The Ultimate Guide To Mold Steel

Mold steel is a popular choice for injection molding dies. It is strong and wear resistant, making it ideal for high-volume production. In this guide, we will discuss the different types of mold steel available, as well as their respective advantages and disadvantages. Whether you are just starting out in plastic injection molding or are looking to upgrade your current die selection, this guide will be sure to help!

What is molds steel?

The creation of molds, such as cold-punching molds, hot forging dies, die-casting molds, and others, uses a substance called mold steel. The mold is the primary processing tool used in the fabrication of radiometers, motors, and other types of machinery.

The Ultimate Guide To Mold Steel

What are the points to consider when selecting mold steel?

Resistance to wear

Its wear resistance is one of the most important characteristics of die steel. Hardness is a key factor that determines wear resistance. In many cases, a high die hardness, minimal loss, and superior corrosion resistance are desirable qualities. Wear resistance varies with carbide type, number, and shape.

Durability

The mold’s working pressure is extremely high. It needs to support a heavy impact load. Therefore, it needs to be very strong and resilient to handle such demanding working conditions. The amount of carbon and the texture affect how tough it is.

Fatigue resistance

Molds that are operated under such high pressure always experience fatigue fracture. The characteristics that affect the fatigue limit are the material’s strength, toughness, hardness, and quantity of inclusions.

Features of high temperature

Working at a high temperature will cause its hardness and strength to decrease, causing the mold to distort and fail. To guarantee that the mold has great strength and hardness when working at high temperatures, the mold steel must have excellent tempering stability.

Corrosion protection

The thermal decomposition will result in the production of HCl, HF, and other etching gasses because the plastic contains chlorine, fluorine, and other elements. This will damage the mold cavity’s surface and harden it, increasing the likelihood that the mold may break.

The following video is intended to help you get a better understanding of the basics of mold steel and what to consider when selecting your own mold steel that meets your specific requirements.

What are different types of molding materials?

There are many different plastic mold material series available because of the variety of working conditions (processing objects), manufacturing processes, precision, and requirements for durability of plastic molds. Steel, copper alloys, aluminum alloys, and zinc alloys are some of the materials utilized to make plastic molds.

Mould steel

Steel is used in the majority of plastic molds. The two main types of steel are carbon steel and alloy steel, with varying chemical compositions. Heat treatment can also alter steel`s strength, plasticity, and toughness. The steel of ordinary carbon composition is typically marked with sulfur and phosphorus content depending on the letter A, B, C, or D; for instance, steel of grade A is stamped with yield strength and the number Q, with the Q indicating the yield point size, and the number for the symbol Q plus the number. Letters F and b go behind the grade for boiling steel, respectively; letters F and b do not go behind the grade for sedimentation steel.

Can steel be injection molded?

Yes, this process is called Metal Injection Molding (MIM) or steel injection molding.  As a result of the laser technology, it can provide intricate parts made from metals such as steel, stainless steel or even high-temperature alloys such as nickel and titanium. Industries such as  medical, electronics, aerospace, etc. are now using this kind of process.

The tables 1 and 2 provide information on the mechanical properties, chemical composition, and grade of common carbon structural steel.

Grades of standard carbon structural steel and its chemical make-up in Table 1

	Chemical composition (mass fraction,%)
Brand	C	Si	Mn	P	S	Ni	Cr	Cu
				≤
08F	0.05 ~ 0.11	≤ 0.03	0.25 ~ 0.50	0.035	0.035	0.25	0.1	0.25
10F	0.07 ~ 0.14	≤ 0.07	0.25 ~ 0.50	0.035	0.035	0.25	0.15	0.25
8	0.05 ~ 0.12	0.17 ~ 0.37	0.35 ~ 0.65	0.035	0.035	0.25	0.1	0.25
10	0.07 ~ 0.14	0.17 ~ 0.37	0.35 ~ 0.65	0.035	0.035	0.25	0.15	0.25
15	0.12 ~ 0.19	0.17 ~ 0.37	0.35 ~ 0.65	0.035	0.035	0.25	0.25	0.25
20	0.17 ~ 0.24	0.17 ~ 0.37	0.35 ~ 0.65	0.035	0.035	0.25	0.25	0.25
25	0.22 ~ 0.30	0.17 ~ 0.37	0.50 ~ 0.80	0.035	0.035	0.25	0.25	0.25
30	0.27 ~ 0.35	0.17 ~ 0.37	0.50 ~ 0.80	0.035	0.035	0.25	0.25	0.25
35	0.32 ~ 0.40	0.17 ~ 0.37	0.50 ~ 0.80	0.035	0.035	0.25	0.25	0.25
40	0.37 ~ 0.45	0.17 ~ 0.37	0.50 ~ 0.80	0.035	0.035	0.25	0.25	0.25
45	0.42 ~ 0.50	0.17 ~ 0.37	0.50 ~ 0.80	0.035	0.035	0.25	0.25	0.25
50	0.47 ~ 0.55	0.17 ~ 0.37	0.50 ~ 0.80	0.035	0.035	0.25	0.25	0.25
55	0.52 ~ 0.60	0.17 ~ 0.37	0.50 ~ 0.80	0.035	0.035	0.25	0.25	0.25
60	0.57 ~ 0.65	0.17 ~ 0.37	0.50 ~ 0.80	0.035	0.035	0.25	0.25	0.25
65	0.62 ~ 0.70	0.17 ~ 0.37	0.50 ~ 0.80	0.035	0.035	0.25	0.25	0.25

Carbon steel is typically used without heat treatment. Q195, Q215, and Q235 steels are known for their low carbon content, superior welding characteristics, and strength. Steel grades Q255 and Q275 are stronger, more flexible, and tougher when welded. Ensure high-quality carbon structural steel’s chemical structure and mechanical properties. The steel grade is a two-digit number that represents its carbon content. Carbon structural steel is crucial to the creation of plastic molds. In Tables 3 and 4, premium carbon structural steel is described chemically and mechanically. Based on its many uses, steel can also be classified as structural steel, tool steel, carburized steel, stainless steel, spring steel, bearing steel, and heat-resistant steel.

Table 3 High-quality carbon structural steel has the following chemical makeup:

Brand	Sample	Recommended heat treatment/℃			Mechanical properties					Hardness of steel in delivery state HBW
	Blank									≤
	Dimensions/	Normalizing	Quenching	Tempering	σ b/MPa			W	AK/J
	Mm					σ/MPa	5
					Not less than					Untreated	Annealed steel
8	25	930			295	175	35	60		131
10	25	930			315	185	33	55		137
8	25	930			325	195	33	60		131
10	25	930			335	205	31	55		137
15	25	920			375	225	27	55		143
20	25	910			410	245	25	55		156
25	25	900	870	600	450	275	23	50	71	170
30	25	880	860	600	490	295	21	50	63	179
35	25	870	850	600	530	315	20	45	55	197
40	25	860	840	600	570	335	19	45	47	217	187
45	25	850	840	600	600	355	16	40	39	229	197
50	25	830	830	600	630	375	14	40	31	241	207
55	25	820	820	600	645	380	13	35		255	217
60	25	810			675	400	12	35		255	229
65	25	810			695	410	10	30		255	229

Table 4 High-quality carbon structural steel’s mechanical characteristics

Steel grade		C	Si	Mn	Cr	W	Mo	V	Others
Carburized plastic die steel	20Cr	0.17 ~	0.17 ~	0.50 ~	0.80 ~
		0.24	0.37	0.8	1.1
	12CrNi2	0.10 ~	0.17 ~	0.30 ~	0.60 ~				Ni1.50 ~ 2.00
		0.17	0.37	0.6	0.9
	12CrNi3	0.10 ~	0.17 ~	0.30 ~	0.60 ~				Ni2.75 ~ 3.25
		0.17	0.37	0.6	0.9
	20Cr2Ni4	0.17 ~	0.17 ~	0.30 ~	1.20 ~				Ni 3.25 ~ 3.75
		0.23	0.37	0.6	1.75
	20CrMnTi	0.17 ~	0.17 ~	0.80 ~	1.00 ~				Ti 0.04 ~ 0.10
		0.23	0.37	1.1	1.3
Pre-hardened plastic die steel	3Cr2Mo	0.28 ~	0.20 ~	0.60 ~	1.40 ~		0.30 ~
		0.4	0.8	1	2		0.55
	3Cr2MnNiMo	0.28 ~	0.20 ~	1.20 ~	1.40 ~		0.30 ~		Ni0.85 ~ 1.15
		0.4	0.4	1.5	2		0.55
	5CrNiMnMoVSCa	0.50 ~	0.20 ~	0.85 ~	1.00 ~		0.30 ~	0.10 ~	Ni0.85 ~ 1.15 Ca, S
		0.6	0.8	1.15	1.3		0.6	0.3
	8Cr2MnWMoVS	0.75 ~	≤ 0.40	1.30 ~	2.30 ~		0.50 ~	0.10 ~	S0.06 ~
		0.85		1.7	2.6		0.8	0.25	0.1
Age-hardening plastic die steel	25CrNi3MoAl	0.20 ~	0.20 ~	0.20 ~	1.20 ~		0.20 ~		Al1.00 ~ 1.60 Ni2.50 ~ 3.00
		0.3	0.5	0.5	1.8		0.4
					1.30 ~		0.90 ~	0.08 ~	TiAl
	06Ni6CrMoVTiAl	≤ 0.06	≤ 0.06	≤ 0.50	1.6		1.2	0.16	1.0/0.5
									Ni 5.50 ~ 6.50
									Ti0. 30/0.50 Al0. 05/0.15 Ni17. 5/18.5 Co7. 00/8.00
	18Ni (250)	≤ 0.03	≤ 0.10	≤ 0.10			4.25 ~
							5.25
Corrosion resistant plastic die steel	4Cr13	0.35 ~	≤ 0.06	≤ 0.80	12.00 ~
		0.45			14
	9Cr18	0.90 ~	0.50 ~	≤ 0.80	17.00 ~
		1	0.9		19
	Cr14Mo	0.90~	0.30~	≤0.80	12.00~		1.40~
		1.05	0.6		14		1.8
	Cr18MoV	1.17~	0.50~	≤0.80	17.50~		0.50~	0.10~
		1.25	0.9		19		0.8	0.2
	1Cr17Ni2	0.11~	≤0.80	≤0.80	16.00~				Ni1.50~2.50
		0.17			18
Integral hardened plastic mould steel	9Mn2V	0.85~	≤0.40	1.70~				0.15~
		0.95		2				0.25
	CrWMn	0.90~	≤0.40	0.80~	0.90~	1.20~
		1.05		1.1	1.2	1.6
	9CrWMn	0.85~	≤0.40	0.90~	0.50~	0.50~
		0.95		1.2	0.8	0.8
	Cr12MoV	1.45~	≤0.40	≤0.40	11.00~		0.40~	0.15~
		1.7			12.5		0.6	0.3
	4Cr5MoSiV1	0.32~	0.80~	0.20~	4.75~		1.10~	0.80~
		0.45	1.2	0.5	5.5		1.75	1.2

Molds made of CrWMn, Cr12MoV, and other alloy tool steels have poor machine performance, have a hard time processing complex cavities, and have short mold lives. Moreover, they cannot deal with the issue of heat treatment deformation. Recent years have seen the introduction of several new plastic mold steels. There are four basic types of steel:

1. Pre-hardened plastic mold steel is forged steel that has been pre-heated to the desired hardness before it is used to create molds. In our alloy tool steel standard, two of the most popular China mold steel are P20 mold steel and 718 mold steel, which increases processing quality and mold life.
2. China has developed sulfur-containing free-cutting pre-hardening plastic mold steel. S, Pb, Cs, etc., are added to pre-hardened plastic mold steel to increase machinability. Large and medium-sized plastic items requiring longer life, complex shapes, and high precision are usually cast in pre-hardened plastic mold steel. 
3. Age-hardening plastic mold steel has been developed by China in recent years. When quenching and heat treating steels, the deformation of the steel is minimized, as the steel is designed to help prevent deformation. 
4. Cold extrusion molding of steel and plastic. Cold extrusion is a very cost-effective processing technique for producing complex plastic mold cavities since it does not require cavity trimming before pressing the hardened products out.  

Injection mold steel materials

420 stainless steel, a high-quality steel with 13% chrome, is a preferred material for custom injection molding in China. Upon hardening, the high carbon content provides maximum corrosion resistance. The edge retains its shape and wears well.

Alloy of aluminum

Plastic molds cannot be made from pure aluminum due to its lack of strength. By adding magnesium, zinc, copper, silicon, and other alloying elements to aluminum, its strength and casting qualities can be improved. It also has high corrosion resistance, wear resistance, thermal conductivity, and electrical conductivity. Plastic molds may change in shape and dimensions under higher temperatures and stresses.

Cobalt alloys

Copper alloys can be classified into three main categories: brass, bronze, and white copper. Alloys with nickel, including white copper, are made from copper. Due to their low melting points, zinc alloys are suitable for making plastic molds using casting techniques. Zinc alloys are often strengthened and corrosion-resistant by adding trace amounts of magnesium.l

Making the right choice for your mold – A guide to steel selection

The rapid development of plastic products makes it possible to categorize plastics into two categories: thermosetting plastics and thermoplastics. Plastic molding molds and the creation of mold materials provide a variety of different needs due to the varied types of diverse polymers, various qualities, and the size, shape, complexity, size accuracy, surface roughness, and production batch of various plastic goods. A choice of plastic mold material should, therefore, be based on some general guidelines, roughly according to the processing method, the service conditions, the product quality, and the batch selection of the material. 

In choosing mold materials, it is important to take into consideration the plastic mold service circumstances and the quality of the product.

1) Carburized plastic mold steel is best for mold cavities requiring high hardness and wear resistance while maintaining a reasonable level of toughness. Carburizing plastic with a steel that has a low hardenability is ideal for injection molds with simple cavities and small sizes. 

2) To ensure accuracy and performance of the mold, it is recommended to use pre-hardened plastic mold steel for complex mold cavities with a high degree of precision and long mold life requirements. This prevents cavity cutting and form because of excessive cavity deformation and ensures long mold life. 

3) Molds of thermoplastic or thermosetting polymer products containing glass fiber as filler or compression molds containing thermosetting polymers have extensive requirements for hardness, wear resistance, compressive strength, toughness, polishability, and electric processability. 

4) The use of PVC, polystyrene, and ABS as raw materials for plastic products will result in corrosive gases being released during the molding process, resulting in mold failure. It is therefore vital that the mold material is corrosion-resistant; corrosion-resistant plastic mold steel can be used to make molds. 

5) For long-section plastic molds, high surface roughness in the cavity, high brightness, precise size and shape, and corrosion resistance, plastic mold steel that hardens over time is recommended. Products with high wear resistance and transparency should also use it. As stated in Table 4, various plastic mold types require different materials.

Table 4: Material choices for plastic molds

Use of plastics or products		Representative raw material	Typical product	Die requirements	Applicable die materials
General thermosetting and thermoplastics	General	ABS	Radiator grille	High strength and high wear resistance	P20, 4Cr5MoV1Si, Cr12MoV, 5NiSCa, 8Cr2MnWMoVS
		Polypropylene	Fan, blade, bucket
	Decorative piece	ABS	Automobile inner panel	High strength, high wear resistance, decorative pattern processability	P20, SM2
	Transparent part	Propylene	Three-dimensional dustproof, automobile taillight	High strength, high wear resistance and good abrasiveness	8Cr2MnWMoVS, 5NiSCa, 18Ni maraging steels
Thermosetting plastics and thermoplastics filled with glass fibers, etc.	Thermoplastics	Polyacetal nylon, polycarbon resin	Engineering plastics products, automobile interior panels, electric tool housings	High abrasion resistance	Cr12MoV, CrWMn, 9Mn2V, 4Cr5MoSiV1, 8Cr2MnWMoVS, 5NiSCa, 7CrSiMnMbV
						Thermosetting plastics	Phenolic epoxy resin	Engineering plastics parts of gears	High abrasion resistance
	Optical lens		Propylene material, polypropylene vinyl	Camera lens, Fresnel lens	High mirror machinability, high strength and rust prevention						18Ni maraging steel, 3Cr13, 4Cr13
	Flame retardant parts		ABS	Television housing, cathode ray tube housing, radio base	Appropriate strength and corrosion resistance	17-4pH, 3Cr13, 4Cr13, P20 chromium plating
Polyvinyl chloride							PVC	Telephone, sink pipe, joint	Appropriate strength and corrosion resistance	17-4pH, P20 chromium plating
Polytetrafluoroethylene		Polytetrafluoroethylene	Sealing gasket, wire coating material	Corrosion resistance	Ni-based superalloy containing tungsten and molybdenum

Carbon steel is typically used without heat treatment. Q195, Q215, and Q235 steels are known for their low carbon content, superior welding characteristics, and strength. Steel grades Q255 and Q275 are stronger, more flexible, and tougher when welded. Ensure high-quality carbon structural steel’s chemical structure and mechanical properties.

The steel grade is a two-digit number that represents its carbon content. Carbon structural steel is crucial to the creation of plastic molds. In Tables 3 and 4, premium carbon structural steel is described chemically and mechanically. Based on its many uses, steel can also be classified as structural steel, tool steel, carburized steel, stainless steel, spring steel, bearing steel, and heat-resistant steel.

If you have any further questions about steels for injection moulds, please contact us! At Go4mould, we have over 20 years of experience in the industry and our team of experts can help you create the perfect mould for your needs, ensuring that your product is manufactured with precision. Talk to an expert!