INJECTION MOLD STEEL TYPES

The mold is a key part of the injection molding process. Every part of the injection molding machine is important in equal measures. But the part where the product finally forms is the mold. The state of the plastic, the pressure of injection, and the gate size. These are some of the factors that affect product quality. The material of fabrication of the mold is also an important factor. It not only affects product quality, it also affects the efficiency of the entire process. The mold material affects the rate of cooling of the plastic, the precision, and surface shine. These amongst others. The quality of the product relates to the inherent properties of the mold material. These are properties like thermal conductivity, wear-resistance, and hardness. 

Mold makers and operators might disagree on the type of steel to use for a type of mold. But one thing they’ll all agree on is that the best material when all factors get considered is steel. Aluminum gets used for prototypes and test runs. These molds are not expected to last long and get used for making one or a few samples of a product. In some cases, the aluminum molds might not give a good representation of the product. This is because the type of mold and mold material affects the product quality a lot. To counter this some mold makers use a combination of steel and aluminum. The aluminum forms the base of the mold while steel gets used for the inserts and/or cavity walls. Aluminum also has better thermal conductivity. This comes in handy when cooling channels cannot fit in either due to time or cost constraints.

It is difficult to say that a particular type of steel should get used for a type of plastic. This is because the choice of material is an interplay of factors. The right approach is to present an understanding of the decision made. For example, one can make general recommendations. Like hardened tool steel for molds that need good wear resistance. But this also means a longer cycle time. This is a result of the lower thermal conductivity of mild steel. The operator can then decide to either use standard tool steel to save cycle time. Otherwise, he can choose to keep longer cycle times. This reduces the cost of mold maintenance and replacement. It also depends on the stage of the company. Small startups might be willing to have a mold that works well in the shorter term. More matured companies would want a longer-lasting mold with lower long term cost.   

Injection Mold Steel Types:

Mold Classification by the numbers

Molds get classified based on the number of shots they run throughout their lifetime. This classification groups different grades of materials based on their durability.

Table listing Mold Classification based on lifetime shots

Understanding the different grades of steel

To select the right type of steel for an injection mold it is important to have the fundamentals on steel. This is of particular importance to early carrier injection molding professionals. Over the years operators and mold makers build experience with different steel types. So over time, each one narrows down to a selection of steel they prefer to work with. Having in mind that there can be more than one grade of steel suitable for a particular application. So other factors other than technical suitability come to play.

There are thousands of steel grades available in different markets. Steel gets made by adding around 2% carbon or less to ferrous compounds. This is alongside several other elements. The added elements include manganese, silicon, and phosphorus. There are four broad categories of steel. These are stainless steel, carbon steel, alloy steel, and tool steel. Chromium gets added at around 11% to make steel stainless. Stainless steel does not rust or get oxidized. Carbon steel is iron with varying amounts of carbon. It has only traced to zero amounts of other elements. Copper, silicon, phosphorus, manganese, and sulfur get added to copper steel. Compositions of these added elements vary from 1.65% to 0.05%. Sulfur is usually in the lowest composition. The carbon content varies from mild to medium to high.  Carbon contents of carbon steel vary from 0.3% in mild steel to over 0.6% for high steel. Alloy steel is iron alloyed with other elements. These can be copper, chromium, cobalt, nickel, and aluminum. These alloys can be a combination of two or more elements. Tool steels contain cobalt, molybdenum, vanadium, and tungsten. These types of steel get made with a focus on heat and wear resistance and durability. They can get machined and used under demanding conditions. Within these four categories are several grades of steel. 

Other than the composition of steel, other factors affect the properties of the steel. So two grades of steel may have the same composition but they get made in different ways. Factors that can vary between grades include the cooling rate and holding time. For example, steel can get hardened by working in a cold state or applying heat treatment. So you’d have it hardened as a different grade. Other factors are the method of combining the different components of steel. There are two main grading systems that are well known globally. One is the American standards and testing methods (ASTM). There’s also the society of automotive engineers (SAE) grading system. The SAE grading system is also interchangeable with the AISI. American iron and steel institute system. 

Typical properties of steel of the four different types

With this in mind, the different grades still vary the compositions of the steel alloy. These various grades achieve different properties which make them suited for specific purposes. Even when narrowed down to injection molding, there are still many options to choose from. There might be more than one option of steel to meet your molding applications. The main thing is not to choose the wrong one. There are a lot of right options and you can’t be too right. But choosing the wrong option can result in loss of time, money, and material. Below are some grades of steel and their composition.

Some steel grades and descriptions

Factors to consider when choosing steel for injection molding

There are several factors to consider when choosing the grade of steel to use for a mold. Some mold makers might tell you they have a preference for some types of material. This could be that they find that specific type of material easier for the machine. It could also be that they have had more experience with that material and know what to expect. The material used for the mold is not only down to the mold maker. The machine operator also gets a say. There often calls for a compromise between initial cost and maintenance cost. Some operators might prefer to run better cycles that need less maintenance. In this case, they would be willing to spend more on a mold that runs more cycles in its lifetime. Other operators might have more access to skill than capital. Those would prefer to use a good mold that retains its quality with regular maintenance. Either way, there is no case where a cheap poor quality mold is acceptable.

The list below summarizes the factors which affect the choice of mold:

• Type of plastic
• Hold pressure
• Temperature of melt
• Mold design  
• Part volume
• Insert or Overmolding
• Cost (initial & maintenance)

The type of plastic refers to different things about plastic. One of the main issues is corrosion. If the plastic is likely to cause metal corrosion, corrosion-resistant steel gets used. Stainless steel grades work well for such. PVC is well known for its corrosion-prone property. This is not to say that corrosion would not occur at all. But it would take longer. Likewise when molding glass fiber plastics where mold wear is a concern. Molder would go for the wear resistance plastics. Tool steels are the best classification for wear resistance. The exact grade is then down to other properties of the plastic. 

Inserts are also used where a combination of properties becomes necessary. This may not be because insert molding is necessary for a particular product. It could be that the internal cavity needs to have certain properties. For example, a mold needs to have good wear resistance but also good thermal conductivity. Alloy steel grade P20 gets used for common plastics that don’t cause much wear and corrosion. Some parts can also get made using aluminum. But these are parts that don’t demand much aesthetics or surface finish. Another option to meet product requirements is to use surface coatings. Lower quality steel gets used for the mold base. This can also be aluminum. This is then coated with a material to aid corrosion and wear resistance. This method comes with the cost of repairing surface coatings.

The design of the mold is also another important factor. The grade of steel has to suit the machine design required. So for molds with complex geometry, grades like stainless steel SUS 420 get used. this also works where high precision is necessary. This of course also depends on the capabilities of the tooling for mold making. Some machines are able to achieve high precision on even the toughest material. In the CAD design of the product, it is important to fillet the edges of the design. This prevents sharp edges which may cause problems in filling or product ejection.

The table below lists some grades of steel that get used for different types of mold. These depend on the geometry and properties of the product getting molded. For example the type of finish required of the plastic and the level of precision required in the part. The values given are examples from some known processes.

Table showing some steel grades and types of mold applications

Conclusion 

Steel remains the best choice for injection mold. There are thousands of grades to choose from for specific products. The type of material and geometry determine the choice of steel grade. Mold makers and mold operators make decisions on steel grade based on different factors. These include maintenance and upfront cost amongst others.