Generation of bad problems related to injection molding is caused by many factors: finished product structure design, mold design and manufacturing, raw material selection (including color matching), injection molding process parameter setting, etc. Molding problem caused by poor finished product design is the most difficult to solve. If molding problem is caused by improper molding conditions, it is the easiest to solve. As long as cause of problem is found and molding parameters are adjusted, problem can be solved. If problem is caused by mold or finished product design, mold needs to be modified at the least, it may become an insurmountable defect at the most serious.
In fact, some problems caused by injection molding conditions alone are not easy to solve, because molding parameters are related to each other. Sometimes some problems can not be solved by adjusting a certain parameter, but must have a holistic understanding of molding parameters and a basic understanding of causes of molding problems. Only then can molding parameters be quickly adjusted to solve problem. Any trial and error method must also be based on such prerequisites to be economical, fast and effective.
In actual production, relying entirely on molding parameters to control poor injection molding requires a prerequisite, that is, stability of molding conditions and large range of parameters that can be adjusted.
Therefore, as an injection molding supervisor foreman, you should have corresponding knowledge of materials, injection molding, mold making, and try to avoid possible failures from source to become an expert in this field.

Possible causes of shrinkage dents
a. Product design;
b. Mold cooling system;
c. Improper molding conditions.
Reason for its formation is volume shrinkage of molten plastic during cooling and solidification. It is more likely to occur in ribs, bosses (BOSS), and places with uneven wall thickness of finished product. Fundamental reason is phenomenon of heat concentration and poor heat dissipation resulting in different shrinkage speeds, shrinkage rates, and shrinkage amounts.
As shown in Figures 1, 2, 3, and 4, heat concentration phenomenon at A is more serious, resulting in a higher temperature at A, slow shrinkage, and a larger shrinkage amount. When plastic material shrinks, it will inevitably shrink and stretch. Because shrinkage at A is slower, adjacent area will be pulled in. Slow shrinkage speed makes shrinkage more complete, shrinkage amount is larger than thickness of mold core wall. During shrinkage process, plastic in adjacent area has been solidified, resulting in a dent as shown in B, which is a general shrinkage dent.

 

When designing a product, try to avoid phenomenon of heat concentration during molding of designed finished product, such as a reasonable thickness ratio (rib thickness/basic wall thickness ratio) and reinforcing ribs, protruding columns should not be too dense, difference in wall thickness should not be too large or wall thickness should not change suddenly, etc. As shown in figure, correct design:

 

1. Non-crystalline plastic:
t≤0.6 T:
2. Crystalline plastic:
t ≤0.5T:
3. Draft angle d =0.5°~1.5°:
4.H<5T(2.5~3T);
5.R=0.25~0.4T

a. Reduce plastic flow length during molding, that is, reduce L/t ratio (ratio of flow length to thickness, the lower ratio, the easier it is to flow), such as using more gates to feed, and make inlet as close to shrinkage depression as possible;
b. Increase gate size to make secondary pressure (holding pressure) filling time longer;
c. In shrinkage depression of finished product, that is, area where heat concentration is more serious or heat is not easy to diffuse, use metal materials with better thermal conductivity (such as beryllium copper) so that heat there can be smoothly conducted out
d. Make mold cooling system pass through or close to shrinkage depression, so that there can be a faster cooling rate and smaller shrinkage (as shown in figure):;
e. If there is a depression in place with uniform thickness at the end of filling, check and optimize mold exhaust

 

a. Increase injection pressure and secondary pressure (holding pressure);
b. Increase injection time;
C. Inject in stages, and reduce injection speed at thick part;
d. Reduce mold temperature and material temperature;
e. Check buffer volume (CUSHION) of material pipe. If buffer volume is insufficient, increase dosage;
f. Rapidly cool finished product after it is taken out, so that surface layer can solidify quickly to resist shrinkage force at thick part;
g. Increase back pressure, and confirm whether check valve of injection molding machine is malfunctioning:
h. Confirm whether machine nozzle and mold gate are in good contact, there is no glue leakage.
Note:
In actual production or debugging, although above countermeasures are beneficial to solving shrinkage depression of finished product, they may also cause new problems such as stress and warping due to changes in molding parameters. In addition, extra injection time after gate solidifies will be futile. Therefore, all these possible effects must be taken into consideration.

Definition: Lines formed when melt wave fronts meet, also known as water lines

 

When wave front meeting angle (Meeting Angle) is less than 135', a Welding Line is formed. When wave front meeting angle is greater than 135", a Melding Line is formed. (As shown in figure below)

 

Compared with Meld line, Weld line has less mutual diffusion of molecules on both sides and is of poorer quality. When meeting angle is between 120 and 150°, surface traces of fusion line gradually disappear. Increase in angle of contact can be achieved by adjusting product thickness, changing position and number of gates, changing position and size of runner, etc.
During plastic injection molding, as long as two or more strands of flowing plastic meet, there will inevitably be a bonding line. That is to say, finished products with openings or more than two points of pouring will inevitably have bonding lines. Therefore, all analysis suggestions here only focus on how to weaken visibility of bonding line or increase strength of bonding line. Therefore, a finished product designer must be able to predict possible location of joint line and possible impact of these joint lines on appearance and strength of finished product. In advance, he must consider using appearance processing techniques such as texturing, painting, and printing to weaken its negative impact on appearance, and change load position of finished product to avoid cracking, deformation, etc. caused by load at joint line position.

a. Set an air vent (AIR VENT) at expected joint line position. If it is not possible to set an air vent on parting surface, you can use nearby ejector pin or slide position;
b. Choose gate position appropriately. Before deciding gate position, you must predict possibility of setting an air vent at possible joint line position and impact of joint line on appearance and strength;
c. Increasing number of gates can reduce L/t ratio of plastic, reduce depth of joint line, change position and strength of joint line, but will increase number of joint lines;
d. Add an overflow groove in joint line area and remove it after molding; (as shown in Figure 1)
e. Optimize design of air vent (number, position, depth, channel) and clean air vent; (as shown in Figures 2 and 3)
f. Add heat medium at joint line position.

 

a. Increase mold temperature, especially core temperature on appearance side;
b. Increase injection pressure, but hold pressure, and appropriately extend holding time;
c. Increase injection speed, but if there is a joint line in area with poor exhaust, injection speed at this position should be reduced;
d. Reduce use of release agent, ensure that mold surface is clean and free of oil stains
e. Increase back pressure to increase density of plastic at joint;
f. Confirm that raw material drying is strictly carried out according to physical property table;
g. Reduce clamping pressure;
h. Appropriately increase temperature of plastic raw materials;
i. Use plastic raw materials with better fluidity.

Silver streaks are formed when water vapor in plastic material adheres to the surface of product and surface of mold cavity along flow direction of plastic during injection molding. As shown in figure

 

a. Improper mold water channel design causes water leakage into mold cavity;
b. Optimize mold exhaust system:
C. Mold flow channel design is too small, resulting in excessive shear heat and raw material cracking.

a. Dry plastic raw materials according to required conditions before molding, and confirm whether there is any mixing:
b. Reduce injection speed, especially when flow is at a vertical corner or flow cross-sectional area is small, and avoid turbulence caused by high injection speed;
c. Increase mold temperature to prevent mold from being too cold so that water vapor adheres to mold surface;
d. Reduce plastic temperature to avoid raw materials from cracking in barrel;
e. Increase back pressure and reduce screw speed (back pressure is too low and screw speed is too fast, which can easily draw air in):
f. Confirm whether release agent has been cleaned;
g. Reduce buffer distance when screw retreats after injection (avoid inhaling too much air).

Flash refers to excess plastic that appears in gap between mold parting surface or ejector, slide, and other molding components, causing irregular edges, corners, and burrs on product at above locations. (As shown in figure below)

 

a. Avoid complex parting surfaces. When designing products, consider how mold is parted, and avoid small steps on adjacent surfaces in the early stage;
b. Design a small break (0.03mm~0.05mm) at parting surface of product to avoid misalignment caused by processing errors and curling at corners of parting surface during discharge machining; (as shown in figure)
c. Design sufficient penetration angles for parts that need to be broken by mold;
d. Uniformity of thickness.

 

a. Avoid unnecessary complex parting surfaces;
b. When finished surface is a mirror surface and is on same surface as broken or inserted surface, pay attention to reserved polishing amount in design;
c. Avoid design of ridges or point sealing glue;
d. Reasonable exhaust. For NCVM or high-gloss products, first-level surface does not have exhaust;
e. Mold cooling system is set evenly to avoid local high temperature of mold;
f. When designing, influence of injection pressure and holding pressure on mold plate should be considered. Mold mplate of appropriate thickness should be selected and arrangement of support columns should be paid attention;
g. Gate position should be reasonable. During injection molding and filling, pressure in mold cavity can meet principle of torque balance;
h. For parts of moving parts that are prone to burrs, influence of burrs on subsequent processes should be considered during design, and direction in which burrs may appear should be adjusted;
i. Hardness of steel selected for mold should be appropriate, and key parts should be plated when necessary.

a. Adjust clamping force of molding machine. If clamping force of machine is insufficient, a larger machine must be replaced:
b. Clean dirt on parting surface;
c. Reduce injection pressure or holding pressure;
d. Reduce injection speed, especially injection speed of last section:
e. Reduce temperature of mold and plastic;
f. Confirm whether holding pressure switching position is appropriate and confirm that metering setting is reasonable;
g. Confirm that there is no cracking of raw material in barrel and residence time is within appropriate range;
h. For parts of moving parts that are prone to burrs, impact of burrs on subsequent processes should be considered during design, and direction in which burrs may appear should be adjusted.
Note: Product clamping force estimation formula:
Clamping force F (tons) = P max (kgf/ cm2) * product projection area (cm2)/1000/0.8
1. P represents filling pressure;
2. 1000 is conversion number of metric tons;
3. 0.8 is safety factor.

Jet lines usually occur at point where casting enters mold cavity. Reason for its formation is that when molten plastic raw material flows through gate, because cross-sectional area of gate is small, according to formula:
Q-V*A (Q: flow rate, V: flow velocity, A: flow path cross-sectional area)
When injection speed remains unchanged, flow rate will become a constant value, and when gate cross-sectional area is small, flow rate of plastic flowing through gate will become faster. At this time, because flow rate of plastic is too fast, and if gate thickness is too large relative to mold cavity height (finished product thickness), front edge of molten plastic (Melt Front) will not be able to effectively adhere to mold surface on both sides to form a skin layer (Skin Layer), plastic will become a jet state. During jet process, plastic of this jet will drop in temperature due to contact with lower temperature air and mold cavity surface. Therefore, it will produce different colors from subsequent plastics due to different material temperatures, forming a snake-like pattern, also known as snake pattern. As shown in figure.

 

a. Increase cross-sectional size of gate;
b. Change pouring method or gate position so that after plastic flows through gate, there is a mold iron material blocking flow in front. Use fan-shaped gates to avoid this;
c. Set an appropriate cold slug well in runner so that defective plastic is retained in cold slug well instead of flowing into mold cavity.

 

a. Reduce injection speed of plastic when it passes through gate;
b. Increase temperature of plastic;
c. Increase temperature of mold

 

Short shot refers to phenomenon that after injection molding process is completed, plastic still cannot completely fill every corner of mold cavity. It mainly occurs in places where thickness is relatively thin, end of plastic flow, and areas where it is difficult to set up exhaust grooves. As shown in figure

 

a. Do not design too thin wall thickness, refer to attached table for recommended wall thickness of commonly used plastics;
b. Choose plastics with better fluidity or choose high fluidity grades for same plastic.
Wall thickness of commonly used plastics (MM)

a. Select appropriate gate position and number to avoid plastic flow path being too long or plastic meeting at an inappropriate position, which will cause difficulty in venting. Attached table shows allowable flow length and wall thickness ratio of common plastics at different finished product thicknesses;
b. Increase gate size:
c. Choose a good venting groove position;
d. Increase cold material of flow channel or vertical runner aperture size
Common plastic maximum flow length and wall thickness ratio

a. Increasing injection speed can improve filling of thin parts, but it will be counterproductive for short shot phenomenon with poor exhaust. Therefore, appropriate injection speed must be determined based on actual test shots
b. Increase mold temperature and plastic temperature;
c. Increase injection pressure and increase injection time;
d. Increase holding pressure and increase holding time:
e. Check whether injection metering is sufficient;
f. Confirm whether screw check valve of molding machine is faulty;
g. Check whether mold and molding machine nozzle are in good contact;
h. Check whether material discharge of molding machine hopper is blocked

After injection, when finished product cools, surface of finished product is cooled and solidified first, while interior of finished product continues to shrink, or when gas is mixed into finished product during molding and cannot escape. Generally speaking, bubbles will exist in thicker wall of finished product or in area where thickness of finished product is uneven. As shown in figure

 

Same as treatment plan for dents, when designing products, try to avoid phenomenon of heat concentration during molding of designed finished products, such as reasonable thickness ratio (rib thickness/basic wall thickness ratio) and reinforcing ribs, protrusions should not be too dense, difference in wall thickness should not be too large or wall thickness should not change suddenly, etc.

a. Increase gate size or inject glue at the thickest part of product:
b. Optimize design of mold exhaust system.

a. Reduce injection speed to make gas easier to discharge;
b. Increase holding pressure and increase injection and holding time:
c. Reduce plastic temperature;
d. Reduce screw speed:
e. Increase back pressure;
f. Dry plastic as required;
g. Increase mold temperature.

Layered peeling refers to ability of surface of a finished plastic product to be peeled off layer by layer. As shown in figure, it mainly occurs during injection molding due to incompatible plastics or excessive moisture, sharp corners in mold gate or runner, etc.

 

a. Reasonably set mold exhaust;
b. Design appropriate gate position, pouring method and gate size, smooth flow channel transition,

a. Pay attention to cleanliness of secondary material crushing process to avoid mixing of foreign materials;
b. Pay attention to cleaning of material pipe when changing materials, especially for incompatible plastics;
c. Reduce proportion of secondary materials;
d. Reduce screw speed;
e. Increase back pressure;
f. Dry plastic as required, increase material temperature and mold temperature:
g. If color powder is added for color adjustment, consider influence of color powder;
h. Reduce use of release agent.

Surface ripples and flow marks refer to surface defects of products that are wavy near gate. This is mainly because during flow of plastic, front material of molten plastic in mold cavity hits mold cavity surface and cools first, rear molten plastic passes over front cold material, hits mold cavity surface again and cools down. In this way, wave-like flow marks will be left on the surface of finished product after injection molding is completed.

 

a. Increase size of gate or runner:
b. Increase or add cold wells;
c. Select gate position so that gate is located at the thickest part of product for pouring:
d. Use selection of gate positions or increase in number of gates to reduce flow length during molding.

a. Increase injection speed;
b. Increase injection pressure:
c. Increase plastic temperature:
d. Increase mold temperature:
e. Increase back pressure;
f. Increase residence time of raw material in material tube; (injection material volume: material tube material = 1:1.5~1:4)
g. When switching injection pressure and position, amplitude should not be too large.

Phenomenon that product sticks to the front or rear mold cavity after opening mold in injection molding machine and cannot be completely removed is called mold sticking. Possible reasons for mold sticking are as follows:
a. Poor polishing causes mold sticking at ribs, pin posts, etc.
b. Deep ribs are not cooled enough.
c. Draft angle is too small or there are undercuts (UNDERCUT) in mold
d. Etched surface and spark pattern surface are too rough.
e. Molding and filling are too full
f. Injection pressure or barrel temperature is too high
g. Pressure holding time is too long
h. Uneven feeding will cause local oversaturation. When there are multiple holes, some holes will be oversaturated.
i. Insufficient cooling time
j. Mold temperature is too high or too low
k. Poor demoulding and exhaust design for deep barrel parts

a. Draft angle should be as large as possible without affecting function and appearance;
b. Ribs generated by glue escape should not be too many, too deep or too thin (depth should not exceed 5T as much as possible, and rib thickness should be at least 0.35mm after demolding)

a. Strengthen polishing, polishing and exhaust of deep rib inserts;
b. Set up a reasonable ejection device to ensure ejection balance and ejection tip is not affected by plastic shrinkage force:
c. Reasonable mold separation and ensure that product-related draft angle is sufficient, ensure that sliding position and inclined ejection stroke are sufficient;
d. Enhance exhaust effect of thin glue to avoid formation of vacuum between product surface and cavity surface;
e. Pay attention to polishing direction, and polishing texture should be consistent with demolding direction as much as possible;
f. Cooling system design should be sufficient.

a. Reduce molding pressure;
b. Extend cooling time and reduce mold temperature:
c. Dry plastic as required and appropriately reduce temperature of material pipe and hot runner;
d. Adjust ejection stroke to ensure that inclined ejector device is out of hook and use release agent reasonably.

So-called burn mark generally includes discoloration of surface of product due to plastic degradation and blackening of filling end of product. As shown in picture:
a. Air trapped in cavity cannot be discharged quickly when plastic melt is filled (trapped air), is compressed and heated up significantly, causing material to burn;
b. Poor exhaust will cause gas compression at the end of product farthest from gate or at feeding and closing parts to generate high temperatures, leading to burning.

 

a. Increase mold exhaust, especially in position that is easy to burn;
b. Increase gate, eliminate sharp obstruction at the corner of gate and runner, and avoid high shear heat when plastic flows

a. Reduce injection pressure;
b. Use multi-stage injection, and slow down in stages at the end of molding to facilitate sufficient gas discharge time;
c. Use a vacuum pump to extract gas in mold cavity to make mold cavity a vacuum state;
d. Enhance exhaust effect of thin glue position to avoid formation of vacuum between product surface and mold cavity surface;
e. Reduce mold temperature;
f. Reduce temperature of hot runner and molding machine material tube;
g. Shorten residence time of plastic in molding machine material tube.

Main reason for black streaks on product is thermal decomposition of plastic material, which is common in materials with poor thermal stability. As shown in figure:

 

Mainly improve molding machine adjustment:
a. When the product is small and barrel size is large, plastic stays for too long and decomposes, and machine should be changed:
b. Check whether proportion of recycled material added is appropriate, otherwise it is easy to decompose by repeated heating;
c. Screw is partially damaged or gap of check ring is large, and materials with high viscosity should be paid special attention;
d. Confirm whether plastic is abnormally heated (such as hot runner damage, etc.) to cause local decomposition of plastic
e. Screw rubber is not biting well and too much air is involved:
f. When plastic lubricant is insufficient, friction is serious, too much shear heat is generated, and poor exhaust causes combustion. Add appropriate lubricant, but at a dosage of 0.2%, flammable volatiles of lubricant make combustion easy and produce black strips;
g. When it occurs suddenly after long-term production, check whether mold water channel is blocked.

Surface of product loses its original gloss, forms a milky white film, and is blurred, which can be called poor surface gloss (Haze or Lusterless).
a. Poor surface gloss of product is mostly caused by surface condition of mold. When surface of mold is poorly polished, surface of product will certainly not have a good gloss. However, when surface condition of mold is good, increasing temperature of material and mold temperature can improve surface gloss;
b. Excessive use of release agents or oily release agents is also one of reasons for poor surface gloss
c. Moisture absorption or contamination of materials containing volatiles and foreign matter can also cause poor surface gloss of product.

a. Check mold surface. For products with mirror surface requirements, if surface gloss is poor, polishing should be strengthened;
b. Confirm etching specifications. Etching can be divided into bright surface and matte surface.

a. Increase material temperature and mold temperature;
b. Confirm whether release agent is used:
c. Increase holding pressure and extend holding time.