TPU injection molding process and mold details

Injection molding tools
TPU mould shall be made of the same kind of steel used for hot forming. Simple small products tend to be made of aluminum alloy. Insert casting resin or die-cast metal can be used normally for sample mold. The following mold types can process TPU:
• Two-piece mold
• Three-piece mold
• Double-parallel mold
• Full pressure die
• Multi-gap mold
Gate
TPU usually uses the following types of gates:
• film gate
• Diaphragm Gate
• Pin Gate
• Annular gate
• Sprue
• Tunnel casting
• Hot runner gate
For gates, the sprue and runner should be 25 - 50% larger than hard plastic (Figure 4.1). Pressure drop shall be particularly avoided in the gate system.
Fig. 4.1 Design of runner bushing
The farther the runner is, the more attention should be paid to the fact that the flow path must be designed to use the entire runner diameter for the whole path, and it should be arranged on one or two half of the mold (Figure 4.2). On the multi-cavity mold, that is, two or three pieces, the runner should be arranged into a flow path with basically the same length (Figure 4.3).
Figure 4.2 Section of sprue
Figure 4.3 Runner of Multi-cavity Mold
The pin-point gate must have a weakness in the gate section to ensure clean tearing. The large pin-point gate must not be associated with any spray or other indentation on the part. For axially symmetrical parts, annular or diaphragm gates may be used to better prevent the formation of streamline. Mold filling and venting must be kept under close control. (Figure 4.4)
Figure 4.4 Gate form
Figure 4.5 "coat hanger type" film gate
If the pouring point is invisible, the tunnel gate is the best solution. (Figure 4.6)
Figure 4.6 Tunnel gate with flat feed
Hot runner technology and hot runner nozzle
Hot runner feeding system is more and more popular for thermoplastics. TPU is also protected from adopting this technology. Figure 4.7 lists the main rules of this method.
Figure 4.7 Requirements for hot runner system
Open nozzle without plastic isolation cap
Processing TPU is not commonly used, because the thermal separation is not good, resulting in "eye circles", halo, adhesion and material dripping.
Figure 4.8 Open nozzle without plastic isolation cap
Open nozzle with plastic isolation cap
The isolation cap and cooling system lead to better thermal separation (Figure 4.9).
Figure 4.9 Open nozzle with plastic isolation cap
Open nozzle of torpedo head
The narrow annular gap leads to excessive shear force. Generally not suitable for TPU (Figure 4.10).
Figure 4.10 Open nozzle with torpedo head
Open nozzle with gate
This system is mainly used for large molds (single/multiple cavities). The specific points are low pressure loss and good thermal separation (Fig. 4.11).
Fig. 4.11 Open nozzle with gate
Nozzle with needle valve
Clean pour points can be placed directly on the parts. This gating system shows effective thermal isolation and low pressure loss (Fig. 4.12).
Figure 4.12 Nozzle with needle valve
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 filled part, the injection speed and the rheological properties of the material. The injection speed varies according to the equipment and technical conditions. The following figure shows the relationship between TPU flow path and melting temperature.
Fig. 4.13 Relationship between flow path length and melting temperature
Mold exhaust
Especially for TPU thick-wall parts, it is important to have good cavity exhaust. The groove with a depth of 0.02 to 0.05mm and a width of 5mm on the parting line is considered appropriate (Fig. 4.14). If the air in a non-mold parting line area is compressed, it is feasible to use a needle and insert appropriate exhaust with appropriate methods.
Figure 4.14 Schematic Diagram of Exhaust Slotting
shrink
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, and its wall thickness and processing conditions have a significant impact on the shrinkage. The general rule of mold design is about 1% shrinkage. The secondary difference is offset by good elastic deformation. The shrinkage of soft-grade TPU and thin-walled articles after molding is greater than that of hard-grade TPU and articles with larger wall thickness. Figure 4.15 shows the situation of total contraction in the whole area.
Figure 4.15 Comparison of TPU shrinkage range
Demoulding
Cavity surface
The surface roughness of a mold cavity reaches 0.5 - 0.6 μ M Reduce mold release and TPU auxiliary spray. The demoulding force is compared with other coarseness or coating in their minimum.
Demoulding taper
The mold taper shall be at least 5 °, and it is obvious to use soft grade TPU. The staggered taper improves the separation from the mold wall (Fig. 4.16).
Figure 4.16 Demoulding taper
Ejector
For TPU, the surface of ejection device should be as large as possible to avoid deformation of finished products (Figure 4.17).
Figure 4 17 Ejector
The high elasticity of TPU allows bottom demoulding and hollow articles. The core can be blown by compressed air (Fig. 4.18).
Figure 4.18 Blow molding
to color
TPU generally only provides natural colors. Coloring is a simple process and can be processed by machines. It is simple and reliable to use TPU primary color master particles. Polystyrene and SAN resin-based masterbatches are barely acceptable, while those based on polyolefin and PVC are not suitable for TPU. Pigments and pastes can be used for coloring. The amount of addition required depends on the wall thickness of the article, the color concentration of the pigment and the specified chroma.
The general addition amount is:
Color masterbatch: 1.0 - 4.0%
Color paste: 0.5 - 1.0%
Pigment: 0.2 - 0.5%
Standard commercial equipment can be used to add and mix colorants when using TPU. For special colors, drum mixer or tipper mixer and other similar devices can be used. Ensure that the color additive or any other additive does not contain any water
Very important. A static mixer tends to greatly improve color dispersion and reduce the amount of colorant.
additive
To improve processing characteristics and special behaviors, we recommend adding common functional masterbatches, such as:
• Anti-adhesive agent
• Demoulding
• UV stabilizer
It must be pre-dried before adding.