Injection Molding – Guide to Buying Manufacturing in China

Injection molding is a great manufacturing process. This is a guide for buyers so finding the right factory in China is faster and more efficient

Vaughn Cook RockWell Window Wells
Jeff Crawshaw
February 3, 2023

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.

Injection molding- Plastic spoon among set of beads
A plastic injection molded spoon among wooden beads

Common design mistakes and how to avoid them

1. Non-uniform walls: If your part has walls of varying thickness, the thinner sections will cool before the thicker ones. This will cause shrinkage and warpage in the thick sections. Walls with uniform thickness not only allow uniform cooling, they also ensure that the plastic flows and fills the mold cavity precisely. If you cannot avoid non-uniform walls due to design limitations, keep the variations as gradual as possible.

2. Thin versus thick walls: Apart from uniform wall thickness, the buyer must also consider the maximum wall thickness for a particular part. Thin walls mean a shorter cycle and lower production cost. Material selection plays a part here, so check with your supplier on the thickness specifications of the materials you are considering. As a general rule, thinner walls call for plastics that flow easily. Additionally, if your mold design requires a longer flow route, then thicker walls are more suitable.

3. Lack of drafts: A draft is a slight angle on the wall of a mold cavity so that its opening is wider than its base. This facilitates easy removal of the part from the mold. Without draft angles, a part can be damaged during ejection. Sometimes, even the mold might be damaged. The absence of draft angles might also result in the part warping or breaking due to stresses caused by cooling or ending up with a poor finish. Ideal draft angles vary from part to part, so make sure to consult your supplier on this. Overlooking draft angles during the design stage is a common mistake, one that should be avoided at all costs.

4. Sharp corners: When molten plastic flows across a sharp corner, it can cause stress formation in the material, leading to warpage and a defective product. Rounding off corners takes care of this problem by ensuring the material flows evenly in the mold cavity. However, stress concentrations vary with radius. Ensure your design incorporates uniform wall thickness with consistent inner and outer radii.

5. Parting line placement: The parting line is the line of separation on the molded part where the two mold halves meet. Its placement is a challenge in any manufacturing process. You’d like it to be as inconspicuous as possible for aesthetic reasons, but without compromising on the part’s functionality. The placement of the parting line requires your full attention because molten plastic tends to flow towards the parting line, which can cause flashes. Additionally, parting line placement impacts mold-making and post-processing costs.

6. Too many undercuts: Undercuts are protrusions or depressions on a molded part that prevent it from sliding out of the mold. They are often used in complicated parts, such as medical equipment components, and in complex molding processes such as overmolding and insert molding. Undercuts add to cost, so they should be used sparingly or eliminated altogether.                       

The plastic and pollution debate

It is impossible to talk about plastics without talking about its impact on the environment. But as we grow into a more environmentally conscious society, the plastic injection molding industry is doing the same. Here’s how:     

  • Recycled plastic plays a big role in plastic injection molding. While its performance might not be as good as new plastic, using it is a planet-friendly decision that can also lower production costs.
  • Similarly, plastic products can be re-purposed and re-used after being recycled. For example, recycled LDPE shopping bags are often put to use a second time as trash cans, floor tiles, furniture and paneling. 
  • Plastic injection molding machines run on power. But, with technological advances, today’s devices are said to consume 20% to 50% less energy than machines from a decade ago. Also, if given a choice, you can pick an electric machine over a hydraulic machine as it is more energy-efficient.

In terms of application, plastic can sometimes be a more environment friendly choice than other material. When used as an alternative to steel in aircraft components, it reduces fuel consumption because of its lower weight. Similarly, when used as insulation material in buildings, it aids in energy conservation.

Sourcing Allies is a team of expert China sourcing agents that has helped western customers manufacture and source products from low-cost regions since 2006.

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