Precautions for TPR Plastic Injection Molding

TPR is a widely used thermoplastic elastomer that combines rubber-like softness with plastic processability. It is commonly used for grips, seals, foot pads, and overmolded parts due to its good elasticity, recyclability, and short molding cycle. However, TPR is highly sensitive to temperature, shear rate, mold structure, and processing parameters. Improper settings often lead to defects such as sticking, shrinkage, bubbles, flow marks, poor bonding, and sticky surfaces. Mastering the key processing points is essential for stable quality, high efficiency, and low rejection rates.

Raw Material Drying and Handling

Most TPR materials absorb moisture easily and must be dried before molding to avoid bubbles, silver streaks, and dull surfaces. The recommended drying condition is 80℃ to 90℃ for 2 to 3 hours, with hopper heating at 60℃ to 70℃. Excessively high temperature causes softening and bridging, resulting in unstable feeding. Avoid long exposure to air. Do not mix TPR of different hardness or from different suppliers, as formula differences may cause inconsistency in appearance, hardness, and process stability.

Temperature Setting

Temperature control is critical for TPR molding. Low temperatures cause insufficient plasticizing, poor flow, obvious weld lines, and hard products. High temperatures lead to decomposition, sticking, burning, odor, and deformation. Barrel temperatures are typically set between 160℃ and 190℃: rear zone 150–160℃, middle zone 160–180℃, front zone 170–190℃, and nozzle 10℃ lower. Softer TPR grades require lower temperatures to prevent shearing and degradation. Mold temperature is usually 20–40℃: low temperature causes rough surfaces, while high temperature slows cooling and increases sticking and warpage.

Injection Process Control

Injection pressure should be moderate, generally 40–70 MPa, since TPR flows easily. Excessively high pressure causes flash, internal stress, and debonding in overmolded parts. Use medium-low speed with a slow-fast-slow profile to avoid jetting, flow marks, and burning. Screw speed should be moderate to prevent excessive shear heat. Back pressure should be low to ensure uniform feeding. Holding pressure and time are set just enough to eliminate sink marks; excessive holding causes stress and demolding difficulties.

Mold Design and Structure

Runners and gates should be smooth and rounded to prevent dead spots and material degradation. Side gates, pin gates, and fan gates are commonly used. The cavity surface must be well-polished for good release. The cooling system should be uniform and sufficient to ensure rapid, stable cooling and reduce warpage. Adequate venting is required at weld lines and flow fronts to prevent burns and short shots. For overmolding, the interface design must ensure reliable bonding between TPR and the substrate.

Demolding and Production Stability

TPR is soft and prone to sticking, so balanced ejection is necessary to avoid deformation and damage. Use a small amount of neutral release agent if needed; overuse affects surface quality, printing, and bonding. Clean the mold regularly to prevent deposits from affecting appearance. Before shutdown, purge the barrel with PE or PP to avoid residue degradation.

Summary

The core of stable TPR injection molding lies in precise temperature control, moderate shearing, smooth flow, and sufficient cooling. With proper drying, reasonable temperatures, balanced process parameters, and well-designed molds, consistent high-quality products can be produced. Stable processing improves productivity, reduces costs, and enhances overall manufacturing efficiency.