For 75 years, Rosti has proudly and successfully contract manufactured millions of useful products that touch people’s everyday lives — from coffee cups to food freshness solutions and bottle caps to vacuum cleaners. Rosti even produces products that help detect illnesses in time to find a cure, help personalize smart homes and help reduce carbon dioxide emissions globally.

Every one of these products is made with technical precision using plastic injection molding.

Precision plastic injection moulding explained

Plastic injection molding is simply a process of melting plastic pellets and injecting the molten plastic into a mold or cavity to form a shape. Once the shape is formed, the material is cooled, ejected from the machine and — voilà — a finished, functional, aesthetically pleasing injection molded product or part is ready!

Of course, the full process is a little more detailed than that.

Figure 1. The Rosti innovation design lab helps bring new concepts to life with precision and efficiency. Source: Rosti

Following the design of a product by an engineer or specialist industrial designer, a mold or tool maker constructs the mold. This mold is then precision-machined into the desired shape and with the precise specifications for the job.

A machine feeds the plastic material for the molded part through to the heated barrel in the injection unit where a screw, driven by a motor, mixes the plastic pellets. The plastic melts and it is then injected to fill the cavity of the mold where it hardens as it cools.

The mold, or tool, can then be used to manufacture millions of parts, if needed.

Which products are plastic injection molded?

The better question to ask is, which products are not plastic injection molded? This is because countless items used every day are produced this way. Examples are everywhere, from electrical switches to wash basins, espresso machines to speakers and cabinets to power tool housings.

Practically every manufacturing sector uses injection molded parts, and these range from micro components to large, single pieces like wheelie bins and cabinets.

Personal hygiene items and medical devices are also plastic injection molded, including disposable razors, bottle lids and closures, and syringes. So too are smart home devices, cashless payment systems, vacuum cleaners and electric tools. Almost 90% of an automobile is precision injection molded.

Most thermoplastics are suitable for precision injection molding. These include nylon, acrylonitrile-butadiene-styrene, polycarbonate, polypropylene and polystyrene.

More about molds

An effective mold is essential for precision injection molding, which is a highly versatile technique that offers a variety of design possibilities, as previously described. The success of this technique relies on the careful design of the part, and the correct choice of components such as the material for the part, the shape, the features, the texture and the material of the mold, and the specifications of the molding machine.

Molds, above all, must be tough. Because each mold will be reused multiple times, it must withstand the impact of melted plastic at high pressures. The mold must also be precise — in some cases, it should be true to the nearest 0.01 mm.

A short history of plastic injection molding

Injection molding has benefited from continuous improvement and innovation throughout time. And it has a surprisingly long history. The first injection molding equipment was patented by American inventor John Wesley Hyatt in 1872.

Compared to the complex industrial machines of today, it was a simple machine that looked like a syringe. A plunger injected celluloid plastic through a heat tube into the mold.

The plastic injection molding industry boomed during the 1940s as World War II increased demand for mass-produced plastic items. An American inventor in 1946 constructed an injection machine using a screw. Next came the gas-assisted procedure for injection molding in the 1970s, and aluminium molds were introduced in the 1990s.

Modern precision injection molding makes use of innovative techniques and procedures to construct the parts needed for modern life.

Important advantages of injection molding

Rosti’s technical capability in precision plastic injection molding, can, from one single process, and in one shot, create a product or part that is repeatable, accurate, cost-effective and functional, with the correct surface finish.

The five main advantages of injection molding are:

• Repeatability: The same shape can be produced in the millions.
• Accuracy: The manufactured product or part is not only repeatable, but it can also be produced to a tight tolerance, consistently.
• Functionality: It is possible to design a number of features into the injection molded part including complex shapes that cannot be practically achieved by other materials or processes.
• Surface finish: Using different injection mold processes coupled with specific tooling, a wide spectrum of surface finishes can be achieved, from high gloss to textured surfaces to simulated natural fibers.
• Cost and weight efficiency: When producing volume parts, the main cost is the tooling. Once invested, the actual production parts are a fraction of the cost to produce versus other options. The weight of the part itself compared to traditional metal parts can also offer cost savings, from functionality to lower logistic costs.

Considerations for effective injection molding

Figure 2. Engineering expertise and innovation are key considerations for selecting an injection molding partner. Source: Rosti

It is important to determine the cost-effectiveness of the manufacturing process when creating a development timeline. The design and prototype of a mold tool is a vital part of the process. It must be precisely achieved, otherwise the aims of the injection molding process may not be met. Most injection-molded products require an initial investment up front, although the repeatability of the process means this investment is usually repaid over time.

Parts should be designed with plastic injection molding in mind right from the start. This streamlines the process and keeps costs down. Small adjustments make a big difference over time when manufacturing millions of components.

It is more cost-effective to increase part quantity in order to give a reduction in price-per-piece. However, a project may not run to millions of parts and this is not problematic. Cost-effective molds are available for small quantities.

Consider the material. Some plastics are easier to mold than others, which lowers the costs. However, it is important to use the right material for the job to maintain functionality and quality.

There is a lot to think about at the start of the injection molding journey.

Rest assured, however, that Rosti is an expert partner throughout the production cycle. A product can be taken from idea to reality, with customers benefiting from decades of expertise and a high level of skill from design to production.

Rosti is strategically located with facilities in China, Germany, Malaysia, Poland, Romania, Sweden, Turkey and the U.K. Contact them today by visiting their website.