Wall Thickness

  • Maintain a uniform wall thickness throughout your parts design
  • Thick wall design is prone to warp or other cosmetic issues
  • 10% Increase in wall thickness will provide 33% more stiffness with most materials.
  • Thick wall areas can sink, warp or contain voids resulting in undesired defects.
  • Using ribs can help to reduce thick wall sections while still giving the part strength.

Check with the material supplier before designing your part.

General Material Wall Thickness

  ABS 0.045 – 0.140
  Acetal 0.030 – 0.120
  Acrylic 0.025 – 0.500
  Liquid crystal polymer 0.030 – 0.120
  Long-fiber reinforced plastics 0.075 – 1.000
  Nylon 0.030 – 0.115
  Polycarbonate 0.040 – 0.150
  Polyester 0.025 – 0.125
  Polyethylene 0.030 – 0.200
  Polyphenylene sulfide 0.020 – 0.180
  Polypropylene 0.025 – 0.150
  Polystyrene 0.035 – 0.150
  Polyurethane 0.080 – 0.750

 

Need More Support?

At Xcentric our aim is to give the design engineer all the tools needed to make a fast educated decision.  That is why we have assigned a technical team to all of our accounts.  Furthermore, we believe that the fastest way to market is by preventing issues early on in the process.  Here are some key points that set us apart from other injection molding companies.

  • Online quote system – Our Online quote system gives our customers instant access to a technical team.  Your team is made up of a tool engineer and a sales rep.  Once a quote is submitted online, you will have a response within 24 hours.  In addition to that, your quote will be managed through our customer portal.
  • Customer Portal – Our online customer portal gives you 24-7 access to Xcentric from anywhere in the world.  In the portal you can;
    • Submit quotes
    • View and interact with a live interactive quote.
    • Purchase Tooling and Parts.
    • Instantly Re-order parts.
    • View history details such as invoices and Purchase Orders
    • Initiate Engineer Changes
    • Organize Parts by type.
    • Update and manage Account Information
    • And many more.  We are always adding features to support our customers needs
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Draft for Injection Molding

Also known as Angles or tapers on walls of plastic part features.  Draft angles are one of the most important design guides for injection molding of plastic parts.

Draft

Parts without draft can still be molded but they can and will have issues during  ejection from the mold.  As the plastic cools it shrinks around the mold core causing an enormous amount of friction.

Pin Push

pin push

While overcoming this friction the ejector pins push into the plastic resulting in pin push.  This results in undesirable marks and distortion of the plastic part.

Drag Marks

drag marks

Drag marks are caused by the plastic adhering to light scratches or textures in the mold side walls.  Thus, when the part is ejected the plastic peels out of these light scratches or texture causing drag marks.  When drag marks are present then the parts are often distorted whether  pin push is evident or not.

Minimum Draft

Regardless of how smooth the surface finish is, it is never a good idea to design a part for injection molding without draft.  There are no minimum draft requirements as each part has different features.  However as a rule of thumb a part that does not have in mold texture should have a minimum of 1 degree draft on side walls.

When adding texture to a cavity of the mold it is a good idea to find out the manufacture spec for minimum draft requirements before starting your part design.  This will eliminate the need to redesign your part as texture usually have a large draft angle requirement, usually 3 degrees and up.

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Injection Molding Process

The plastic injection molding process is a manufacturing method for producing custom plastic parts.  The service takes place at injection molding companies often referred to as a custom molder.  Before the process begins, an experienced mold maker must construct a mold or tool in order to produce a part.  The construction of the mold will include 2 halves and contain all the geometry and features that make up the part specifications.

Once the mold is constructed, it is then loaded into an injection molding machine where the process begins.  A hopper is use to hold and feed plastic virgin material into the barrel of the machine, where it becomes molten.  From here, a reciprocating screw will continue to feed and mix the proper amount of material.  After this, the screw will ram inject the material into the mold cavity.  Once the material enters the mold, it begins to cool and harden to conform to the geometry of the mold.  After the material cools, it is safe to remove the part from the mold, and this completes the cycle.  Plastic Injection Molding Machine

The injection molding cycle is as follows;

  1. Material Enters Barrel
  2. Material melts and mixes
  3. Volume of material (Shot sizes in barrel is created)
  4. Mold closes
  5. Injection of the plastic into the mold cavity
  6. Molten material cooled (during this process steps 1-3 are preparing for next cycle)
  7. Mold Opens
  8. Part Ejects
  9. Jump to step 4

Calculating an injection molding cycle is as follows;

Cycle = Mo+Mc+I+C

Mc = Time to close the mold (this is the time it takes to actually close the tool)

I = Time to inject material into the mold

C = Cooling Time (Time to solidify molten material)

To = Time to open a mold and eject the part (these can overlap and together make up total open time)

 

Design Characteristics of Plastic Injection Molded Parts

Custom components for the molding process, should be designed and engineered by an experienced industrial designer or engineer.  Producing a dimensional and stable part requires many factors to be considered.  Failure to follow the design guidelines for injection molding can end up with undesirable results.  Many factors to consider are as follows;

  • Material Selection
  • Shrink Rate
  • Draft
  • Ribs
  • Bosses
  • Undercuts
  • Integrated Fasteners
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Mold quality materials

With the modern industrials development, the plastic products industry, agriculture and daily life, and other fields are widely used; quality requirements have become more sophisticated. Plastic products, the production and quality of mold design, advanced mold manufacturing equipment and reasonable processing, mold quality materials and modern equipment are molding quality plastic parts forming an important condition.

In the Injection plastic mould process, hot molten plastic is forced under pressure by a hydraulic ram into a closed mold.  The mold is cooled to freeze the plastic in the desired shape, and no chemical reaction takes place.

Plastic injection mould Process, including pressure plastic, blow, extrusion, etc., plastic injection mouldprocessing is the most commonly used methods, apply to all parts of thermoplastic and thermosetting plastics. With the injection mold industry, mold cavity mold and shape the increasingly complex, precision die increasingly high demands, the production cycle requirements become increasingly short.

Plastic injection mould process demands precise control of melt temperature, melt viscosity, injection speed, injection follow-up pressure, switch over point from speed to pressure, cycle time. It is found that different polymers have different characteristics and different limitations in processing. Shear rate and shear stress influence melt temperature, viscosity, density and flow behavior of polymer. Some polymers are hygroscopic, some polymer have limited thermally stable time which is different at different temperature. Such polymers have limited residence time. The changes in each parameter has its own influences on other parameters.

Plastic injection mould process includes a number of factors, some of them are important. They play a decisive role for the quality in the plastic injection mould processing. Such as freezing time and injection time; maximum injection speed; maximum injection pressure; injection power; plasticizing rate; etc.

And plastic injection mould machine application shaping products directly affect the efficiency of production, quality and cost.

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Plastic mold polishing process

Plastic mold polishing the basic procedures in order to obtain high-quality polishing, the most important thing is to have a
Plastic mold
Quality of Whetstone, sandpaper and diamond grinding tools and polishing ointment aids.The polishing process depends on the choice of pre-processed surface conditions, such asmachining, EDM, grinding, and so on.
Plastic mold polishing the general process is as follows:
1, fine polishing
Fine polishing using diamond polishing paste key. If the polishing cloth with a mixture ofdiamond abrasive wheel abrasive powder or paste for grinding, then grind the usual order of9��m (# 1800) ~ 6��m (# 3000) ~ 3��m (# 8000). 9��m diamond polishing paste and polishing cloth wheel is used to remove the # 1200 and # 1500 sandpaper grinding marks left by the hairy. Then use the sticky carpet and polishing the diamond abrasive paste, in order of1��m (# 14000) ~ 1/2��m (# 60000) ~ 1/4��m (# 100000). Accuracy in more than1��m (including 1��m) of the polishing process in the mold shop in a clean room can bepolished. If a more sophisticated finish is absolutely necessary for a clean space. Dust,smoke, dandruff and saliva foam are likely to scrap a few hours after work to get the high-precision polishing surface.
2, rough polishing
After milling, EDM, surface grinding and other processes can be selected after the 35 000-40 000 rpm speed rotation of the surface grinding machine polishing machine or ultrasonicpolishing. Commonly used methods are the use of diameter ��3mm, WA # 400 of the sparkwheel to remove the white layer. And then grinding by hand Whetstone, Whetstone strip pluskerosene as coolant or lubricant. The use of general order # 180 ~ # 240 ~ # 320 ~ # 400 ~ #600 ~ # 800 ~ # 1000. Many mold makers in order to save time and the choice of startingfrom # 400.
3, semi-fine polishing
Semi-fine sandpaper and polishing the main use of kerosene. Sandpaper numbers were: #400 ~ # 600 ~ # 800 ~ # 1000 ~ # 1200 ~ # 1500. # 1500 sandpaper actually only suitable forhardened tool steel (52HRC above) does not apply to pre-hardened steel, as this may result in pre-hardened steel surface burns.

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Why Molds are Expensive

Introduction:

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

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

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

Materials:

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

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

Molds:

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

Engineering and Design:

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

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

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

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

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

Manufacturing:

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

Conclusion:

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

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Simulate Your Way to a Better Mold

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

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

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

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

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

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

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

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

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Gate Location Analysis

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

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

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Flow Analysis

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

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

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Cool Analysis

Why we need optimize cooling ?

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

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

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

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