Fault analysis and improvement method for flow marks on the surface of injection moulding products

Time:2019-10-05 10:00:51 / Popularity: / Source:


A linear trace on the surface of shaped part that indicates direction in which melt flows.
flow marks 

Fault analysis and troubleshooting

1. Poor flow of melt causes surface of plastic part to produce annual ring-shaped wave flow marks centered on the gate. Mold and nozzle temperatures can be increased separately, injection rate and filling speed can be increased, injection pressure, holding pressure and pressurization time can be increased. It is also possible to provide a heater at the gate to increase local temperature of gate portion. It is also possible to appropriately enlarge gate and runner area, gate and runner sections are preferably circular, which allows for optimum filling. However, if a gate is placed in a weak area of plastic part, a square cross section should be used. In addition, bottom of injection port and end of sub runner should be provided with a large cold hole. The greater influence of material temperature on flow properties of melt, the more attention should be paid to size of cold feed hole. Position of cold feed hole must be set at the end of melt along flow direction of injection port.
flow marks 
2. Flow of melt in runner is not smooth, resulting in spiral wave flow marks on the surface of plastic part. When melt flows from a narrow cross section of flow path into a cavity of a larger cross section, or flow path of mold is narrow and finish is poor, flow is likely to form a turbulent flow, resulting in a spiral wave flow on the surface of plastic part. In this regard, injection speed can be appropriately reduced or controlled slowly, quickly, and slowly. Gate of mold should be placed in a thick wall or directly on wall side. Gate form is preferably shank, fan or diaphragm. It is also possible to appropriately enlarge flow passage and gate cross section to reduce flow resistance.
3. Volatile gases cause cloud-like wave flow marks on the surface of plastic parts. When ABS or other copolymerized resin raw materials are used, if processing temperature is high, volatile gases generated by resin and lubricant may cause cloud-like wave flow marks on the surface of plastic part. In this regard, mold and barrel temperature should be appropriately reduced, exhaust condition of mold should be improved, material temperature and filling rate should be lowered, gate cross section should be appropriately enlarged, and type of lubricant should be replaced or amount of lubricant should be reduced.

Analysis and solution of several cases of injection moulding products

Product insufficiency is a problem often encountered in production, but it is relatively easy to solve. When it can't be solved by process, it can be improved from design and manufacture of mold, which can generally be solved. Tip: When such problems occur, it is necessary to observe movement stroke of screw of injection molding machine while molding. Pay special attention to position where screw is located when pressure is completed and injection is completed.
Case 1:
Position of screw melt of injection molding machine reaches set value, position of injection molding pressure is 0 (residual injection amount is 0). Possible causes:
1, Leaking. In this case, first check barrel of injection molding machine for leaking; whether matching position between injection molding machine nozzle and main bushing of mold leaks. If there is leakage, product is a natural phenomenon and should be excluded first. Following checks are performed only if it is confirmed that there is no melt leakage.
flow marks 
When product is under-injected and there is no colloid leakage, it may be that amount of material is insufficient. At this time, it is necessary to increase melt stroke and increase amount of material injected. After increasing melter stroke, continue to observe residual material amount of screw, melt stroke and injection of product.
2. Amount of material is not enough. After increasing melt stroke, product is increased in injection, indicating that amount of material is not enough, and it is necessary to continue to increase melt stroke. When melt stroke reaches maximum and product is still owed, you need to replace large injection molding machine.
3. Material meson leaks. For example, as melt stroke increases, amount of material injected into product during injection molding does not increase correspondingly, which may be material meson leakage of injection molding machine.
Check material meson leaking method:
(1) Keep product of a certain injection cycle in cavity, use manual injection. Injection pressure is changed from small to large. Observing advancement of screw can judge leakage of material meson. When it is manually injected, screw does not advance, indicating that material meson is well sealed and does not leak. Under a large injection pressure, screw has a small amount of advancement, which is allowed. At this time, it is not a melt leakage, but a shrinkage of melt.
(2) In the case of product underfill, firstly, it is necessary to check whether there is leakage between injection molding machine barrel and nozzle; secondly, check material meson of front end of injection molding machine for leaking. Only after exclusion of these two situations can following aspects be determined.
Case 2:
Screw of injection molding machine can reach set melt stroke, but it can not reach set injection stroke (residual material amount). In this case, there is material in barrel, but it does not enter cavity. Possible causes:
1. Fluidity of melt adhesive is not enough.
Fluidity of melt is insufficient, flow resistance is large, and material is difficult to fill into mold. This may happen because of following reasons:
a, Raw material itself has poor mobility;
b. Processing temperature of barrel is too low or temperature does not reach set value, such as temperature display is not true.
c. Mold temperature is low. Although colloidal fluidity in barrel is good, after flowing into mold runner, cooling is too fast, fluidity in cavity in mold runner is poor, resulting in dissatisfaction of filling.
2. Improper setting of injection molding process parameters.
Injection pressure is low, colloid can not overcome filling resistance; injection speed is low, melting time of material is long when filling, fluidity is reduced, resulting in a large filling resistance; injection holding time is short, filling is stopped before product is filled. All will resulting in a product owing.
3. Filling channel is blocked.
Lack of material due to cold material, impurities or other substances blocking filling channel can also cause product to be underfilled. At this time, it is necessary to check whether injection nozzle, main channel, shunt channel, gate of injection molding machine are unblocked. If it is blocked, it needs to be cleaned up.
flow marks 
4. Mold casting system is defective.
Defects of mold casting system are divided into following aspects:
a. Runner is too small, too thin or too long, increasing fluid resistance;
b. Runners and gates are rough, flawed, or have sharp angles, surface roughness is poor, which affects flow;
c. Cold well is improperly set or not set, and cold glue collection is incomplete, resulting in blockage of runner or gate;
d. Position or quantity of gate is improperly set. Filling resistance is greater than injection pressure, material is not filled into mold.
E. Mold is poorly exhausted or exhaust position is improperly set, mold is partially trapped and product is not filled.
5. Poor product design.
Product structure is poorly designed, material position design is not uniform, and local material position is too thin, resulting in product filling dissatisfaction.

What should we do with gas grain in the gate of injection moulding products?

In point gate design of vertical part direction, a radiation system consisting of different color depths and glosses, called ray line, it is formed on the surface of part during injection molding.
flow marks 
Most of these defects occur when injecting polystyrene and modified polystyrene mixtures, which are related to following factors: two materials differ in rheology, coloration, etc. Flow system stratosphere and turbulent layer have different flow rates and heat conditions. Plastics are burnt due to thermal decomposition. Interference of gaseous substances when plastics enter mold.
(1) When using mixed plastic, particle size of plastic should be same and uniform.
(2) Plastic and colorant should be mixed evenly, and if necessary, a suitable dispersing agent should be added and mechanically mixed.
(3) Plasticization should be complete, plasticizing performance of machine should be good.
(4) Reduce injection pressure and speed, shorten injection and holding time, increase mold temperature and nozzle temperature, and reduce temperature of front furnace.
(5) Melts and coking substances that prevent degradation of plastics and increase viscosity: If there is a dead angle at the screw and barrel, or if heating system is out of control, improper processing will cause plastic to decompose for a long time. It is possible to pass through inner surface of front end of screw and barrel.
(6) Improve gate design, such as enlargement of gate diameter, change gate position, change gate to a rounded transition, try to locally heat gate, and add a cold well at runner end.

What should I do if there are spots on the gate of injection moulding products?

Many black streaks or black spots appear on the surface or inside of plastic part, called smear or char, which is usually caused by "melt breakage" due to expansion of injection mold.
flow marks 

(1) Injection molding problem:

1 increase temperature of barrel;
2 increase nozzle temperature;
3 slow down injection speed;
4 increase injection pressure;
5 change injection time;
6 reduces lubricant or exchanges lubricant.

(2) Mold problem:

1 increase mold temperature;
2 increase gate size
3 change shape of gate (fan gate);
4 increase cold hole;
5 increase size of sub-runner;
6 change gate position;
7 Improve exhaust.
(3) Material problems:
1 dry material;
2 Remove contaminants from material.

How to control color difference of injection products?

Chromatic aberration is a common defect in injection molding. It is not uncommon for injection molding machine to be scrapped in batches due to difference in color of matching parts. There are many factors affecting chromatic aberration, including raw material resin, color masterbatch (or toner), mixing of color masterbatch with raw materials, injection molding process, injection molding machine, mold, etc. Because of wide range of exposure, color difference control technology is also recognized as one of hard-to-master technologies in injection molding.
flow marks 
In actual production process, we generally control color difference from following six aspects:

1. Eliminate influence of injection molding machine and mold factors

To select an injection molding machine with same capacity as injection molding products, it is best to replace equipment if there is a problem with dead angle of injection molding machine. For chromatic aberration caused by mold casting system, exhaust groove, etc., it can be solved by mold maintenance of corresponding part. Injection molding machine and mold problem must be solved before production can be organized to reduce complexity of problem.

2. Eliminate influence of raw resin and color masterbatch

(1) Controlling raw materials is the key to completely solving chromatic aberration. Therefore, especially in the production of light-colored articles, significant influence of difference in thermal stability of raw material resin on color fluctuation of product cannot be ignored.
(2) In view of fact that most injection molding manufacturers do not produce plastic masterbatch or color masterbatch themselves, focus of attention can be placed on production management and raw material inspection. That is to strengthen inspection of raw materials; same product in production should be produced by same injection molding manufacturers, same brand masterbatch and color masterbatch;
(3) For color masterbatch, we must carry out sampling test color before mass production, which must be compared with last proofreading, and also in this comparison. If color difference is not large, it can be considered qualified. As batch color master has slight color difference, color master can be remixed and used to reduce color difference caused by uneven mixing of color master. At the same time, we also need to focus on thermal stability of raw material resin and color masterbatch. For raw materials with poor thermal stability, we recommend that manufacturers be required to exchange.

3. Eliminate the influence of uneven mixing of masterbatch

(1) If plastic masterbatch is not well mixed, color of product will be changed. After masterbatch are mechanically mixed, when hopper is fed through lower suction, color masterbatch is separated due to action of static electricity, and is easily adsorbed on hopper wall, which is bound to cause a change in the amount of masterbatch during injection molding cycle, thereby generating chromatic aberration.
(2) In this case, raw material can be taken into hopper and then manually stirred. For addition of toner to produce colored products, the most effective way is to not use suction machine, but use a hot air dryer to manually prevent color difference caused by separation of toner from masterbatch.

4. Reduce influence of barrel temperature on chromatic aberration

In production, it is often encountered that a certain heating coil is damaged or heating part is out of control, long burning causes barrel temperature to change drastically and chromatic aberration. Color difference caused by this kind of reason is easy to judge. Generally, heating ring damages causes chromatic aberration and is accompanied by uneven plasticization, and uncontrolled long burning of heating control part is often accompanied by product speckle, severe discoloration or even coking. Therefore, heating part should be inspected frequently during production. If heating part is damaged or out of control, it needs to be replaced and repaired in time to reduce possibility of such color difference.

5. Reduce impact of injection moulding process adjustment

(1) it is necessary to adjust injection moulding process parameters for non-color difference reasons, injection temperature, back pressure, injection cycle and amount of color masterbatch are not changed as much as possible. Adjustment also needs to observe influence of process parameter change on color. If color difference is found, it should be adjusted in time.
(2) Avoid using high-injection speed, high back pressure and other injection moulding process that cause strong shearing, to prevent chromatic aberration caused by local overheating or thermal decomposition. Strictly control temperature of each heating section of barrel, especially nozzle and heating part next to nozzle.

6. Grasp influence of barrel temperature and color masterbatch on product color change

It is also necessary to know trend of color of product as a function of temperature and color masterbatch before performing color difference adjustment. Different color masterbatch varies with temperature or color masterbatch of product, color change law of product is different. Color change process can be used to determine law of change.

Go To Top