(Updated April 2019)
It pays to understand the processes and possible snags within the injection molding industry to make a more educated decision when purchasing services for your project. Buyers should know they will need to establish a mechanism to monitor and assess the supplier's output - such as ISO certifications or FDA registration. This helps to verify the injection molder meets industry standards. Buying custom injection molded plastic parts can vary based on the quantity, material and complexity of the part. Here are a few tips to help you through the purchasing process for your plastic component or device – note most of these decisions are determined during the design phase of your project so it is important to partner with an injection molder, such as Crescent Industries, that designs for manufacturability!
Cost of the Injection Mold
The injection mold is the most visible expense of your project. Size and the complexity of the part to be molded and the number of cavities of the injection mold are major influences on the initial price of the mold. Therefore, there are a few approaches to mold fabrication that can be considered.
To further understand the purchasing of an injection mold - you need to understand what mold bases are and why they are required. The mold base holds all of the mold cavities, inserts and components together. The cost of the mold base is established based on the size, type of steel and the customization required. Most mold bases come in standard sizes, and are further machined to meet the requirements of a specific project.
Commercial injection mold bases are typically made from pre-hardened steel where the part geometry is machined either directly into the mold plates or into inserts specific only to that mold. Sometimes aluminum or mild steel mold bases are used to manufacture prototype injection molds or short run injection molds to keep costs down.
In addition to the part, runner and gate geometry, all the mechanical elements of the mold (such as the leader pins and bushings, ejector system, sprue bushing, support pillars, etc.) need to be designed, machined and installed for each new mold.
Depending on your project requirements - you can purchase only the custom injection mold cavity to manufacture your part. You are purchasing a type of removable tool set - which is not a complete mold. This means that you rely on the molder to use his mold base to house your tool set during production runs. This will provide cost savings, because only the tool set needs to be designed and machined instead of the entire mold base. This significantly reduces the material used, labor needed and design costs. Typically, this approach is used for smaller production runs.
Costs of Raw Material
Raw material is a major ongoing cost of the injection molded part which affects long term costs. The mold designer will need to know the material to be used up front in order to accurately determine manufacturability and mold design. Different resins have different shrink rates, corrosiveness, etc., so it may not be practical to change resins after the mold is manufactured.
There are hundreds of resins to choose from for your product, but most resins fall into two categories: commodity resins or engineering resins.
- Commodity resins are usually lower cost, lower performance resins that are widely available from many suppliers - such as polyethylene, polypropylene and resins that contain polystyrene.
- Engineering resins are resins that have superior properties over commodity resins. The cost can vary greatly depending on the physical properties like chemical resistance, electrical and thermal properties which mean prices may vary greatly. Some of these include the Nylon family, polycarbonate grades and PEEK.
Costs of Custom Injection Molding the Part
After the cost of the raw material, the manufacturing costs are determined by the machine hourly rate and the number of cavities in the mold. The machine hourly rate depends on the size of the injection molding press. To determine the size of the injection molding press - you will need a few points to consider for your project:
- Plastic injection molding presses are classified based on tonnage or clamping pressure or force - presses can run from 5 tons of clamping pressure to over 4000 tons! This clamping pressure is what keeps the mold closed during the injection molding process.
- The higher press ton rating - the larger the machine.
- MFI (melt flow index) is a measure of the ease of flow of the melt of a thermoplastic polymer - the higher MFI, the higher the pressure needed.
- The size of the part will also determine the press size. It is typically 2.5 times the surface square inches of the part to be produced
- Many calculations determine press size, including the platen size. The platens are the heavy steel plates to which the halves of the mold are attached. One of these is typically a stationary platen and the other is mounted so that it can be moved with the clamp mechanism. The molds are attached to these platens by directly bolting or by using clamps.
Your plastic injection molder will determine what press size would be best suited for your project.
Why Crescent Industries?
If you are not working with a completed mold, contact Crescent Industries to discuss the design and fabrication of your plastic injection molding project. Crescent will streamline your manufacturing process and offer tooling solutions that meet your production requirements while optimizing project costs.
For additional information, please fill out the form below to get our white paper “10 Factors that Impact Tooling Costs”.