Why Molds are Expensive

Introduction:

The simple answer is that Injection Molds are expensive because they are very complex mechanical systems. Molds require: Engineering and design, special materials, machinery and highly skilled personnel to manufacture, assemble and test them.
The injection molding process is one where molten plastic material is forced into a mold cavity under high pressure. The mold cavity is an exact hollow negative of the part to be produced. In order for the part to be released, the mold must open at the widest place on the part. The molten plastic pressure during injection ranges from 5,000 to over 20,000 psi. This pressure multiplied by the area of the part gives rise to huge forces seeking to open the mold. The mold must be constructed to withstand the very high clamping forces exerted by the injection molding machine to contain this pressure

The injection molding process is capable of rapidly producing large quantities of parts with very high precision. Tolerances of a few thousandths of an inch are routinely achieved. With the right combination of material, part design and mold construction, even sub one thousandth inch tolerances can be achieved for small features.

The cost of injection molds can range from a few thousand dollars to hundreds of thousands of dollars.

Materials:

The materials used to construct injection molds range from aluminum to hardened steel:
Aluminum for simple low production prototypes.

The relative low strength of aluminum that makes it quicker to fabricate into molds likewise limits its useful life. Aluminum molds are typically intended to produce from a few thousand to a few hundred thousand parts with relatively simple features.
Prehardened tool steel for moderate production, more complex molds.
Prehardened tool steel molds are much stronger and more durable, yet still soft enough to be worked by conventional machining processes such as milling and turning. Prehardened tool steel molds are typically intended to produce from one hundred thousand to five hundred thousand parts, and can have a wide array of features such as slides and more intricate shapes that might break in an aluminum mold.
Hardened tool steel for high production, long life molds.
Hardened tool steel molds are the most durable and expensive because part way through fabrication their components are heat treated to achieve a hardness greater than can be machined. From that point on, the fabrication must continue using grinding and EDM processes.
Hardened steel molds are intended to produce one million or more parts. Their hardness enables them to resist wear from their own operation and the abrasion of the plastic material, particularly glass fiber reinforced materials. Hybrid construction is very common, where steel parts are used in an aluminum mold to add strength to a slender feature, or parts of a steel mold are hardened to prevent wear at a rotating or sliding mold feature.

Molds:

Single cavity molds offer the lowest tooling costs and highest precision at the penalty of higher unit costs. Multi-cavity molds are utilized to increase capacity and lower unit costs.
Family molds, multi-cavity molds with different items together, offer both the lowest mold cost and low unit cost. However, they present other problems of matching the process conditions for each part and balancing supply when the product mix or yield at a later manufacturing step varies.

Engineering and Design:

The design of injection molds begins with a review of part specifications including: Aesthetics: color, clarity, high gloss, matte, special texture, etc. Material: strength, toughness, hardness, chemical and environmental resistance Interaction with mating parts: fits and tolerances Demand and unit cost goals
From this review process the mold design concept is evolved and decisions are made resulting in a mold specification:

Single, multiple cavity or family molds The grade of mold: aluminum, prehardened tool steel or hardened tool steel Material flow considerations Parting lines and gates Finish: high gloss, texturing, embedded text and graphics, etc. Accuracy and tolerances Cooling passages Ejection system Runners or runnerless system design

The next step is the actual design of the mold. Highly skilled designers using very complex and expensive computer software programs perform this. The design tasks include:

Modeling of the products and mold components in 3D. Mold flow analysis CNC tool path design and calculation Mold materials procurement list

Early in the design process, materials and components are ordered so that manufacturing can commence as soon as possible.

Manufacturing:

Once the design is completed manufacturing begins. Mold making involves many steps, most of which are very exacting work requiring highly skilled moldmakers. One mistake can ruin or cause major repair expense to a work piece that has undergone a series of manufacturing steps over several weeks. The processes employed in mold making include:
Milling and turning
Heat-treating
Grinding and honing
Electrical discharge machining
Polishing and texturing
To save cost, common mold components are purchased from suppliers. Frequently, outside services are required from subcontractors, which use specialty equipment such as thread grinding, etc.
When all of the parts are completed the next step is to fit, assemble and test the mold. All of the mold component parts must fit together precisely to achieve an aesthetic result on the product and for the mold to not wear out rapidly or break. The mold must be fluid tight to contain the molten plastic. Yet, at the same time the mold must have venting features added to allow the air to escape. The behavior of the plastic material when molded has been anticipated, however there can be some variance in the actual result. The mold must be tested to insure the products are correct and that the mold is performing properly. Where high accuracy is required, the mold may intentionally be made “metal safe” with the final adjustments coming after the first molding trial.

Conclusion:

As can be seen from the above, the engineering and creation of injection molds is a time consuming process. The work is demanding in terms of knowledge, skills and exacting attention to details. This will always be expensive, however this expense must be viewed in terms of what is achieved: Unsurpassed sophistication in part design and aesthetic appearance with low cost mass production.
Consider the Desk Telephone. The injection molds for these half a dozen parts likely costs a quarter of a million dollars. Amortizing that expense over the hundreds of thousands of units produced brings the mold cost to pennies per phone. For this type of product, no other manufacturing process can approach the level of design, functionality and cost effectiveness of an injection molded article.

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Simulate Your Way to a Better Mold

How incorporating simulation of the cooling stage of the injection molding process can help mold designers and molders make their tool as efficient as possible.

The use of simulation software has become a ubiquitous tool for injection molded plastic parts and mold design. When used early in the design stage, simulation can provide designers and engineers with useful insight on part performance and how to manufacture plastic parts.  ‘The use of simulation allows our company to predict shrinkage, suggest product or mold changes to control warp, and verify that we have sufficient and balanced cooling to meet or beat the targeted cycle time. It also allows us to ensure that the mold will function to the customers’ expectation upon delivery.’

However, the rise of the ever-expanding global economy demands more than just manufacturing acceptable parts. The new world standard is quality parts, faster. Moldmakers and molders are constantly asked to cut seconds off the cycle time. Although this is a common demand it can be a daunting task that makes it difficult to know where to start. Through the use of commercial mold filling packages engineers can determine how to optimize the process and if it can be improved.

Examining the injection molding process reveals that the majority of the cycle time is dedicated to cooling the part. Often accounting for two-thirds of overall cycle time, the cooling stage of the process is the most beneficial stage to optimize and improve to reduce cycle time. Yet often the cooling layout is one of the last aspects of the mold design to be addressed. By optimizing the mold design and cooling layout you can improve your productivity and your bottom line. So what information can be derived from a cooling analysis?

Optimized Coolant Conditions
An injection mold is essentially a heat exchanger. The molten plastic introduces heat into the mold and the coolant extracts the heat out of the mold. Most mold designers will concentrate cooling circuits in high heat load areas, and reduce the number of circuits in low heat load areas. Once the initial mold design and cooling layout have been established it is important to maximize the heat transfer between the part and the mold. With all other parameters fixed the most important factor in determining how much heat can be extracted by the coolant is the flow rate through the cooling channels. Simulation software can allow the user to determine the required flow rate through the cooling channels (Figure 1), the temperature of the coolant in the cooling circuit (Figure 2) and the pressure required to maintain that flow rate (Figure 3).

Flow rate: The important word when determining the flow rate required to maximize heat transfer in a mold is turbulent. Ensuring turbulent flow maximizes the heat transfer between the mold and the coolant and helps maintain a more uniform coolant temperature from inlet to outlet. Engineers use a dimensionless number called Reynolds number to help them determine when the flow has transitioned from the laminar regime to the turbulent regime. The Reynolds number is directly proportional to the flow rate and density of the coolant, and is inversely proportional to the viscosity of the coolant. This transition occurs when the Reynolds number is greater than 5,000. If the coolant used and the diameter of the cooling channel are fixed the only way to increase the Reynolds number and ensure turbulent flow is to increase the flow rate.

Temperature: In addition to maximizing the heat transfer between the coolant and the mold, ensuring turbulent flow can also help maintain a more uniform coolant temperature through the cooling circuit. General guidelines suggest that the coolant temperature should not rise more than 5 F while in the circuit. Minimizing the temperature rise allows the cooling rates throughout the circuit to remain more uniform, which helps maintain a more uniform mold temperature. Figure 2 shows that if we have a flow rate that creates a laminar flow (i.e. Reynolds number less than 5,000), the coolant temperature increases about 8’F. However, if we increase the flow rate so the flow is turbulent, the temperature rise is less than 2 F.

Pressure: So what is restricting a molder from simply pushing as much coolant through a mold as possible? The answer is pressure. As the flow rate increases, for a cooling circuit the pressure required to maintain that flow rate also increases, and this requires greater pumping power. Additionally, the longer the cooling circuit and the more restrictions (i.e. change in flow direction and use of quick connects), the more power that is required to maintain that flow. Once the flow has become turbulent, increasing the flow rate further provides diminishing returns in heat extraction while increasing the pumping power substantially. This extra energy consumption can reduce profit margins, while providing minimal benefit. Therefore, these three variables should be considered collectively when designing the cooling layout.

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Gate Location Analysis

Placing a gate correctly can be one of the most critical factors in determining the final quality of the part. The location of the gate may have many requirements and restrictions including part design, usage, aesthetics, and tool construction.

How to determine the gate location?
There are several considerations for determining the gate location for a part, including:
Place gates to achieve balanced filling.
Place gates to achieve unidirectional filling.
Place gates in thicker areas.
Place gates far from thin features.
Place gates against a wall to prevent jetting.
Place gates to prevent weld lines from:
Forming in weak regions of the part.
Forming where they will be visible.
Place additional gates as necessary to reduce pressure.
Place additional gates to prevent overpacking.
The type of tool being used.  Is it a 2 or 3-plate mold?
Hot or cold runners, or a combination?
The type of gate that is desired; edge, tunnel, etc.
Restrictions on gate location due to part function.

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Flow Analysis

Every part that is analyzed has a different set of constraints in the form of objectives, restrictions, and guidelines. These constraints must be taken into consideration when doing an analysis.

Will the part fill?
What material will work best for my part with regards to fill properties, i.e. pressure, shear stress, temperature distribution, etc.
What processing conditions should be used to mold this part?
Where should the gate be located?
How many gates are required?
Where will the weld lines be, and will they be of high quality?
Will there be any air traps?
How thick can the part be made?
Is the flow balanced within the part with the fixed gate location?
Are ribs too thin to fill completely?
Are ribs so thick that they shrink too much?
Can the part be packed out well enough?
Will this snap fit break during use?
Can the part be filled and packed in the press specified for the job?
Are the runners balanced?
What size do the runners need to be to balance the fill?
Is the runner volume as small as it can be?
Is the gate too big or too small?

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Cool Analysis

Why we need optimize cooling ?

There are two major reasons why cooling should be optimized.  The first is part quality and the second is cycle time.  When considering quality, several possible issues arise.  The first is surface finish.  The mold temperature can affect the appearance of the part.  As mold temperature changes, so does the gloss level of the part.  A higher mold temperature tends to lead to a glossier finish on the part.  For certain applications, this can be a major issue.

Residual stress and thermal bending are other quality issues.  As the mold temperature goes up, the cooling rate slows and more stress is relieved from the part, lowering the warpage.  When the temperatures on either side of the plastic cross-section are different, this can lead to the part warping or bending due to the non-uniform shrinkage of the part.

Molders are very interested in cycle time.  They want to make parts as fast as they can to keep production costs down.  When injection molds are optimized for cooling, the cooling of the part is reduced to the minimum time possible.  This often means that the part can be ejected hotter and still not warp too much, plus the part will still meet its critical dimensions.

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Warp Analysis

When looking critically at the causes of warpage, you could conclude that virtually any aspect of the molding process will have some effect on the warpage of the part. The four major categories that contribute to warpage are listed below:
Part design.
Mold design .
Processing conditions.
Material.

Once you know that a part is going to warp outside the design criteria for that part, something will need to be changed with the part design, injection mold design, or processing to reduce the warpage.

Reducing warpage to within design criteria is critical for a successful part. The earlier this can be done in the product design cycle the better. Much of the time, reducing warpage involves changing the part design, however, changing the tooling design can also be an important component in reducing warpage. The cost of change is least when it is done early in the design cycle and will also mean less time to production.

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Runner Balance Analysis

The runner system should be designed so that all of the parts finish filling at the same time. To do this it might be necessary to balance the runner system.

Before you design the runners for a family injection mold, analyze each part on its own. Once you know that each cavity will fill, then you can design the runner system to create balanced fill paths in each cavity. If the runners are not balanced, molding problems such as hesitation, underflow and overpacking may occur.

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Optimizing Product Function-injection mould

The injection mould process affords many opportunities to enhance part functionality and reduce product cost. For example, the per-part mold costs associated with adding functional details to the part design are usually insignificant. Molds reproduce many features practically for free. Carefully review all aspects of your design with an eye toward optimization, including part and hardware consolidation, finishing considerations, and needed markings and logos, which are discussed in this section.

Consolidation

Within the constraints of good molding practice and practical mold construction, look for opportunities to reduce the number of parts in an assembly through part consolidation. A single molded part can often combine the functionality of two or more parts.

Hardware

Clever part design can often eliminate or reduce the need for hardware fasteners such as screws, nuts, washers, and spacers. Molded-in hinges can replace metal ones in many applications. Molded-in cable guides perform the same function as metal ones at virtually no added cost. Reducing hardware lessens material and assembly costs, and simplifies dismantling for recycling.

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Polymers Used in Plastic Moulding

The term “polymer” describes classes of molecules with large numbers of repeating structural units connected through covalent bonds. The major identifying feature distinguishing polymers from other molecules is the repetition of many similar, identical, or complementary subunits.

Most manufacturers decide upon polyethylene as the polymer of choice for the plastic moulding process because of its availability, ease of use, and suitable properties. According to recent reports 80% – 90% of all polymers used in the plastic moulding industry are polyethylene compounds (HDPE, LPDE and LLPDE). Although PVC, nylons, and polypropylene compounds are also used.
Nylons
Polyethylene
Polypropylene
PVC

A paper written by J.D. Ratzlaff of Chevron Phillips Chemical Company LP in 2004 entitled “Polyethylene: Process Sensitivity in Rotational Moulding” presents the results of a study of the impact sensitivity of polyethylene to processing conditions and discusses methods to maintain high impact standards.

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Technical Comparison of seven big countries die

In addition in many areas of Japan to control the core technology, but also hold the most profitable industrial chain link, to most other global markets, a joint venture assembly plant have only a part of the profits, this “industrial nation” is the economic model Chinese enterprises should learn from. The current need to guard against the capitalists by international hedge funds and create public opinion, exaggerating the losses, took the opportunity to take away the money, and that the development of the global economy will have serious impact, if the Japanese capital was pulled out, the global industry association will be very large body of  impact.

The United States has about 7,000 mold making supplier, 90% were less than 50 small businesses. Since the height of the development of industrialization, the United States die industry has become a mature high-tech industries in the world. The United States die steel production and supply have been standardized, universal application of die design and manufacturing CAD / CAE / CAM technology, processing technology, supporting the advanced inspection equipment, large-scale, complex, precision, long life, high performance tooling to advanced level of development. However, since since the 90s of last century the U.S. economy faces big adjustments post-industrial era, major changes, but also the face of strong international competition – from the cost pressures, time pressures and competitive pressures.

Germany has always produced excellent processing skills and precision machinery, tools, known for its mold industry, but also fully embodies this characteristic. The content of the complex for the mold industry, after years of practice and exploration, German mold manufacturers formed a consensus: that the industry must be coordinated and work together to tap the development potential, with a spirit of innovation, technological progress together, learn from each other, play well overall advantage in order to achieve industry success. In addition, to meet the current demand for rapid development of new products, not only the big companies in Germany, established a new development center, and many small and medium enterprises to do the same, take the initiative to do research and development work for clients. In research has always been very active in Germany, as its unbeaten in the international market an important foundation. In the fierce competition, the German mold industry for many years lived in the international market to maintain a strong position in the export rate has been stable at around 33%. According to the German workers, the mold industry organizations – the German Machinery Manufacturers Association (VDMA) Tool and Die Association statistics, Germany has about 5,000 mold companies, in 2003 the output value of the German Die 48 billion euros. One (VDMA) has 90 member enterprises die, this 90 key business value of the mold die output of Germany accounted for 90%.

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