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Die Casting – Guide to Buying Manufacturing in China

Die casting 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
November 25, 2020

Die casting, as we know, is a process where molten metal is injected into a specially designed steel mold – called a die – at high pressure. When the liquid metal cools and solidifies, it takes the shape of the die. These parts, called castings, are a big part of our daily lives. You will find them in the appliances you use at home, the car you drive to work, the machines and devices you operate at your workplace, your mobile phone, your skincare products, and in numerous other objects and places.

This blog explores what you, as a buyer looking for die casting services, must keep in mind in order to receive a final product that ticks all your boxes. As a China sourcing agent, Sourcing Allies can help connect you with trustworthy manufacturers and factories in the world’s die casting capital. Just remember, your collaboration is crucial to a happy sourcing experience.

What is my role as a buyer when it comes to die casting?

In die casting, a buyer is not merely a customer who receives the manufactured goods at the end but rather, an active collaborator who works with the manufacturer from the onset. When Sourcing Allies helps you get cast parts from manufacturers in China, we will require specific design inputs from you. These include:

3D files

The first step in the product development process is the creation of a 3D model. For simple products, we can work with 2D drawings that show the part’s basic dimensions, weights and tolerances. (Tolerance is the acceptable degree of variance in dimensions). However, for a more intricately designed part, we will require a 3D file. A 3D file comes in various formats. A STEP file is one such format. It doesn’t really matter what format you submit your 3D file in. We will then send the 3D file to the manufacturer. Based on it, the manufacturer lets us know if the part can be produced and extracted from the tool without problems and how much each part will cost.

Samples

Along with design files, some of our customers provide us with an existing sample of a product. We might use the sample to highlight defects and make improvements or simply to emphasize the surface finish requested by the buyer.

Specific requirements

Initially, we ask for a clear 2D drawing showing the essentials, such as material type, surface finish and tolerances. But it is useful to have other details, such as the product’s final intended use or what type of environment it will be used in (say, a corrosive or particularly humid environment or one with lots of vibration). It also helps to know the product’s history – if it breaks, cracks or distorts in certain areas under certain conditions. For example, a buyer might tell us that their die cast aluminium part is perfectly fine when it comes out of the tool and when it is machined but suddenly distorts when it is powder coated during surface treatment. Details like these help us make decisions that improve the manufacturing process. A buyer must be as specific as possible about their requirements – whether it be their choice of metal, or the wall thickness or surface finish they are looking for. This helps us communicate their requirements to the manufacturer clearly and precisely.

Die casting tooling being made in China
There are a number of factors to consider when buying tooling.

What is tooling, and why is it important?

It is important to get the tool design right. Sometimes, a customer will specify that they want a tool made in a particular way for a particular reason. However, generally, the tool design is left to the supplier’s specialist toolmaker. This process is called tooling. Apart from dies, additional tools used could include jigs, fixtures, mold inserts, cutting tools and so on. A common machining technique used in tooling is Electrical Discharge Machining (EDM), which uses electric sparks to cut and shape metal. Thereafter, the tools are heat-treated and surface texture, if required, is added.

Some points to remember when designing tools:

  • Strength: Tools must be built to withstand severe operating conditions, such as high temperatures and wear and tear. They must also have good thermal conductivity and high ductility (the ability to stretch without breaking under tensile stress). This is why they should be made of the highest grade tool steel, even though it costs more than regular steel.
  • Life: The tools can be used for years. They can last up to 70,000 cycles. In some cases, even 100,000 cycles. But the life of a tool depends on the material it is made of, the cost at which it is built, the material injected in it, and the structure of the casting.
  • Lead time: It usually takes 20 to 60 days on average to build a production tool. A specific surface finish or texture can greatly add to the lead time. Remember to budget for it.
  • Price: Tooling costs range from a few hundred dollars to hundreds of thousands of dollars, depending on complexity and size. You might be tempted to go with the manufacturer offering you the lowest tooling cost. But in the long haul, it is advisable to invest in a tool designed for longevity and to make sure it receives regular maintenance.

Common design mistakes in a part, and how to avoid them

1. Thick, non-uniform walls

A part with thick walls means a longer casting cycle, which drives up production cost. Reduce wall thickness to cut down cycle time. The alloys used are capable of walls as thin as 1 mm. Thinner walls also make the part lighter, which means that you save on material. Additionally, if your walls are uniform, the alloy will cool evenly and your product will have fewer defects. Many defects occur during the metal cooling stage.

2. Sharp corners

Sharp corners/angles hinder metal flow and increase stress concentration, leading to cracks. Filleting or rounding off corners will reduce stress concentration and give you a more durable part. In tooling, it will increase the longevity of the die and save you the expense of frequent repairs.

3. Wrong parting line placement

A die has two halves. The part where the halves meet is called the parting line, which can be straight or broken. Getting the parting line placement wrong might require a redesign and add to costs. A few things to consider:

  • Parting line surfaces have an inferior finish. If placed in a visible area, it can affect the part’s aesthetic quality and require additional polishing, which is expensive.
  • The parting line is where the gate inlet is located. The inlet controls metal flow into the die. Improper metal flow can lead to defects. Thus, both the parting line and inlet must be located in a place that allows optimum metal flow.
  • Keep cores and undercuts to a minimum. Cores are holes in the casting. Undercuts are protrusions or depressions that stop the casting from being ejected from the die. Place the parting line in such a way that it requires minimum undercuts. This will, in turn, reduce the number of cores required and lower tooling costs.

4. Ejection problems

If your casting gets stuck in the die during ejection, you could have issues with draft angles. A draft is a slight angle or slope in the walls, cores or other parts of the die cavity that facilitates easy release of the casting without damaging it. Draft angles usually depend on the alloy used.

5. Tolerance troubles

Producing products with a tight tolerance (minimum dimensional variance) is a tall order. If not done well, it can result in an inferior product and necessitate a redesign. If the functionality of your part does not depend on perfect dimensional accuracy, you would be wise not to insist on it.

6. Letters and logos

Often, buyers want some form of identification – letters, logos, trademarks – on the die cast part. Manufacturers use either raised letters on the casting or depress them into the metal. The former is more cost-effective and the letters last longer. The latter is more expensive and the letters are more prone to wear and tear.

Remember, MOQ matters

In a mass production process, minimum order quantity (MOQ) matters greatly to both the buyer and manufacturer.

For the manufacturer, it means the difference between profit and loss. When your supplier asks you to buy a certain amount of material, just a portion of that goes into making your products. The rest is scrapped, or recycled for use another time. Some degree of wastage is inevitable. For example, if you’re pouring molten metal into a production tool, there will be metal left in the inlet and feedlines, which comes out on the part straight from the tool. This metal obviously gets removed

before the next step in the process. The manufacturer accounts for all this wastage when accepting an order.

For the buyer, not meeting an MOQ might mean losing the manufacturer’s goodwill and cooperation. On the flipside, the right MOQ will give you greater say with your manufacturer, drive your costs down and your quality.


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.

For more on China sourcing visit our website or write to us at info@sourcingallies.com.

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