There are many methods of TPU molding process: including injection molding, blow molding, compression molding, extrusion molding, etc., among which injection molding is the most commonly used.

Barrel of injection machine is lined with copper-aluminum alloy, and screw is chrome-plated to prevent wear. Screw length-to-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. Injection volume should be 40%~80% of rated volume. Screw speed is 20~50r/min.

Pay attention to following points when designing molds:

Shrinkage is affected by molding conditions such as hardness of raw material, thickness and shape of part, molding temperature and mold temperature. Usually shrinkage range is 0.005~0.5px/cm. For example, a 100×10×2mm rectangular test piece is gated in length direction and shrinks in flow direction. Hardness of 75A is 2 to 3 times greater than that of 60D. When TPU hardness is between 78A and 90A, shrinkage rate of part decreases as thickness increases; when hardness is between 95A and 74D, shrinkage rate of part increases slightly as thickness increases.

Main runner is a passage in mold that connects injection machine nozzle to runner or cavity. Diameter should be expanded inward to an angle of more than 2° to facilitate demoulding of runner debris. Sub runner is channel connecting main runner and each cavity in multi-slot mold. Arrangement on mold should be symmetrical and equidistant. Flow channel can be circular, semicircular, or rectangular, and diameter is preferably 6 to 9 mm. Runner surface must be polished like mold cavity to reduce flow resistance and provide faster mold filling.
Cold slug cavity is a cavity located at the end of main flow channel to capture cold slug generated between two injections at the end of nozzle to prevent clogging of runner or gate. Cold material is mixed into mold cavity, and product is prone to internal stress. Diameter of cold material cavity is 8~10mm and depth is about 6mm.

 

Gate is channel that connects main channel or runner with mold cavity. Its cross-sectional area is usually smaller than flow channel, it is the smallest part of flow channel system, and its length should be short. Shape of gate is rectangular or circular, and size increases with thickness of product. For products with a thickness of less than 4mm, diameter is 1mm; for products with a thickness of 4~8mm, diameter is 1.4mm; for products with a thickness above 8mm, diameter is 2.0~2.7mm. Gate position is generally selected where product is thickest without affecting appearance and use, at right angles to mold wall to prevent shrinkage holes and swirl marks.
Exhaust product is a groove-shaped air outlet opened in mold to prevent molten material entering mold from being entrained in gas and to discharge gas in cavity out of mold. Otherwise, product will have pores, poor welding, unsatisfactory mold filling, and even product will be burned due to high temperature generated by compression of air, resulting in internal stress in product. Exhaust port can be located at the end of melt flow in cavity or on parting surface of mold. It is a 0.15mm deep and 6mm wide sprue. Care must be taken to control mold temperature as uniformly as possible to avoid warping and twisting of parts.
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The most important molding conditions for TPU are temperature, pressure and time that affect plasticization flow and cooling. These parameters will affect appearance and performance of TPU parts. Good processing conditions should result in uniform white to beige parts.

Temperatures that need to be controlled during molding TPU process include barrel temperature, nozzle temperature and mold temperature. The first two temperatures mainly affect plasticization and flow of TPU, the latter temperature affects 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 and maximum temperature at the end of barrel is also high. Temperature range of barrel used for processing TPU is 170~250℃. Temperature distribution of barrel generally increases gradually from one side of hopper (rear end) to nozzle (front end), so that TPU temperature rises steadily to achieve 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 a self-locking nozzle is used to prevent drooling, nozzle temperature can also be controlled within maximum temperature range of barrel.
c. Mold temperature: Mold temperature has a great impact on intrinsic performance and apparent quality of TPU products. Its level depends on many factors such as crystallinity of TPU and size of product. Mold temperature of TPU products is generally between 40 and 80℃. If mold temperature is low, molten material will freeze prematurely, resulting in streamlines, which is not conducive to growth of spherulites, resulting in low crystallinity of product and a late crystallization process, which will cause post-shrinkage and performance changes of product.

Pressure in injection molding process includes plasticizing pressure (back pressure) and injection pressure. When screw retreats, pressure on molten material at the top is back pressure, which is adjusted through relief valve. Increasing back pressure will increase melt temperature, reduce plasticization speed, make melt temperature uniform, color materials are evenly mixed, and melt gas is discharged, but it will prolong molding cycle. Back pressure of TPU is usually 0.3~2MPa.
Injection pressure is pressure exerted by the top of screw on TPU. Its function is to overcome flow resistance of TPU from barrel to cavity, increase filling rate of molten material, and compact molten material. TPU flow resistance and mold filling rate are closely related to viscosity of melt, viscosity of melt is directly related to hardness of TPU and temperature of melt. That is, viscosity of melt is not only determined by temperature and pressure, but also by hardness and deformation rate of TPU. The higher shear rate, the lower viscosity; when shear rate remains unchanged, the higher TPU hardness, the greater viscosity.
Under condition of constant shear rate, viscosity decreases with increasing temperature, but at high shear rate, viscosity is not as affected by temperature 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 1.4MPa to make TPU plasticized evenly.

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 (mold opening, demoulding, mold closing, etc.), which directly affects labor productivity and equipment utilization. Molding cycle of TPU usually depends on hardness, part thickness and configuration. TPU with high hardness has a short cycle, thick plastic parts have a long cycle, and plastic parts with complex configuration have a long cycle. Molding cycle is also related to mold temperature. TPU molding cycle is generally between 20 and 60 seconds.

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

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

Since TPU may degrade under high temperature for extended periods of time, it should be cleaned with PS, PE, acrylic plastic or ABS after shutdown. If machine is shut down for more than 1 hour, heating should be turned off.

Due to uneven plasticization in barrel or different cooling rates in mold cavity, TPU often produces uneven crystallization, orientation and shrinkage, resulting in internal stress in product, which is more prominent in thick-walled products or products with metal inserts. Products with internal stress often experience a decrease in mechanical properties during storage and use, with silver streaks or even deformation and cracking on the surface. The way to solve these problems in production is to anneal products. Annealing temperature depends on hardness of TPU product. Products with high hardness have higher annealing temperatures, and products with low hardness have lower annealing temperatures. Too high a temperature may cause product to warp or deform, while too low a temperature cannot eliminate internal stress.
TPU should be annealed at low temperature for a long time. Products with lower hardness can achieve optimal performance by leaving them at room temperature for several weeks. For hardness below Shore A85, anneal at 80℃×20h, and for hardness above A85, anneal at 100℃×20h. Annealing can be carried out in a hot air oven. Pay attention to placement position so that local overheating does not cause product to deform.
Annealing can not only eliminate internal stress, but also improve mechanical properties. Since 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. There must be enough time for it to separate and form micro-domains to obtain the best performance.

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 complete product. TPU products with inserts have large differences in thermal properties and shrinkage rates between metal inserts and TPU, resulting in weak bonding between inserts and TPU. Solution is to preheat metal insert, because preheated insert reduces temperature difference of molten material, so that molten material around insert can cool more slowly during injection process, shrink more evenly, cause a certain amount of hot material feeding to prevent excessive internal stress around insert. TPU inlay molding is relatively easy, and shape of insert is not limited. As long as insert is peeled off and heated at 200-230℃ for 1.5-2 minutes, peeling strength can reach 6-9kg/25mm. For a stronger bond, adhesive can be applied to insert, then heated to 120℃ before injection. In addition, it should be noted that TPU used cannot contain lubricants.

During TPU processing process, waste materials such as main channels, shunt channels, unqualified products can be recycled and reused. Judging from experimental results, 100% recycled materials are not blended with new materials, mechanical properties are not significantly reduced, so they can be fully utilized. However, in order to maintain physical and mechanical properties and injection conditions at optimal level, it is recommended that proportion of recycled materials be 25% to 30%. It should be noted that varieties and specifications of recycled materials and new materials should be same. Avoid using contaminated or annealed recycled materials. Do not store recycled materials for too long. It is best to granulate them immediately and use them dry. Melt viscosity of recycled materials generally needs to be reduced, and molding conditions need to be adjusted.

Depressions on the surface of TPU products will reduce quality and strength of finished product, and will also affect appearance of product. Cause of dents is related to raw materials used, molding technology and mold design, such as shrinkage rate of raw materials, injection pressure, mold design and cooling device, etc.
Table 1 shows possible causes and treatment methods of dents

During injection molding process, products sometimes have many bubbles, which affects their strength and mechanical properties, and also greatly reduces appearance of product. Usually, due to uneven thickness of product, or when mold has protruding ribs, material cools at different rates in mold, resulting in uneven shrinkage and easy formation of bubbles, so special attention must be paid to mold design.

 

In addition, raw materials are not fully dried and still contain some moisture. When materials are melted, they are heated and decompose into gases, which can easily enter mold cavity and form bubbles. Therefore, when bubbles appear in product, following factors can be checked and dealt with.
Table 2 shows possible causes and treatment methods of bubbles

Cracks are a fatal phenomenon of TPU products, which usually appear as hair-like cracks on the surface of product. When products have sharp edges and corners, hard-to-see fine cracks often occur in this area, which is very dangerous to product. Main reasons for cracks during production are as follows:
1. Difficulty in demoulding;
2. Overfilling;
3. Mold temperature is too low;
4. Defects in product structure.
To avoid cracks caused by poor demoulding, mold forming space must be equipped with a sufficient demoulding slope, size, position, and form of ejector pin must be appropriate. When ejecting, demoulding resistance of each part of finished product should be uniform.
Overfilling is caused by applying too much injection pressure or too much material, which causes excessive internal stress in product and causes cracks during demoulding. In this state, deformation of mold parts also increases, making it more difficult to demould and promoting occurrence of cracks (or even rupture). At this time, injection pressure should be reduced to prevent overfilling.
Excessive internal stress is often left in gate area, and vicinity of gate is prone to embrittlement, especially direct gate part, which is prone to cracking due to internal stress.
Table 3 shows possible causes and treatment methods of cracks

Reasons for warpage and deformation of TPU injection molded products are too short cooling and shaping time, excessive mold temperature, unevenness and asymmetry of flow channel system. Therefore, following points should be avoided when designing mold:

1. Thickness of same plastic part varies greatly;

2. There are excessively sharp angles;
3. Buffer zone is too short, resulting in a huge difference between thick and thin turns;
In addition, attention should also be paid to setting appropriate number of ejector pins and properly designed mold cavity cooling channels.
Table 4 shows the possible causes and treatment methods of warpage and deformation.

Scorched spots or black streaks refer to phenomenon of black spots or black stripes on products. Main reason for their occurrence is poor thermal stability of raw materials, which is caused by thermal decomposition of raw materials.
Effective countermeasures to prevent occurrence of scorch spots or black lines are to prevent raw material temperature in melt barrel from being too high and slow down injection speed. If there are scratches or gaps on inner wall of melt barrel or screw, part of raw material will adhere to it, and this part of raw material will cause thermal decomposition due to overheating. In addition, check valves can also cause thermal decomposition due to retention of raw materials. Therefore, when using raw materials with high viscosity or easy decomposition, special attention should be paid to preventing occurrence of scorch spots or black lines.

 

Table 5 shows possible causes and treatment methods of burnt spots or black lines.

Burrs are a common problem encountered in TPU products. When pressure of raw material in mold cavity is too great, mold parting force generated is greater than mold clamping force, thus forcing mold open, causing raw material to overflow and form burrs. There may be many reasons for the formation of burrs, such as problems with raw materials, problems with injection molding machine, improper adjustment, or even the mold itself. Therefore, when determining cause of burrs, proceed from easy to difficult.
1. Check whether raw materials are thoroughly dried, whether they are mixed with impurities, whether they are mixed with different types of raw materials, and whether they are affected by viscosity of raw materials;
2. Correctly adjust pressure control system of injection molding machine and adjustment of injection speed must match clamping force used;
3. Whether there is wear in some parts of mold, whether vent hole is blocked, and whether flow channel design is reasonable;
4. Whether there is any deviation in parallelism between mold plates of injection molding machine, whether force distribution of mold plate tie rods is uniform, whether screw check ring and melt barrel are worn.
Table 6 shows possible causes and treatment methods of burrs.

When TPU products stick to mold during injection molding, you must first consider whether injection pressure or holding pressure is too high. Because if injection pressure is too high, product will be over-saturated, causing raw material to fill in other gaps and causing product to get stuck in mold cavity, causing difficulty in demoulding. Secondly, when temperature of melt barrel is too high, raw materials will be thermally decomposed and deteriorated, causing fragmentation or breakage during demoulding process, causing mold sticking. As for mold problems, such as unbalanced feed inlets, inconsistent cooling rates of products, and mold sticking during demoulding.
Table 7 shows possible causes and treatment methods of mold sticking.

Toughness is energy required to break a material. Main factors causing reduction in toughness are: raw materials, recycled materials, temperature and molds, etc. Reduced toughness of product will directly affect strength and mechanical properties of product.
Table 8 shows possible causes and treatment methods of reduced toughness.

Underfilling of TPU products refers to phenomenon that molten material does not completely flow through all corners of molded body. Reasons for insufficient filling include improper setting of molding conditions, imperfect mold design and production, thick flesh and thin walls of molded products, etc. Countermeasures in terms of molding conditions are to increase temperature of material and mold, increase injection pressure, injection speed and improve fluidity of material. In terms of mold, size of sprue or runner can be increased, or position, size, quantity, etc. of gate can be adjusted and modified to make molten material flow smoothly. Furthermore, in order to smoothly evacuate gas in forming space, exhaust holes can be provided at appropriate locations.

 

Table 9 shows possible causes and treatment methods of insufficient filling.

Bonding line is a thin line formed by confluence of two or more lines of molten material. It is also usually called a welding line. Bonding lines not only affect appearance of product, but are also detrimental to strength of product. Main reasons for occurrence of bonding lines are:
1. Flow pattern of materials caused by shape of product (mold structure);
2. Poor confluence of molten materials;
3. Air, volatile matter or refractory matter is mixed into confluence of molten materials.
Raising material and mold temperatures can minimize degree of bond lines. At the same time, change position and number of gates to move position of bonding line to another place; or set up exhaust holes in fusion part to quickly evacuate air and volatile matter in this part; or set up a material overflow pool near fusion part, moving bonding line to overflow pool, then cutting it off are all effective countermeasures to eliminate bonding line.
Table 10 shows possible causes and treatment methods of bonding lines.

Surface of TPU products loses original luster of material, forms a film or a hazy state, etc., which can be called poor surface gloss.
Poor surface gloss of product is mostly caused by poor grinding of mold forming surface. When surface condition of forming space is good, increasing temperature of material and mold can enhance surface gloss of product. Use of too much refractory agent or greasy refractory agent is also cause of poor surface gloss. At the same time, material absorbs moisture or contains volatile matter and foreign matter mixed with pollution, which are also reasons for poor surface gloss of product. Therefore, special attention should be paid to mold and material factors.
Table 11 shows the possible causes and treatment methods of poor surface gloss.

Flow marks are traces of flow of molten material, appearing as stripes centered on gate.
Flow marks are caused by material that initially flows into forming space cooling too quickly and forming a boundary with material that flows later. In order to prevent flow marks, material temperature can be increased, material fluidity can be improved, and injection speed can be adjusted.

 

If cold material remaining at the front end of nozzle directly enters forming space, it will cause flow marks. Therefore, setting up sufficient stagnant areas at junction of runner and runner can effectively prevent occurrence of flow marks. At the same time, flow marks can also be prevented by increasing gate size.
Table 12 shows possible causes and treatment methods of flow marks.

Table 13 shows possible causes and treatment methods of screw slippage.

Table 14 shows possible causes and solutions for failure of screw to rotate.

Table 15 shows possible causes and treatment methods of injection nozzle leakage.

Table 16 shows possible causes and treatment methods of unmelted materials.