INJECTION MOULD DESIGN

 INJECTION MOULD DESIGN

Injection Mould

Injection mold determines the size, shape, dimensions, finish and often the physical properties of the final product.

Cross sectional view 

• Function of mould components

• The sprue is a channel in the stationary platen that transports the melt from the plasticizing barrel to the runner system. The runner system basically transports the melt from the sprue to the gate. The gate further transports the melt into the cavity.

Parting line

Every mold has two basic parts to contain the cavities and cores. These are stationary mold-half (on the side from where the plastic melt is injected) and a moving mould-half (on the ejector side of the machine). The line on a plastic part which separates the two mould halves is called the parting line. 

Types of injection moulds

• There are many different types of  molds, designed to meet different product requirements. However, there are six basic types of injection molds for use with thermoplastics.
• These types are (1) two-plate mold (2) three-plate mold (3) hot-runner mold (4) insulated hot-runner mold (5) hot-manifold mold and (6) stacked mold.
• A two-plate mold consists of two plates with the cavity and cores mounted in either plate. The plates are fastened to the press platens, and the moving half of the mold usually contains the ejector mechanism and runner system. A two-plate mold is the most logical type of tool to use for parts that require large gates. 

• The three-plate mold is made up of three plate runner plate, cavity plate and force plate.
• The stationary (or runner plate) is attached to the stationary platen and usually contains the sprue and half of the runner. The middle (or cavity plate) contains half of the runner and gate and is allowed to float when the mold is open. The movable (or force plate) contains the molded part and ejector system for the removal of the molded part.

• In a hot runner mould, the runners are kept hot in order to keep the molten plastic in a liquid state all the time. It is also known as "runner-less" molding process. In such molds, the runner is contained in a seperate plate. Hot-runner molds are similar to three-plate injection molds, except that the runner section of the mold is not opened during the molding cycle. The heated runner plate is insulated from the rest of the cooled mold.
• Runner-less molding has several advantages over conventional cold-runner type molding. There are no molded side products (gates, runners or sprue) to be disposed off.

• Insulated runner mould is a variation of the hot-runner mold. In this type of molding, the outer surface of the material in the runner acts as an insulator for the molten material to pass through.
• In the insulated mould, the molding material remains molten by retaining its own heat. This type of mold is ideal for multi-cavity center-gated parts. The diameter of the runner is almost twice that in a cold-runner system.

• The hot manifold mould is a variation of the hot runner mould. In the hot-manifold, only the runner is heated (runner plate is not heated). This is done by using electric-cartridge insert probes in sprue, runners, and gates.

• Stacked mold is a multiple two-plate mold, with the molds placed one on top of the other. This type of construction can also be used with three-plate, hot-runner, and insulated hot-runner moulds.

Runner system

• The runner should be short and free of bends
• The supply of melt should reach to each cavity simultaneously
• Types of runner designs

• The cross sections of runners in multi cavity molds must be large enough to convey the plastic melt rapidly to the gates without excessive chilling by the relatively cool mold for thermoplastics. Runner cross sections that are too small require higher injection pressure and more time to fill the cavities.
• The pressure loss in the runner system must be as low as possible.
• Large runners produce a better finish on the molded parts and minimize weld lines, flow lines, sink marks, and internal stresses. However, excessively large runners should be avoided.
• Correct and incorrect runner designs

• Multi cavity mould of a bottle cap

Gates

• The gate is given a  smaller cross section than the runner so that the molding can be easily degated (separated from the runner).
• Large-diameter gates, require mechanical degating after ejection and always leave a mark on the product.
• The gate must be located in such a way that rapid and uniform mold filling is ensured.
• Gate preferably be located at the thickest part of the molding.
• The positioning and dimensioning of gates are critical, and sometimes the gates must be modified after initial trials with the mold.
• Types of gates designs are standard gate, Ring gate, Submarine gate etc.

• Standard gate

• Ring  gate

• Submarine gate

• Tab gate

• Disc gate

• Film gate

• Fan gate

• Spider gate

• Sprue gate

• Pin point tab gate

Mould temperature and melt temperature

• Mould temperature and melt temperature for injection moulding of crystalline, semi-crystalline, and amorphous thermoplastics

Ejector Systems

• The conventional mold ejector system moves between the clamp plate and support plate. The ejector plate carrying the ejector pins must move freely in the mold.
• The ejector plate and pin plate are guided by returns pins. Most mold bases use four or more return pins. 
• The number and location of the ejector pins are determined by the size and shape of the moulded part.
• Ejection in a two-plate mould