During the market promotion of polyvinyl acetate produced by Nitchen Group, some SMC industry customers still use LDPE as a low shrinkage agent due to its low surface requirements and the desire to maintain their original production and sales. However, with the development of the global economy and China’s increasingly prominent role in the world economy, especially in the manufacturing industry, a more efficient low shrinkage solution has become an inevitable trend. The method of replacing LDPE with polyvinyl acetate low shrinkage agent and the comparison of the effects after replacement are briefly listed below. This solution is limited to the low shrinkage additive polyvinyl acetate solid resin PVAC produced by Nitchen Group.

1. Implementation method of replacing LDPE with PVAC
2. Formula replacement ratio: Based on the addition amount of LDPE in the original SMC formula (typically 5-10% of the unsaturated resin (UP) mass), PVAC is substituted using an equal mass or adjusted as needed. The recommended initial addition amount is 6-9% of the UP mass (it can be slightly adjusted according to the dimensional accuracy requirements of the product, and can be increased to 8-10% for high-end precision parts).
3. Hybrid Process: During the preparation stage of SMC sheets, PVAC is added to a mixing device along with raw materials such as UP resin, chopped carbon fibers, initiators, and accelerators, and stirred at low speed for 5-8 minutes to ensure thorough wetting. PVAC exhibits excellent compatibility with UP resin, allowing for uniform dispersion without the need for additional compatibilizers.
4. Core Advantages after Replacement
5. Ultimate low shrinkage control: It can reduce the volume shrinkage rate of UP resin from 6-10% during molding to 0.05%, completely solving problems such as product cracks, bending, and dimensional deviations, and significantly improving dimensional accuracy and molding stability.
6. High-end surface performance: The surface glossiness of the product can reach Grade A, without flow marks or ripple defects, which does not affect subsequent secondary processes such as painting and coating. It is suitable for high-surface-quality applications such as automotive headlights and high-end electrical accessories.
7. Mechanical property upgrade: Effectively enhance the flexibility and impact strength of the product, prevent cracking during drilling and punching, and improve the durability of the product.
8. Strong process adaptability: Its compatibility with UP resin is far superior to that with LDPE, eliminating the need to change the existing core processes of mixing and sheet preparation, thereby reducing process switching costs.

III. Corresponding process adjustment plan

1. Optimization of molding parameters

• Temperature: Maintain the original molding temperature range (130-150℃), as PVAC can fully soften and exhibit its low shrinkage effect at this temperature. For thick products with a thickness of >5mm, the mold temperature can be increased by 5-10℃ to ensure uniform molding of the internal material.
• Pressure: Maintain the SMC standard molding pressure of 10-25 MPa. For complex structures or high-precision products, the pressure can be adjusted upward to 20-25 MPa to ensure that the material completely fills the mold cavity.
• Holding time: Due to the extremely low shrinkage rate of PVAC, the holding time can be extended by 10-15% (e.g., from the original 10 minutes to 11-11.5 minutes), further ensuring the dimensional stability of the product. The curing time should follow the original process and does not require significant changes.

2. Material feeding and molding adjustment

• Feed quantity: The original LDPE formula requires a shrinkage allowance. After replacing with PVAC, the shrinkage can be neglected. The feed quantity is adjusted to be the product volume × actual finished product density × (1+0.1%), reducing raw material waste.
• Feeding position and area: The main feeding position remains in the middle of the mold cavity; as PVAC can improve material flowability, the feeding area for complex structural products can be appropriately reduced to 65-70% to avoid uneven fiber orientation.

3. Mold and post-processing adjustment

• Mold release management: PVAC products have a high degree of surface smoothness, making mold release smoother. The amount of mold release agent can be reduced by 10-15%, while still ensuring that the mold surface is clean and the mold release agent is applied evenly.
• Post-processing: After demolding, the product can proceed to the subsequent process without additional aging treatment, thereby shortening the production cycle.

1. Trial production verification plan

During the trial production stage, the shrinkage rate (as per DIN16901 standard), surface glossiness, and impact strength of each batch of products are tested to ensure that the shrinkage rate is ≤0.1%, the surface reaches Grade A, and the impact strength is improved by ≥15%. If uneven local shrinkage occurs, the amount of PVAC added can be increased by 0.5-1% or the holding time can be extended by 30-60 seconds. If there are surface defects, the amount of release agent or mold temperature can be slightly adjusted.