01
The product has depressions
The depression on the surface of TPU products can reduce the quality and strength of the finished product, and also affect the appearance of the product. The cause of the depression is related to the raw materials used, molding technology, and mold design, such as the shrinkage rate of the raw materials, injection pressure, mold design, and cooling device.
Table 1 shows the possible causes and treatment methods of depressions
Methods for handling the causes of occurrence
Insufficient mold feed increases feed volume
High melting temperature reduces melting temperature
Short injection time increases injection time
Low injection pressure increases injection pressure
Insufficient clamping pressure, appropriately increase the clamping pressure
Improper adjustment of mold temperature to appropriate temperature
Adjusting the size or position of the mold inlet for asymmetric gate adjustment
Poor exhaust in the concave area, with exhaust holes installed in the concave area
Insufficient mold cooling time prolongs cooling time
Worn and replaced screw check ring
Uneven thickness of the product increases injection pressure
02
The product has bubbles
During the injection molding process, products may sometimes appear with many bubbles, which can affect their strength and mechanical properties, and also greatly compromise the appearance of the products. Usually, when the thickness of the product is uneven or the mold has protruding ribs, the cooling speed of the material in the mold is different, resulting in uneven shrinkage and the formation of bubbles. Therefore, special attention should be paid to mold design.
In addition, the raw materials are not fully dried and still contain some water, which decomposes into gas when heated during melting, making it easy to enter the mold cavity and form bubbles. So when bubbles appear in the product, the following factors can be checked and treated.
Table 2 shows the possible causes and treatment methods of bubbles
Methods for handling the causes of occurrence
Wet and thoroughly baked raw materials
Insufficient injection inspection temperature, injection pressure, and injection time
Injection speed too fast Reduce injection speed
Excessive raw material temperature reduces melt temperature
Low back pressure, increase back pressure to appropriate level
Change the design or overflow position of the finished product due to excessive thickness of the finished section, rib or column
The overflow of the gate is too small, and the gate and entrance are increased
Uneven mold temperature adjustment to uniform mold temperature
The screw retreats too fast, reducing the screw retreating speed
03
The product has cracks
Cracks are a fatal phenomenon in TPU products, usually manifested as hairlike cracks on the surface of the product. When the product has sharp edges and corners, small cracks that are not easily visible often occur in this area, which is very dangerous for the product. The main reasons for cracks occurring during the production process are as follows:
1. Difficulty in demolding;
2. Overfilling;
3. The mold temperature is too low;
4. Defects in product structure.
To avoid cracks caused by poor demolding, the mold forming space must have sufficient demolding slope, and the size, position, and form of the ejector pin should be appropriate. When ejecting, the demolding resistance of each part of the finished product should be uniform.
Overfilling is caused by excessive injection pressure or excessive material measurement, resulting in excessive internal stress in the product and causing cracks during demolding. In this state, the deformation of mold accessories also increases, making it more difficult to demould and promoting the occurrence of cracks (or even fractures). At this time, the injection pressure should be lowered to prevent overfilling.
The gate area is often prone to residual excessive internal stress, and the vicinity of the gate is prone to embrittlement, especially in the direct gate area, which is prone to cracking due to internal stress.
Table 3 shows the possible causes and treatment methods of cracks
Methods for handling the causes of occurrence
Excessive injection pressure reduces injection pressure, time, and speed
Excessive reduction in raw material measurement with fillers
The temperature of the molten material cylinder is too low, increasing the temperature of the molten material cylinder
Insufficient demolding angle Adjusting demolding angle
Improper ejection method for mold maintenance
Adjusting or modifying the relationship between metal embedded parts and molds
If the mold temperature is too low, increase the mold temperature
The gate is too small or the form is improperly modified
Partial demolding angle is insufficient for mold maintenance
Maintenance mold with demolding chamfer
The finished product cannot be balanced and detached from the maintenance mold
When demolding, the mold generates vacuum phenomenon. When opening or ejecting, the mold is slowly filled with air
04
Product warping and deformation
The reasons for warping and deformation of TPU injection molded products are short cooling setting time, high mold temperature, unevenness, and asymmetric flow channel system. Therefore, in mold design, the following points should be avoided as much as possible:
1. The thickness difference in the same plastic part is too large;
2. There are excessive sharp corners;
3. The buffer zone is too short, resulting in a significant difference in thickness during turns;
In addition, it is also important to set an appropriate number of ejector pins and design a reasonable cooling channel for the mold cavity.
Table 4 shows the possible causes and treatment methods of warping and deformation
Methods for handling the causes of occurrence
Extended cooling time when the product is not cooled during demolding
The shape and thickness of the product are asymmetrical, and the molding design is changed or reinforced ribs are added
Excessive filling reduces injection pressure, speed, time, and raw material dosage
Changing the gate or increasing the number of gates due to uneven feeding at the gate
Unbalanced adjustment of the ejection system and the position of the ejection device
Adjust the mold temperature to equilibrium due to uneven mold temperature
Excessive buffering of raw materials reduces buffering of raw materials
05
The product has burnt spots or black lines
Focal spots or black stripes refer to the phenomenon of black spots or black stripes on products, which mainly occurs due to poor thermal stability of raw materials, caused by their thermal decomposition.
The effective countermeasure to prevent the occurrence of scorch spots or black lines is to prevent the raw material temperature inside the melting barrel from being too high and slow down the injection speed. If there are scratches or gaps on the inner wall or screw of the melting cylinder, some raw materials will be attached, which will cause thermal decomposition due to overheating. In addition, check valves can also cause thermal decomposition due to the retention of raw materials. Therefore, when using materials with high viscosity or easy decomposition, special attention should be paid to preventing the occurrence of burnt spots or black lines.
Table 5 shows the possible causes and treatment methods of focal spots or black lines
Methods for handling the causes of occurrence
Excessive raw material temperature reduces melt temperature
Injection pressure too high to reduce injection pressure
Screw speed too fast Reduce screw speed
Readjust the eccentricity between the screw and the material pipe
Friction heat maintenance machine
If the nozzle hole is too small or the temperature is too high, adjust the aperture or temperature again
Overhaul or replace the heating tube with burnt black raw materials (high-temperature quenching part)
Filter or replace the mixed raw materials again
Improper exhaust of the mold and appropriate increase of exhaust holes
06
The product has rough edges
Rough edges are a common problem encountered in TPU products. When the pressure of the raw material in the mold cavity is too high, the resulting parting force is greater than the locking force, forcing the mold to open, causing the raw material to overflow and form burrs. There may be various reasons for the formation of burrs, such as problems with raw materials, injection molding machines, improper alignment, and even the mold itself. So, when determining the cause of burrs, it is necessary to proceed from easy to difficult.
1. Check whether the raw materials are thoroughly baked, whether impurities are mixed, whether different types of raw materials are mixed, and whether the viscosity of the raw materials is affected;
2. Correct adjustment of the pressure control system and injection speed of the injection molding machine must match the locking force used;
3. Whether there is wear on certain parts of the mold, whether the exhaust holes are blocked, and whether the flow channel design is reasonable;
4. Check if there is any deviation in the parallelism between the injection molding machine templates, whether the force distribution of the template pull rod is uniform, and whether the screw check ring and the melt barrel are worn.
Table 6 shows the possible causes and treatment methods of burrs
Methods for handling the causes of occurrence
Wet and thoroughly baked raw materials
Raw materials are contaminated. Check the raw materials and any impurities to identify the source of contamination
Raw material viscosity is too high or too low. Check the viscosity of the raw material and the operating conditions of the injection molding machine
Check the pressure value and adjust if the locking force is too low
Check the set value and adjust if the injection and pressure maintaining pressures are too high
Injection pressure conversion too late Check the conversion pressure position and readjust the early conversion
Check and adjust the flow control valve if the injection speed is too fast or too slow
Check the electric heating system and screw speed if the temperature is too high or too low
Insufficient rigidity of the template, inspection of locking force and adjustment
Repair or replace the wear and tear of the melting barrel, screw or check ring
Repair or replace the worn back pressure valve
Check the tension rod for uneven locking force
Template not aligned in parallel
Cleaning of mold exhaust hole blockage
Mold wear inspection, mold usage frequency and locking force, repair or replacement
Check if the relative position of the mold is offset due to mismatched mold splitting, and adjust it again
Design and modification of mold runner imbalance inspection
Check and repair the electric heating system for low mold temperature and uneven heating
07
The product has adhesive mold (difficult to demould)
When TPU experiences product sticking during injection molding, the first consideration should be whether the injection pressure or holding pressure is too high. Because too much injection pressure can cause excessive saturation of the product, causing the raw material to fill other gaps and making the product stuck in the mold cavity, causing difficulty in demolding. Secondly, when the temperature of the melting barrel is too high, it can cause the raw material to decompose and deteriorate under heat, resulting in fragmentation or fracture during the demolding process, causing mold sticking. As for mold related issues, such as imbalanced feeding ports that cause inconsistent cooling rates of products, it can also cause mold sticking during demolding.
Table 7 shows the possible causes and treatment methods of mold sticking
Methods for handling the causes of occurrence
Excessive injection pressure or melting barrel temperature reduces injection pressure or melting barrel temperature
Excessive holding time reduces holding time
Insufficient cooling increases cooling cycle time
Adjust the mold temperature and relative temperature on both sides if the mold temperature is too high or too low
There is a demolding chamfer inside the mold. Repair the mold and remove the chamfer
Imbalance of the mold feed port restricts the raw material flow, making it as close as possible to the mainstream channel
Improper design of mold exhaust and reasonable installation of exhaust holes
Mold core misalignment adjustment mold core
The mold surface is too smooth to improve the mold surface
When the lack of release agent does not affect secondary processing, use release agent
08
Reduced product toughness
Toughness is the energy required to break a material. The main factors that cause a decrease in toughness include raw materials, recycled materials, temperature, and molds. The decrease in toughness of products will directly affect their strength and mechanical properties.
Table 8 shows the possible causes and treatment methods for toughness reduction
Methods for handling the causes of occurrence
Wet and thoroughly baked raw materials
Excessive mixing ratio of recycled materials reduces the mixing ratio of recycled materials
Adjusting the melt temperature if it is too high or too low
The mold gate is too small, increasing the gate size
Excessive length of the mold gate joint area reduces the length of the gate joint area
The mold temperature is too low, increasing the mold temperature
09
Insufficient filling of products
Insufficient filling of TPU products refers to the phenomenon where the molten material does not fully flow through the corners of the formed container. The reasons for insufficient filling include improper setting of forming conditions, incomplete design and production of molds, and thick flesh and thin walls of formed products. The countermeasures in terms of molding conditions are to increase the temperature of materials and molds, increase injection pressure, injection speed, and improve the fluidity of materials. In terms of molds, the size of the runner or runner can be increased, or the position, size, quantity, etc. of the runner can be adjusted and modified to ensure smooth flow of molten materials. Furthermore, in order to ensure the smooth evacuation of gas in the forming space, exhaust holes can be set up at appropriate locations.
Table 9 shows the possible causes and treatment methods of insufficient filling
Methods for handling the causes of occurrence
Insufficient supply increases supply
Premature solidification of products to increase mold temperature
The temperature of the molten material cylinder is too low, increasing the temperature of the molten material cylinder
Low injection pressure increases injection pressure
Slow injection speed Increase injection speed
Short injection time increases injection time
Low or uneven mold temperature adjustment
Removal and cleaning of nozzle or funnel blockage
Improper adjustment and change of gate position
Small and enlarged flow channel
Increase the size of the sprue or overflow port by increasing the size of the sprue or overflow port
Worn and replaced screw check ring
The gas in the forming space has not been discharged and an exhaust hole has been added at an appropriate position
10
The product has a bonding line
A bonding line is a thin line formed by the merging of two or more layers of molten material, commonly known as a welding line. The bonding line not only affects the appearance of the product, but also hinders its strength. The main reasons for the occurrence of the combination line are:
1. The flow mode of materials caused by the shape of the product (mold structure);
2. Poor confluence of molten materials;
3. Air, volatiles, or refractory materials are mixed at the confluence of molten materials.
Increasing the temperature of the material and mold can minimize the degree of bonding. At the same time, change the position and quantity of the gate to move the position of the bonding line to another location; Or set exhaust holes in the fusion section to quickly evacuate the air and volatile substances in this area; Alternatively, setting up a material overflow pool near the fusion section, moving the bonding line to the overflow pool, and then cutting it off are effective measures to eliminate the bonding line.
Table 10 shows the possible causes and handling methods of the combination line
Methods for handling the causes of occurrence
Insufficient injection pressure and time increase injection pressure and time
Injection speed too slow Increase injection speed
Increase the temperature of the melt barrel when the melt temperature is low
Low back pressure, slow screw speed Increase back pressure, screw speed
Improper gate position, small gate and runner, changing gate position or adjusting mold inlet size
The mold temperature is too low, increasing the mold temperature
Excessive curing speed of materials reduces the curing speed of materials
Poor material fluidity increases the temperature of the melt barrel and improves material fluidity
The material has hygroscopicity, increases exhaust holes, and controls material quality
If the air in the mold is not discharged smoothly, increase the exhaust hole or check if the exhaust hole is blocked
Raw materials are unclean or mixed with other materials. Check the raw materials
What is the dosage of release agent? Use release agent or try not to use it as much as possible
11
Poor surface gloss of the product
The loss of the material’s original luster, formation of a layer or blurry state on the surface of TPU products can be referred to as poor surface gloss.
Poor surface gloss of products is mostly caused by poor grinding of the mold forming surface. When the surface condition of the forming space is good, increasing the material and mold temperature can enhance the surface luster of the product. Excessive use of refractory agents or oily refractory agents is also a cause of poor surface gloss. At the same time, material moisture absorption or contamination with volatile and heterogeneous substances is also the reason for poor surface gloss of products. So, special attention should be paid to factors related to molds and materials.
Table 11 shows the possible causes and treatment methods for poor surface gloss
Methods for handling the causes of occurrence
Adjust the injection pressure and speed appropriately if they are too low
The mold temperature is too low, increasing the mold temperature
The surface of the mold forming space is contaminated with water or grease and wiped clean
Insufficient surface grinding of mold forming space, mold polishing
Mixing different materials or foreign objects into the cleaning cylinder to filter the raw materials
Raw materials containing volatile substances increase the temperature of the melt
The raw materials have hygroscopicity, control the preheating time of the raw materials, and thoroughly bake the raw materials
Insufficient dosage of raw materials increases injection pressure, speed, time, and raw material dosage
12
The product has flow marks
Flow marks are traces of the flow of molten materials, with stripes appearing at the center of the gate.
Flow marks are caused by the rapid cooling of the material that initially flows into the forming space, and the formation of a boundary between it and the material that subsequently flows into it. To prevent flow marks, the material temperature can be increased, the material fluidity can be improved, and the injection speed can be adjusted.
If the cold material remaining at the front end of the nozzle directly enters the forming space, it will cause flow marks. Therefore, setting sufficient lagging areas at the junction of the sprue and runner, or at the junction of the runner and splitter, can effectively prevent the occurrence of flow marks. At the same time, the occurrence of flow marks can also be prevented by increasing the size of the gate.
Table 12 shows the possible causes and treatment methods of flow marks
Methods for handling the causes of occurrence
Poor melting of raw materials increases melt temperature and back pressure, accelerates screw speed
The raw materials are unclean or mixed with other materials, and the drying is insufficient. Check the raw materials and thoroughly bake them
The mold temperature is too low, increasing the mold temperature
The temperature near the gate is too low to increase the temperature
The gate is too small or improperly positioned. Increase the gate or change its position
Short holding time and extended holding time
Improper adjustment of injection pressure or speed to appropriate level
The thickness difference of the finished product section is too large, and the finished product design is changed
13
Injection molding machine screw slipping (unable to feed)
Table 13 shows the possible causes and treatment methods of screw slipping
Methods for handling the causes of occurrence
If the temperature of the rear section of the material pipe is too high, check the cooling system and reduce the temperature of the rear section of the material pipe
Incomplete and thorough drying of raw materials and appropriate addition of lubricants
Repair or replace worn material pipes and screws
Troubleshooting the feeding part of the hopper
The screw recedes too quickly, reducing the screw receding speed
The material barrel was not cleaned thoroughly. Cleaning the material barrel
Excessive particle size of raw materials reduces particle size
14
The screw of the injection molding machine cannot rotate
Table 14 shows the possible reasons and treatment methods for the inability of the screw to rotate
Methods for handling the causes of occurrence
Low melt temperature increases melt temperature
Excessive back pressure reduces back pressure
Insufficient lubrication of the screw and appropriate addition of lubricant
15
Material leakage from the injection nozzle of the injection molding machine
Table 15 shows the possible causes and treatment methods of injection nozzle leakage
Methods for handling the causes of occurrence
Excessive temperature of the material pipe reduces the temperature of the material pipe, especially in the nozzle section
Improper adjustment of back pressure and appropriate reduction of back pressure and screw speed
Main channel cold material disconnection time early delay cold material disconnection time
Insufficient release travel to increase release time, changing nozzle design
16
The material is not completely dissolved
Table 16 shows the possible causes and treatment methods for incomplete melting of materials
Methods for handling the causes of occurrence
Low melt temperature increases melt temperature
Low back pressure increases back pressure
Lower part of the hopper is too cold. Close the lower part of the hopper cooling system
Short molding cycle increases molding cycle
Insufficient drying of the material, thorough baking of the material
Post time: Sep-11-2023