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Tool Tips: Materials, Manifolds & More
Make sure you get what you really want with a little care on the injection mold design.
By Design News Staff -- Design News, November 7, 2005
You design a part, build prototype tooling and test the prototype parts. You're going to get exactly the same part from the production tool . . . correct? Not necessarily. The transition from prototype tool to production tool can be fraught with problems.
There's a wide gamut of issues that must be considered, including construction materials, gate design, venting, and runner design. Sometimes, you'll know after the first shot in the production tool that you have problems because the tool failed to fill properly, shrinkage rates varied from prototype tests, parts warped, or cosmetic surfaces were marred by sink marks. And then again, you might not discover damaging problems until four to six months later when you get a call at 4 a.m. from your QC manager in Souchou, China.
Start with a careful review of the specification for the materials used to make the production tool. Most prototype tools are made from aluminum or inexpensive, easy-to-machine steels. You need better for production tools, particularly so if you're molding high-temperature, high-performance, filled plastic compounds. The tremendous run-up in tool steel prices over the past 18 months has caused a few purchasers to cut corners on steel specs. You'll find out in time—possibly it will take a few months—if you made a serious mistake.
Filled resins will wear down the cheaper tool steel at gates and other vulnerable locations. In general, steel with a hardness of Rc 55 should be used for production runs of 50,000 shots or more. For applications with potential parting line damage, the bulk hardness of the cavities should be above Rc50. The highest-rated materials for superior abrasion resistance are H-13 and S-7 steel as well as hardened SS 420. Poor choices for abrasion-resistance applications would be nickel-plated P-20 and D-2. One way to guard against excessive wear from highly filled materials is to fabricate several gate inserts that can be replaced easily after signs of wear.
The next major issue in tool design to consider is gate design. General guidelines for gate design include:
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Land length of gates should be 0.030 to 0.060 inch.
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For proper packing of the tool, the smallest gate dimension should be at least half of the part wall thickness. The gate should be located at the thickest section of the part.
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Corners should have generous radii to reduce shear. This is particularly important for compounds with glass reinforcement.
Good venting is also required in production tools. Tools fill rapidly under high pressure in actual part production. Air trapped in the mold cavity needs to escape quickly to avoid burning, short shots, tool damage and other problems. Vents should be as large as possible. Be sure to follow material suppliers' recommendations on vent dimensions to avoid burn marks or flash. Smaller vents can be used on parts, such as connectors, with a small shot size.
Relief channels need to extend all the way to the outer edge of the tool. Pay particular attention to venting on insert-molded parts. In prototype operations, metal inserts are often hand loaded and there are plenty of air escape routes in the tool. Sealing is tight on production tools. Recommendation: Design venting in the production tool to the minimum level recommended by your materials' supplier and plan to add vents where you need them. A good way to determine areas that need more venting is to run the first test runs in natural (unpigmented) material to easily spot burn marks.
Runners used to convey melt from the injection barrel to the tool cavity require special attention. Productions tools increasingly use hot manifolds that generate no scrap. Pay attention to heat distribution in a runnerless tool for semi-crystalline resins, such as AMODEL® polyphthalamide. Temperature control at each gate is essential. Inconsistent temperatures can cause drool, flash, part-weight variations, high scrap rates and other problems. Consider using a hot-to-cold runner system. It has the benefits of a hot runner, yet uses a very small cold runner to catch the cold slug.
Hot runner temperature controls can monitor from two to 192 zones. The latest and greatest on display at the last giant plastics emporium in Düsseldorf, Germany (the K Fair) used closed-loop systems to measure thermocouples 20 times a second. Something simpler should work fine for most parts. Reflecting the current economic climate in the global injection molding business, emphasis in hot runner temperature controllers is increasingly on more control at lower cost.
For tools with cold manifolds, design systems that minimize scrap but fill the tools as efficiently as possible with minimal pressure drop. In situations that require maximum part properties, the runner system diameter should be designed larger than the part wall thickness. Reason: Resin in the cavity should set up fully before the runner freezes off to insure that the part is fully packed out. The draft angle for the sprue must be between 3 and 4 degrees. A polished surface will also help keep the sprue from sticking to the tool manifold. The sprue feeds a network of runners that connect to other cavities in the tool. Design tip: Under size the cross section of the runners. They can be enlarged later if you encounter filling difficulties. Remember that a balanced system is absolutely essential so that all cavities fill at the same time with the same pressure. The cold slug (see diagram) has a reverse taper in order to pull the sprue out of the stationary side as the mold opens.
To access The Design Engineer's Portal of High Performance Plastics, go to http://rbi.ims.ca/4400-506.
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