Design, processing, and production of plastic caps, plastic cap molds

CAP & closure

Design, processing, and production of plastic caps, plastic cap molds


Around the world, people use millions of lids every day. The lid has a variety of applications in daily life. It appears on household medicine bottles, cooking oil bottles, sauces, or helps keep water in beverage bottles. So, where and how each bottle cap is produced, please refer to the article below.


A plastic cap is a part to cover an object, ensuring that the contents inside are always kept there until used. The outside of the lid is often grainy instead of smooth to make it easier to open and sometimes has protective threads. The top of the cap can be left flat or embossed with the company logo, the product’s brand. In some cases (pharmaceutical products), the manufacturer will print the opening method here.

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The vast majority of plastic bottle caps are made from recyclable plastics. Here are the four most common types.

PET (polyethylene terephthalate): PET is durable and lightweight. It is suitable for water/soft drink bottle caps.

HDPE (high-density polyethylene): HDPE is tough, stiff, yet still light.

LDPE (low-density polyethylene): LDPE is flexible, with high strength and impact resistance.

PP (polypropylene): PP is hard, strong, and durable. It is commonly used for all kinds of bottle caps.

Bottle caps are popular items in everyday life. So it is often produced with materials that are easily recycled. The choice of material for the production of bottle caps needs to consider factors such as time of use, products contained inside (water, powder, sauces, etc.), and other standards. Bottle caps used in pharmaceuticals, food, or beverages will have to be made from polymers that meet the requirements of food safety and hygiene and are certified by reputable organizations.


Most plastic bottle caps are manufactured through a process known as injection molding. This process enables each finished one to achieve high precision, uniformity in size, shape, and high surface finishing.

In this process, the plastic particles are heated in a metal hopper to the point that they become very soft and pliable. It is possible to add color to the plastic at this stage. The softened plastic is then injected under high pressure into a complex structural steel or aluminum mold. Here the glue is filled into the cavities in the mold and cooled enough to harden. Finally, the mold will open for bottle cap removal.


The injection machine includes a material hopper, screw rod/ plunger, and a heating element. Molds are clamped onto the machine’s roller, and plastic is injected into the mold through the injection hole. Injection molding machines are classified or rated based on tonnage (the maximum clamping force an injection molding machine can offer to keep the injection tool tightly closed during the injection process). The “tonnage” can vary from less than 5 tons to 6,000 tons. However, manufacturers rarely use large tonnage machines. Instead, they often use ones with tonnage from 60 tons to 450 tons in the production of bottle caps.



Two sides of the mold are held in place by a clamping device. The hydraulic rod keeps the mold firmly in place so that the cylinders feed the plastic into the mold.


From the storage hopper, the plastic granules are put into the hot chamber through the movement of the cylinder. After that, plastic granules will turn into glue and be pumped into the mold to fill the empty cavities.


The cooling process helps the molten plastic turn into a solid according to the shape of the mold. Each individual mold has a different cooling time for each plastic product design depending on the wall thickness, thermal properties, melt index, and mechanical properties of each resin.

Removing products

When the mold opens, a part of the mold helps push the products out when they have achieved the desired shape.


Mold Material

Molds are used to produce a lot of caps during the manufacturing process. Molds are usually made from hardened steel, pre-hardened steel, aluminum, or copper-beryllium alloy. The selection of materials is mainly based on cost and desired mold strength. In general, the cost of materials is higher, but their longer service life offsets the initial investment cost.

Pre-hardened steel molds are less prone to wear and suitable for high-volume production requirements. The steel hardness commonly used in molds is 38–45 on the Rockwell-C scale. The hardened steel mold is heat-treated after machining to increase hardness and service life. Their hardness ranges from 50 to 60 Rockwell-C (HRC). In some cases, manufacturers can also use aluminum. If processed accurately by modern machines and technology, aluminum mold can meet hundreds of thousands of production cycles. Beryllium copper is suitable for areas of the mold that require rapid cooling or areas where high-temperature treatment is needed. Molds can be manufactured by CNC or electrical discharge machining (EDM) processes.

Mold durability

When put into use, molds will run thousands of cycles. There are even molds that are used year-round with millions of cycles. As shown above, each mold is designed to produce a lot of bottle caps per cycle. These molds not only create the shape of the bottle caps but are also subjected to a total pressure of hundreds of tons to ensure durability (each square centimeter of the mold contact area receives an impact force from 0.3 to 0.6 tons ). The mold also integrates a part to push the products out so that they are not deformed and scratched. Nowadays, modern molds are also integrated with various technologies to minimize plastic waste or close the lid right on the mold.

Injection molds are created with high precision and have near-perfect surface finishes. Therefore, it has high requirements for the qualifications, experience, and skills of engineers in the production process.

Creating a bottle cap is not easy. It takes a methodical investment by the manufacturer, considering factors affecting the user experience such as size, shape, features, standards for cap diameter, neck height, thread pitch, external decorative elements, product protection film, etc. In addition, the use of materials suitable for the products contained inside and safety standards also needs to be considered. Therefore, manufacturers need to spend a lot of effort to create them. Click here for more information/services of DuyTan Precision Mold!

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