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Everything you need to know about TPU injection molding

Table of Contents

There are various methods of TPU molding process: injection molding, blow molding, compression molding, extrusion molding, etc., among which injection molding is the most common.

The function of injection molding is to process TPU into the required parts, which is divided into a discontinuous process of pre-molding, injection and ejection in three stages.

There are two types of injection molding machines, plunger type and screw type, and the screw type injection molding machine is recommended because it has the ability to provide uniform speed, plasticization and melting.

Design of TPU material injection machine

The injection machine barrel is lined with copper-aluminum alloy, and the screw is chromium-plated to prevent wear. Screw L/D ratio L/D=16~20 is good, at least 15; compression ratio 2.5/1~3.0/1. Feeding section length 0.5L, compression section 0.3L, metering section 0.2L. A non-return ring should be installed near the top of the screw to prevent backflow and maintain maximum pressure.

TPU should be processed with a self-flowing nozzle with an inverted cone outlet, with a nozzle diameter of 4mm or more, less than 0.68mm than the inlet of the main flow path collar, and the nozzle should be equipped with controlled heating tape to prevent material solidification.

From the economic point of view, the injection volume should be 40% to 80% of the rated volume. The screw speed should be 20~50r/min.

TPU material mold design

Shrinkage of molded TPU parts

Shrinkage is influenced by the hardness of the raw material, the thickness and shape of the part, the molding temperature and mold temperature, and other injection molding conditions.

Usually the shrinkage rate ranges from 0.005 to 0.020cm/cm.For example, 100 × 10 × 2mm rectangular test piece, in the length direction gating, flow direction shrinkage, hardness 75A than 60D 2 to 3 times greater.

TPU hardness between 78A to 90A, the shrinkage rate of the parts decreases with the increase of thickness; hardness in 95A to 74D, the shrinkage rate of the parts increases slightly with the increase of thickness.

Runner and cold cavity

The main runner is a section of the mold connecting the injector nozzle to the manifold or cavity, the diameter should be expanded inward, at an angle of 2o or more, to facilitate the flow of superfluous material off the mold.

The manifold is a channel connecting the main flow channel and each cavity in a multi-groove mold, and the arrangement of the mold should be symmetrical and equidistant.

The runner can be round, semi-circular, 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.

The cold material cavity is a cavity located at the end of the main flow channel to trap the cold material generated between injections at the end of the nozzle, thus preventing blockage of the manifold or gate.

Cold material mixed into the cavity, the product is easy to produce internal stress. The cold material cavity is 8~10mm in diameter and about 6mm in depth.

Gate and exhaust port

The gate is the channel that connects the main flow channel 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 rectangle or circle, the size increases with the thickness of the product, the thickness of the product is below 4mm, the diameter is 1mm; the thickness is 4~8mm, the diameter is 1.4mm; the thickness is above 8mm, the diameter is 2.0~2.7mm.

The location of the gate is generally chosen in the thickest of the product without affecting the appearance and use, and at right angles to the mold wall to prevent shrinkage and avoid spin lines.

The air vent is a kind of slot-shaped air outlet in the mold, which is used to prevent the melt entering the mold from getting involved in the gas and to discharge the gas from the cavity.

Otherwise, it will make the product with air holes, poor fusion, mold filling, and even the product will be burned due to the high temperature generated by the air compression, and the internal stress of the product.

The vent can be located at the end of the melt flow in the cavity or on the mold parting surface, for 0.15mm deep, 6mm wide trough must pay attention to the mold temperature control as uniform as possible, so as to avoid warping and twisting of the parts.

TPU Material Molding Conditions

The most important molding conditions for TPU are the temperature, pressure and time that affect the plasticizing flow and cooling. These parameters will affect the appearance and performance of the TPU part. Good processing conditions should result in uniform white to beige parts.

Temperature

The temperatures that need to be controlled in the TPU molding process are the barrel temperature, the nozzle temperature and the 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.

a. 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 distribution of the barrel temperature is generally from the hopper side (back end) to the nozzle (front end), gradually increasing, so that the TPU temperature rises steadily to achieve the purpose of uniform plasticization.

b. Nozzle temperature

It is usually slightly lower than the maximum barrel temperature to prevent possible salivation of the melt in the straight-through nozzle.

If a self-locking nozzle is used to eliminate salivation, the nozzle temperature can be controlled within the maximum temperature of the barrel.

c. Mold temperature

The mold temperature has a great impact on the intrinsic properties and apparent quality of TPU products. Its level is determined by many factors such as the crystallinity of TPU and the size of the product.

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 480A, mold temperature 20-30℃.

Hardness 591A, mold temperature 30 ~ 50 ℃; hardness 355D, mold temperature 40 ~ 65 ℃. TPU products mold temperature is generally in 10 ~ 60 ℃.

Mold temperature is low, the melt freezes prematurely and produces flow lines, and is not conducive to the growth of spherical crystal, so that the product crystallization is low, there will be a late crystallization process, which causes post-shrinkage of the product and changes in performance.

Pressure

The injection process is pressure includes plasticizing pressure (back pressure) and injection pressure. The pressure on the top of the melt 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 to 4 MPa.

Injection pressure is the pressure applied to TPU by the top of the screw, and its function is to overcome the flow resistance of TPU from the barrel to the cavity, to give the rate of melt filling, and to compact the melt.

TPU flow resistance and mold filling rate is closely related to melt viscosity, and 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 determined by TPU hardness and deformation rate.

The higher the shear rate, the lower the viscosity; shear rate remains unchanged, the higher the TPU hardness, the higher the viscosity.

Under the condition of constant shear rate, viscosity decreases with increasing temperature, but at high shear rate, viscosity is not affected by temperature as much as low shear rate.

The injection pressure of TPU is generally 20~110MPa. the holding pressure is about half of the injection pressure, and the back pressure should be below 1.4MPa to make TPU plasticized evenly.

Time

The time required to complete an injection process is called the injection molding cycle.

The molding cycle includes mold filling time, holding time, cooling time and other time (mold opening, mold release, mold closing, etc.), which directly affects labor productivity and equipment utilization.

TPU injection molding cycle is usually determined by hardness, part thickness and configuration, TPU hardness cycle is short, thick plastic parts cycle is long, complex plastic part configuration cycle is long, molding cycle is also related to the mold temperature.

Injection speed

The injection speed is mainly determined by the configuration of TPU injection molded products. Products with thick end face need lower injection speed, while thin end face needs faster injection speed.

Screw speed

Processing TPU products usually requires low shear rate, so a lower screw speed is appropriate. 20~80r/min for TPU is the general screw speed, 20~40r/min is preferred.

Post-treatment of TPU injection molded products

TPU due to uneven plasticization in the barrel or different cooling rates in the mold cavity, often produce 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. Annealing temperature depends on the hardness of TPU injection molded products, high hardness of the product annealing temperature is also higher, low hardness temperature is also low; too high a temperature may make the product warp or deformation, too low to achieve the purpose of eliminating internal stress.

TPU annealing should be used for a long time at low temperature, the products with low hardness 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 of the position not to local overheating and deformation of the product. Annealing can not only eliminate the internal stress, but also improve the mechanical properties.

As TPU is a two-phase form, the mixing of phases occurs during TPU thermal processing, and during rapid cooling, the phase separation is slow due to the high viscosity of TPU, and there must be enough time for the separation to form micro-areas to obtain the best performance.

TPU material inlay injection molding

To meet the needs of assembly and use strength, TPU parts are embedded with metal inserts. The metal insert is first placed in a predetermined position in the injection mold and then injected into a whole product.

TPU products with inserts are not firmly bonded to 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 melt around the insert can be cooled more slowly during the injection process, the shrinkage is more uniform, and a certain amount of hot material shrinkage occurs to prevent excessive internal stress around the insert.

TPU inlay molding is relatively easy, the shape of the insert is not limited, as long as the insert degreasing, it will be in 200 ~ 230 ℃ heating treatment 1.5 ~ 2min, peel strength up to 6 ~ 9kg / 25mm.

To obtain a firmer bond, the insert can be coated with adhesive, then heated at 120°C and then injected. In addition, it should be noted that the TPU used should not contain lubricant.

Reuse of TPU recycling material

In the TPU processing process, the waste materials such as main stream channel, manifold channel and unqualified products can be recycled and reused.

From the experimental results, 100% recycled material is not mixed with new material, the mechanical properties of the decline is not too serious, can be fully utilized, 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 same species specifications, have been contaminated or have been annealed to avoid the use of 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.

TPU covering process to pay attention to which points?

Polyurethane is the abbreviation of polyurethane, which is made by the reaction of polyisocyanate and polyol, and contains many repeating carbamate groups (-NH-CO-O-) in the molecular chain.

Thermoplastic polyurethane rubber (TPU) is a (AB) n-type block linear polymer, A represents high molecular weight polyester or polyether (molecular weight of 1000 to 6000), called long chain, B represents diol containing 2 to 12 straight chain carbon atoms for short chain, AB inter-chain segment chemical bonding is with diisocyanate.

And in the TPU field of overmolding, is a special overmolding. TPU overmolding is plastic over plastic, either soft plastic material over hard plastic or soft plastic material over soft plastic material, and TPU overmolding has the following 5 points for your reference from the process.

1. Two-color injection molding (2K molding) is better than the effect of the second injection molding (2-step molding).
Because, the first step of hard plastic injection is still hot, immediately into the second cavity of the 2K injection molding machine, the intermediate transfer process also did not absorb moisture, both easy to be high temperature TPU melt in the surface ablation of ultra-thin layer, but also no water vapor in the surface is adsorbed impact.

in the TPU material processing allowed range, as far as possible to choose a higher melt temperature of the material, otherwise TPU can not have enough heat to ablate the surface of the hard plastic material.

The second step of the cladding, hard plastic as the inserts of the cavity as high as possible to use the mold temperature. Hard plastic temperature is high, TPU high temperature melt cooling will be slow, there is enough heat, enough time to ablate hard plastic to form an interdigitated ultra-thin layer.

In the second step of overmolding, the TPU melt is shot out at a high injection speed as possible without producing TPU flying edge.

a. High injection speed, TPU spreading time on the hard plastic surface is short, and there is more time to ablate the hard plastic surface in the molding cycle.

b. With high injection speed, the friction between TPU and hard plastic surface leads to frictional heat generation and slower cooling of TPU melt temperature, which is used to contact and ablate the hard plastic surface for a longer period of time.

c. High injection speed, most TPU (except TPE, TPE melt viscosity is more sensitive to temperature) melt viscosity shear thinning, surface tension decreases, conducive to spreading in the hard plastic surface.

To avoid moisture absorption or staining of the hard plastic surface, especially in the two-step injection molding, hard plastic intermediate after the stop hair, transfer process, is more likely to happen.

Polar hard plastics such as PC, PBT, PET, POM especially strong polar hard plastics such as nylon 6, nylon 66, surface moisture absorption leads to the formation of hydrogen bonds with the adsorbed water molecules trip.

TPU can not effectively form intermolecular bonding with the hard plastic surface molecules, the natural bonding force of the cladding is reduced; the surface of the hard plastic parts manufacturing is stained, such as workshop workers gloves are not clean or gloves cotton fibers adhered to the surface of polar hard plastic parts, naturally affect the TPU melt cladding.

Wrapping polar hard plastic TPU, certainly also polar formula, do not pay attention to moisture, the same impact on the bonding effect of the package.

Summary

This article analyzes the characteristics of TPU materials, molding conditions, molding methods, and molding equipment, as well as the points to note when molding, which will hopefully help your TPU material project.

The article mentions TPU overmolding and TPU insert molding processes, which require high experience of injection mold suppliers and injection molding product suppliers.

If you have a project involving these two processes, it is recommended to confirm that the injection mold factory and injection molding product factory have experience in making similar products to ensure the smooth implementation of the project.

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