The steels for plastic injection mold

Due to the enormous diversity of plastic materials and the widely varied requirements for plastic products, various requirements for the performance of plastic injection mold steel have been laid down. Therefore, many industrially developed countries have created an extensive range of plastic mold steel series, including carbon steel, carburized plastic mold steel, aging hardening plastic mold steel, corrosion resistant plastic mold steel, free machining plastic mold steel, through-hardening plastic mold steel, maraging plastic mold steel and mirror polishing plastic mold steel, etc.


Plastic injection Molds can be classified into 5 classes by service cycle, and they impose the following requirements on steel materials:


According to the length of mold life, molds can be classified into 5 classes. Class 1 mold runs 1 million or more shots, class 2 mold runs 0.5 – 1 million shots, class 3 mold runs 0.3 – 0.5 million shots, class 4 mold runs 0.1 – 0.3 million shots, and class 5 mold runs less than 0.1 million shots.


Class 1 and class 2 molds require the steel materials that can be harden up, with a hardness around HRC50, or the molds will wear easily, leading to out-of-tolerance injection molded products. As a result, the selected steel materials need to possess good heat treatment properties and machinability despite the high hardness. Of course, there are some other considerations, too.


Usually, the steel choices include Swedish 8407 and S136; American 420 and H13; European 2316, 2344 and 2083 and Japanese SKD61. For strongly corrosive plastics, S136, 2316 and 420 steels are normally chosen. In addition to S136, 2316 and 420 steels, low corrosion plastics can also choose SKD61, NAK80, PAK90 and 718M steels. Product appearance also has a great influence on the mold materials. S136, 2316, 718S, NAK80, PAK90 and 420 steel materials are suitable for transparent and mirror polishing products, while highly transparent products should primarily select the S136, and secondarily the 420 steel materials.


Class 3 molds mostly use pre-hardened steels, such as S136H, 2316H, 718H and 083H, with a hardness ranging from HB270 to 340.


Class 4 and class 5 molds usually use the P20, 718, 738, 618, 2311 and 2711 steel materials. For molds with extremely low requirements, S50C and 45# steels may be used, i.e. creating a cavity directly in the mold base


The spec. of plastic injection mold steel

1.USA standard:  AISI


P1-P19:Low Carbon Steel

P20-P39:Low Carbon, High Alloy Steel

2XX,3XX,4XX,6XX:Stainless Steel

H1-H19:Chromium base

Wx:Water Hardening Steel

Sx:Shock Resisting Steel

Ox:Oil Hardening Steel

Ax:Air Hardening Steel

Dx:High Carbon, High Chromium Steel

Mx:Molybdenum base (H.S.S.)

2.German standard:  DIN


1.2738:Low carbon, high alloy (P20)

1.2311:Low carbon, high alloy (P20)

1.2312:Low carbon, high alloy, free Machine (P20)

1.2083:Stainless Steel (420)

1.2316:High performance stainless Steel (420)

1.2343:Chromium base (H11)

1.2344:Chromium base (H13)

1.2510:Low alloy steel (O1)

1.2379:High carbon, high chromium steel (D2)

3.Japan standard: JIS


SxxC:Plain Carbon steel(S55C)

SUSxx:Stainless Steel (420)

SCrx:Chromium Steel

SCMx:Chromium Molybdenum Steel(P20)

SKx:Carbon Tool Steel

SKSx:Low Alloy Steel (- O1)

SKD11:Medium High Alloy Steel(D2)

SKD6:Medium High Alloy Steel(H11)

SKD61:Medium High Alloy Steel(H13)

SKHxx:High Speed Steel (M 2)

SUMx:Free Cutting Steel

SUJx:Bearing Steel

Common imported mold steels and their parameters & performance


ASSAB STAVAXESR-S136 – Highly abrasion/corrosion resistant pre-hardened mirror finish steel

  • The status of Ex-factory︰ HB215
  • Equivalent to Buderus standard︰ 2316
  • Equivalent to Bohler standard︰ M310
  • Equivalent to Hitachi standard︰ HPM38
  • Equivalent to DAIDO standard︰ PAK90
  • Abrasive resistance︰ ★★★☆☆
  • Tenacity︰ ★★★☆☆
  • Dimension stability︰ ★★★☆☆
  • Machinability︰ ★★★☆☆
  • Polish︰ ★★★★★
  • Corrosion Resistance︰ ★★★★☆
  • Product Description: The high grade stainless tool steel that possesses excellent anti-corrosion, polishing, anti-abrasion and machining properties. Electrical discharge machining (EDM) can create good mirror finishing effects and high quality surface finish; great stability will be showed when hardening. The cavity is able to maintain the original smoothness despite long-term mold production. Special care is not needed when the mold is operated or stored in a moist environment. So, it is recommended for molds that have high polishing requirements, as well as corrosive plastic molds.

 ASSAB 8407 – High grade hot-work tool steel

  • The status of Ex-factory︰ HB 185
  • Equivalent to Buderus standard︰ 2344ESR
  • Equivalent to Hitachi standard︰ DAC
  • Equivalent to DAIDO standard︰ DHA1
  • Abrasive resistance︰ ★★★☆☆
  • Tenacity︰ ★★★☆☆
  • Machinability︰ ★★★★☆
  • Product Description: The Cr-Mo-V tool steel is a high-purity fine steel material that is produced via special steelmaking technologies and under stringent quality control. The isotropy (physical properties are identical in all directions.) of the 8407 steel is better than the conventional H13, which brings great benefits of mechanical fatigue resistant and thermal stress fatigue resistant properties to the molds like die casting molds, forging molds and extrusion molds etc. As a result, the hardness of the 8407 molds is 1 – 2 HRC higher than that of the ordinary H13 without comprising the toughness. The high hardness is able to reduce the occurrence of crazing, thus improve mold life cycle. So, it is applicable for various die casting metal molds, extrusion molds, and plastic molds that have requirements for high quality.

 ASSAB 718HH — High-polishing pre-hardened precision plastic mold steel

  • The status of Ex-factory︰ HB 330-370
  • Equivalent to Buderus standard︰ 2711
  • Abrasive resistance︰ ★★★☆☆
  • Tenacity︰ ★★★☆☆
  • Machinability︰ ★★★☆☆
  • Polish︰ ★★★★☆
  • Corrosion Resistance︰ ★★★☆☆
  • Product Description: The pre-hardened Cr-Ni-Mo plastic mold steel that is produced under vacuum melting for improved properties. Before leaving the factory, it has undergone the hardening and tempering processes, so there is no risk of quenching cracks or heat treating deformation, because it does not need heat treatment, but employs nitrogentreatment and flame hardening treatment to enhance the surface hardness and abrasion resistance of the molds. The excellent polishing and anti-abrasion properties allow it to be used for thermal plastic injection molds and extrusion molds, high polishing plastic product molds, as well as blow molds, forming molds, structural components and shafts, etc.

 DAIDO NAK80 – Pre-hardened mirror surface precision plastic mold steel

  • The status of Ex-factory︰ HRC37-43
  • Equivalent to Hitachi standard︰ HPM50
  • Abrasive resistance︰ ★★★☆☆
  • Tenacity︰ ★★★☆☆
  • Machinability︰ ★★★☆☆
  • Polish︰ ★★★★☆
  • Corrosion Resistance︰ ★★★☆☆
  • Product Description: The pre-hardened steel (36-43 HRC) can be machined directly without heat treatment. Its hardness is quite uniform from the surface to the core with great machinability; possesses excellent electrical discharge machinability, and very easy to grind after EDM because the uniform surface hardness and lower white layer hardness are ensured after EDM; good mirror polishing features; excellent welding performance; great etching properties; and dimensional stability, making it suitable for precision parts production as well as mass production. As restricted by its chemical components, this steel material is quite brittle. When used for complicated molds, cracks tend to appear on the area where processing stress concentrates. Due to its high thermal sensitivity, preheating, heat reservation, post weld heat treatment (PWHT) and stress relieving treatment are required during welding, or weld failure might occur. What’s worth noting is that when treating temperature exceeds 520℃, dimensional changes can take place.

 DAIDO DHA1 – High performance hot-work tool steel JIS SKD61

  • The status of Ex-factory︰ HB229
  • Equivalent to Buderus standard︰ 2344
  • Equivalent to Bohler standard︰ W302
  • Equivalent to hitachi standard︰ DAC
  • Equivalent to ASSAB standard︰ 8402
  • Abrasive resistance︰ ★★★☆☆
  • Tenacity︰ ★★★☆☆
  • Dimension stability︰ ★★★☆☆
  • Machinability︰ ★★★☆☆
  • Polish︰ ★★☆☆☆
  • Product Description: The DAIDO DHA1 steel is widely used for Mg & Al die casting molds. As a common hot-work mold steel, it possesses great machinability and balanced heat resistant features. DHA1 is mainly used for Mg & Al die casting molds, related parts of die casting molds, hot stamping molds, hot extrusion molds and hot shearing blades, etc.


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Plastic injection molding gate types

For plastic injection mold design, one of the most important factors is how and where the gate should be located. As the mold opening, the gate is where the molten plastic flows into the final part. It serves as the boundary between the part and the scrap, so its location, size, and shape play an important role in how everything should be constructed, from structural integrity to exterior appearance of the finished product.Below is gate type we often choose:

Direct Gate(Sprue gate):

1. Little pressure loss;
2. Easy preparation.

1. High stress around the gate;
2. Gate (runner) needs to be trimmed manually;
3. Obvious gate scars will be left on the surface.

1. Suitable for production of large and deep barrel-shaped plastic parts. However, warping can easily occur due to contractibility   and stress when applied on shallow and flat plastic parts.

2,For plastic parts that do not allow gate marks on the exterior, the gate can be designed on the inner surface of the parts.


Side Gate:

1. Simple structure, easy processing;
2. Easier to remove the gate.

1. Automatic separation of the part and the gate is not allowed;
2. Gate marks are easily left on the plastic part.

1. Gate width W = (1.5~5.0)mm. Usually W = 2H, which may be appropriately increased for large and transparent plastic parts.
2. Height H = (0.5~1.5)mm. Specifically speaking, usually H = (0.4~0.6)d for commonly seen ABS and HIPS. Among them, d refers to the basic wall thickness of the plastic part; H = (0.6~0.8)d for materials with poor fluidity, like PC and PMMA; the suggested gate height for POM and PA is H = (0.6~0.8)d, so as to help avoid shrink marks and wrinkles by guaranteeing sufficient pressure holding, because though these materials possess good fluidity, they become solid very fast with larger contractibility; for materials like PE and PP, gate height H = (0.4~  0.5)d, because the small-sized gate is helpful for molten plastic shear thinning, thus reducing stickiness.

1. Suitable for production of plastic parts of various shapes, but it is will not be selected for slender barrel-shaped parts.


Tab Gate:

1. It is a form evolved form the side gate, so it shares the various advantages of the side gate;
2. It is a typical impingement gate that can effectively prevent molten plastic jetting.

1. Automatic separation of the part and the gate is not  allowed;
2. Obvious gate scars are easily left on the surface.

Refer to the side gate parameters for application.

Suitable for flat plastic parts that impose requirements on surface finish.


Fan Gate:

1. The horizontal distribution of the molten plastic is more uniform when passing through the gate, helpful for reduction of plastic part stress;
2. Lower the possibility of air getting into the cavity, to avoid the occurrence of defects, like silver lines and bubbles, etc.

1. Automatic separation of the part and the gate is not allowed;
2. Long gate marks are left on the edge of the plastic part, which need to be flattened by a tool.

1. The commonly used height H = (0.25~1.60) mm;
2. Width W = 8.00 mm to ¼ of the cavity width at the gate end.
3. The section area of the gate should be larger than that of the sub-runner.

Usually used for production of wide but thin plastic parts, as well as transparent plastic parts and those with poor fluidity, like PC and PMMA, etc.


Submarine Gate:

1. Flexible choices of gate location;

Automatic separation of the part and the gate is allowed;
3. Smaller gate marks;
4. Applicable for both 2-plate and 3-plate molds.

1. Plastic powder is easily dragged at the gate position;
2. Stress mark is easily created at water entry;
3. Plastic films need to be sheared manually;
4. Great pressure loss from the gate to the cavity.

1. Gate diameter d = 0.8~1.5mm;
2. The plastic flow direction and the vertical direction form an angle a between 30°and 60°;
3. The taper b is between 15° and 25°;
4. Distance to the cavity A = (1.5~3.0)mm.

Suitable for plastic parts that do not allow exposed gate marks on the exterior. For a multi cavity mold, the resistances from the gate to each cavity should be kept as close as possible, so as to avoid viscous flow and obtain better flow balance.


Banana Gate:

1. Automatic separation of the part and the gate is allowed;
2. The gate area does not need additional processing;
3. No gate marks will be left on the exterior of the plastic parts.

1. Stress marks may show on the surface;
2. Complicated processing;
3. Easily broken and thus blocking the gate if not appropriately designed.

1. Gate diameter at water entry end d = (Φ0.8~Φ1.2) mm, length = (1.0~1.2) mm;
2. A = approx. 2.5D;
3. Φ2.5min* refers to the gradual transition from the large end 0.8D to the small end Φ2.5.

Normally used for ABS and HIPS, suitable for neither crystalline materials like POM and PBT, nor high-rigidity materials like PC and PMMA, so as to avoid the curvy runner from being broken and thus blocking the gate.


Point Gate:

1. Flexible choices of gate location;
2. Automatic separation of the part and the gate is allowed;
3. Smaller gate marks;
4. Low stress around the gate.

1. High injection pressure;
2. Complicated structure, usually employing the 3-plate structure.

1. Usually the gate diameter d = (0.8~1.5) mm;
3.The gate length L = (0.8~1.2) mm;
4. To help pull the gate broken from the root, a taper should be set for the gate, a = approx. 15°~20°;

the gate and the runner are joined by arc R1 to ensure that the plastic part is not damaged when pulling the point gate broken; R2 = (1.5~2.0) mm; R3 = (2.5~3.0) mm; height h = (0.6~0.8) mm.

Usually used for the production of large plates and bottom cases. The proper distribution of gate can help reduce the flow distance of molten plastic and thus guarantee satisfactory distribution of melting marks; also able to be used for production of long barrel-shaped plastic parts to improve ventilation.


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Why the venting is so important for a plastic injection mold?

The venting slot serves two purposes: 1. Expel air from the plastic injection mold cavity during the injection process of the molten plastic material; 2. Get rid of the various gases produced during injection molding processing. The setup of venting slots is of great importance, especially for thin-walled products or the locations far away from the gate. In addition, close attention should also be paid to venting slots for the production of small-size or precision parts, because they are able to protect the products from surface burns, insufficient filling, as well as other defects.

So, what is sufficient air venting for plastic injection molding? Generally speaking, if no burn marks are left on the product at the highest molten plastic injection speed, then the venting effect of the mold cavity will be considered sufficient.


Venting Methods:

There are lots of ways to vent the mold cavity, but each of them has to guarantee that: while performing venting functions, the size of the vent slot needs to be able to prevent material from entering the slot, as well as clogging at the same time. However, if there are too many slots, it will do more harm than good, because if the clamping force against the mold cavity parting surface area without vent slots is too high, the cavity material will be prone to cracks, which is very dangerous. In addition to being designed on the parting surface of the mold cavity, the vent slot can also be machined in the end section of the runner system. The clearance around the ejector can also act as a way to let the trapped air out. If the height, width and position of the vent are not appropriately defined, flash will be caused, thus affecting product aesthetics and precision. As a result, the clearance design should be able to prevent flash from occurring around the ejector pin. In particular, it is worth noting that molded parts like gears expect no flash at all. Therefore, the following venting methods should be employed:

  1. Completely eliminate the air in the runner;
  2. Apply peening treatment to the parting surface with the 200# silicon carbide abrasive, and open vent slots in the end section of the runner system, mainly referring to machining slots in the end section of the sub-channel, of which the width should be equal to that of the sub-channel while the height may vary from material from material.

Design Approach:

Based on the years of injection mold design and mold trial experience, this article is aimed to generally explain the design principles of several slot types. For parts with complicated geometric shapes, the vent slot positions should be identified after several mold trials. If a mold design adopts the integrated structure, poor ventilation will be its biggest disadvantage. So, for molds with integrated cavity and core, the following venting methods can be adopted:

(1) Make use of the slot or insert location in the mold cavity;

(2) Make use of the lateral insert crevice;

(3) Machine the local part into the spiral shape;

(4) When it is extremely difficult to expel the air out of the mold, an insert should be adopted. If it is not easy to machine a vent slot in some locations of a mold, such as in the corners, the insert molding process may be appropriately applied on condition that product appearance and precision are not affected. This method not only helps with venting, but is also able to lower the difficulty level for machining, and convenient for maintenance, too.

Vent Slot Design Dimensions:

The width of the vent slot ranges from 1.5 to 6mm, while the depth design should be able guarantee that the plastic material will not get into the slot to cause flash. Its value is dependent on the viscosity of the molten plastic, but usually its applicable range is from 0.013 to 0.05mm.



Appropriately designed venting slots are able to drastically reduce injection pressure, injection time, pressure holding time and clamping force, thus making the plastic injection molding process much easier by improving production efficiency, lowering production costs and saving the energy consumed by machine.



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The texture for plastic injection mold

While our everyday life is filled with more and more plastic products, people start to be aware that they do not want them to look like “plastic” products. Therefore, the plastic injection molds decorated with the texture process are more welcomed because they cater to people’s interests.

Purpose of Texture

(1)Improve product appearance. The texture process is able to camouflage part of the shrinkage, welding line, parting line and steps of slider, etc.

(2)Product surface strength can be improved via texturing and sandblasting.

(3)Improve the look and feel of plastic products, to allow the creation of diversified and/or brand-new product designs.


Principle of Texturing
Allow chemical agents (e.g. sulfuric acid and nitric acid, etc.) to chemically react with mold steels, and at the same time control the reaction process to obtain various desired effects.


Process of Texturing
Mold Preparation- Filmmaking – Film Application – Exposure to Light – Corrosion – Cleansing – Post Treatment


Categories of Texturing
Sand pattern, satin pattern, leather pattern, stone pattern, geometric pattern, HANDS and HN3D, etc.


Procedure of Texturing

  1. Cleansing: Clean the mold cavity surface, to remove surface oil/grease.
  2. Sealing: Apply adhesive paper or corrosion resistant coating to the cavity surface that does not need to be textured, so as to prevent corrosion. This is the most time consuming step, during which the 3 commonly used sealing materials include: Thick adhesive paper, to cover the majority part of the cavity surface; thin adhesive paper, to seal the details; and corrosion resistant coating, to cover the area that adhesive paper fails to cover, e.g. complicated curvy surfaces.
  3. Drying: Dry the anti-corrosion coating.
  4. Surface treatment: Carefully wipe the cavity surface to be textured using absorbent cotton, to make it free from any dirt, thus ensuring the texturing effect.
  5. Texturing: Apply a coating to the cavity surface to be textured and then soak it in the corrosive fluid. During this process, attention should be paid to the texturing status. Repeated soaking is required to get the desired textures.
  6. Sandblasting: Sandblasting serves 2 purposes: A). To remove the residue liquid on the cavity surface after cleansing, with ammonia and pressure washer; B). To tune the gloss of the texture; different levels of gloss can be achieved by using different sands and different pressure levels.
  7. Post treatment: Cleanse the cavity surface and apply rust protection agent before delivering the mold parts back to the mold manufacturer.

Pre-texturing Requirements on Molds

The pre-texturing treatment of a mold plays an important role in defining the final texturing effects. As a result, every detail should carefully considered:

  1. Requirement on the draft angle (lower than 500mm): At least 1 degree for each 13μm of texture finish depth (excluding special textures).
  1. Polishing Requirements:
  • Apply 1,200+ sand paper for depth of around 5μm
  • Apply 1,000 sand paper for depth of around 10μm
  • Apply 800 sand paper for depth of around 25μm
  • Apply 600 sand paper for depth of around 50μm
  1. Parting line treatment: A 0.2 – 1mm margin is suggested; chrome plating is required after texturing, and it is also suggested to deepen the texture by 10μm.
  1. Pre-texturing requirements on mold surface:
  • No machining marks
  • No welding marks
  • No polishing marks
  • No EDM marks
  • Smooth mold surface
  • Mold surface allows texturing


Common Post-texturing Problems

  • Due to the fact that the mold cavity surface is roughened after texturing, the most common problems like scratches and stickiness to the cavity may arise. In some areas, the originally small draft angle will be made smaller after texturing, or even resulting in a undercut sometimes, so scratches are often caused. During the ejection process, ejector marks tend to appear due to unfavorable mold release, thus greatly affecting the part appearance.
  • To resolve the problem of scratches and ensure smooth mold release, the textured surface usually needs to be sandblasted to reduce the texture depth and at the same time eliminate the acute angles caused by texturing. In the practical production scenario, it is very difficult to resolve the mold release problem by adjusting injection parameters, so release agent is usually applied to the textured surface to facilitate production. From the perspective of mold, the situation may be improved by increasing the draft angle in the scratched surface area/increasing the number of ejector pins.



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