Injection Molding – Guide to Buying Manufacturing in China

Plastic injection molding, as you might already know, is a quick and efficient way of mass-producing plastic products such as food containers, furniture, retail packaging for beauty products, and toys. It is very likely that any common plastic object you come across is a product of this manufacturing process. A molded part is created by injecting molten plastic at high speed and pressure into a metal mold and letting it cool and solidify to take on the shape of the mold.

What is my role as a buyer?

If you’re looking to buy plastic injection moldings, you must first understand that this kind of purchase requires a deeper involvement in the process. For example, if you are sourcing a particularly complex molded part, you might need to be involved in its design. There are numerous design decisions you need to take as these will determine your plastic injection molding price and quality. Taking informed decisions early on is critical as it will help you find and fix potential problems.  

The process of buying a custom plastic injection molding starts with you, the buyer, providing the supplier with detailed 2D drawings showing material type, surface finish, tolerances (the accepted amount of dimensional deviation) and any specific critical dimensions along with a 3D design file of the part. If you go through a sourcing agent such as Sourcing Allies, we will act as the go-between. Most suppliers prefer 3D files in the STEP or IGES formats, which are the most widely accepted. Your initial design might need some tweaks and improvements, which can be made in collaboration with your supplier and sourcing agent.

One point to remember when buying plastic injection moldings from China or anywhere else is to be very specific about your requirements – in terms of plastic injection molding price, quality, lead time and so on – to your supplier and preferably to put it all in writing.

Material selection, and why it is important

A key decision a buyer must make pre-purchase is selecting the right plastic raw material. After all, the strength, durability, flexibility, functionality, appearance, and cost of the part all hinge on this choice. Given that there are more than 85,000 varieties of plastics out there, this isn’t an easy choice to make. But you can start by familiarizing yourself with the most popular plastics used in plastic injection molding and their properties:

Acrylonitrile butadiene styrene (ABS): It solidifies quickly, has good impact resistance, is tough and sturdy, has a glossy appearance and doesn’t shrink easily when molded. ABS is used to make light and rigid parts such as pipes, toys and golf stick heads. However, it melts or warps at temperatures above 93 degrees Celsius, making it unsuitable for parts that are subject to sustained and intense heat.  Polycarbonates (PC): These are a group of hard, stiff, and tough thermoplastics that are also known for their thermal stability. Being naturally transparent, polycarbonates are used to make safety goggles, riot shields, compact discs, and baby bottles among others. However, they are not ideal for products that require a certain degree of flexibility or color aesthetics.       Polypropylene (PP): This highly pliable thermoplastic is used for products that are flexible, lightweight, heat-resistant, and water-resistant. Food packaging, pipes, cables, bottle caps, washing machine drums and toys, for example. It can also be spun into fiber to make upholstery, carpets and rope. Polypropylene is often called the “steel of plastics” due to its versatility. However, it is difficult to dye or paint, is prone to UV degradation and is flammable.       Nylons: A class of plastics with high heat, chemical and wear resistance, nylons are also flexible, durable and flow easily around complex inserts. Toothbrushes, fasteners, and food packaging are some examples of nylon molded parts. However, nylons are prone to shrinkage. They also have poor dimensional stability, water absorption and heat resistance. Acetal: This stiff and strong thermoplastic has high dimensional stability, is easy to machine and has good wear resistance. It also offers great friction resistance, can be dyed easily and is pocket-friendly. Garden hose heads, zippers and valves are some products molded from Acetal. This plastic is unsuitable for products that will be used in environments where they are exposed to acids, UV rays and high temperatures.Polyethylene (PE): Polyethylene is moisture-resistant and perfect for waterproof molded parts. Think plastic shopping bags. It is also lightweight, flexible, soft, and affordable. It comes in a high-density and low-density version. Low-density polyethylene (LDPE) is used to make six-pack rings, containers, and computer parts. It is not suitable for use in high temperatures or harsh weather. High-density polyethylene (HDPE) is highly versatile with both common (lotion bottles, pipes) and uncommon applications (used in facial reconstruction procedures).                   

Remember, material selection doesn’t end with picking the right plastic. As a buyer, you must be aware of the right molding conditions for your material and product. This includes injection temperature and pressure, mold temperature, ejection temperature, cooling time and cycle time. Given that plastic injection molded parts are getting more intricate by the day, it is also important to be aware of draft angles and undercuts (more on these in the next section). Naturally, all these decisions must be taken early to avoid delays, additional expenses, and stress mid- and post-production.