Injection molds are made of various materials, such as polystyrene, High-density polyethylene (HDPE), Acetalacetal, and PVC. Here we will go over the differences between these materials. Then you can decide whether or not you want to use one. To make the decision easier, we will break down the different types of materials into their properties. In this way, you will know what your best option is.


The process of molding polystyrene involves using heat and pressure to shape the material into a specific shape. The initial charge of polystyrene or related resin is usually a preformed billet or block. The material is heated to a temperature above 120 degrees Celsius and is then compressed into a mold cavity. The mold cavity is then filled with the resin, and the part is ejected.

Another thermoplastic material for injection molds is polypropylene. Polypropylene has high impact strength and is resistant to warping. It is ideal for industrial applications requiring excellent rigidity and dimensional stability. It is also strong and stiff, making it a popular choice for injection molds. This versatile thermoplastic also exhibits excellent machinability and dimensional stability. It is also recyclable and can be used to manufacture live hinges and other items that withstand high temperatures.

High-density polyethylene

Polyethylene is a versatile plastic that is used for injection molding that is used in a defense & government injection mold builder. It is non-toxic and offers a range of properties that make it suitable for low and high-production applications. It is also cost-effective and environmentally friendly, making it a popular choice for various industries. Due to its melting temperature of 120 to 180 degrees Celsius and its high level of strength and stiffness, polyethylene is ideal for multiple applications, including molds.

The properties of HDPE allow it to be molded into parts of varying shapes and sizes. The material is first melted and transferred into the mold cavity consisting of “A” and “B” parts. The part is then ejected from the mold when it has cooled to the desired temperature. The process can be repeated endlessly. Finally, the mold is custom-made for a specific project.


Polypropylene, also known as PP, is a thermoplastic addition polymer formed by combining several propylene monomers. This plastic has many uses, including as a replacement for polyacetal and as a contact point in furniture. However, because polypropylene is not very adhesive, it may not be suitable for injection molding in high-friction environments.

To control polypropylene shrinkage and warpage, use the proper injection pressure for the part’s construction. Injection pressure should be high enough to fill about 99% of the part. However, a low injection pressure can result in flashing or sticking. Therefore, accurate injection is the most important factor for warpage and shrinkage control. On the other hand, if the polypropylene material has too much pressure, it may be subject to warpage and flashing.


The abrasion resistance, stiffness, and wear properties of acetal resins make them perfect for making injection molds. This thermoplastic material is also highly resistant to water, heat, and solvents. During its production, acetal injection molds are highly durable. They are used to produce various parts for autos, trucks, bicycles, and other machinery. The molded acetal parts are highly durable, and the acetal plastic is available in many colors.

When using Acetalacetal, the material should be poured at a minimum of 0.001 inches of its nominal wall thickness. The variation in wall thickness must be uniform and smooth. The viscosity of the acetal resin affects its shrinking behavior. Thicker walls shrink faster than thinner ones, which can be problematic when tight tolerances are required. Acetal can also fracture in places with a sharp break in the surface. A radius of 25 percent must be applied on the inside corners to reduce the chances of fracture.


When using nylon to make injection molds, it is necessary to be careful about gassing. Gas is a common problem in nylon molding, resulting in poor appearance and shorts. It can also lead to inadequacy in filling in details and ribs. Nylon is often treated with a UV stabilizer before injection molding to reduce these problems.

Nylon is a common thermoplastic with silk-like qualities. Chemists invented the material at DuPont, which sold 4 million pairs in two days. Although nylon is amorphous, the material is highly stable and impact-resistant. It can also be injected with glass fills to make it even stronger. Nylon is also recyclable and can be remolded. The main advantage of using nylon for injection molding is its recycling ability.