OverMolding Service

OverMolding or Insert molding is the process of injection molding molten plastic around any substrate or insert.

Over-molding Services Provided

  • Plastic over Plastic Substrate
  • Rubber over Hard Substrate Overmolding
  • Plastic over Metal
  • Multi Color
  • Plastic Over Custom Inserts of any Material
  • Chemical Bond Overmolding
  • Plastic over Threaded Inserts
  • Mechanical Bond Overmolding
  • Medical Instrument Overmolding
  • Electronic Overmolding
  • Wire Overmolding

Custom Insert Overmolding.

The part below was created using a custom metal insert supplied by our customer.  Because of our skilled mold design engineers we were able to  mechanically place the insert into the mold and inject the plastic around it.

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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.


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


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.


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.


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.


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
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.


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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Production Calculators

The production calculators below will calculate the answers to 3 of the most common scheduling questions in an injection molding plant:

  1. What is the production rate?
  2. How long will it take to complete each production run?
  3. How much plastic material will be required for each production run?

Production Rate

Use this calculator to find the number of parts produced per hour given cycle time & number of cavities in the mould.

Reduce the cycle time by just 1 second to see how many more parts can be made per hour. The more parts made per hour the lower the part cost.

Calculation For Part Count Per hour
Cycle time (seconds):
Mold cavitation:
Parts per hour:

Production Time

Use this calculator to find the time required to complete a production run given the part quantity, cycle time & number of cavities in the injection mold.

Reduce the cycle time by a small amount to see the overall reduction in the production time.  For example, if a single cavity mould is required to produce 20,000 parts with a 19 second cycle time then the number of hours required is 105.6hours. However, if the cycle time is reduced to 18 seconds then the time is reduced to 100.0 hour. That’s more than 5 hours less which not only makes the production run cheaper per part but also makes the moulding machine available much sooner for the next production run.

Calculation For Number of Hours Required to Make x Parts
Quantity of parts required:
Cycle time (seconds):
Mold cavitation:
Hours required:


Mold Design Services

Click here to see a video example

Production Calculators for
Plastic Material Requirement

One of the most useful production calculators is a material requirement calculator.

Use this calculator to not only find out quantity of material required to produce a given number of parts  but also to work out material savings when comparing a hot runner mould to a cold runner mould.  For example, a 2 cavity cold runner mould making handles for buckets – the handle weight is 20 grams and the runner weight is 8 grams per shot. So for a 50,000 production run 1200kgs of material  is required.  However, if the same 2 cavity mold is converted to a hot runner then the amount of material required is 1000kg which is 200kgs less resulting in a saving of hundreds of dollars.

Although the cost of a hot runner mould is more than a cold runner mould this cost difference must be compared against the cost of managing the regrind (paying employees to feed cold runners into a grinding machine then blending it with virgin material). On top of this, the cost of a grinding machine itself along with the electricity cost must be included.

If the quantity of parts required are large enough then the extra cost of a hot runner will be justified. Even better, hot runners will generally have shorter cycle times than cold runner moulds meaning your production time will be lower.

Calculation For Plastic Material Requirement
Preferred unit of measurement:
Quantity of parts required:
Part weight (grams):
Mold cavitation:
Runner weight per shot (zero for hotrunner):
Material requirement (kg) :


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Estimating Cycle Time

Estimating cycle time in injection molding can be difficult because every injection molding company has a different way of doing things. Different levels of operator training and machine capability play a significant role in cycle time.

Material selection and part design also have an influence.


Part Design

The wall thickness of a part will directly affect the cycle time; thicker walls create longer cycle times while thinner walls create shorter cycles that’s why thin wall molding has cycle times in the 2 to 5 seconds range.  Click here to learn more about thin wall injection molding. The fact is, the part designer needs to make the wall thickness as thin and uniform as possible while still maintaining adequate strength in line with the application.

The height of a part also effects cycle time. The higher the part the further the machine platen has to open and the more time required to eject it off the mould core.

Mold Design

The cooling system within a mould is a heat exchange system.  There must be a sufficient number of cooling channels and sufficient flow rate of cooling fluid (usually water) to be able to extract the heat energy out of the plastic part within a specified time – this is called the cooling time of the moulding process and this directly effects cycle time. Different injection mold designers will design cooling systems that have different capabilities. This means cycle times will be different when using different designers.

Another aspect of mould design that will directly effect cycle time is the ejector system.  The ejector system should be rigid enough to remove the part off the mould core without delay.  Slow moving ejectors will only add to cycle time unnecessarily.  What’s more, it is important to know when to use ejector pins and when to use a stripper plate for ejection as there is a big difference in the cost of the mould.

The cycle time calculator does not take into consideration hydraulically or pneumatically operated moving slides and cores. This method will add considerable time to mould ejection.

Also, the use of compressed air in many applications is vital to achieving quick cycle times because this allows the vacuum between the part and the core to be removed before the ejector pins (or stripper plate) take over.  Without air, ejection is slower and will likely cause quality issues such as part distortion.

Machine Capability

A moulding machine will affect the cycle time in a number of ways:

  • New technology machines are faster than old technology machines.
  • Machines that have been well maintained will be faster and more reliable than machines that have not been maintained.
  • Smaller machines generally cycle faster than larger machines.
  • Machines with toggle clamps will give shorter cycle times than machines with hydraulic clamping units because platen speed will be faster.
  • All-Electric machines have the fastest platen speeds so cycle time will be the shortest (everything else being equal)

Temperature Controller

The temperature controller (chiller) needs to be able to extract the heat energy from the cooling fluid (usually water) fast enough to maintain mould temperature for the required cycle time.

Adequate flow rate is also essential so the pump must have sufficient power. Centralized systems can be inadequate especially when additional moulding machines are installed without regard to temperature controller capacity.

Technician Capability

Ask 5 different technicians to set the same mould and you will probably get five different cycle times.Different technicians have been trained differently and have different experiences and understandings so the chance of variation is huge.

Runner System

When estimating cycle time, the type of runner system will play a role. Hot runners and insulated runners (thick runners) are faster than cold runners (used in 2 plate and 3 plate moulds). This is because there isn’t a runner to eject between each cycle and the injection time is less since the flow length is shorter.

Whats more, hot runners and insulated runners are not affected by cooling time whereas cold runners, when too large, require more cooling time than the part needs.

Cycle Time Calculator

Use the following cycle time calculator for general purpose molding to get an estimation keeping in mind that the points above can have considerable influence. This calculator is to be used as a guide and the results are not guaranteed.

It accounts for parts with wall thicknesses between 0.8mm and 5.0mm  (0.032 and 0.196 inches).

Refer to https://www.improve-your-injection-molding.com/

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Plastic Dryers, Moisture Measurement And Part Quality

Plastic dryers are used to remove moisture (water) that has been absorbed either onto the surface of plastic pallets or into the plastic pallets internal structure before they go into the injection molding machine to make parts.

There are 2 groups of plastics. The first group will only hold water on its surface (such as polypropylene) while the second group will absorb it into its internal structure (such as PET polyester).

Drying is required to ensure good quality parts are made consistently. Plastics with excess moisture levels will react when processed in the molding machine barrel and will produce by-products that will effect such things as impact strength in the finished molded part.

Plastics That Need To Be Dried Before Processing:

1.       SAN

2.       PVC

3.       ABS

4.       PPO

5.       ACRYLIC

6.       ACETAL

7.       PPS



10.   PEI


12.   NYLON


Plastics  1 to 7 need to be dried for cosmetic reasons only. Excess moisture will cause bubbles, flow lines or surface defects in the molded part. However, their mechanical properties are not affected by moisture.

Excess moisture in the plastics 8 to 13 will affect the mechanical properties in the molded part. The part will have reduced impact and tensile strength but will not show any cosmetic defects.  This fact is very important to know so that moulders do not rely upon visual checks to ensure the quality of the molded part.

 Why It Is Important To Measure Moisture Level After Drying.

Processing plastic pallets in an injection molding machine that have not been dried to the required level can lead to disastrous consequences in the field.  Parts that look good are not necessarily strong enough to function properly. This means you are not just wasting time producing rejects but more importantly are likely to damage your company’s reputation as a quality supplier.

Just because the plastic material is dried at the recommended temperature and length of time doesn’t mean the material is dry enough to process. If the desiccant dryer has not been maintained properly then the plastic material may still have too much moisture and may need to be dried for a longer period of time.

That’s why it is critical that the moisture content is measured before processing. This should be done on a daily basis so that moisture can be eliminated as a reason for any part quality issues.

Moisture Measurement Methods.

There are 2 different systems of moisture measurement – massed based and sensor based instruments.

Sensor based (usually Karl Fischer) are the most accurate because they will only measure the moisture level in the pallets. However, the massed based systems will measure the moisture level along with other volatiles produced during the analysing process giving a false reading.

The appeal of massed based systems are their lower purchase price compared with sensor based and are easier to use. But these are not reasons to use this type of instruments because they do not give accurate and repeatable results.

Additional Comments

As an injection moulder you spend millions of dollars in equipment such as  molding machines, moulds and chillers so there is no excuse not to spend a bit more on quality plastic dryers and moisture measuring equipment.

What’s more, material cost is an injection moulders largest ongoing expense so it is critical that waste is eliminated.

Refer to https://www.improve-your-injection-molding.com/

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