In the process of plastic injection molding, the adjustment of the process is the necessary way to improve the quality and output of products. Since the injection molding cycle itself is very short, if the process conditions are not well mastered, the scrap will be incessant. When adjusting the process, it is better to change one condition at a time and observe several times. If the pressure, temperature, and time are adjusted together, it is easy to cause confusion and misunderstanding, and we do not know what is the reason for the problem.
Translated with www.DeepL.com/Translator (free version)The measures and means to adjust the process are many. For example, there are more than ten possible solutions to solve the problem of insufficient product injection, to choose one or two major solutions to the problem, in order to really solve the problem. In addition, should also pay attention to the solution in the discriminatory relationship. For example, the product has a dent, sometimes to raise the material temperature, sometimes to lower the material temperature; sometimes to increase the amount of material, sometimes to reduce the amount of material. To recognize the feasibility of reverse measures to solve the problem.
What should I do if I encounter incomplete molding of plastic parts?
This is a frequently encountered problem, but also relatively easy to solve. When the process means really can not solve, can be considered from the mold design and manufacturing to improve, generally can be solved.
I. Equipment.
(1) injection molding machine plasticizing capacity is small.
When the product quality exceeds the actual maximum injection quality of the injection molding machine, it is obvious that the amount of material supply is not enough. If the product quality is close to the actual injection quality of the injection molding machine, there is a problem of insufficient plasticization, the material in the barrel of the heat time is not enough, as a result, can not provide the mold with the appropriate melt in a timely manner.
This situation can only be solved by replacing the injection molding machine with a larger capacity. Some plastics such as nylon (especially nylon 66) have a narrow melting range and a large specific heat, so it is necessary to use an injection molding machine with a large plasticizing capacity to ensure the supply of material.
(2) The temperature displayed by the thermometer is not true, the bright high and low, causing the material temperature to be too low.
This is due to temperature control devices such as thermocouples and their lines or temperature difference millivolt meter failure, or due to the aging or burning of the electric heating ring away from the temperature measurement point, heating failure and has not been found or not timely repair and replacement.
(3) nozzle bore diameter is too large or too small.
Too small, due to the small diameter of the circulation, the specific volume of the strip increases, easy to cold, blocking the feed channel or consumption of injection pressure; if too large, the circulation cross-sectional area is large, the pressure per unit area of plastic into the mold is low, the formation of small injection force conditions.
At the same time, non-Newtonian plastics such as ABS do not have access to large shear heat and cannot make the viscosity drop causing difficulties in filling the mold. Nozzle and the main channel inlet with poor, often occur outside the mold overflow material, the mold is not filled with the phenomenon. The nozzle itself has a high resistance to flow or is clogged with foreign matter, plastic carbonized deposits, etc.
Nozzle or the main channel inlet spherical damage, deformation, affecting the good cooperation with each other; Note seat mechanical failure or deviation, so that the nozzle and the main channel axis produce tilt displacement or axial compression surface detachment; nozzle ball diameter than the main channel inlet ball diameter, due to the edge of the gap, gradually increase the nozzle axial push open force under the overflow squeeze will cause the product injection is not enough.
(4) plastic lump blocking the feeding channel.
Due to plastic in the hopper dryer local melting lumps or barrel feeding section temperature is too high, or plastic grade selection is not appropriate, or plastic containing too much lubricant will make plastic in the entry
Inlet reduction position or screw start screw end of the deep groove prematurely melted, granules and molten material stick to each other to form a "bridge", blocking the channel or wrap the screw, with the screw rotation for the circumference of the slide, can not move forward, resulting in interruptions or irregular fluctuations in the supply. This situation can only be fundamentally solved after chiseling through the channel and excluding the material block.
(5) Nozzle cold material into the mold.
Injection molding machines are usually installed only straight-through nozzle to take into account the pressure loss. However, if the barrel front and nozzle temperature is too high, or in the high-pressure state of the barrel front storage too much material, resulting in "salivation", so that the plastic in the case of open injection and mold, accidentally preempted into the main channel entrance and hardened under the cooling effect of the template, and prevent the molten material to enter the cavity smoothly.
In this case, the temperature of the front end of the barrel and the nozzle should be reduced as well as the storage volume of the barrel, and the backpressure should be reduced to avoid excessive density of the molten material at the front end of the barrel.
(6) The injection cycle is too short.
Due to the short cycle, the material temperature will not be able to keep up with the shortage of material, especially when the voltage fluctuation is obvious. The cycle should be adjusted accordingly according to the supply voltage.
Adjustment generally does not consider the injection and holding time, the main consideration is to adjust the time from the completion of the holding pressure to the return of the screw, not only does not affect the mold filling molding conditions but also to extend or shorten the preheating time of the material in the barrel.
Second, mold aspects.
(1) The mold casting system has defects.
The runner is too small, too thin or too long, which increases the fluid resistance. The main runner should increase its diameter, and the runner and manifold should cause round better.
The runner or more mouth is too big and the shooting force is not enough; the runner and gate are blocked by impurities, foreign matter or carbonide; the runner and gate are rough and scarred or have sharp angle and bad surface roughness, which affects the material flow not smoothly; the runner does not open the cold material well or the cold material well is too small and the opening direction is not right.
For multi-cavity molds, the balance of runner and gate size distribution should be carefully arranged, otherwise only the cavities near the main flow channel or the cavities with thick and short gates can be filled while other cavities cannot be filled. Should be appropriate to thicken the diameter of the runner, so that the flow to the end of the runner to reduce the pressure drop of the melt, but also to increase the gate far from the mainstream cavity, so that the injection pressure and material flow speed of each cavity is basically the same.
(2) The mold design is not reasonable.
Overly complex mold, more turns, improper selection of inlet, too narrow runner, insufficient number of gates or improper form; products with thin local section, should increase the thickness of the whole product or local, or set auxiliary runner or gate near the underfilled place.
It is not uncommon to see inadequate exhaust measures in the mold cavity causing dissatisfaction of the part. Most of these defects occur at the turns of the product, at deep depressions, at thin-walled parts surrounded by thick-walled parts and at the bottom of thin bottom shells molded with side gates. The design to eliminate this defect includes opening effective air venting orifices, choosing a reasonable gate location so that air can be easily discharged in advance and, if necessary, deliberately making an insert in a part of the cavity where air is trapped so that air escapes through the insert gap.
For multi-cavity molds are prone to unbalanced gate distribution, if necessary, the number of injection cavities should be reduced to ensure that other cavities are qualified.
third, the process.
(1) Improper adjustment of feeding, lack of material or more material
Inaccurate metering or charging control system operation is not normal, injection molding machine or mold or operating conditions lead to injection cycle anomalies, pre-plastic backpressure is small or the barrel of the material grain density is small may cause a lack of material, for large particles, more gaps in the grain material and crystalline plastic such as polyethylene, polypropylene, nylon, etc. and viscosity of plastics such as ABS should be adjusted to a higher amount of material, the material temperature should be adjusted to a higher amount. When the material temperature is high, the amount of material should be adjusted to a larger amount.
When the end of the barrel stock has too much material, an injection screw consume additional injection pressure to compress, push the barrel of the excess stock, which greatly reduces the effective injection pressure of plastic into the cavity and makes the product difficult to fill.
(2) The injection pressure is too low, the injection time is short, and the plunger or screw is returned too early.
The molten plastic has high viscosity and poor fluidity at low working temperatures, so it should be injected with higher pressure and speed. For example, when making ABS color parts, the colorant's intolerance of high temperature limits the barrel's heating temperature, which should be compensated by a higher injection pressure and longer injection time than usual.
(3) Slow injection speed
Injection speed for some complex shapes, thickness changes, long process products, as well as the viscosity of plastics such as toughness ABS, etc. has a very prominent significance. When the use of high pressure can not be filled with products, should consider the use of high-speed injection in order to overcome the problem of insufficient injection.
(4) The material temperature is too low
The front end of the barrel temperature is low, the melt into the cavity due to the cooling effect of the mold and the viscosity of the premature rise to the point where it is difficult to flow, preventing the distal end of the filling; barrel temperature is low, the viscosity of the plastic flow difficulties, hindering the forward movement of the screw, resulting in seemingly sufficient pressure gauge pressure and the actual melt into the cavity at low pressure and low speed.
Low nozzle temperature may be fixed when the nozzle is in contact with the cold mold for a long time to dissipate heat, or nozzle heating ring heating or poor contact caused by low material temperature, may block the mold feed channel; if the mold without cold material well, with self-locking nozzle, the use of post-feeding procedures, the nozzle can maintain the necessary temperature; just start the nozzle is too cold sometimes you can use a flame gun to do external heating to accelerate the nozzle warming.
Fourth, the raw materials.
(1) poor plastic flow
Plastic plants often use recycled shredded material, and recycled shredded material will often reflect the tendency to increase viscosity. Experiments point out that: due to oxidative cleavage generated by the molecular chain brake unit volume density increased, which increases the viscosity of the flow in the barrel and cavity, regeneration of shredded material to promote the generation of more gaseous substances, so that the injection pressure loss increased, resulting in mold filling difficulties.
In order to improve the flowability of plastics, the addition of external lubricants such as stearic acid or its salts should be considered, preferably with silicone oil (viscosity 300~600cm2/s). The addition of lubricant not only improves the fluidity of plastic but also improves the stability and reduces the gas resistance of gaseous substances.