Process sharing-TPU injection molding process

Time:2020-05-14 10:39:08 / Popularity: / Source:

Hardness of TPU is usually expressed by internationally recognized Shore hardness. In general, A is expressed below 100 degrees, such as 80A, 90A, 95A, 98A, etc., and D is expressed above 100 degrees, such as: 50D, 60D, 64D, 71D, 74D, etc.
There are many methods for TPU molding process: including injection molding, blow molding, compression molding, extrusion molding, etc. Among them, injection molding is the most commonly used. Function of injection molding is to process TPU into required parts, which are divided into three discrete processes: pre-plasticization, injection and demould. Injection machine is divided into two types: plunger type and screw type. It is recommended to use screw injection machine, because it provides uniform speed, plasticization and melting.
TPU injection molding 

1. Design of injection machine

Barrel of injection machine is lined with copper and aluminum alloy, screw is chrome plated to prevent wear. Screw length-diameter ratio L / D = 16 ~ 20 is better, at least 15; compression ratio is 2.5 / 1 ~ 3.0 / 1. Length of feeding section is 0.5L, compression section is 0.3L, and metering section is 0.2L. Check ring should be installed near top of screw to prevent backflow and maintain maximum pressure.
For processing TPU, self-flowing nozzles should be used. Outlet is inverted cone, diameter of nozzle is more than 4mm, which is smaller than 0.68mm of inlet of main runner. Nozzle should be equipped with a controllable heating belt to prevent material from solidifying.
From an economic point of view, injection volume should be 40% to 80% of rated volume. Screw speed is 20 ~ 50r / min.
TPU molding 

2. Mold design

Pay attention to following points in mold design:

(1) Shrinkage of molded TPU parts

Shrinkage is affected by molding condition such as hardness of raw material, thickness, shape, molding temperature and mold temperature of part. Usually shrinkage range is 0.005 ~ 0.020cm / cm. For example, a rectangular test piece of 100 * 10 * 2 mm has a gate in longitudinal direction and shrinks in flow direction, hardness of 75A is 2 to 3 times greater than that of 60D. Impact of TPU hardness and thickness on shrinkage is shown in Figure 1. It can be seen that when TPU hardness is between 78A and 90A, shrinkage of workpiece decreases with increasing thickness; when hardness is 95A to 74D, shrinkage of workpiece increases slightly with increasing thickness.

(2) Runner and cold cavity

Main runner is a section of mold that connects nozzle of injection machine to sub runner or cavity. Diameter should be enlarged inward at an angle of more than 20 to facilitate demould of  mold from runner. Sub runner is channel connecting main runner and each cavity in multi-slot mold. Layout on mold should be symmetrical and equidistant. Runner can be round, semi-circular, or rectangular, with a diameter of 6-9 mm. Surface of runner must be polished like a cavity to reduce flow resistance and provide a faster filling speed.
Cold cavity is a cavity located at the end of main runner to capture cold material generated between two injections at the end of nozzle, thereby preventing clogging of runner or gate. Cold material is mixed into cavity, product is prone to internal stress. Cold cavity has a diameter of 8 ~ 10mm and a depth of about 6mm.

(3) Gate and exhaust

Gate is a channel that connects main runner or sub runner and cavity. Cross-sectional area is usually smaller than runner, which is the smallest part of runner system, and length should be short. Shape of gate is rectangular or round, and size increases with thickness of product. Thickness of product is less than 4mm and diameter is 1mm; thickness is 4 ~ 8mm and diameter is 1.4mm; Gate position is generally selected at the thickest part of product without affecting appearance and use, at a right angle to mold wall to prevent shrinkage and avoid swirling.
Exhaust port is a groove-shaped air outlet opened in mold to prevent molt entering mold from being caught in gas and expelling gas in cavity from mold. Otherwise, product will have air holes, poor welding, underfilling, or even burn product due to high temperature caused by compression of air, internal stress in workpiece. Exhaust port can be set at the end of melt flow in cavity or on mold parting surface. It is a 0.15mm deep and 6mm wide sprue.
Care must be taken to control mold temperature as uniformly as possible to avoid warpage and distortion of parts.
molding condition 

3. Molding condition

The most important molding condition of TPU are temperature, pressure and time that affect flow and cooling of plasticization. These parameters will affect appearance and performance of TPU parts. Good processing conditions should be able to obtain uniform white to beige parts.

(1) Temperature

Temperature to be controlled in molding TPU process is barrel temperature, nozzle temperature and mold temperature. The first two temperatures mainly affect plasticization and flow of TPU, the latter two temperatures affect flow and cooling of TPU.
A. Barrel temperature
Choice of barrel temperature is related to hardness of TPU. TPU with high hardness has a high melting temperature, maximum temperature at the end of barrel is also high. Temperature range of barrel used for processing TPU is 177 ~ 232 ℃. Temperature distribution of barrel is generally from side of hopper (rear end) to nozzle (front end), gradually increasing, so that temperature of TPU rises smoothly to achieve purpose of uniform plasticization.

B. Nozzle temperature 

Nozzle temperature is usually slightly lower than maximum temperature of barrel, to prevent molten material from drooling in straight-through nozzle. If self-locking nozzles are used to prevent drooling, nozzle temperature can also be controlled within maximum temperature range of barrel.

C. Mold temperature

Mold temperature has a great influence on internal performance and apparent quality of TPU products. Its height depends on many factors such as crystallinity of TPU and size of product. Mold temperature is usually controlled by a constant-temperature cooling medium such as water. TPU has high hardness, high crystallinity, and high mold temperature. For example, Texin, hardness 480A, mold temperature 20~30 ℃; hardness 591A, mold temperature 30~50 ℃; hardness 355D, mold temperature 40~65 ℃. TPU product mold temperature is generally 10 ~ 60 ℃. Mold temperature is low, melt freezes prematurely to produce streamlines, it is not conducive to growth of spherulites, which makes crystallinity of product low, and the later crystallization process will occur, which will cause product to shrink and change in performance.

(2) Pressure

Injection pressure includes plasticizing pressure (back pressure) and injection pressure. When screw retreats, pressure on top melt is back pressure, which is adjusted by overflow valve. Increasing back pressure will increase melt temperature, reduce plasticizing speed, make melt temperature uniform, color materials mix evenly, and discharge melt gas, but it will prolong molding cycle. Back pressure of TPU is usually 0.3 to 4MPa.
Injection pressure is pressure exerted on TPU by top of screw. Its function is to overcome flow resistance of TPU from barrel to cavity, fill mold of melt, and compact melt. TPU flow resistance and mold filling rate are closely related to melt viscosity, melt viscosity is directly related to TPU hardness and melt temperature, that is, melt viscosity is not only determined by temperature and pressure, but also by TPU hardness and deformation rate. The higher shear rate, the lower viscosity; constant shear rate, the higher TPU hardness, the greater viscosity.
Under condition of constant shear rate, viscosity decreases with increasing temperature, but under high shear rate, viscosity is not affected by temperature as much as low shear rate. Injection pressure of TPU is generally 20 ~ 110MPa. Holding pressure is about half of injection pressure, and back pressure should be below 1.4 MPa to make TPU plasticize evenly.

(3) Time

Time required to complete an injection process is called molding cycle. Molding cycle includes mold filling time, pressure holding time, cooling time and other times (opening mold, demolding, closing mold, etc.), which directly affects labor productivity and equipment utilization. Molding cycle of TPU usually depends on hardness, thickness of part and configuration. TPU hardness is high, cycle is short; thick plastic parts, long cycle; complex plastic parts configuration, long cycle, molding cycle is also related to mold temperature. TPU molding cycle is generally between 20 ~ 60s.

(4) Injection speed

Injection speed mainly depends on configuration of TPU product. Products with thick end faces require lower injection speed, while thin end faces provide faster injection speed.

(5) Screw speed

Processing TPU products usually requires a low shear rate, so a lower screw speed is appropriate. Screw speed of TPU is generally 20 to 80 r / min, preferably 20 to 40 r / min.

4. Stop processing

Since TPU may degrade for a prolonged time under high temperature, after shutdown, it should be cleaned with PS, PE, acrylic plastic or ABS; if it is shut down for more than 1 hour, heating should be turned off.
molding condition 

5. Post-processing of products

Due to uneven plasticization in barrel or different cooling rates in mold cavity, TPU often produces uneven crystallization, orientation, and shrinkage, which results in internal stress in products. This is more prominent in thick-walled products or products with metal inserts. Products with internal stress often suffer from a decrease in mechanical properties during storage and use, surface has silver streaks or even deformation and cracking. Way to solve these problems in production is to anneal products. Annealing temperature depends on hardness of TPU product. Annealing temperature of product with high hardness is also higher, and temperature of low hardness is lower. Temperature is too high may cause product to warp or deform, too low to achieve purpose of eliminating internal stress. Annealing of TPU should use low temperature for a long time, and products with lower hardness can reach the best performance after being placed at room temperature for several weeks. When hardness is below Shore A85, it is annealed at 80 ℃ for 20h, and above A85 at 100 ℃ for 20h. Annealing can be carried out in a hot air oven, taking care not to overheat locally and deform product.
Annealing can not only eliminate internal stress, but also improve mechanical properties. Because TPU is a two-phase form, phase mixing occurs during TPU thermal processing. During rapid cooling, due to high viscosity of TPU, phase separation is very slow, and there must be sufficient time for it to separate and form micro-domains to obtain optimal performance.

6. Mosaic injection

In order to meet needs of assembly and use strength, metal inserts need to be embedded in TPU parts. Metal insert is first placed in a predetermined position in mold, and then injected into a whole product. For TPU products with inserts, due to large difference in thermal performance and shrinkage between metal insert and TPU, insert and TPU are not bonded well. Solution is to pre-heat metal insert, because insert reduces temperature difference of melt after preheating, so that melt around insert can be cooled slowly during injection process, and shrinkage is more uniform. A certain amount of hot material shrinkage occurs to prevent excessive internal stress around insert. TPU insert molding is relatively easy, and shape of insert is not limited. As long as insert is degreased, it is heated at 200 to 230 ° C for 1.5 to 2 minutes, peel strength can reach 6 to 9 kg / 25 mm. For a stronger bond, apply adhesive to insert, heat at 120 ° C, and then inject. In addition, it should be noted that TPU used must not contain lubricants.

7. Reuse of recycled materials

In the process of TPU processing, waste materials such as main runner, sub runner, and substandard products can be recycled. From experimental results, 100% recycled materials are not blended with new materials, mechanical properties decline is not too serious. It can be fully used, but in order to maintain physical, mechanical properties and injection conditions at the optimal level, it is recommended that proportion of recycled materials be 25% to 30%.. It should be noted that specifications of reclaimed material and new material are preferably same. Contaminated or annealed reclaimed material should be avoided. Reclaimed material should not be stored for too long. It is best to granulate and dry it immediately. Melt viscosity of recycled materials is generally reduced, and molding conditions are adjusted.

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