Thermoplastic polyurethane TPU has excellent wear resistance and resilience. It is widely used in some fields with requirements for cushioning, sealing, shock absorption, wear resistance, etc. TPU is usually processed by injection molding or extrusion, but its application in the field of two-color injection molding is also relatively common.
1. TPU two-color injection molding of hard plastic matching selection
TPU is used for two-color injection molding, and the hard plastics that match it are mainly ABS, PC, PC+ABS, PBT, SAN and other plastics. The main reason is that TPU has similar molecular polarity with these plastics. This provides a basis for strong coating and bonding.
2. Analysis of the advantages and disadvantages of TPU in two-color injection molding
2.1 Advantages
a Excellent adhesion to plastics such as ABS, PC, PC+ABS;
b Excellent tensile resilience, with elasticity higher than TPE and PVC;
c Excellent surface scratch resistance, no scratches by nails;
2.2 Disadvantages and shortcomings
a Sensitive to temperature, high process control requirements during molding operation;
b Hard feel, not smooth and soft enough;
c When used for encapsulation of lower hardness (75A and below), the cost is high (usually modified TPE is used as encapsulation material);
3. Selection of soft TPU coating materials for two-color injection molding
3.1 For products with high hardness requirements (above 85A), high resilience and surface scratch resistance, TPU is recommended as the first choice. At the same time, hard plastics are usually ABS, PC, PC+ABS, etc., and plastics such as PP and PE have poor adhesion to TPU;
3.2 For products with lower hardness (below 80A) and soft touch, using TPU is a problem in terms of performance and cost. It is recommended to use modified TPE coating materials.
4. Main applications of TPU overmolding
The hard plastics for TPU overmolding are mainly ABS, PC, PC+ABS. The main application areas are automotive parts, electronic appliances, and communication equipment.
5. TPU shrinkage rate
The shrinkage rate of TPU is about 0.8~1.6%, and the temperature range of two-color injection overmolding is 180~230℃. For the same type of TPU, the higher the hardness, the greater the viscosity, the higher the molding temperature, and the smaller the shrinkage rate.
6 Common problems and solutions of TPU encapsulation
6.1 Water break
Reason: a. The encapsulation molding temperature is too high; b. The nozzle design is unreasonable for the new mold design.
Solution: Appropriately reduce the injection molding temperature of the encapsulation; modify the position of the polished nozzle.
6.2 Sticky mold
Reason: a. The material itself; b. The mold is too smooth
Solution: Appropriate modification to improve the demoulding property of the material; the mold is treated with sun grain when designing; as a simple method, mold release agent can also be applied, but it will affect production efficiency. Sometimes the mold release agent remains on the surface of the product, which takes time to clean.
6.3 Surface air grain material flower
Possible reasons: a. The TPU material is not dried before encapsulation injection molding; b. If it is a color matching product, check whether the masterbatch is well dispersed and compatible; c. The mold venting is not good
Solution: Dry the material before molding; improve the masterbatch dispersion, consider internal color extraction; improve mold venting.
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