Tool RecomMendation

All conventional gating systems are possible. The cross-section of a cold runner should be large enough to guarantee solidification shortly after gate sealing. In general, runner cross-section should be about 1mm larger than the thickest point in the molded part. For cold runner, draft angles between 1° and 3° have proven successful. For multi cavity molds, shear heating and balanced filling should be evaluated using injection molding process simulation.

Cold Runner Geoemtry
Runners should have a round or trapezoidal design (largest possible flow cross-section at the smallest surface area). Semi-round or rectangular manifold geometries are not recommended.

Gates, gating systems

The gating system depends on the melt volume, number of mold cavities, and part geometry. All common gating systems are possible. However, small tunnel gates freeze quickly and should be used only for parts requiring short holding pressure times (part with thin wall thicknesses).

Gating locations should be chosen carefully, in order to avoid jetting. Gating at the largest wall thickness minimizes sink marks.

Tunnel gate

Tunnel gates can be used, although the gate variant with conical bucket gate. Freezes more slowly, enables a better packing efficiency, decreases pressure loss and shearing.

In hot runner systems material residence time, shear rates, and melt temperatures should be evaluated. Shear rate should kept constant or should increase from hot runner entry to nozzle tip. Particular attention should be paid to the thermal interaction between the hot runner and the injection mold, in order to guarantee homogeneous temperature distribution in the molding compound. Close cooperation between mold designer and hot runner supplier avoids design errors. Complex systems should undergo thermal-rheological evaluation utilizing injection molding process simulation, in order to avoid any substantial temperature differences in the hot runner / sprue gate and molded part. Inside a hot runner system, melt temperature should not deviate more than 5 °C from the target temperature. Thermally critical locations inside the hot runner are often located far from the temperature measurement points and remain undiscovered.

For molding compounds with high filler contents (e.g. glass fibers) open hot runner nozzles should be preffered to valve gate nozzles. The insulating gap between mold and hot runner nozzle, should be filled with unfilled material in order to guarantee proper thermal insulation.

When processing TROGAMID® Care, an intensive part cooling is recommended in order to avoid optical defects(thick-walled parts (> 8 mm) can lead to cloudy areas).

To meet high quality requirements, mold temperatures must be distributed evenly throughout the cavity surface.

In the right half, tempering channels are located too close to the cavity. An inhomogeneous cavity temperature occurs. A rough guideline for tempering channel designs is given in table below. A precise mold temperature evaluation requires injection molding process simulation.

Cavity should have a proper venting. Venting slots should be 0.03 - 0.05 mm deep and 4 - 5 mm wide.

To estimate molding shrinkage, the following values can be used (determined as per ISO 294-4):

In general, draft should be 0.5° - 3°. For textured surfaces, about 1° draft angle / 0.025 mm texture depth should be added.Surface structures should be aligned with demolding direction.

Steel grade examples:

• 1.1730 for general mold plates, centering flange, ejector plates, clamping plates
• 1.2379 for sprue bushings
• 1.2312 for (milled) mold plates and mold inserts
• 1.2343 for cavity plates, mold inserts, and slides
• 1.2767 for mold plates and (polished, etched, eroded) mold inserts and slides

In practice, Ni-P-PTFE and TiAlOx have proven successful as surface coatings. Copper beryllium alloys can be used for mold inserts with high heat flux requirements. These inserts can be chrome and nickel coated.

Process monitoring and control:

• It is recommended to use a cavity pressure sensor for switching from filling to packing.
• Monitoring injection, plastication and cavity pressures as well as screw strokes helps to analyse injection molding processes.