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.

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.

sositarmold How To

How to Make Your Mold Shop Stronger

In March of 2001 the U.S. economy slid into a recession. By the time the powers that be decided to make the announcement in 2002 it was old news. Many organizations, including those in the moldmaking sector, had already taken a hard hit from the economic downturn, and were taking measures to survive long before it was made formal. It’s now almost 2004 and they are still in a survival mode.  There’s no need to announce that the economy is taking longer to recover than anticipated.

Today’s companies are facing issues and problems exacerbated by the economic downturn.

Downsizing, reorganization, restructuring, cost cuts, hiring freezes and other measures have been taken to stay in business in a tough economy. Managerial calls to employees to embrace change have sometimes been met with resistance and uncertainty. Low employee morale is prevalent. Although overall competition has always been tough, today competition from foreign outsourcing is fierce.   These are just some of many overwhelming challenges organizations must face. Most companies are trying to figure out what to do until things turn around. But who knows the shape of things to come or the duration of the change?
Although each company has its own issues and challenges, here’s what you can do now to help drive business growth:

Accept that times have changed and what has worked in the past may not work in the future. We must use change to provide the creative tension to keep us moving forward. Think of tough times as opportunities to tap into your creativity; get your organization running like a well-oiled machine and seek new ways of doing business.

Rethink your entire business strategy. Ask these questions:
Have we been complacent for too long?
Where might there be opportunities to expand our services?
What more can we offer?
How can we penetrate new markets?
How can we do more with less?

Take Advantage of Foreign Competition

Business is looking very interesting on an international basis for companies in the U.S., especially in certain emerging markets like Russia and specific markets in Latin America such as Brazil.  Also, the Chinese market continues to do quite well despite the fact that their currency is overvalued.

Speed and execution are the keys to taking advantage of these markets.  Companies in the moldmaking sector that respond to these emerging situations will be the most competitive. This will be especially true if growth accelerates and companies find themselves with inadequate capacity to meet demand. Smart companies are moving forward aggressively in terms of new markets, emerging growth sectors and product opportunities through strategic alliances on an international basis.

You must be receptive to seeking out innovative new products that will enhance your current product line and stimulate business growth.  Learn what new products and technology are available that can help to expand your business. You need to accept that finding products overseas can enhance your current product or add to your product line. Thoroughly explore the alternatives. Some companies have found critical new products and technologies from outside companies. If you are not receptive and highly conscious to outside new technology and overseas markets, you may quickly find yourself acting in a defensive position simply to survive.

Invest in New Technologies

Invest in technology that reduces your operational cost, improves quality and increases your contact with the customer. Pay attention to customer-relationship management regardless of the size of your company.  There are new technologies being created everyday, use this as the driver and the tool for business growth.

Maximize Your Business Strategies

Make this economic downturn a good time to re-examine your core business values. It helps to seek input from your employees by involving them on a day-to-day basis. Asking them for their extra effort allows you to crosstrain people in various positions. Make sure they know that they play a strong role in developing the company’s reputation and establishing customer loyalty. When things do turn around, you will have a competitive edge because your employees will be more supportive and understand the big picture to a greater degree.

Going back to basics means working harder than ever to sustain and develop strong business relationships. Talk with your customers about where they see their business in the future and identify how you can assist them in selling more of their products. No matter how often you’ve heard about the importance of customer service, it always warrants reinforcement. If you want to obtain customer loyalty and remain competitive, your level of service must far surpass your competition. If what you sell or offer is essentially the same as other competitors, differentiate your approach to marketing and sales. Be willing to take a risk. Tap into your creativity and think about what you can do to stand out in the minds of your customers. Innovation will make you sharper, better and bolder by definition.  By giving them the help that they need, you will create quality customer relationships that mean long-term profitability.

Companies that institute smart strategies and believe in their employees, products and services are able to introduce new products and differentiate themselves from their competitors by portraying a higher level of service in every aspect of the customer experience. This can gain a competitive edge until things turn around. But keep in mind, that when they do, business will be very different. Determine your future and its potential.

Moldfolow Analysis Will Help Avoiding Potential Problem in Mold

Over the past two to three years, substantial advancements have been made in injection molding CAEsoftware. What started out as a tool to give designers a general idea of how a simple plastic part will fill, can now accurately analyze packing, cooling, warpage, fiber orientation in any complex part geometry and conditions in the mold.

Identifying Problems in the Mold

The goals of flow analysis can range from basic knit-line location prediction to measuring the exact displacement due to anisotropic conditions on a low-tolerance part.  When performed early in the design process, users of the technology save thousands of dollars in startup costs, and thousands more by improving part quality, eliminating downtime and reducing cycle times and scrap rates. Flow simulation helps to perfect the part design by reducing or eliminating conditions that may lead to gas traps, burns, sink marks, voids or excessive warp. This is done by optimizing factors such as gate size and location, runner balancing in multicavity tools, mold design including inserts and cooling line circuitry, material selection and process conditions. The technology identifies problems in the mold before they become problems in the part, and the mold designer and moldmaker are the first line of defense in eliminating these costly tool and part problems.

mold-analytics

Fiber orientation in the weld line of a pressure vessel.

The simulation serves as the perfect medium for trial-and-error techniques that are very expensive and time-consuming to perform on the mold, and must be used early in the design process to gain the most ROI. The reputations of tool designers and builders depend on how well a tool performs when it is placed in the press for the first time, and profits are gained by not having to go back and recut the tool multiple times. The competition in today’s market requires vendors to reduce costs while improving quality. Flow analysis allows moldmakers to reduce the mold building cost by 10 to 30 percent, shave weeks off the delivery time and reduce piece-part cost; this is all while improving the quality of the end product for their customer. This competitive edge represents the difference between profit and loss since the days of building in the extra startup costs of time and tool recutting are over. Modern plastic products have extreme performance standards with very strict tolerances, often involving hybrid blends of materials with many additives and stabilizers that make it impossible to know exactly what the final molded part will look like. Without understanding the characteristics of these new materials upfront, the design criteria may be outside the physical scope of cut and try tooling, which in some cases requires a complete rebuild.

Midplane Analysis

The most significant breakthrough in flow simulation technology was the advent of true 3-D solid element analysis in 1999. Prior to this, the only way to perform a plastics flow analysis was using midplane technology based on the Hele-Shaw approximation. In a midplane analysis, often referred to as 2.5-D, the part model is represented by a shell of 2-D triangular mesh elements, which are then assigned an appropriate thickness. Similarly, runner systems and cooling lines are modeled with 1-D beam elements. Since each element represents conditions through its entire thickness, many assumptions are made within the predictive software code, which may or may not skew the final results. Extracting a midplane mesh is a time-consuming, arduous and ambiguous process that can take several days, in some cases accounting for up to 80 percent of the man-hours that go into a given flow analysis project. While this approach works well for simple part geometries with uniformly thin walls, it does not capture the true phenomena occurring in the runner system and mold base. Significant accuracy can be lost on parts with a moderate to high level of detail, variable wall thickness and/or thick and bulky areas.

mold-flow

Clipped view of temperature distribution throughout the mold.

Bulky parts with varying wall thickness cannot be accurately represented with midplane technology and require a more advanced solution. In a true 3-D simulation, solid mesh elements, predominantly tetrahedral, fill the entire volume of the part geometry, without the modifications and assumptions associated with a midplane. This results in a much better representation of the original part file, and therefore much more accurate simulation results. The 3-D meshing process is highly automated allowing users to create a solid element model in a fraction of the time spent generating a midplane. However, 3-D models have far more elements than midplane models and require longer solve-times and more computer hardware. This is a small price to pay, considering that 3-D analyses can run on an unmanned PC overnight and offer significantly better results.

The Fountain Effect

Users can observe the true volumetric fountain effect of the melt front, including jetting and gravitational effects. With several element layers per thickness, gradients of results such as temperature, pressure, volumetric shrinkage and fiber orientation can be seen across all the thicknesses. When modeling the true mold base geometry with solid elements, moldmakers can incorporate multiple insert materials, such as beryllium copper, into the analyses. By reviewing outputs such as temperature distribution, cycle-average heat flux and part warpage, users can evaluate the effectiveness of these inserts prior to building the tool.

melt-front-advancement

Melt front advancement.

Saving Costs

In addition to identifying and eliminating potential problems, flow analysis helps to optimize overall part and mold design while preventing overengineering. This occurs when too much effort is put into part/mold design. The design often ends up being more complicated, using more plastic than necessary. Flow analysis can prevent unnecessary overengineering by proving that a simpler part/mold design is sufficient. Today’s technology allows the moldmaker to evaluate different water layout designs to reduce flow rates and optimize temperature distribution throughout the part and mold, ultimately minimizing differential shrinkage, warpage and cycle time. Different water layout designs refer to the cooling line circuitry in a mold. Without running a cooling analysis first, moldmakers often have to redesign and modify the cooling lines after they prove to be ineffective in the machine.

What effects will changing the tool steel have on the part?  Does it need a hot runner system or will a three-plate mold be sufficient?  By running a flow analysis upfront, these types of questions can be answered, saving thousands of dollars in unnecessary tooling costs.