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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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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Core Shift Analysis

Core shift is the spatial deviation in the position of the core from its original position in the injection mold before plastic is injected into the cavity. It is a frequent problem with long, slender, and not necessarily thin-walled products, such as vials, test tubes, pen barrels. It is also experienced often in molds for thin-walled containers.

Core shift can result in undesirable variations in wall thickness which will affect the final shape and mechanical performance of the part. The Core shift simulation provides detailed information about the movement of the mold core and its interaction with the polymer flow process as the plastic is being injected. Designers can use this information to correct for the core shift phenomenon, for example, by modifying the design of the part, or adjusting process conditions such as the gate location or core/mold temperatures.

There are three main causes of core shift:
Inaccuracies in the machining or setting of the injection mold which leads to alignment problems when the mold is closed.
Deflection of the platens or mold plates due to insufficient strength under the high injection pressures experienced during molding.

Deflection of the core as a result of pressure differentials on opposing mold walls. These differentials arise as a result of the gate location or variations in part thickness.

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Mold Heat Treatment and prevention of defects

1. Mold surface has soft spots

After heat treatment, surface mold soft spots, mold will affect the wear resistance and reduce die life.

(1) causes

1) die in the heat treatment before the surface oxide skin, pitting and local decarburization.

2) die quenching heating, cooling quenching medium the inappropriate choice of quenching impurities in the medium too much or aging.

(2) Preventive measures

1) The injection mold should be removed before heat treatment oxide skin, pitting, in the quenching adequate protection when heated mold surface, should maximize the use of a vacuum furnace, salt bath furnace and the protection of the atmosphere oven.

2) die quenching heated cooling, they should choose a suitable cooling medium, cooling medium to the long-term use should always be filtered, or periodic replacement. 2. Die poor organization before heat treatment

Mold the final ball of the organization coarse uneven, the ball of imperfect organization mesh, ribbon and chain carbon, which will die in quenched easy to crack, resulting in mold scrapped.

(1) causes

1) die steel materials, the original organizations, there are serious carbide segregation.

2) The forging process poor, such as forging heating temperature is too high, distortion is small, stop forging temperatures are high, and slow cooling rate after forging to forging the organization and a coarse mesh, ribbon, and chain-like carbon exists, so that the ball annealing it difficult to eliminate.

3) spheroidizing poor, such as the annealing temperature is too high or too low, such as temperature and shorter annealing time, annealing can cause the ball or the ball of the organization inequality bad.

(2) Preventive measures

1) The general conditions of work should be based on the mold, the production volume and material toughening its own performance, as we choose a good quality mold steel.

2) Improving the forging process or prepared using normalizing heat treatment, to eliminate the raw material in the mesh and chain carbide and carbide heterogeneity.

3) can not be forged high-carbon carbide segregation serious die steel can be refined solution heat treatment.

4) after the die forging billet formulate the correct spheroidizing specification, can be used quenched and tempered heat treatment and rapid absorption fine ball annealing work.

5) reasonable loading furnace to ensure that mold billet furnace temperature uniformity. 3. Mold produces quenching cracks

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Guide to Plastic Injection Molds Key Terms

Use plastic injection molds terminology with confidence
When it comes to plastic molds, if you’re talking to those involved in the industry, you had better know that a blush at the parting line refers not to classic romance movies, but to artifacts of injection molding on the finished product. More importantly, if you’re just starting out or if you’ll be hiring an injection molding company to manufacture your product, you’ll need to know the ins and outs of the process itself to have success with plastic injection molding.

Whether you’re an inventor and want to use injection molds for plastic to develop a product prototype, or you’re a manufacturer in the market for a production technology, get familiar with some basic plastic injection molds key terms to make the process easier. Learn the terms as you need them, or study plastic injection molding in-depth, depending on your relationship to the industry.

1. Talk plastic injection with ease when ordering prototypes.

2. Know enough about injection molds for plastic to avoid expensive errors and delayed production.

3. Use plastic injection molds key terms when buying or selling molding equipment.

Action Steps
The best contacts and resources to help you get it done

Order prototypes from plastic injection mold technology companies
You’ll need certain plastic injection mold information just to order a quote for prototype production. First of all, you need to know the number of cavities and their purposes within the product design. You will need to provide the molding company with information about whether the runner system will use hot or cold sprue, what kind of polymer you want used for production, and what type of finish — high-gloss, sandblasted or textured, for instance — you’ll want your prototype to exhibit.

Research plastic injection molding solutions to save money on production
As affordable plastic injection molds go, it may still be more cost-effective for your business to order molded parts from an experienced manufacturer. What you don’t know could cost you money, however. Your part design may not allow for a standard draft angle for removal from the mold, for instance, but if you adjust the design slightly to allow a small opening in the part, you can avoid serious retooling lead time and extra expense.

I recommend: View examples of plastic injection molding solutions and schematics through eFunda. Read up on computer-aided design (CAD) considerations and see a sample CAD rendering of a boss structure from Engineer’s Edge.
Find information on plastic injection molds when in the market for special orders
Traditional plastic injection molding companies may not always fit the bill; when you need rapid-fire production or have orders that require quick redesign, retooling and prototyping, you have to understand exactly what’s involved in the process to avoid extra charges. Also, understanding the technology will help you remain realistic as you search for a plastic injection molding solution to your current dilemma.
I recommend: Get in-depth information on rapid prototyping and test your understanding of plastic injection molding company terminology with 3-Dimensional Services Group. Or, learn more with “A Practical Guide to Prototyping” from Design Prototyping Technologies.

Tips & Tactics
Helpful advice for making the most of this Guide
• When ordering prototypes made through plastic injection molds, know what type of resin you’ll be using in production before you design the CAD model. The type of resin used will determine the thickness of your prototype’s walls, which should only be thin on smaller parts.

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Basic knowledge of plastic mold 2

injection molding is the most commonly used plastic processing method. The method is applicable to all parts of thermoplastics and thermosetting plastics, plastic products obtained large quantities unmatched by other molding methods, injection molding process as one of the main tools for plastic injection molds, precision in quality, manufacturing cycle and injection molding process in the level of production efficiency, etc., directly affect the product quality, yield, cost and product updates, but also determines the competition in the market responsiveness and speed.

injection mold by a number of steel plates with the various component parts, and basically divided into:
A molding device (die, punch)
B positioning device (guide post, guide sleeve)
C fixtures (the word board, code-mode pits)
D cooling system (carrying water hole)
E constant temperature system (heat pipe, heater)
F flow system (pump nozzle holes and flow channel grooves, flow hole)
G top of the system (thimble, top stick)

5, the system according to the different type of gating system can mold into three categories:
(1) large outlet die: the flow channel and gate at the parting line, with products in the mold with the mold release time to design the most simple, easy processing, low cost, so the more people using a large nozzle system operation.

(2) fine nozzle mold: flow channel and the gate is not the parting line, usually directly on the product, so much to design a set of outlet parting line, the design is more complex, more difficult process, generally used depending on the product requirements fine nozzle system.

(3) hot runner mold: This mold structure and fine nozzle substantially the same, the biggest difference is that the flow in one or more of a constant temperature of the hot runner plate and heat pump mouth, no cold material ejection, and the water flow port directly on the product, so the flow does not require stripping, this system is also known as non-outlet system can save raw materials for more expensive raw materials, products require a higher situation, design and processing difficulties, mold costs are higher.

Hot runner systems, also known as the hot runner system, mainly by the hot sprue bush, hot runner plate, temperature control electric box composition. The most common hot runner hot runner system has a single point and multi-point thermal gate two forms. Single hot runner gate is a single set of heat directly to the molten plastic injected into the cavity, it applies to a single gate single cavity plastic mold; more hot runner hot runner plate through the molten material to each branch sub-set of the hot runner and then go to the cavity, it is more suitable for feeding a single cavity or multi-cavity mold.

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Basic Knowledge of Plastic Mold

1,Introduction
Our everyday life and production used a variety of tools and products to the large to the machine base, the body shell, an embryo as small screws, buttons and various household appliances in the shell, all have a close relationship with the mold. The shape of the mold determines the shape of these products, the quality and precision mold processing will determine the quality of these products. Material for various products, appearance, specifications and use of different mold into the casting mold, forging mold, die casting, stamping die and other non-plastic mold and plastic mold.

In recent years, with the rapid development of plastics and general industrial and engineering plastics in the intensity and precision, continuous improvement, plastic products, also expanding range of applications, such as: household electrical appliances, instruments, construction equipment, automotive, Japan with hardware and other fields, the proportion of plastic products is rapidly increasing. A well-designed plastic parts can often replace the more traditional metal parts. Plastic household products, industrial products and the trend is increasing.

2, the general definition of the mold: in industrial production, mounted by a variety of presses and press the special tools, through pressure or non-metallic materials, the metal shape of the required parts or products, this special tool referred to as mold.

3, the injection molding process Description: Mold is a tool for the production of plastic products. It consists of several groups of components parts, the combination of the successful mold cavity. Injection, the injection mold clamping in injection molding machine, molten plastic is injected into the mold cavity forming and cooling in the cavity shape, and then separated from the upper and lower mold through the top of the system will be products from the top of the left mold cavity, the mold is closed again, the last the next injection, the injection molding process is conducted in cycles.

4, the general categories of mold: Mold can be divided into plastic and non plastic mold:
(1) Non-plastic molds are: die casting, forging die, die stamping, die casting molds.
A. Casting mold – faucets, cast iron platform
B. Forging Die – Auto Body
C. Stamping die – computer panel
D. Die casting mold – super alloys, cylinder

(2) Plastic mold production technology and production based on the different products are divided into:
A. Injection Molding – TV cabinet, keyboard buttons (the most common)
B. Blow mold – drink bottles
C. Compression Molding – bakelite switch, science bowl dish
D. Transfer Molding – IC products
E. Extrusion die – glue tubes, plastic bags
F. Hot Forming Die – transparent shell molding packaging
G. Rotary molding – soft toy doll

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What is Injection Mold Design

In the fast paced world of plastic molding, injection mold design is one of the most interesting and challenging jobs to be found. You will draw upon all of your creative abilities as you develop the design for new products.

It might look easy because of the powerful CAD programs, but in reality, they are just tools to help you. In the field of injection mold design you often must develop new and original methods of plastic molding. This sometimes requires a lot of creativity and inventiveness. What is a typical day like for injection mold design? Most injection mold designers follow a schedule similar to the mold makers. Because their mold designs are soon going to be manufactured by the mold makers, there is a very close relationship between these two.

A mold designer spends most of his time at his computer, using powerful CAD programs such as Unigraphics, AutoCAd, SolidWorks, MasterCam and many, many others. Nowadays, the programs are incredibly fast and powerful.

Very often the mold designer will be required to communicate with the various mold makers, CNC programmers, WEDM operators, etc.. This rapport is critical for a successful career as an injection mold designer.

Usually the designer does not work quite as many hours per week as the mold maker. Often mold designers have a shop background and help out in the mold making shop as well. This is especially common if there is a slowdown in design and a lot of work in the shop. How do you become an injection mold designer? Essentially, there are two paths in the USA. One is to learn on the job and the other is to learn at a design school. Both are common and work well.

Many plastic molding designers come from a mold making background. This is especially helpful to provide a realistic approach to mold design. There is no substitute for practical experience!

Several tech schools and universities offer excellent courses on plastic injection mold design. A background in mechanics, spatial relationships, ability to visualize 3D parts, and mathematics are all essential.

Is there a future in injection mold design?

Like everything else associated with the plastics industry, the answer is yes and no. Yes, because the plastics field is growing all the time and skilled designers are in high demand and low supply.

No, because the field is so competitive on a global scale. In this electronic age the designer does not even need to be in the same country as the mold maker. I had this experience at one shop; the designer was in Canada and we were in the USA. It worked well, but required considerable phone time on the part of the project manager. Conclusion Working in injection mold design is challenging, interesting, secure, well paying and in demand. Anyone interested will find many good courses available and companies seeking qualified designers.

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How Plastic is Molded

Plastic has, quite literally, become the cornerstone of our society. We make so many things from plastic that it is hard to imagine what our lives would be like if it was never invented. With so many of our everyday products being made of plastic, it is easy to understand why plastic injection molding is such a huge industry.

Approximately 30% of all plastic products are produced using an injection molding process. Of this 30%, a large amount of these products are produced by using custom injection molding technology. Six steps are involved in the injection molding process, after the prototype has been made and approved.

The first step to the injection molding process is the clamping of the injection mold. This clamping unit is one of three standard parts of the injection machine. They are the mold, the clamping unit and the injection unit. The clamp is what actually holds the mold while the melted plastic is being injected, the mold is held under pressure while the injected plastic is cooling.

Next is the actual injection of the melted plastic. The plastic usually begins this process as pellets that are put into a large hopper. The pellets are then fed to a cylinder; here they are heated until they become molten plastic that is easily forced into the mold. The plastic stays in the mold, where it is being clamped under pressure until it cools.

The next couple of steps consist of the dwelling phase, which is basically making sure that all of the cavities of the mold are filled with the melted plastic. After the dwelling phase, the cooling process begins and continues until the plastic becomes solid inside the form. Finally, the mold is opened and the newly formed plastic part is ejected from its mold. The part is cleaned of any extra plastic from the mold.

As with any process, there are advantages and disadvantages associated with plastic injection molding. The advantages outweigh the disadvantages for most companies; they include being able to keep up high levels of production, being able to replicate a high tolerance level in the products being produced, and lower costs for labor as the bulk of the work is done by machine. Plastic injection molding also has the added benefit of lower scrap costs because the mold is so precisely made.

However, the disadvantages can be a deal breaker for smaller companies that would like to utilize plastic injection molding as a way to produce parts. These disadvantages are, that they equipment needed is expensive, therefore, increasing operating costs.

Thankfully, for these smaller companies, there are businesses that specialize in custom plastic injection molding. They will make a mock up mold to the exact specifications, run it through the complete process and present the completed piece along with an estimate to complete the job to the customer.

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Mold Polishing Tips For Injection Molds

Mold polishing is primarily a manual process requiring a high level of skill and knowledge.  There are a number of finishes available to mould makers and the final finish depends upon the plastic part requirements.

Mirror polished spoon cavity

Types of Finishes in Mold Polishing


The most basic type of finish is a “stoned” finish which uses stone abrasives to get a matt finish. They range from 100 grit(roughest) to 1200 grit (finest). This is the cheapest & quickest form of polishing. Normally used to remove machining marks for the purpose of easy part ejection during molding.

Also, when a grit blasted finish is required, the surface must be stone finished first in order to get a quality grit blasted surface finish.

Then there are “paper finishes” which is essentially a fine abrasive attached to paper which is used after stoning to get  some level of gloss on the mold tool surface. The plastic part will also replicate this same level of gloss on its surface.

Finally there is the mirror polish. There are different levels of mirror polishing the highest level looks just like a glass mirror and is used for lens parts. This is the most expensive and time consuming type of finish to achieve and needs the first 2 stages (stoning and papering) to be completed before mirror polishing can begin. Mirror polishing requires the use of a buffing wheel & is often referred to as buffing.

Know The Minimum Type of Finish Required.


In order save costs and time a mould maker must know the minimum type of finish required in mold polishing.  The best way to do this is to find several sample parts of different finishes and get your customer to approve the minimum requirement. This sample can then be passed on to the mould polisher so he can replicate the required finish.

Also, knowing if the polished finish is for functional or cosmetic reasons helps in deciding the type of finish. A stoned finished is usually the minimum requirement for functional reasons (for easy ejection off the mold tool core). Mirror polished mold tool surfaces will more likely give part ejection difficulties especially for deep parts with little or no draft. This will result in longer cycle times and more part quality issues. So avoid mirror polishes if possible.

Polishing H13 Tool Steel versus 420 Stainless Tool Steel.


Different grades of tool steels will require a different approach when mirror polishing. For example: it takes more time to polish stainless steel than H13 steel. Before the mirror polish stage can be started on H13 steel a stoned finish of  600 grit is required whereas on stainless steel a stoned finish of 1000 grit is required which is 3 grades finer than 600 grit stone abrasive.  This takes more time.

In addition to this, at the start of the mirror polishing stage, stainless steel must begin with a coarser grade of diamond paste (18micron paste) compared to H13 (8micron paste). So this stage also takes more time so expect to pay a premium.

Orange Peel Effect


The orange peel finish sometimes seen on plastic parts is a result of poor polishing techniques. If this type of finish is not adequate for your parts then the mold tool surface needs to be repolished with a lot more care.

The Polishers Ability.


A person’s ability to polish is very important when it comes to the final quality. Different polishers have slightly different ways to polish which they have developed during years of practice.  The ultimate guide of a good polisher is one who can deliver the required finish on time at a reasonable price. Unfortunately, these types of polishers are becoming harder & harder to find.

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