Overview Of TPU(Thermoplastic Polyurethanes)
TPU has excellent high tension, high tensile strength, toughness, and anti-aging properties, a type of environmental protection material. TPU has been widely used in medical and health care, electronics and electrical appliances, industry, sports, etc.
It has the characteristics of high strength, good toughness, wear resistance, cold resistance, oil resistance, water resistance, aging resistance, etc.,
It also has many excellent functions, such as moisture permeability resistance, windproof, cold resistance, antibacterial, mold resistance, warmth, and UV resistance.
Critical Factors Affecting The Injection Mold And Injection Process Of TPU Products
TPU can be injection molded, blow molded, compression molded, or extruded, but the most commonly used process is injection molding.
We recommend using the screw-type injection machine to process TPU injection molding because its injection speed, plasticization, and melting process are more uniform and can adapt to the various characteristics of TPU material.
TPU plastic has a wide range of hardness, it can get different hardness specifications of TPU plastic by changing the ratio of each reaction group of TPU, and it can still maintain good elasticity as the hardness increases.
The most critical concern in designing TPU injection molds and injection molding process is hardness, affecting every aspect of TPU injection molding and molds.
The hardness of TPU is usually expressed by Shore hardness, which is generally expressed by A below 100 degrees, such as 80A, 90A, 95A, 98A, etc., and D above 100 degrees, such as 50D, 60D, 64D, 71D, 74D, etc.
Concerns For Designing TPU Plastic Product Molds
Shrinkage Of Molded TPU Parts
Shrinkage is influenced by the hardness of the raw material, thickness, and shape of the part, molding temperature and mold temperature, and other molding conditions. Usually, the shrinkage range is 0.005~0.020cm/cm. For example, 100×10×2mm rectangular test piece, in the length direction gate, flow direction shrinkage, hardness 75A is 2~3 times larger than 60D.
Runners And Cold Cavities
The main runner is a mold section connecting the injection molding machine nozzle to the manifold or cavity. The diameter should be expanded inward at ∠ 20 or more to facilitate the runner release. The manifold is a channel connecting the main runner and each cavity in a multi-groove mold, and the arrangement on the mold should be symmetrical and equidistant. The runner can be round, semi-circular, or rectangular, with a diameter of 6~9mm. The surface of the runner must be polished like the cavity to reduce the flow resistance and to provide faster mold filling speed.
Cold Cavities
The cold material cavity is located at the end of the main runner to trap the cold material generated between injections at the end of the nozzle, thus preventing blockage of the manifold or gate. If cold material is mixed into the cavity, the product is easy produces internal stress. The cold material cavity is 8~10mm in diameter and about 6mm in depth.
Gate
The injection molding gate is the channel that connects the main runner or manifold to the cavity. Its cross-sectional area is usually smaller than the runner and is the smallest part of the runner system, and the length should be short. The shape of the gate is rectangular or circular; the size increases with the thickness of the product.
| The Thickness Of The Product | Gate Diameter |
|---|---|
| below 4mm | diameter 1mm |
| thickness 4~8mm | diameter 1.4mm |
| thickness above 8mm | diameter 2.0~2.7mm |
The gate location is usually chosen in the thickest part of the product without affecting the appearance and function, at right angles to the wall of the mold to prevent shrinkage and avoid spin lines.
Venting port
The venting port is a kind of slot-shaped air outlet opened in the mold to prevent the melt entering the mold from being involved in the gas, and the gas in the cavity is discharged from the mold. Otherwise, it will make the product with air holes, poor fusion, and mold filling, and even the air is compressed to produce high temperature and the product will be burned, causing the internal stress of the part. The air vent can be located at the end of the melt flow in the cavity or on the mold parting surface, which is 0.15mm deep and 6mm wide.
Molding Conditions
The essential molding conditions for TPUs are the temperature, pressure and time, which affect the plasticizing flow and cooling. These parameters will affect the appearance and performance of TPU parts. Good processing conditions should result in uniform white to beige-colored features.
Temperature
The temperatures that need to be controlled in the molding TPU process are barrel, nozzle, and mold temperature. The first two temperatures mainly affect the plasticization and flow of TPU, and the latter temperature affects the flow and cooling of TPU.
- Barrel Temperature
The choice of barrel temperature is related to the hardness of TPU. The melt temperature of TPU with high hardness is high, and the maximum temperature at the end of the barrel is also high. The barrel temperature range for TPU processing is 177~232℃. The barrel temperature is generally distributed from the hopper side (back end) to the nozzle (front end), gradually increasing to make the TPU temperature rise smoothly to achieve uniform plasticization.
- Nozzle temperature
The nozzle temperature is usually slightly lower than the maximum barrel temperature to prevent possible salivation of the melt in a straight-through nozzle. If a self-locking nozzle is used to eliminate salivation, the nozzle temperature can also be controlled within the maximum temperature of the barrel.
- Mold temperature
Mold temperature has a significant influence on the intrinsic properties and apparent quality of TPU products. Many factors, such as the crystallinity of TPU and the size of the product determine its level. Mold temperature is usually controlled by a constant temperature cooling medium such as water, and the mold temperature is high for TPU with high hardness and crystallinity. For example, Texin:
| Hardness | Mold temperature |
| 480A | 20 to 30°C |
| 591A | 30～50℃ |
| 355D | 40～65℃ |
Pressure
The injection process pressure includes plasticizing pressure (back pressure) and injection pressure. The pressure to which the top melt is subjected when the screw is backed off is the back pressure, which is regulated by the relief valve. Increasing the back pressure will increase the melt temperature, reduce the plasticizing speed, make the melt temperature uniform, mix the color material evenly, and discharge the melt gas, but it will prolong the molding cycle. The back pressure of TPU is usually in 0.3~4MPa.
TPU flow resistance and mold filling rate are closely related to melting viscosity, which in turn is directly related to TPU hardness and melt temperature, i.e., melt viscosity is not only determined by temperature and pressure but also by TPU hardness and deformation rate. The higher the shear rate, the lower the viscosity; the shear rate is constant, and the higher the TPU hardness, the greater the viscosity.
Under the condition of a constant shear rate, viscosity decreases with increasing temperature. Still, at a high shear rate, viscosity is not affected by temperature as much as low shear rate. The injection pressure of TPU is generally 20~110MPa. Holding pressure is about half of the injection pressure; back pressure should be below 1.4MPa to make TPU plasticized evenly.
Molding Cycle
The time required to complete an injection process is called the molding cycle. The molding cycle includes mold filling time, holding time, cooling time, and other times (mold opening, mold release, mold closing, etc.), which directly affects productivity. The hardness, part thickness, and shape usually determine the molding cycle of TPU. The cycle time of TPU is short for high hardness, long for thick parts, and long for complex part shapes, and the molding cycle is also related to mold temperature. The molding cycle of TPU is generally between 20~60 seconds.
Injection Speed
The configuration of TPU products mainly determines the injection speed. Products with a thick-end face need a lower injection speed, while a thin-end face needs a faster injection speed.
Screw Speed
Processing TPU products requires a low shear rate, so a lower screw speed is appropriate. 20~80r/min for TPU screw speed is generally preferred to 20~40r/min.
Shutdown Processing
As TPU may degrade under high temperatures for an extended period of time, it should be cleaned with PS, PE, acrylate plastic, or ABS after shutdown; shut down for more than 1 hour; and the heating should be turned off.
Post-processing of products
TPU, due to uneven plasticization in the barrel or different cooling rates in the mold cavity, often produces uneven crystallization, orientation, and shrinkage, resulting in the existence of internal stress in the product, which is more prominent in thick-walled products or products with metal inserts. In storage and use, the products with internal stress often suffer from mechanical property degradation, surface silvering, and even deformation and cracking. The solution to these problems in production is to anneal the products. The annealing temperature depends on the hardness of TPU products, high hardness products’ annealing temperature is also higher, and low hardness temperature is also low; too high a temperature may make the product warp or deformation, and too low to achieve the purpose of eliminating internal stress. TPU annealing should be used for a long time at low temperatures, low hardness products can be placed at room temperature for several weeks to achieve the best performance. Hardness in Shore A85 annealed below 80 ℃ × 20h, A85 above 100 ℃ × 20h can be. Annealing can be carried out in the hot air oven; pay attention to the placement position, not to local overheating and deformation of the product.
Annealing eliminates not only internal stress but also improves mechanical properties. Since TPU is a two-phase form, the mixing of phases occurs during TPU thermal processing, and phase separation is slow during rapid cooling due to the high viscosity of TPU; there must be enough time for them to separate and form micro-zones to obtain the best properties.
Inlay Injection Molding
To meet the needs of assembly, TPU parts need to be embedded in metal inserts. The metal insert is first put into a predetermined position in the mold and then injected into a whole product. TPU products with inserts are not firmly bonded with TPU due to the difference in thermal properties and shrinkage rate between metal inserts and TPU. The solution is to preheat the metal insert because after preheating, the insert reduces the temperature difference of the melt so that the injection process can make the melt around the insert cool slower, shrinkage is more uniform, the occurrence of a certain amount of hot material shrinkage, to prevent excessive internal stress around the insert. TPU inlay molding is relatively easy, and the shape of the insert is not limited; as long as the insert is degreasing, it will be in the 200 ~ 230 ℃ heating treatment for 1.5 ~ 2 minutes. To obtain a firmer bond, the insert can be coated with adhesive, heated at 120°C, and injected. In addition, it should be noted that the TPU used should not contain lubricant.
Reuse Of Recycled Materials
In the TPU processing process, the main runner, manifold, unqualified products, and other waste materials can be recycled and reused. From the experimental results, 100% recycled material is not mixed with new material, and the mechanical properties of the decline are not too serious. They can be used, but to maintain the physical and mechanical properties and injection conditions at the best level, the recommended proportion of recycled material in 25% to 30% is good. It should be noted that the recycled material and the new material of the exact species specifications have been contaminated or have been annealed to avoid using recycled material; recycled material should not be stored for too long; the best immediately granulated, dry use. The melt viscosity of the recycled material should generally be reduced, and the molding conditions should be adjusted.
Causes Of Injection Defects And Treatment
TPU molded products are, in principle, manufactured according to standard specifications. However, it still varies quite extensively and abruptly. Sometimes, dents, bubbles, cracks, deformations, and other substandard products are produced during production. Therefore, it is necessary to understand the problem and propose a solution to the defective products, which is the accumulation of expertise and practical experience. Sometimes it is only required to change the operating conditions or make slight processing and adjustment in raw materials, molds, and machines to solve the problem. We have prepared “The causes and treatment of injection defects of TPU molding products,” hoping to guide you to find the causes of product defects and choose a suitable method to solve them.