|Aluminum molds were considered to be too soft for high volume production so far, because the tooling was unable to withstand the rigors of high-temperature, high-volume injection molding. This is changing as OEMs look more closely at tooling costs and cycle times to reduce piece-part pricing, and overall manufacturing cost. Many companies are now using aluminum for production instead of just prototype work. Aluminum tooling offers a myriad of benefits: it is easier to cut and it cools at a much more rapid rate than tool steels, which reduce cycle times-resulting in reduced costs at the OEM level.
Aluminum cools at a much quicker and even rate than steel, reducing cycle time and saving money. Because of its lightness in weight, aluminum has the advantage of faster machining. Aluminum dissipates heat at a very even rate, which allows for great dimensional stability due to less distortion. Longevity of aluminum tooling can be improved by surface coatings. Some aluminum molds are capable of producing 2 mln cycles. Some of the major benefits of aluminum molds versus P-20 steel are:
• 10% saving in cost of manufacturing an aluminum mold
• 20-35% gain in cycle time resulting in higher output
• Lower injection and clamp pressure with aluminum molds as compared to steel tooling
• Has 4-5 times better thermal heat transfer. With better thermal conductivity, the position and number of water lines is less critical, allowing for the placement of additional ejector pins where needed
• Demonstrated productivity improvement of 20-30% in cycle time savings
• Has outstanding machining and polishing characteristics. With some high strength aluminum alloys, machining ratings can be 8 times faster than steel
• Does not have to be sent out for heat treating or stress relieving, as this can be done during its manufacturing process
Aluminum mold can be designed depending on the abrasiveness of polymer material to produce from 100 K 1 million components. Aluminum alloys such as 7075, Alumold, QC-10 and Hokotol have a Brinell hardness range of 150-180 and Rockwell hardness range of B82-87. Case hardening of aluminum mold or inserting certain areas of mold base with steel will enhance the production cycle of most abrasive material like glass filled compounds. Almost any finish that can be applied to a steel alloy mold can be applied to an aluminum mold. This includes finishes and surface grades from SPI, Mold-Tech, Raytheon or any other industry or OEM finish specification. Various grades of aluminum are available, and utilizing the correct grade of aluminum for the number of parts required and plastic requirements will result in cost savings, averaging on the order of 10-12% and as high as 15-25%.
Why does plastic flow better in aluminum injection molds?
1) Plastic material flows longer distances with less injection pressure, when compared to steel
2) Molds fill faster and more efficiently
3) Parts have minimal warp and better dimensional stability
4) Aluminum�s thermal conductivity is nearly 5 times greater than that of steel
According to moldmakingtechnology.com, aluminum alloys are differentiated by the manner in which they are produced: when manufactured, they are either a non-heat treated cast material or a heat-treated wrought material. Non-heat treated material is ideal for low-strength mold applications. Cast products typically display lower strength and provide excellent dimensional stability, excellent machinability and minimal residual stress. Though not engineered for high strength, cast material is suitable for some low-pressure injection molds and straight injection molds requiring less than a few thousand shots. The high strength, heat-treated wrought aluminum alloys are technologically advanced alloys with enhancements in heat treating, ageing practice or a combination of both. These products exhibit high strength (relative to cast product), excellent machining and polishing characteristics, as well as dimensional through-thickness consistency. These aluminum alloys are suitable to mold nearly 75% of all unfilled resins�including PP, HDPE, Nylon, PET, ABS and PE. Molds manufactured using high strength aluminum has been known to successfully run millions of shots. Obviously, proper tool design, setup, running and maintenance of the mold must be adhered to. In the middle of the spectrum are what can be referred to as common alloys�such as 2024, 6013, 6061, 7050 and 7075. Common alloys are manufactured at facilities in accordance with the Aluminum Association procedures for manufacturing these materials.
When designing a production aluminum mold, several factors need to be taken into consideration for a successful mold. They are
1. Is the resin conducive for aluminum?
2. What is the geometry of the part to be molded?
3. What is the quantity?
4. What is the desired cycle time?
5. How long does the mold need to last?