Plastic Injection Molding FAQs

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To let our customer know more about our plastic injection molding service, we make 49 FAQs in here


1. Causes and solutions for product burrs(flash) formed when the injection machine first starts, as well as the incomplete filling after a while of production:
When the machine first starts, the melting in the injection machine barrel flows smoothly with a lower stickiness thanks to a longer period of heating, so product burrs tend to occur. After a while of production, since the melting continuously takes the heat away, the product will not be fully injected due to great stickiness and poor liquidity. This problem can be solved by gradually increasing the temperature of the barrel after some time into the production.


◆2. In the production process, the product cannot be fully injected, even increasing injection pressure and velocity won’t work sometimes. Why and how to solve it:
In the production process, the melting continuously takes the heat away, so the product will not be fully injected due to great stickiness and poor liquidity. This problem can be solved by increasing the temperature of the injection machine barrel.


◆3.Causes and solutions for product becoming oval:
A product becomes oval is because the melting is not evenly fed, causing uneven pressure on the circumference of the product thus making it oval. The 3-point injection approach can be adapted to allow for even feeding.


◆4. Precision products require the mold to be:
They require the mold to be made of high-rigidity materials, with a smaller elastic deformation and lower thermal expansion coefficient.


◆5. What changes would occur to the plastics when the right or wrong back pressure is applied:
When the screw is rotating, the resistant force is called the back pressure. The purpose of the back pressure is to thoroughly remove the air from the raw materials during the delivery and compressing a process that happens inside the screw so that the raw materials will have a higher density and the injected products will be more stable. Since the raw materials are repeatedly stirred in the barrel, the heat of fusion will increase. For products with poor color mixing or gas mark or products that need rapid color or plastic material change, the application of back pressure will bring about a significant effect. Therefore, when the back pressure is too low, there will be bubbles inside the product or silver streaks on the product surface; if the back pressure is too high, the raw materials will be overheated, the feed opening will be clogged, the screw will fail to return, the cycle will be extended and the nozzle will overflow. The control of back pressure can be achieved by the application of 2 devices - the throttle valve or the pressure regulator.


◆6.Causes and solutions for metal insert cracks when the product is under stress:
During the insert molding process, when the hot melting meets with the cold insert, an internal stress will be created, which weakens the product strength and thus results in cracks. It is suggested that the insert is pre-heated during the production process.


◆7. Reasonableness and selection method of injection mold vents:
If mold vents are not reasonably designed, not only ventilation will be greatly affected, also product deformation or dimensional change will be caused. As a result, the vents need to be properly designed. When selecting mold vents, it is suggested that vents are opened at the points where the product is fully injected or the air is trapped.


◆8. Causes and solutions for easy cracking in plastic injection product:
The product cracks easily because too many degraded and/or substandard materials are used, or because the materials stay too long in the barrel, resulting in material aging and thus product cracking. The solution would be increasing proportion of new materials and reduce the recycling of degraded materials (usually no more than 3 cycles), to prevent the materials from staying too long in the barrel.


◆9.Causes and solutions for suffused fiber on glass-fiber reinforced products:
Because the low temperature of the melting or the mold leads to insufficient injection pressure, the glass fiber cannot blend with the plastic in a very satisfactory way, causing fiber suffusion. This can be solved by increasing melting temperature, mold temperature, and injection pressure.


◆10.How does gate temperature influences the product:
If the gate temperature is too high or too low, recycled materials and feeding amount will not be stable enough to guarantee product dimensions and appearances.


◆11.Causes and solutions for white stains on transparent products:
The white stains on transparent products are caused by cold melting in the products or dirt in the materials. This can be solved by raising nozzle temperature, adding cold slug well, and storing materials in a good way to keep them from dirt.


◆12. Requirements for injection machine hydraulic fluids:
(1)Appropriate stickiness and good viscosity-temperature features;
(2)Good lubrication and anti-rust features;
(3)Good chemical stability, won’t gasify into the plastic mass;
(4)Foam stability;
(5)Little corrosion to machine parts and the sealing unit;
(6)Ignition temperature (flash point) requirement; low solidification point.


◆13. Hydraulic fluid’s influence on an injection machine:
When the system works in a higher temperature environment, the fluid with a stronger stickiness should be employed; and vice versa; when the system works in a higher pressure environment, the fluid with a stronger stickiness should be employed, because it is not easy to seal up under the high pressure – leakage will be one of the main issues; on the contrary, when the system is working in a lower pressure environment, the fluid with a weaker stickiness is recommended. When the hydraulic system parts are running at a high speed, so will the flow of the fluid. Pressure loss will increase in such circumstances, so it is better to choose the fluid with a weaker stickiness. On the contrary, when hydraulic system parts are running at lower speed, the fluid with a strong stickiness should be applied.


◆14. Key benefits of gas-assisted molding (GAM):
Able to exhaust the core of thick materials to produce hollow tubes, which helps save materials and shorten the cycle. The gas applied in the injection molding process is helpful for even pressure distribution. When plastics is cooled and solidified, the gas is able to compensate plastic shrinkage via expansion, so as to reduce the internal stress of the plastic product and thus improve dimensional stability, eliminating deformation and warping.


◆15. Spherical surface cracks after screen printing:
The screen printing cracks because there is a stress on the product surface. This can be solved by raising mold temperature and reducing the stress (by annealing).


◆16. For electric casing with 4 fixing pillars, cracks occur when tightening the screws:
The weld lines on the pillars tend to cause the cracks on the pillars. This issue can be solved by improving ventilation and raising mold temperature properly, so as to improve injection speed and thus reduce the weld lines.


◆17. Product deformation:
Product deformation is mainly caused by imbalanced product thermal shrinkage, or by the internal stress of the product itself.


◆18. Bubbles in transparent PC casing:
The raw materials are not dry enough; uneven plastic thickness, poor mold ventilation and decomposability of raw materials may also cause bubbles in the product. This can be solved by fully drying the materials, improving mold ventilation, and minimizing uneven plastic thickness.


◆19. Product plastic coating, support pin shrinkage:
Support pin shrinkage may be caused by poor mold ventilation, low injection speed, insufficient pressure and/or pressure holding time. So, this can be solved by improving mold ventilation, properly increasing injection speed, and increasing pressure and/or pressure holding time.


◆20. Product cracks caused by internal stress after a period of time:
Some internal stress remains inside the product, so after a period of time, the product will crack under the stress. In the injection molding process, this can be solved by raising mold temperature and reducing injection pressure, so as to eliminate the stress. Product stress can be eliminated by annealing.


◆21. When using black masterbatch for the ABS, the product is prone to crack and peeling:
Product peeling is caused by the too much carbon powder contained in the pigment of the masterbatch. This can be solved by changing master batch pigments.


◆22. After 3 years of service, the 100-ton hydraulic toggle machine often fails to open after being clamped.
This is caused by toggle machine wear, leading to the imbalanced opening. So, the mold often fails to open after being clamped.


◆23. After 2 years of service, an injection molding machine is not able to maintain injection stability – burrs and/or the incomplete filling will occur. Seals and nozzle tips have been replaced and the system is able to maintain a stable pressure, but it just doesn’t work properly:
Screw wear or damage causes uneven material recycling, so injection is not stable.


◆24. A 150-ton injection molding machine has been put into the production of PP drainage opening material for half a year. Melting time is increased from 3s to the current 6s:
Screw wear leads to slower material recycling.


◆25. When calibrating the mold, the product is not fully injected. It remains the same when speed or pressure is raised a little, but burrs will occur when speed or pressure is further raised:
The clamping unit of the machine is worn, causing gaps in the clamp, so burrs occur.


◆26. After 2 years of service, the temperature of the machine barrel is a bit too high during the injection molding process. Even, turning off the power will not help:
The screw is worn and roughened, so in the injection recycling process, the friction will produce heat, making the temperature in the midsection of the barrel a bit too high.


◆27. How does a power saving pump work:
A power saving pump refers to a variable delivery pump. When the machine is working, the hydraulic fluid pushes the machine into action via valve and cylinder, and then returns to the hydraulic tank. When the machine is not working, the fluid returns to the hydraulic tank directly.


◆28. Causes of intractable seal leakage and how to prevent it:
The seal is damaged by the worn core of the oil cylinder, leakage is thus caused. The cylinder core should be kept clean to avoid abrasion, so that it will not damage the oil seal and cause oil leakage.


◆29. Causes and improvement measures for slowed production cycle:
The production cycle is slowed due in large part to the prolonged cooling time, as well as the extended recycling time caused by screw wear. So, the production cycle can be quickened by improving mold cooling effect, shortening cooling time, and replacing the worn screw to reduce the recycling time.


◆30. Causes and solutions for squeaks in the melting process:
In the melting process, squeaks are usually caused by the friction between the screw and the grinder or between the screw and the barrel. To solve this problem, the screw can be polished or plated to make the surface smoother and thus reduce friction; or adjust the centrality of the screw to avoid friction with the barrel.


◆31. Causes and preventive measures for a broken tie bar:
A broken tie bar is mainly caused by the unevenness of the clamp. So, we can adjust the parallelism of the clamp to prevent the tie bar from being broken.


◆32. Causes and preventive measures for broken screw and nozzle tip:
The screw and nozzle tip is broken because the plastics have reached the melting temperature or there are metal blocks stuck in the barrel, causing the screw and nozzle tip to be broken under great pressure.


◆33. Analysis of cooler water leakage:
The coolant is too acidic or alkaline, which erodes the pipes, and thus causing cooler leakage.


◆34. Solutions for insufficient melting torque and injection pressure when using engineering plastics:
When using engineering plastics, insufficient melting torque can be solved by the application of a larger melting motor; and the insufficient injection pressure can be solved by the application of a smaller-diameter screw.


◆35. Causes and solutions for the too high temperature in the midsection of the barrel:
The temperature in the midsection of the barrel is a bit too high because the screw surface is not smooth enough, so the friction between the screw and the barrel will cause the temperature to rise. As a result, the screw surface should be polished or plated, and recycling back pressure reduced.


◆36. During the production process, the hydraulic toggle injection molding machine witnesses decreased the clamping force and loosened mold. The causes and solutions are:
This is mainly caused by toggle wear and clamp cylinder seal aging. In such circumstances, the copper bushing of the toggle, as well as the clamp cylinder seal needs to be replaced.


◆37. Lubricating oil VS grease:
Lubricating oil and grease are introduced to smoothen the joints of a machine. Compared with the grease, it is easier for the oil to reach the joints of a machine, while the grease is able to stay at the joints of a machine for a longer period of time.


◆38. Electronic ruler VS decoder:
Both the electronic ruler and the decoder are scales to measure machine movements – the electronic ruler is not as accurate as the decoder, but it is much more stable and the base point will not change. The decoder is more accurate than the electronic ruler, but it is not as stable – its base point changes easily.


◆39. Analysis of louder oil pump noise:
Oil pump noise becoming louder is mainly caused by oil pump wear, or pump bearing wear.


◆40. Cause analysis for frequent nozzle heater burnout under normal production conditions:
The wire is burnt out because the nozzle heater is not in good contact, or the heating wire doesn’t resist high temperature, so it is easily burnt out.


◆41. The oil cylinder fixing screw is frequently broken. The causes and solutions are:
The main cause is that the fixing screw is not strong enough, or the fixing screw is loosened due to vibration. So, this can be solved by choosing a stronger fixing screw; or when necessary, fix an anti-slip ring on the screw, so as to prevent it from vibrating loose.


◆42. Analysis of hydraulic oil whitening and deterioration:
The hydraulic oil is whitened because the oil is mixed with water, causing it to deteriorate.


◆43. Causes and solutions for product weight deviations:
Product weight increases mainly because the clamp of the injection molding machine is not tightened, or the clamping mechanism is worn, leading to excessive injection pressure or excessive back pressure which increases the product weight. Check whether the clamping mechanism is worn or whether the mold is finely adjusted, so as to reduce injection pressure or back pressure.


◆44. Causes and solutions for frequent mold opening failure:
When the mold fails to open, check whether the oil circuit is stuck or blocked and whether the clamping mechanism is worn flat which causes imbalance. If the mold is clamped too tight and for too long, opening failure will also occur.


◆45. The distinctive features of the thermoplastics and thermosetting plastics are:
When heated to the softening temperature, thermoplastics becomes the melting, which can be solidified and molded after being cooled, and this process can be repeatedly reversed. Thermosetting plastics are molded after being heated and retains the shape after cooling. The process is not reversible.


◆46. Why additives are added to plastics:
Additives are added to meet the needs of product features because additives are able to improve multiple product features and functions.


◆47. How to set clamping force:
The clamping force is set as per the projection area of the product * the pressure coefficient of the plastic material. Refer to the (Working Principles of an Injection Machine).


◆48. How to properly set the switchover position:
The right way would be switching to the holding pressure when the product is 98% injected.


◆49. What changes would occur to the plastics inside the screw when the screw rotates at a high or low speed:
When the screw rotates at a high speed, the plasticization level of the melting will be improved. High-speed rotation is able to increase melting shearing force, so the stickiness will be weakened, thus good for molding. However, friction heat will build up, thus often causing melting decomposition and temperature runaway. When the screw rotates at a low speed, the plasticization level of the melting will be reduced. However, the lower friction heat is good for plastic performances.