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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Stress analysis

The Stress analysis program performs isotropic and orthotropic stress analysis of normal or fiber-reinforced thermoplastic materials. Stress analysis for injection molding predicts actual molding stiffness. It analyzes a product for possible structural defects or failure points when the product is exposed to a load.
It is used to identify structural related problems, typically with the strength, stiffness, and life expectancy of plastic products.

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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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What is Gas-assisted injection molding

Gas-Assisted Injection Molding is a process where an inert gas is introduced at pressure, into the polymer melt stream at the end of the polymer injection phase. The gas injection displaces the molten polymer core ahead of the gas, into the as yet unfilled sections of the mold, and compensates for the effects of volumetric shrinkage, thus completing the filling and packing phases of the cycle and producing a hollow part.

Traditionally, injection molded components have been designed with a relatively constant wall thickness throughout the component. This design guideline helps to avoid major flaws or defects such as sink marks and warpage. However, apart from the simplest of parts, it is impossible to design a component where all sections are of identical thickness. These variations in wall thickness result in different sections of the part packing differently, which in turn means that there will be differentials in shrinkage throughout the molding and that subsequently distortion and sinkage can often occur in these situations.

By coring out the melt center, gas injection molding enables the packing force (which compensates for differential shrinkage) to be transmitted directly to those areas of the molding which require attention. This dramatically reduces differentials in shrinkage and thus the sinkage. In addition, the internal stresses are kept to a minimum, considerably reducing any distortion that may otherwise have taken place.

Maximum clamp pressures are normally required during the packing phase of a molding cycle. This is due to the force which has to be exerted at the polymer gate in order to pack melt into the extremities of the mold cavity in an effort to compensate for the volumetric shrinkage of the solidifying melt. In comparison to compact injection molding, gas injection molding typically has considerably shorter distance over which the solidifying melt is required to be packed because of the gas core. This means that proportionally lower packing pressures are required to achieve the same results and in turn, lower machine clamp forces are required.

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What is Overmolding

Overmolding is an injection molding process where two materials are molded together. Types of overmolding include two shot sequential overmolding, multi-shot injection molding or insert overmolding.

Multi-shot injection molding injects multiple materials into the cavity during the same molding cycle. Insert overmolding uses a pre-molded insert placed into the mold before injecting the second material. Two shot sequential overmolding is where the molding machine injects the first material into a closed cavity, and then moves the mold or cores to create a second cavity, using the first component as an insert for the second shot using a different material.

Materials are usually chosen specifically to bond together, using the heat from the injection of the second material to form that bond. This avoids the use of adhesives or assembly of the completed part. It can result in a robust multi-material part with a high quality finish.

When designing an overmolded part, wall thicknesses of both the insert and the overmolded component should be as uniform as possible to ensure an even and robust bond. Avoid ribs and sharp corners to reduce flow problems.

Overmolded parts take longer to cool than single shot injection molded part, and cooling systems are less effective. The insert acts as an insulator and heat is less efficiently extracted from the part. However, optimising the cooling system can help reduce the cycle time.

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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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Procedures for control of injection pressure

1.4.3.2 procedures for control of injection pressure
The control of the injection pressure is usually divided into an injection pressure, the second injection pressure (holding pressure) or more than three times the injection pressure control. Switching time pressure if it is appropriate to prevent mold pressure is too high, to prevent overflow or lack of material are all very important. Mold packing stage the specific volume depends on the gate closed when the melt pressure and temperature. If the holding pressure time to switch to products from the cooling phase of the same pressure and temperature, then the specific volume of products will not change. Molding at a constant temperature to determine product size is the most important parameters of the packing pressure, affecting the product dimensional tolerances of the most important variable is the holding pressure and temperature. For example: after the end of the filling, packing pressure decreased immediately, when the surface to form a certain thickness, packing pressure to rise again, thus forming a low clamping force of large thick-walled products, elimination of collapse pits and flash.

Holding pressure and the speed is usually highest when the plastic cavity filling pressure and speed of 50% to 65%, which is holding pressure lower than the injection pressure of about 0.6 ~ 0.8MPa. As the packing pressure lower than the injection pressure, holding pressure in the considerable time, the pump load is low, solid pump life is extended, while oil pump motor power consumption is also reduced.

Three pressure injection filling both the smooth parts, and weld line does not appear, depression, flash and warpage. For thin-walled parts, small pieces of long, long process of molding large parts or even the cavity mold configuration is not balanced and not too close parts of the molding as a whole.

1.4.3.3 the amount injected into the mold cavity filling process control of plastic
Regulate the use of certain measures in advance so near the end of the injection stroke, the screw end is still a small amount of residual melt (buffer capacity), according to the mold filling was further applied injection pressure (injection pressure two or three times) , add little melt. This can prevent depression or adjustment products product shrinkage.

1.4.3.4 screw back pressure and speed of the process control
High back pressure can be intense shear melt at low speeds, will also get a long plastic tube in the machine plasticizing time. Therefore the current use of more back pressure and speed of parallel programming control. For example: measurement of the whole trip in the screw first high speed, low back pressure, and then switch to a lower speed, high back pressure, and then switch to high back pressure, low speed, and finally in the low back-pressure, low speed for plastics , so that the front screw melt most of the pressure be released, reducing the rotational inertia of the screw, thereby improving the accuracy of measurement of the screw. Often result in excessive back pressure increases the degree of stain color; pre-plastic bodies in increased mechanical wear screw and barrel; pre-plastic periodic extension of decline in production efficiency; nozzle prone to drooling, regeneration feeding amount increased; even with self-locking nozzle, if the back pressure higher than the design of the spring closure pressure may also cause fatigue failure. So, back pressure must be properly adjusted.
As technology advances, the small computer into the injection molding making machine control system, using a computer to control the injection molding process has become possible. Japan Steel Works N-PACS (micro-computer control system) can do four feedback control (holding pressure adjustment, molded adjustment, automatic measurement adjustment, resin temperature adjustment) and four process control (injection speed process control, pressure inspection, screw speed process control, process control back pressure).

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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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