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Views: 0 Author: Site Editor Publish Time: 2025-10-22 Origin: Site
Molding thermoplastic polyurethane (TPU) parts presents unique challenges that can significantly impact quality and performance. Achieving flawless TPU components requires meticulous attention to various factors, including melt temperature, mold temperature, injection speed, and pressure. Even minor deviations can lead to defects such as shrinkage, warpage, and sticking, compromising the integrity of the final product. This article explores essential techniques and best practices for molding TPU parts effectively, ensuring consistent quality and minimizing defects throughout the production process.
Melt temperature plays a huge role in how TPU turns out. If it's too low, the material stays thick and sticky, making it tough to fill the mold completely. This leads to short shots or parts with rough surfaces. On the other hand, too high a melt temperature can break down the TPU molecular structure. This degradation causes discoloration, brittleness, or even foaming if moisture is present. So, the key is to keep the melt temperature just right—hot enough to flow smoothly, but not so hot that it damages the material.
Incorrect melt temperature causes several common defects:
● Short shots: When the TPU is too thick, it can't fill the mold fully.
● Weld lines: Poor flow due to low temperature creates weak spots where flow fronts meet.
● Burn marks: Overheating causes the material to scorch near gates or thin sections.
● Flash: Excessively runny TPU leaks out at mold parting lines.
● Brittleness and discoloration: Degraded TPU loses flexibility and changes color.
● Foaming or bubbles: Moisture combined with high melt temperature generates steam inside parts.
These defects reduce part quality and may cause failure in use.
To nail the melt temperature for TPU:
● Start with the supplier's recommended range and set the melt temperature near the middle.
● Make small adjustments—even 5°C changes can impact flow and quality.
● Use precise temperature controllers to avoid fluctuations.
● Monitor part appearance and mechanical properties regularly during production.
● Avoid overheating by checking barrel zones and screw speed to prevent excessive shear heat.
● Ensure the material is fully dried; moisture worsens degradation at high temperatures.
For example, a TPU grade might recommend 200°C to 220°C melt temperature. Begin at 210°C and adjust based on part quality. If you see burn marks, lower the temperature slightly. If short shots appear, increase it a bit.
Always verify melt temperature stability with accurate sensors and maintain it within the supplier's range to prevent TPU degradation and ensure consistent part quality.
Mold temperature plays a crucial role in shaping TPU parts accurately. When the mold is too cold, TPU solidifies quickly, which can cause uneven shrinkage. This leads to warpage—where parts twist or bend—and dimensional inaccuracies. Parts might come out smaller or distorted, failing to meet specifications.
On the flip side, a mold that's too warm slows down cooling. This can reduce internal stresses, resulting in better dimensional stability. However, it also extends cycle time, affecting production efficiency. If the mold temperature isn't consistent, parts can vary from shot to shot, making quality control difficult.
For example, a mold temperature around 40°C often balances these effects well for many TPU grades. This temperature helps the material flow smoothly, fills details nicely, and cools evenly, reducing shrinkage and warpage risks.
Finding the sweet spot for mold temperature means balancing quality and speed. Lower mold temps speed up cycle time but risk defects like warpage and poor surface finish. Higher mold temps improve part quality but slow production.
To optimize:
● Start within the TPU supplier's recommended mold temperature range.
● Adjust gradually, monitoring part dimensions and surface quality.
● Use temperature controllers to keep mold temperature steady.
● Consider part thickness: thicker sections may need higher mold temps to cool evenly.
● Factor in part complexity; intricate shapes benefit from warmer molds to reduce internal stresses.
This balance ensures parts meet quality standards without sacrificing too much productivity.
Case Study 1: A manufacturer producing TPU gaskets noticed warpage after molding at 25°C mold temperature. Increasing the mold temperature to 38°C reduced warpage significantly. The warmer mold allowed more uniform cooling, minimizing internal stresses.
Case Study 2: For a TPU phone case, running the mold at 45°C improved surface gloss and detail sharpness. Though cycle time increased by 10%, customer satisfaction rose due to the premium finish.
Case Study 3: A TPU handle molded at 50°C showed sticking issues. Lowering mold temperature to 35°C helped parts eject cleanly without damage, demonstrating the need to balance mold temp to avoid sticking while controlling warpage.
Use precise mold temperature control and monitor part dimensions closely; small temperature tweaks can dramatically reduce shrinkage and warpage in TPU parts.
Injection speed and pressure are like the heartbeat of TPU molding. Too fast, and you risk overheating the material, causing burn marks or jetting. Too slow, and the TPU might start cooling too soon, leading to short shots or weak weld lines. The goal Find a sweet spot where the material flows smoothly without degrading.
For TPU, a moderate injection speed usually works best. It lets the material fill the mold evenly, avoiding cold flow fronts that cause weld lines or voids. High speeds can generate shear heat, damaging the TPU molecular structure. Low speeds might not push the material far enough before it cools, especially in thin or complex sections.
Pressure plays a key role too. Enough pressure ensures the mold fills completely and packs out details, but too much can cause flash or sticking. It also affects how well the material compensates for shrinkage during cooling. Balancing injection pressure with speed helps maintain consistent part quality.
Injection speed directly influences how TPU moves through the mold. At the start, slower speeds help the material pass through gates without excessive shear. Then, increasing speed in the main cavity helps fill larger volumes quickly before cooling sets in. This staged or profiled injection speed approach reduces stress on the material and improves surface finish.
If you inject too fast, the TPU turn to the cavity, creating flow lines or burn marks. This happens because the melt shoots ahead, trapping air or causing turbulence. Slow injection speeds reduce these issues but may increase cycle time.
Material viscosity also changes with temperature. At optimal melt temperatures, TPU flows easier, allowing slightly faster injection without damage. But if the melt temp is low, slower injection speed is necessary to avoid short shots.
Injection pressure must overcome flow resistance from the mold's runners, gates, and cavity geometry. TPU's higher viscosity means it generally needs moderate to high injection pressures compared to other thermoplastics.
Start with pressure near the TPU supplier's recommended range, then fine-tune based on part quality. If you see short shots or incomplete filling, increase pressure slightly. Watch for flash or sticking as signs pressure may be too high.
Holding pressure (packing pressure) is also critical. After the cavity fills, holding pressure pushes extra material to compensate for shrinkage, preventing sink marks or voids. The holding time should last until the gate freezes off, locking the part shape.
Pressure must be balanced carefully. Too low holding pressure leads to internal defects; too high causes overpacking, increasing stresses and risk of flash. Adjusting both injection and holding pressures together achieves optimal results.
Use a profiled injection speed—start slow through the gate, speed up in the cavity, then slow near the end—to reduce shear and ensure smooth TPU flow without defects.
Holding pressure, also called packing pressure, is the steady force applied after the mold cavity fills. It compensates for TPU shrinkage during cooling. Without enough holding pressure, parts develop sink marks—those dimples or depressions often seen opposite thick sections. Internal voids or bubbles may also form inside the part. Both reduce strength and spoil appearance.
Too much holding pressure causes issues too. It can overpack the mold, pushing TPU too hard against mold surfaces. This leads to flash—thin plastic layers squeezing out where mold halves meet—and sticking, making parts hard to eject. Overpacking also raises internal stresses, risking warpage or cracking later.
The key: balance holding pressure carefully. It should be high enough to pack out shrinkage but not so high it damages parts or mold. Holding time matters too. It must last until the gate freezes off, sealing the cavity and preventing material backflow. Too short a hold time means shrinkage compensation stops early, causing sinks or voids. Too long wastes cycle time and risks overpacking.
Back pressure is the resistance applied to the screw during its rotation and retraction before injection. It may seem odd to push back on the screw, but it's crucial for melt quality. Proper back pressure mixes the TPU melt thoroughly, ensuring uniform temperature and density.
Consistent melt density means parts have even mechanical properties and surface finish. Back pressure also helps expel air and volatiles trapped in the melt, reducing bubbles or splay defects. It pre-compresses the material, making injection smoother and more predictable.
However, too much back pressure raises screw torque and cycle time. It can generate shear heat, risking TPU degradation. Too little back pressure leads to inconsistent melt, causing color streaks or weak spots.
For TPU, moderate back pressure usually works best. Adjustments depend on material grade, machine, and part complexity.
Fine-tuning holding and back pressure is a final, critical step to perfect TPU molding. Here are some practical tips:
● Start with supplier recommendations for both pressures.
● Adjust holding pressure upward if sink marks or voids appear.
● Lower holding pressure if flash or sticking occurs.
● Set holding time to just beyond gate freeze-off, verified by trial shots.
● Increase back pressure slightly if melt inconsistencies or color streaks show.
● Reduce back pressure if cycle time or screw torque becomes excessive.
● Monitor parts closely for surface finish, dimensional accuracy, and mechanical properties during adjustments.
● Use process data logging to track pressure stability and correlate with part quality.
For example, a TPU grade might recommend holding pressure around 60–90 MPa and back pressure at 5–15 MPa. Start in the middle, then tweak based on defects observed.
Always verify gate freeze-off time experimentally to set precise holding time, ensuring shrinkage compensation without overpacking in TPU molding.
Moisture in TPU is a silent killer. Even tiny amounts cause big problems during molding. When wet TPU hits the hot barrel, moisture turns into steam, creating bubbles inside the melt. This leads to splay marks—those ugly silver streaks on parts—and internal voids that weaken the structure. Worse, moisture triggers hydrolysis, breaking down TPU's polymer chains. The result? Brittle, discolored parts that fail mechanical tests, even if they look fine at first. Moisture also worsens foaming defects and black spots caused by overheating. So, drying isn’t just a step—it's critical to avoid these costly defects.
Drying TPU right means following some key steps:
● Use a desiccant dryer: It blows super-dry air at controlled temperatures, removing moisture efficiently. Hot air ovens aren't enough for TPU.
● Follow recommended drying time and temperature: Usually 2 to 4 hours at 80–110°C (175–230°F). Check your TPU supplier's datasheet for exact specs.
● Verify dryness with a moisture analyzer: Don't guess. Measure moisture content to confirm it's below 0.05% or even 0.02% for sensitive grades.
● Dry regrind separately and thoroughly: Recycled TPU must be as dry as virgin material to avoid defects.
● Keep material dry after drying: Use heated hoppers and sealed containers. Minimize exposure to humid air during handling and processing.
Proper handling prevents moisture pickup after drying:
● Store TPU pellets in airtight containers or vacuum-sealed bags.
● Use dry rooms or climate-controlled storage areas if possible.
● Maintain hopper temperatures around 80–90°C to keep pellets dry before processing.
● Minimize time pellets sit in the hopper; long residence times increase moisture absorption.
● Avoid mixing dried and undried material; always dry mixed batches thoroughly.
● Train operators on the importance of moisture control and proper material handling.
By following these practices, you protect your TPU from moisture-related defects and ensure consistent, high-quality parts.
Always verify TPU moisture content before molding using a moisture analyzer to prevent costly defects like splay, bubbles, and brittleness.
When molding TPU, gate and runner design is crucial. TPU is more viscous than many plastics, so gates must be sized and placed carefully to ensure smooth flow. The gate controls how TPU enters the mold cavity. Too small, and you risk high shear that damages the material. Too large, and you might get flash or poor surface finish.
For TPU, tab and fan gates work well on larger, flat parts. They spread flow evenly and reduce shear stress. Pinpoint gates suit smaller or multi-cavity molds but must be sized to avoid freeze-off or stringing. Place gates at the thickest sections to help pack out the part and reduce voids.
Runners act like highways for molten TPU. Use full-round runners to minimize pressure drop and shear heating. Avoid sharp corners or abrupt size changes, which cause turbulence and degrade TPU. Keep runner lengths short and balanced to ensure all cavities fill evenly in multi-cavity molds.
Venting plays a vital role in TPU molding. TPU's viscosity tends to trap air as it flows, causing defects like burn marks, voids, or short shots. Proper venting lets trapped air escape, ensuring complete cavity fill and defect-free parts.
Vents should be shallow—about 0.015 to 0.03 mm deep—to avoid TPU flashing into the vents. Place vents at likely air trap points, such as the last fill areas, parting lines, or near ribs and inserts. Wide vent channels help air escape quickly without risking part damage.
Keep vents clean and polished along the flow direction to prevent clogging. Regular maintenance is essential, as TPU residue can build up and block vents, leading to air entrapment and defects.
Wall thickness impacts TPU part quality and performance. Too thin walls cause filling problems and weak spots. Too thick walls risk sink marks, longer cooling times, and warpage.
Ideal TPU wall thickness usually ranges from 1.0 to 3.0 mm. Softer TPU grades may go thinner, down to 0.5 mm, but require careful processing. Harder grades can handle thicker walls but watch for uneven cooling.
Keep wall thickness uniform to avoid differential shrinkage and warpage. If thickness changes are necessary, make transitions gradual with generous radii. Ribs can add strength without increasing overall thickness but should be about 50–60% of the nominal wall thickness to prevent sinks.
Design gates larger than usual and use full-round runners for TPU to reduce shear and ensure smooth flow, preventing common defects like jetting and short shots.
Precise control over melt temperature and mold temperature is crucial for defect-free TPU molding. Proper injection speed, pressure, and drying techniques further ensure high-quality parts. Future trends in TPU molding technology may offer advanced solutions for even better results. Mastering TPU molding requires attention to detail and adherence to best practices. YEESHINE TECHNOLOGY CO. offers innovative products that simplify the molding process, ensuring consistent quality and performance. Their expertise provides valuable support for achieving flawless TPU parts.
A: The TPU Part is a component made from Thermoplastic Polyurethane, known for its flexibility, durability, and resistance to abrasion and impact.
A: To prevent defects in TPU Parts, control melt and mold temperatures precisely, ensure proper drying, and optimize injection speed and pressure.
A: Mold temperature affects the dimensions and quality of TPU Parts, preventing shrinkage and warpage by ensuring even cooling.
A: Reduce mold temperature and adjust injection pressure to troubleshoot sticking issues in TPU Parts, ensuring smooth ejection.
