Location and structure of gates in mold design (essential knowledge for mold design)

April 10, 2022

By siedery

The gate, also known as the inlet, is the channel that connects the manifold to the melt in the cavity. The choice of the gate is directly related to whether the injection moulded product can be injected in good quality or not. The location of the gate plays a decisive role in the shape of the melt flow front and the effect of the holding pressure, therefore also determining the strength and other properties of the injection moulded product.

Types and positions of gates

In injection mould design, according to the structural form and characteristics of the gate, the following 11 types of gates are commonly used.

1. Straight sprue

This is the main flow gate, which is a non-restrictive gate.

Advantages: the plastic melt enters the cavity directly from the large end of the main flow channel, therefore it is characterised by low resistance to flow, short flow and long recharge time. The melt flows from the centre of the bottom of the cavity to the parting surface, which is conducive to the exhaust of air.

Disadvantages: there is a large residual stress at the feed, which can easily lead to warpage and deformation of the injection moulded product, and the gate is large, which makes it difficult to remove the gate marks and affects the aesthetics, therefore, this type of gate is mostly used for injection moulding of large and medium-sized long flow, deep cavity, barrel or shell shaped injection moulded products, especially for high viscosity plastics such as polycarbonate and polysulfone. In addition, this type of gate is only suitable for single cavity moulds.

In the design of these gates, in order to reduce the gating area in contact with the injection moulded product and to prevent defects such as shrinkage and deformation, on the one hand a smaller taper of the main flow taper angle (2-4°) should be used as far as possible, on the other hand the thickness of the fixed template and fixed mould base should be reduced as far as possible.

2. Side sprue

Abroad, the side gate is called the standard gate.

The side gate is generally opened on the parting surface, the plastic melt is filled with the mould cavity from the inside or outside, its cross-sectional shape is mostly rectangular (flat groove), change the width and thickness of the gate, you can adjust the shear rate of the melt and the freezing time of the gate. This type of gate can be positioned according to the shape of the injection moulded product and is easy to process and trim, which is why it is so widely used.

Advantages: small gate cross-section, reduced melt consumption in the pouring system, easy to remove the gate and less visible traces. It is suitable for all shapes of injection moulded products, but not for long and thin barrel shaped injection moulded products.

Disadvantages: the injection moulded product and the gate cannot be separated by themselves, there are fusion marks, the injection pressure loss is large and it is not good for the exhaust of deep cavity injection moulded products.

3. Fan gate

The fan-shaped gate is generally opened on the parting surface and is fed from the outer side of the cavity. The gate is gradually widened along the direction of feeding and the thickness is gradually thinned. The plastic melt entering the cavity from the gate is straighter in front of the wave, which reduces warpage and deformation, and is suitable for moulding plastic products with larger widths.

Gate position and structure picture1
Gate position and structure picture1

4. Sheet gating

Also known as a flat slit gate, the distribution runner of the gate is parallel to the side of the cavity and its length is usually greater than the width of the plastic product.

Advantages: the molten plastic enters the cavity at a low speed and evenly through the flake gate, the material flow is parallel and warpage can be avoided.

Disadvantage: it is difficult to remove the gate, thus increasing the production cost of plastic products.

5. Lug gates

The lug gate is mainly used for highly transparent flat plastic products and for plastic products with minimal deformation requirements.

Advantages: the lug gate is a lug groove on the side of the cavity, where the melt impinges on the side of the lug through the gate to produce frictional heat, thus improving the flow, and after adjusting the direction and speed, it enters the cavity evenly and smoothly at the lug, avoiding the jet.

Disadvantages: more difficult to remove the gate and larger gate marks.

6. Spot gates

Point gates are particularly suitable for barrel, shell and box-shaped plastic products. For large flat plastic products, multiple point gates can be set to reduce warpage deformation; for thin-walled plastic products, the shear rate near the gate is too high and residual stresses are high, making them prone to cracking, so the wall thickness at the gate can be increased locally.

Advantages: small restrictions on the location of the point gate, small traces of the gate, the gate can be automatically pulled off when the mould is opened, which is conducive to automatic operation.

Disadvantages: the injection pressure is high, in most cases, a three-plate mould structure must be used, the mould is relatively complex and the moulding cycle is long.

Gate position and structure picture2

7. Submerged gates

Advantages: The submerged gate is flexible and can be used to feed plastic products on both internal and external surfaces. The runner is opened on the parting surface, the gate is submerged under the parting surface and the melt enters the cavity diagonally. The gate is automatically cut off when the mould is opened and the runner is automatically dislodged. At the same time, its mould structure is simpler than that of the three-plate mould, which greatly improves production efficiency and reduces costs.

Disadvantages: not suitable for over tough (e.g. PA) or over brittle (e.g. PS) plastics, the former is not easy to cut off, the latter is easy to fracture and easy to block the gate.

8. Moon shaped gates

The moon gate is in fact an arc-shaped submerged gate structure, generally used in two-plate moulds, using a direct glue feed from the parting surface, made of two pieces of inserts, the manifold and the gate are designed on the inserts.

The advantages: the glue can be fed at the bottom of the product, it has the characteristics of a point sprue, the traces of the sprue are small, it can be automatically pulled off when ejecting, and it is easy to realise automation. If the head of the ejector pin is made conical, a push-out mechanism is set up in the runner to facilitate ejecting.

Disadvantages: complex shape, need to use electrodes to process the gate.

9. Ring gate

A gate that uses a circular form of feed for the filling of the cavity is called a ring gate.

Advantages: uniform feed, approximately the same flow rate everywhere around the circumference, good flow, easy to exclude the air in the cavity and avoid fusion marks. As the gate is designed on the core, the annular gate is mainly used for moulding cylindrical bottomless plastic products.

Disadvantages: high material consumption in the pouring system, difficulty in removing the gate and obvious gate marks.

Gate position and structure picture2
Gate position and structure picture2

10. Umbrella Gates

The umbrella gate is a special form of ring gate, mainly used for short thick tubular plastic products with very high-quality requirements.

Advantages: uniform feed, no fusion marks, good exhaust.

Disadvantages: the removal of the runner must be done by cutting, which increases the cost.

11. Disc gates

The disc gate is in fact an umbrella gate with a top angle of 180°. It is used for cylindrical plastic products with a large bore, or for plastic products with a large rectangular bore, where the gate is around the entire bore.

Advantages: with the characteristics of an umbrella gate, the plastic melt is injected into the cavity from the perimeter of the bore in a roughly synchronous manner, the core is evenly stressed, fusion marks can be avoided and the exhaust is smooth.

Disadvantage: it leaves a visible gate mark on the inner edge of the plastic product.

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