is no exception. After knowing stakes. Now, let’s analyze occurrence and reasons of misjudgment of plastic product quality. We all know that defects of plastic products include: broken ejector pins, white ejection pins, top height, deformation, out-of-tolerance size, shrinkage, uneven glue running, gate flushing, mixed colors, miscellaneous spots, glue, whitening, cracking, pin deflection, ejection cracks, mold sticking, trapped air, air marks, drag, scratches and scratches caused by human factors, scratches, abrasions, pressure deformation, fingerprints, oil leakage, rubber wire residue, dust, etc.
Then, factors that are most likely to cause us to misjudge are: broken ejector pins, white ejection pins, top height, deformation, out-of-tolerance size, shrinkage, uneven glue running, gate flushing, color mixing, color difference, miscellaneous spots, glue, mold sticking, trachoma, whitening, cracking, pin deflection, trapped air, air marks, drag, pick-up, criticism, scratches, and abrasions. Now, let’s analyze them one by one.

So-called broken pins refer to: there is too much glue in post hole in semi-finished product of plastic product or depth is not enough, causing screws to be unable to be driven in place or not screwed at all during assembly. At this time, we can completely determine that part is NG and unacceptable. However, what we need to pay attention to is whether screws can be driven into place. Of course, if post hole is completely blocked, it will definitely be scrapped. If length of M3 or M4 screw is shorter than depth at which column is completely broken, then of course we will accept it as OK when making quality control decisions. In another case, depth of part is required, and customer receives goods according to depth requirement of part pillar without affecting customer's assembly. At this time, what we need to do on site is to first open an abnormal document, immediately isolate abnormal parts, then notify quality engineer, QA engineer or business engineer to report situation. Seeking to reduce product scrapping, final acceptance or rejection, or need for immediate mold repair, must be made after listening to opinions of relevant engineers.

Colleagues who have worked on quality of plastic products will definitely be familiar with this. It is nothing more than white circle shape left on the surface of product after mold ejector pin ejects product when plastic product is released from mold. We can directly determine NG or scrap at the first time, but when there is a seal, we must first understand whether semi-finished plastic part needs to be sprayed with oil, what kind of oil to spray, and whether it can be covered; secondly, what level of appearance does plastic part have, how is it assembled, and whether it will be exposed in a conspicuous place on the surface of finished product after assembly. This is after we analyze it before making a final judgment.

 

That is to say, a white circle appears on the surface of plastic product and is higher than plane of product. It feels like a bulge when touched by hand. To be precise, it is NG and needs to be scrapped, but we still need to know whether product after lifting is really going to be scrapped. Make a comprehensive judgment from several aspects: 1. Whether plastic semi-product is an internal component, whether it will affect use and function of product after assembly. 2. In general, height of product should be of a certain level, but if it is a first-level appearance, it will not work. As long as assembly, function, appearance, and size of the entire product are not affected, we still require production department to improve before re-production when doing quality control on site. Make confirmation before shipping!

Deformation can be divided into several situations: backlog deformation before packaging, extrusion deformation after packaging, and deformation after product is released from mold during production. First of all, product deformation has positive and negative tolerances. If there is, it will be judged based on plus and minus tolerances. If not, it will be judged based on whether it affects assembly and appearance of finished product during assembly. During process quality control, we should try our best to ensure that there is no backlog of products; packaging materials must also meet packaging material specifications specified by PE project. However, if cause of deformation during production is unreasonable mold design or deformation during mold trial, then production will be based on prototype. Because any plastic product must first go through mold testing, sample signing, trial production and then production, so you don’t need to pay too much attention to it. Of course, company or customer's engineering research and development side may also overlook this point. During production, our quality personnel can make suggestions as much as possible.

 

Dimensional out-of-tolerance is generally caused by several aspects:
1. Have we found measurement datum points indicated on engineering controlled drawings accurately?
2. Is measuring tool expired or accuracy damaged? Or is scale unclear? Or is it because we did not reset measuring tool to zero?
3. Are measurement techniques and readings wrong? For example, when using a caliper to measure length of a plastic semi-finished product shell, do we ensure that measured plastic part and caliper are vertical and parallel? Has it been achieved that external measurement point has maximum value, and internal measurement point has minimum value? Are we using too much force when measuring data? If it is hardware, are we stuck too loosely? Have you carefully looked at minimum scale value of measuring tool before reading it?
4. Are measured locations and measuring tools we use accurate?
For example, we usually use a knife-edge micrometer and cut it off with a hacksaw to measure slot tolerance of a column. Do we use an ordinary micrometer to measure it?
When measuring depth of a blind hole, is outer diameter of pin gauge we use different from inner diameter of blind hole too far apart? Is it possible to read and take number after bottom end of pin gauge and blind hole are in place? Of course, if there are no problems with measurement techniques, controlled engineering drawings, and measurement tools used when we do accurate measurement of process, then problem lies with molding parameters and mold. For example, after length and width dimensions of a plastic semi-product meet requirements of controlled engineering drawings during molding, serious drag occurs on its secondary appearance surface (i.e., side of product), which will affect the overall appearance of finished product, which is also unacceptable. This may also allow dimensional deviations in engineering, but what we have to do is to issue an exception contact sheet to notify relevant engineering personnel for confirmation before making a final judgment. If depth of blind hole is out of tolerance, then it is a problem with mold pin, but it does not affect assembly. We also need to ask relevant engineering personnel to confirm before making final judgment!

Shrinkage of plastic semi-finished products refers to phenomenon of depression on the surface of product, which is actually unacceptable. However, we make a comprehensive judgment based on following situations:
1. Engineering: If surface of plastic semi-finished product does not shrink, its size will be out of tolerance and will not be accepted.
2. In terms of molding, molding parameters may not be set properly during shrinkage or performance of injection molding machine cannot be controlled.
3. In terms of mold, it may be caused by glue level being too thick somewhere in semi-finished plastic parts. It is also possible that when mold is released, plastic part is pulled and dented, or if it does not shrink, it will cause secondary surface drag.

 

Once above three situations occur, our on-site quality control personnel will immediately notify relevant staff in production department, such as production foreman, production adjustment technician or senior production technician, etc. Then, abnormal information is fed back to relevant engineering personnel for comprehensive judgment before acceptance or rejection action is taken.

Our peers all know that uneven glue supply means that semi-finished parts are missing glue. So, are all glue shortages considered scrap NG? Do we need to adjust molding parameters for all semi-finished products if glue is uneven? Answer: Not all products with uneven glue flow need to adjust molding parameters or be scrapped! Whether molding parameters need to be adjusted due to uneven glue running, we need to understand it fundamentally.

 

First, is uneven glue flow caused by mold exhaust not being opened properly?
Second, is it because glue level of semi-finished product itself is too thin?
Third, although glue is uneven, will it affect assembly and function and appearance of finished product after installation?
Fourth, is machine performance uncontrollable?
In the first three cases, we need to refer to customer signature sample, limit sample, and AOD sample for confirmation! In last case, we will directly judge NG, but it is necessary to combine it with previous three to make a comprehensive judgment. Reason is very simple, because same set of molds produced on different machines have different quality conditions, or similar quality conditions have occurred before. At the same time, we also use bad quality ledger to confirm! When necessary, we can notify relevant engineering personnel to provide technical assistance. Our colleagues also know that even if same model, same machine, but different materials are used, this kind of situation will inevitably happen, so avoid it!

Gate is location of glue inlet. Once punching appear, they will appear on the surface of product. There are obvious and non-obvious differences. In same way, we should not beat quality of process to death. Once punching appear, it does not mean it is NG. What we need to do is to make a correct judgment. Of course, once such a situation occurs, production cannot continue anyway, molding parameters must be adjusted immediately or whether there is a problem with mold. At the same time, when our relevant personnel in process quality control are unsure, they must notify relevant quality assistants or relevant project management personnel for confirmation.

 

Color mixing and color difference are what we are most familiar with in process quality control. At the same time, it is also an important factor in increasing production costs. Semi-finished plastic parts that are most likely to have mixed colors are those produced by mixing toner with raw materials. First of all, we need to determine whether mixed color is NG. First, we need to find out customer's acceptance standards and whether raw materials come from same supplier each time, as well as ratio of toner to raw materials. At the same time, we must also pay attention to whether machine is suitable for making plastic semi-finished products produced by mixing toner raw materials.

 

We also know that color differences will inevitably occur if raw materials and toners used are not supplied by same supplier. In the past, we were always complained by customers about why colors were inconsistent every time we made a batch of orders. After all, it is because we are too sensitive to production costs. If we use pumped pellet materials to produce, we think its cost is higher than production of toner mixing raw materials. However, scrap rate of mixed colors and color differences in production of pelletized materials is low, color is stable, and scrap rate of production of toner mixed raw materials is high. According to incomplete statistics, scrap rate in production of toner mixing raw materials is as high as 20% or more. It will be clear which one has higher cost in long run. In terms of process quality control, we can let quality engineering and business engineering provide technical support. For example, color plates, color difference values, colorimeter or color matching light source box, etc. I think that in process quality control, it is best for mixed-color plastic semi-finished products to have control samples, AOD samples and NG samples signed by relevant projects for long-term storage. Any discrepancies must be sealed and not referenced! !

During manufacturing process, our colleagues often say that there are foreign objects or black spots on the first-level appearance surface of semi-finished plastic parts. So here I will collectively call them miscellaneous dots!

 

Under different customer requirements, some miscellaneous items can be accepted, while others must be scrapped! Depending on actual production situation, customer will also provide corresponding technical assistance. For example, provide foreign object film. This is also not enough. We usually know that some foreign objects appear to exceed standards. For example, Toshiba and Apple customers require a foreign body diameter of 0.1mm, but actual foreign body diameter is 0.15mm. However, it appears in a blurry state at a depth of 0.2mm in inner plastic layer of first-level exterior surface of plastic semi-finished product. Even if you blink 5 times under a 40W or 60W standard fluorescent light source from a distance of 60cm to 45cm, you may not be able to see it clearly. This situation is also acceptable. Another situation is distance requirement between two points. If distance requirement between two foreign objects is 50 to 45mm, two points with ≤0.15mm are acceptable, but it is very unclear what about two small points with a distance of <50 or <45mm? Should it be judged as NG? This will have to be answered by engineering side. I think you may not be able to see clearly even if you blink your eyes 5 times under a 40W or 60W standard fluorescent light source, and it is still acceptable.

Gluing and mold sticking of plastic semi-finished products are two different concepts. Gluing refers to irregular scar-like gravure marks on the primary and secondary appearance surfaces of semi-finished plastic parts; mold sticking means that screw posts, positioning posts and positioning bones of semi-finished plastic parts are completely stuck in mold cavity. However, degree of adhesive depends on degree. Generally speaking, NG is completely unacceptable, including mold gluing. If such a situation is not discovered in our process quality control, it can only mean that we have made a low-level mistake.

Among plastic semi-finished products, some trachoma has nothing to do with mold, and some has a lot to do with mold itself. It will also appear on the surface or sides of semi-finished plastic parts. If it is not a problem with material of mold itself, then it depends on size or just NG. Why do we say this? First, we need to see whether customers have complained about such problems in plastic semi-finished products. Second, plastic semi-finished products with trachoma will affect appearance after they are finished. The most important thing is to look at customer. If foreign matter is very small and size is ≤0.1mm or ≤0.15mm, and depth is within 0.1mm, it should be acceptable. However, post-production improvements are required during production. If problem is caused by material of mold itself, abnormal information will be issued immediately and fed back to relevant engineering personnel for follow-up and improvement to seek the best solution for final judgment.

Some of our colleagues have been doing plastic quality control for several years now, and customers must have complained about abnormalities such as whitening and cracking! So let’s first understand where whitening and cracking usually appear in plastic semi-finished products? In general, there are following aspects:
1. Bone position of product
2. Insert position of product (that is, side)
3. Root of pillar
4. Buckle position
In fact, comparing the two, whitening situation is not as serious as cracking. There are also degrees of whitening. Large customers like Kyocera require our suppliers to whiten them no matter what level they are. It will affect service life of finished product after assembly. Everyone knows that Toshiba and Kyocera are large Japanese-owned Fortune 500 multinational companies, so if semi-finished plastic parts are whitened, they will judge them as NG. However, we will also observe that plastic semi-finished products are whitened due to mold positioning problems. However, we process and ship plastic semi-finished products when bones are not stretched or deformed. It will also not affect service life of finished product after being assembled, but it will be scrapped if it breaks! As long as there are no cracks at insert position, base of pillar, and buckle position, it can be processed and shipped if it is only whitened but does not affect assembly and screwing functions and does not affect life of product. Specific situation also depends on customer's requirements. During production, we will also issue abnormal reports to inform relevant engineering personnel for follow-up processing.

Here we say that pin deviation refers to deviation of pin in mold, which causes screw hole of plastic semi-finished product to be too large and hole diameter to be too large. During product assembly, screws will slide, which seriously affects function and use of product. Colleagues who are doing quality control of dimensions of manufactured products should pay attention to fact that aperture size must be controlled within tolerance range. If aperture size is large, quality assistant engineer or above should be notified to senior management for judgment and tracking, and the best quality judgment should be sought before shipment.

Air trapping in plastic semi-finished products refers to situation that occurs in mold cavity due to improper exhaust system. Phenomenon of trapped air in plastic semi-finished products includes following situations:
1. Lack of glue on the edge of bone
2. There is a burnt look somewhere on the edge of bone
3. Buckle position is not satisfied with glue removal.
4. There is a bifurcated line somewhere on secondary appearance surface.
5. Semi-circular bone area is burnt white or black.
6. Positioning column is not full of glue.
Taken together, we can come up with these problems: lack of glue in bone position, burnt bone position, white or black burnt bone position, lack of glue in buckle position, clamping line, and lack of glue in positioning column. Is it possible to give all judgments NG? Answer depends on situation. First of all, we need to pay attention to whether there have been complaints about these types of anomalies before. Are their degrees same? Then, in sample management, we have limited sample, AOD samples and engineering confirmation samples, including customer sign sample, to see if there are any similar abnormalities such as missing bone, burned bone, white or black burned bone position, lack of glue in buckle position, pinched wires, or missing glue on positioning column. If there is such a sample, we must strictly abide by such sample in process quality control to accept and reject. If there is no such sample, our process quality control personnel will issue a quality abnormality document as soon as possible and contact relevant engineering management personnel to make a comprehensive judgment before making a final judgment.

So-called air pattern is a small piece of mist-like luster on primary appearance surface or secondary appearance surface corresponding to position of plastic inlet in plastic semi-finished product, which is very sub-circular or oval-shaped. This kind of situation is also closely related to size of mold inlet and molding parameters. If it is obvious on both first-level appearance surface and second-level appearance surface, then you need to carefully check customer-signed sample before deciding whether it is NG or OK. Generally speaking, it is required to improve production before reproducing. However, if design of mold itself is unreasonable rather than a problem with molding parameters and we require production adjustment, then subsequent work will be troublesome.

 

Anyone who has worked in same field knows the word "gain and loss" very well, that is, if there is a gain, there must be a loss, and if there is a loss, there must be a gain. When you want to adjust air pattern, their production technicians will speed up injection speed to remove air pattern, but if you do this, line clipping somewhere in semi-finished plastic part will be more obvious than the one just now, and it may not match sample. Here I suggest that process quality control personnel check samples first and then decide whether to let production technicians adjust molding parameters. Specific handling method is to first issue a quality abnormality information feedback form to senior production technicians to find the best solution and let them assist in quality processing of abnormality. At the same time, we also need to inform relevant engineering management personnel to provide technical support to quality side!

Let’s first analyze, where are dragging and lifting parts of plastic products more common?
1. Secondary appearance or side texture
2. Insert position, pin position
3. Touch the hole position
4. Inclined thimble position
5. Heart pumping position
6. Mold position
7. Raised identification position on primary appearance surface
Depending on product and customer's requirements, if a certain plastic part is scratched but not lifted, we usually ask production department to improve it before production. There is an exception here: the entire grain surface on secondary appearance surface or side surface is scratched. If our quality control personnel ask production technicians to make improvements, it will affect length, size and appearance of the entire part and cause shrinkage. Here I still suggest that our quality control personnel should carefully check sample or drag limit plate or AOD sample before suggesting improvements to production technician, otherwise new problems will arise and need to be confirmed. Generally, sample management office of quality department of plastic parts with scratched plastic parts will have relevant samples. Let's take a look at them first and then draw a conclusion as to whether they really need to be improved. Of course, there is also a situation where a single drag mark has a deep drag mark, which requires mold polishing. At this time, finished parts need to be considered whether they can be shipped. However, I think if goods cannot be shipped, it can be directly judged as NG. But we still notified quality assistant or quality engineer to seek judgment. If there is a slight scratch on insert position, pin position, bump hole position, inclined ejector pin position, centering position, mold slider, or raised mark position on first-level appearance surface, as long as dragged out rubber powder is wiped clean with a white cloth, it is acceptable but production department must be required to make immediate improvements before reproducing. If improvements cannot be made, an exception order will be issued to notify relevant engineering personnel to follow up and improve such quality abnormalities.

Speaking of plastic capes, I would like to talk about what is your concept of scratches?

 

In the company where I work, some customers have following requirements for plastic parts: for example, take Toshiba and Kyocera, two of the world's top 500 customers, if they give edge of ≤0.15mm on periphery of semi-finished plastic parts provided by our suppliers, it is considered OK. Today, when plastic industry is mature, we must also consider production costs in production. If plastic parts have to be processed if they have scratches, then our company will spend a lot of manpower processing caps, scratch processing will also bring many uncontrollable factors to our on-site quality control. For example, if there is pressure damage or pressure deformation during processing of scratch, it will flow into good product, resulting in secondary complaints! Although its defective rate only accounts for about 5%, if calculated based on PPM, it is a very high defective rate. So, where will a plastic cape appear? Place where a cape is most likely to occur is where front and rear molds are separated. For example: hole position, penetration position, slider, buckle position, inclined thimble pin position, silk barrel pin position, etc. There are several factors that cause growth of semi-finished plastic parts:
1. Molding parameters of machine are not adjusted stably.
2. Caused by mold exhaust slot being enlarged
3. Caused by loosening of bobbin pin
4. Caused by loose thimble
5. Caused by collision damage to insert position
Above five anomalies that cause plastic parts to have long edges are not allowed. Especially the first abnormality, in serious cases, will cause semi-finished plastic parts to be NG in size and unusable. Category 2, 3, 4, and 5 abnormalities that cause chipping still require processing. After order is completed, on-site quality control personnel must promptly notify senior production technician to follow up and optimize such chipping phenomena.

If there are plastic parts that are processed, there will be abnormal injuries. Generally speaking, if secondary appearance surface is damaged during processing, but grain surface of parts is not damaged and gaps during assembly are not affected, it is acceptable. However, if part is completely an internal component, then it is okay if it is slightly damaged and does not affect assembly! However, some customers also have certain requirements for concept of plastic semi-finished products, which must be comprehensively judged based on customer's acceptance standards before shipment.

Scratches and abrasions will not be accepted in quality control of process. Especially for products produced by some polishing molds, operator is required not to accumulate too many parts on the table when working, and there are still many parts on work surface, anti-scratch and abrasion rubber pads are placed on work surface to prevent scratches and abrasions on plastic parts. In another situation, our process quality control personnel should pay attention. When such products are reworked due to other factors, they should pay attention to scratches and abrasions. I suggest that for this type of product, if there are not too serious defects among good products, you should not ask operators to rework to reduce number of scratched defective products. It is best to work harder and increase frequency of inspections on machine table to control flow of other defective products into good products to reduce rework.
Finally, we also know that when it comes to process quality control, we should not say OK or NG easily, and we should not easily ask production department to adjust molding parameters at will to improve existing quality situation. We should consult more relevant project management personnel, check more samples, AOD samples, engineering samples and customer-signed project samples, do less judgment and more legwork to seek the best answer from engineering side. reduce our own misjudgment rate, improve our own work efficiency and professional skills.