As we mentioned in our first post, choosing the proper material for your next injection mold can often be as critical as choosing the right plastic resin for your part.
In this post, we’ll answer those important questions to uncover when and why you may choose hardened steels for your next injection mold project.
General standards for the three most common injection mold materials
1. How many parts are expected to be molded?
Common grades of hardened steel include H13, S7 and 420SS. Similar to pre-hard steel, hardened steel offers a wide range of versatility in the injection mold making industry. Typically found with a Rockwell rating of RC 46-62 (S7 tends to be a little harder than H13), it can be a great material for creating core and cavity detail. With hardened steel molds, one can easily achieve 1,000,000 cycles for unfilled materials and 250,000-500,000 cycles with filled materials. Once again, this range is dependent on the amount of filler present in the plastic material.
2. What surface finish requirements are expected of the molded part?
Much like pre-hard steel, hardened steel has the ability to accept a multitude of different surface treatments. This may include light to aggressive textures, EDM finishes or polish. An A-1 or better polish can be achieved but this is grade specific. While materials like H13 is capable of achieving an A-1 finish it is generally preferred to use 420 SS. It is important that the grain structure of the material be at 90° to the mold surface to achieve best results.
3. What steel conditions exist as a result of the part design and are they conducive of excess wear?
With hardened steels you can decrease the amount of draft of wiping shut-offs to about 3 degrees. When dissimilar cavity and core steels are used in conjunction with pre-alignment features (straight side locks), less draft can be used for shut-off angles. While hardened steels perform better under these circumstances, don’t be alarmed if you mold maker and molder take exception to certain details with less than 3 degrees of draft from a “guarantee” perspective. Because of their increased hardness, it is typically not necessary to have different hardness/alloys materials where you have moving components (like when using pre-hard steel).
With the pressures and heats seen during the injection molding process, small, intricate and tall details are inherently a potential weakness for any steel and should always be taken into consideration. Because S7 and H13 will not lose hardness over time, this material becomes an ideal candidate for such detail. It is generally recommended not to exceed a 5:1 aspect ratio for standing core details. However, if this aspect ratio is exceeded, it is best to insert the detail in the mold. As always, circumstances like this can often be a balancing act, so it is important that you consult your mold maker to understand any risks associated.
As we touched on in the previous post, it is important to understand the compression strength. Compression strength is critical in mold construction as it dictates not only the strength of the steel but also the dimensional stability. Of the 3 materials being reviewed in this 3-part series, hardened steel has very good to excellent compression strength. Because of this, hardened steel is a great candidate for higher volume production tooling, including insert molding and overmolding. Lastly, hardened steel is considered to have excellent dimensional stability after heat treat and is not susceptible to warpage during the machining process.
4. What type of plastic resin will be used on the molded part?
When molding high heat resins like PEI, PPS, PPSU, PPA and PEEK at higher volumes, hardened steels should be your go-to steel. Because of the way S7 and H13 are alloyed, they do not lose their hardness over constant heating and cooling cycles. Because of its inherent resistance to heat cycling, many companies use S7 and H13 for components within hot manifolds. Hardened steels will also hold up better to highly abrasive materials. However, even with hardened steels, the abrasive nature of some materials can still wear on finer details. For highly abrasive materials, with volume expectations that exceed 250,000-500,000 cycles, a great “insurance policy” is to have the core and cavity surfaces of the mold coated/plated. When this very thin layer of material is applied to the core and cavity surfaces (typically .0003”-.0005”), it can greatly increase the Rockwell hardness of hard steel. According to Bales Metal Surface Solutions, Rockwell coating of 45RC-85RC can be achieved with various coatings. If molding highly corrosive molded resins, like PVC, common metals can quickly corrode, so 420SS is highly recommended. There are many options that offer different levels of hardness, as well as corrosion resistance, so it is recommended that you consult with your mold maker or coating professional.
A wide variety of mold materials are readily available for the construction of injection molds. By considering these 4 basic questions, you can make a more educated choice as to what the correct material is for you. While choosing the best option for your mold is not complicated once you understand the pros and cons of each material, it is a critical part of the process that can affect the success of your project. Research done prior to mold construction, and more importantly vendor selection, can help ensure you receive the performance you expect from your next mold. Tune into the next article for information related to aluminum mold materials.