The Ultimate Injection Molding Design Guide

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    The Ultimate Injection Molding Design Guide

    Making quality products comes from a proper understanding of the processes involved in making such a product. In rapid prototyping, this statement is true of all the different processes it encompasses e.g., CNC machining, injection molding, 3D printing, etc. Of these previously mentioned processes, injection molding is one with a unique mechanism. Consequently, this necessitates a good understanding of an injection molding design guide.

    injection molding design

    This article will act as an ultimate injection molding design guide where you can learn 99% of all you need to know about injection molding. It will introduce what injection molding is and how it works, the different materials you can use, plastic mold design, gate design, and defects in injection molding design. By going through half of this injection molding design guide, you will get to know a lot about injection molding than you do before you clicked that link – and that is even half. Read on!!!

    What is Injection Molding?

    what is injection molding?

    Calling this article, the ultimate injection molding design guide would be wrong without a short introduction to the process. Injection molding is a rapid prototyping process commonly used in the mass production of products. It involves the production of products by the injection of the molten form of the selected materials into an already made mold. On cooling and solidification, the materials take the shape of the predesigned mold. The injection molding cycle includes:

    · Fill time

    The filling time corresponding to the minimum injection pressure is required to fill the mold.

    · Clamping

    The two halves of the injection molding unit are closed by a clamping unit before the mold is injected with material

    · Injection

    The resin is fed into pellet forms by a hopper into the injection molding machine. The time for the injection process is estimated by other factors like power and shot volume, injection pressure, and machine type.

    · Cooling

    The plastic inside the mold is cooled down after it is molded. As the plastic cools down, it takes the desired shape.

    · Ejection

    This is the last stage of the injection molding cycle, where the final part is pushed out of the mold.

    Materials Compatible with Injection Molding

    materials compatible with injection molding

    Aside from “injection molding design,” another common phrase related to injection molding is “plastic mold design.” It makes you wonder about the different materials you can incorporate into your injection molding plan. Is plastic the only material compatible with injection molding? No, there are many other materials compatible with injection molding. For example, metals, glasses, elastomers, etc., are compatible. However, plastics are the most common materials used in rapid prototyping.

    What then should you look out for? The first is molten-ability. However, below is an exquisite introduction and illumination on plastic materials used in injection molding.

    What is Plastic?

    Plastic is a polymer material comprising of resin as the main component. It may be with or without adding other compounding materials (auxiliaries), and you can mold it under heat and pressure. The basic raw materials for plastics are carbon and hydrogen compounds of low molecular weight. The main sources of synthetic plastics include natural materials such as crude oil, natural gas, coal, salt, etc.

    Classification Of Plastic Materials

    There are many plastic materials you can consider in your injection molding design. However, choosing one comes with understanding their different categories. Below are the classifications of plastics ideal for plastic mold design.

    Classification based on Basic Performance  

    This category of plastics you can factor into your injection molding design is based on their basic performance. Below are the categories and the respective plastic types you can use.

    • General Purpose Plastics
    general purpose plastics

    Plastic materials: Polyethylene (PE), Polypropylene (PP), Polystyrene (PS), Polyvinyl Chloride (PVC), ABS, etc.

    Features: This category of plastics have a large output, low price, and are usually high on general performance

    • Engineering Plastics
    engineering plastics

    Plastic materials:Polyamide (PA), Polycarbonate (PC), Polyoxymethylene, Polyphenylene Ether

    Features: It has excellent mechanical properties, electrical properties, chemical properties, heat resistance, wear-resistance and dimensional stability. Production of engineering plastics is relatively small and so, the price is high.

    • Functional Plastic

    Plastic materials: Medical plastic, Photosensitive plastic

    Features: This type of plastic has special functions.

    Classification based on Heat

    This category of plastics that you can factor into your injection molding design is based on their heat performance. Below are the categories and the respective plastic types you can use.

    • Thermoset Plastic

    In this type of plastic, there is irreversible hardening of a soft solid or viscous resin by curing over a specified temperature range. Such plastics comprise of material (resin) obtained by the polymer reaction and do not cause chemical cross-linking when heated. Thus, it is still malleable even when you heat it again.

    These types of plastics include polyethylene, polypropylene, polystyrene, polycarbonate, etc., and they are an important part of many injections molding designs.

    • Thermoplastic

    It is a plastic that can be pliable when heated above a specific temperature and hardens when it cools. This type of plastic comes from a resin obtained by a polycondensation reaction, which is a chemical compound that undergoes a chemical change upon heating and this causes the conversion of a linear molecular structure resin into a bulk structure of a polymer compound. When heated again, it no longer has plasticity.

    This category includes plastics such as Phenolic plastics and Aminoplast and they are also an important part of any injection molding design.

    Classification Of Plastic Materials According to Molecular Heating Characteristics

    Classification can also be based on the molecular heating characteristics of the plastic. This is an important type of plastic material to consider in injection molding design. Below are the classifications underneath this category.

    • Advanced Engineering Plastics: FluoroResin, Teflon, Tefzel, PSU, PES, PPSU, etc. 
    • Engineering plastics: It includes Polycarbonate, PPO, etc.
    • General Purpose Plastic: It includes ABS, PVC, PMMA, Polystyrene, SAN, etc.
    • Advanced Engineering Plastics: It includes Polyurethane, PTFE, Liquid-crystal Polymer (LCP)- Zenite, High-temperature Nylon (HTN)- Zytel, etc.
    • Engineering Plastics: It includes PA nylon (Zytel & Minlon), polyformaldehyde (POM) (Delrin), PBT (crastin & rynite), PET, TPE (Hytrel- Polyester Elastomer) etc.
    • General Plastic: It includes PP-GF, PE-UHMW, PP, PE-LD, PE-HD, etc.

    The Difference Between Amorphous Plastics and Crystalline Plastics

    Amorphous and crystalline plastics are an important part of any plastic molding design setup. They are the two major types of plastics used in the injection molding process. Below are the characteristics of the two types of plastics.

    the difference between amorphous plastics and crystalline plastics

    Injection Molding Design Guidelines That You Can Use in Your Project

    Getting the best plastic mold design or injection molding design comes with understanding the process involves. However, it also gets better with some tips. Below are some injection molding guidelines that you can use in your project.

    · The Importance of Uniform Wall Thickness in Injection Molded Parts

    With wall thickness changes, there can be a sudden change in direction and flow rate of molding which can cause defects like sink marks and flow lines. If the molding process is done with the wrong gate, it is almost impossible to correct the resulting consequences during the forming process. Also, regardless of the position of the mold gate, it is possible to cause poor molding.

    The design of the injection molded parts should be properly considered as rounded corners in the design of the mold ensures even wall thickness that improves the flow of the melt around a corner. It also increases the strength of the molded part.

    Plastic parts should have high consideration for uniform wall thickness when designing injection mold parts. Ribs are also ideal for improving the wall thickness of plastic parts and their durability as well. The uniform wall thickness of a mold vastly affects its structural design.

    · Mold strength 

    You should also consider the mold strength in injection molding designs of plastic structure designs with elongated slots. Designs with elongated round holes offer better strength than plastic structure designs with sharp corners.

    · Simplification of the mold design

    You should consider in injection molding design that simplification of mold design is important. Therefore, by reducing the mold movement using simplified mold designs, side mold cored plastic structure designs or plastic structure designs with sharp corners are easy to produce.

    · Reasonable design of Ribs

    reasonable design of ribs

    In injection molding design, the cross structure is the best because it can handle many different load arrangement changes. Therefore, a properly designed cross structure that can withstand the expected stress ensures a uniform distribution of stress across the article. The nodes formed at the intersection represent the accumulation of material. However, you can hollow out the center of the node to prevent problems. You must note that material accumulation does not occur where the intersection intersects the edges of the component.

    Types of Injection Molds

    types of injection molds

    Another important aspect this injection molding guide will discuss is the different types of injection mold you have. Below is an extensive discussion on the various types of a plastic injection mold that you can use.

    Single Cavity Injection Mold

    single cavity injection mold

    A single cavity injection mold has only one cavity. Therefore, it can only produce a single component per injection cycle. You should incorporate the single cavity injection mold into your plastic molding guidelines if you deal with small production.

    Advantages

    • It has a lesser cost than multi-cavity injection mold
    • The mold is easy to design and manufacture
    • There is better control of the injection cycle which improves parts’ quality

    Disadvantages

    • It has lower productivity.

    Multi Cavity Injection Mold

    multi cavity injection mold

    A multi-cavity injection mold has more than one cavity. Therefore, it can only produce multiple components per injection cycle. You should incorporate the single cavity injection mold into your plastic molding guidelines dealing with large production.

    Advantages

    • It has a lesser cost per unit
    • It has a higher productivity

    Disadvantages

    • The mold is hard to design and manufacture, which increases lead time
    • Higher cost of manufacturing a multi-cavity mold

    Family Mold

    A family mold has more than one core and cavities for producing different parts. Therefore, you can mold separate components in one injection cycle

    Advantages

    • It has a lesser cost of manufacturing parts
    • It has a high productivity

    Disadvantages              

    • The mold is hard to design and manufacture, which increases lead time
    • Higher cost of manufacturing a multi-cavity mold

    Male and Female Injection Molds

    male and female injection molds

    Molds that you can utilize in injection plastic mold design can also be of two types, male molds, and female molds. On the one hand, male molds, the thermoplastic sheet location is over the mold, and in female molds, the thermoplastic sheet is inside the mold. Male molds are generally applicable when the inner dimension of the plastic part is a priority and Female molds are applicable when the part’s outer dimensions are more important than its inner dimensions.

    Male Molds

    • Less expensive than female molds
    • Parts formed over a male mold retain the color and texture of the extruded sheet better. 
    • Require greater draft angles than female molds
    • Male (concave, fixed) mold (exhaust groove)

    Female Molds

    • More expensive than male molds
    • Can produce highly detailed parts 
    • Used for production of the exterior surface of the finished parts
    • Inside corners of a female mold could be tight
    • Female (convex, moving) mold

    Two Plate Mold

    The components of a two-plate mold include movable side mounting, backing plate, a support plate, movable side formwork, fixed side formwork, fixed side mounting plate, a positioning ring, runner bushing, guide bushing, guidepost, return lever, ejector bottom plate

    Three Plate Mold

    The components of a three-plate mold include movable side mounting, backing plate, a support plate, movable side formwork, fixed side formwork, spore splitter plate, fixed side mounting plate, a positioning ring, runner, runner bushing, guide bushing, guidepost, return lever, Ejector bottom plate.

    comparing the two-plate and three-plate mold in injection molding design

    Gate Design 

    gate design in injection molding design

    The gate has a direct connection to the plastic part, which is melted into the cavity. Therefore, the gate is the key part of the gating system. The shape and size of the gate have a great influence on the plastic parts. In most cases, the gate is the smallest part of the entire channel system, which controls the filling flow.

    Gates are vital in injection molding as they serve as the boundary between the part and the scrap. Therefore, their location, size, and shape play an important role in how everything should be constructed, from structural integrity to the exterior appearance of the finished product. 

    Classification Of Gates

    According to the shape of the fracture:

    • Round- Like Common needle gates, latent gates, and mainstream gate gates, etc.;
    • Rectangle- Like Common edge gates, tab gates, etc.;
    • Slit shape- Like Fan-shaped, film gates, etc.;
    • Circular section: Like main runner gate, pin point gate, latent gate etc.;
    • Rectangular section: Like edge gate, tab gate, overlapping gate, etc.;
    • Slit profile: Like fan gate, film gate, disc gate, etc.

    Gate Design Principle

    • Design the gate to spread the melted material evenly and in a single direction to fill the cavity and obtain a proper set time to cool the part.
    • The gate should be at a suitable location, such as a non-functional area or a non-appearance area.
    • The gate should be in the thickest part of the plastic, allowing the plastic to flow from the thick area to the thin area, which helps to obtain a good flow path and pressure maintaining path.
    • Place the gate in the center of the plastic part so that the melt flow to the end of the plastic part has the same flow length.
    • The position of the gate must allow the gas in the cavity to escape during injection molding.
    • The design of the position and size of the gate should also avoid jet flow.
    • The design of the gates should prevent melt backflow.
    • Numerical simulation of injection molding is an effective tool for comparing the effects of different gate designs.

    Types of Gates

    Below are the types of gates you can factor into your injection molding design guide. This will be in terms of their advantages and disadvantages.

    Direct Gate/Sprue gate

    Pros:   
    1. Little pressure loss;
    2. Easy preparation.

    Cons:  
    1. High stress around the gate;
    2. Gate scars can be left on the surface.

    Side Gate:

    Pros:
    1. Easy processing, Simple structure;
    2. Easier to remove the gate.

    Cons:
    1. There is no allowing of automatic separation of the part and the gate;
    2. Gate marks are easily left on the plastic part.

    Tab Gate:

    Pros: 
    1. It is a form evolved form of the side gate, so it shares the various advantages of the side gate;
    2. It can prevent molten plastic jetting.

    Cons: 
    1. Automatic separation of the part and the gate is not allowed;
    2. Obvious gate scars are easily left on the surface.

    Fan Gate:

    Pros:
    1. The horizontal distribution of the molten plastic is more uniform when passing through the gate, helpful for reduction of plastic part stress;
    2. Lower the possibility of air getting into the cavity to avoid the occurrence of defects, like silver lines and bubbles, etc.

    Cons:
    1. Automatic separation of the part and the gate is not allowed;
    2. Long gate marks are left on the edge of the plastic part, which needs to be flattened by a tool.

    Submarine Gate:

    Pros:
    1. Flexible choices of gate location;

    Automatic separation of the part and the gate is allowed;
    3. Smaller gate marks;
    4. Applicable for both two-plate and three-plate molds.

    Cons:
    1. You can easily drag the plastic powder at the gate position;
    2. It is easy to create stress mark at water entry;
    3. Plastic films need to be sheared manually;
    4. Great pressure loss from the gate to the cavity.

    Point Gate:

    Pros: 
    1. Flexible choices of gate location;
    2. Automatic separation of the part and the gate is allowed;
    3. Smaller gate marks;
    4. Low stress around the gate.

    Cons:
    1. High injection pressure;
    2. Complicated structure, usually employing the 3-plate structure.

    Injection Molding Design’s Parts Defects

    injection molding design parts defects

    Here, we will discuss the different defects that you can encounter in injection molding. This will be based on the causes of the defect and the appropriate countermeasures. Below are the common injection molding design defects that you can encounter.

    Insufficient Filling

    Insufficient filling is an injection molding design defect that you must take note of. It occurs when the resin is not filled into the corner of the mold cavity. Insufficient filling can lead to the following:

    • Lack fluidity of resin and insufficient internal pressure, increase maximum injection pressure and injection speed, increase mold temperature and the resin temperature
    • Insufficient plasticization
    • Increase back pressure and increase the temperature at the rear of the barrel
    • Gas, air causes insufficient injection
    • The mold is insufficiently exhausted, which is more likely to occur in a thinner part where the thickness of the mold is not uniform. Slowing down the injection speed can make the exhaust gas unobstructed. Sometimes, the mold clamping ability can be reduced to solve the exhaust failure to eliminate the filling. 
    • A mold with multiple holes, partially filled
    • Speed ​​up the shot so that it fills evenly. If the peak pressure is caused after the speed is increased, the pressure-holding position should be adjusted to suppress the peak pressure.

    In the case of insufficient filling of the molded product, the flow in the mold is good, and the resin temperature, the mold temperature, the 2-1 underfill, and the burr are simultaneously present.

    Countermeasures

    There are burrs near the runner and the gate, and the end portion of the molded article is insufficiently filled to prevent it-

    • 1.Set multi-segment injection, formed by short shots, changing the speed of underfill and flashing.
    • 2.Increase mold temperature, resin temperature, improve fluidity and slow down injection speed. If the filling is insufficient after slowing down the injection speed, the injection speed of this part is accelerated.
    • 3.Confirm the balance of the mold gate
    • Bubbles or Holes

    The voids are appearing inside the molded article cause voids in the thickness portion due to the difference in volume shrinkage of the molded article; the moisture and gas in the resin become bubbles after being bubbled.

    Flash

    The interval between the split surface into which the molten resin flows and the joint surface of the core, etc., and burrs occur after molding

    Countermeasures:

    1. 1.Check if burrs occur under normal clamping force
    2. 2.Reduce resin temperature and mold temperature, slow down injection speed
    3. 3.Change the pressure holding switching position and reduce the peak pressure

    Countermeasures

    • For the bubble, the resin temperature is lowered to prevent thermal decomposition of the resin, and back pressure is applied to prevent air from entering the resin.
    • For voids, the mold setting causes many voids, which can extend the holding time and increase the mold temperature.
    • To prevent the inhalation of air, reduce the retraction distance of the screw and slow down the retreating speed.
    • For molded articles with large thickness variations, it is difficult to discharge the gas in the mold or to take in air to slow down the injection speed.

    Sink Mark

    It is the deformation on the surface of the molded product.

    Countermeasures

    • Reduce mold temperature and resin temperature, slow down injection speed and extend dwell time

    Flow Marks

    Centering on the gate direction, the traces of the resin flow are engraved on the surface of the molded product in a concentric shape.

    Countermeasures

    • Increase resin temperature and mold temperature, and change injection speed, if necessary
    • If you cannot eliminate the error after changing the injection speed, you should inspect or repair the mold.
    • In the case of water ripples, the speed of the water ripple portion is increased, mainly because the injection speed is slow, cooling during the filling process, and wrinkles appear due to high viscosity. 
    • To lose the luster of the gate, use multiple shots to slow down this part.

    Spray Marks

    The ejection of the cavity occurs at high speed, and the molding material is ejected and cooled in contact with the wall surface of the mold. This material cannot be fused with the filling material, and you cannot obtain the desired gloss.

    Reason

    The injection speed is too fast, and the resin temperature and the mold temperature are too low.

    Countermeasures

    • Use multiple shots to slow down the exit speed of the opening and increase resin temperature and mold temperature.

    Cracking and Whitening

    There is little cracking on the surface of the molded article, especially for products with sharp corners. Therefore, the whitening phenomenon occurs as a result of the unnecessary pressure exerted by the mold release to make this part white.

    Countermeasures

    • When you open the mold, the mold release is poor, the filling is too high, and the resin and the mold temperature will be low.
    • Use a release agent to slow down the injection speed.

    Distortion and Distortion

    The molded article taken out from the mold is called “deformation” if it is a deformation in the equilibrium direction and “twisted” if it is a diagonal deformation.

    Countermeasures

    • Mold design and mold temperature adjustment are available.
    • Reduce resin temperature and mold temperature to increase pressure retention.
    • When the filling pressure is too high, increase the resin temperature, and the injection speed.
    • Once you reduce the pressure in the first stage, you should increase the pressure isn the second stage.
    • If you have uneven density inside the molded product, the deformation will occur.

    Weld Marks

    Weld marks come to be when there is a confluence of the two resins. Here, the viscosity of the resin is too high, and the injection speed is slow.

    Countermeasures

    • Increase resin temperature and mold temperature.
    • Speed ​​up injection
    • If the air and volatile components in the cavity interfere with the joining of the resin, reduce it.

    Mold Release

    The molded article is stuck to the master or the male mold of the main mold, and de-molding becomes difficult.

    Causes

    Poor mold design, injection speed is not fast.

    Countermeasures

    • Reduce the maximum injection pressure and slow down the injection speed.
    • Lower the resin temperature and use a release agent.
    • If it is stuck to the fixed side of the mold, detaching the injection port is effective.

    Charred

    The surface of the molded article is discoloured due to overheating of the resin, and the convex portion of the head of the molded article is scorched and blackened by the resistance of the resin.

    Causes

    The resin temperature is too high, or you cannot discharge the air trapped in the cavity, causing adiabatic compression and burning of the resin.

    Countermeasure

    • Slow down the speed of shooting
    • Reducing the resin temperature, the resin should not stay in the barrel for a long time.
    • Sweep or increase the mold vent.
    • If you have a worn screw or screw tip, replace the wear parts.

    Residual Stress

    When forming a molded article (either during or after forming), you generate stress. That is a strain phenomenon of the molded article, which engineers generally refer to as residual stress.

    Causes Of Residual Stress

    When you perform injection molding, if you inject an excessive resin into the cavity during pressure retention, the uneven thickness may cause a difference in shrinkage rate, which may occur during de-molding and insertion.

    Countermeasure

    • Increase resin temperature, mold temperature.
    • Reduce the maximum injection pressure and shorten the pressure holding time through the shape of the gate, such as multi-point gates, edge gates, and sheet gates.

    Conclusion

    Injection molding is an important rapid prototyping process known for its ease of operation, custom material support, and flexibility. Consequently, it is an important process in many industries. Due to the nature of the process, there is a need for a perfect understanding on what you need and how to go about the process. Therefore, this article became an injection molding guideline as it discussed almost everything you need to in injection molding.

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