Why is injection molding so expensive?
If you have ever been involved in injection molding projects, you know that they can easily be the most expensive part of a project. Of the many factors that influence the cost of injection molded parts, three rise to the top of the list: the size and intricacy of the part, the number of parts, and the work necessary to make the injection mold.
The pricing for all manufactured goods depends on the cost of raw materials and labor. Since the price of molded parts depends on the price of plastic and the labor involved throughout the operation, we will concentrate on one of the most crucial aspects of this process, the injection mold tooling. We will then briefly discuss the vast array of choices in plastics.
Injection Mold Tooling
An injection mold, also known as a die or injection mold tooling, is the single most expensive item built specifically to produce a plastic part. The injection molding process is quite simple: pressured injectors squirt hot plastic into a mold and, as the plastic cools, the part forms and the mold splits apart so the parts drop out. One closing-injection-opening operation of the mold is called a cycle.
However, as they say, the devil is in the details. The process involves intricate, instantaneous movement of many moving parts, some of which with tolerances in the micron range. With that said, the injection die plays a crucial part in this process. They come in many sizes and shapes, each designed for one integral part that makes up a whole product. All molds have a cavity (many have two or more cavities so that each cycle produces multiple parts) that accepts the hot plastic. This cavity is machined, ground and polished to extremely tight tolerances.
The process of making a mold begins on an engineering table with the concept that includes the most effective size and number of cavities for a given project. The size and shape of the overall mold are the main factors in manufacturing the mold. Large, intricate plastic parts require intricately machined and polished cavities. Molds that have two parts that simply open and shut against each other are less expensive and most common. However, molds can get pretty intricate with the addition of features such as slides lifters, and cams, which are devices that assist with the production of undercuts. Most large-sized molds must be made on large, expensive CNC machines by experts in metalworking.
Mold Tooling Material
The next cost factor considered is the material for the mold. Most injection molds are machined from steel. Both the type and grade of steel are important. Since tolerances for parts are very tight, the interior cavities of the mold must be nearly wear-free. They cannot deform or pit.
Image shows tooling being carved out of steel — the most popular choice for mold tooling.
In order to avoid any deformation and ensure minimal wear on molds, hardened and pre-hardened tool steels are the best choices for molds, according to expert mold makers1. Steels and other metals wear for a variety of reasons: heat, friction, hot-cold changes, pressure, and fatigue. The choice of material for the mold will likely depend on the number of parts to be produced. The more parts to be produced, the better and more expensive grade of steel should be used. Keep in mind that the cost of the raw material is only one factor given that the cost of machining and finishing the steel will depend on the characteristics of the chosen material.
Company size and expertise
Tooling used to machine steel varies widely in cost, as well. Plastic parts are so varied in size and shape that mold making plants need a big inventory of cutting tools, abrasives and other materials for finishing molds. Companies that specialize in mold making have the expertise needed to recommend and manufacture many kinds of molds. However, keep in mind that there may be situations where a company is asked to build a product they have never done before. This can add significantly to costs.
Plastic as raw material
The number of products that use plastic parts is uncountable. Throughout the world, more and more products are designed to use plastic for its properties of strength, light weight and infinite design applications.
Plastic pellets ready to be used for injection molding
In various industries, certain plastics are used regularly. In electronics and computing, there are requirements that materials must meet; whereas in the toy industry, the requirements could be entirely different. Printed circuit boards require specific materials; electrical tubing parameters are very different.
Prices of plastics used in specific industries vary immensely as well. Like other products bought in bulk, the price per unit might drop as the quantity purchased goes up. However, the ingredients of plastics are both simple and complex, depending on what industry it is being manufactured for, as discussed above. Plastics are manufactured to meet specific needs such as precision cutting, chemical resistance, electrical resistance, heat resistance and much more.
The cost of injection molded plastic parts is partly defined by bulk material costs; the greater the number of parts, the less customers pay per pound of material. However, with material comes some hidden costs, such as storage. Generally, storage of bulk material is expensive, so the faster a customer can receive parts, the lower the cost of storage. These numbers are included in the final cost per part, or cost per cycle.
There are other factors in the cost of injection molding that fall into categories typical of all manufacturing, such as maintenance and utilities. For instance, the entire process of plastic injection molding uses a lot of electricity. Machines are running constantly, thermoplastics are heated to melt, and chillers cool the mold and products after the cycle. These all add up to high electricity spending for each time the machine runs a cycle.
Overview of injection molding facility: electricity can be a major factor of utilities cost.
As you can see, injection molding costs encompass an array of things: from simpler things like raw materials and storage to more complex ones such as engineering and manufacturing. That is why having a sound design optimized for manufacturability from the beginning will help customers save time and money throughout the process.
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