In injection molding, plastic, in the form of a granulate or powder, is melted into a viscous mass and injected into a mold under high pressure. The plastic solidifies by cooling and the desired product remains. Injection molding is done using a mold specially manufactured for the product. Often this mold is many times larger than the desired product, and as complex as, for example, the engine block of a car. For example, there are often complex cooling systems on the mold, also comparable to that of an engine block, because the cooling has a direct influence on the open-close time. This complexity creates an often more expensive mold. As a result, injection molding is usually more feasible at higher numbers, thousands or more.
Although the principle of plastic injection molding is simple, the implementation and the choices to be made is a profession in itself. There are a lot of variables that determine the final price-performance ratio: for example, the wall thickness determines the cooling speed, and there are countless different materials with different properties and price advantages to choose from. Therefore, a lot of attention should be paid to these choices during the design phase, to avoid later disappointments or costly mistakes.
Rotation Casting uses a steel mold with an opening. This mold is usually quite simple, and therefore relatively cheaper. The mold is filled with plastic, closed, and then heated while rotating. This causes the plastic to melt and to move towards the sides under the influence of centrifugal forces (just like laundry in a washing machine). The wall thickness of the product is limited by the content of the mold, because the amount of plastic material that fits in the hollow shape is limited. Products that are produced in this way are almost always made of the material LLDPE (high impact strength) or the more expensive and harder variant HDPE. Additions can be made to this (at a higher cost price), for example to have a fire-retardant effect or to achieve more color-fast properties. Rotation casting is usually used with larger products, such as large drums or water-filled roadway partitions. Often (mistakenly) overlooked in low quantities from a few to thousands.
Using this production technique, plastic is melted above a vertically divisible mold, and then drops through a round discharge opening. This creates a hollow preform, which is then inflated under pressure (like a balloon) against the wall of the mold. After cooling, this produces a hollow plastic shape, for example a drinking bottle, a jerry can or a baby seat. The mold is relatively cheap compared to injection molding and offers the possibility of relatively complex hollow shapes, which makes this method attractive from a few hundred to many thousands of products.