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Molds: make ’em rugged

Molds: make ’em rugged
Update Time:2019-01-03

Speed is one of the key attributes of successful thin-wall molding. Faster filling and higher pressures are required to drive molten thermoplastic material into thinner cavities at a sufficient rate to prevent freeze off. If a standard part is filled in 2 sec, then a reduction in thickness of 25% potentially can require a drop in fill time of 50% to just 1 sec.

One benefit of thin-wall molding is that as wall sections drop, there is less material to cool. Cycle times can drop by 50% with aggressive wall-thickness reduction. Careful management of the melt-delivery system can keep runners and sprues from diminishing that cycle-time advantage. Hot runners and heated sprue bushings are often used in thin-wall molding to help minimize cycle time.

Mold material should be reviewed too. P20 steel is used extensively in conventional applications, but due to the higher pressures of thin-wall molding, molds must be built more robustly. H-13 and other tough steels add an extra degree of safety for thin-wall tools. (If possible, you will also want to select a molding material that doesn’t accelerate mold wear when injected into the cavity at high speeds.)

However, robust tools cost money—possibly even 30% to 40% more than a standard mold. Yet the cost is often offset by increased productivity. In fact, the thin-wall approach is frequently used to save money on tooling. A 100% increase in productivity can mean that fewer molds need to be built, thereby saving money over the life of a program.

Here are some more tips on tool design for thin walls:

  • For aggressive thin-wall applications, use steel harder than P20, especially when high wear and erosion are expected. H-13 and D-2 steels have been successful in gate inserts.
  • Mold interlocks sometimes can stave off flexing and misalignment.
  • Cores that telescope into the cavity can help reduce core shifting and breakage.
  • Use heavier support plates (often 2 to 3 in. thick) with support pillars (typically preloaded 0.005 in.) under the cavities and sprue.
  • Use more and larger ejector pins than with conventional molds to reduce pin pushing.
  • Consider strategic placement of sleeve and blade knockouts.
  • No. 2 diamond polish on cores and ribs can eliminate problems of part sticking. Mold surface treatments, such as nickel-PTFE can also improve part release.
  • Venting is critical and can be facilitated with vented core pins and ejector pins, as well as venting along up to 30% of the parting line around the part. Vents are typically 0.0008 to 0.0012 in. deep and 0.200 to 0.0400 in. wide. While not usually necessary, some processors have sealed the parting line with an O-ring in order to pull a vacuum on the cavity for quick gas evacuation.
  • With higher injection speeds, gates larger than the nominal walls help reduce material shear and gate wear and help prevent freeze-off before good packing is achieved.
  • Gate inserts with a Rockwell (Rc) hardness greater than 55 are typically used to withstand high injection pressures.
  • When gating directly onto a thin wall with a sprue, pinpoint, or hot-drop, use gate wells to reduce stress at the gate, aid filling, and reduce part damage when degating.
  • Hot manifolds can help reduce pressure loss in runner systems, but they require at least 0.5-in.-diam. inner passages with no sharp corners or dead zones. Manifolds should have external, not internal, heaters. Valve gates, if used, must be non-restrictive and built to take high pressure.

In addition, cooling of the cores and cavities is more critical and challenging in thin-wall applications. Two important guidelines are:

  1. Non-looping cooling lines should usually be located directly in the core and cavity blocks to help keep the mold surface temperature as consistent as possible.
  2. Instead of decreasing coolant temperature to maintain the desired steel temperature, it is generally better to increase the amount of coolant flow through the tool. As a rule of thumb, the difference in temperature between the delivery coolant and return coolant should be no more than 5° to 10° F.