Defects in injection molded parts are often characterized by surface marks and are formed by shrinkage of plastic from mold surface.
2. Reasons for possible problems (1). Melting temperature is either too high or too low. (2). There is insufficient plastic in cavity. (3). During cooling stage, surface contacting plastic is overheated. (4). Runner is unreasonable and gate section is too small. (5). Whether mold temperature is compatible with characteristics of plastic. (6). Product structure is unreasonable (reinforcement is too high, too thick, and thickness is obviously different). (7). Cooling effect is not good, and product continues to shrink after demolding.
3. Remedy (1). Adjust temperature of injection cylinder. (2). Adjust screw speed to obtain correct screw surface speed. (3). Increase injection volume. (4). Make sure to use correct gasket; increase screw forward time; increase injection pressure; increase injection speed. (5). Check whether stop valve is installed correctly, because abnormal operation will cause pressure loss. (6). Reduce mold surface temperature. (7) Correct flow channel to avoid excessive pressure loss; according to actual needs, appropriately expand cross-sectional size. (8). Mold temperature is properly controlled according to characteristics of plastic used and product structure. (9) Improve product structure where possible. (10). Try to let product have enough cooling.

 

First, characteristics of injection molding defects can be easily seen in "air trap" of transparent injection molding parts but can also appear in opaque plastics, which are related to thickness and are often caused by plastic shrinkage away from center of injection molding parts.
Second, possible reasons for problem (1). Mold is not fully filled. (2). Abnormal operation of stop valve. (3).Plastic is not completely dried. (4). Pre-molding or injection speed is too fast. (5). Some special materials are produced by special equipment.
Third, remedy (1). Increase amount of material shot. (2). Increase injection pressure. (3). Increase screw forward time. (4). Lower melting temperature. (5). Reduce or increase injection speed. (For example, increase speed by 45% for amorphous plastics. (6). Check whether check valve is cracked or not functioning. (7). Drying conditions should be improved according to characteristics of plastic, and plastic should be thoroughly dried. (8). Appropriately reduce screw speed and increase back pressure, or reduce injection speed.

Characteristics of defects in injection molded parts 1. Change in weight and dimension during injection molding process exceeds production capacity of mold, injection molding machine, and plastic combination.
2. Reasons for possible problems (1). Plastic input into injection cylinder is uneven. (2). Temperature or fluctuation range of injection cylinder is too large. (3). Capacity of injection molding machine is too small. (4). Injection pressure is unstable. (5). Screw reset is unstable. (6). Change of operating time and inconsistency of solution viscosity. (7). Injection speed (flow control) is unstable. (8). Plastic varieties that are not suitable for mold are used. (9) Consider influence of mold temperature, injection pressure, speed, time and pressure on product.
3. Remedy (1). Check whether there is sufficient cooling water flowing through hopper throat to maintain correct temperature. (2). Check for inferior or loose thermocouples. (3). Check that thermocouple used with temperature controller is of correct type. (4) Check injection volume and plasticizing capacity of injection molding machine, then compare it with actual injection volume and injection volume per hour. (5). Check whether there is a stable molten hot material for each operation. (6). Check backflow prevention valve for leakage, and replace it if necessary. (7). Check whether feed setting is wrong. (8). Ensure that screw is stable in return position of each operation, that is, change is not more than 0.4mm. (9). Check for inconsistencies in operating hours. (10). Use back pressure. (11). Check whether hydraulic system operates normally and whether oil temperature is too high or too low (25-60'C). (12). Select plastic varieties suitable for mold (mainly from shrinkage rate and mechanical strength). (13). Readjust the entire production process.

Possible reasons for problem 1) Plastic contains moisture and volatiles; 2) Material temperature is too high or too low; 3) Injection pressure is too small; 4) Size of runner and gate is too large; 5) Insert is not preheated well, and temperature is too low;6) Internal stress of product is too large.

Characteristics of defects in injection molded parts, sprue is held by sprue sleeve.
Second, possible reasons for problem (1). Nozzle sleeve and nozzle are not aligned. (2). Excessive packing of plastic in nozzle sleeve. (3). Nozzle temperature is too low. (4). Plastic is not completely solidified in nozzle, especially nozzle with a larger diameter. (5) Arc surface of nozzle sleeve and arc surface of nozzle do not match properly, and a runner that looks like a "mushroom" appears. (6). Runner is not enough to pull out slope.

3. Remedy (1). Re-align nozzle and nozzle sleeve. (2). Reduce injection pressure. (3). Reduce forward time of screw. (4). Increase nozzle temperature or use an independent temperature controller to heat nozzle. (5) Increase cooling time, but it is better to use a nozzle sleeve with a smaller nozzle instead of original nozzle sleeve. (6). Correct mating surface of nozzle sleeve and nozzle. (7). Properly expand pull-out slope of runner.

 

First, characteristics of injection molding defects: Shape of injection molded part is similar to mold cavity but it is a distorted version of mold cavity shape.
2. Reasons for possible problems (1). Bending is due to excessive internal stress in injection molded parts. (2). Mold filling speed is slow. (3). There is insufficient plastic in cavity. (4). Plastic temperature is too low or inconsistent. (5). Injection molded parts are too hot when ejected. (6). Insufficient cooling or inconsistent temperature of moving and fixed molds. (7). Structure of injection molded parts is unreasonable (for example, reinforcing ribs are concentrated on one side, but far apart).
Third, remedy (1). Reduce injection pressure. (2). Reduce forward time of screw. (3). Increase cycle time (especially cooling time). Immerse in warm water (38oC) immediately after ejection from mold (especially thicker parts) to cool part slowly. (4). Increase injection speed. (5). Increase plastic temperature. (6). Use cooling equipment. (7) Appropriately increase cooling time or improve cooling conditions to ensure that mold temperature of movable and fixed molds is consistent as much as possible. (8) Improve structure of plastic parts according to actual situation.

Characteristics of Injection Molded Part Defects: Injection molding process is incomplete because mold cavity is not filled with plastic or injection molding process lacks certain details.
2. Reasons for possible problems (1). Insufficient injection speed. (2). Plastic shortage. (3). Screw does not leave screw gasket at the end of stroke. (4). Running time changes. (5). Temperature of injection cylinder is too low. (6). Insufficient injection pressure. (7). Nozzle part is sealed. (8). Heater outside nozzle or injection tank cannot work. (9). Injection time is too short. (10). Plastic is attached to throat wall of hopper. (11). Capacity of injection molding machine is too small (ie injection weight or plasticizing capacity). (12). Mold temperature is too low. (13).Anti-rust oil that does not clean mold. (14). Backstop ring is damaged, and molten material has a backflow phenomenon.
Third, remedy (1). Increase injection speed. (2). Check amount of plastic in hopper. (3). Check whether injection stroke is set correctly, and change it if necessary. (4). Check whether check valve is worn or cracked. (5). Check whether operation is stable. (6). Increase melt temperature. (7). Increase back pressure. (8). Increase injection speed. (9). Check whether there is foreign matter or unplasticized plastic in nozzle hole. (10). Check all heater outer layers with an ammeter to verify that energy output is correct. (11). Increase screw forward time. (12). Increase cooling capacity in throat area of hopper, or reduce temperature in rear area of injection cylinder. (13). Use a larger injection molding machine. (14). Properly increase mold temperature. (15). Clean up rust inhibitor in mold. (16). Check or replace backstop ring.

Possible reasons for problem 1) Temperature of barrel, nozzle and mold is too high; 2) Injection pressure is too high, and clamping force is too small; 3) Mold is not tightly sealed, there are debris or mold plate is deformed; 4) Cavity is poorly vented 5) Fluidity of plastic is too good;

Possible reasons for problem 1) Material temperature is too low, and fluidity of plastic is poor; 2) Injection pressure is too small; 3) Injection speed is too slow; 4) Mold temperature is too low; 5) Cavity is poorly vented; 6) Plastic is polluted.

"Dent" is caused by local internal shrinkage after gate sealing or under injection. Dimples or dents on the surface of injection molded parts are an old problem in injection molding process. Dent is generally caused by local increase in shrinkage rate of product caused by increase in wall thickness of plastic product. It may appear near outer sharp corner or at sudden change of wall thickness, such as protrusions, ribs, or behind support, and sometimes in unusual locations. Root cause of dents is thermal expansion and contraction of material, since thermoplastics have a fairly high coefficient of thermal expansion. Degree of expansion and contraction depends on many factors, of which properties of plastic, maximum and minimum temperature ranges, and cavity holding pressure are the most important factors. There are also size and shape of injection molded parts, as well as cooling rate and uniformity.
Amount of expansion and contraction during molding process of plastic materials is related to thermal expansion coefficient of plastic being processed, thermal expansion coefficient during molding process is called "molding shrinkage". As molded part cools and shrinks, molded part loses close contact with cooling surface of cavity. At this time, cooling efficiency decreases. After molded part continues to cool, molded part continues to shrink. Amount of shrinkage depends on combined effect of various factors. Sharp corners on molded part cool the fastest and harden earlier than other parts. Thick part near center of molded part is the farthest from cooling surface of cavity and becomes last part of molded part to release heat. After solidified material is solidified, as melt near center of part cools, molded part will continue to shrink, and plane between sharp corners can only be cooled on one side, and its strength is not as high as that of material at sharp corners. Cooling shrinkage of plastic material at the center of part pulls inward relatively weak surface between partially cooled and more cooled sharp corners. In this way, dents are created on the surface of injection molded part. Presence of dents indicates that molding shrinkage here is higher than shrinkage of surrounding areas. If molded part shrinks more in one place than the other, that is cause of molded part warping. In-mold residual stress reduces impact strength and temperature resistance of molded part. In some cases, adjusting process conditions can avoid sink marks. For example, during holding process of molded part, mold cavity is additionally injected with plastic material to compensate for molding shrinkage. In most cases, gate is much thinner than the rest of part. While molded part is still hot and continues to shrink, small gate has cured. After curing, holding pressure has no effect on molded part in cavity.

 

Semi-crystalline plastic materials have high molding shrinkage, which exacerbates dent problem; non-crystalline materials have lower molding shrinkage, which minimizes dents; fill and maintain reinforcement materials, which have shrinkage rates lower and less likely to dent.
Thick injection molded parts have a long cooling time and will produce larger shrinkage. Therefore, large thickness is root cause of dents. Attention should be paid to design. Thick-walled parts should be avoided as much as possible. If thick wall cannot be avoided, it should be designed to be hollow Yes, thick parts are smoothed to nominal wall thickness, and large arcs are used instead of sharp corners to eliminate or minimize dents near sharp corners.