Guide for Selecting the Best Steel for Your Mold
There are many different metals which can be used to create molds. The best option for one project may not be the best option for another. There are also short-term and long-term costs to consider. Some tools and molds may be less expensive to make but cost more to maintain while, for others, the opposite true. When choosing mold metals a balance must be achieved between these two considerations. Where that balance lies may not always be intuitive because some metals may react differently to different resins.
For lower volume tools, aluminum can be a cost-effective choice. This is because aluminum is less expensive and can reduce cycle times by allowing faster cooling times during production. Tools can be made more quickly from aluminum than from many other metals. For high-volume production, hardened tool steels are usually a better choice. This steel is wear-resistant and less thermally conductive. Although these steels do not require a coating or surface treatment to reduce wear one can be added if desired.
Hardened steel is wear-resistant and typically low maintenance. Processing glass filled materials may lead to more rapid wear and erosion. Because stainless steel is resistant to corrosion it is best for projects requiring such abrasive materials. Other options may be used in place of stainless steel, but they require special treatment to address the risk of corrosion. Unlike most other wear-resistant steel varieties carbide inserts have high thermal conductivity. However, their costs can be prohibitively high and they can be difficult to replace.
Aluminum molds can help keep building costs down and speed up cycle times. Many different variations of aluminum are used in molding. Assuming that because these variants are similar means that they are interchangeable can be a significant mistake. Each of these aluminum types has different thermal and structural properties. Aluminum is a soft metal, and for any design forged in aluminum, it is necessary to take extra care to make sure that its structure is sufficiently robust.
For high-volume parts that have cosmetic requirements or very precise measurements, aluminum is not recommended. Using aluminum for prototype tooling may be useful for testing, however, even if the final tool will be cut from a different metal. If the prototype design hasn’t been proved out, and significant changes need to be made, it’s better to build an inexpensive aluminum tool until you’ve proved your design.
Alloys are significantly more robust than aluminum and many offer excellent thermal conductivity. A wide variety is available with varying hardness which can be especially helpful when molding certain materials. As the hardness of an alloy increases its thermal conductivity decreases. Depending on the material being molded this may be either positive or negative. Alloys are more expensive than aluminum but they don’t crack, break, or warp as easily. Carbide inserts, as mentioned above, are alloys usually containing tungsten.
Steel is a versatile metal for molding. Like aluminum, it comes in many varieties, and all types should not be considered to be the same. Although steel molds can be used equally well for high-volume and low-volume parts. In general, steel must be protected from erosion with a coating or through surface hardening. This can have drawbacks both in manufacture and maintenance depending on the demands of a project. The most common steel in the middle of the spectrum is P-20. When using plastics without abrasive additives, this steel, or one with similar features is usually the first choice. For high-volume, tight-tolerance parts, a better choice is S-7 steel. It is impact resistant and stable in the heat treating process.
S-7 steel expands and contracts less than other steels that are comparable to it which makes maintaining tolerances easier. When creating medical tooling or highly polished cavities stainless steel is best. It can also be used for PVC since stainless steel is resistant to corrosion. The most common variant of steel for these uses is H-13. Stainless steel usually does not require protection from erosion as do other variants of steel, but it can erode under certain circumstances.
If a tool isn’t hardened properly it can experience expedited wear. Tools with sharp edges are more susceptible to these defects. To mitigate damage pockets for slides and lifters should have smooth internal corners and radii within specific cavities. Because most CAD models will not add these radii, and cosmetic care is essential to product quality, partnering with a skilled and competent molder is a wise option to ensure proper part manufacturing.
When finding an appropriate metal for your mold consider its function, life expectancy, and manufacturing cost. The best option often is to find a skilled molder, such as New Berlin Plastics, to assist you with mold design and build. If you would like to know more reach out to New Berlin Plastics at 262.784.3120.
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