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.