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Do you know the name of the process by which most plastic solid materials are produced? It is called Injection molding. It is one of the best molding processes to make millions of injection molding parts in a very short time. However, The initial injection mold tooling cost is pretty high than other machining methods, but this injection tooling cost will be recovered by the large production later, and this process has less or even no waste rate.
Injection molding (or Injection Moulding) is a manufacturing technology for producing products from plastics. Injecting the molten plastic resin at high pressure into an injection mould, which the mold is made according to the desired part shape, the part shape was created by a designer using some CAD design software ( such as UG, Solidworks, etc), The mould is made by a mould company (or mold maker) from metal material or aluminum, and precision-machined to form the features of the desired part by some high technical machines like CNC machine, EDM machine, lather machine, grinding machine, wire-cutting machine, etc, step by step to make final mould cavity base on exactly desired part shape and size, which we called an injection mold.
The injection molding process is widely used for producing a variety of plastic products, from the smallest component to the big bumpers of cars. It is the most common technology to produce molding products on the world today, with some commonly made products including food containers, buckets, storage bins, house cooking equipment, outdoor furniture, automotive components, medical components, molding toys, more and more.
If you want to run your business with injection molding, below some basic information you may need to know
2.1. Injection molding machine
- 2.1.1 Horizontal or vertical machines
- 2.1.2 Clamping unit
3.1. Cold runner mold
- 3.1.1. Two plate mold
- 3.1.2. Three plate mold
3.2. Hot runner mold
- 4.1 Injection Molding Cycle
- 4.2 Injection Molding trial
- 4.3 Injection Molding Defects
- 4.3.1 Issue No. I: Short shot defects
- 4.3.2 Issue No. II: Trimming (Flashing or burrs) Defects
- 4.3.3 Issue No. III. Welding Line (Joint Line) Defects
- 4.3.4 Issue No. IV: Warp Distortion
- 4.3.5 Issue No. V: Sink Mark Defects
- 4.3.6 Issue No. VI: Flow Mark
- 4.3.7 Issue No. VII: Glass fiber streaks
- 4.3.8 Issue No. VIII: Ejector marks
Types of Injection Molding – Basically 7 types of injection molding process as below
Injection molding equipment
Injection molding machine
Injection molding machine, American normally called injection press (or press), press our custom made injection mold in the machine. The injection machine is rated by tonnage, which indicates the amount of clamping force that the press can generate. This clamping force keeps the mold closed during the injection molding process. There are Various specifications of injection molding machines from less than 5 tons to 6000 tons, or even bigger.
Horizontal or vertical machines
There are normally two types of injection molding machines, Horizontal & vertical molding machines,
This means molding machines fasten the mold in either a horizontal or vertical position. The majority is horizontally injection molding machines but vertical machines are used in some niche applications such as cable insert molding, insert molding or some special molding process requirement, there are some injection machines can produce two colors, three or four colors molded parts at one step, we call double-shot injection molding machine or 2K injection molding machine (more color will be 3K or 4K molding machine),
Machines are classified primarily by the type of driving systems they use: hydraulic, electric, or hybrid. Hydraulic presses have historically been the only option available to molders until Nissei introduced the first all-electric machine in 1983. The electric press, also known as Electric Machine Technology (EMT), reduces operation costs by cutting energy consumption and also addresses some of the environmental concerns surrounding the hydraulic press. Electric presses have been shown to be quieter, faster, and have higher accuracy, however, the machines are more expensive. Hybrid injection molding machines take advantage of the best features of both hydraulic and electric systems. Hydraulic machines are the predominant type in most of the world, with the exception of Japan.
Injection Mold- Types of injection molds
Simply explain the injection mold is custom made of desired part shape by cutting the steel or aluminum, and produce the mold that can be used in the injection molding machine, which we called injection mold or plastic injection mold, go to our plastic molding section to know more about plastic injection mold manufacturing, but to make injection mould actually is not easy, you need to have a professional team (mold maker, mold designer) and mold manufacturing equipment like CNC machines, EDM machines, wire-cutting machines, etc
There are two main types of injection molds: cold runner mold (two plate and three plate designs) and hot runner mold (the more common of the runner-less molds). The significant difference is the presence of sprue and runner with every molded part in the cold runner type. This extra molded component must be separated from the desired molded part, the hot runner basically does not have any runner waste of have small runner waste.
Cold runner mold
Developed to provide for injection of thermoset material either directly into the cavity or through sprue and a small sub-runner and gate into the mold cavity, there are basically two types of the cold runner which mostly used in the mold industry, two plate mold and Three plate mold.
Two plate mold
The conventional two-plate mold consists of two halves fastened to the two platens of the molding machine’s clamping unit. When the clamping unit is opened, the two mold halves open, as shown in (b). The most obvious feature of the mold is the cavity, which is usually formed by removing metal from the mating surfaces of the two halves. Molds can contain a single cavity or multiple cavities to produce more than one part in a single shot. The figure shows a mold with two cavities. The parting surfaces (or parting line in a cross-sectional view of the mold) is where the mold opens to remove the part(s).
In addition to the cavity, there are other features of the mold that serve indispensable functions during the molding cycle. The mold must have a distribution channel through which the polymer melt flows from the nozzle of the injection barrel into the mold cavity. The distribution channel consists of (1) a sprue, which leads from the nozzle into the mold; (2) runners, which lead from the sprue to the cavity (or cavities); and (3) gates that constrict the flow of plastic into the cavity. There are one or more gates for each cavity in the mold.
Three plate mold
The two-plate mold is the most common mold in injection molding. An alternative is a three-plate mold. There are advantages to this mold design. First, the flow of molten plastic is through a gate located at the base of the cup-shaped part, rather than at the side. This allows more even distribution of melt into the sides of the cup. In the side gate design in the two-plate the plastic must flow around the core and join on the opposite side, possibly creating a weakness at the weld line. Second, the three-plate mold allows more automatic operation of the molding machine. As the mold opens, it divides into three plates with two openings between them. This forces the disconnection of runner and parts, which drop by gravity (with possible assistance from blown air or a robotic arm) into different containers beneath the mold.
Hot Runner Mold
Hot runner molding has parts that are physically heated. These types of molding help transfer the molten plastic quickly from the machine directly feeding into the mold cavity. It can also be known as the runner-less mold. The hot runner system is very useful for some of the high volumes of products, that will save huge production costs by using the hot runner mold system. The sprue and runner in a conventional two-plate or three-plate mold represent waste material. In many instances they can be ground and reused; however, in some cases, the product must be made of “virgin” plastic (original raw plastic material) or there is multiple cavity mold ( such as 24 cavities, 48 cavities. 96 cavities, 128 cavities or even more cavities mold). The hot-runner mold eliminates the solidification of the sprue and runner by locating heaters around the corresponding runner channels. While the plastic in the mold cavity solidifies, the material in the sprue and runner channels remains molten, ready to be injected into the cavity in the next cycle.
Type of hot runner system.
Basically there are two types of hot runner systems, one is cal hot sprue mold (without the manifold plate and hot runner plate) and one is hot runner mold (with the manifold plate and hot runner plate).
Hot sprue mold (without the manifold plate and hot runner plate) is using the hot nozzle (sprue) feeding the material into the mold cavity directly or indirectly.
hot runner mold (with the manifold plate and hot runner plate) means the hot runner system has the hot runner plate, manifold plate, and sub-hot runner sprue. The below pictures are simple explaining for two types of hot runner systems.
Advantages and Disadvantages of Cold Runner Molding
There are a few, amazing advantages of cold runner molding such as:
- Cold runner molding is cheaper and easier to maintain.
- You are able to quickly change colors.
- It has a faster cycle time.
- It is more flexible than hot runner molding.
- The gate locations can easily be changed or fixed.
Although there are many advantages, there are also some disadvantages. The disadvantages of cold runner molding are:
- You have to have thicker dimensions compare to the hot runner mold.
- You can only use certain types of nozzles, fittings, and manifolds.
- Cold runner molding can result in slower production time when you remove sprues and runners.
- You must manually separate the runner and parts after molding.
- You may waste the plastic materials if you do not reset after each run.
If you want to know more information, please go to the cold runner mold page to check more detail.
Advantages and Disadvantages of Hot Runner Molding
Hot runner molding has a few advantages such as:
- Hot runner molding has a very quick cycle time.
- You can save production costs by using hot runner molding.
- Less pressure is needed to inject the molding.
- You have more control over the hot runner molding.
- Hot runner molding can fit a wide variety of gates.
- multiple cavities mold can be easily done by using the hot runner system.
The disadvantages to using hot runner moldings are:
- It is more expensive to make the hot runner mold than cold runner mold.
- It is difficult to maintain and fix the hot runner mold.
- You are not able to use hot runner molding on materials that are thermally sensitive.
- You will need to get your machines inspected more often than cold runner molding machines.
- It is hard to change the colors to the hot runner mold system.
Want to know more information, welcome to the hot runner mold section to know more.
Injection Molding Processing?
Injection molding is one of the best ways to shape plastic products by injecting a thermoplastic material, During the process of injection molding, the plastic material in the injection molding machine and use the melt system of the injection unit to melt the plastics into the liquid, then use high pressure injecting the liquid material into a mold (custom manufacturing mold) which assembled in that injection molding machine. The mold is made up of any metal such as steel or aluminum. The molten form is then allowed to cool down and set it into a solid form.
The plastic material thus formed is then ejected out of the plastic mold. The actual process of plastic molding is just an expansion of this basic mechanism. The plastic is let into a barrel or chamber under gravity or force-fed. As it moves down, the increasing temperature melts the plastic resin. Then, the molten plastic is forcibly injected into the mold under the barrel with an appropriate volume. As the plastic cools, it solidifies. The injection molded parts like this has a reverse shape than that of the mold. A variety of shapes both 2D and 3D can be produced by the process.
The process of plastic molding is cheap due to the simplicity involved and the quality of the plastic material is modifiable by changing the factors involved in the custom injection molding process: – The pressure of injection can be changed to change the hardness of the final product. – The thickness of the mold also governs the quality of the article produced. – The temperature for melting and cooling determine the quality of the plastic formed. ADVANTAGES The major advantage of injection molding is that it is very cost-effective and fast. Apart from this, unlike the cutting processes, this process rules out any undesired sharp edges. Also, this process produces smooth and finished products that require no further finishing. Check below for the detail advantages and disadvantages.
Advantages of injection moulding
Although injection molding is used by many different companies, and there is no doubt this is one of the most popular methods to produce the injection molding products, there are some of the advantages of using them such as:
- Precision and aesthetics – because in this injection moulding process you can make your plastic part with any shape and surface finished ( texture and high gloss finishing), some of the special surface finishing still can meet by the secondary surface finishing process, injection moulding part is the repeatability of their shapes and dimensions.
- Efficiency and speed – a single production process, even for the most complex products, lasts from a few to several dozen seconds,
The possibility of full automation of the production process – which in the case of companies dealing with the production of plastic components translates into low production effort and the possibility of mass production.
- Ecology – because compared to metalworking, we are dealing with a significant reduction in the number of technological operations, less direct energy and water consumption and low emissions of compounds harmful to the environment.
Plastics are materials that, although known relatively recently, have even become indispensable in our lives, and thanks to increasingly modern production processes from year to year they will contribute even more to saving energy and other natural resources.
Disadvantages of injection molding
- The high cost of injection molding machines and often the cost of tooling (molds) that equals it resulting in extended depreciation time and high costs of starting production.
- Due to the above, injection technology is only cost-effective for mass production.
- The need for high qualifications of technical supervision employees who must know the specifics of injection molding processing.
- The need for high technical requirements for injection mold making
- The need to maintain narrow tolerances of processing parameters.
- A long time of preparation of the production due to the labor-intensive implementation of the injection molds.
Injection Molding Cycle Time
The basic injection cycle time is including Mold close – injection carriage forward –plastics filling time – metering – carriage retracts – holding pressure – cooling time – mold open – eject part(s)
The mold is closed shut by the injection molding machine, and the melted plastics is forced by the pressure of the injection screw to inject into the mold. The cooling channels then assist in cooling the mold and the liquid plastics become solid into the desired plastic part. The cooling system is one of the most important parts in the mold, inappropriate cooling can result in distorted molding products and the cycle time will be increased, this will increase the injection molding cost as well.
When the injection mold has been made by mold maker, the first thing we need to do the mold trial, this is the only way to check the mold quality if made according to custom requirement or not, to trial the mold we normally fill the plastics the molding step by step, use short shot filling at the first and increase the material weight little by little until the mold is 95 to 99% full. After meeting this status a small amount of holding pressure will be added and holding time increased until the gate freeze off has been occurred, then holding pressure is increased until the molding part is free of sink marks and part weight has been stables. Once the part is good enough and has passed any specific technical tests, a machine parameter sheet needs to be recorded for massive production in the future.
Plastic Injection Molding Defects
Injection molding is a complex technology and problems may happen every time. A new custom made of an injection mold has some issue is very normal, to solve the mold issue we need to fix and test the mold several times, normally two or three times trial can completely solve all o issue, for some of the cases only need one-time mold trial can be approved the samples. and finally, all of the issues are solved completely. Below are most of the injection molding defects and the troubleshooting skills to solve those issues.
Issue No. I: Short shot defects- What is a short shot issue?
Injecting material into the cavity, molten material does not completely fill the cavity, resulting in the product lacking material. This is called Short Molding or Short Shot as shown in the picture, there are lots of reason to cause the short shot issues,
Fault Analysis and Method for Correction the defects
- Improper selection of injection molding machine – When choosing plastic injection machines, the maximum shot weight of the plastic injection machine must be bigger than the weight of the product. During verification, the total injection volume (including the plastic product, runner and trimming) shall not be more than 85% of the plasticizing capacity of the machine.
- Insufficient supply of material -The bottom of the feed position might have “bridging the hole” phenomena. The shot stroke of the injection plunger should be added to increase the supply of material.
- Poor flow factor of raw material – improve mold injection system, for example, by proper design of runner location, by enlarging gates, runner and feeder size, and by using a bigger nozzle, etc. In the meantime, the additive can be added to the raw material to improve the flow rate of the resin or change the material with a better flow rate.
- Overdose of using the lubricant – Reduce the lubricant and adjust the gap between barrel and injection plunger to recover the machine, or fix the mold so that is no need any lubricant during the molding process
- Cold foreign substances blocked the runner – This issue is normally happened to hot runner systems, dismount and clear the nozzle of hot runner tip, or enlarge the cold material cavity and runner cross-section area.
- Improper design of injection feeding system – When designing the injection system, pay attention to gates balance, product weight of each cavity should be in proportion of gates size, so as to make each cavity can be full filled simultaneously and gates should be positioned in thick walls. A balanced separate runners scheme can also be adopted. If the gate or runner is small, thin, or long, the molten material pressure will be reduced too much during running and the flow rate is blocked, which will result in the poor filling. To solve this problem, the cross-sections of the gate and runner should be enlarged and use multiple gates when necessary.
- Lack of venting – Check if there is cold-slug well or if the position of cold-slug well is correct. For mold with a deep cavity or deep ribs, venting slots or venting grooves should be added at positions of short molding (end of feeding area). Basically, there is always have venting grooves on the parting line, venting grooves size can be 0.02-0.04mm and 5-10mm width, 3mm close to the sealing area, and venting opening should be at the end of filling the position. When using raw materials with excessive moisture and volatile content, a large amount of gas (air) will also be generated, causing the air trap in the mold cavity. In this case, raw material should be dried and cleared in volatile substances. In addition, during the injection process operation, poor venting can be addressed through increasing mold temperature, low injection speed, reducing injection system obstruction and reducing mold clamping force, and enlarging gaps between molds. But the short shot issue happens to the deep ribs area, to release the air out, you need to add venting insert to solve this air trap and short shot issues.
- The mold temperature is too low – Before starting molding production, mold should be heated up to the required temperature. In the beginning, should connect all of the cooling channels and check if the cooling line is working well, especially for some special material, like PC, PA66, PA66+GF, PPS, etc, the perfect cooling design is the must for those special plastics material.
- Molten material temperature is too low – In proper molding process window, the temperature of the material is in the proportion of filling length, low-temperature molten material is poor in fluidity and the filling length is shortened. It should be noted that after the feed barrel is heated to the required temperature, it should stay constant for a while before start molding production. In case that low-temperature injection must be used to prevent molten material being resolved, the injection cycle time can be prolonged to overcome short shot, if you have professional molding operator he should know this very well
- Nozzle temperature is too low – When open mold, the nozzle should be part away from the mold spure to reduce the influence of mold temperature on the nozzle temperature and keep the nozzle temperature within the range of the molding process requires.
- Insufficient injection pressure or holding pressure – Injection pressure is close to a positive proportion to the filling distance. The injection pressure is too low, the filling distance is short, and the cavity cannot be fully filled, increasing the injection pressure and holding pressure can improve this issue
- Injection speed is too slow – Mold filling speed is directly related to injection speed. If injection speed is too low, filling of molten material is slow while slow-flowing molten is easy to cool hence the flow properties further decrease and result in short injection. For this reason, the injection speed should be enhanced properly.
- Plastic product design is not reasonable – If the wall thickness is out of proportion to the length of the plastic product and the product shape is very complex and the forming area is large, melt material is easily blocked at the thin wall of the product and leading to insufficient filling. Therefore, when designing the shape and structure of the plastic products, note that wall thickness is in direct relation to melting limit filling length. During injection molding, product thickness should range between 1-3mm and 3-6mm for large products. Generally, it is not good for injection molding if the wall thickness is over 8mm or less than 0.4mm, so this kind of thickness should be avoided in design.
Issue No. II: Trimming (Flashing or burrs) Defects
I. What is the flashing or Burrs?
When extra plastic melt material is a force out of mold cavity from the mold joint and forming thin sheet, trimming is generated, if the thin sheet is big in size, it is called flashing.
II. Fault Analysis and Method for Correction
- Mold clamping force is not sufficient. Check whether the booster is over pressurizing and verify whether the product of the projected area of the plastic part and the forming pressure exceeds the clamping force of the equipment. Forming pressure is the average pressure in the mold, normally is 40Mpa. If the calculation product is larger than the mold clamping force, it indicates that the clamping force is insufficient or the injection positioning pressure is too high. In this case, the injection pressure or the section area of the injection gate size should be reduced, pressure keeping and pressurization time can also be shortened, or reduce injection plunger stroke, or reduce the number of injection cavity or use mold injection machine with larger tonnage.
- Material temperature is too high. The temperature of feed barrel, nozzle and mold should be decreased properly and reduce the injection cycle. For melts of low viscosity, like polyamide, it is difficult to solve overflow flashing defects by simply changing injection molding parameters. To solve this issue completely, fix the mold is the best way, like do better mold fitting and make the parting line and shot off area more precisely.
- Mold defect. Mold defect is the main reason for overflow flashing. The mold must be carefully examined and re-verified the mold parting line to ensure the pre-centering of mold. Check whether the parting line fits well, whether the gap between sliding parts in the cavity and core is out of tolerance, whether there is adhesion of foreign matter on the parting line, whether the mold plates are flat and whether there is bending or deformation, whether the distance between mold pate is adjusted to fit the thickness of the mold, whether the surface mold block is damaged, whether the pull rod is deformed unevenly, and whether the venting slot or grooves is too large or too deep.
- Improper of the molding process. If injection speed is too high, injection time is too long, injection pressure in the mold cavity is unbalanced, mold filling speed is not constant, or overfeeding of material, overdosage of lubricant can lead to flashing, therefore corresponding measures should be taken according to the specific situation during operation.
Issue No. III. Welding Line (Joint Line) Defects
I. What is the welding line defect?
When filling the mold cavity with molten plastic material, if two or more flows of molten material have cooled down in advance before confluence t the joint area, the flows will not be able to totally integrate and a liner is produced at the confluence, thereby a welding line is formed, also called joint line
II. Fault Analysis and Method for Correction
- Material temperature is too low. Low-temperature molten material flows are poor in confluence performance and the welding line is easily formed. If welding marks appear at the same position of both interior and exterior of a plastic product, usually it is inappropriate welding caused by the low temperature of the material. To address this problem, feeding barrel and nozzle temperature can be properly increased or the injection cycle can be prolonged to increase the material temperature. In the meantime, the coolant flow inside the mold should be regulated to properly increase mold temperature. Generally, the strength of the plastic product welding line is relatively low, if the position of the mold with the welding line can be partially heated to partially increase the temperature at the welding position, the strength at the welding line can be enhanced. When low-temperature injection molding process is used for special needs, injection speed and injection pressure can be increased to improve confluence performance, a small dosage of lubricant can also be added to the raw material formula to increase molten flow performance.
- Mold Defect. The fewer numbers of the gate should be adopted and the position of the gate should be reasonable to avoid inconsistent filling speed and interruption of molten flow. Where possible, one point gate should be adopted. To prevent low-temperature molten material generates welding mark after injected into the mold cavity, mold temperature and add more cold well in the mold
- Poor mold venting solution. Check whether the venting slot is blocked by solidified plastic or other substance at first (especially happened to some glass fiber material), and check whether there is a foreign substance at the gate. If there still are carbonation spots after removing the extra blocks, add venting groove at the flow convergence in the mold, or change the gate location, reduce mold clamping force to increase venting intervals to speed up the convergence of material flows. In terms of the molding process, reducing material temperature and mold temperature, shortening high-pressure injection time, decreasing injection pressure can be taken.
- Improper use of release agents. In injection molding, usually, a small quantity of release agent is evenly applied at the thread and other position not easy to demould. In principle, the use of the release agent should be reduced as far as possible. In the massive production should never use a release agent.
- The structure of plastic products is not reasonably designed. If the wall of the plastic product is too thin or the thickness differs greatly, or there are too many inserts, it will cause poor welding. When designing a plastic product, it shall be ensured that the thinnest part of the product must be greater than the minimum wall thickness allowed during forming. In addition, reduce the number of inserts and make the wall thickness to be uniform as possible.
- The welding angle is too small. Each kind of plastic has its own limit welding angle. When two flows of molten plastic converge, the welding mark will appear if the converging angle is smaller than the limit welding angle and will disappear if the converging angle is bigger than the limit welding angle. Usually, the limit welding angle is around 135 degrees.
- Other causes. Different degree of poor welding can be caused by use of raw material with excessive moisture and volatile content, oil stains in the mold that are not cleaned up, cold material in mold cavity or uneven distribution of fiber filler in the molten material, unreasonable design of mold cooling system, fast solidification of the melt, low temperature of the insert, small nozzle hole, insufficient plasticizing capacity of the injection machine, or large pressure loss in the plunger or barrel of the machine. To solve these problems, different measures, such as pre-drying of raw materials, regular cleaning of mold, changing the design of mold cooling channel, controlling the flow of cooling water, increasing the temperature of inserts, replacing nozzles with larger apertures, and using injection machines with larger specifications, can be taken in the process of operation.
Issue No. IV: Warp Distortion – What is warp distortion?
Due to internal shrinkage of the product is inconsistent, the internal stress is different and distortion occurs.
Fault Analysis and Method for Correction
1. The molecular orientation is unbalanced. In order to minimize warp distortion caused by diversification of molecular orientation, create conditions to reduce the flow orientation and relax orientation stress, the most effective method is to reduce molten material temperature and mold temperature. When this method is used, it is better to combine with heat treatment of the plastic parts, otherwise, the effect of reducing molecular orientation diversification is often of short duration. The method of heat treatment is: after demoulding, keep the plastic product at a high temperature for some time and then cool to room temperature gradually, in this way, the orientation stress in the plastic product can be largely eliminated.
2. Improper cooling. When designing a plastic product structure, the cross-section of each position should be consistent. Plastic must keep in the mold for a sufficient time for cooling and forming. For the design of a mold cooling system, cooling pipelines should be at positions where the temperature is easy to rise and the heat is relatively concentrated, as for the positions easily cool down, gradual cooling should be adopted to ensure balanced cooling of each position of the product.
3. The gating system of mold is not properly designed. When determining the gate position, be aware that the molten material shall not directly impact the core, and be sure the stress on both sides of the core is the same. For large flat rectangular plastic parts, membrane gate or multi-point gate shall be used for resin raw materials with wide molecular orientation and shrinkage, and side gate shall not be used; for ring parts, disk gate or wheel gate shall be used, and side gate or pinpoint gate shall not be used; for housing parts, the straight gate shall be used, and side gate shall not be used as far as possible.
4. Demolding and venting system is not properly designed. In-mold design, draft angle, position, and the number of ejectors should be reasonably designed to improve mold strength and positioning accuracy. For small and medium-sized molds, anti warping molds can be designed and made according to warping behavior. In respect of mold operation, ejection speed or ejection stroke should be properly reduced.
5. Improper operation process. The process parameter shall be adjusted according to the actual situation.
Issue No. V: Sink Mark Defects – What is the sink mark?
Sink marks are unevenly shrinking of the surface caused by the inconsistent wall thickness of the plastic product.
Fault Analysis and Method for Correction
- The injection molding condition is not properly controlled. Properly increase injection pressure and speed, increase molten material compression density, prolong injection and pressure keeping time, compensate the sinking of the molten, and increase the buffering capacity of injection. However, the pressure should not be too high, otherwise, the convex mark will appear. If sink marks are around the gate, prolong the pressure keeping time can eliminate the sink marks; if sink marks are at the thick wall, prolong the cooling time of the plastic product in the mold; if sinks around insert are caused by partial shrinkage of molten, the main reason is that the temperature of the insert is too low, try to increase the temperature of the insert to eliminate the sink marks; if sink marks are caused by insufficient material feeding, increase the material. Besides all of this, the plastic product must be fully cooled in the mold.
- Mold defects. According to the actual situation, properly enlarge gate and runner cross-section, and the gate should be at a symmetrical position, feed inlet should be in the thick wall. If sink marks appear away from the gate, the cause usually is that flow of molten material is not smooth at some position of the mold, which hampers the transmission of pressure. To solve this problem, enlarge the injection system size to allow the runner to extend to the position of the sink marks. For products with thick walls, wing type gate is preferred.
- Raw material cannot meet molding requirements. For plastic products with high finish standards, resin with low shrinkage shall be used or the appropriate dosage of lubricant can also be added to the raw material.
- Improper design of product structure. The wall thickness of the product shall be uniform, if the wall thickness differs a lot, the structure parameter of the injection system or the wall thickness shall be adjusted.
Issue No. VI: Flow Mark－What is Flow Mark?
Flow mark is a linear trace on the surface of a molding product that shows the flow direction of molten material.
Fault Analysis and Method for Correction
- Ring shape flow mark on the surface of the plastic part with the gate as the center caused by poor flow motion. To address this kind of flow marks, increase the temperature of mould and nozzle, increase injection rate and filling speed and prolong pressure keeping time, or add a heater at the gate to raise the temperature around the gate. Appropriately expand the gate and runner area can also work, while the gate and runner section is preferably circular, which can guarantee the best filling. However, if the gate is in the weak area of the plastic part, it shall be square. In addition, a large cold-slug well should be set at the bottom of the injection port, and at the end of the runner; the greater the influence of material temperature on the flow performance of the melt, the more attention should be paid to the size of the cold-slug well. The cold-slug well must be set at the end of the melt flow direction from the injection port.
- Whirl flow marks on the surface of the plastic part caused by the unsmooth flow of molten material in the runner. When the molten material flows from the runner with a narrow section to the cavity with a larger section or the mold runner is narrow and the finish is poor, the material flow is easy to form turbulence, resulting in whirl flow mark on the surface of the plastic part. To address this kind of flow marks, reduce injection speed appropriately, or control the injection speed in slow-fast-slow mode. Gate of mold shall be in the thick wall and preferably in the form of handle type, fan type, or film type. Runner and gate can be enlarged to reduce material flow resistance.
- Cloud-like flow marks on the surface of the plastic part caused by volatile gas. When ABS or other copolymerized resin are used, if the processing temperature is high, the volatile gas produced by resin and lubricant will form cloud-like ripple marks on the surface of the product. In order to solve this problem, it is necessary to reduce the temperature of mold and barrel, improve the venting of mold, reduce the material temperature and filling speed, enlarge the gate section properly, and consider changing the type of lubricant or reducing the use of lubricant.
Issue No. VII: Glass fiber streaks – What is Glass fiber streaks
Surface Appearance: Plastic molding products with glass fiber have various surface defects, such as dim and dreary in color, coarse in texture, and metal bright spots, etc. these are especially obvious in the convex part of the material flow area, close to the joint line where the fluid meets again.
If the injection temperature and the mold temperature is too low, the material containing glass fiber tend to solidify fast on the mold surface, and the glass fiber will not be melt in the material again. When two flows meet, the orientation of glass fiber is in the direction of each flow, which will lead to irregular surface texture at the intersection, resulting in the formation of joint seams or flow lines.
This type of defect is more obvious if the molted material is not fully mixed in the barrel, for example, if the stroke of the screw is too long, it will cause the under mixed material is also injected.
Causes related to process parameter and improvements can be taken:
- Injection speed is too low: increase injection speed, consider use multi-step injection method like slow-fast mode.
- The temperature of mold is low: increase mold temperature could improve the glass fiber streaks
- Molten material temperature is too low: increase barrel temperature and increase screw back pressure to improve.
- The temperature of molten material varies a lot: if the molten material is not fully mixed, increase screw back pressure, reduce screw speed, and use the longer barrel to shorten stroke.
Issue No. VIII: Ejector marks – What is Ejector marks
Surface Appearance: Stress whitening and stress rising phenomena found on the side of the product which is facing the nozzle, i.e. where the ejector rod is located on the ejector side of the mold.
If the demoulding force is too high or the surface of the ejector rod is relatively small, the surface pressure here will be very high, causing deformation and eventually causing whitening at the ejection area.
Causes related to process parameter and improvements can be taken:
- Holding pressure is too high: decrease pressure keeping pressure
- holding pressure time is too long: shorten pressure keeping time
- holding pressure switch time is too late: advance the pressure keeping switch
- Cooling time is too short: increasing the cooling time
Causes related to mold design and improvements can be taken:
- The draft angle is no enough: increase the draft angle according to specification, especially in the area of the ejector mark.
- The surface finish is too rough: mold shall be well polished on the demolding direction.
- A vacuum is formed on the ejection side: install an air valve in the core.
Due to the specific properties of plastics, injection moulding is a very complex technological process; unlike the seemingly related process of the metal die casting, it is not a mechanical process but a mechanical-physical one. In the injection molding process, a molded piece is obtained. It is characterized not only by a specific shape but also by a specific structure resulting from the flow of the plasticized material in the mold and the course of its solidification. Because these processes occur in the form of injection, the designer of this tool must take into account, in addition to typically mechanical issues, also issues related to the physical nature of the material transformation. Constructing a rationally working form requires at the same time from the designer a thorough knowledge of the technical capabilities of the injection molding machine, because it is a machine with extremely rich possibilities provided by its equipment and numerous work programs.
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