During the injection molding process, defects such as insufficient injection, dents, burrs, bubbles, cracks, warping deformation, and dimensional changes may often occur in the product due to poor raw material handling, unreasonable product or mold design, operator failure to master appropriate process operating conditions, or mechanical reasons.

There are three main aspects to evaluate plastic products. The first is the appearance quality, including integrity, color, gloss, etc; The second is the accuracy between size and relative position; The third is the mechanical, chemical, and electrical properties corresponding to the purpose. These quality requirements vary depending on the usage occasion of the product and the required scale.

Production practice has proven that the defects of products mainly lie in the design, manufacturing accuracy, and wear degree of the mold. But in fact, the technical personnel of plastic processing plants often face the difficult situation of using process methods to compensate for the problems caused by mold defects without achieving much success.

The adjustment of processes during the production process is a necessary way to improve product quality and output. Due to the short injection molding cycle itself, if the process conditions are not well controlled, waste products will continue to flow in endlessly. When adjusting the process, it is best to only change one condition at a time and observe multiple times. If the pressure, temperature, and time are all adjusted together, it is easy to cause confusion and misunderstandings, and if there is a problem, the reason is unknown. The measures and means for adjusting the process are multifaceted. For example, there are more than ten possible solutions to solve the problem of insufficient product quality. To truly solve the problem, it is necessary to choose one or two main solutions to solve the problem. In addition, attention should also be paid to the dialectical relationship in the solution. For example, if the product has a depression, sometimes the material temperature needs to be raised, and sometimes the material temperature needs to be lowered; Sometimes it is necessary to increase the material quantity, and sometimes it is necessary to reduce the material quantity. To acknowledge the feasibility of reverse measures in solving problems.

1.6.1 Incomplete plastic molding

This is a frequently encountered problem, but it is also relatively easy to solve. When the problem cannot be solved through technological means, improvements can be considered from the perspective of mold design and manufacturing, which can generally be solved.

1、 In terms of equipment:

(1) The injection molding machine has a small plasticizing capacity. When the quality of the product exceeds the actual maximum injection quality of the injection molding machine, it is obvious that the material supply cannot make ends meet. If the quality of the product is close to the actual injection quality of the injection molding machine, there is a problem of insufficient plasticization. The material is not heated enough in the barrel, resulting in the inability to provide appropriate molten material to the mold 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 high specific heat, requiring injection molding machines with large plasticizing capacity to ensure material supply.

(2) The temperature displayed by the thermometer is not true, indicating high but actually low, causing the material temperature to be too low. This is due to the malfunction of temperature control devices such as thermocouples and their circuits or temperature difference millivolt meters, or due to the aging or burning of electric heating coils far from the temperature measurement point, heating failure that has not been detected or repaired or replaced in a timely manner.

(3) The diameter of the nozzle inner hole is too large or too small. If it is too small, the specific volume of the material bar increases due to the small flow diameter, which can easily cause cooling, block the feeding channel, or consume injection pressure; If it is too large, the cross-sectional area of flow is large, and the pressure per unit area of plastic injection into the mold is low, resulting in a situation of low injection force. At the same time, non Newtonian plastics such as ABS cannot reduce viscosity due to the lack of significant shear heat, resulting in difficulty in mold filling. Poor coordination between the nozzle and the inlet of the main channel often leads to overflow outside the mold and insufficient filling inside the mold. The nozzle itself has high flow resistance or is blocked by foreign objects, plastic carbonization deposits, etc; The spherical surface of the nozzle or main channel inlet is damaged or deformed, which affects the good coordination with the other side; Mechanical failure or deviation of the injection seat, causing tilting displacement or detachment of the axial compression surface between the nozzle and the main channel axis; The diameter of the nozzle ball is larger than that of the inlet ball of the main channel. Due to the appearance of gaps at the edges, gradually increasing the axial pushing force of the nozzle under the pressure of overflow can cause insufficient injection of the product.

(4) Plastic frits block the feeding channel. Due to local melting and agglomeration of plastic in the hopper dryer, excessive temperature in the feeding section of the machine barrel, improper selection of plastic grade, or excessive lubricant contained in the plastic, it can cause premature melting of plastic in the position of reducing the diameter of the feeding port or deep groove at the screw starting end. The particles and molten materials bond with each other to form a "bridge", blocking the channel or wrapping the screw, and rotating with the screw to slide in a circular manner without moving forward, Causing supply interruption or irregular fluctuations. This situation can only be fundamentally resolved after drilling the channel and removing the material blocks.

(5) The nozzle cools the material into the mold. Injection molding machines are usually only equipped with straight through nozzles due to pressure loss considerations. However, if the temperature of the front end of the barrel and the nozzle is too high, or there is too much material stored in the front end of the barrel under high pressure, it will produce "salivation", causing the plastic to unexpectedly enter the main channel inlet before starting injection and the mold to open, and become hard under the cooling effect of the template, thereby hindering the smooth entry of the molten material into the mold cavity. At this point, the temperature of the front end of the barrel and the nozzle should be lowered, as well as the storage capacity of the barrel should be reduced, and the back pressure should be reduced to avoid excessive melt density at the front end of the barrel.

(6) The injection molding cycle is too short. Due to the short cycle, not being able to keep up with the material temperature can also cause a shortage of materials, especially when the voltage fluctuates greatly. Adjust the cycle accordingly based on the power supply voltage. When adjusting, the injection and holding time are generally not considered, and the main consideration is to adjust the time from the completion of holding to the return of the screw, which does not affect the filling and molding conditions, and can also extend or shorten the preheating time of the material particles in the machine barrel.

2、 Mold

(1) There is a defect in the mold pouring system. The flow channel is too small, too thin, or too long, which increases fluid resistance. The diameter of the main flow channel should be increased, and the flow channel and diversion channel should be made circular. The flow channel or nozzle is too large, resulting in insufficient ejection force; There are impurities, foreign objects or carbonized substances blocking the flow channel and gate; The runner and gate are rough with scratches or sharp corners, and the surface roughness is poor, which affects the smooth material flow; The flow channel is not equipped with a cold material well or the cold material well is too small, and the direction of opening is incorrect; For multi cavity molds, it is necessary to carefully arrange the balanced distribution of runner and gate sizes, otherwise there may be situations where only the cavities near the main runner or with thick and short gates can be filled while other cavities cannot be filled. The diameter of the flow channel should be appropriately enlarged to reduce the pressure drop of the molten material flowing to the end of the channel. It is also necessary to increase the gate of the cavity far from the main channel, so that the injection pressure and material flow speed of each cavity are basically consistent.

(2) Unreasonable mold design. The mold is excessively complex, with many transitions, improper selection of feed ports, narrow flow channels, insufficient number or improper form of gates; The local section of the product is very thin, and the thickness of the entire product or local area should be increased, or auxiliary runners or gates should be set up near the insufficient filling area; It is not uncommon for inadequate exhaust measures in the mold cavity to cause dissatisfaction with the product. These defects mostly occur at the corners, deep depressions, thin-walled parts surrounded by thick walls, and the bottom of thin bottom shells formed with side gates. The design to eliminate this defect includes opening an effective exhaust duct, selecting a reasonable gate position to make it easy for air to be discharged in advance, and if necessary, deliberately making a part of the trapped area of the mold cavity into an insert to allow air to overflow from the gap of the insert; For multi cavity molds, it is easy to have imbalanced gate distribution. If necessary, the number of injection cavities should be reduced to ensure that other cavity parts are qualified.

3、 Process aspect

(1) Improper adjustment of feed, shortage or excess of material. Inaccurate feeding measurement or abnormal operation of the feeding control system, abnormal injection cycle caused by injection molding machine or mold or operating conditions, small pre molding back pressure, or low particle density inside the machine barrel can all cause material shortage. For large particles, porous particles, and plastics with large changes in crystallinity, such as polyethylene, polypropylene, nylon, and plastics with high viscosity, such as ABS, the material quantity should be adjusted higher. When the material temperature is high, the material quantity should be increased.

When there is too much material stored at the end of the barrel, the screw needs to consume additional injection pressure during injection to compress and push the excess material stored inside the barrel, which greatly reduces the effective injection pressure of plastic entering the mold cavity and makes it difficult for the product to fill.

(2) The injection pressure is too low, the injection time is short, and the plunger or screw returns too early. Molten plastic has a high viscosity and poor flowability at low working temperatures, and should be injected at high pressure and speed. For example, when making ABS colored parts, the lack of high-temperature resistance of colorants limits the heating temperature of the machine barrel, which needs to be compensated for by higher injection pressure and longer injection time than usual.

(3) The injection speed is slow. The injection speed is of great significance for some products with complex shapes, large thickness changes, and long processes, as well as for plastics with high viscosity such as toughened ABS. When high-pressure injection cannot fully fill the product, high-speed injection should be considered to overcome the problem of insufficient injection.

(4) The material temperature is too low. The temperature at the front end of the barrel is low, and the viscosity of the molten material entering the mold cavity increases prematurely to the point where it is difficult to flow due to the cooling effect of the mold, which hinders the filling of the far end of the mold; The low temperature and high viscosity of the plastic in the rear section of the barrel make it difficult to flow, hindering the forward movement of the screw. As a result, it appears that the pressure displayed on the pressure gauge is sufficient, but in reality, the molten material enters the mold cavity at low pressure and low speed; If the temperature of the nozzle is low, it may be due to the prolonged contact between the nozzle and the cold mold during fixed feeding, or the insufficient heating or poor contact of the nozzle heating ring causing low material temperature, which may block the feeding channel of the mold; If the mold does not have a cold well, use a self-locking nozzle and use a post feeding program to maintain the necessary temperature; When the nozzle is too cold at the beginning of startup, sometimes a flame gun can be used for external heating to accelerate the temperature rise of the nozzle.

Four aspects of raw materials

Plastic has poor fluidity. Plastic factories often use recycled crushed materials, which often reflect a tendency to increase viscosity. The experiment pointed out that due to the increase in the unit volume density of molecular chain breaks generated by oxidative cracking, the viscosity of the flow in the barrel and mold cavity is increased, and the regenerated crushed material promotes the production of more gaseous substances, resulting in an increase in injection pressure loss and difficulty in mold filling. In order to improve the fluidity of plastics, external lubricants such as stearic acid or its salts should be considered, preferably silicone oil (viscosity 300-600cm2/s). The addition of lubricants not only improves the fluidity of plastics, but also enhances stability, reducing the gas resistance of gaseous substances.

1.6.2 Overflow (flying edge)

Overflow, also known as flash, overflow, or edge, mostly occurs at the parting and closing positions of the mold, such as the parting surface of the mold, the sliding parts of the slider, the gaps in the inserts, and the pores in the top rod. If the overflow is not resolved in a timely manner, it will further expand, leading to local collapse of the embossing mold and causing permanent damage. The overflow of the gaps in the insert and the holes in the top rod can also cause the product to get stuck on the mold, affecting demolding.

In terms of equipment

(1) The actual clamping force of the machine is insufficient. When selecting an injection molding machine, the rated clamping force of the machine must be higher than the tension formed by the longitudinal projected area of the injection molded product during injection, otherwise it will cause mold expansion and flash.

(2) The adjustment of the clamping device is poor, the elbow mechanism is not straight, resulting in uneven clamping of the left, right, or up and down molds, and the parallelism of the mold cannot be achieved. This causes one side of the mold to be tightly closed while the other side is not tightly attached, resulting in flash during injection.

(3) The poor parallelism of the mold itself, or the installation being not parallel, or the template being not parallel, or the uneven distribution of force and deformation of the pull rod, all of which will cause the mold to not close tightly and produce flash.

(4) Severe wear of the check ring; Spring nozzle spring failure; Excessive wear of the material barrel or screw; The failure of the cooling system at the inlet leads to the phenomenon of "bridging"; Insufficient injection amount of the machine barrel setting, too small buffer pads, etc. may cause repeated flash occurrence, and it is necessary to repair or replace accessories in a timely manner.

II. Mould

(1) The accuracy of the mold parting surface is poor. Deformation and warping of activity templates (such as middle plates); There are foreign objects on the parting surface or protruding skid marks and burrs around the mold frame; The old mold suffered fatigue collapse around the cavity due to earlier flash extrusion.

(2) Unreasonable mold design. The opening position of the mold cavity is too deviated, which can cause tension on one side of the mold during injection and cause flash; Plastics with good flowability, such as polyethylene, polypropylene, nylon, etc., have low viscosity in the molten state and are easy to enter movable or fixed gaps, requiring high manufacturing accuracy of molds; On the premise of not affecting the integrity of the product, it should be placed as much as possible on the center of mass symmetry, and the material should be fed into the thick parts of the product to prevent the situation of missing materials and flying edges at the same time;

When there are forming holes in or near the center of the product, it is customary to set up side gates on the holes. Under high injection pressure, if the clamping force is insufficient and the supporting force of this part of the mold is insufficient, slight warping may occur, resulting in flash. For example, when there are movable components on the side of the mold, the projected area on the side is also affected by the forming pressure. If the supporting force is insufficient, it may also cause flash; Poor fitting accuracy of sliding core or deviation of fixed core and cavity installation position resulting in flash; Poor exhaust of the mold cavity, failure to open exhaust channels on the parting surface of the mold, shallow or deep exhaust channels, or blockage by foreign objects will cause flash; For multi cavity molds, attention should be paid to the reasonable design of the flow divider and gate, otherwise it will cause uneven filling force and flash.

Three process aspects

(1) Injection pressure is too high or injection speed is too fast. Due to high pressure and high speed, the opening force of the mold increases, resulting in material overflow. The injection speed and injection time should be adjusted based on the thickness of the product. Thin products should be quickly filled with high speed and no longer injected after being filled; Thick products should be molded at low speed and the skin should be roughly fixed before reaching the final pressure.

(2) Excessive feeding caused flash. It is worth noting not to inject too much molten material to prevent dents, as dents may not necessarily be "filled out" and flash edges may appear. This situation should be solved by extending the injection time or holding time.

(3) If the temperature of the barrel or nozzle is too high or the temperature of the mold is too high, the viscosity of the plastic will decrease, the fluidity will increase, and flash will be caused when the mold is smoothly inserted.

Four aspects of raw materials

(1) If the viscosity of plastic is too high or too low, flash may occur. Low viscosity plastics such as nylon, polyethylene, polypropylene, etc. should increase the clamping force; Plastics with strong water absorption or sensitive to water can significantly reduce flow viscosity at high temperatures, increasing the possibility of flash. These plastics must be thoroughly dried; If too much recycled material is added to the plastic, its viscosity will also decrease, and if necessary, residual components should be supplemented. If the viscosity of plastic is too high, the flow resistance increases, resulting in a large back pressure that increases the pressure in the mold cavity, resulting in insufficient clamping force and flash.

(2) When the particle size of plastic raw materials is uneven, the amount of material added may vary, resulting in incomplete parts or flash.

1.6.3 Dents (collapses, indentations)

Shrinkage and depression caused by plastic cooling and hardening mainly occur in thick wall positions, ribs, casings, and the back of nut inserts.

In terms of equipment

(1) Insufficient supply of materials. The screw or plunger is severely worn, causing leakage of molten material during injection and pressure maintaining, reducing the filling pressure and material volume, resulting in insufficient molten material.

(2) The nozzle hole is too large or too small. If it is too small, it is easy to block the feeding channel, while if it is too large, it will reduce the injection force and make it difficult to fill the mold.

II. Mould

(1) If the gate is too small or the runner is too narrow or too shallow, the runner efficiency is low, the resistance is high, and the molten material cools too early. The gate should not be too large, otherwise the shear rate will be lost and the viscosity of the material will be high, which cannot make the product full. The gate should be set on the thick wall of the product. Setting up necessary cold material wells with sufficient capacity in the flow channel can exclude cold material from entering the mold cavity and ensure continuous mold filling. The gate length of point gate and needle gate must be controlled below 1mm, otherwise the plastic will solidify quickly at the gate and affect pressure transmission; If necessary, the number or location of point gates can be increased to meet actual needs; When the flow channel is long and thick, exhaust grooves should be set at the edge of the channel to reduce the blocking effect of air on the material flow.

(2) To adjust the filling speed of each gate in a multi gate mold, it is best to symmetrically open the gate.

(3) The key parts of the mold should be effectively equipped with cooling channels to ensure that the cooling of the mold has a good effect on eliminating or reducing shrinkage.

(4) The entire mold should be free of burrs and have reliable mold sealing, capable of withstanding high pressure, high speed, and low viscosity melt filling.

Three process aspects

(1) Increase injection pressure, maintain pressure, and extend injection time. For plastics with high fluidity, high pressure can cause flash and collapse. The material temperature should be appropriately reduced, including the temperature of the front section of the barrel and nozzle, to reduce the volume change of the molten material entering the mold cavity and facilitate cold solidification; For high viscosity plastics, the barrel temperature should be increased to make mold filling easier. When shrinkage occurs in the gate area, the holding time should be extended.

(2) Increasing the injection speed can conveniently fill the workpiece and eliminate most of the shrinkage.

(3) Thin walled parts should increase the mold temperature to ensure smooth material flow; Thick walled parts should reduce mold temperature to accelerate the solidification and shaping of the skin.

(4) Extending the cooling retention time of the workpiece in the mold, maintaining a uniform production cycle, increasing back pressure, and retaining a certain buffer pad in the front section of the screw are all beneficial for reducing shrinkage.

(5) Low precision products should be molded early and allowed to cool slowly in air or hot water to ensure smooth shrinkage and indentation without affecting use.

In terms of four raw materials: If the raw materials are too soft, it is easy to cause dents. An effective method is to add nucleating agents to the plastic to accelerate crystallization.

In terms of product design: The product design should ensure uniform wall thickness and avoid changes in wall thickness as much as possible. For plastics with large shrinkage such as polypropylene, when the thickness change exceeds 50%, it is best to replace the thickened parts with ribs.

1.6.4 Silver lines, bubbles, and pores

During the molding process, plastic is often disturbed by gas, resulting in silver streaks or small bubbles on the surface of the product or the formation of bubbles within the thick walls of the product. The main source of these gases is the presence of moisture or volatile substances in the raw materials or excessive lubricants, or it may be due to the degradation of plastic due to high material temperature and prolonged heating time, resulting in the generation of pyrolysis gases.

On the one hand, in terms of equipment, the nozzle hole is too small, materials are salivating or drawing at the nozzle, there are obstacles or burrs in the barrel or nozzle, and friction heat is generated when high-speed material flow passes through, causing material decomposition.

II. Mould:

(1) Due to design defects such as poor gate position, too small gate, asymmetric gate layout for multi gate components, small flow channels, and unreasonable mold cooling system causing significant temperature differences, the flow of molten material in the mold cavity is discontinuous, blocking the air passage.

(2) The mold parting surface lacks necessary exhaust channels, or the exhaust channels are insufficient, blocked, or poorly positioned, and there are no machining gaps such as inserts or thimbles for exhaust, resulting in the air in the mold cavity not being able to leave at the same time as plastic enters.

(3) The surface roughness of the mold is poor, and the frictional resistance is high, causing local overheating and causing the plastic passing through to decompose.

Three process aspects

(1) The material temperature is too high, causing decomposition. If the barrel temperature is too high or the heating is out of balance, the barrel temperature should be reduced step by step. The temperature of the feeding section is too high, causing a portion of plastic to melt prematurely and fill the screw groove, making it impossible for air to be discharged from the feeding port.

(2) The injection pressure is low and the holding time is short, making the molten material and the surface of the mold cavity not closely attached.

(3) The injection speed is too fast, causing the molten plastic to decompose under the action of large shear, producing decomposition gas; The injection speed is too slow to fill the mold cavity in a timely manner, resulting in insufficient surface density and silver lines on the product.

(4) Insufficient material quantity, excessive feeding buffer pad, low material temperature or mold temperature can all affect the flow of molten material and forming pressure, resulting in bubbles.

(5) Using multi-stage injection to reduce silver streaks: medium speed injection to fill the runner → slow speed filling of the gate → fast injection → low pressure slow speed filling of the mold, so that the gas in the mold can be promptly eliminated in each section.

(6) When the screw is pre molded, the back pressure is too low and the rotation speed is too high, which causes the screw to retract too quickly, making it easy for air to be pushed towards the front end of the barrel along with the material.

Four aspects of raw materials

(1) Mixing different types of plastic or a large amount of powder into the raw material can easily entrain air during melting, and sometimes silver lines may appear. When the raw materials are contaminated or contain harmful chips, they are prone to thermal decomposition.

(2) The structure of the recycled material particles is loose, and there is a large amount of air stored in the micropores; The regeneration frequency of recycled materials is too high or the proportion with new materials is too high (generally less than 20%)

(3) The raw materials contain volatile solvents or the liquid additives in the raw materials, such as dyeing aid white oil, lubricant silicone oil, plasticizer dibutyl ester, stabilizer, antistatic agent, etc., are too much or mixed unevenly, and enter the cavity in an accumulated state to form crazes.

(4) Plastic is not dried or absorbs moisture from the atmosphere. The raw materials should be thoroughly dried and a drying hopper should be used.

(5) Some brands of plastic cannot withstand higher temperatures or longer heating times themselves. Especially when it contains trace amounts of water, catalytic cracking reactions may occur. For this type of plastic, it is necessary to consider adding external lubricants such as stearic acid and its salts (up to 50g per 10kg of material) to minimize its processing temperature.

In terms of product design, if the wall thickness is too thick, the cooling speed inside and outside the watch may vary. During mold manufacturing, the size of the main flow channel, splitter channel, and gate should be appropriately increased.

1.6.5 Weld marks

When molten plastic meets multiple strands in the mold cavity due to encountering inserts, holes, areas with discontinuous flow rates, areas with interrupted filling material flow, and when gate injection filling occurs, linear fusion marks are generated due to incomplete fusion. The presence of fusion marks greatly weakens the mechanical strength of the product. The method of overcoming welding marks is basically the same as the method of reducing product depressions.

In terms of equipment, poor plasticization and uneven melt temperature can prolong the molding cycle, making plasticization more complete. If necessary, replace the machine with a large plasticization capacity.

II. Mould

(1) If the mold temperature is too low, the mold temperature should be appropriately increased or the local temperature at the fusion joint should be purposefully increased.

(2) The flow channel is small, too narrow or too shallow, and the cold material well is small. The size of the flow channel should be increased to improve its efficiency, while also increasing the volume of the cold material well.

(3) Expand or reduce the gate section and change the gate position. The opening of the gate should try to avoid the flow of melt around the inserts and holes. Efforts should be made to correct, relocate, or buffer the gate that undergoes injection molding. Try to avoid or minimize the use of multiple gates.

(4) Poor exhaust or lack of exhaust holes. Exhaust channels should be opened, expanded or unblocked, including the use of inserts and ejector pin gaps for exhaust.

Three process aspects

(1) Increase injection pressure and extend injection time.

(2) Adjust the injection speed: High speed allows the molten material to reach the confluence before it can cool down, while low speed allows time for the air in the mold cavity to be discharged.

(3) Adjust the temperature of the machine barrel and nozzle: high temperature reduces the viscosity of plastic, smooth flow, and fine welding marks; Low temperature reduces the decomposition of gaseous substances.

(4) Release agents should be used as little as possible, especially those containing silicon, otherwise the material flow may not merge.

(5) Reduce the clamping force to facilitate exhaust.

(6) Increase the screw speed to reduce the viscosity of the plastic; Increase back pressure to increase plastic density.

Four aspects of raw materials

(1) The raw materials should be dry and the liquid additives in the formula should be minimized as much as possible.

(2) Add lubricants and stabilizers appropriately to plastics with poor flowability or high heat sensitivity, and if necessary, switch to plastics with good flowability or high heat resistance.

In terms of product design

(1) The wall thickness is small, and the parts should be thickened to avoid premature solidification.

(2) The position of the insert is incorrect and should be adjusted accordingly.

1.6.6 Brittleness

The brittleness of products is largely caused by internal stress. There are many reasons for product brittleness, mainly including:

In terms of equipment

(1) There are dead corners or obstacles inside the barrel, which can easily promote the degradation of molten materials.

(2) The plasticizing capacity of the machine is too small, and the plastic is not fully plasticized in the machine barrel; The plasticizing capacity of the machine is too large, and the plastic is subjected to heat and shear in the machine barrel for too long, which can easily age and make the product brittle.

(3) The ejection device is tilted or unbalanced, with a small cross-sectional area or improper distribution of the top stem.

II. Mould

(1) The gate is too small, it should be considered to adjust the gate size or add an auxiliary gate.

(2) If the diversion channel is too small or improperly configured, it should be arranged as balanced and reasonable as possible or the size of the diversion channel should be increased.

(3) Poor mold structure causes abnormal injection molding cycles.

Three process aspects

(1) The temperature of the barrel and nozzle is too low, please raise it. If the material is prone to degradation, the temperature of the barrel and nozzle should be increased.

(2) Reduce the back pressure and rotation speed of the screw pre molding to slightly loosen the material and reduce the degradation of the plastic due to shear overheating.

(3) The mold temperature is too high, making it difficult to demould; If the mold temperature is too low, the plastic will cool too early, and the fusion of fusion joints will be poor, making it easy to crack, especially for high melting point plastics such as polycarbonate.

(4) The cavity and core should have an appropriate demolding angle. When the core is difficult to demould, it is necessary to increase the cavity temperature and shorten the cooling time; When the mold cavity is difficult to detach, it is necessary to lower the mold cavity temperature and extend the cooling time.

(5) Try to minimize the use of metal inserts, and brittle plastics such as polystyrene with high cold and hot specific capacities should not be added to insert injection molding.

Four aspects of raw materials

(1) When the raw material is mixed with other impurities or doped with inappropriate or excessive solvents or other additives.

(2) Some plastics, such as ABS, can undergo catalytic cracking reactions with water vapor when heated under damp conditions, causing significant strain on the parts.

(3) Too many times of plastic recycling, too high content of recycled materials, or too long heating time in the machine barrel can promote brittle cracking of the parts.

(4) Plastic itself has poor quality, such as a large molecular weight distribution and excessive occupancy of components with uneven structures such as rigid molecular chains; Or being contaminated by other plastic additives, harmful additives, dust impurities, etc. is also the cause of brittleness.

In terms of product design

(1) The product has sharp corners, notches, or areas with significant thickness differences that are prone to stress cracking.

(2) The product design is too thin or has too many hollows.

1.6.7 Discoloration

The reasons for discoloration are also multifaceted, mainly including:

In terms of equipment

(1) The equipment is not clean. Dust or other dust deposits on the hopper, causing the material to become contaminated and discolored.

(2) Thermocouples, temperature controllers, or heating systems malfunction due to misalignment.

(3) There are obstacles in the barrel that can easily promote plastic degradation; There are metal foreign objects stuck in the barrel or screw groove, and continuous grinding causes discoloration of the plastic.

II. Mould

(1) The mold exhaust is poor, and the plastic is adiabatic and compressed, which reacts violently with oxygen under high temperature and pressure, burning the plastic.

(2) The mold gate is too small.

(3) There are too many lubricants and release agents in the material or mold. If necessary, the material barrel should be cleaned regularly to remove additives such as anti-static properties that are worse than plastic's heat resistance.

(4) The size of the nozzle hole, main flow channel, and diversion channel is too small.

Three process aspects

(1) The screw speed is too high and the pre molded back pressure is too high.

(2) The temperature of the barrel and nozzle is too high.

(3) The injection pressure is too high, the time is too long, and the injection speed is too fast, which causes discoloration of the product.

Four aspects of raw materials

(1) The material is contaminated.

(2) High moisture and volatile content.

(3) Decomposition of colorants and additives.

1.6.8 Black spots or black liquid

The main reasons for this defect are in terms of equipment and raw materials:

In terms of equipment

(1) There is burnt black material in the barrel.

(2) The barrel has cracks.

(3) Worn screw or plunger.

(4) The area near the hopper is not clean.

II. Mould

(1) There is oil in the mold cavity.

(2) Oil seeps into the ejection device.

In terms of three raw materials:

(1) The raw materials are not clean.

(2) Insufficient lubricant.

1.6.9 Burned Dark Lines

In terms of equipment:

After injecting thermosensitive plastic, the machine barrel was not cleaned thoroughly or there was a material pad at the nozzle, which caused poor exhaust at the beginning of the injection.

II. Mould:

(1) Poor exhaust.

(2) The gate is small or improperly positioned.

(3) The local resistance in the cavity is high, causing slow convergence of material flow and making it difficult to exhaust.

In terms of three processes:

(1) The temperature of the barrel and nozzle is too high.

(2) Injection pressure or pre molded back pressure is too high.

(3) The injection speed is too fast or the injection cycle is too long.

In terms of four raw materials:

(1) The particles are uneven and contain powder.

(2) The volatile content in the raw materials is high.

(3) Excessive use of lubricants and release agents.

1.6.10 Poor gloss

In terms of equipment:

(1) Insufficient supply of materials.

(2) The cylinder was not cleaned thoroughly during refueling.

II. Mould:

(1) The gate is too small or the runner is too thin.

(2) The surface roughness of the cavity is poor.

(3) Poor exhaust or low mold temperature.

(4) There is no cold well.

In terms of three processes:

(1) Uneven heating of the barrel, high or low temperature of the barrel.

(2) The nozzle is too small or the pre molded back pressure is too low.

(3) Injection speed too high or too low.

(4) Uneven plasticization.

In terms of four raw materials:

(1) The raw material has not been dried.

(2) Contains volatile substances.

(3) Excessive use of additives or release agents.

1.6.11 Difficulty in demolding (gate or plastic part compressed inside the mold)

On the one hand, in terms of equipment: insufficient jacking force.

II. Mould:

(1) Unreasonable demolding structure or improper position.

(2) Insufficient demolding slope.

(3) Excessive mold temperature or poor ventilation.

(4) The surface of the sprue wall or cavity is rough.

(5) The nozzle does not fit well with the mold feed port or the nozzle diameter is greater than the feed port diameter.

In terms of three processes:

(1) The barrel temperature is too high or there is too much injection volume.

(2) The injection pressure is too high or the holding and cooling times are long.

In terms of four raw materials: insufficient lubricant.

1.6.12 Warping deformation

I Mould:

(1) Improper gate position or insufficient quantity.

(2) Improper ejection position or uneven force on the product.

In terms of technology:

(1) The temperature of the mold and barrel is too high.

(2) The injection pressure is too high or the injection speed is too fast.

(3) The holding time is too long or the cooling time is too short.

In terms of three raw materials: phthalocyanine pigments can affect the crystallinity of polyethylene and lead to product damage

Deformation.

In terms of product design:

(1) Uneven wall thickness, sudden changes or too small wall thickness.

(2) Improper structural design of the product.

1.6.13 Dimensional instability

In terms of equipment:

(1) The feeding system is not functioning properly.

(2) Unstable back pressure or unstable temperature control.

(3) The hydraulic system has malfunctioned.

II. Mould:

(1) Uneven size of gates and runners.

(2) The size of the cavity is not accurate.

In terms of three processes:

(1) Unreasonable mold temperature control due to uneven mold temperature or improper cooling circuit.

(2) Low injection pressure.

(3) Insufficient or fluctuating injection pressure holding time.

(4) High barrel temperature or unstable injection cycle.

In terms of four raw materials:

(1) During batch production, there are changes in resin properties.

(2) The particle size of the material is irregular.

(3) High moisture content.

(4) Replacing additives has an impact on the shrinkage law.

1.6.14 Cracking vapor white

On the one hand, the ejection mechanism is poor.

In terms of technology:

(1) The barrel temperature is low or the mold temperature is low.

(2) Injection pressure is high.

(3) Long holding time.

In terms of three raw materials:

(1) Improper or excessive use of lubricants or release agents.

(2) Brand and grade not applicable.

In terms of product design, unreasonable product design leads to local stress concentration.

1.6.15 Layered Peeling

In terms of technology:

(1) The temperature of the barrel and nozzle is low.

(2) Low back pressure.

(3) For PVC plastics, excessive injection speed or low mold temperature may also cause delamination and peeling.

In terms of raw materials:

(1) Raw materials contaminated or mixed with foreign objects.

(2) Different plastics are mixed.

1.6.16 Swelling and Bubbling

Some plastic products quickly experience swelling and bubbling on the back of metal inserts or in particularly thick areas after molding and demolding. This is due to the gas expansion released by the plastic that has not fully cooled and hardened under internal pressure.

Solution:

(1) Reduce mold temperature and extend mold opening time.

(2) Reduce the drying temperature and processing temperature of the material; Reduce the filling rate; Reduce the molding cycle; Reduce flow resistance.

(3) Increase the holding pressure and time.

(4) Improve the situation where the wall surface of the product is too thick or has significant changes in thickness.

1.6.17 Slow production

(1) Plastic has a high temperature and a long cooling time for products. The temperature of the barrel should be lowered, the screw speed or back pressure should be reduced, and the temperature of each section of the barrel should be adjusted properly.

(2) The high temperature of the mold affects the shaping process and also causes stoppage due to jamming and clamping of the parts. Targeted measures should be taken to strengthen the cooling of waterways.

(3) The molding time is unstable. Automatic or semi-automatic operation should be used.

(4) The heat supply of the machine barrel is insufficient. Machines with high plasticizing capacity should be used or preheating of materials should be strengthened.

(5) Improve the production conditions of the machine, such as oil pressure, oil quantity, and clamping force.

(6) The nozzle is salivating. The temperature of the barrel and nozzle should be controlled properly or self locking spray should be used instead

Mouth.

(7) The wall thickness of the product is too thick. The mold should be improved to reduce wall thickness.