There are many methods for TPU materials molding, such as injection molding, blow molding, compression molding, extrusion molding, etc. Among them, injection molding is the most commonly used. The function of injection molding is to process TPU into the required parts, which is a discontinuous process that consists of three stages: pre-forming, injection molding, and machining. Injection hammers are divided into plunger type and screw type. Screw type injection molding machines are recommended because they provide uniform speed, plasticization, and melting.
The cylinder of the injection molding machine is lined with copper-aluminum alloy, and the screw is chrome-plated to prevent wear. The length-to-diameter ratio of the screw should be 16-20, at least 15; The compression ratio is 2.5/1 to 3.0/1. The length of the feeding section is 0.5L, the length of the compression section is 0.3L, and the length of the metering section is 0.2L. The check ring should be installed near the top of the screw to prevent backflow and maintain the maximum pressure. A gravity nozzle should be used to process TPU materials. The outlet is conical, and the nozzle diameter is larger than the inlet of the pouring ring by 0.68mm. The nozzle should be equipped with a controllable heating tape to prevent the material from solidifying.
From an economic perspective, the injection volume should be 40% to 80% of the fixed amount. The screw speed is 20-50r/min.
In mold design, pay attention to the following points:
The shrinkage is affected by molding conditions such as the hardness of the raw material, the thickness and shape of the parts, molding temperature, and mold temperature. The general shrinkage range is 0.005~0.020cm/cm. For example, a 100×10×2mm rectangular specimen is cast in the length direction and shrinks in the flow direction. The hardness of 75A is 2-3 times that of 60D.
The gate is a passage that connects the injection molding machine nozzle to the split channel or cavity in the mold. The diameter should expand inward at an angle greater than 20 degrees for easy removal of wheel debris. The dividing channel is a channel that connects the main channel and each cavity in the multi-slot mold. The arrangement on the plastic mold should be symmetrical and equidistant. The runner can be circular, semi-circular, or rectangular, and the diameter should be 6-9mm. The surface of the runner must be polished like the mold cavity to reduce flow resistance and provide faster mold filling speed.
The cold material cavity is the end cavity of the main channel, used to capture the cold material produced between the nozzle at the end of two injections to prevent the split channel or gate from blocking. After the cold material is mixed into the cavity, the product is prone to internal stress. The diameter of the cold material hole is 8-10mm, and the depth is about 6mm.
The gate is a channel that connects the main channel or dividing channel with the cavity. Its cross-sectional area is usually smaller than the runner, and the runner is the smallest part of the wheel system, and its length should be shorter. The gate shape can be rectangular or circular, and its size increases with the thickness of the product. If the product thickness is less than 4mm, the diameter is 1mm; if the thickness is 4-8mm, the diameter is 1.4mm; if the thickness is greater than 8mm, the diameter is 2.0-2.7mm. The gate is usually located where the thickest part of the product does not affect the appearance and use, and is at a right angle to the mold wall to prevent shrinkage and rotation.
The vent is a groove-shaped vent that opens in the mold to prevent the melt material entering the mold from being rolled into gas and discharged from the cavity. Otherwise, the product will have pores, poor welding, insufficient filling, and even burn due to high temperature caused by compressed air, resulting in internal stress of the product, etc. The vent can be set at the end of the flow of molten material in the cavity or plastic mold split surface, which is a pouring groove 0.15mm deep and 6mm wide.
It must be noted that the mold temperature should be controlled as evenly as possible to avoid warping and twisting of the workpiece.