TPU (Thermoplastic polyurethane) vowin.cn/' target='_blank'>plastic injection vowin.cn/en/News/news1251.html' target='_blank'>molding process has many methods, including injection molding, blow molding, compression molding, extrusion molding, etc., among which injection molding is most commonly used. use the injection molding process to mold TPU into required TPU injection molding parts, which are divided into three stages: pre plasticizing, injection, and ejecting out. The injection machine is divided into plunger type and screw type. Screw type injection machine is recommended because it provides uniform speed, plasticization, and melting.

TPU phone cover molding

1. Design of injection machine

The barrel of the injection machine is lined with copper-aluminum alloy, and the screw is chrome plated to prevent wear. The length diameter ratio of screw L / D = 16 ~ 20 is better, at least 15; the compression ratio is 2.5/1 ~ 3.0/1. The length of the feeding section is 0.5L, the compression section is 0.3L, and the metering section is 0.2L. The check ring shall be installed near the top of the screw to prevent backflow and maintain the maximum pressure.

The TPU should be processed with a self flow nozzle, the outlet is an inverted cone, the nozzle diameter is more than 4mm, less than 0.68mm of the main channel collar inlet, and the nozzle should be equipped with a controllable heating belt to prevent material solidification.

From an economic point of view, the injection volume should be 40% – 80% of the quantitative amount. The screw speed is 20-50 R / min.

2. Mold design

Mold design should pay attention to the following points:

(1) shrinkage of molded TPU parts

Shrinkage is affected by the hardness of raw materials, thickness, shape, molding temperature, mold temperature, and other molding conditions. Generally, the shrinkage range is 0.005-0.020cm/cm. For example, a 100 × 10 × 2mm rectangular test piece shrinks in the length direction of the gate and the flow direction, and the hardness of 75A is 2-3 times larger than that of 60d. The effect of hardness and thickness of TPU on shrinkage is shown in Figure 1. It can be seen that when the hardness of TPU is between 78a and 90a, the shrinkage decreases with the increase of thickness; when the hardness is between 95A and 74d, the shrinkage slightly increases with the increase of thickness.

(2) Runner and cold slot well

The main runner is a section of the runner connecting the injector nozzle to the shunt channel or cavity in the mold. The diameter should be expanded inward, with an angle of more than 2O, so as to facilitate the removal of flow channel vegetations. The shunt channel is the channel connecting the main channel and each cavity in the multi-groove mold, and its arrangement on the mold should be symmetrical and equidistant. The flow channel can be circular, semi-circular and rectangular, with a diameter of 6-9mm. The runner surface must be polished like the cavity to reduce flow resistance and provide a faster filling speed.

A cold well is an empty place (extra extend runner) at the end of the main runner, which is used to catch the cold material produced between the two injections at the end of the nozzle, so as to prevent the diversion runner or gate from blocking by cold material. When the cold material is mixed into the mold cavity, the internal stress of the product is easy to occur. The diameter of the cold material hole is 8-10 mm, and the size is about 6 mm long.

(3) gate and vent

The gate is the runner connecting the main flow channel or the shunt channel and the cavity. Its cross-sectional area is usually smaller than the runner passage, which is the smallest part of the runner system, and its length should be short. The gate shape is rectangular or circular, and the size increases with the thickness of the product. The thickness of the product is less than 4mm, with a diameter of 1mm; the thickness of the gate is 4-8mm, with a diameter of 1.4mm; the thickness of the gate is more than 8mm, with a diameter of 2.0-2.7mm. The gate position is generally selected at the thickest part of the product, which does not affect the appearance and use, and is at right angles to the mold, so as to prevent shrinkage and avoid spiral pattern.

Exhaust or venting slot is a kind of slot type air outlet opened in the mold, which is used to prevent the molten material from entering the mold from getting involved in the gas and to discharge the gas from the mold cavity. Otherwise, the products will have air holes, poor fusion, insufficient filling or air trap, and even burn the products due to high temperatures caused by air compression, resulting in internal stress of the products. The exhaust port can be set at the end of the melt flow in the mold cavity or on the parting line of the plastic mold, which is a 0.15mm deep and 6mm wide pouring slot.

It is necessary to control the mold temperature as evenly as possible to avoid warping and twisting of the parts, below are some TPU plastic injection molding products we made before. If you have any requirement for TPU or TPE molded products, welcome to contact us.