Why Do Plastic Injection Molds Cost So Much

Why Do Plastic Injection Molds Cost So Much?

People often ask why plastic injection molds cost so much? Here is the answer –

Producing high quality plastic parts can only be achieved by using a high quality built mold. Molds for plastic injection consist of precisely machined components made from various metals such as aircraft grade aluminum or hardened mold steels.

These molds are designed and made by highly skilled and well paid people categorically called “mold makers”. They have spent years and possibly even decades being trained in the mold making trade.

Additionally, mold makers need very expensive tools to perform their job, such as very expensive software, CNC machinery, tooling, and precision fixtures. The amount of time that mold makers need to finish a plastic injection mold can range from a few days to several weeks depending on the complexity and size of the end product.

Mold Construction Requirements

In addition to the costs that are associated to molds from the skilled people and machinery that make them, the construction requirements for an injection mold to function properly during the injection molding process is quite astounding. Although the molds are summarized as halving “two halves”, a cavity side and a core side, there are often dozens of precision parts that make up each half.

Almost all of the precisely machined mold components that will come together and function to manufacture your custom molded parts are machined to tolerances of +/- 0.001″ or 0.025mm. A standard piece of copy paper is 0.0035″ or 0.089mm thick. So just imagine slicing your copy paper into three ultra-thin pieces as a reference to how precise a mold maker needs to be to properly build your mold.

Mold Design

And finally, the design of your plastic injection mold has a very large impact on its cost. The plastic injection molding process requires enormous amounts of pressure when the plastic is injected into the mold cavities by the machine. Without these high pressures the molded parts will not have nice surface finishes and potentially will not be dimensionally correct.

Mold Materials

In order to withstand the pressures your mold will see during the injection molding process it must be made with high quality aluminum and steel grades, and be designed to withstand clamping and injection forces that can range from 20 tons for a small precision part to thousands of tons for a residential recycling bin or garbage can.

Lifetime Warranty

Whatever type of plastic injection mold you need, we understand that your injection mold purchase will become a significant asset to your business. For that reason, we warranty the production life of molds we build for our customers for the life of their production requirements.

Injection Molding Process

Plastic injection molding requires three primary components – an injection molding machine, a mold, and raw plastic material. Molds for plastic injection consist of high strength aluminum and steel components that have been machined to operate in two halves. The mold halves come together inside the molding machine to form your custom plastic part.

The machine injects molten plastic into the mold, where it solidifies to become the final product. The injection molding process is actually a complex process with many variables of speed, time, temperatures and pressures. The complete process cycle for making each custom part can range from no more than a few seconds to several minutes. Below we offer you a very brief explanation of four steps of the molding process.

Clamping – Before the plastic is injected into the mold, the machine closes the two halves of the injection mold with tremendous forces which prevent the mold from opening during the plastic injection step of the process.

Injection – Raw plastic, generally in the form of small pellets, is fed into the injection molding machine at the feed zone area of a reciprocating screw. The plastic material heats up by temperature and compression as the screw conveys the plastic pellets through heated zones of the machine barrel. The amount of melted plastic that is conveyed to the front of the screw is a strictly controlled dosage because that will be the amount of plastic which will become the final part after injection. Once the proper dosage of melted plastic reaches the front of the screw and the mold is fully clamped, the machine injects it into the mold, pushing it into the endpoints of the mold cavity under high pressures.

Cooling – As soon as the molten plastic contacts the internal mold surfaces, it begins to cool. The cooling process solidifies the shape and rigidity of the newly molded plastic part. The cooling time requirements for every plastic molded part depends on the thermodynamic properties of the plastic, the wall thickness of the part, and the dimensional requirements for the finished part.

Ejection – After the part is cooled inside the mold and the screw has prepared a new shot of plastic for the next part, the machine will unclamp and open the plastic injection mold. The machine is equipped with mechanical provisions that work with mechanical features designed within the plastic injection mold to eject the part. The custom molded part is pushed out of the mold during this stage and once the new part is fully ejected, the mold is ready for use on the next part.

Many plastic molded parts are fully completed after the are ejected from the mold and simply fall into their final carton to be shipped in, and other plastic part designs require post operations after they are injection molded. Every custom injection molding project is different!

We hope this information helps you better understand plastic injection mold construction and their cost. Remember the quality of your custom plastic parts will first depend on the quality of your mold. Let us quote your next injection molding project and we will work closely with you to make your project a success!