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What are the main points of TPU injection molding?

What are the main points of TPU injection molding?
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  • Categories:Industry news
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  • Time of issue:2021-06-07 11:28
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(Summary description)

What are the main points of TPU injection molding?
Top

(Summary description)

  • Categories:Industry news
  • Author:
  • Origin:
  • Time of issue:2021-06-07 11:28
  • Views:
Information

 

 

1. Injection molding
The most suitable processing means of TPU is screw injection molding machine. With a single thread of normal length, a three-section screw can produce a good plasticized homogeneous melt. If high plasticizing capacity (production) is required, longer screws can be used. Screw rods in short compression zones are not suitable due to high shear forces. TPU plasticizing requires high energy and high torque drive of the screw. Insufficient torque will lead to screw speed fluctuations and uneven plasticization. Within limits, higher cylinder temperatures will produce good results, although there is a risk of overheating the material. Access to nozzle and screw head shall be designed to have no dead ends so that the material can be shot in without thermal damage. Accurate temperature control of the heating system of the screw and nozzle is essential. Take care to ensure that the nozzle is heated evenly over the entire length. Beware of local overheating and possible cold exposure of molten material in the passage. Melted TPU is neither corrosive nor wear - free type. For this reason, the screw does not require any special alloy steel or reinforced coating.

 

2. Temperature setting for screw and die

2.1 The injection unit TPU shall be processed at a melting temperature between 190 °C and 220 °C. For some hard grades the melting temperature may increase to 240°C. The melting temperature range for a specific grade of TPU can be found in the relevant product information table.

 

2.2 Mold temperature is mainly affected by surface quality and demoulding behavior. It also affects the final structure's shrinkage and internal stress when exposed to cold. The normal mold temperature should be 20-40 °C. But with some modified TPU grades and the use of glass filament filling to strengthen the TPU, the mold temperature should be raised to 60 °C to ensure the best surface quality. For thick-walled items, a reduction of approximately 5°C can reduce cycle time.

 

2.3 Plasticizing For plasticizing speed, the circumferential rate should not exceed 0.3m /s. The metering stroke should be between 1 D and 4 D. Shown is the maximum velocity at different screw diameters. Practical experience is that 30-75% of the capacity of the screw barrel is optimal. If the screw capacity corresponding to injection volume is very low, the melt will remain in the plasticizing unit for too long. Can cause thermal damage of the molten material.

 

2.4 Injecting pressure, maintaining pressure, back pressure and injection speed For ideal processing, the key is to control the pressure and injection speed uninterrupted. It shall be able to control injection and maintain pressure in the range of 100 to 1200 bar. In order to homogenize there must be back pressure, which is usually set between 1 and 2.5% of the injection pressure. The injection rate is mainly based on the wall thickness. Usually molds for thick-walled items need to be filled slowly, while thin-walled items need to be filled quickly. Reasonable wall thickness and cavity type are essential, but the importance of exhaust can not be ignored. When the exhaust is not good, the melt of high speed and high pressure will cause the "internal combustion engine effect" in the mold cavity, so that the product may appear charred, yellow or even underinjection. Cooling and demoulding of TPE parts should also be taken into account when setting rate of fire and pressure retention. The high orientation and high pressure brought by too fast injection speed will affect the cooling and shaping of the plastic parts, or make the plastic parts deform seriously. When the injection is complete, there should be enough holding pressure to support the mold setting and prevent the shrinkage of the mold as it cools. As a rule of thumb, the retention pressure for TPE molding is about 50% of the injection pressure.

 

The cycle time is determined by the shape of the item, the thickness of the wall, the cooling of the mold and the material itself.

2.6 The replication mould of demoulding TPU needs to be described in detail. Soft level TPUs are very peculiar in that they can produce very unusual wall items. This must be considered in the design of the mould. Demoulding agent can be used. Silicyl release agents such as Baysilon M have good effect. Silane-free release agents are also available, but must be used frequently.

 

 

3. Waste recycling
Waste, sprue, unqualified products made of crushing materials if clean and dry can be made into particles for recycling. For injection molding, the addition of less than 30% pulverizer to the pure material will not affect the performance of the product. If you process and crush the material yourself, the molded item must be tested as required to determine whether it meets the performance requirements. It is not appropriate to mix the pulverized material with the extruded pure material (because of the viscosity difference). If it is homogeneous and pure crushing material for extrusion, it is not absolutely not possible, but it is completely possible for injection molding.

 

4. Mold design/exterior structure

4.1 TPU mould for injection molding tool shall be made of the same steel used for hot forming. Simple small products tend to be made of aluminum. For sample molds you can normally insert cast resin or die cast metal. The following die types can be used to process TPU: • Two-piece die, • Three-piece die, • Double-parallel die, • Full pressure die, • Multiple gap die

 

The following types of gates are commonly used for TPUs: • Film gates • Diaphragm gates • Pin-point gates • Circular gates • Straight-line gates • Tunnel gates • Hot gate For gates, the gates and runways should be 25-50 percent larger than hard plastics. Pressure drop should be especially avoided in the gate system. The further the runner is, the more attention should be paid to the flow path that must be designed to the full path using the entire diameter of one runner and arranged on one or two half die molds. On multi-die hole moulds, i.e., two or three pieces, the runner should be arranged into a flow path of essentially the same length. The pintle point gate must have a weak point in the gate section to ensure a clean tear off. Large pin-point gates must be free of any injection or other indentation on the parts. For axially symmetrical parts, a ring or diaphragm gate may be used to better prevent streamlining. Mold filling and venting must be kept under tight control. If the pouring point is not visible, the tunnel gate is the best solution.

 

4.3 Hot runner technology and hot runner nozzle Hot runner feeding systems are becoming more and more popular for thermoplastics. The same goes for TPU, which is a technology that has been adopted in a defensive manner. 1. Flow technology II. Temperature control 3. Mechanical state

 

4.4 Flow characteristics of mold The flow behavior of TPU is basically the same as that of other thermoplastic materials. The length of the flow path depends on the melting temperature, the wall thickness of the filling part, the injection rate and the rheological properties of the material. Injection rates vary according to equipment and technical conditions.

 

4.5 Mold exhaust especially for TPU thick wall parts, it is important to have good die cavity exhaust. Grooves of 0.02 to 0.05mm depth and 5mm width on the parting line are considered appropriate. If the air is compressed in an area of a non-die parting line, it is feasible by using a needle and inserting a suitable exhaust in a suitable way.

 

4.6 Shrinkage For thermoplastic polyurethane, the shrinkage data can only be modified within a certain limit based on the shape of the article in the mold design stage. Both the wall thickness and the processing conditions have a significant impact on the shrinkage. The rough rules of die design set a shrinkage of about 1%. The minor difference is offset by good elastic deformation. The shrinkage of an object molded with soft TPU and thin wall is greater than that molded with hard TPU and large wall thickness.

 

4.7 Demoulding 1 cavity surface The roughness of cavity surface of a mold should reach 0.5-0.6 m to reduce demoulding efficiency and TPU auxiliary spray. The demoulding force is compared with other dryness or coatings at their minimum. 2 Demoulding taper The taper of the mold should be at least 5°, which is more obvious when it is manufactured with soft TPU. Interlaced taper improves detachment from mold wall. 3 ejection device For TPU, the surface of ejection device should be as large as possible to avoid deformation of the finished product.

 

4.8 Colored TPU generally only provides natural color. Coloring is a simple process and can be machine-handled. Using TPU base color masterbatch granules is simple and reliable. Polystyrene and SAN resin-based masterbatches are fine, while those of polyolefin and PVC-based masterbatches are not suitable for TPU. Pigments and pigments can be used for coloring. The amount required depends on the wall thickness of the item, the color concentration of the pigment and the specified shade. The general addition amount is: Masterbatch: 1.0-4.0% Paste: 0.5-1.0% Pigment: 0.2-0.5%Colorants can be added and mixed when using TPU using standard commercial equipment. For special colors you can use a drum mixer or a tilting bucket mixer and similar devices. It is very important to ensure that the colorant or any other additive does not contain any moisture. A static mixer tends to greatly improve color dispersion and reduce the amount of colorant used.

 

4.9 Additives In order to improve the processing characteristics and special behavior, we recommend the addition of commonly used functional master batch such as: • Anti-adhesion, Demoulding, UV Stabilizer must be pre-dried before addition.

 

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