This article discusses two-color mold design from three aspects: core elements, design details, and post-design self-checks.
Because two-color and multi-color products involve a wide range of industries and technologies, some technical requirements may be suitable for one product, injection molding machine, or mold type, but not for another product, injection molding machine, or mold type. Therefore, this guide is for reference only and requires adjustments based on actual conditions; it should not be copied verbatim.

 

The key to successful two-color injection mold design lies in precision, collaboration, and foresight. Following points integrate classic design principles and practical experience to systematically guide design process.

1. Basic Principles Must Not Be Violated:
- Hard plastic first, soft plastic later: Ensure stability of hard plastic as framework.
- Transparent material first, then opaque material: To avoid contamination or damage to transparent parts during secondary injection.
- High-temperature material first, then low-temperature material: To prevent first-molded part from deforming due to heat during secondary injection.
- Violating these principles will likely result in mold failure.
2. Injection Molding Machine Parameters are Foundation of Design:
- Nozzle Spacing: Center distance of product placement must strictly match fixed (or adjustable) nozzle spacing of injection molding machine. Most domestic machines are non-adjustable.
- Nozzle Direction: Determine whether nozzles are arranged along X-axis or Y-axis, thus determining product layout direction within mold.
- Ejector Roller Spacing: Spacing of ejector roller holes on mold base plate must match injection molding machine. If they differ, oblong holes must be made to accommodate this.
3. Planning of Operating Side and Non-Operating Side:
- Product injected for the first time should be placed on non-operating side. After rotating 180 degrees, product injected for second time will be positioned on operating side, facilitating automated part removal.

4. Mold Base and Guiding System:
- A dedicated two-color mold base must be used. Guide pillars, guide bushings, mold bases, etc., must be completely symmetrical in all directions to ensure precise alignment with front mold after rear mold rotates 180 degrees.
- Side locks (straight-body locks) must be positioned symmetrically on the four sides of mold center; otherwise, they will not align after rotation and will lose their positioning function.
5. Rotation and Ejection System:
- Rear mold rotates 180 degrees while front mold remains fixed. Two rear mold cores must be identical (mirror image), while two front mold cores (Cavity) must be different.
- Two independent ejection systems must be provided, corresponding to two ejector pins. Ejector pins are rotationally symmetrically positioned on rear mold.
- Ejector plate reset must only use springs; forced reset with screws is strictly prohibited to avoid interfering with rotational motion.
6. Parting Surface and Sealing Design:
- Rear Mold Parting Surface: The overall parting surface after merging two products.
- Front Mold Parting Surface: Designed separately for each individual product.
- Sealing Priority: Use near-break sealing whenever possible, avoiding penetration sealing, to increase lifespan and reliability of sealing surface. If necessary, consider modifying product design.
7. Cooling Water Channel Layout:
- All water inlets and outlets must be located on the top/bottom sides, inlet and outlet of same water channel must be on same side. "Top in, bottom out" is strictly prohibited, as water channels will become disordered after rear mold rotates.
- The overall height of mold base must consider location of injection molding machine's water channel to avoid interference and prevent water pipe connection.
8. Critical Tolerances and Datums:
- Strict tolerances (e.g., ±0.01mm) must be applied to critical dimensions such as center distance of guide pillars and guide bushings, four sides of mold base, and depth to prevent burrs caused by accumulated errors after rotation.
- When machining mold core, center of four guide pillars and bushings must be used as reference to ensure concentricity.

9. Gating System Design:
- First Injection (Hard Rubber): A submarine gate or spot gate is preferred to achieve automatic cut-off between runner and product.
- Second Injection (Soft Rubber): Gate location should avoid direct impact of molten material onto already molded first injection product. To prevent residue from first injection point from penetrating second injection material, a "wave" structure can be made at spot gate.
- Hot Runner Application: For complex or large products, this is an effective solution for optimizing filling and reducing waste.
10. Anti-Interference and Protection Design:
- Product Clearance: When designing second injection cavity, appropriate clearance can be made for first injection product in non-sealing area to prevent insertion damage. However, strength of sealing area must be strictly checked to avoid deformation and flash under injection pressure.
- Anti-impact molding: Ensure A/B plates (moving and fixed molds) close first, allowing front mold's slider or ejector to reset, preventing product damage.
- Anti-sticking: Product size of first injection can be slightly enlarged to ensure tighter compression in second injection, enhancing sealing effect and preventing deformation from second molten material impact.

11. Material Selection and Bonding:
- Common combinations include ABS/TPE, PC/ABS, etc. If two materials have poor adhesion, a rough texture (etched), serrated, or grooved mechanical interlocking structure should be designed on the first injection bonding surface.
- The smoother mold surface, the weaker bond between soft and hard plastics may be. Appropriate roughness helps with "bonding."
12. Shrinkage Management:
- Basic principle: The overall shrinkage rate is usually determined by material of first injection. When soft plastic completely encapsulates hard plastic, only hard plastic's shrinkage is considered. If contours meet, shrinkage rates of both must be considered separately.
- Special Cases: For transparent shells (ABS/PMMA), an inverted mold structure is often used, with higher-temperature PMMA (transparent part) injected first.

13. Design Experience for Transparent Two-Color Parts:
- Wall Thickness: Transparent part ≥ 0.8mm, non-transparent part ≥ 0.7mm. Use lighter colors for non-transparent parts as much as possible, and pay attention to LED light shielding.
- Structure: Simplify structure of two-color part itself. Complex ribs should be placed on mating parts. Width of ribs on non-transparent parts should be 0.5-0.6mm to prevent shrinkage.
- Appearance: Parting lines and hole edge widths must be consistent (0.5mm recommended) and curved surfaces should be smoothly joined to avoid uneven light transmission.
- Testing Focus: Drop tests and thermal shock tests (e.g., -40℃~65℃) are crucial for verifying bonding strength between two materials, as failure is very likely to occur at these temperatures.

1. Case 1: Reversed Molding Sequence Leads to Deformation and Failure of Hard Plastic
A 3C product's two-color casing was designed with soft plastic as first injection and hard plastic as second injection to simplify gate. During trial molding, molten hard plastic directly impacted soft plastic skeleton, causing sidewall deformation and dimensional errors, rendering mold unusable.
Warning: Molding sequence is crucial for two-color molds; core principle of "hard plastic first, soft plastic later" must not be violated for sake of convenience.
2. Case 2: Incorrect Water System Layout Causes Inability to Connect Water After Rotation
A home appliance handle's two-color mold had its water inlet and outlet located on the left and right sides. After rotating mold 180 degrees, inlet and outlet positions were completely reversed, making it impossible to connect with injection molding machine's water pipes. Temporary modifications to water system took 7 days, delaying project delivery.
Warning: Two-color mold water systems must strictly adhere to principle of "inlet and outlet on the top and bottom sides, same side, same group." Connection status after rotation must be repeatedly checked during initial design phase.
3. Case 3: Ejection Reset Interference, Mold Damage Due to Forced Rotation
A two-color mold for a medical accessory used a screw-driven reset structure for ejector plate. During trial molding, reset was not timely, causing rear mold to collide with front mold during rotation, resulting in bent guide pillars and chipped mold cores, causing direct losses exceeding 50,000 yuan.
Warning: Two-color mold ejection reset must use springs; rigid reset structures are strictly prohibited. Interference risks from rotational movements must be simulated beforehand.
4. Case 4: Smooth Material Bonding Surfaces, Product Delamination After Thermal Shock
A two-color automotive interior part used a combination of PP and TPE, with a high-gloss finish on bonding surface. During thermal shock testing, a large area of TPE layer peeled off. Cause was analyzed as insufficient adhesion due to lack of a mechanical interlocking structure on smooth surface.
Warning: When material compatibility is poor, textured or grooved structures must be designed on the first injection bonding surface, prioritizing strong bonding over high-gloss finishes.

 

Before finalizing design, please verify following high-risk items:
Principle Verification: Does molding sequence conform to "hard before soft, transparent before opaque, high temperature before low temperature"?
Machine Compatibility: Are product spacing, ejector pin spacing, and nozzle direction fully compatible with specified injection molding machine?
Rotational Symmetry: After rear mold rotates 180 degrees, can all positioning systems (guide pillars, side locks) and cooling water channels precisely align with front mold?
Ejection Safety: Is ejection system independent and symmetrical? Is reset solely by springs?
Sealing Reliability: Is a breakable sealant preferred? Is sealant strength sufficient at second injection position?
Gate Separation: Can first injection gate automatically cut off? Will second injection gate impact first injection product?
Anti-sticking and Anti-impact: Is an anti-rotation structure designed? Are there measures to prevent second injection from misaligning with first injection?
Joint Reinforcement: When materials are incompatible, has joint surface of first injection been roughened or equipped with a mechanical interlocking structure?
By systematically applying above points, common pitfalls in two-color mold design can be avoided to the greatest extent, ensuring smooth mold development and stable production.

1. Basic Principles of Two-Color Mold Design:
(1) Hard plastic is molded once, soft plastic twice;
(2) Transparent plastic is molded once, non-transparent plastic twice;
(3) Plastics with high molding temperatures are molded once, plastics with low molding temperatures twice;
These are basic principles for making two-color molds; otherwise, mold will be wasted. Additionally, when sealing, try to use a method of breaking sealant rather than inserting it, even when suggesting modifications to product.
2. Mold base guide pillars and bushings must be symmetrical in all directions, front and rear molds must be symmetrical.
3. Rear mold must be rotated 180 degrees, while front mold remains stationary.
4. Product spacing must be based on nozzle spacing of injection molding machine. Some foreign two-color injection molding machines have adjustable nozzle spacing, while others do not; domestic machines do not have adjustable nozzle spacing.
5. Two independent ejection systems, each with two ejector pins. Two products in rear mold are identical, and ejector pins are same; they are in a rotational relationship, not a translational one.
6. Ejector plate can only be reset by a spring, not by force with screws, because rear mold will rotate.
7. Side locks must be on all four sides of the mold center, front and rear molds must be symmetrical; otherwise, when rear mold rotates 180 degrees, it will not align with front mold.
8. If injection point spacing and injection molding machine nozzle spacing are different, ejector pin holes must be made oblong, because injection molding machine ejector pin spacing is not adjustable. Note that most domestic two-color injection molding machines have non-adjustable injection nozzles.
9. Pay attention to direction of parallel nozzles provided by customer, whether it is x-axis or y-axis, to determine product layout.
10. Water inlet and outlet directions must be on the top and bottom sides, each water circulation must enter and exit from same surface. Water cannot enter from top and exit from bottom, as rear mold will rotate 180 degrees. Ensure mold base size does not exceed height of injection molding machine's water outlet trough; otherwise, water cannot be connected.
11. Product being injected for the first time should be placed on non-operating side. After first injection, product will rotate 180 degrees for second injection, which will place it on operating side for easy product removal.
12. Exit mold's mounting position should be on operating side and non-operating side, not on the top and bottom sides, because their products require fully automatic operation.
13. Parting surface considerations: Rear mold parting surface should be parting surface obtained by merging two products; front mold parting surface should be parting surface of a single product, not parting surface of a merged product.
14. Tolerance for front and rear flanges is -0.05mm; tolerance for distance between two flanges is ±0.02mm; clearance between ejector pin and ejector pin hole is 0.1mm on each side; center distance tolerance for front and rear mold guide bushings and guide pillars is ±0.01mm; tolerances must be added to four sides and depth of mold base, otherwise, when rear mold rotates 180 degrees, unevenness will cause burrs. Frame depth tolerance is -0.02mm.
15. If mold base has already been machined at mold base factory, when our factory processes sprue and ejector pin holes, distance between center of four guide pillar and guide bushing holes must be used as reference; otherwise, too much deviation may easily cause mold to jam. When ordering mold base, it must be specified that it is a two-color mold base, with four guide pillars, guide bushings, and frame symmetrical, so that rear mold can match front mold after rotating 180 degrees.
16. For a two-color mold with a rotating rear mold, it's much simpler. Both front mold cores are identical. After injecting hard plastic on one side, rotate it 180 degrees (ensure product doesn't fall during rotation; gate should automatically separate). Then inject soft plastic on the other side. No ejector pins are needed for hard plastic injection; simply place them on soft plastic side of mold core. Pay attention to shrinkage: if soft plastic completely wraps around hard plastic, only hard plastic's shrinkage needs to be accounted for; if contours meet, both hard and soft plastic shrinkage needs to be accounted for.
17. For a two-color mold with one set of molds, there's one straight injection barrel and one 90-degree injection barrel. No rotation is needed; only one mold core is required. Separation of soft and hard plastic is achieved through slide sealing.
18. Two shapes of cavity are different, each molding one type of product. Two shapes of core are identical.
19. Front and rear molds must align after rotating 180 degrees around center. This check must be performed during design phase.
20. Pay attention to position of ejector pin holes; minimum spacing should be 210mm. Larger molds require an appropriate increase in the number of ejector pin holes. Furthermore, since ejector pins provided with injection molding machine are not long enough, our mold must be designed with extended ejector pins, extending approximately 150mm beyond mold base plate.
21. Two locating rings must be designed on the rear mold base plate.
22. The total thickness of front mold panel plus A-plate should not be less than 170mm. Please carefully review other reference data for this type of injection molding machine, such as maximum mold thickness, minimum mold thickness, and ejector pin hole spacing.
23. Depth of front sprue should not exceed 65mm. Distance from top of upper (large gate) sprue to center of mold base should not be less than 150mm.
24. When designing cavity for second injection, to prevent cavity from inserting (or scratching) previously molded product, a partial clearance can be designed. However, strength of each sealing point must be carefully considered. Specifically, during injection molding, is there a possibility that plastic will deform under high injection pressure, potentially causing flash during second injection?
25. During injection molding, product size of first injection molded part can be slightly larger so that it can be pressed more tightly against the other cavity during second molding, achieving sealing effect.
26. Before closing A and B plates, consider whether front mold slider or lifter will reset first and damage product. Therefore, a method must be found to ensure that A and B plates close first, allowing front mold slider or lifter to reset.
27. Water distribution of two cavities and core should be as sufficient, balanced, and uniform as possible.
28. In 99% of cases, hard plastic parts of product are injected first, followed by soft plastic parts. This is because soft plastic is easily deformed.
29. During second injection, be aware of whether movement of plastic might agitate already molded product from first injection, causing deformation of molded area. If this is possible, find a way to mitigate the issue.
30. Carefully select gate location for two-color molds. For the first injection, a submarine gate is best, allowing for automatic cut-off between product and runner. When a submarine gate is not possible, consider a three-platen mold or a hot runner mold. If first injection uses a point gate, create a wave-shaped gate to prevent residue from the first injection point from piercing second injection.
31. Two-color injection molds often use rotary injection molds. Punches/dies in both positions of a rotary injection mold must have consistent dimensions and precision, and a good fit with dies/punches. When ejector mechanism on two-color injection molding machine cannot be used, a hydraulic ejector mechanism must be installed on rotary table.
32. Two-color injection molding typically uses same type of plastic in different colors, or it can use two different plastic raw materials. In this case, interfacial interaction between two materials, difference in shrinkage rate, and processing parameters must be considered.
Two-color injection molded products generally use hard plastics such as ABS and PC combined with soft plastics such as TPE. Due to cost or application considerations, it is crucial to fully consider potential lack of good adhesion and fusion between two materials. Many issues need to be addressed, including mold treatment at joint between two materials (usually resulting in embossing or requiring a sealing groove), and thickness of materials. To ensure a tighter bond between two plastics, adhesion between materials and roughness of mold surface must be considered. Two-color injection molding uses specialized TPU materials; the smoother mold surface, the tighter they "stick."

 

33. Generally, shrinkage rate of two-color molds depends on primary material. Because primary material already supports outline of plastic product, secondary material won't shrink much. Determining primary and secondary materials involves many factors, such as raw material flowability and shape of plastic product.
34. Pay attention to positioning of front and rear molds; difference in slope between all insertion and break surfaces should be as large as possible, at least 0.1mm.
35. For ABS/PC, ABS/PC+ABS, and ABS/PMMA two-color injection molding, higher-temperature PC, PC+ABS, or PMMA should be injected first. For transparent shell molds, inverted mold structures are mostly used.
36. For large, transparent two-color injection molded parts, following structural design considerations are necessary:
Gate location must be agreed upon with mold manufacturer beforehand.
Recommended material thickness is 0.8mm or thicker for transparent parts and 0.7mm or thicker for non-transparent parts. Use lighter colors for non-transparent parts and take precautions to block LED lights.
Parting line and holes of transparent parts should have consistent width, ideally 0.5mm. Ensure smooth transitions between curved surfaces to avoid excessive light penetration from side, which is aesthetically pleasing. Minimize number of holes.
Thickness of non-transparent parts is limited. Ribs and other structural elements should ideally be between 0.5-0.6mm to avoid shrinkage.
Currently, two-color injection molding with a main lens is not recommended due to high mold and product costs. It also places significant demands on mold manufacturer's equipment and technical expertise. If this is undertaken, a detailed solution for the entire process must be provided for evaluation, and each step must be reviewed.
For large-area two-color injection molded parts, two key tests are drop and thermal shock. In these tests, transparent and non-transparent parts are prone to separation. Thermal shock -40℃ -65, time 48 hours.
Simplify structure of two-color parts as much as possible; complex structures should be placed on mating parts. Width of ribs on non-transparent parts should be kept between 0.5-0.6 mm to avoid shrinkage.

Precision of two-color mold design determines success or failure of trial molding. A minor oversight can lead to tens or even hundreds of thousands of dollars in mold modification costs and delays in project delivery. After design is completed, a systematic self-check to eliminate potential problems is a key step in reducing trial molding risks. Following 8 core self-check dimensions cover the entire process from mold architecture to process adaptation, helping designers efficiently complete self-correction and rectification.

1. Verify that molding sequence follows three core principles: hard plastic first, soft plastic second; transparent first, non-transparent third; high-temperature material first, low-temperature material third. There should be no arbitrary reversal of order due to convenient gate design.
2. Confirm the compatibility of two injection molding materials. If adhesion is poor, check if the first injection joint surface is designed with mechanical interlocking structures such as textured surfaces, serrations, or grooves to prevent delamination and detachment of finished product.
2. Check if shrinkage rate setting is accurate. Ensure the overall shrinkage rate is based on the first injection material. When soft plastic completely encapsulates hard plastic, is only hard plastic's shrinkage factor considered? For two-color parts with intersecting contours, are shrinkage parameters of both materials considered separately?

1. Verify that product's center distance is completely consistent with injection molding machine's nozzle spacing. This is especially important for domestic models with non-adjustable nozzles. There should be no risk of injection position deviation due to spacing discrepancies.
2. Confirm that nozzle arrangement direction (X-axis or Y-axis) matches product layout direction within mold. There should be no mold installation problems caused by incorrect orientation.
3. Check if ejector pin hole spacing on mold base plate matches injection molding machine's ejector pin position. If they do not match, check if a slotted hole has been designed to prevent ejection system from malfunctioning.

1. Confirm that mold base is a dedicated two-color mold base, and that key components such as guide pillars, guide bushings, mold bases are completely symmetrical horizontally and vertically. Rear mold should precisely match front mold after rotating 180 degrees, with no potential positioning deviation.
2. Verify that side locks (straight-body locks) are positioned on four sides of mold center, symmetrically distributed between front and rear molds to prevent loss of positioning function after rotation, which could lead to mold misalignment upon mold closing.
3. Check that two rear mold cores are completely mirror images of each other, and that two front mold cores correspond to different cavities of two-color product, with no errors in mold core machining.

1. Confirm that mold is equipped with two independent ejection systems, with ejector pins arranged in a rotationally symmetrical relationship on the rear mold, meeting ejection requirements of both stations.
2. Verify that ejector plate reset method uses only spring reset, without a screw-driven forced reset structure, to avoid interference with rear mold rotation during reset.
3. Check that ejector pins, ejector tubes, and other ejection components do not interfere with cavity and core, that ejection trajectory is smooth, and that there is no risk of product damage or deformation due to uneven ejection.

1. Confirm that all water inlets and outlets are located on the top/bottom sides of mold, that inlet and outlet of same water circuit are on same side, avoiding erroneous "top in, bottom out" design to prevent water circuit confusion after rear mold rotates.
2. Verify that the total height of mold base takes into account location of injection molding machine's water channel, and that there are no issues with water pipes not connecting due to height discrepancies.
3. Check that water circuit covers all cavity and core areas, that cooling is uniform, and that there are no potential risks of uneven product shrinkage or warping due to insufficient localized cooling.

1. Confirm that rear mold parting surface is the overall parting surface after two products are combined, and that front mold parting surface is designed separately for each individual product, with clear and unambiguous parting surfaces.
2. Verify that sealing structure prioritizes use of puncture-resistant sealants, avoiding use of piercing sealants. If puncture-resistant sealants are necessary, ensure sealant surface strength has been reinforced to prevent deformation and flash under injection pressure.
3. Check that second injection cavity provides adequate clearance for the first injection product in non-sealing areas, with reasonable clearance dimensions and no risk of damage to first injection product. Simultaneously, ensure sealant strength meets injection pressure requirements.

1. Confirm that first injection (rigid plastic) gate prioritizes use of a submarine gate or a spot gate, enabling automatic cut-off between runner and product, eliminating need for manual runner trimming.
2. Verify that second injection (soft plastic) gate location avoids critical surfaces of first injection product, preventing deformation and damage caused by direct impact of molten material on molded product; check if a "wave" structure is designed at spot gate to prevent residual material from the first injection point from penetrating secondary material.
3. Check temperature control and gate location of hot runner system (if applicable) to ensure they are reasonable and that there are no product quality issues caused by hot runner leakage or uneven temperature.

1. Confirm that mold design includes an anti-blowout structure to ensure that A/B plates (moving and fixed molds) close first, and front mold slide or ejector resets, eliminating risk of slide or ejector damaging product.
2. Verify that dimensions of first injection product are slightly enlarged to ensure it is compressed within second injection cavity, enhancing sealing effect and preventing deformation caused by impact of second injection molten material.
3. Check if mold has a reasonable venting structure, that venting grooves are appropriately positioned and sized, without any issues such as insufficient venting leading to missing glue, bubbles, or burning in product.

Self-inspection after designing a two-color mold focuses on three key principles: "symmetry, matching, and anti-interference." This involves verifying each aspect from four levels: materials, machinery, mold structure, and process details. Through comprehensive self-inspection and correction across these eight dimensions, over 90% of trial molding errors can be effectively avoided, reducing mold modification costs, shortening mold development cycle, and ensuring smooth production of two-color mold.