mold design

Plastic Injection Molding Design Guidelines in 2017

Designing Your Plastic Part

When designing parts for injection molding, the manufacturing process is an important consideration. Injection molding is a process in which solid thermoplastic resin pellets are melted, injected into a mold, and then cooled back to a solid state in a new form. During Boss both the injection and cooling stages of the manufacturing process, there are several factors that may affect the quality of the final product and the repeatability of the manufacturing process. Although it is not always possible to follow all recommendations, outlined on the following pages are some of the most fundamental guidelines when designing parts for injection molding.

Radii

An inside radius should be at least 50 percent of the nominal wall thickness. An outside radius should be the nominal wall thickness plus the inside radius (150 percent of nominal wall). Sharp corners at the base of bosses and ribs can be stress concentrators. The edge where a boss meets the nominal wall should be radiused to reduce the sharp corner without increasing the wall thickness enough that it creates a sink problem. The radius at the base of a boss should be ¼ of the nominal wall with a minimum radius of 0.015”.

 

 

Design Recommendations:

Wall Thickness

  • Maintain a wall thickness of less than 5mm because thick walls can lead to long cycle times and poor mechanical

properties.

  • Avoid large variations in wall thicknesses in order to simplify flow pattern and minimize variations in shrinkage that can lead to warpage.
  • Avoid abrupt changes in wall thickness, as this can create stress concentration areas that may reduce a part’s impact strength. Wall thickness changes should have transition zones that reduce the possibility of stress concentrations, sinks, voids, and warp.
  • Avoid gating near an area with a large variation in wall thickness because hesitation and race tracking can create non-uniform flow and shrinkage.

 

Ribs

  • Maximum rib thickness should be 0.5 to 0.75 of the nominal wall to avoid creating areas of sink.
  • To avoid thin sections of steel in your mold, the distance between ribs should be at least two and a half times the nominal wall thickness.
  • Ribs should have a draft angle of at least ½˚ per side in order to accommodate easier ejection from the mold.
  • Maximum rib height should be no greater than three times the nominal wall thickness in order to avoid large variations in wall thickness.
  • Balance ribs on both sides of the nominal wall to avoid non-uniform shrink that can lead to warpage.

 

Bosses

  • Stand-alone bosses should be designed following the design guidelines for ribs

(see more information under the “Ribs” section).

  • Use connecting ribs and/or supporting gussets if possible to stiffen structural parts. Connecting ribs should be 0.6 times the nominal wall thickness at their base to avoid sink.
  • To maintain uniform wall thickness, bosses should be cored to the bottom of the boss.

 

Threads

Plastic threads used for joining parts can be machined or molded-in

  • When designing molded-in threads, avoid feathered edges and include radiused roots in order to minimize stress concentrations and to keep the walls uniform.
  • Sharp edges can be stress concentrators in plastic parts. Thread designs should consider this.

 

Draft Angle

  • Design parts with a minimum of ½˚ per side draft in order to accommodate easier ejection from the mold.

 

Amorphous Versus Semi-Crystalline Materials:

In amorphous materials, molecules are randomly oriented and intertwined. Polymer molecules have no ordered structure. These materials have no identifiable “melting point” but progressively soften through a broad temperature range. Unfilled amorphous materials are typically isotropic, shrinking equally in the flow and transverse directions. Even fiber-filled amorphous materials typically have low shrink and good dimensional control.

An Overview & Future Development Trend of the Chinese Plastic Injection Molding Industry

Recent years, plastic injection industry have a huge change in the environment of consumption upgrade. product requirements are getting an increase. In the past, large-scale production of precision-less plastic industry, now also need to rely on the mold industry to improve the accuracy of product demand.

The production of plastic products starts with the design and development of plastic molds, which are a basic tool for manufacturing of plastic products. As a result, the mold precision has a direct influence on the structural integrity and dimensional accuracy of the end plastic products.

The production of all plastic products has to go through the several steps that include design & making of plastic molds, injection molding, and surface finishing, among which the precision mold features a complex structure and therefore imposes high requirements for surface quality and technical standards.

Since it is a technology-intensive as well as capital-intensive product, many technologies need to be involved in its production process, including high-speed machining, super-finishing, rapid prototyping, automatic control, processing and applications of polymeric materials, etc., covering the areas of machinery, metal materials, polymeric materials, electronics & electrics and automatic control, imposing high comprehensive requirements on technologies.

 

1.Overview of the Plastic Industry

Plastics are malleable (seldom non-malleable) materials that are consisting of synthetic or natural high molecular compounds. They are produced under certain temperature and pressure conditions, assisted with additives of plasticizer, filler, lubricant, injection agent and more.

 

Plastics are widely used in modern society thanks to its lightweight, high plasticity, low production cost and diversified functions. Currently, as one of the four basic materials that support the development of modern society (the other three are steel, wood, and cement), the high molecular compound material has become a new material that is indispensable to human life.

Its application covers the many areas of information, energy, industry, agriculture, and transportation, etc. In recent years, the plastic manufacturing industry of China has witnessed a rapid development with an ever-increasing scale – it is now one of the essential parts of the Chinese economy.

 

  • In 2013, production of Chinese plastic products totaled 61.886,6 million tons, representing a year-on-year increase of 8.02%;
  • As of December 2014, the number of above-scale plastic manufacturing enterprises reached 14,062;
  • Prime operating revenue hit 2,039.239 billion CNY, representing a year-on-year increase of 8.92%;
  • Total profit reached 118.286 billion CNY, a year-on-year increase of 4.24%;
  • Export delivery value reached 226.066 billion CNY, a year-on-year increase of 2.82%.

Therefore, it is obvious that the production scale of the Chinese plastic manufacturing industry is being continuously expanded and production is increasing year by year, though year-on-year increase rate slows down a little bit over the past two years.

 

If viewed from the different segments of the plastic manufacturing industry, the most widely used plastic products are plastic pipes & fittings, plastic wires, and plastic weaving products, followed by plastic films and plastic packaging containers, while household plastic products and plastic parts rank third.

Among them, the prime operating revenue and export delivery value of plastic parts has reflected a rising trend over the past five years. The increase rate was particularly high during the 2010-2011 period, with prime operating revenue increasing from 237.282 billion CNY to 709.559 billion CNY,

representing an increase of nearly 200%, while export delivery value increased from 63.579 billion CNY to 191.426 billion CNY;

from 2012 to 2014, though increase rate slowed down slightly, the prime operating revenue was still on the rise, which hit 939.413 billion CNY as of 2014.

 

2. Overview of Mold Manufacturing Industry

As an intermediate product used for the production of the end product, molds are basic processing equipment that is machined by using a wide range of materials, such as plastic, rubber, metal, powder, glass, and FRP, etc.

Mold processing is one of the most important ways of material molding. Compared with machining, its benefits include fewer process steps, higher material utilization ratio, less energy consumption, ease of production, higher profitability and more. Thus, it is widely used in the manufacturing of automobile, energy, machinery, electronics, information, aviation, as well as household commodities. According to statistics, 75% parts of the rough processing industry and 50% parts of the precision processing industry are molded, while 80% parts of the home appliance industry and over 70% parts of the ME industry are also mold-processed.

As the mold industry is helpful for driving the development of other related industries, it is often dubbed the profit amplifier.

injection molding market

image: injection molding market

Though the mold industry of China had an early start, it was kept as an affiliate of downstream production-oriented enterprises, which restricted the development of molding enterprises, and molds were not listed in the ME product catalog until the year of 1987. At that time, there were approx.

6,000 mold factories in China, which yielded a total production value of 3 billion CNY. After more than two decades of development, the Chinese mold industry has made a remarkable progress. As of 2013, the number of mold companies had reached 30,000, with a total output value of approx. 220 billion CNY and an annual export value of nearly 5 billion CNY.

Currently, the largest markets of Chinese molding lie in the following industries of automobile, IT, home appliance & office equipment, machinery and building materials. Along with the rapid growth of national economy and improvement of people’s income level, there is a growing demand for automobiles, consumer electronics, and home appliances, allowing these industries to embrace a speedy development phase in recent years. This constitutes one of the most important reasons for the rapid development of China’s mold industry.

The following diagram shows the 2007 – 2013 mold production value of China. We can see that the production value of China’s mold industry has witnessed a rapid growth in recent years, increasing from 87 billion CNY in 2007 to 210.6 billion CNY in 2013, with an increased rate much higher than the overall level of the world mold industry.

 

3. Development Trend of the Mold & Plastic Industry

Along with the gradual industrial transformation and upgrade, as well as the increasing proportion of high-end products and continuous improvement of basic supporting services, there’s still much value in plastic production. Though it is witnessing a slower increase rate of production, the output value is still increasing at a relatively fast pace.

We can tell that the plastic processing industry still shows a sign of rising. As one of the indicators to measure the development level of a country’s plastic industry, the ratio between plastic and metal is only 30:70 in China, falling far behind the world average (50:50) and the developed countries like Germany (63:37) and America (70:30). Hence, with technological advancement and consumption upgrade, the plastic industry of China will maintain a steady development pace.

 

The mold industry which forms a basic industry in the national economy, is specifically involved in machinery, automobile, optical industry, electronics, chemistry, metallurgy, building materials and more industries, representing a very wide and comprehensive scope of application. China’s industry is developing rapidly, among which, the development of automotive manufacturing, IT and the medical device offers a great development opportunity for the entire mold industry.

 

Currently, China is experiencing an industrial restructuring, with national objectives for emerging industries planned as: by the year of 2020, the increased value of strategic emerging industries is aimed to account for about 15% of GDP. To achieve this 15% target, among all the seven strategic emerging industries, as the pillar of national economy, the high-end equipment manufacturing industry will be vigorously developed, so the mold industry which is dubbed “the mother of modern industry”, will definitely benefit from the development of high-end equipment manufacturing.

injectionmolding company

International Mold Supply Chain Further Leans Towards China Mold Companies

When compared to the past, the technological level of the Chinese mold industry has been greatly improved, but the proportions of high, medium and low-end molds produced by Chinese manufacturers are extremely imbalanced, which severely hampers the development of the Chinese mold industry.

Over the past few years, tremendous changes have taken place in the industrial structure as well as the system of the Chinese mold industry, which are mainly represented by: medium- & high-level molds, large sizes, precision, complexity and long lifecycle, but the self-sufficiency rate of medium- & high-end molds falls below 60%, due to the fact that China has an enormous demand for medium- and low-level molds. It is easy to see the irrationality lying in this situation.

The industry has pointed out that the proportions of high, medium and low-level molds produced by Chinese manufacturers are extremely imbalanced, which severely hampers the development of the Chinese mold industry. The pace of restructuring needs to be quickened urgently with emphasis on the development of the high-end market segment, so as to keep pace with the international market.

How to keep pace with the international market? The mold industry in China needs to speed up restructuring, while at the same time making efforts to improve technological level.

It is mainly reflected in the following aspects: i. Restraints, such as mold steel; ii. The level of standardization needs further improvement; iii. Top-level mold talents need to be cultivated desperately; iv. Quicken the pace of mold product restructuring; v. Increase investment to enhance innovation capability; vi. Promote consolidation and restructuring among mold enterprises; vii. Further, deepen the exploration of the overseas market.

The downstream suppliers, especially the automotive industry, have a high degree of dependence on importation when the key and core molds are concerned. As a result, a host of key and core injection molded parts that OEM products need are also supplied by these world-renowned mold enterprises.

internation supply chain

image:international supply chain (click to view the full image)

In recent years, Chinese mold companies have continuously improved mold technologies and products. Therefore, some plastic molds and injection molded parts start to be distributed internationally, have successfully made it to the supply chain system of some high-end industries and even replaced some imported products, dependence on importation is thus eliminated. However, it is undeniable that, the companies that have got into the international high-end market are few in number.

In the Chinese mold industry, along with the improvement of technological level and gradual increase of product lines, some international OEM industries are now further leaning towards Chinese companies, which presents not only an opportunity but also a challenge. It is believed that by 2015, the mold self-sufficiency rate in the Chinese market will reach 85%, among which the self-sufficiency rate of high-end molds will witness a remarkable increase.

According to experts, in addition to continuous improvement of productivity, the Chinese mold industry needs to pay more attention to internal restructuring, as well as technological advancement. The focus is mainly on realization of a more specialized enterprise structure; a product structure that is oriented towards higher end molds; improvement towards the import & export structure; molding analysis & structural improvement of medium- & high-end automotive panels; application of multifunctional composite mold, combined machining technology and laser technology in design and manufacturing of molds; as well as the development orientation towards high-speed cutting, ultra-precision processing, polishing technology and informationization, etc.

With the thorough transformation of the machine tool industry, the Chinese mold industry is experiencing technological innovation and product upgrade, while also proactively exploring the international market. In the competition with international competitors, the Chinese mold industry is making the most of its strengths, in an attempt to secure a place in the international market. So, what advantages does Chinese mold industry have over the international mold industry?

Advantage

  1. Strong national policy support should not be ignored;
  2. Gradual improvement of infrastructure creates possibilities for the ever increasing market demand;
  3. Transfer of world mold industry center to China, as well as the wide application of e-commerce, will create extraordinary business opportunities for the Chinese mold industry;
  4. Compared with European and American countries, the low labor and material costs will make China the center of attention under the current circumstances that there’s an urgent need to bring down cost in the international market.

How to Choose the Right Plated Coating for Improved Mold Performance

Many mold makers and molders have had great success with one particular coating and have used it for all of their applications. Unfortunately, there is not a magical coating out there that works in every situation. Many times, excellent coatings are used in the wrong applications. Speaking from a player’s viewpoint, one of the major challenges is correcting problems caused by another plating company applying the wrong coating for a particular application. When plating is used properly, the positive results can be stunning.

There are many causes of wear and corrosion in plastic molds that can contribute to greatly reducing mold life. Plating can solve many of these situations. However, these coatings also can improve the performance of molds in many other ways. A basic knowledge of plating is necessary to make an educated choice for properly solving your particular problems. There are more than eight different plated coatings for molds. It is the intent of this article to help molders and mold makers take advantage of these benefits.

Types of Plating

 

Types of Plating

There are two basic types of plating – electrolytic and electroless. Electrolytic plating requires electricity to make the process occur. There is a positive and negative charge. The positive is called the anode and the negative is the cathode. The part to be plated gets the negative charge so it becomes the cathode. The anode is made of conductive metal – such as lead – and becomes the source of the positive charge. Hard chrome plating is one of the oldest electrolytic plating processes. The chromic acid solution is the medium by which the current transfers. When the plating process occurs, the negative and positive ions transfer in the solution causing a metal (chrome) to reduce the base metal of the part being plated. A good analogy would be to look at it as the reverse of EDM. Because the process uses electricity, the plater is constantly fighting against the laws of nature.

The old rule still holds true: electricity travels in straight lines and goes to the closest point. On sharp corners, there will be a heavy buildup of plating. In the recesses, the plating will tend to be thinner.

Electroless plating, such as electroless nickel, is just the opposite of electrolytic plating. Additives in the solution take the place of electricity. These additives are known as reducing agents. Since all metals have a charge, when the reducing agents detect the base metal charge in the plating bath, they start to react. This causes the metal in the bath solution – in this case, nickel – to reduce the base material of the part being plated. No electrical current is required. Wherever the plating solution touches the base material, the plating will adhere. This gives the plated part a very uniform deposit and the plating thickness can be controlled within .00005-.0001, even on complex shapes.

Within these two families, you have several types of deposits. These different types include composite and alloy deposits. As a generality, if you need a perfectly uniform deposit in a complex shape, electroless nickel deposits are best. However, hard chromium is the hardest deposit and has an excellent release.

How to Choose

When choosing to plate for your mold or mold components, you must first identify the problem to be solved or the problem to be prevented. The first question you should ask is “What is the cause of this problem? Is this problem the result of corrosion or wear?” If the component has previously been plated, this may be difficult to determine. Have your plating vendor look at the component to determine if it has been plated and if so, what type of plating was used.

Abrasive wear can occur on compression molds that use mineral or glass-filled materials. These materials can cause a scouring action on the mold surface. In transfer and injection molding of thermosetting materials, wear often is detected in the high flow areas such as in the sprues, runners, gates, and portions of the cavities and cores that are directly opposite the gates. In injection molds for thermoplastics, wear most commonly appears on the surface opposite the gate.

Most damage results from continuing to run the mold after flashing occurs. However, there are other sources of damage of which to be wary. These sources can include water contacting unplated surfaces, causing corrosion; water condensing in the molds; seepage through porous metals; and leaky pipe fittings and “O” rings. Where chillers are used for mold temperature control, condensation of moisture on the mold surfaces can sometimes occur even while they are in full operation. Careless handling of hoses and feed lines during hookup can leave water on the mold surface. Corrosion is progressive and even if the molds are stored after being sprayed with an antioxidant, a few drops of water or condensation can cause tremendous and costly damage.

Another source of damage attacks from acids. These acids may form after exposure to corrosive materials generated by thermoplastics decomposition (often due to overheating). Overheating can occur in the plasticizing cylinder, the hot runner system or in the mold cavities as the result of two small gates, or inadequate venting or cooling systems. During the molding of PVC, a small amount of hydrochloric acid is formed, which is extremely corrosive to the mold cavity.

Electroless nickel, by its very nature, is an excellent corrosion barrier for most mineral acids, whereas these acids will attack chrome. Stainless steels also can be susceptible to attack by chlorine or fluorine containing plastic, leading to pitting or stress corrosion cracking. This can be eliminated with a high phosphorous electroless nickel deposit over the stainless steel.

With so many different types of molding and even more types of materials available, there is no easy answer as to which coating will best enhance performance. An investigation into these coatings is worthwhile because if your molding operation is left unprotected, it can create corrosion, erosion, materials flow or release problem. The proper plating can make all the difference. Before you make that critical decision, call your plating vendor of choice and ask which coating will work best for your application.

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