What is the right injection mold tool steel? That depends. What works for one job does not necessarily work for the next and the correct choice of material can make or break the injection mold making job. Once you have made the core and cavity blocks, slides, wear plate and gibs, there is no turning back.
Typically plastic injection molds are manufactured from either heat-treated tool steel or pre-hardened steel. Other materials include aluminum and epoxy-like composite materials, which work well for low-volume projects.
Machining costs can vary dramatically as well: compare surface grinding D-2 versus S-7, for example.
Plastic Mold Tool Steel Considerations
- What is the production volume? How many plastic parts will the mold be making? Obviously, heat-treated tool steels will last much longer than pre-hard or aluminum.
- What are the surface finish requirements? A highly polished lens requires a hardened tool steel, such as 420 SS or H-13. Some steels simply will not allow for the high lustre of a mirror finish.
- What type of plastic will the mold run? Some materials are very abrasive and will quickly destroy the wrong material.
- Is the mold highly detailed? Tiny details are, by nature, very weak. A soft steel will not stand up to the molding pressures and will fail.
- What machining methods will be used? D-2 is much more difficult to machine than PX-5, for example and requires better equipment, higher horsepower and higher quality cutters. QC-10 aluminum, on the other hand, is very easily machined and can easily be cut with HHS.
What about the injection molding process?
- What will be the molding temperatures? Certain alloys are designed specifically for high temperature production and will withstand the abuse. The wrong choice will cause the tool to fail.
- Does the mold require moving parts, such as slides and lifters? Usually designers combine dis-similar steels of differing hardness to prevent galling or premature wear.
- Will the finished mold be coated? New coatings come on the market regularly and some work on particular steels better than others.
- Is the thermal conductivity of the mold important? Beryllium copper, for instance, is highly conductive, as is aluminum.
- Does thermal expansion of the mold affect the quality of the part? Silicon molds are heated during production and thus the mold components are often all made of the same material. This enables the injection mold designer to predict more closely how the mold will expand when heated.
- What type of plastic molding will be used? Besides injection molding, there are many other methods of producing a plastic part. RIM, extrusion, vacuum, overmolding, insert, two color, stack, blow and compression mold are some examples that require unique mold materials.
Beware of “traditions”: just because you have always done it this way does not mean it is the best way.
Build a good working relationship with the steel supplier
Be sure to nurture a good working relationship with the steel supplier as he is crucial to the process. Stick with one or two suppliers, rather than bottom fishing for the cheapest price. In reality the steel represents only a small fraction of the entire cost of a mold, so it makes no sense to bargain hunt for cheap injection mold components.
A good supplier will have a knowledgeable applications engineer available to help solve problems and troubleshoot. Their heat-treating guidelines and recommendations are invaluable and can save a lot of time and money.
Types of injection mold tool steel
Types of injection mold aluminum
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