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

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.

 

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.

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.

Latch lock

The Functions of a Latch Lock:

 

In the two-step parting, two-step ejection or cavity ejection mechanism, a latch lock unit is always employed to control the opening sequence and stroke of each mold plate.

Though latch locks perform similar functions, they have a diversity of different structures. So, it is necessary to select the right latch lock for the right circumstance according the characteristics of the latch lock. The several commonly seen latch lock structures are explained as below.

 

1,Daido Latch Lock: 

  1. Pros: The Daido latch lock features a simple and concise structure, and requires little machining of the mold base. It takes up a little space and can be mounted on the mold freely, so the chances of interfering with the other parts on the mold base are minimized. Daido latch lock is able to control the mold opening stroke flexibly – a stop block can be mounted on the mold base to realize opening stroke control. In addition, it is inexpensive.
  2. Cons: Daido latch lock uses a spring to press the steel ball into contact with the control lever to realize mold closing. As a result, it is not easy to control the clamping force. There are cases of unhooking before reaching the required stroke. It is not so reliable, nor is it easy to adjust.
  3. Applicable Scenario: This type of latch lock is suitable for medium-/small-sized plastic injection molds, which do not require a large clamping force or a high reliability. Usually, it is not selected due to its poor reliability. See figure I: Used for the 2-step ejection of the ejector plate.

Note: The Daido latch lock uses a spring to press the steel ball into contact with the control lever to realize mold closing. As a result, it is not easy to control the clamping force. It is only applicable for medium-/small-sized molds, which do not require a large clamping force.

2,HASCO Latch Lock: 

  1. Pros: Sold at a moderate price, the HASCO latch lock boasts diversified specifications and a wide range of available options; the mechanical closing mechanism is secure, reliable, convenient to mount and requires little machining of the mold base. The lengths of its control lever and hook can be adjusted as per actual conditions.
  2. Cons: When mounting, machining is needed after the location of pin holes and screw holes are determined. It is not easy to change the mold opening stroke after it is designed. Also, the stroke range is limited by the length of the control lever.
  3. Applicable Scenario: The mechanical closing mechanism is secure and reliable, so it is suitable for various large-/medium-/small-sized molds.

 

Note: It is not easy to change the mold opening stroke after the stroke is designed, and the pin hole and screw hole locations are machined. Also, the stroke range is limited by the length of the control lever.

3,STRACK Latch Lock:

 

  1. Pros: The mechanical closing mechanism is secure and reliable. A wider stroke range can be covered by changing the relative location between the control lever and the hook. After the latch lock is mounted, the mold opening stroke is still able to be adjusted by moving the adjusting slider on the control lever.
  2. Cons: Occupy a larger space; complicated mounting process; require a lot of machining of the mold base; high price.
  3. Applicable Scenario: Suitable for large-size molds, and able to cover a wide opening stroke range. Usually, it is not selected due to its high price, unless specified by the client.

Note: After the latch lock is mounted, the mold opening stroke is still able to be adjusted by moving the adjusting slider on the control lever.

4,RABOURDIN Latch Lock

  1. Pros: With a large clamping force, the mechanical closing mechanism is secure and reliable. Its biggest benefit is that the mold opening stroke is able to be adjusted by moving the adjusting slider after the latch lock is mounted, so a wider stroke range can be covered.
  2. Cons: Occupy a larger space; complicated mounting process; require a lot of machining of the mold base; high price.

3. Applicable Scenario: Suitable for large-size plastic injection molds, and able to cover a wide opening stroke range. Usually, it is not selected due to its high price, unless specified by the client.

 

Note: After the latch lock is mounted, the mold opening stroke is still able to be adjusted by moving the adjusting slider on the control lever.