mold steels

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

Code:

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

Code:

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

Code:

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

mold steels

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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Hot runner system types and their application in plastic injection mold

1,How the Hot Runner System Works?

The cold runner of a plastic injection mold refers to the section between the mold inlet and the gate. The molten plastic maintains its fluidity inside the runner by virtue of the injection pressure and its own heat. However, as a part of the molding material, the runner is not a product. As a result, when designing a mold, we need to consider not only the filling effects, but also the material-saving effect produced by shortening or downsizing the runner; but in actuality, it is not easy for us to have the best of both worlds.this is why we need hot runner mold

Also known as the runner-less system, the hot runner is a system in which the molten plastic is not solidified after each injection, so when releasing the plastic part, the gate inside the runner does not have to be released. Since the plastic inside the runner is not solidified, the runner still stays unblocked when the next injection takes place.

hot runner system

2,The advantage of Hot Runner Molds compare to cold runner molds

Why has the hot runner system been developed? What benefits can the hot runner system bring us? Engineers familiar with the plastic injection molding process all know that the traditional injection molding is always associated with the following disadvantages:

a.Hard to fill; b. thin-walled large size parts easy to deform; c. material waste in the runner; d. inconsistent plastic part quality when using a multi-cavity mold, and so forth.

The appearance of the hot runner system has provided an almost perfect solution for these problems. Generally speaking, the hot runner system offers the following benefits:

■ Shorter cycle time

As it is not limited by the runner cooling time, the plastic part can be ejected in a timely manner after being molded and solidified. For the production of thin-walled small parts, the molding cycle can be kept within 5s.

■ Material saving

In a complete hot runner mold, material waste will not occur thanks to the absence of a cold runner. This is of a greater significance to the application projects of the more costly plastic materials. As a matter of fact, global major hot runner manufacturers had witnessed leap forward development during the years when world crude oil and raw plastic materials were expensive. This is because the hot runner technology is an effective way to reduce waste materials and thus lower raw material costs.

■ Reducing defective products, and improving product quality

The temperature of the molten plastic inside the runner system can be accurately controlled during the hot runner injection molding process, so that the plastic is able to flow into each cavity in a more consistent manner, thus guaranteeing consistent quality products. The hot runner gate is of a higher quality, so after mold release, the part produced will have a smaller internal stress, leading to a smaller part deformation. Therefore, many high quality products in the market are all produced with the hot runner mold.

■ Eliminating following steps, helpful for production automation

After being molded with a hot runner mold, the part will be the final product, so there will be no follow-up steps like gate trimming, recycling and runner cooling, thus helpful for production automation. Many manufacturers from both home and abroad have combined the hot runner system with automation, so as to greatly improve production efficiency.

3,The Hot Runner system types

  1. The mold that employs a hot sprue for direct or indirect material feeding, is referred to as a hot sprue mold.

direct hot runner

 

2,The mold that is designed with a hot runner manifold and a secondary hot sprue, is referred to as a hot runner mold.

indirect hot runner

Examples of hot sprue molds

(1) The hot sprue mold structure that feeds materials through the point gate. It is only suitable for the single-cavity mold, and is limited by the gate location.

point tip hot runner

2) The hot sprue mold structure with the end surface engaged in injection molding. It is suitable for the single-cavity mold, but sprue marks will be left on the part surface. The end surface of the hot sprue is machinable.

The hot sprue mold structure with the end surface hot runner

(3) The hot sprue/cold runner mold structure with a bit of the traditional runner features. This structure is able to produce multiple plastic parts at the same time, but some cold slugs will be produced.

The hot sprue/cold runner mold structure

Besides open gate hot runner as above,Now people start to choose valve gate hot runner for high class product.

What is valve gate hot runner system ?

Mount the hot sprue and the manifold into the injection mold, and then by way of heating, keep the fluidity of the plastic material after it is out of the barrel. The result is, the product gate seems to be able to contact with the nozzle directly. So, when releasing the product, the sprue is closed under the action of the needle valve. The application of one or more cold runners is thus avoided, realizing wasteless production.

valve gate hot runner

The advantages of the valve gate hot runner system include:

  • Shorten production cycle, improve production efficiency;
  • Since the cold runner is replaced, cold slugs will not be produced, thus saving a large amount of raw materials;
  • Improve product consistency, free from obvious gate marks;
  • Remarkably improve product appearance;
  • Help reduce product stress, minimize product deformation and structural brittleness, so the production capacity and life cycle of complex products are improved;
  • Offer more process control, allow accurate adjustment to the injection molding process;
  • Not limited by the change of plastic injection molding materials.

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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):

Pros:  
1. Little pressure loss;
2. Easy preparation.

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

Application:
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:

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

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

Parameters:
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.

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

 

Tab Gate:

Pros:
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.

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

Parameters:
Refer to the side gate parameters for application.

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

 

Fan Gate:


Pros:
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.

Cons:
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.

Parameters:
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.

Application:
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:

Pros:
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.

Cons:
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.

Parameters:
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.

Application:
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:

Pros:
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.

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

Parameters:
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.

Application:
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:

Pros:
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.

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

Parameters:
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.

Application:
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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Hot runner mold vs Cold runner mold(Two plate mold vs Three plate mold)

Hot runner mold VS Two plate mold VS Three plate mold

Hot runner mold vs cold runner mold ( two plate mold vs three plate mold)

 

1.Cold Runner Mold:

 

Refers to the portion between the mold inlet and the product gate. The molten plastic is kept flowing in the runner under the effect of the injection pressure and its own heat. The runner only functions as part of the molding material, but is not a part of the final product. (The runner part is the residue of plastic injection molding.)

Characteristics: The advantages of the cold runner system are that it is easy to use, and able to well meet the aesthetic requirements in some circumstances. The cold runner is able to reduce the acrylate or polycarbonate used for light transmission in the injection mold runner, so as to avoid the visible strips caused by the injection molding in some parts. Disadvantages of the cold runner: (1) More waste of raw materials; (2) Additional steps in the whole production process;

the cold runner mold include two series mold: two plate mold and three plate mold

Two plate mold:

When the mold opens, the cavity plate and the core plate are separated to release the molded product. This is referred to as a single parting surface mold, AKA a two plate mold. Being most widely used, it is the simplest and most basic form of the injection mold, and can be designed as a single-cavity or a multi-cavity injection mold.

Three plate mold

Three plate mold has two parting surfaces. Compared with two plate mold, it adds a partially movable intermediate plate (also called a movable runner plate, on which the gate, runner, as well as other parts and components required for the core are designed) into the fixed mold half, so it is also known as a three-plate (cavity plate, runner plate and core plate) injection mold, which is often used for the single-cavity or multi-cavity pinpoint gate plastic injection mold. When the mold opens, the runner plate is separated from the core plate for a fixed distance on the guide pin in the core, so that the sprue can be removed from between the two plates. The three plate mold features a complicated structure, a high manufacturing cost, and difficult part processing, so usually it is not used for injection molding of large or extra large plastic products

 

2.Hot Runner mold

As a common part of the injection molding system, the hot runner ensures that the plastic material in the runner and gate keeps the molten state by heating. Since the heating rod and the heating ring are arranged near or in the center of the runner, the whole runner – from the nozzle of the injection molding machine to the gate – is kept in a high temperature status, so that the plastic in the runner is kept molten. Usually, it is unnecessary to open the runner to remove the sprue after shutdown. It is only necessary to heat the runner to the required temperature after the machine restarts. Therefore, the hot runner process is sometimes referred to as hot-manifold system or runner-less injection molding.

Characteristics: The hot runner technology boasts the following advantages when compared with the conventional cold runner: (1) Saving the raw material and reducing production cost; (2) Shortening the molding cycle and improving machine efficiency; (3) Improving product surface quality and mechanical properties; (4) Allowing the application of the pinpoint gate without the three-plate mold system; (5) Economically molding a single product with the edge gate; (6) Improving the degree of automation; (7) Controlling gate freezing with a needle valve gate; (8) Consistent injection molded product quality of the multi-cavity mold; (9) Improving the surface aesthetics of the injection molded product.

 Disadvantages of the hot runner mold:

(1) The mold structure is complex, the manufacturing cost and maintenance cost are high; (2) It takes a certain period of time for the start-up process to be stable, resulting in more waste products; (3) When there is melt leakage or heating element failure, product quality and production schedule are greatly impacted. The 3 mentioned disadvantages can be improved through the purchase of high-quality heating elements, hot runner plates and nozzles, with careful maintenance during production.

Advantages of the Hot runner mold

(1). The diameter of a hot runner is generally larger, and the melt is kept at a high temperature in the hot runner. Therefore, the shear stress and pressure drop of the molten plastic in the hot runner are much lower than those flowing in the cold runner, thus able to deliver the homogenous (relatively identical pressure and temperature) melt to all the gates, which is advantageous for the injection molding of high quality products (especially thin-walled products), as well as the development of multi-cavity molds.

(2). The cold runner mold tends to have a too long cycle time because it is too thick compared to the product. In such circumstances, a hot runner has a shorter cycle time because it does not have to be cooled and solidified.

(3). Saving the raw material and reducing product cost are the most prominent features of the hot runner mold. In the ordinary gating system, a large number of sprues are generated, and in the production of small-sized products, the weight of the gating system sprue may exceed the weight of the product. Since the plastic is always in a molten state in a hot runner mold, the product gate does not need to be trimmed, and basically no waste is generated, so that a large amount of raw materials can be saved. Since no waste recycling, selection, crushing, dyeing or other process steps are required, it is able to save labor, time, and energy, thus reducing consumption.

(4). When combined with the valve gate, the hot runner system is able to change the filling pattern, eliminate or transfer weld lines, air traps, and the like.

(5). Simplified operation and reduced molding cycle are also important features of a hot runner mold. Compared with the cold runner, the opening and closing stroke is reduced, which not only shortens the demolding and molding cycles of the workpiece, but also facilitates automated production. According to statistics, compared with the cold runner system, the molding cycle after switching to the hot runner system can usually be improved by 30%.