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April 25, 1999

10 Min Read
The selection, care and feeding of soft tools for a long life

Editor's note: Richard Caufman is the principal of RC Marketing Inc. (Warren, PA), a part and tool design shop that also provides rapid tooling and prototyping services. Caufman has 16 years' experience in injection and compression/transfer molding. He offers these guidelines for molding with and caring for soft tools.

Ask any moldmaker or molder what the properties of the ideal tool steel would be, and the description might look like this: the material should be free machining to reduce tool costs, capable of a number one finish with five minutes of number 120 stoning, as hard as a diamond, capable of flexing 30° for five million cycles without fracture, have 10 times the thermal conductivity of water, and cost less than $.02/cu inch. Because we have yet to find this elusive material, we are stuck with making choices. Every choice affects the well-being of tooling, process, and manufacturer. It also affects the profitability of molders and their customers and gives us something to talk about.

With today's choices ranging from computer-generated resin tooling to specialty alloys like Crucible's CPM 10V or even pure carbide, everyone from purchasing agents to shop personnel must consider the ramifications of tool hardness and construction. Because the softest tool that will do the job is usually the least expensive to build, here are a few ways you can get the most life from the least tool.

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Molds cut from soft steel can last a long time, but they must be well maintained. This aluminum base has steel wear plates along the inside edge that weren't cleaned. After producing 20,000 HIPS parts, there are obvious signs of grimy buildup caused by venting. The sprue bushing also has a steel insert and shows some signs of corrosion.

Define Your Parameters
There are basic requirements that should be determined in advance.

  • Program life. Start by considering the life expectancy of the tool: the number of parts per order, number of orders per year, and number of years in the program. Also, consider the aging your tool will experience because of startup stress. A mold that runs infrequently is more thermally stressed than one that runs continuously. Infrequently used tools, especially those molding high-temperature resins such as PPS, often make fewer parts than expected, thanks to thermal stress.

  • Resin properties. Is the resin abrasive, corrosive, flash prone (at .0001 inch), or in need of a hot mold (more than 250F)? What kind of fill velocity and venting are required? What surface finish is needed for reasonable release and ejection? Moldmakers know each increase in tool performance requires an increase in tool strength and precision. This necessitates a harder, or sometimes tougher, tool material, but not every toolmaker has experience with the material you will be using. If you don't have a particular tool material selected, give your designer and moldmaker enough information to help them make good decisions.

  • Part design. Next, determine what kind and number of shutoffs are required. Is it a speaker grille, a 600-pin connector with four shutoffs per pin, or a bread box with a flat parting line? Each step away from a simple shutoff should be accompanied by a change in hardness or surface coatings (a simple shutoff has no interlocking angles of less than 10°). Additionally, ejection systems, side actions, and objects loaded into the tool (insert molding) influence tool material selection.

  • Productivity issues. What kind of throughput is needed? Does adding a second to the mold open or ejection time cause accounting to go crazy? What about preventive maintenance? Is it a way of life or the slogan of the day? Given the program life, does it cost more to trim parts or build a better tool? What does the customer think?

  • Tool maintenance costs. Does your customer pay for maintenance on an as-needed basis, or are you working under a mold warranty agreement? Customers who buy parts from supplier-maintained tooling often pay more for tooling initially because the supplier wants the best tool possible.

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Soft tools do offer versatility. These PPS parts were molded in an aluminum prototype tool. The halfmoon part has a highly polished surface. The larger part is unpolished.

Its Built; Keep It Running
Once the tool has been constructed, there are many things that can be done to extend its useful life.

  • Keep it clean. Soft tools are sensitive to compression and abrasion. Residue buildup can gall ejector pin bores or hob parting lines. This leads to flash, which, if left on the parting line, can cause further damage. How often should a tool be cleaned? It depends on the material being molded and the tool design, but most parting lines would be glad to be cleaned twice per shift. Some materials, sticky by nature or degraded by processing pressures, require cleaning hourly. Most tools can survive on one cleaning per shift. To clean, use a mold cleaner designed to loosen normal parting line and vent residue. The cleaning fluid should be compatible with the tooling material. (I once saw a number one finish on an aluminum tool ruined by an incompatible cleaner.) Wipe parting lines with a lint-free rag and use a soft brush to reach the corners-as long as the brush doesn't scratch the tool surface. Spray again to remove loosened material. If using an evaporative spray, let it dry. Second, treat the ejector and return pins or mechanisms the same way by pushing them to full stroke and cleaning the sides. This keeps the ejector bores clean on conventional systems, and it is especially important on stripper plate systems as residue buildup can affect shut height or seriously damage this expensive ejector system. For side action components, treat the jibs as you did the ejector systems, then lubricate after cleaning (medical or cleanroom molding is the exception; use prelubricated components instead). Finally, at least once a day, lubricate the return pins, leader pins, and guided ejector system.

  • Process gently. Fill velocity was mentioned earlier. Softer materials abrade more rapidly than their harder cousins, so use the lowest possible velocity, pressure, and clamp pressure that produce good parts. Unfortunately, sometimes financial pressures tempt us to push harder. Which ultimately costs more: two more seconds per cycle, adding an operator to trim parts, or repairing the tool? Don't forget to include lost productivity for downtime in your cost calculations.

  • Balance mold temperatures. Because the tool was fitted at a common temperature, it stands to reason that achieving a uniform temperature between halves would keep the fits as they were at room temperature. This does not mean running the same temperature water in all cooling lines. It means using a pyrometer to verify temperatures at various locations and adjusting cooling flow accordingly.Another aspect of temperature control concerns the startup procedure. Give the tool time to stabilize at the molding temperature before cycling the press. This is especially important when the tool has marginal cooling. Typically the areas most likely to wear are farthest from the cooling media and reach temperature last. How long should it take to stabilize? As a rule of thumb, the mold stabilization time should equal the screw soak time.

  • Use wear-resistant coatings. While a coating does not increase the hardness of the parent metal, it can reduce wear from abrasion, injection turbulence, vent burning, and ejection. Titanium nitriding (cold process), chrome, and nickel- or Teflon-filled coatings are compatible with most alloys; hard anodizing is widely available for aluminum.Also, consider decorative anodizing to reveal high wear areas. The identification of wear before damage occurs to the parent tool is an underused benefit of surface coating. By inspecting for this wear, molders can replate as required or modify the tool with wear-resistant inserts in the now-identified wear areas.

  • Replace high wear areas with harder materials. About 90 percent of any well-made tool lasts a million cycles or more. But gates, vents, and ejector bores can wear in as little as a thousand cycles. (I saw one 40 percent glass-filled PPS part whose gate, machined from H-13, lasted less than eight hours.) When a problem area is identified, let the repair requirement become an upgrade opportunity. Install a hardened tooling insert instead of reinstalling the same alloy. Some customers may object to the witness lines caused by installing the insert, but they are fewer than the customers who want to keep getting parts without repair costs.How hard should you go? For one part molded from a highly abrasive material, I saw cavities made of D-2 ultimately receive solid carbide inserts with TiN coating. This combination was good for up to a month of run time. The unplated D-2 inserts lasted less than five days.

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Abusing soft tools in the press can not only damage the mold but produce bad parts as well. This is the top edge of a display box from a soft mold that was clamped too hard. Parting line damage caused the part edge to roll-an aesthetic violation.

A Word about Welding
Welding fixes accidents; it does not remedy problems. Using weld makes sense to finish runs, correct machining errors, or save costly components. But if the problem is wear related, welding is not a solution. It is like using pans to catch leaks instead of repairing the seals.

The compromises required of molders who run soft tools are the same as those of the hard tool molders, but they are apparent earlier. With reasonable care and foresight, not every tool has to be a fully hardened, specialty alloy work of metallurgical marvel.

Table 1. Categorizing tool materials for injection molds

Material

Epoxy, filled

Sintered metal

QC-7 aluminum

P-20 steel

Viscount 44

S-7

H-13

D-2

CPM 10V, CPM 9V

Carbide

This list is far from complete. There are several factors beside hardness that determine the material of choice for a specific program. Consult your tool builders, molding material suppliers, and especially tool material suppliers for additional advice.

Contact Information
RC Marketing Inc.
Warren, PA
Richard Caufman
Phone:(814) 726-7442
Fax:(814) 726-7443

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