Choosing the proper material for your next injection mold can often be as critical as choosing the right plastic resin for your part.
With many different types of mold material options available, all formulated to meet specific requirements, ensuring the material used to produce your injection mold has a positive impact on the success of your next project. Similar to plastic resins being formulated to meet performance criteria in various applications, steel is also alloyed to meet specific requirements.
Circumstances that effect choosing the proper injection mold material could include:
- ensuring parting line integrity;
- wear resistance because of mechanical fatigue;
- wear resistance because of abrasive or corrosive plastic resins; or
- inclusion of thermal properties needed to successfully mold your plastic part.
The most common materials used to construct plastic injection molds are pre-hard steel, hardened steel, and aluminum. This 3-part series will explore each and answer the important questions you should consider before choosing your material:
- How many parts are expected to be molded?
- What surface finish requirements are expected of the molded part?
- What steel conditions exist as a result of the part design and are they conducive of excess wear
- What type of plastic resin will be used on the molded part?
In this first post, we’ll examine pre-hard steel and when it’s the appropriate material to choose.
General standards for the three most common injection mold materials
1. How many parts are expected to be molded?
Pre-hard steel is often considered “the work horse” for the plastic injection mold making industry. This is because pre-hard steel has a broad range of favorable characteristics and can be a great material for creating core and cavity detail. Typically found with a Rockwell rating of RC 30-42 (in its pre-hardened state), this material is quite tough, easy to machine and relatively inexpensive. 250,000-500,000 cycles can easily be achieved when molding unfilled materials, and 50,000-100,000 cycles can be obtained with filled materials. The range is dependent on the amount of filler present in the plastic material. When total expected volume is less than 250,000 cycle’s, pre-hard steel is the way to go.
2. What surface finish requirements are expected of the molded part?
Because of its versatility, pre-hard steel has the ability to accept a multitude of different surface treatments. So if you plan to incorporate a finish, including light to aggressive textures, EDM finishes, or even polish, this material can often be your best bet. However, when considering a polish finish on your mold and subsequent molded part, certain steels are only capable of producing certain levels of polish.
In the case of most pre-hard steels, you can achieve an A-3 finish without any additional treatment to the steel. There are specialized vacuum arc re-melted grades of pre-hard steel that can achieve very close to A-1 finish with optical lapping. So as long as your requirements do not exceed that of an A-3 requirement, pre-hard steel is once again a great choice.
3. What steel conditions exist as a result of the part design and are they conducive of excess wear?
When metal-to-metal wiping shut-offs exist, it is a common practice to have 5 degrees of draft on those wiping surfaces with pre-hard steel. When 5 degrees of draft is achieved, pre-hard steel is often a good material choice as excess wear is kept to a minimum. In addition to adequate draft and where slides and lifters (moving components) may be required to create detail, it is suggested the moving components be made of a different alloy and/or hardness to further reduce any galling or excess wear. As the total expected volume of the mold increase, this becomes more of a common practice and is recommended to alleviate future, unforeseen problems.
It is important to look at the detail in the part design that will also drive the detail required in the mold. With the pressures and heats seen during the injection molding process, small, intricate and tall details are inherently potential weak points for any steel and should be taken into consideration. The size of such details (width and height) can be of the most concern, and as a general rule of thumb you should aim to keep your aspect ratio below 5:1 for pre hard steel. This will alleviate issues down the road related to the failure of such details (cracking, breaking, bending, etc.). When the aspect ratio for standing core details exceeds this ratio you can diminish some of the risk of failure by inserting the mold in this area, but should always review with injection molder and mold maker prior to doing so.
It’s also important to understand the compression strength of your material. 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, pre-hard steel falls in the middle and has fair compression strength. Because of this, pre-hard steel is a great candidate for lower volume production tooling, including insert molding and overmolding. It is important to note, however, that you can still “hob” the steel if the molded part or insert is not properly placed and aligned in the mold. Lastly, pre-hard steel is considered to have good to excellent dimensional stability depending on the grade being considered. You can still experience some warpage of the material during rough milling, so care should be taken.
4. What type of plastic resin will be used on the molded part?
High heat resins such as PEI, PPS, PPSU, PPA, and PEEK, as well as filled materials (long and short glass fiber, carbon, stainless steel, etc.), can often be tough on pre-hard steels from a wear perspective. While certain steps can be taken, like inserting the gate area for easy maintenance and replacement, it should be expected that the life of the mold will decrease in these circumstances. If you find that your project requires between 50,000-100,000 parts over the life of your program, pre-hard steel is certainly adequate. If your part requires a high level of filler, or if you wish to extend the life of your pre-hard steel mold, a great “insurance policy” is to have the core and cavity surfaces of the mold coated/plated.
Here, a very thin layer of material is applied to the core and cavity surfaces (typically .0003”-.0005”), which can greatly increase the Rockwell hardness of pre-hard steel. According to Bales Metal Surface Solutions, Rockwell ratings of 45RC-85RC can be achieved with various coatings. 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 hardened steel mold materials.