What are forms of gate and advantages and disadvantages of each gate?

 What are forms of gate and advantages and disadvantages of each gate?

“Gate” has a large influence on formability and internal stress of injection molding mold. Usually, appropriate form is determined according to shape of molded products, and it can be classified into two types: “restricted gate” and “unrestricted gate”. The former is made into a narrow part at the entrance of runner and cavity, is easy to process and easy to cut molded article from runner, which can reduce residual stress. Gates of most of plurality of shaped products are easily formed at one time, cavity is easily balanced. Plastics are not easy to countercurrent, and this form is generally used. It can also be divided into Side Gate, Overlap Gate, Tab Gate, Fan Gate, Film Gate, Ring Gate, Disk Gate, Point Gate, Submarine Gate and so on. The latter is a gate that directly injects plastic into cavity from down sprue as a representative of unrestricted gate. Type, position, size, and number of gates directly affect appearance, deformation, forming shrinkage, and strength of molded products. Therefore following items should be considered in design:

1. Gate shape: Shape of gate affects fluidity of molten resin in cavity, appearance of molded products, and flow direction of material. Therefore, when selecting type of gate, shape of material or shape of molded products should be considered, and influence of flow alignment should be considered.

2. Gate location and number:

(1) Position that molten material can sufficiently circumscribe each part of master mold can be selected, try to choose center or thick meat part of molded products.

(2) Hole of molded products will be inserted into injection molding mold, and inflowing material should not be bent or offset.

(3) When there are more than two places, do not allow weld line or air bubbles to damage appearance or reduce strength of product.

(4) Residual stress during molding is likely to concentrate around gate portion, and sometimes it becomes brittle and ruptures.

(5) Selecting position that appearance of product is not conspicuous, and it is easy to process position of gate.

3. Gate type (shape): Gates can be divided into “restricted gate” and “unrestricted gate” according to their functions. The former is made into a narrow part at the junction of gate and master mold, which hinders material flow; The latter runner (vertical gate) directly serves as inlet of material to master mold, and is generally used as restricted gate. Features, advantages, disadvantages and uses of various gates are as follows:

• Unrestricted gates – direct gate / vertical gate (Direct Gate)
  

Characteristic

1. Direct gates are representative of unrestricted gates.

2. Vertical sprue is inflow of material to master mold.

3. Nozzle diameter of molding machine is limited.

4. Material is well filled, and even glass fiber-filled material is easily formed, and shrinkage of surface of molded article is less.

Advantage

1. Good liquidity.

2. Simple construction.

3. Wide range of applicable resins.

4. Material filling is good.

5. Surface of molded products shrinks less.

6. Omit processing of flow path.

7. Less pressure loss.

8. It can form large or deeper molded products.

Disadvantage

1. Only one molded product can be formed at a time, and it is not possible to take several multi-point gates unless a multi-nozzle molding machine is used.

2. There are traces of gate residue that affect appearance and increase post-processing.

3. Flat and shallow molded articles are easy to warp and twist.

4. Gate cycle must be determined.

5. Residual stress near gate is large, which may cause cracking or deformation.

Usage

1. Suitable for containers of large objects and deep objects.

2. Applicable plastics: PVC, PE, PP, PC, PS, PA, POM, AS, ABS, PMMA.

• Restricted gates – Standard Gate / Side Gate / Edge Gate

Characteristic

1. The most representative gate.

2. Take a majority of multi-point gates.

3. Avoid important positions of molded products.

4. Gate is provided on the end face of mold and side (end face) of molded products.

5. It is convenient to cure material quickly after forming and reduce residual stress in gate.

Advantage

1. Low residual stress.

2. Gate size is correct (rectangular section).

3. Gate is easy to separate from the molded product.

4. Prevent material from flowing backwards.

5. Part of gate generates frictional heat, which can raise temperature of material again and promote filling.

Disadvantage

1. Flow resistance is large.

2. Pressure loss is large.

3. Materials with poor fluidity can easily cause insufficient filling or halfway curing.

4. A flat or large-sized molded products is likely to cause bubbles or flow marks due to small size of gate.

Usage

1. Applicable plastic: PVC, PE, PE, PP, PC.

• Limit gates – Overlap Gate

Characteristic

1. A type of side gate.

2. A portion of gate overlaps thickness of molded article.

Advantage

1. Appearance of gate is not easy to see, and it can prevent flow of molded article.

2. Gate is easy to separate from molded products.

Disadvantage

1. Pay attention to gate processing.

2. Pressure loss is large.

Usage

1. Applicable plastic: POM

• Limit gates – Fan Gate

Characteristic

1. A type of tab gate.

2. Gate is fanned to master mold and has same application range as film gate.

3. Resin is easily dispersed in a large area and filled evenly.

4. Avoid air bubbles and residual flow marks.

5. There is a need for post processing.

Advantage

1. Good liquidity.

2. It can be uniformly filled to prevent deformation of molded products.

3. Gate alignment is low.

4. There are almost no defects in molded products with good appearance.

Disadvantage

1. Gate processing takes time.

2. It is slightly more difficult to cut gate part.

Usage

1. It is suitable for thin and large flat, disc-shaped or large-sized molded products.

2. Applicable plastics: PP, POM, ABS, especially for composites with strong alignment.

• Limit gates – Diaphragm Gate /Film Gate

Characteristic

1. Plastic of gate flows in parallel direction in master mold to uniformly fill master mold to prevent deformation.

2. It is suitable for crystalline plastics with strong mobile alignment, filling materials strengthened with glass fiber, thermosetting materials etc., which are easily deformed by flow direction of filling material.

3. It is easy to shrink evenly on plate-shaped finished product.

Advantage

1. Good liquidity.

2. Precision of round molded products is good.

3. It can be uniformly filled to prevent deformation of molded products.

Disadvantage

1. It takes time to process after gate.

2. It is slightly more difficult to cut gate part.

Usage

1. Discs, cylinders (gears, etc.) or large sheet metal products.

2. Applicable plastic: Polypropylene (PP)

• Limit gates – Ring Gate

Characteristic

1. To prevent formation of weld marks, an overflow well shall be provided for toroidal gate.

2. When gate is provided from outside of cylindrical product, an annular auxiliary runner is provided, and product is connected by a thin annular gate from runner, and two-type gate can prevent deformation or fusion of molded article.

3. It can evenly fill cylinder forming product to avoid deformation and eccentricity caused by weld line and partial filling.

Advantage

1. Prevents flow marks from occurring.

Disadvantage

1. It is a little difficult to cut gate.

Usage

1. Applicable plastic: POM, ABS.

• Restricted gates – Disk Gate

Characteristic

1. Gate is placed on a thin circular plate gate on the inside of tube or annular molded products, and portion of circular plate is cut off after gate.

2. With direct gate characteristics.

3. Using small cylinder deep into finished center ejector pin and can directly form chassis of disc gate.

Advantage

1. Good liquidity.

2. Precision of round molded products is good.

3. It can prevent occurrence of flow marks.

4. Eliminate processing of flow path.

Disadvantage

1. It takes time to process after gate.

2. Gate is slightly difficult to cut.

3. Only one molded product can be formed at a time.

4. Center of hole of molded products shall correspond to sprue.

Usage

1. Can be used for discs, cylinders (gears or deep cylinders)

2. Applicable plastic: PS, PA, AS, ABS, staple fiber plastic.

• Limit gate – point gate / pin gate

Characteristic

1. Connect master mold with small dots, gate trace is small, and it is easy to remove runner from molded product.

2. If it is used for a three-plate mold, multiple gates are placed in items with large projected areas, filling condition of each gate can be adjusted, and an unobtrusive gate can be provided on the bottom of cup or bottom of box-shaped article.

3. Take majority and multi-point gates.

4. The smaller pinhole of pin point, the higher frictional heat caused by material flow, which can reduce viscosity, but loss of injection pressure is also increased, generally 0.8 to 1.0.

5. Post-processing is easy, gate position can be freely selected, and it is a three-plate mold structure.

Advantage

1. Have plasticizing ability.

2. Gate is cut off by itself.

3. Gate trace is small and can be exempted from post processing.

4. Gate location is free to choose.

5. Gate can be injected from several points with less stress and strain.

6. Suitable for molding of most molded products.

7. Has advantage of restrict gate.

Disadvantage

1. Flow resistance is large.

2. It is easy to overheat.

3. Mold construction is complicated.

4. Resin yield is low.

5. There is no resin available.

6. Pressure loss is large.

Usage

1. Applicable plastics: PE, PP, PC, PS, PA, POM, AS, ABS).

2. For application of multi-gate, single forming, multiple forming at one time.

• Restricted gates – Submarine Gate / Tunnel Gate

Characteristic

1. Side gate automation.

2. Note drop of secondary gate.

3. Gate sneaked into template on fixed or movable side to reach wall of product or to reach secondary runner located on ejector pin.

4. When molded product is ejected, it is automatically cut off and is suitable for fully automatic forming.

5. A gate may be provided inside ring-shaped article, or a secondary runner may not be provided for ejector pin, and hub of molded product or other hub portion may be used, and a gate is provided here, and this portion is removed afterwards.

6. Automatically remove gate part after forming, saving post processing.

7. Moulding process is more difficult than others.

Advantage

1. Have plasticizing ability.

2. Gate is cut off by itself and is exempt from post-processing.

3. Gate traces are small.

4. Gate position can be set freely on the outside or inside of molded products.

Disadvantage

1. Flow resistance is large.

2. Machined surface is not easy to machine.

3. Pressure loss is large.

Usage

1. Applicable plastic: PS, PA, POM, ABS

2. Without post-processing, feeding system can be used automatically.