The essential molding conditions for TPU injection molded products are the temperature, pressure, and time that affect the plasticization flow and cooling.
The characteristics of TPU, especially its processing temperature fluctuation range, are particularly narrow, requiring the injection molding machine must have accurate temperature regulation and control system. Otherwise, it will cause product defects due to various factors.
Four factors cause TPU product defects: raw materials, molds, injection molding machines, and injection conditions.
We have made a quick checklist to summarize the rules and experience so that we can quickly find the causes of similar problems and make slight adjustments to the raw materials, molds, and machines to solve the problem.
This article is based on the screw injection molding machine to produce TPU products and is suitable for those in the same conditions.
The Products Have Depression
The dent on the surface of TPU products will reduce the quality and strength of the finished product and also affect the appearance of the product. The causes of denting are related to the raw materials used, molding technology, and mold design, such as the shrinkage rate of raw materials, injection pressure, mold design, cooling device, etc.
| Causes | Solution |
|---|---|
| Insufficient mold feed | Increase the feeding volume |
| High melt temperature | Lower melt temperature |
| Short injection time | Increased injection time |
| Low injection pressure | Increase injection pressure |
| Insufficient clamping pressure | Increase the clamping pressure appropriately |
| Improper mold temperature | Adjust to the proper temperature |
| Gating asymmetry | Adjustment of mold entrance size or position |
| Poor venting of depressed areas | Ventilation holes in recessed areas |
| Insufficient cooling time of the mold | Extended cooling time |
| Screw-stop collar wear | Replacement |
| Uneven thickness of the product | Increase injection pressure |
The Product Has Bubbles
In the injection molding process, the products sometimes have a lot of bubbles, affecting their strength and mechanical properties, and the product’s appearance is also significantly reduced. Usually, consequences due to uneven thickness or mold have a prominent rib, and the material in the mold cooling rate is different, resulting in uneven shrinkage and easy-to-form bubbles, so the mold design must pay special attention.
In addition, the raw material is not sufficiently dried and still contains some water in the melt material. When the heat decomposes into gas, easy to enter the mold cavity to form bubbles. So when the product appears bubble, you can check the following factors and deal with it.
| Causes | Solution |
|---|---|
| Wet raw materials | Thoroughly roasted |
| Insufficient material injection | Check temperature, injection pressure, and injection time |
| Injected too fast | Reduced injection speed |
| High raw material temperature | Lower melt temperature |
| Low back pressure | Increase back pressure to appropriate |
| Finished section, rib, or column too thick | Change of finished product design or spillway location |
| Sprue overflow is too small | Enlarged waterway and entrance |
| Uneven mold temperature | Adjust mold temperature to uniform |
| Screw back speed is too fast | Reduced screw setback speed |
The Product Has Cracks
Cracking is a fatal phenomenon of TPU products, usually manifested as hair-like cracks on the product’s surface. When the product has sharp edges, this part often does not have obvious thin cracks, which is very serious for the product. The main reasons for crashes occurring during the production process are as follows.
- Difficulty in demolding.
- Overfilling.
- Low mold temperature.
- Bad construction design of the product
To avoid cracks caused by poor demolding, the mold’s cavity must be equipped with a sufficient demolding slope, and the ejector’s size, position, and form must be appropriate. When ejecting, the release resistance of each part of the finished product should be uniform.
Overfilling is due to the application of excessive injection pressure or excessive material measurement so that the internal stress of the product is too large, resulting in cracks when the mold is released; in this state, the amount of molded part variation also increases, making it more difficult to release from the mold, so we should reduce the injection pressure to prevent overfilling.
The injection molding gate area is often prone to residual excessive internal stress, and the vicinity of the gate is accessible to embrittlement, especially the part of the direct gate, which is easy to rupture due to internal pressure.
| Causes | Solution |
|---|---|
| High injection pressure | Reduce injection pressure and speed |
| Excessive packing | Reduction of raw material measurement |
| The melting barrel temperature is too low | Increase melt barrel temperature |
| Insufficient release slope | Adjustment of release slope |
| Improper ejection method | Inspection and repair of molds |
| Metal-embedded parts related | Adjustment or modification of molds |
| The mold temperature is too low | Increase mold temperature |
| Gates are too small or improperly shaped | Modification |
| Part of the draft is not enough | Inspection and repair of molds |
| With release chamfer | Check and repair molds |
| The product cannot be balanced by demolding | Check and repair molds |
| Vacuum in the mold during demolding | Slow speed when demolding or ejecting, and add air intake equipment |
Warpage And Deformation Of Products
The reasons for the warping and deformation of TPU injection molded products are too short-term cooling and shaping time, too high mold temperature, non-uniformity, and asymmetry of the runner system. Therefore, we should avoid the following elements in the mold design.
- Too much difference in thickness in the same plastic part.
- The existence of excessive sharp corners.
- The buffer zone is too shallow, so the difference between thick and thin turns is too significant.
In addition, we should pay attention to setting the correct number of ejector pins and designing a proper cooling flow channel for the mold cavity.
| Causes | Solution |
|---|---|
| The product is not cooled when it is demolded | Extended cooling time |
| Asymmetric shape and thickness of the product | Change of molding design or addition of reinforcement ribs |
| Excessive packing | Reduced injection pressure, speed, time, and material |
| Uneven gate feed | Change the gate or increase the number of gates |
| Ejector system imbalance | Adjustment of the ejector position |
| Uneven mold temperature | Adjust mold temperature to equalize |
| Excessive buffering of raw materials | Reduced raw material buffer |
Products With Scorched Spots Or Black Stripes
The main reason for the occurrence of scorched spots or black lines is the poor thermal stability of raw materials caused by the thermal decomposition of raw materials.
The effective way to prevent the occurrence of scorched spots or black lines is to prevent the temperature of the raw material in the barrel from getting too hot and to slow down the injection speed. If the inner wall of the barrel or the screw is scarred or chipped, some of the raw material will be trapped, which will cause thermal decomposition due to overheating. In addition, the non-return valve can also cause thermal decomposition due to stagnant raw materials. Therefore, when using high-viscosity or easily decomposed raw materials, we should pay special attention to prevent the occurrence of scorched spots or black lines.
| Causes | Solution |
|---|---|
| High raw material temperature | Lower melt temperature |
| The injection pressure is too high | Lower injection pressure |
| Screw speed is too fast | Slow down the screw speed |
| The eccentricity of the screw and material tube | Returning |
| Frictional heat | Inspection and repair of machines |
| The nozzle hole is too small, or the temperature is too high | Readjustment of orifice or temperature |
| Heated material tube with burned black material (high-temperature quenching part) | Inspection or replacement of the material tube |
| Mixed ingredients | Re-filtering or replacing raw materials |
| Improper mold exhaust | Appropriately increase the air venting |
Products With Raw Edges
Burrs are a common problem for TPU products. When the pressure of raw materials in the mold cavity is too enormous, the resulting parting force is greater than the clamping force, thus forcing open the mold and causing the raw materials to overflow and form a burr. There may be various reasons for the formation of burrs, such as problems with the raw material, the injection molding machine, improper adjustment, or even the mold itself may be possible. Therefore, when determining the cause of burrs, we should proceed from easy to difficult.
Check whether the raw materials are roasted thoroughly, mixed with debris, or combined with different kinds of raw materials and whether the viscosity of raw materials affects.
Correct adjustment of the pressure control system of the injection molding machine and the adjustment of the injection speed must match the clamping force used.
- Checking whether the raw materials are thoroughly roasted, mixed with debris, or mixed with different kinds of raw materials, and the influence of raw material viscosity.
- Correct adjustment of the pressure control system of the injection molding machine and the adjustment of the injection speed must match the clamping force used.
- Whether certain parts of the mold are worn, whether the exhaust holes are blocked, and whether the design of the flow channels is reasonable.
- Whether there is any deviation in the parallelism between the injection molding machine templates, whether the force distribution of the template ties is uniform, and whether the screw stop ring and the melt barrel are worn.
| Causes | Solution |
|---|---|
| Wet raw materials | Thoroughly roasted |
| Contamination of raw materials | Check raw materials and any impurities to find the source of contamination |
| The viscosity of raw material is too high or too low | Check the viscosity of the raw material and the operating conditions of the injection molding machine. |
| The clamping force is too low | Check pressure value and adjustment |
| Injection and holding pressure too high | Checking the setting value and adjustment |
| Injection pressure changeover too late | Check the changeover pressure position and re-adjust for early changeover |
| Injecting too fast or too slow | Checking and adjusting flow control valves |
| Temperature too high or too low | Check the electric heating system and screw speed |
| Insufficient rigidity of the stencil | Check clamping force and adjustment |
| Wear the melt barrel, screw, or non-return ring | Repair or replacement |
| Backpressure valve wear | Repair or replacement |
| Uneven clamping force | Checking the force on the tie rod |
| Template not parallel | Correction |
| Mold vent blockage | Clearance |
| Mold wear | Check the number of times the mold is used and the clamping force, and repair or replace it. |
| Mold parting does not fit. | Check whether the relative position of the mold is offset and re-adjust |
| Mold runner imbalance | Check design and modification |
| Low mold temperature and uneven heating | Inspection of the electrical heating system and repair |
The Products Have Sticky Mold (Difficult To Demolding)
TPU in the injection molding products sticky mold, the first thing to consider is whether the injection or pressure-holding pressure is too high. Because the injection pressure will cause excessive saturation of the product so that the raw materials fill in other voids and make the product stuck in the mold cavity, caused of difficulty in release; secondly, when the molten barrel temperature is too high, it will make the raw material decomposition of heat degradation, broken or fractured in the demolding process, resulting in sticky mold. As for mold problems, such as an unbalanced inlet, the rate of product cooling is not consistent, which will also cause the products to demolding of the sticky mold phenomenon.
| Causes | Solution |
|---|---|
| High injection pressure or melt cylinder temperature | Lower injection pressure or melt barrel temperature |
| Too long holding time | Reduced pressure-holding time |
| Insufficient cooling | Increase cooling cycle time |
| Mold temperature is too high or too low | Adjustment of mold temperature and relative temperature on both sides |
| Release chamfer inside the mold | Tooling repair to remove chamfers |
| Mold inlet imbalance | Limit the melt flow as close as possible to the main runner |
| Poor mold exhaust design | Set reasonable air venting |
| Cavity misalignment | Adjustment of Cavity |
| The mold surface is too smooth | Improvement of the mold surface |
| Lack of mold release agent | Use a release agent when it does not affect secondary processing |
Product Toughness Is Reduced
Toughness is the energy required to make the material fracture. The main factors that cause the reduction of toughness are raw materials, recycling materials, temperature, mold, etc. toughness of the product is reduced, which will directly affect the strength and mechanical properties of the product.
| Causes | Solution |
|---|---|
| Wet raw materials | Thoroughly roasted ingredients |
| The proportion of recycled material is too large | Reduce the proportion of recycled materials incorporated |
| Melt temperature too high or too low | Adjustment of melt temperature |
| The mold gate is too narrow | Increase gate size |
| Long gate joint area in the mold | Reducing the length of the gate joint area |
| The mold temperature is too low | Increase mold temperature |
Insufficient Filling Of Products
Underfilling of TPU products refers to the phenomenon that the molten material does not ultimately flow through all corners of the cavity. The reasons for underfilling are improperly set molding conditions, poor mold design, and thin wall thickness of the molded product. The countermeasures for the molding conditions are to increase the material and mold temperature, increase the injection pressure and speed, and improve the flow of the metal. The mold can grow the size of the sprue or runner or adjust the location, size, and number of gates to make the molten material flow smoothly. Furthermore, air venting holes can be set at appropriate places to make the gas evacuate smoothly.
| Causes | Solution |
|---|---|
| Insufficient supply of material | Increase the feeding volume |
| Premature curing of the product | Increase mold temperature |
| The melting barrel temperature is too low | Increase melt barrel temperature |
| Low injection pressure | Increase injection pressure |
| Slow injection speed | Increase injection speed |
| Short injection time | Increased injection time |
| Low or uneven mold temperature | Adjusting mold temperature |
| Nozzle or funnel blockage | Dismantling and cleaning |
| Improper gating position | Adjustment changes |
| Small runners | Enlarged runners |
| Small sprue or overflow | Enlarge sprue or overflow |
| Screw-stop collar wear | Replacement |
| Gas is not discharged from the forming space | Adding air venting holes at appropriate locations |
The Product Has A Bonding Line
The bonding line is a thin line formed by two or more merging parts of the molten material, usually known as the welding line. A combined line not only affects the product’s appearance but is also detrimental to the strength of the product. The main reasons for the occurrence of bond lines are.
- The product’s shape (mold configuration) is due to the material’s flow.
- Poor merging of the molten material.
- Air, volatiles, or refractory materials are mixed in the molten material flow.
Raise the temperature of the material and mold to minimize the degree of bonding line. At the same time, changing the location and number of gates to move the bonding line to another location, setting up an air vent in the fusion part to quickly evacuate the air and volatiles in this area, or setting up a material overflow pool near the fusion part, moving the bonding line to the overflow pool and then removing it are effective countermeasures to eliminate the bonding line.
| Causes | Solution |
|---|---|
| Insufficient injection pressure and time | Increase injection pressure and time |
| The injection speed is too slow | Increase injection speed |
| Low melt temperature | Increase melt barrel temperature |
| Low back pressure, slow screw speed | Increase back pressure, screw speed |
| Improper gate position, too small gate, and runner | Change the gate position or adjust the mold entrance size |
| The mold temperature is too low | Increase mold temperature |
| Material curing too fast | Reduced material curing speed |
| Poor material flow | Increase melt barrel temperature to improve material flow |
| Material is hygroscopic | Adding air venting holes to control material quality |
| Poor venting in the mold | Increase the air vent or check if the air vent is blocked |
| Raw materials are unclean or mixed with other materials | Checking raw materials |
| Too many release agent | Use less mold release agent or try not to use it |
No Luster On The Surface Of The Product
TPU product surface loss of the original material luster, the formation of layers of film or fuzzy state, etc., can be called surface gloss.
Product surface gloss is due mainly to mold-forming surface grinding. Increasing the material and mold temperature can enhance the product surface gloss when the forming space surface condition is good. Too much refractory or greasy refractory is also the cause of poor surface gloss. At the same time, the material absorbs moisture or contains volatiles, and foreign matter mixed with pollution is also the cause of the low surface gloss of products. Therefore, we should pay special attention to mold and material factors.
| Causes | Solution |
|---|---|
| Injection pressure and speed are too low | Appropriate adjustments |
| The mold temperature is too low | Increase mold temperature |
| Water or grease contamination on the mold-forming space surface | Wipe clean |
| Insufficient surface grinding of mold-forming space | Mold polishing |
| Different materials or foreign objects mixed in | Cleaning the barrel and filtering the material |
| Raw materials contain volatiles | Increase melt temperature |
| The material is hygroscopic | Control the preheating time of raw materials to dry them thoroughly |
| Insufficient raw materials | Increase injection pressure, speed, time, and raw materials |
The Product Has Flow Marks
Flow marks are the traces of molten material flowing, with the gate as the center and presenting the striped pattern.
Flow marks are caused by the initial inflow of material into the mold cavity cooling too fast and forming a boundary between the material flowing after it. We can increase the material temperature and improve the flow and injection speed to prevent the flow marks.
Cold material remaining at the front of the nozzle can cause flow marks if it enters the mold cavity. Therefore, we can effectively prevent flow marks by setting up sufficient stagnant parts at the confluence of the sprue and runner or the intersection of the runner and manifold. Also, an Enlarged gate size can prevent flow marks.
| Causes | Solution |
|---|---|
| Poor melting of raw materials | Increase melt temperature, back pressure, and speed up screw speed |
| Raw materials are unclean or mixed with impurities, insufficient drying | Check raw materials and dry them thoroughly |
| The mold temperature is too low | Increase mold temperature |
| The temperature near the gate is too low | Increase temperature |
| Gates are too small or improperly positioned | Increase the gate or change the position |
| Short pressure retention time | Extended pressure-holding time |
| Improper injection pressure or speed | Adjust to appropriate |
| The thickness of the finished section varies too much | Change of finished product design |
Injection Molding Machine’s Screw Slippage (Can Not Feed)
| Causes | Solution |
|---|---|
| The excessive temperature at the rear section of the material pipe | Check the cooling system to reduce the temperature of the rear section of the material pipe |
| Incomplete drying of raw materials | Thorough roasting of raw materials and proper lubrication |
| Material tube and screw wear | Repair or replacement |
| Hopper feeding part failure | Troubleshooting of feeding parts |
| Screw backing off too fast | Reduced screw setback speed |
| The barrel is contaminated | Cleaning barrel |
| Oversized raw material particles | Reducing particle size |
The injection Molding Machine Screw Can Not Rotate
| Causes | Solution |
|---|---|
| Low melt temperature | Increase melt temperature |
| High back pressure | Reduced back pressure |
| Insufficient lubrication of the screw | Appropriate addition of lubricant |
Leakage Of Injection Nozzle Of Injection Molding Machine
| Causes | Solution |
|---|---|
| The excessive temperature of the material tube | Reduce the material tube temperature, especially in the injection nozzle section. |
| Improper back pressure adjustment | Appropriate reduction of back pressure and screw speed |
| Main flow channel cold material break off time early | Delayed cold material break-off time |
| Insufficient loosening and receding travel | Increase the release time and change the nozzle head design |
Materials Not Dissolved
| Causes | Solution |
|---|---|
| Low melt temperature | Increase melt temperature |
| Low back pressure | Increase back pressure |
| Subcooling of the lower part of the hopper | Shut down the lower hopper cooling system |
| Short molding cycle | Increased molding cycle time |
| Insufficient material drying | Thorough drying of the material |
We have listed the common defects of TPU plastic injection molding and how to deal with them. Debugging the mold and injection molding conditions is a process we must go through, from mold design and production to mass production. To shorten the process and reduce the difficulty of solving problems, we still need to plan everything from the very beginning of the mold design stage.
We have prepared a “TPU Injection Molding Guide” for you, hoping to help you reduce the detours and execute a successful TPU injection molding project.