Plastic products come in all shapes and sizes. Some products have simple shapes, like a flower pot or a plastic spoon. Products are often complex in shape such as plastic parts for cars. A separate mold is made for each plastic product. During the design and production of a mold, the requirements for the mold are taken into account such as raw material, durability, recycling, dimensions, tolerances, lifespan, and color. A mold is a complex and expensive tool. The price of a mold can even be higher than that of an injection molding machine.
The simplest form of an injection mold consists of two blocks of metal. The right part is called the pre-mold. The pre-mold is mounted against the fixed clamping plate. The left part is called the back die. This mold half is mounted against the movable clamping plate. The mold opens or closes by moving this clamping plate. The plane where the two parts touch is called the closing plane or part plane. A space is saved between the two mold halves. This space has the shape of the product to be made.
The red area in the mold is plastic. The two semicircles are the two plastic products, this mold is also called a double mold. The space of one product is called the product cavity. The sprue channel is a cup-shaped recess, also called radius. The nozzle of the machine fits into this radius.
The image shows a sprue channel, which is a filling channel that is conical in shape. This is also known as a runner channel. The sprue is the plastic part that is needed to fill the product cavity(ies). The sprue is therefore not part of the product. Immediately after injection, the warm mass cools down. When the product, with the sprue, is sufficiently dimensionally stable, the mold can be opened. After that, the product can be taken out or ejected.
Mold shrinkage
The plastic in the mold shrinks when it cools down. This shrinkage continues after the product has been demolded from the mold. This shrinkage makes the final product smaller than the product cavity of the mold. When making the mold, it is very important that this shrinkage is taken into account, as different plastics often have a different shrinkage.
Demolding means that the product is removed from the mold. The moment the mold opens, the product should separate from the pre-mold and remain attached to the back mold. As the product cools and shrinks, it loosens in the pre-mold.
At the same time, due to shrinkage, the product remains fixed on the core of the back mold. When the mold opens, the product moves along with the back mold. In order to be able to demold the product properly and without damage, the machine is equipped with an ejection mechanism together with the mold. Figure 1 shows several ejection pins.
Mold discharge
When making a design of the mold, it is important to consider the release of the products. It is common practice to make the product cavity slightly conical in both the front mold and the back mold. The inclination that the mold has in the closing direction is called the draft angle. Due to this release angle, the product easily releases from the pre-mold. The release angle on the back mold is provided in order to be able to demold the product without damage. The design of the product determines whether a product is easy or difficult to release. The raw material used is also important when it comes to unloading. A brittle material such as PMMA cracks or breaks easily, which is why an optimum release is required for a material like this. Less release is required for a more elastic material.
The mold surface
The injection mold is composed of several parts. A special type of steel is often used for the molding plates, which can withstand a high surface pressure. The product cavities are often made of a special steel and are usually hardened. The steel surface that forms the product is specially processed. This surface is usually smooth and polished. If there is damage such as a scratch on this surface, you will always see this reflected in the product.
With a smoothly polished surface, any product deviation such as plunge or a flow seam is clearly visible. Therefore, for some products the mold surface is slightly roughened on purpose. The product surface is in this case not glossy, but has a matte structure, which is called texturing the mold.
Deaeration of the mold
When plastic is injected, the warm mass pushes the air present in front of it. Especially if the melt is injected quickly, the air will be strongly compressed. If this air cannot escape fast enough, this will cause all kinds of problems.
The air partially escapes through the ejection pins and cores or slides. In order to give the rest of the air the opportunity to escape, the mold is provided with venting channels. These venting channels lie on the partial surface of the mould.
The mold cooling
In the case of injection molding, the products must be cooled down as quickly as possible after injection. That is why the mold is equipped with special cooling channels. Water flows through these channels. This water dissipates the excess heat as quickly as possible. Immediately upon injection, the warm mass touches the cold walls of the product cavity. The mass then immediately starts to cool and forms a cold layer.
Figure 2 – Simulation of the cooling of a product
Plastics have a low thermal conductivity coefficient. This means that the heat is poorly transmitted, which makes them a good insulator. This is favorable when injecting, because the mass remains uniform in temperature and can be injected more easily. This is a disadvantage for cooling. As a result, the product stays warm in the middle longer. As a result, a longer cooling time must be applied for thicker products. The rate of cooling depends on the wall thickness of the product, temperature of mold and plastic, the type of plastic, and the material of the mold.
